Search

Close encounter between a star and a supermassive black hole (SMBH) results in the tidal disruption of the star, known as a tidal disruption event (TDE). Recently, a few TDEs, e.g., ASASSN-15oi and AT2018hyz, have shown late-time (hundreds of days after their UV/optical peaks) radio flares with radio luminosities of $10^{38\sim39}$ erg/s. The super-Eddington fallback or accretion in a TDE may generate a mass outflow. Here we investigate a scenario that the late-time radio flares come from the interaction of the outflow with the circum-nuclear gaseous clouds, in addition to the slow-evolving emission component due to the outflow-diffuse medium interaction. We calculate the associated radio temporal and spectral signatures and find that they reproduce well the observations. The outflows have the inferred velocity of 0.2$\sim0.8$ c, the total mass of $10^{-3}\sim10^{-1}$ $\mathrm{M_{\odot}}$ and the ejection duration of a month to a year. The distances of the clouds to the SMBH are $0.1\sim1$ pc. This scenario has advantages in explaining the long delay, sharpness of the rise and the multiplicity of the late radio flares. Future observations may build up a much larger sample of late-time radio flares and enable their use as a probe of the TDE physics and the host circumnuclear environment., Comment: 13 pages, 13 figures. Submitted to ApJ. A new version with some modifications. Comments are welcome

Expanding X-ray halo or rings appear when short pulses of X-ray radiation from a background source are scattered by clouds of dust in the Milky Way. We study the X-ray rings of the brightest gamma-ray burst (GRB) 221009A, detected by the {\it Swift} X-Ray Telescope. The rings center on the GRB position and their angular radii increase with time. We identify five major expanding rings, and our modeling of their expansion history suggests that they are scattered off, respectively, from five dusty clouds at distances of 0.4-13 kpc from the observer. Given an assumed prompt X-ray fluence of this GRB, the fluxes of those rings suggest that these clouds have dust grain column densities of $10^{7\sim8}~\mathrm{cm^{-2}}$. More interestingly, our time-dependent spectral analysis of these rings show that they all experience spectral softening, i.e., getting softer as they expand, with spectral indices ranging from 2.2 to 5, consistent with what the dust scattering model predicts., Comment: 9 pages, 6 figures, 2 tables, Accepted for publication in ApJ

Pulsar-black hole (BH) close binary systems, which have not been found yet, are unique laboratories for testing theories of gravity and understanding the formation channels of gravitational-wave sources. We study the self-gravitational lensing effect in a pulsar-BH system on the pulsar's emission. Because this effect occurs once per orbital period for almost edge-on binaries, we find that it could generate apparently ultra-long period (minutes to hours) radio signals when the intrinsic pulsar signal is too weak to detect. Each of such lensed signals, or 'pulse', is composed of a number of amplified intrinsic pulsar pulses. We estimate that a radio telescope with a sensitivity of $10\,\rm mJy$ could detect $\sim$ a few systems that emit such signals in our galaxy. The model is applied to three recently found puzzling long-period radio sources: GLEAM-X J1627, PSR J0901-4046, and GPM J1839-10. To explain their observed signal durations and periods, the masses of their lensing components would be $\sim10^4\,\rm M_{\odot}$, $\sim4\,\rm M_{\odot}$ and $10^{3-6}\,\rm M_{\odot}$, respectively, with their binary coalescence times ranging from a few tens to thousands of years. However, the implied merger rates (as high as $\sim 10^{3-4}\,\rm Myr^{-1}$ per galaxy) and the large period decay rates ($>10^{-8}\,\rm s\,s^{-1}$) tend to disfavour this self-lensing scenario for these three sources. Finally, for a binary containing a millisecond pulsar and a stellar-mass BH, the Shapiro delay effect would cause a $\geq10\%$ variation of the profile width for the sub-pulses in such lensed signals., Comment: 13 pages, 6 figures, 1 table, Accepted for publication in ApJ

ASASSN-14ko was identified as an abnormal periodic nuclear transient with a potential decreasing period. Its outbursts in the optical and UV bands have displayed a consistent and smooth "fast-rise and slow-decay" pattern since its discovery, which has recently experienced an unexpected alteration in the last two epochs, as revealed by our proposed high-cadence Swift observations. The new light curve profiles show a bump during the rising stages and a rebrightening during the declining stages, making them much broader and symmetrical than the previous ones. In the last two epochs, there is no significant difference in the X-ray spectral slope compared to the previous one, and its overall luminosity is lower than those of the previous epochs. The energy released in the early bump and rebrightening phases ($\sim10^{50}$ erg) could be due to collision of the stripped stream from partial tidal disruption events (pTDEs) with an expanded accretion disk. We also discussed other potential explanations, such as disk instability and star-disk collisions. Further high-cadence multi-wavelength observations of subsequent cycles are encouraged to comprehend the unique periodic source with its new intriguing features., Comment: Accepted for publication in ApJL, 10 pages, 6 figures

