Search

With the discovery of gravitational waves (GWs), the disks of Active Galactic Nuclei (AGN) have emerged as an interesting environment for hosting a fraction of their sources. AGN disks are conducive to forming both long and short Gamma-Ray Bursts (GRBs), and their anticipated cosmological occurrence within these disks has potential to serve as an independent tool for probing and calibrating the population of stars and compact objects within them, and their contribution to the GW-detected population. In this study, we employ Monte Carlo methods in conjunction with models for GRB electromagnetic emission in extremely dense media to simulate the cosmological occurrence of both long and short GRBs within AGN disks, while also estimating their detectability across a range of wavelengths, from gamma-rays to radio frequencies. {We investigate two extreme scenarios: ``undiffused", in which the radiation escapes without significant scattering (i.e. if the progenitor has excavated a funnel within the disk), and ``diffused", in which the radiation is propagated through the high-density medium, potentially scattered and absorbed. {In the diffused case,} we find that the majority of detectable GRBs are likely to originate from relatively low redshifts, and from the outermost regions of large supermassive black hole (SMBH) masses, $\gtrsim 10^{7.5} \rm M_{\odot}$. In the undiffused case, we expect a similar trend, but with a considerable contribution from the intermediate regions of lower SMBH masses. Detectable emission is generally expected to be dominant in prompt $\gamma$-rays if diffusion is not dominant, and X-ray afterglow if diffusion is important; however, the nature of the dominant observable signal highly depends on the specific AGN disk model, hence making GRBs in AGN disks also potential probes of the structure of the disks themselves., Comment: 18 pages, 16 figures

The fate of the binary neutron star (NS) merger remnants hinges sensitively upon the NS equation of state and the threshold mass, $M_{\rm ls}$, that separates a long-lived from a short-lived NS remnant. The nature of the electromagnetic counterparts is also influenced by the remnant type, particularly in determining whether a gamma-ray burst from a compact binary merger (cbGRB) is of short or long duration. We propose a novel approach to probe the threshold mass by linking it to the estimated observed ratio of long to short cbGRBs. We find that current observations broadly favour a relatively high value for this transition, $M_{\rm ls}\simeq 1.3 M_{\rm TOV}$, for which $ M_{\rm TOV} \lesssim 2.6\,M_\odot $, consistent with numerical simulations, as also shown here. Our results disfavour nuclear physics scenarios that would lead to catastrophic pressure loss at a few times nuclear density and temperatures of tens of MeV, leading to a rapid gravitational collapse of binaries with total mass $M \lesssim 1.3 M_{\rm TOV}$. Future individual gravitational wave events with on-axis cbGRBs can further bound $M_{\rm ls}$., Comment: 10 pages, 5 figures. Comments are welcome

One of the most surprising gamma-ray burst (GRB) features discovered with the Swift-X ray telescope (XRT) is a plateau phase in the early X-ray afterglow light curves. These plateaus are observed in the majority of long GRBs, while their incidence in short GRBs (SGRBs) is still uncertain due to their fainter X-ray afterglow luminosity with respect to long GRBs. An accurate estimate of the fraction of SGRBs with plateaus is of utmost relevance given the implications that the plateau may have for our understanding of the jet structure and possibly of the nature of the binary neutron star (BNS) merger remnant. This work presents the results of an extensive data analysis of the largest and most up-to-date sample of short GRBs observed with the XRT, and for which the redshift has been measured. We find a plateau incidence of 18-37% in short GRBs, which is a significantly lower fraction than that measured in long GRBs (>50%). Although still debated, the plateau phase could be explained as energy injection from the spin-down power of a newly born magnetized neutron star (NS; magnetar). We show that this scenario can nicely reproduce the observed short GRB plateaus, while at the same time providing a natural explanation for the different plateau fractions between short and long GRBs. In particular, our findings may imply that only a minority of BNS mergers generating short GRBs leave behind a sufficiently stable or long-lived neutron star (NS) to form a plateau. From the probability distribution of the BNS remnant mass, a fraction 18-37% of short GRB plateaus implies a maximum NS mass in the range $\sim$ 2.3 - 2.35 M$_{\odot}$., Comment: 13 pages, 9 figures

The disks of Active Galactic Nuclei (AGNs) are expected to be populated by numerous stars, either formed in the outer regions of the disk via gravitational instability, or captured from the nearby nuclear star cluster. Regardless of their formation mechanism, these stars experience altered evolutionary paths, mostly shaped by the accretion of dense disk material. In this study, through the comparison of different timescales, we chart the evolutionary outcomes of these AGN stars as a function of disk radius and across a range of supermassive black hole (SMBH) masses, spanning from $10^6$ to $10^9 \rm M_\odot$, for two popular AGN disk models. We find that, in the outer regions of the disk, stars evolve similarly to those in the interstellar medium, but in the inner and denser regions accretion quickly turns low-mass stars into massive stars, and their fate depends on just how quickly they accrete. If accretion occurs at a faster rate than nuclear burning, they can reach a quasi-steady `immortal' state. If stars accrete faster than they can thermally adjust, runaway accretion occurs, potentially preventing a quasi-steady state and altering the disk structure. During the AGN lifetime, in the regions of the disk that produce massive stars, supernovae (SNe) and Gamma-Ray Bursts (GRBs) may occur within the disk over a wide range of optical depths and ambient densities. Subsequently, in the final phase of the AGN, as the disk becomes depleted, formerly immortal stars will be unable to replenish their fuel, leading to additional SNe and GRBs., Comment: 14 pages, 7 figures

Star-forming regions, characterized by dense environments, experience frequent encounters that significantly influence binary systems, leading to their hardening, softening, or ionization. We extend the Hut \& Bahcall formalism to derive an analytical expression for the ionization cross-section in extreme mass ratio binary systems, allowing us to investigate the orbital evolution and population dynamics of binary planets and binary brown dwarfs in star clusters, while considering ongoing binary system formation. Our findings reveal that for low-mass soft binaries, the semi-major axis distribution asymptotes to a universal power law between $\propto a^{-8/3}$ and $\propto a^{-5/3}$ over the derived ionization timescale. We also discuss the implications of our results for the candidate Jupiter-mass binary objects putatively reported in the Trapezium cluster. We demonstrate that if their existence is verified, they likely form continuously with a spectrum proportional to $a^{1}$, aligning better with the ejection mechanism than with the in-situ formation mechanism, which predicts a distribution roughly proportional to $a^{-1}$. However, this implies an impractically high ejection formation rate. Alternatively, if these objects are binary brown dwarfs, continuous in-situ formation ($\propto a^{-1}$) with an initial minimal semi-major axis around 20 AU and a formation rate of 100 Myr$^{-1}$ plausibly matches the observed number of single objects, binary number, binary fraction, and semi-major axis distribution., Comment: Comments are welcome

The brightest steady sources of radiation in the universe, active galactic nuclei (AGN), are powered by gas accretion onto a central supermassive black hole (SMBH). The large sizes and accretion rates implicated in AGN accretion disks are expected to lead to gravitational instability and fragmentation, effectively cutting off mass inflow to the SMBH. Radiative feedback from disk-embedded stars has been invoked to yield marginally stable, steady-state solutions in the outer disks. Here, we examine the consequences of this star formation with a semi-analytical model in which stellar-mass black hole (sBH) remnants in the disk provide an additional source of stabilizing radiative feedback. Assuming star formation seeds the embedded sBH population, we model the time-evolving feedback from both stars and the growing population of accreting sBHs. We find that in the outer disk, the luminosity of the sBHs quickly dominates that of their parent stars. However, because sBHs consume less gas than stars to stabilize the disk, the presence of the sBHs enhances the mass flux to the inner disk. As a result, star formation persists over the lifetime of the AGN, damped in the outer disk, but amplified in a narrow ring in the inner disk. Heating from the embedded sBHs significantly modifies the disk's temperature profile and hardens its spectral energy distribution, and direct emission from the sBHs adds a new hard X ray component, resembling a Compton reflection "hump"., Comment: 21 pages, 15 figures

