Search

Synchrotron maser emission is a leading candidate to explain the coherent emission from Fast Radio Bursts (FRBs). This mechanism requires a population inversion in order to operate. We show that nonresonant interactions between Alfv\'{e}n waves and a relativistic plasma result in the formation of population inversions across a wide range of magnetizations, $\sigma\gtrsim10^{-4}$, and temperatures, $10^{-2} \leq k_bT/mc^2 \leq 3$, spanning the parameters expected in FRB environments. We calculate the fraction of energy contained in the inversion across the whole of this parameter space for the first time and we show that energy fractions of $f_{inv}\sim 0.3$ are achieved for high magnetizations $\sigma >1$. The population inversion forms on time-scales compatible with the typical dynamical time-scales of magnetars for all magnetizations. Furthermore, we provide physical explanations for the behaviour of the interaction in different magnetization regimes, and identify the important characteristic values at which this behaviour changes. We also show that the mechanism is capable of producing an FRB signal at GHz frequencies in a relativistic magnetar wind close to the light cylinder and that this signal can escape the magnetar environment without significant damping., Comment: 19 pages, 21 figures, submitted to MNRAS

We explore a new mechanism for photon energy gain in a relativistic plasma with velocity shear. This process takes place in optically thick plasma and resembles conventional Fermi acceleration, where photons undergo multiple scatterings between regions with varying Lorentz factors, leading to an overall energy increase. The resulting high-energy spectra from the escaped photons exhibit a power-law form. The mechanism is an alternative to the classical radiation spectrum from power-law accelerated particles, which can produce power-law spectra in sources like Gamma-ray bursts (GRBs) and Active Galactic Nuclei (AGNs). By employing both numerical simulations and theoretical analysis, we calculate the expected spectra for GRBs, and show that they match the observed photon indices ($\beta$) at high energies., Comment: Proceeding in Marcel Grossman 17, Pescara, Italy

Positive spectral lags are commonly observed in gamma-ray burst (GRB) prompt phase where soft photons lag behind hard ones in their spectral studies. Opposite to this pattern, a fraction of GRBs show a negative spectral lag where hard photons arrive later compared to soft photons. Similarly, recent Fermi-LAT observations show a late onset of high-energy photons in most GRB observations. A fraction of GRBs show a transition from positive to negative lags. Such negative lags and the spectral lag transition have no convincing explanation. We show that a structured GRB jet with velocity shear naturally produces both positive and negative spectral lags. s gain energy from repeated scattering with shearing layers and subsequently escape from higher altitudes. Hence, these photons are delayed compared to soft photons producing a negative spectral lag. The inner jet has no shear and a positive lag appears providing a unified picture of spectral lags in GRBs. The theory predicts a flip in spectral lag from positive to negative within the evolution of the prompt phase. Comparison of the observed lags with the prediction of the theory limits the possible range of GRB jet Lorentz factors to be a few tens., Comment: Submitted to ApJL

The brightest of all time (BOAT) GRB221009A show evidence for a narrow, evolving MeV emission line. Here, we show that this line can be explained as due to pair annihilation in the prompt emission region, and that its temporal evolution is naturally explained as the high-latitude emission (emission from higher angles from the line of sight) after prompt emission is over. We consider both the high and low optical depth for pair production regimes, and find acceptable solutions, with the GRB Lorentz factor $\Gamma \approx 600$ and the emission radius $r \gtrsim 10^{16.5}$~cm. We discuss the conditions for the appearance of such a line, and show that a unique combination of high luminosity and Lorentz factor that is in a fairly narrow range are required for the line detection. This explains why such an annihilation line is rarely observed in GRBs., Comment: typo corrected; extended discussion on pair contribution. Published in Ap.J. Lett