Short-duration gamma-ray bursts (sGRBs) are commonly attributed to the mergers of double neutron stars (NSs) or the mergers of a neutron star with a black hole (BH). While the former scenario was confirmed by the event GW170817, the latter remains elusive. Here, we consider the latter scenario in which, a NS is tidally disrupted by a fast spinning low-mass BH and the accretion onto the BH launches a relativistic jet and hence produces a sGRB. The merging binary's orbit is likely misaligned with the BH's spin. Hence, the Lense-Thirring precession around the BH may cause a hyper-accreting thick disk to precess in a solid-body manner. We propose that a jet, initially aligned with the BH spin, is deflected and collimated by the wind from the disk, therefore being forced to precess along with the disk. This would result in a quasi-periodic oscillation or modulation in the gamma-ray light curve of the sGRB, with a quasi-period of $\sim 0.01-0.1$ s. The appearance of the modulation may be delayed respective to the triggering of the light curve. This feature, unique to the BH-NS merger, may have already revealed itself in a few observed sGRBs (such as GRB 130310A), and it carries the spin-obit orientation information of the merging system. Identification of this feature would be a new approach to reveal spin-orbit-misaligned merging BH-NS systems, which are likely missed by the current gravitational-wave searching strategy principally targeting aligned systems., Comment: 13 pages, 8 figures, Accepted for publication in ApJ

During the inspiralling of a white dwarf (WD) into an intermediate-mass black hole (~ 10^{2-5} M_sun), both gravitational waves (GWs) and electromagnetic (EM) radiation are emitted. Once the eccentric orbit's pericenter radius approaches the tidal radius, the WD would be tidally stripped upon each pericenter passage. The accretion of these stripped mass would produce EM radiation. It is suspected that the recently discovered new types of transients, namely the quasi-periodic eruptions and the fast ultraluminous X-ray bursts, might originate from such systems. Modeling these flares requires a prediction of the amount of stripped mass from the WD and the details of the mass supply to the accretion disk. We run hydrodynamical simulations to study the orbital parameter dependence of the stripped mass. We find that our results match the analytical estimate that the stripped mass is proportional to z^{5/2}, where z is the excess depth by which the WD overfills its instantaneous Roche lobe at the pericenter. The corresponding fallback rate of the stripped mass is calculated, which may be useful in interpreting the individual flaring light curve in candidate EM sources. We further calculate the long-term mass-loss evolution of a WD during its inspiral and the detectability of the GW and EM signals. The EM signal from the mass-loss stage can be easily detected: the limiting distance is ~ 320(M_h/10^4 M_sun)^{1/2} Mpc for Einstein Probe. The GW signal, for the space-borne detectors such as Laser Interferometer Space Antenna or TianQin, can be detected only within the Local Supercluster (~ 33 Mpc)., Comment: 18 pages, 13 figures, Accepted for publication in ApJ

The fast blue optical transients (FBOTs) are a new population of extragalactic transients of unclear physical origin. A variety of mechanisms have been proposed including failed supernova explosion, shock interaction with a dense medium, young magnetar, accretion onto a compact object, and stellar tidal disruption event, but none is conclusive. Here we report the discovery of a possible X-ray quasi-periodicity signal with a period of $\sim$250 second (at a significance level of 99.76%) in the brightest FBOT AT2018cow through the analysis of XMM-Newton/PN data. The signal is independently detected at the same frequency in the average power density spectrum from data taken from the Swift telescope, with observations covering from 6 to 37 days after the optical discovery, though the significance level is lower (94.26%). This suggests that the QPO frequency may be stable over at least 1.1$\times$ 10$^{4}$ cycles. Assuming the $\sim$250 second QPO to be a scaled-down analogue of that typically seen in stellar mass black holes, a black hole mass of $\sim10^{3}-10^{5}$ solar masses could be inferred. The overall X-ray luminosity evolution could be modeled with the stellar tidal disruption by a black hole of $\sim10^4$ solar masses, providing a viable mechanism to produce AT2018cow. Our findings suggest that other bright FBOTs may also harbor intermediate-mass black holes., Comment: 18 pages, 10 figures. Accepted for publication in Research in Astronomy and Astrophysics