At least three members of the recently identified class of fast luminous blue optical transient show evidence of late-time electromagnetic activity in great excess of what predicted by an extrapolation of the early time emission. In particular, AT2022tsd displays fast, bright optical fluctuations approximately a month after the initial detection. Here, we propose that these transients are produced by exploding stars in black hole binary systems, and that the late-time activity is due to the accretion of clumpy ejecta onto the companion black hole. We derive the energetics and timescales involved, compute the emission spectrum, and discuss whether the ensuing emission is diffused or not in the remnant. We find that this model can explain the observed range of behaviors for reasonable ranges of the orbital separation and the ejecta velocity and clumpiness. Close separation and clumpy, high velocity ejecta result in bright variable emission, as seen in AT2022tsd. A wider separation and smaller ejecta velocity, conversely, give rise to fairly constant emission at a lower luminosity. We suggest that high-cadence, simultaneous, panchromatic monitoring of future transients should be carried out to better understand the origin of the late emission and the role of binarity in the diversity of explosive stellar transients., Comment: ApJL in press. Formally a revised version, but practically a completely new work that focuses on interpreting late activity in FLBOTs

The inflated radii observed in hundreds of Hot Jupiters represent a long-standing open issue. The observed correlation between radii and irradiation strength, and the occasional extreme cases, nearly double the size of Jupiter, remain without a comprehensive quantitative explanation. In this investigation, we delve into this issue within the framework of Ohmic dissipation, one of the most promising mechanisms for explaining the radius anomaly. Using the evolutionary code MESA, we simulate the evolution of irradiated giant planets, spanning the range 1 to 8 Jupiter masses, incorporating an internal source of Ohmic dissipation located beneath the radiative-convective boundary. Our modeling is based on physical parameters, and accounts for the approximated conductivity and the evolution of the magnetic fields, utilizing widely-used scaling laws. We compute the radius evolution across a spectrum of masses and equilibrium temperatures, considering varying amounts of Ohmic dissipation, calculated with the internal conductivity profile and an effective parametrization of the currents, based on the typical radius of curvature of the field lines. Our analysis reveals that this internal Ohmic dissipation can broadly reproduce the range of observed radii using values of radius of curvature up to about one order of magnitude lower than what we estimate from the Juno measurements of the Jovian magnetosphere and from MHD dynamo simulations presented herein. The observed trend with equilibrium temperature can be explained if the highly-irradiated planets have more intense and more small-scale magnetic fields. This suggests the possibility of an interplay between atmospherically induced currents and the interior, via turbulence, in agreement with recent box simulations of turbulent MHD in atmospheric columns., Comment: 13 pages, 14 figures

Nuclear star clusters (NSCs), made up of a dense concentrations of stars and the compact objects they leave behind, are ubiquitous in the central regions of galaxies, surrounding the central supermassive black hole (SMBH). Close interactions between stars and stellar-mass black holes (sBH) lead to tidal disruption events (TDEs). We uncover an interesting new phenomenon: For a subset of these, the unbound debris (to the sBH) remain bound to the SMBH, accreting at a later time, and thus giving rise to a second flare. We compute the rate of such events, and find them ranging within $10^{-6}$ -$10^{-3}$ yr$^{-1}$gal$^{-1}$ for SMBH mass $\simeq 10^{6}-10^{9}M_\odot$. Time delays between the two flares spread over a wide range, from less than a year to hundreds of years. The temporal evolution of the light curves of the second flare can vary between the standard $t^{-5/3}$ power-law to much steeper decays, providing a natural explanation for observed light curves in tension with the classical TDE model. Our predictions have implications for learning about NSC properties and calibrating its sBH population. Some double flares may be electromagnetic counterparts to LISA Extreme-Mass-Ratio-Inspiral (EMRI) sources. Another important implication is the possible existence of TDE-like events in very massive SMBHs, where TDEs are not expected. Such flares can affect spin measurements relying on TDEs in the upper SMBH range., Comment: Accepted for publication in ApJL

The thermal evolution of isolated neutron stars is a key element in unraveling their internal structure and composition and establishing evolutionary connections among different observational subclasses. Previous studies have predominantly focused on one-dimensional or axisymmetric two-dimensional models. In this study, we present the thermal evolution component of the novel three-dimensional magnetothermal code MATINS (MAgneto-Thermal evolution of Isolated Neutron Star). MATINS employs a finite volume scheme and integrates a realistic background structure, along with state-of-the-art microphysical calculations for the conductivities, neutrino emissivities, heat capacity, and superfluid gap models. This paper outlines the methodology employed to solve the thermal evolution equations in MATINS, along with the microphysical implementation which is essential for the thermal component. We test the accuracy of the code and present simulations with non-evolving magnetic fields of different configurations (all with electrical currents confined to the crust and a magnetic field that does not thread the core), to produce temperature maps of the neutron star surface. Additionally, for a specific magnetic field configuration, we show one fully coupled evolution of magnetic field and temperature. Subsequently, we use a ray-tracing code to link the neutron star surface temperature maps obtained by MATINS with the phase-resolved spectra and pulsed profiles that would be detected by distant observers. This study, together with our previous article focused on the magnetic formalism, presents in detail the most advanced evolutionary code for isolated neutron stars, with the aim of comparison with their timing properties, thermal luminosities and the associated X-ray light curves., Comment: 26 pages, 16 figures; published in MNRAS

The astrophysical origin of stellar-mass black hole (BH) mergers discovered through gravitational waves (GWs) is widely debated. Mergers in the disks of active galactic nuclei (AGN) represent promising environments for at least a fraction of these events, with possible observational clues in the GW data. An additional clue to unveil AGN merger environments is provided by possible electromagnetic emission from post-merger accreting BHs. Associated with BH mergers in AGN disks, emission from shocks emerging around jets launched by accreting merger remnants is expected. In this paper we compute the properties of the emission produced during breakout and the subsequent adiabatic expansion phase of the shocks, and we then apply this model to optical flares suggested to be possibly associated with GW events. We find that the majority of the reported flares can be explained by the breakout and the shock cooling emission. If these events are real, then the merging locations of binaries are constrained depending on the emission processes. If the optical flares are produced by shock cooling emission, they would display moderate color evolution, possibly color variations among different events, a positive correlation between the delay time and the duration of flares, and accompanying breakout emission in X-ray bands before the optical flares. If the breakout emission dominates the observed lightcurve, it is expected that the color is distributed in a narrow range in the optical band, and the delay time from GW to electromagnetic emission is longer than $\sim 2$ days. Hence, further explorations of the distributions of delay times, color evolution of the flares, and associated X-ray emission will be useful to test the proposed emission model for the observed flares., Comment: 15 pages, 3 figures

The discovery of planetary systems beyond our solar system has challenged established theories of planetary formation. Planetary orbits display a variety of unexpected architectures, and free-floating planets appear ubiquitous. The recent detection of candidate Jupiter Mass Binary Objects (JuMBOs) by the James Webb Space Telescope (JWST) has added another puzzling layer. Here, through direct few-body simulations, we demonstrate that JuMBOs could arise from the ejection of double giant planets following a close encounter with a passing star, if the two planets are nearly aligned at the closest approach. These ejected JuMBOs typically possess an average semi-major axis approximately three times the orbital separation within their original planetary system and a high eccentricity, characterized by a superthermal distribution that sets them apart from those formed primordially. We estimate the JuMBO formation rate per planetary system in typical and densely populated clusters, revealing a significant environmental dependence. In dense clusters, this formation rate can reach a few percent for wide planetary systems. Comparative analysis of JuMBO rates and properties with current and forthcoming JWST observations across various environments promises insights into the conditions under which these giant planets formed in protoplanetary disks, thereby imposing constraints on theories of giant planet formation., Comment: Matches version published in Nature Astronomy; Videos are available at https://yihanwangastro.github.io/#posts2