Progenitor stars of long gamma-ray bursts (GRBs) could be surrounded by a significant and complex nebula structure lying at a parsec scale distance. After the initial release of energy from the GRB jet, the jet will interact with this nebula environment. We show here that for a large, plausible parameter space region, the interaction between the jet blastwave and the wind termination (reverse) shock is expected to be weak, and may be associated with a precursor emission. As the jet blast wave encounters the contact discontinuity separating the shocked wind and the shocked interstellar medium, we find that a bright flash of synchrotron emission from the newly-formed reverse shock is produced. This flash is expected to be observed at around ~100 s after the initial explosion and precursor. Such a delayed emission thus constitutes a circumburst medium (CBM) phase in a GRB, having a physically distinct origin from the preceding prompt phase and the succeeding afterglow phase. The CBM phase emission may thus provide a natural explanation to bursts observed to have a precursor followed by an intense, synchrotron-dominated main episode that is found in a substantial minority, ~10% of GRBs. A correct identification of the emission phase is thus required to infer the properties of the flow and of the immediate environment around GRB progenitors., Comment: 14 pages, 3 figures. Submitted for publication in the Astrophys. J

Despite being studied for many years, the structure of collisionless shocks is still not fully determined. Such shocks are known to be accelerators of cosmic rays, which, in turn, modify the shock structure. The shock width $\lambda$ is known to be connected to the cosmic rays (CRs) spectral index, $a$. Here, we use an instability analysis to derive the shock width in the presence of CRs. We obtain an analytical expression connecting the shock width to the CRs index and to the fraction of upstream particles that are accelerated. We find that when this fraction becomes larger than $\sim$~30\%, a new instability becomes dominant. The shock undergoes a transition where its width increases by a factor $\sim 8- 10$, and the CRs acceleration effectively ends. Our analysis is valid for strong, non-relativistic and unmagnetized shocks. We discuss the implication of these results to the expected range of CRs spectra and flux observed, and on the structure of non-relativistic collisionless shocks., Comment: 14 pages, 6 figures, to appear in ApJ

Modeling the multiwavelength spectral energy distributions (SEDs) of blazars provides key insights into the underlying physical processes responsible for the emission. While SED modeling with self-consistent models is computationally demanding, it is essential for a comprehensive understanding of these astrophysical objects. We introduce a novel, efficient method for modeling the SEDs of blazars by the mean of a convolutional neural network (CNN). In this paper, we trained the CNN on a leptonic model that incorporates synchrotron and inverse Compton emissions, as well as self-consistent electron cooling and pair creation-annihilation processes. The CNN is capable of reproducing the radiative signatures of blazars with high accuracy. This approach significantly reduces computational time, thereby enabling real-time fitting to multi-wavelength datasets. As a demonstration, we used the trained CNN with MultiNest to fit the broadband SEDs of Mrk 421 and 1ES 1959+650, successfully obtaining their parameter posterior distributions. This novel framework for fitting the SEDs of blazars will be further extended to incorporate more sophisticated models based on external Compton and hadronic scenarios, allowing for multi-messenger constraints in the analysis. The models will be made publicly available via a web interface, the Markarian Multiwavelength Datacenter, to facilitate self-consistent modeling of multi-messenger data from blazar observations., Comment: 16 pages, submitted to ApJ. The model will be publicly available from the 20th of November on https://mmdc.am/

The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA., Comment: Presented at the 38th International Cosmic Ray Conference (ICRC 2023), 2023 (arXiv:2309.08219)

Observations of long duration gamma-ray bursts (GRBs) with TeV emission during their afterglow have been on the rise. Recently, GRB 221009A, the most energetic GRB ever observed, was detected by the {LHAASO} experiment in the energy band 0.2 - 7 TeV. Here, we interpret its afterglow in the context of a hybrid model in which the TeV spectral component is explained by the proton-synchrotron process while the low energy emission from optical to X-ray is due to synchrotron radiation from electrons. We constrained the model parameters using the observed optical, X-ray and TeV data. By comparing the parameters of this burst and of GRB 190114C, we deduce that the VHE emission at energies $\geq$ 1 TeV in the GRB afterglow requires large explosion kinetic energy, $E \gtrsim 10^{54}$~erg and a reasonable circumburst density, $n\gtrsim 10$~cm$^{-3}$. This results in a small injection fractions of particles accelerated to a power-law, $\sim 10^{-2}$. {A significant fraction of shock energy must be allocated to a near equipartition magnetic field, $\epsilon_B \sim 10^{-1}$, while electrons should only carry a small fraction of this energy, $\epsilon_e \sim 10^{-3}$. Under these conditions required for a proton synchrotron model, namely $\epsilon_B \gg \epsilon_e$, the SSC component is substantially sub-dominant over proton-synchrotron as a source of TeV photons.} These results lead us to suggest that proton-synchrotron process is a strong contender for the radiative mechanisms explaining GRB afterglows in the TeV band., Comment: 15 pages and 4 figures (Accepted to be published in ApJ)