Recently, the red giant V723 Mon is reported to have an unseen companion with a mass of $3.04\pm0.06M_{\odot}$, but question remains about whether it is a single (thus the so-called mass-gap) black hole or an inner binary of two more ordinary compact objects (neutron stars or white dwarfs). In this work, we estimate the X-ray emission by considering the wind-fed accretion from V723 Mon onto the compact companion. We analyze three different scenarios of the dark companion, i.e., a single black hole, binary neutron stars and binary of a neutron star and a white dwarf. We show that the single black hole is the most favored scenario. We also calculate the synchrotron emission from the bow shock caused by the interaction of the compact companion with the wind. We find this emission peaks at $\sim$ 0.1-1 GHz, with a flux density $\sim$ 1 mJy, which is expected to be detected by observations with higher angular resolution in the future., Comment: 8 pages, 2 figures, 1 table, submitted to MNRAS

Fast Blue Optical Transients (FBOTs) are luminous transients with fast evolving (typically $t_{\rm rise}<12\ \rm days$) light curve and blue color (usually $\rm {-0.2\ >\ g-r\ >\ -0.3}$) that cannot be explained by a supernova-like explosion. We propose a radiative diffusion in a time-dependent outflow model to interpret such special transients. In this model, we assume a central engine ejects continuous outflow during a few days. We consider the ejection of the outflow to be time-dependent. The outflow is optically thick initially and photons are frozen in it. As the outflow expands over time, photons gradually escape, and our work is to model such an evolution. Numerical and analytical calculations are considered separately, and the results are consistent. We apply the model to three typical FBOTs: PS1-10bjp, ZTF18abukavn, and ATLAS19dqr. The modeling finds the total mass of the outflow ($\sim 1-5 {M_{\odot}}$), and the total time of the ejection ($\sim$ a few days) for them, leading us to speculate that they may be the result of the collapse of massive stars., Comment: 10 pages, 6 figures, accepted for publication on Research in Astronomy & Astrophysics. Proof-read version

Tidal disruption events (TDEs) can uncover the quiescent supermassive black holes (SMBHs) at the center of galaxies. After the disruption of a star by a SMBH, the highly elliptical orbit of the debris stream will be gradually circularized due to the self-crossing, and then the circularized debris will form an accretion disk. The recent TDE candidate AT 2019avd has double peaks in its optical light curve, and the X-ray emerges near the second peak. The durations of the peaks are ~400 and 600 days, respectively, and the separation between them is ~700 days. We fit its spectral energy distribution (SED) and analyze its light curves in the optical/UV, mid-infrared, and X-ray bands. We find that this source can be interpreted as a two-phase scenario in which the first phase is dominated by the stream circularization, and the second phase is the delayed accretion. We use the succession of the self-crossing model and the delayed accretion model to fit the first and the second peaks, respectively. The fitting result implies that AT 2019avd can be interpreted by the partial disruption of a 0.9 M_sun star by a 7 * 10^6 M_sun SMBH, but this result is sensitive to the stellar model. Furthermore, we find the large-amplitude (by factors up to $\sim 5$) X-ray variability in AT 2019avd can be interpreted as the rigid-body precession of the misaligned disk due to the Lense--Thirring effect of a spinning SMBH, with the precession period of 10 - 25 days., Comment: 18 pages, 12 figures, 4 tables, Accepted by ApJ

A late (t $\sim$ 1,500 days) multi-wavelength (UV, optical, IR, and X-ray) flare was found in PS1-10adi, a tidal disruption event (TDE) candidate that took place in an active galactic nucleus (AGN). TDEs usually involve super-Eddington accretion, which drives fast mass outflow (disk wind). So here we explore a possible scenario that such a flare might be produced by the interaction of the disk wind with a dusty torus for TDEs in AGN. Due to the high velocity of the disk wind, strong shocks will emerge and convert the bulk of the kinetic energy of the disk wind to radiation. We calculate the dynamics and then predict the associated radiation signatures, taking into account the widths of the wind and torus. We compare our model with the bolometric light curve of the late flare in PS1-10adi constructed from observations. We find from our modeling that the disk wind has a total kinetic energy of about $10^{51}$ erg and a velocity of 0.1 c (i.e., a mass of 0.3 $M_{\odot}$); the gas number density of the clouds in the torus is $3\times 10^{7}$ $\rm cm^{-3}$. Observation of such a late flare can be an evidence of the disk wind in TDEs and can be used as a tool to explore the nuclear environment of the host., Comment: 12 pages, 7 figures. Accepted for publication in Journal of High Energy Astrophysics