Tidal Disruption Events (TDEs) are routinely observed in quiescent galaxies, as stars from the nuclear star cluster are scattered into the loss cone of the central supermassive black hole (SMBH). TDEs are also expected to occur in Active Galactic Nuclei (AGN), due to scattering or orbital eccentricity pumping of stars embedded in the innermost regions of the AGN accretion disk. Encounters with embedded stellar-mass black holes (BH) can result in AGN $\mu$TDEs. AGN TDEs and $\mu$TDEs could therefore account for a fraction of observed AGN variability. Here, by performing scattering experiments with the few-body code {\tt SpaceHub}, we compute the probability of AGN TDEs and $\mu$TDEs as a result of 3-body interactions between stars and binary BHs. We find that AGN TDEs are more probable during the early life of the AGNs, when rates are $\sim (6\times 10^{-5}-5 \times 10^{-2}) (f_\bullet/0.01)$ $~\rm{AGN}^{-1}$~yr$^{-1}$ (where $f_\bullet$ is the ratio between the number density of BHs and stars), generally higher than in quiescent galactic nuclei. By contrast, $\mu$TDEs should occur throughout the AGN lifetime at a rate of $\sim (1\times 10^{-4} - 4\times 10^{-2} (f_\bullet/0.01)$ $~\rm{AGN}^{-1}$~yr$^{-1}$. Detection and characterization of AGN TDEs and $\mu$ AGN TDEs with future surveys using {\em Rubin} and {\em Roman} will help constrain the populations of stars and compact objects embedded in AGN disks, a key input for the LVK AGN channel., Comment: Final version accepted by MNRAS

Multi-body dynamical interactions of binaries with other objects are one of the main driving mechanisms for the evolution of star clusters. It is thus important to bring our understanding of three-body interactions beyond the commonly employed point-particle approximation. To this end we here investigate the hydrodynamics of three-body encounters between star-black hole (BH) binaries and single stars, focusing on the identification of final outcomes and their long-term evolution and observational properties, using the moving-mesh hydrodynamics code AREPO. This type of encounters produces five types of outcomes: stellar disruption, stellar collision, weak perturbation of the original binary, binary member exchange, and triple formation. The two decisive parameters are the binary phase angle, which determines which two objects meet at the first closest approach, and the impact parameter, which sets the boundary between violent and non-violent interactions. When the impact parameter is smaller than the semimajor axis of the binary, tidal disruptions and star-BH collisions frequently occur when the BH and the incoming star first meet, while the two stars mostly merge when the two stars meet first instead. In both cases, the BHs accrete from an accretion disk at super-Eddington rates, possibly generating flares luminous enough to be observed. The stellar collision products either form a binary with the BH or remain unbound to the BH. Upon collision, the merged stars are hotter and larger than main sequence stars of the same mass at similar age. Even after recovering their thermal equilibrium state, stellar collision products, if isolated, would remain hotter and brighter than main sequence stars until becoming giants., Comment: 16 pages, 10 figures, 2 tables. Submitted to MNRAS. Movie here: https://studio.youtube.com/playlist/PLxLK3qI02cQf_x96DSKQWVS251Jog7IOE/videos

We present the first numerical simulations that track the evolution of a black hole-neutron star (BH-NS) merger from pre-merger to $r\gtrsim10^{11}\,{\rm cm}$. The disk that forms after a merger of mass ratio $q=2$ ejects massive disk winds ($3-5\times10^{-2}\,M_{\odot}$). We introduce various post-merger magnetic configurations and find that initial poloidal fields lead to jet launching shortly after the merger. The jet maintains a constant power due to the constancy of the large-scale BH magnetic flux until the disk becomes magnetically arrested (MAD), where the jet power falls off as $L_j\sim t^{-2}$. All jets inevitably exhibit either excessive luminosity due to rapid MAD activation when the accretion rate is high or excessive duration due to delayed MAD activation compared to typical short gamma-ray bursts (sGRBs). This provides a natural explanation for long sGRBs such as GRB 211211A but also raises a fundamental challenge to our understanding of jet formation in binary mergers. One possible implication is the necessity of higher binary mass ratios or moderate BH spins to launch typical sGRB jets. For post-merger disks with a toroidal magnetic field, dynamo processes delay jet launching such that the jets break out of the disk winds after several seconds. We show for the first time that sGRB jets with initial magnetization $\sigma_0>100$ retain significant magnetization ($\sigma\gg1$) at $r>10^{10}\,{\rm cm}$, emphasizing the importance of magnetic processes in the prompt emission. The jet-wind interaction leads to a power-law angular energy distribution by inflating an energetic cocoon whose emission is studied in a companion paper., Comment: For movies of the simulations, see https://oregottlieb.com/bhns.html

The ongoing LIGO-Virgo-KAGRA observing run O4 provides an opportunity to discover new multi-messenger events, including binary neutron star (BNS) mergers such as GW170817, and the highly anticipated first detection of a multi-messenger black hole-neutron star (BH-NS) merger. While BNS mergers were predicted to exhibit early optical emission from mildly relativistic outflows, it has remained uncertain whether the BH-NS merger ejecta provides the conditions for similar signals to emerge. We present the first modeling of early near-ultraviolet/optical emission from mildly relativistic outflows in BH-NS mergers. Adopting optimal binary properties: a mass ratio of $q=2$ and a rapidly rotating BH, we utilize numerical relativity and general relativistic magnetohydrodynamic (GRMHD) simulations to follow the binary's evolution from pre-merger to homologous expansion. We use an M1 neutrino transport GRMHD simulation to self-consistently estimate the opacity distribution in the outflows and find a bright near-ultraviolet/optical signal that emerges due to jet-powered cocoon cooling emission, outshining the kilonova emission at early time. The signal peaks at an absolute magnitude of $\sim -15$ a few hours after the merger, longer than previous estimates, which did not consider the first principles-based jet launching. By late 2024, the Rubin Observatory will have the capability to track the entire signal evolution or detect its peak up to distances of $\gtrsim1$ Gpc. In 2026, ULTRASAT will conduct all-sky surveys within minutes, detecting some of these events within $\sim 200$ Mpc. The BH-NS mergers with higher mass ratios or lower BH spins would produce shorter and fainter signals., Comment: For movies of the simulations, see https://oregottlieb.com/bhns.html

Gamma-ray bursts (GRBs), both long and short, are explosive events whose inner engine is generally expected to be a black hole or a highly magnetic neutron star (magnetar) accreting high density matter. Recognizing the nature of GRB central engines, and in particular the formation of neutron stars (NSs), is of high astrophysical significance. A possible signature of NSs in GRBs is the presence of a plateau in the early X-ray afterglow. Here we carefully select a subset of long and short GRBs with a clear plateau, and look for an additional NS signature in their prompt emission, namely a transition between accretion and propeller in analogy with accreting, magnetic compact objects in other astrophysical sources. We estimate from the prompt emission the minimum accretion luminosity below which the propeller mechanism sets in, and the NS magnetic field and spin period from the plateau. We demonstrate that these three quantities obey the same universal relation in GRBs as in other accreting compact objects switching from accretion to propeller. This relation provides also an estimate of the radiative efficiency of GRBs, which we find to be several times lower than radiatively efficient accretion in X-ray binaries and in agreement with theoretical expectations. These results provide additional support to the idea that at least some GRBs are powered by magnetars surrounded by an accretion disc., Comment: 15 pages, 5 figures, accepted for publication in The Astrophysical Journal Letters