The jet cores in blazars are resolved and found to harbour an edge brightened structure where the jet base appears extended at sides compared to its propagation axis. This peculiar phenomenon invites various explanations. We show that the photosphere of an optically thick jet base in Active Galactic Nuclei (AGNs) is observed edge brightened if the jet Lorentz factor harbours an angular dependence. The jet assumes a higher Lorentz factor along the jet axis and decreases following a power law along its polar angle. For an observer near the jet axis, the jet has a lower optical depth along its propagation axis compared to off axis regions. Higher optical depths at the outer region makes the jet photosphere appear to extend to larger radii compared to a deeper photosphere along its propagation axis. We tackle the problem both analytically and numerically, confirming the edge brightening through Monte Carlo simulations. Other than the edge brightening, the outcomes are significant as they provide a unique tool to determine the jet structure and associated parameters by their resolved observed cores. The study paves way to explore the spectral properties of optically thick cores with structured Lorentz factors in the future., Comment: Submitted to APJL, 9 pages. Suggestions, comments or feedback will be appreciated

We show that an extragalactic jet with a velocity shear gives rise to Fermi like acceleration process for photons scattering withing the shear layers of the jet. Such photons then gain energy to produce a high energy power law. These power law spectra at high energies are frequently observed in several extragalactic objects such as Gamma Ray Bursts (GRBs). We implement the model on GRBs to show that the obtained range of the photon indices are well within their observed values. The analytic results are confirmed with numerical simulations following Monte Carlo approach., Comment: 4 pages, 2 figures

We solve the full quasilinear kinetic equation governing nonresonant interactions of Alfv\'en waves with relativistic plasmas. This work was motivated by the need to determine the energy available for the synchrotron maser in the context of Fast Radio Bursts (FRBs). This interaction can result in plasma heating and the formation of population inversions necessary for the maser. We find that population inversions containing $\sim 1-10\%$ of the distribution's energy form in the relativistic regime, providing an explanation for the formation of the inversion in the environment expected near FRBs., Comment: 7 pages, 2 figures, Accepted for publication in PRD

Gamma-ray bursts (GRBs) are the most powerful explosions in the universe. How efficiently the jet converts its energy to radiation is a long-standing problem and it is poorly constrained. The standard model invokes a relativistic fireball with a bright photosphere emission component. A definitive diagnosis of GRB radiation components and measurement of GRB radiative efficiency require prompt emission and afterglow data with high-resolution and wide-band coverage in time and energy. Here we report a comprehensive temporal and spectral analysis of the TeV-emitting bright GRB 190114C. Its fluence is one of the highest of all GRBs detected so far, which allows us to perform a high-resolution study of the prompt emission spectral properties and their temporal evolution down to a timescale of about 0.1 s. We observe that each of the initial pulses has a thermal component contributing $\sim20\%$ of the total energy, the corresponding temperature and the inferred Lorentz factor of the photosphere evolve following broken power-law shapes. From the observation of the non-thermal spectra and the light curve, the onset of afterglow corresponding to the deceleration of the fireball is considered at $\sim 6$~s. By incorporating the thermal and the non-thermal observations, as well as the photosphere and the synchrotron radiative mechanisms, we can directly derive the fireball energy budget with little dependence on hypothetical parameters and to measure a $\sim 16\%$ radiative efficiency for this GRB. With the fireball energy budget derived, the afterglow microphysics parameters can also be constrained directly from the data., Comment: 27 pages, 8 figures (including 14 panels), 6 tables, accepted for publication in The Astrophysical Journal Letters