The variable source OGLE-UCXB-01 is the first OGLE-discovered ultracompact X-ray binary (UCXB). The 12-year long-term OGLE optical photometry of this source shows a period of P= 12.8 min and a fast period decreasing rate Pdot= -9.2E-11 s s^-1. At a luminosity of L_X ~ 4E33 erg s^-1, its X-ray emission is also variable and correlated with the optical variability. To determine the nature of this variable source, specifically the masses and types of its binary components, we consider first an attractive possibility that the optical variation is due to the secondary's ellipsoidal variation and a strong gravitational wave emission drives the orbital decay. However, we can not find an allowable solution to the secondary that satisfies simultaneously the three constraints: an ultra-tight orbit, the bright absolute magnitude, and the large amplitude of the brightness variation. Moreover, the inferred mass transfer rate is too high. This scenario is therefore ruled out. We then find the system is fully consistent with an "intermediate polar" model, in which the optical and X-ray emission comes from a magnetized white dwarf (WD) accreting from a low-mass (<~ 0.7 M_sun) main-sequence secondary. The observed period decay is the accretion-driven spin-up of the WD. The WD spin period is 12.8 min and the orbital period is shorter than 10 hr. The method presented here can be applied to other UCXB candidates or impostors with time-domain data available only., Comment: 7 pages, 7 figures, accepted for publication on ApJ

Tidal disruption events (TDEs) can uncover the quiescent black holes (BHs) at the center of galaxies and also offer a promising method to study them. In a partial TDE (PTDE), the BH's tidal force cannot fully disrupt the star, so the stellar core survives and only a varied portion of the stellar mass is bound to the BH and feeds it. We calculate the event rate of PTDEs and full TDEs (FTDEs). In general, the event rate of PTDEs is higher than that of FTDEs, especially for the larger BHs. And the detection rate of PTDEs is about dozens per year by Zwicky Transient Factory (ZTF). During the circularization process of the debris stream in PTDEs, no outflow can be launched due to the efficient radiative diffusion. The circularized debris ring then experiences viscous evolution and forms an accretion disk. We calculate the light curves of PTDEs contributed by these two processes, along with their radiation temperature evolution. The light curves have double peaks and the spectra peak in UV. Without obscuration or reprocessing of the radiation by an outflow, PTDEs provide a clean environment to study the circularization and transient disk formation in TDEs., Comment: 15 pages, 15 figures, accepted for publication in ApJ

Compact binary mergers that involve at least one neutron star, either binary neutron star or black hole--neutron star coalescences, are thought to be the potential sources of electromagnetic emission due to the material ejected during the merger or those left outside the central object after the merger. Since the intensity of these electromagnetic transients decay rapidly with time, one should pay more attention to early emissions from such events, which are useful in revealing the nature of these mergers. In this work, we study the early emission of kilonovae, short $\gamma$-ray bursts and cocoons that could be produced in those mergers. We estimate their luminosities and time scales as functions of the chirp mass which is the most readily constrained parameter from the gravitational wave detections of these events. We focus on the range of chirp mass as $1.3M_{\odot} -2.7M_{\odot}$ which is compatible with one of the merging component being a so-called `mass gap' black hole. We show that the electromagnetic observation of these transients could be used to distinguish the types of the mergers when the detected chirp mass falls in the range of $1.5M_{\odot}-1.7M_{\odot}$. Applying our analysis to the sub-threshold GRB GBM-190816, we found that for this particular event the effective spin should be larger than 0.6 and the mass of the heavier object might be larger than 5.5$M_{\odot}$ for the SFHo equation of state., Comment: 16 pages, 6 Figures. Accepted for publication in ApJ

We report the discovery of a Tidal Disruption Event (TDE) candidate occurring in the Active Galactic Nucleus SDSS J022700.77-042020.6. A sudden increase in flux of J0227-0420 during the second half of 2009 is clearly shown in the long-term optical, UV, and NIR light curves. A plateau phase, following an initial decline, is seen in the NUV and optical u, g, r, i light curves. Moreover, we find possible evidence that the plateau phase in the NUV band may lag behind the optical ones by approximately 70-80 days with also a much shorter duration, i.e. $\sim$7-15 days against $\sim$40-50 days. The long-term NUV/optical (after the plateau phase), NIR and MIR light curves can be well characterized with a form of $L(t)\propto t^{-\beta}$, consistent with the expectation of a TDE. The plateaus can be explained if the stellar streams collide with the pre-existing AGN disk at different radii. Though the overall fallback rate decreases, the material in the outer disk gradually drifts inward and increases the local accretion rate at the inner region, producing the optical and UV plateaus. The possible lag between the optical and NUV plateaus can then be attributed to viscosity delay. The index $\beta$ of the NIR $J, H, K_s$ bands ($\sim1.4-3.3$) is steeper than that of the UV/optical ($\sim0.7-1.3$) and MIR bands ($\sim0.9-1.8$), which may suggest that a certain fraction of the dust in the inner region of the dusty torus may be sublimated during the TDE phase. Our results indicate that, due to collisions between stellar debris and pre-existing disk, the light curves of TDEs occurring in AGN may show distinctive features, which may shed new light on the accretion process., Comment: 15 pages, 7 figures, submitted to ApJ after minor revision