While magnetism in exoplanets remains largely unknown, Hot Jupiters have been considered as natural candidates to harbour intense magnetic fields, both due to their large masses and their high energy budgets coming from irradiation as a consequence of their vicinity to their host stars. In this work we perform MHD simulations of a narrow day-side atmospheric column of ultra-hot Jupiters, suitable for very high local temperatures (T > 3000 K). Since the conductivity in this regime is very high, the dominant effect is winding due to the intense zonal winds. By including a forcing that mimics the wind profiles obtained in global circulation models, the shear layer induces a strong toroidal magnetic field (locally reaching hundreds of gauss), supported by meridional currents. Such fields and the sustaining currents don$'$t depend on the internally generated field, but are all confined in the thin (less than a scale-height) shear layer around 1 bar. Additionally, we add random perturbations that induce turbulent motions, which lead to further (but much smaller) magnetic field generation to a broader range of depths. These results allow an evaluation of the currents induced by the atmospheric dynamo. Although here we use ideal MHD and the only resistivity comes from the numerical scheme, we estimate a-posteriori the amount of Ohmic heat deposited in the outer layers, which could be employed in evolutionary models for Hot Jupiters' inflated radii., Comment: Accepted for publication in MNRAS

Dynamical interactions involving binaries play a crucial role in the evolution of star clusters and galaxies. We continue our investigation of the hydrodynamics of three-body encounters, focusing on binary black hole (BBH) formation, stellar disruption, and electromagnetic (EM) emission in dynamical interactions between a BH-star binary and a stellar-mass BH, using the moving-mesh hydrodynamics code {\small AREPO}. This type of encounters can be divided into two classes depending on whether the final outcome includes BBHs. This outcome is primarily determined by which two objects meet at the first closest approach. BBHs are more likely to form when the star and the incoming BH encounter first with an impact parameter smaller than the binary's semimajor axis. In this case, the star is frequently disrupted. On the other hand, when the two BHs encounter first, frequent consequences are an orbit perturbation of the original binary or a binary member exchange. For the parameters chosen in this study, BBH formation, accompanied by stellar disruption, happens in roughly 1 out of 4 encounters. The close correlation between BBH formation and stellar disruption has possible implications for EM counterparts at the binary's merger. The BH that disrupts the star is promptly surrounded by an optically and geometrically thick disk with accretion rates exceeding the Eddington limit. If the debris disk cools fast enough to become long-lived, EM counterparts can be produced at the time of the BBH merger., Comment: 16 pages, 11 figures, 2 table. Accepted for publication in MNRAS, movie here: https://studio.youtube.com/playlist/PLxLK3qI02cQfXhvhRZp9gkvts_E46t0kH/videos

Long and short gamma-ray bursts (GRBs), canonically separated at around 2 seconds duration, are associated with different progenitors: the collapse of a massive star and the merger of two compact objects, respectively. GRB 191019A was a long GRB ($T_{90}\sim64$ s). Despite the relatively small redshift z=0.248 and HST followup observations, an accompanying supernova was not detected. In addition, the host galaxy did not have significant star formation activity. Here we propose that GRB 191019A was produced by a binary compact merger, whose prompt emission was stretched in time by the interaction with a dense external medium. This would be expected if the burst progenitor was located in the disk of an active galactic nucleus, as supported by the burst localization close to the center of its host galaxy. We show that the light curve of GRB 191019A can be well modeled by a burst of intrinsic duration t=1.1 s and of energy $E_{\rm{iso}}=10^{51}$ erg seen moderately off-axis, exploding in a medium of density $10^7-10^8$ cm$^{-3}$. The double-peaked light curve carries the telltale features predicted for GRBs in high-density media, where the first peak is produced by the photosphere, and the second by the overlap of reverse shocks that take place before the internal shocks could happen. This would make GRB 191019A the first confirmed stellar explosion from within an accretion disk, with important implications for the formation and evolution of stars in accretion flows and for gravitational waves source populations., Comment: 6 pages, 2 figures. Accepted for publication in ApJL

Close encounters between stellar-mass black holes (BHs) and stars occur frequently in dense star clusters and in the disks of active galactic nuclei (AGNs). Recent studies have shown that in highly eccentric close encounters, the star can be tidally disrupted by the BH (micro-tidal disruption event, or micro-TDE), resulting in rapid mass accretion and possibly bright electromagnetic signatures. Here we consider a scenario in which the star might approach the stellar-mass BH in a gradual, nearly circular inspiral, under the influence of dynamical friction on a circum-binary gas disk or three-body interactions in a star cluster. We perform hydro-dynamical simulations of this scenario using the smoothed particle hydrodynamics code PHANTOM. We find that the mass of the star is slowly stripped away by the BH. We call this gradual tidal disruption a "tidal-peeling event", or a TPE. Depending on the initial distance and eccentricity of the encounter, TPEs might exhibit significant accretion rates and orbital evolution distinct from those of a typical (eccentric) micro-TDE.

Intermediate-mass black holes (IMBHs) are the missing link between stellar-mass and supermassive black holes, widely believed to reside in at least some dense star clusters, but not yet observed directly. Tidal disruptions of white dwarfs (WDs) are luminous only for black holes less massive than $\sim 10^5\,M_{\odot}$, therefore providing a unique smoking gun that could finally prove the existence of IMBHs beyond any reasonable doubt. Here, we investigate the tidal captures of WDs by IMBHs in dense star clusters, and estimate a typical rate of $\sim 1\,{\rm Myr}^{-1}$ for galactic nuclei and $\sim 0.01\,{\rm Myr}^{-1}$ for globular clusters. Following the capture, the WD inspirals onto the IMBH producing gravitational waves detectable out to $\sim100$ Mpc by LISA for $\sim 10^4\,M_{\odot}$ IMBHs. The subsequent tidal stripping/disruption of the WD can also release bright X-ray and gamma-ray emission with luminosities of at least $\gtrsim10^{40}\,\rm{erg\,s^{-1}}$, detectable by \textit{Chandra}, \textit{Swift}, and upcoming telescopes, such as the \textit{Einstein Probe}., Comment: 13 pages, 8 figures. Published at ApJ

Two-body scatterings under the potential of a massive object are very common in astrophysics. If the massive body is far enough away that the two small bodies are in their own gravitational sphere of influence, the gravity of the massive body can be temporarily ignored. However, this requires the scattering process to be fast enough that the small objects do not spend too much time at distances near the surface of the sphere of influence. In this paper, we derive the validation criteria for effective two-body scattering and establish a simple analytical solution for this process, which we verify through numerical scattering experiments. We use this solution to study star-black hole scatterings in the disks of Active Galactic Nuclei and planet-planet scatterings in planetary systems, and calculate their one-dimensional cross-section analytically. Our solution will be valuable in reducing computational time when treating two-body scatterings under the potential of a much more massive third body, provided that the problem settings are in the valid parameter space region identified by our study., Comment: Comments are Welcome

Stellar-mass black holes (BHs) are predicted to be embedded in the disks of active galactic nuclei (AGN) due to gravitational drag and in-situ star formation. However, clear evidence for AGN disk-embedded BHs is currently lacking. Here, as possible electromagnetic signatures of these BHs, we investigate breakout emission from shocks emerging around Blandford-Znajek jets launched from accreting BHs in AGN disks. We assume that the majority of the highly super-Eddington flow reaches the BH, produces a strong jet, and the jet produces feedback that shuts off accretion and thus leads to episodic flaring. While these assumptions are highly uncertain at present, they predict a breakout emission characterized by luminous thermal emission in the X-ray bands, and bright, broadband non-thermal emission from the infrared to the gamma-ray bands. The flare duration depends on the BH's distance $r$ from the central supermassive BH, varying between $10^3-10^6$ s for $r \sim 0.01-1$ pc. This emission can be discovered by current and future infrared, optical, and X-ray wide-field surveys and monitoring campaigns of nearby AGNs., Comment: 10 pages, 4 figures, accepted in ApJL