We consider here a proton-synchrotron model to explain the MAGIC observation of GRB 190114C afterglow in the energy band $0.2 - 1$~TeV, while the X-ray spectra are explained by electron-synchrotron emission. Given the uncertainty of the particle acceleration process, we consider several variations of the model, and show that they all match the data very well. We find that the values of the uncertain model parameters are reasonable: {isotropic} explosion energy $\sim 10^{54.5}$~erg, ambient density $\sim 10-100~{\rm cm^{-3}}$, and fraction of electrons/protons accelerated to a high energy power law of a few per-cents. All these values are directly derived from the observed TeV and X-ray fluxes. The model also requires that protons be accelerated to observed energies as high as a few $10^{20}$~eV. Further, assuming that the jet break takes place after $10^6$~s gives the beaming-corrected energy of the burst to be $\approx 10^{53}$~erg, which is one to two orders of magnitude higher than usually inferred, because of the small fraction of electrons accelerated. Our modeling is consistent with both late time data at all bands, from optical to X-rays, and with numerical models of particle acceleration. Our results thus demonstrate the relevance of proton-synchrotron emission to the high energy observations of GRBs during their afterglow phase., Comment: Revised version with the added subsections and figures (accepted for publication in ApJ)

A high energy power law is a common feature in the spectra of many astrophysical objects. We show that the photons in a relativistic plasma with a variable Lorentz factor go through repeated scattering with electrons to gain energy. The escaped population of photons naturally produces a power-law-shaped spectrum making it an anisotropic analogue to the conventional Fermi acceleration mechanism of charged particles. Thus, this mechanism provides a natural alternative to current explanations of high energy power-law spectra via synchrotron or thermal Comptonization. The model is applicable to any relativistic plasma beam with an arbitrary Lorentz factor profile. We implement the theory to GRB prompt phase and show that the obtained range of the photon indices is compatible with the observed values. Therefore, the observed high energy spectral indices provide a unique indicator of the jet structure., Comment: 6 pages, 3 figures, Submitted to PRL

In this paper, we expand upon previous work that argued for the possibility of a sub-equipartition magnetic field in the accretion flow of a black hole binary system. Using X-ray observations of the three well-known sources A0620-00, XTE J1118+480 and V404 Cyg during the quiescent state, we compare the theoretically expected spectral shape with the observed data in order to verify that the parameters of the sub-equipartition model are plausible. In all three cases, we find that it is possible to reproduce the spectral shape of the X-ray observations with a sub-equipartition flow. These findings support the idea that the quiescent state spectrum of X-ray binary systems is produced by a weakly-magnetized accretion flow. A sub-equipartition flow would pose a significant challenge to our current understanding of jet-launching, which relies on the presence of a strong magnetic field to power the jet., Comment: 7 pages, 3 figures, submitted to ApJ

We introduce a new GPU-accelerated general-relativistic magneto-hydrodynamic (GR-MHD) code based on HARM which we call cuHARM. The code is written in CUDA-C and uses OpenMP to parallelize multi-GPU setups. Our code allows us to run high resolution simulations of accretion disks and the formation and structure of jets without the need of multi-node supercomputer infrastructure. A $256^3$ simulation is well within the reach of an Nvidia DGX-V100 server, with the computation being a factor about 10 times faster if only the CPU was used. We use this code to examine several disk structures all in the SANE state. We find that: (i) increasing the magnetic field, while in the SANE state does not affect the mass accretion rate; (ii) Simultaneous increase of the disk size and the magnetic field, while keeping the ratio of energies fixed, lead to the destruction of the jet once the magnetic flux through the horizon decrease below a certain limit. This demonstrates that the existence of the jet is not a linear function of the initial magnetic field strength; (iii) the structure of the jet is a weak function of the adiabatic index of the gas, with relativistic gas tend to have a wider jet., Comment: 33 pages, 16 figures, submitted to ApJ

We study the effect of Bethe-Heitler (BeHe) pair production on a proton synchrotron model for the prompt emission in gamma-ray bursts (GRBs). The possible parameter space of the model is constrained by consideration of the synchrotron radiation from the secondary BeHe pairs. We find two regimes of interest. 1) At high bulk Lorentz factor, large radius and low luminosity, proton synchrotron emission dominates and produces a spectrum in agreement with observations. For part of this parameter space, a subdominant (in the MeV band) power-law is created by the synchrotron emission of the BeHe pairs. This power-law extends up to few tens or hundreds of MeV. Such a signature is a natural expectation in a proton synchrotron model, and it is seen in some GRBs, including GRB 190114C recently observed by the MAGIC observatory. 2) At low bulk Lorentz factor, small radius and high luminosity, BeHe cooling dominates. The spectrum achieves the shape of a single power-law with spectral index $\alpha = -3/2$ extending across the entire GBM/Swift energy window, incompatible with observations. Our theoretical results can be used to further constrain the spectral analysis of GRBs in the guise of proton synchrotron models., Comment: Submitted to ApJ

The joint detection of gravitational waves and the gamma-ray counterpart of a binary neutron star merger event, GW170817, unambiguously validates the connection between short gamma-ray bursts and compact binary object (CBO) mergers. We focus on a special scenario where short gamma-ray bursts produced by CBO mergers are embedded in disks of active galactic nuclei (AGN), and we investigate the $\gamma$-ray emission produced in the internal dissipation region via synchrotron, synchrotron self-Compton and external inverse-Compton (EIC) processes. In this scenario, isotropic thermal photons from the AGN disks contribute to the EIC component. We show that a low-density cavity can be formed in the migration traps, leading to the embedded mergers producing successful GRB jets. We find that the EIC component would dominate the GeV emission for typical CBO mergers with an isotropic-equivalent luminosity of $L_{j,\rm iso}=10^{48.5}~\rm erg~s^{-1}$ which are located close to the central supermassive black hole. Considering a long-lasting jet of duration $T_{\rm dur}\sim10^2-10^3$ s, we find that the future CTA will be able to detect its $25-100$ GeV emission out to a redshift $z=1.0$. {In the optimistic case, it is possible to detect the on-axis extended emission simultaneously with GWs within one decade using MAGIC, H.E.S.S., VERITAS, CTA, and LHAASO-WCDA. Early diagnosis of prompt emissions with Fermi-GBM and HAWC can provide valuable directional information for the follow-up observations., Comment: 11 pages, 5 figures. Accepted for publication in The Astrophysical Journal

We propose a backscattering dominated prompt emission model for gamma-ray bursts (GRB) prompt phase in which the photons generated through pair annihilation at the centre of the burst are backscattered through Compton scattering by an outflowing stellar cork. We show that the obtained spectra are capable of explaining the low and high energy slopes as well as the distribution of spectral peak energies in their observed prompt spectra., Comment: 6 pages, 5 figures

Multi-messenger astrophysics is becoming a major avenue to explore the Universe, with the potential to span a vast range of redshifts. The growing synergies between different probes is opening new frontiers, which promise profound insights into several aspects of fundamental physics and cosmology. In this context, THESEUS will play a central role during the 2030s in detecting and localizing the electromagnetic counterparts of gravitational wave and neutrino sources that the unprecedented sensitivity of next generation detectors will discover at much higher rates than the present. Here, we review the most important target signals from multi-messenger sources that THESEUS will be able to detect and characterize, discussing detection rate expectations and scientific impact., Comment: Submitted to Experimental Astronomy

We present new results of the backscattering dominated prompt emission model in which the photons generated through pair annihilation at the centre of a gamma ray burst (GRB) are backscattered through Compton scattering by an outflowing stellar cork. %We show that the produced spectral features are capable of producing spectral features that are widely observed in GRBs. Using Comptonized pair annihilation spectrum accompanied by bremsstrahlung radiation for seed photons, we show that the obtained spectra produce low energy photon index in the range $\alpha \sim -1.95$ to $-1.1$, steeper high energy slopes $\beta \sim -3.5$ to $-2.4$ and spectral peak energies $\sim$ few KeV to few $10\times$ MeV. These findings are consistent with the values covered in GRB prompt phase observations., Comment: 7 Pages, 5 Figures, Submitted to ApJL