The origin of stellar-mass black hole mergers discovered through gravitational waves is being widely debated. Mergers in the disks of active galactic nuclei (AGN) represent a promising source of origin, with possible observational clues in the gravitational wave data. Beyond gravitational waves, a unique signature of AGN-assisted mergers is electromagnetic emission from the accreting black holes. Here we show that jets launched by accreting black holes merging in an AGN disk can be detected as peculiar transients by infrared, optical, and X-ray observatories We further show that this emission mechanism can explain the possible associations between gravitational wave events and the optical transient ZTF19abanrhr and the proposed gamma-ray counterparts GW150914-GBM and LVT151012-GBM. We demonstrate how these associations, if genuine, can be used to reconstruct the properties of these events' environments. Searching for infrared and X-ray counterparts to similar electromagnetic transients in the future, once host galaxies are localized by optical observations, could provide a smoking gun signature of the mergers' AGN origin., Comment: 21 pages, 6 figures, accepted in ApJ

Strong dynamical interactions among stars and compact objects are expected in a variety of astrophysical settings, such as star clusters and the disks of active galactic nuclei. Via a suite of 3D hydrodynamics simulations using the moving-mesh code AREPO, we investigate the formation of transient phenomena and their properties in close encounters between an $2M_{\odot}$ or $20M_{\odot}$ equal-mass circular binary star and single $20M_{\odot}$ black hole (BH). Stars can be disrupted by the BH during dynamical interactions, naturally producing electromagnetic transient phenomena. Encounters with impact parameters smaller than the semimajor axis of the initial binary frequently lead to a variety of transients whose electromagnetic signatures are qualitatively different from those of ordinary disruption events involving just two bodies. These include the simultaneous or successive disruptions of both stars and one full disruption of one star accompanied by successive partial disruptions of the other star. On the other hand, when the impact parameter is larger than the semimajor axis of the initial binary, the binary is either simply tidally perturbed or dissociated into bound and unbound single stars ("micro-Hills" mechanism). The dissociation of $20M_{\odot}$ binaries can produce a runaway star and an active BH moving away from one another. Also, the binary dissociation can either produce an interacting binary with the BH, or a non-interacting, hard binary; both could be candidates of BH high- and low-mass X-ray binaries. Hence our simulations especially confirm that strong encounters can lead to the formation of the (generally difficult to form) BH low-mass X-ray binaries., Comment: 13 pages, 8 figures, 2 tables. Accepted for publication in MNRAS. Movies: https://www.youtube.com/watch?v=DGz2WokfZLA&list=PLxLK3qI02cQeEc3ktlYAOLY4RHVX1V_nZ

Both long and short gamma-ray bursts (GRBs) are expected to occur in the dense environments of active galactic nuclei (AGN) accretion disks. As these bursts propagate through the disks they live in, they photoionize the medium causing time-dependent opacity that results in transients with unique spectral evolution. In this paper we use a line-of-sight radiation transfer code coupling metal and dust evolution to simulate the time-dependent absorption that occurs in the case of both long and short GRBs. Through these simulations, we investigate the parameter space in which dense environments leave a potentially observable imprint on the bursts. Our numerical investigation reveals that time dependent spectral evolution is expected for central supermassive black hole masses between $10^5$ and $5 \times 10^7$ solar masses in the case of long GRBs, and between $10^4$ and $10^7$ solar masses in the case of short GRBs. Our findings can lead to the identification of bursts exploding in AGN disk environments through their unique spectral evolution coupled with a central location. In addition, the study of the time-dependent evolution would allow for studying the disk structure, once the identification with an AGN has been established. Finally, our findings lead to insight into whether GRBs contribute to the AGN emission, and which kind, thus helping to answer the question of whether GRBs can be the cause of some of the as-of-yet unexplained AGN time variability.

In addition to a supermassive black hole (SMBH), the central parsec of the Milky Way hosts over a hundred of massive, high velocity young stars whose existence, and organisation of a subset of them in one, or possibly two, mis-aligned disks, is puzzling. Due to a combination of low medium density and strong tidal forces in the vicinity of Sgr A*, stars are not expected to form. Here we propose a novel scenario for their in-situ formation: a jetted tidal disruption event (TDE) from an older wandering star triggers an episode of positive feedback of star formation in the plane perpendicular to the jet, as demonstrated via numerical simulations in the context of jet-induced feedback in galactic outflows. An over-pressured cocoon surrounding the jet shock-compresses clumps to densities high enough to resist the SMBH tidal field. The TDE rate of $10^{-5}-10^{-4}$ yr$^{-1}$ per galaxy, out of which a few percent events are jetted, implies a jetted TDE event per galaxy to occur every few million years. This timescale is interestingly of the same order of the age of the disk stars. The mass function predicted by our mechanism is top-heavy. Additionally, since TDEs are isotropic, our model predicts a random orientation for the disk of stars with respect to the plane of the galaxy and, due to the relatively high TDE rate, it can account for multiple disks of stars with uncorrelated orientations., Comment: ApJL in press

Long and short gamma-ray bursts are traditionally associated with galactic environments, where circumburst densities are small or moderate (few to hundreds of protons per cubic cm). However, both are also expected to occur in the disks of Active Galactic Nuclei, where the ambient medium density can be much larger. In this work we study, via semi-analytical methods, the propagation of the GRB outflow, its interaction with the external material, and the ensuing prompt radiation. In particular, we focus on the case in which the external shock develops early in the evolution, at a radius that is smaller than the internal shock one. We find that bursts in such high density environments are likely characterized by a single, long emission episode that is due to the superposition of individual pulses, with a characteristic hard to soft evolution irrespective of the light curve luminosity. While multi-pulse light curves are not impossible, they would require the central engine to go dormant for a long time before re-igniting. In addition, short GRB engines would produce bursts with prompt duration that would exceed the canonical 2 s separation threshold and would likely be incorrectly classified as long events, even though they would not be accompanied by a simultaneous supernova. Finally, these events have a large dynamical efficiency which would produce a bright prompt emission followed by a somewhat dim afterglow., Comment: 9 pages, 5 figures, under review for AAS Journals

The disks of Active Galactic Nuclei (AGNs) have emerged as rich environments for the production and capture of stars and the compact objects that they leave behind. These stars produce long Gamma-Ray Bursts (LGRBs) at their deaths, while frequent interactions among compact objects form binary neutron stars and neutron star-black hole binaries, leading to short GRBs (SGRBs) upon their merger. Predicting the properties of these transients as they emerge from the dense environments of AGN disks is key to their proper identification and to better constrain the star and compact object population in AGN disks. Some of these transients would appear unusual because they take place in much higher densities than the interstellar medium. Others, which are the subject of this paper, would additionally be modified by radiation diffusion since they are generated within optically thick regions of the accretion disks. Here we compute the GRB afterglow light curves for diffused GRB sources for a representative variety of central black-hole masses and disk locations. We find that the radiation from radio to UV and soft X-rays can be strongly suppressed by synchrotron self-absorption in the dense medium of the AGN disk. In addition, photon diffusion can significantly delay the emergence of the emission peak, turning a beamed, fast transient into a slow, isotropic, and dimmer one. These would appear as broadband-correlated AGN variability with dominance at the higher frequencies. Their properties can constrain both the stellar populations within AGN disks as well as the disk structure., Comment: Accepted for publication in MNRAS

Many astrophysical environments, from star clusters and globular clusters to the disks of Active Galactic Nuclei, are characterized by frequent interactions between stars and the compact objects that they leave behind. Here, using a suite of $3-D$ hydrodynamics simulations, we explore the outcome of close interactions between $1M_{\odot}$ stars and binary black holes (BBHs) in the gravitational wave regime, resulting in a tidal disruption event (TDE) or a pure scattering, focusing on the accretion rates, the back reaction on the BH binary orbital parameters and the increase in the binary BH effective spin. We find that TDEs can make a significant impact on the binary orbit, which is often different from that of pure scattering. Binaries experiencing a prograde (retrograde) TDE tend to be widened (hardened) by up to $\simeq 20\%$. Initially circular binaries become more eccentric by $\lesssim 10\%$ by a prograde or retrograde TDE, whereas the eccentricity of initially eccentric binaries increases (decreases) by a retrograde (prograde) TDE by $\lesssim 5\%$. Overall a single TDE can generally result in changes of the gravitational wave-driven merger time scale by order unity. The accretion rates of both black holes are very highly super-Eddington, showing modulations (preferentially for retrograde TDEs) on a time scale of the orbital period, which can be a characteristic feature of BBH-driven TDEs. Prograde TDEs result in the effective spin parameter $\chi$ to vary by $\lesssim 0.02$ while $\chi\gtrsim -0.005$ for retrograde TDEs., Comment: 14 pages, 12 figures, 2 tables. Accepted for publication in MNRAS

Stars that pass close to the supermassive black holes located in the center of galaxies can be violently disrupted by tidal forces, leading to flares that are observed as bright transient events in sky surveys. The rate for these events to occur depends on the black hole spins, which in turn can be affected by ultra-light bosons due to superradiance. We perform a detailed analysis of these effects and show that searches for stellar tidal disruptions have a significant potential to uncover the existence of ultra-light bosons. In particular, we find that upcoming stellar tidal disruption rate measurements by the Vera Rubin Observatory's Legacy Survey of Space and Time can be used to either discover or rule out bosons with masses ranging from $10^{-20}$ to $10^{-18}$ eV. Our analysis also indicates that these measurements may be used to constrain a variety of supermassive black hole spin distributions and determine if close-to maximal spins are preferred., Comment: 7 pages, 5 figures

The detection of electromagnetic radiation (EM) accompanying the gravitational wave (GW) signal from the binary neutron star (BNS) merger GW170817 has revealed that these systems constitute at least a fraction of the progenitors of short gamma-ray bursts (SGRBs). As gravitational wave detectors keep pushing their detection horizons, it is important to assess coupled GW/EM probabilities, and how to maximize observational prospects. Here we perform population synthesis calculations of BNS evolution with the code MOBSE, and seed the binaries in galaxies at three representative redshifts (z=0.01,0.1,1) of the Illustris TNG50 simulation. The binaries are evolved and their locations numerically tracked in the host galactic potentials until merger. Adopting the astrophysical parameters of GRB170817A as a prototype, we numerically compute the broadband lightcurves of jets from BNS mergers, with the afterglow brightness depending on the local medium density at the merger sites. We perform Monte Carlo simulations of the resulting EM population assuming either a random viewing angle with respect to the jet, or a jet aligned with the orbital angular momentum of the binary, which biases the viewing angle probability for GW-triggered events. We find that ~70-80% of BNSs from z=0.01 should be detectable in gamma-rays. The afterglow detection probabilities of GW-triggered BNS mergers vary between ~0.3-0.7%, with higher values for jets aligned with the BNS angular momentum, and are comparable across the high and low-energy bands, unlike gamma-ray-triggered events (cosmological SGRBs) which are significantly brighter at higher energies. We further quantify observational biases with respect to host galaxy masses., Comment: 15 pages, 12 figures, submitted to MNRAS

Stellar-mass BHs (sBHs) are predicted to be embedded in active galactic nuclei (AGN) disks due to gravitational drag and in-situ star formation. However, we find that due to a high gas density in an AGN disk environment, compact objects may rapidly grow to intermediate-mass BHs and deplete matter from the AGN disk unless accretion is suppressed by some feedback process(es). These consequences are inconsistent with AGN observations and the dynamics of the Galactic center. Here we consider mechanical feedback mechanisms for the reduction of gas accretion. Rapidly accreting sBHs launch winds and/or jets via the Blandford-Znajek mechanism, which produce high-pressure shocks and cocoons. Such a shock and cocoon can spread laterally in the plane of the disk, eject the outer regions of a circum-sBH disk (CsBD) and puncture a hole in the AGN disk with horizontal size comparable to the disk scale-height. Since the depletion timescale of the bound CsBD is much shorter than the resupply timescale of gas to the sBH, the time-averaged accretion rate onto sBHs is reduced by this process by a factor of $\sim 10$--$100$. This feedback mechanism can therefore help alleviate the sBH over-growth and AGN-disk depletion problems. On the other hand, we find that cocoons of jets can unbind a large fraction of the gas accreting in the disks of less massive SMBHs, which may help explain the dearth of high-Eddington ratio AGNs with SMBH mass $\lesssim10^5{\rm M_\odot}$., Comment: 19 pages, 9 figures, revised version after the first referee report

Active galactic nucleus (AGN) disks may be important sites of binary black hole (BBH) mergers. Here we show via numerical experiments with the high-accuracy, high precision code {\tt SpaceHub} that broken symmetry in dynamical encounters in AGN disks can lead to an asymmetry between prograde and retrograde BBH mergers. The direction of the hardening asymmetry depends on the initial binary semi-major axis. An asymmetric distribution of mass-weighted projected spin $\chi_{\rm eff}$ should therefore be expected in LIGO-Virgo detections of BBH mergers from AGN disks. This channel further predicts that negative $\chi_{\rm eff}$ BBH mergers are most likely for massive binaries., Comment: submitted to ApJL. Comments are welcome

Exoplanetary observations reveal that the occurrence rate of hot Jupiters is correlated with star clustering. In star clusters, interactions between planetary systems and close fly-by stars can significantly change the architecture of primordially coplanar, circular planetary systems. Flybys in dense clusters have a significant impact on hot Jupiter formation via activation of high eccentricity excitation mechanisms such as the Zeipel-Lidov-Kozai (ZLK) effect and planet-planet scattering. Previous studies have shown that if there are two giant planets in the planetary system, close flybys can efficiently activate the ZLK mechanism, thus triggering high eccentricity tidal migration and ultimately form hot Jupiters in star clusters. Here we extend our previous study with a multi-planet (triple) system. We perform high precision, high-accuracy few-body simulations of stellar flybys and subsequent planetary migration within the perturbed planetary systems using the code {\tt SpaceHub}. Our simulations demonstrate that a single close flyby on a multi-planet system in a cluster can activate secular chaos and ultimately lead to hot Jupiter formation via high eccentricity migration. We find that the hot Jupiter formation rate per system increases with both the size of the planetary system as well as with the mass of the outer planet, and we quantify the relative formation fractions for a range of parameters. Hot Jupiters formed via secular chaos are expected to be accompanied by massive companions with very long periods. Our study further shows that this flyby-induced secular chaos is preferred in low-density clusters where multi-planet systems are more likely to survive, and that it contributes a significant fraction of the hot Jupiter formation in star clusters compared to the flyby-induced ZLK mechanism., Comment: submitted to MNRAS