While the front of a fluid shock is a few mean-free-paths thick, the front of a collisionless shock can be orders of magnitude thinner. By bridging between a collisional and a collisionless formalism, we assess the transition between these two regimes. We consider non-relativistic, un-magnetized, planar shocks in electron/ion plasmas. In addition, our treatment of the collisionless regime is restricted to high Mach number electrostatic shocks. We find that the transition can be parameterized by the upstream plasma parameter $\Lambda$ which measures the coupling of the upstream medium. For $\Lambda \lesssim 1.12$, the upstream is collisional, i.e. strongly coupled, and the strong shock front is about $\mathcal{M}_1 \lambda_{\mathrm{mfp},1}$ thick, where $\lambda_{\mathrm{mfp},1}$ and $\mathcal{M}_1 $ are the upstream mean-free-path and Mach number respectively. A transition occurs for $\Lambda \sim 1.12$ beyond which the front is $\sim \mathcal{M}_1\lambda_{\mathrm{mfp},1}\ln \Lambda/\Lambda$ thick for $\Lambda\gtrsim 1.12$. Considering $\Lambda$ can reach billions in astrophysical settings, this allows to understand how the front of a collisionless shock can be orders of magnitude smaller than the mean-free-path, and how physics transitions continuously between these 2 extremes., Comment: 13 pages, 4 figures, Under consideration for publication in J. Plasma Phys

A common assumption used in the study of accretion disks is that the magnetic energy density and the kinetic energy density should be in equipartition. This assumption relies on the faster growth rate of the magnetic field strength against the kinetic energy of the particles in the flow, for decreasing radius, combined with a dissipation mechanism that tends towards equipartition. In this paper, we examine this assumption by modeling the radio, mm and optical spectra of several black hole binaries in their quiescent state. We use a standard two-component disk model, consisting of an inner geometrically thick and optically thin disk, emitting thermal synchrotron radiation, along with an outer, thin disk, which radiates as a multicolor blackbody. We find that at the low accretion rates typical of the quiescent state, the spectral shape is qualitatively reproduced using magnetic fields that are between 0.1% and 1% of the equipartition value, considerably smaller than previously thought. We discuss our findings in view of (1) the launching of jets in these objects, which is commonly believed to rely on the presence of a strong magnetic field in the central region of the disk; and (2) the role of magnetic dissipation in the structure of the inflow., Comment: 11 pages, 6 figures. Refereed version, accepted for publication in ApJ. Corrections to parameters of V404 Cyg and other minor changes

Gamma-ray bursts (GRBs) are highly variable and exhibit strong spectral evolution. In particular, the emission properties vary from pulse to pulse in multipulse bursts. Here we present a time-resolved Bayesian spectral analysis of a compilation of GRB pulses observed by the {\it Fermi}/Gamma-ray Burst Monitor. The pulses are selected to have at least four timebins with a high statistical significance, which ensures that the spectral fits are well determined and that spectral correlations can be established. The sample consists of 39 bursts, 117 pulses, and 1228 spectra. We confirm the general trend that pulses become softer over time, with mainly the low-energy power-law index $\alpha$ becoming smaller. A few exceptions to this trend exist, with the hardest pulse occurring at late times. The first pulse in a burst is clearly different from the later pulses; three-fourths of them violate the synchrotron line of death, while around half of them significantly prefer photospheric emission. These fractions decrease for subsequent pulses. We also find that in two-thirds of the pulses, the spectral parameters ($\alpha$ and peak energy) track the light-curve variations. This is a larger fraction compared to what is found in previous samples. In conclusion, emission compatible with the GRB photosphere is typically found close to the trigger time, while the chance of detecting synchrotron emission is greatest at late times. This allows for the coexistence of emission mechanisms at late times., Comment: 112 pages, 17 figures including 318 panels, 46 tables. Published in ApJS on 2021 June 7. Data release at this Zenodo link ( [\href{https://zenodo.org/record/4746267}{this https URL}])

As gamma-ray burst (GRB) jet drills its way through the collapsing star, it traps a baryonic cork ahead of it. Here we explore a prompt emission model for GRBs in which the jet does not cross the cork, but rather photons that are emitted deep in the flow largely by pair annihilation are scattered inside the expanding cork and escape largely from the back end of it as they push it from behind. Due to the relativistic motion of the cork, these photons are easily seen by an observer close to the jet axis peaking at $\varepsilon_{peak}\sim~few \times 100 keV$. We show that this model naturally explains several key observational features including: (1) High energy power law index $\beta_1 \sim -2 {~\rm to~} -5$ with an intermediate thermal spectral region; (2) decay of the prompt emission light curve as $\sim t^{-2}$; (3) Delay of soft photons; (4) peak energy - isotropic energy (the so-called Amati) correlation, $\varepsilon_{peak} \sim \varepsilon_{iso}^m$, with $m\sim 0.45$, resulting from different viewing angles. At low luminosities, our model predicts an observable turn off in the Amati relation. (4) An anti-correlation between the spectral full width half maxima (FWHM) and time as $t^{-1}$. (5) Temporal evolution $\varepsilon_{peak} \sim t^{-1}$, accompanied by an increase of the high energy spectral slope with time. (6) Distribution of peak energies $\varepsilon_{peak}$ in the observed GRB population. The model is applicable for a single pulse GRB lightcurves and respective spectra. We discuss the consequence of our model in view of the current and future prompt emission observations., Comment: Accepted for publication in ApJ, 20 pages, 17 Figures

We study the connection between low-luminosity gamma-ray bursts (llGRBs) and ultra-high-energy cosmic rays (UHECRs) using the canonical low-luminosity GRB 060218 as a proxy. We focus on the consequential synchrotron emission from electrons that are coaccelerated in the UHECR acceleration region, comparing this emission to observations. Both the prompt and afterglow phases are considered. For the prompt phase, we assume the coaccelerated electrons are injected with a power law distribution instantaneously (without additional heating or reacceleration), which results in bright optical-UV emission in tension with observations. For the afterglow phase, we constrain the total kinetic energy of the blast wave by comparing electron thermal synchrotron radiation to available radio data at $\sim~3$ days. Considering mildly relativistic outflows with bulk Lorentz factor $\Gamma \gtrsim 2$ (slower trans-relativistic outflows are not treated), we find that the limited available energy does not allow for GRB 060218-like afterglows to be the main origin of UHECRs. This analysis independently constrains the prompt phase as a major UHECR source as well, given that the prompt energy budget is comparable to that of the afterglow kinetic energy. More generally, our study demonstrates that synchrotron emission from thermal electrons is a powerful diagnostic of the physics of mildly relativistic shocks., Comment: Published as ApJ 902:148 (2020). Updated to match published version. Discussion added on inverse Compton effects during the prompt phase. Main conclusions unchanged

There is no complete description of the emission physics during the prompt phase in gamma-ray bursts. Spectral analyses, however, indicate that many spectra are narrower than what is expected for non-thermal emission models. Here, we reanalyse the sample of 37 bursts in \citet{Yu2019}, by fitting the narrowest time-resolved spectrum in each burst. We perform model comparison between a photospheric and a synchrotron emission model based on Bayesian evidence. We choose to compare the shape of the narrowest expected spectra: emission from the photosphere in a non-dissipative flow and slow-cooled synchrotron emission from a narrow electron distribution. We find that the photospheric spectral shape is preferred by $54 \pm 8 \%$ of the spectra (20/37), while $38 \pm 8 \%$ of the spectra (14/37) prefer the synchrotron spectral shape; three spectra are inconclusive. We hence conclude that GRB spectra are indeed very narrow and that more than half of the bursts have a photospheric emission episode. We also find that a third of all analysed spectra, not only prefer, but are also compatible with a non-dissipative photosphere, confirming previous similar findings. Furthermore, we notice that the spectra, that prefer the photospheric model, all have a low-energy power-law indices $\alpha > -0.5$. This means that $\alpha$ is a good estimator of which model is preferred by the data. Finally, we argue that the spectra which statistically prefer the synchrotron model, could equally well be caused by subphotospheric dissipation. If that is the case, photospheric emission during the early, prompt phase would be even more dominant., Comment: Accepted: ApJ