The association of GRB170817A with GW170817 has confirmed the long-standing hypothesis that binary neutron star (BNS) mergers are the progenitors of at least some short gamma-ray bursts (SGRBs). This connection has ushered in an era in which broadband observations of SGRBs, together with measurements of the time delay between the gravitational waves and the electromagnetic radiation, allow to probe the properties of the emitting outflow and its engine to an unprecedented detail. Since the structure of the radiating outflow is molded by the interaction of a relativistic jet with the binary ejecta, it is of paramount importance to study the system in a realistic setting. Here we present a three-dimensional hydrodynamic simulation of a relativistic jet propagating in the ejecta of a BNS merger, which were computed with a general relativistic magnetohydrodynamic simulation. We find that the jet's centroid oscillates around the axis of the system, due to inhomogeneities encountered in the propagation. These oscillations allow the jet to find the path of least resistance and travel faster than an identical jet in smooth ejecta. In our setup the breakout time is ~0.6 sec, comparable to the expected central engine duration in SGRBs and possibly a non-negligible fraction of the total delay between the gravitational and gamma-ray signals. Our simulation also shows that energy is carried in roughly equal amounts by the jet and by the cocoon, and that about 20 per cent of the injected energy is transferred to the ejecta via mechanical work., Comment: Under review for publication in ApJL

We present the open source few-body gravity integration toolkit {\tt SpaceHub}. {\tt SpaceHub} offers a variety of algorithmic methods, including the unique algorithms AR-Radau, AR-Sym6, AR-ABITS and AR-chain$^+$ which we show out-perform other methods in the literature and allow for fast, precise and accurate computations to deal with few-body problems ranging from interacting black holes to planetary dynamics. We show that AR-Sym6 and AR-chain$^+$, with algorithmic regularization, chain algorithm, active round-off error compensation and a symplectic kernel implementation, are the fastest and most accurate algorithms to treat black hole dynamics with extreme mass ratios, extreme eccentricities and very close encounters. AR-Radau, the first regularized Radau integrator with round off error control down to 64 bits floating point machine precision, has the ability to handle extremely eccentric orbits and close approaches in long-term integrations. AR-ABITS, a bit efficient arbitrary precision method, achieves any precision with the least CPU cost compared to other open source arbitrary precision few-body codes. With the implementation of deep numerical and code optimization, these new algorithms in {\tt SpaceHub} prove superior to other popular high precision few-body codes in terms of performance, accuracy and speed., Comment: Submitted to MNRAS. Comments are welcome

The disks of active galactic nuclei (AGNs) have emerged as a rich environment for the evolution of stars and their compact remnants. The very dense medium favors rapid accretion, while torques and migration traps enhance binary formation and mergers. Both long and short gamma-ray bursts (GRBs) are hence expected. We show that AGN disks constitute an ideal environment for another interesting phenomenon: the accretion induced collapse (AIC) of neutron stars (NSs) to black holes (BHs). Rapid accretion in the dense disks can cause NSs to grow to the point of exceeding the maximum mass allowed by their equation of state. General relativistic magnetohydrodynamical simulations have shown that electromagnetic signatures are expected if the NS is surrounded by a mini-disk prior to collapse, which then rapidly accretes onto the BH, and/or if the NS is highly magnetized, from reconnection of the magnetosphere during collapse. Here we compute the rates of AICs and their locations within the disks for both isolated NSs, and for (initially stable) NSs formed from NS-NS mergers. We find that the global AIC rates are $\sim 0.07-20$~Gpc$^{-3}$~yr$^{-1}$, and we discuss their observable prospects and signatures as they emerge from the dense disk environments., Comment: Submitted to AAS Journals. Comments are welcome

In dense star clusters, such as globular and open clusters, dynamical interactions between stars and black holes (BHs) can be extremely frequent, leading to various astrophysical transients. Close encounters between a star and a stellar mass BH make it possible for the star to be tidally disrupted by the BH. Due to the relative low mass of the BH and the small cross section of the tidal disruption event (TDE) for cases with high penetration, disruptions caused by close encounters are usually partial disruptions. The existence of the remnant stellar core and its non-negligible mass compared to the stellar mass BH alters the accretion process significantly. We study this problem with SPH simulations using the code {\tt Phantom}, with the inclusion of radiation pressure, which is important for small mass BHs. Additionally, we develop a new, more general method of computing the fallback rate which does not rely on any approximation. Our study shows that the powerlaw slope of the fallback rate has a strong dependence on the mass of the BH in the stellar mass BH regime. Furthermore, in this regime, self-gravity of the fallback stream and local instabilities become more significant, and cause the disrupted material to collapse into small clumps before returning to the BH. This results in an abrupt increase of the fallback rate, which can significantly deviate from a powerlaw. Our results will help in the identification of TDEs by stellar mass BHs in dense clusters., Comment: Accepted for publication in MNRAS

GRB 190114C was a bright burst that occurred in the local Universe (z=0.425). It was the first gamma-ray burst (GRB) ever detected at TeV energies, thanks to MAGIC. We characterize the ambient medium properties of the host galaxy through the study of the absorbing X-ray column density. Joining Swift, XMM-Newton, and NuSTAR observations, we find that the GRB X-ray spectrum is characterized by a high column density that is well in excess of the expected Milky Way value and decreases, by a factor of ~2, around ~$10^5$ s. Such a variability is not common in GRBs. The most straightforward interpretation of the variability in terms of photoionization of the ambient medium is not able to account for the decrease at such late times, when the source flux is less intense. Instead, we interpret the decrease as due to a clumped absorber, denser along the line of sight and surrounded by lower-density gas. After the detection at TeV energies of GRB 190114C, two other GRBs were promptly detected. They share a high value of the intrinsic column density and there are hints for a decrease of the column density, too. We speculate that a high local column density might be a common ingredient for TeV-detected GRBs., Comment: 6 pages, 4 figures. Accepted for publication in A&A

Stars can either be formed in or captured by the accretion disks in Active Galactic Nuclei (AGN). These AGN stars are irradiated and subject to extreme levels of accretion, which can turn even low-mass stars into very massive ones ($M > 100 {\rm M}_\odot$) whose evolution may result in the formation of massive compact objects ($M > 10 {\rm M}_\odot$). Here we explore the spins of these AGN stars and the remnants they leave behind. We find that AGN stars rapidly spin up via accretion, eventually reaching near-critical rotation rates. They further maintain near-critical rotation even as they shed their envelopes, become compact, and undergo late stages of burning. This makes them good candidates to produce high-spin massive black holes, such as the ones seen by LIGO-Virgo in GW190521g, as well as long Gamma Ray Bursts (GRBs) and the associated chemical pollution of the AGN disk., Comment: 19 pages, 7 figures. Submitted to ApJ, comments welcome!

The disks of Active Galactic Nuclei (AGNs), traditionally studied as the feeders of the supermassive black holes (SMBHs) at their centers, have recently triggered a lot of interest also as hosts to massive stars and hence their neutron star (NS) and black hole (BH) remnants. Migration traps and gas torques in these disks favor binary formation and enhance the rate of compact object mergers. In these environments both long gamma-ray bursts (GRBs) from the death of massive stars and short GRBs from NS-NS and NS-BH mergers are expected. However, their properties in the unique environments of AGN disks have never been studied. Here we show that GRBs in AGNs can display unique features, owing to the unusual relative position of the shocks that characterize the burst evolution and the Thomson photosphere of the AGN disk. In dense environments, for example, the external shock develops before the internal shocks, leading to prompt emission powered by a relativistic reverse shock. The transient's time evolution is also compressed, yielding afterglow emission that is much brighter and peaks much earlier than for GRBs in the interstellar medium. Additionally, in regions of the disk that are sufficiently dense and extended, the light curves are dominated by diffusion, since the fireball is trapped inside the disc photosphere. These diffusion-dominated transients emerge on timescales of days in disks around SMBHs of $\sim 10^6 M_\odot$ to years for SMBHs of $\sim 10^8 M_\odot$. Finally, a large fraction of events, especially in AGNs with SMBHs $\lesssim 10^7 M_\odot$, display time-variable absorption in the X-ray band., Comment: Submitted to ApJ Letters

The discovery of high incidence of hot Jupiters in dense clusters challenges the field-based hot Jupiter formation theory. In dense clusters, interactions between planetary systems and flyby stars are relatively common. This has a significant impact on planetary systems, dominating hot Jupiter formation. In this paper, we perform high precision, few-body simulations of stellar flybys and subsequent planet migration in clusters. A large parameter space exploration demonstrates that close flybys that change the architecture of the planetary system can activate high eccentricity migration mechanisms: Lidov-Kozai and planet-planet scattering, leading to high hot Jupiter formation rate in dense clusters. Our simulations predict that many of the hot Jupiters are accompanied by "ultra-cold Saturns", expelled to apastra of thousands of AU. This increase is particularly remarkable for planetary systems originally hosting two giant planets with semi-major axis ratios $\sim$ 4 and the flyby star approaching nearly perpendicular to the planetary orbital plane. The estimated lower limit to the hot Jupiter formation rate of a virialized cluster is $\sim 1.6\times10^{-4}({\sigma}/{\rm 1kms^{-1}})^5({a_{\rm p}}/{\rm 20 AU})({M_{\rm c}}/{\rm 1000M_\odot})^{-2}$Gyr$^{-1}$ per star, where $\sigma$ is the cluster velocity dispersion, $a_{\rm p}$ is the size of the planetary system and $M_{\rm c}$ is the mass of the cluster. Our simulations yield a hot Jupiter abundance which is $\sim$ 50 times smaller than that observed in the old open cluster M67. We expect that interactions involving binary stars, as well as a third or more giant planets, will close the discrepancy., Comment: Accepted for publication in ApJS

The activity of magnetars is powered by their intense and dynamic magnetic fields and has been proposed as the trigger to extragalactic Fast Radio Bursts. Here we estimate the frequency of crustal failures in young magnetars, by computing the magnetic stresses in detailed magneto-thermal simulations including Hall drift and Ohmic dissipation. The initial internal topology at birth is poorly known but is likely to be much more complex than a dipole. Thus, we explore a wide range of initial configurations, finding that the expected rate of crustal failures varies by orders of magnitude depending on the initial magnetic configuration. Our results show that this rate scales with the crustal magnetic energy, rather than with the often used surface value of the dipolar component related to the spin-down torque. The estimated frequency of crustal failures for a given dipolar component can vary by orders of magnitude for different initial conditions, depending on how much magnetic energy is distributed in the crustal non-dipolar components, likely dominant in newborn magnetars. The quantitative reliability of the expected event rate could be improved by a better treatment of the magnetic evolution in the core and the elastic/plastic crustal response, here not included. Regardless of that, our results are useful inputs in modelling the outburst rate of young Galactic magnetars, and their relation with the Fast Radio Bursts in our and other galaxies., Comment: 9 pages, 4 figures, ApJ Letter in press

As the sensitivity of gravitational wave (GW) instruments improves and new networks start operating, hundreds of merging stellar-mass black holes (SBHs) and intermediate-mass black holes (IMBHs) are expected to be observed in the next few years. The origin and distribution of SBH and IMBH binaries in various dynamical environments is a fundamental scientific question in GW astronomy. In this paper, we discuss ways tidal disruption events (TDEs) may provide a unique electromagnetic window into the assembly and merger of binary SBHs and IMBHs in nuclear star clusters (NSCs). We discuss how the host NSC mass and density and the slope of the black-hole mass function set the orbital properties and the masses of the binaries that undergo a TDE. For typical NSC properties, we predict a TDE rate of $\sim 10^{-6}$--$10^{-7}\ {\rm yr}^{-1}$ per galaxy. The lightcurve of TDEs in NSCs could be interrupted and modulated by the companion black hole on the orbital period of the binary. These should be readily detectable by optical transient surveys such as the Zwicky Transient Facility and LSST., Comment: 13 pages, 7 figures, 1 table, accepted by MNRAS

GRB 170817A was a subluminous short gamma-ray burst detected about 1.74 s after the gravitational wave signal GW170817 from a binary neutron star (BNS) merger. It is now understood as an off-axis event powered by the cocoon of a relativistic jet pointing 15 to 30 degrees away from the direction of observation. The cocoon was energized by the interaction of the incipient jet with the non-relativistic baryon wind from the merger remnant, resulting in a structured outflow with a narrow core and broad wings. In this paper, we couple the observational constraints on the structured outflow with a model for the jet-wind interaction to constrain the intrinsic properties with which the jet was launched by the central engine, including its time delay from the merger event. Using wind prescriptions inspired by magnetized BNS merger simulations, we find that the jet was launched within about 0.4 s from the merger, implying that the 1.74 s observed delay was dominated by the fireball propagation up to the photospheric radius. We also constrain, for the first time for any gamma-ray burst, the jet opening angle at injection and set a lower limit to its asymptotic Lorentz factor. These findings suggest an initially Poynting-flux dominated jet, launched via electromagnetic processes. If the jet was powered by an accreting black hole, they also provide a significant constraint on the survival time of the metastable neutron star remnant., Comment: Accepted for publication in ApJ

The discovery of planetary systems outside of the solar system has challenged some of the tenets of planetary formation. Among the difficult-to-explain observations, are systems with a giant planet orbiting a very-low mass star, such as the recently discovered GJ~3512b planetary system, where a Jupiter-like planet orbits an $M$-star in a tight and eccentric orbit. Systems such as this one are not predicted by the core accretion theory of planet formation. Here we suggest a novel mechanism, in which the giant planet is born around a more typical Sun-like star ($M_{*,1}$), but is subsequently exchanged during a dynamical interaction with a flyby low-mass star ($M_{*,2}$). We perform state-of-the-art $N$-body simulations with $M_{*,1}=1M_\odot$ and $M_{*,2}=0.1M_\odot$ to study the statistical outcomes of this interaction, and show that exchanges result in high eccentricities for the new orbit around the low-mass star, while about half of the outcomes result in tighter orbits than the planet had around its birth star. We numerically compute the cross section for planet exchange, and show that an upper limit for the probability per planetary system to have undergone such an event is $\Gamma\sim 4.4(M_{\rm c}/100M_\odot)^{-2}(a_{\rm p}/{\rm AU}) (\sigma/1\,{\rm km}\,{\rm s}^{-1})^{5}$Gyr$^{-1}$, where $a_{\rm p}$ is the planet semi-major axis around the birth star, $\sigma$ the velocity dispersion of the star cluster, and $M_{\rm c}$ the total mass of the star cluster. Hence these planet exchanges could be relatively common for stars born in open clusters and groups, should already be observed in the exoplanet database, and provide new avenues to create unexpected planetary architectures., Comment: Accepted to ApJ Letters

The discovery of Exoplanetary Systems has challenged some of the theories of planet formation, which assume unperturbed evolution of the host star and its planets. However, in star clusters the interactions with flyby stars and binaries may be relatively common during the lifetime of a planetary system. Here, via high-resolution $N$-body simulations of star-planet systems perturbed by interlopers (stars and binaries), we explore the reconfiguration to the planetary system due to the encounters. In particular, via an exploration focused on the strong scattering regime, we derive the fraction of encounters which result in planet ejections, planet transfers and collisions by the interloper star/binary, as a function of the characteristics of the environment (density, velocity dispersion), and for different masses of the flyby star/binary. We find that binary interlopers can significantly increase the cross section of planet ejections and collisions, while they only slightly change the cross section for planet transfers. Therefore, in environments with high binary fractions, floating planets are expected to be relatively common, while in environments with low binary fractions, where the cross sections of planet ejection and transfer are comparable, the rate of planet exchanges between two stars will be comparable to the rate of production of free-floating planets., Comment: Submitted to MNRAS. Comments are welcome