In order to better understand the physical origin of short duration gamma-ray bursts (GRBs), we perform time-resolved spectral analysis on a sample of 70 pulses in 68 short GRBs with burst duration $T_{90}\lesssim2$ s detected by the \textit{Fermi}/GBM. We apply a Bayesian analysis to all spectra that have statistical significance $S\ge15$ within each pulse and apply a cut-off power law (CPL) model. We then select in each pulse the timebin that has the maximal value of the low energy spectral index, %$\alpha_{\rm max}$, for further analysis. Under the assumption that the main emission mechanism is the same throughout each pulse, such an analysis is indicative of pulse emission. We find that $\sim$1/3 of short GRBs are consistent with a pure, non-dissipative photospheric model, at least, around the peak of the pulse. This fraction is larger compare to the corresponding one (1/4) obtained for long GRBs. For these bursts, we find (i) a bi-modal distribution in the values of the Lorentz factors and the hardness ratios; (ii) an anti-correlation between $T_{90}$ and the peak energy, $E_{\rm pk}$: $T_{90} \propto E_{\rm pk}^{-0.50\pm0.19}$. This correlation disappears when we consider the entire sample. Our results thus imply that the short GRB population may in fact be composed of two separate populations: one being a continuation of the long GRB population to shorter durations, and the other one being distinctly separate with different physical properties. Furthermore, thermal emission is initially ubiquitous, but is accompanied at longer times by additional radiation (likely synchrotron)., Comment: 24 pages, 20 figures, one table, accepted for publication in Ap.J. on the 4th of June 2020

We present post-jet-break \textit{HST}, VLA and \textit{Chandra} observations of the afterglow of the long $\gamma$-ray bursts GRB 160625B (between 69 and 209 days) and GRB 160509A (between 35 and 80 days). We calculate the post-jet-break decline rates of the light curves, and find the afterglow of GRB 160625B inconsistent with a simple $t^{-3/4}$ steepening over the break, expected from the geometric effect of the jet edge entering our line of sight. However, the favored optical post-break decline ($f_{\nu} \propto t^{-1.96 \pm 0.07}$) is also inconsistent with the $f_{\nu} \propto t^{-p}$ decline (where $p \approx 2.3$ from the pre-break light curve), which is expected from exponential lateral expansion of the jet; perhaps suggesting lateral expansion that only affects a fraction of the jet. The post-break decline of GRB 160509A is consistent with both the $t^{-3/4}$ steepening and with $f_{\nu} \propto t^{-p}$. We also use {\sc boxfit} to fit afterglow models to both light curves and find both to be energetically consistent with a millisecond magnetar central engine, although the magnetar parameters need to be extreme (i.e. $E \sim 3 \times 10^{52}$ erg). Finally, the late-time radio light curves of both afterglows are not reproduced well by {\sc boxfit} and are inconsistent with predictions from the standard jet model; instead both are well represented by a single power law decline (roughly $f_{\nu} \propto t^{-1}$) with no breaks. This requires a highly chromatic jet break ($t_{j,\mathrm{radio}} > 10 \times t_{j,\mathrm{optical}}$) and possibly a two-component jet for both bursts., Comment: 19 pages, 9 figures. Revised version; accepted for publication in ApJ

The jet photosphere has been proposed as the origin for the gamma-ray burst (GRB) prompt emission. In many such models, characteristic features in the spectra appear below the energy range of the $\textit{Fermi}$ GBM detectors, so joint fits with X-ray data are important in order to assess the photospheric scenario. Here we consider a particular photospheric model which assumes localized subphotospheric dissipation by internal shocks in a non-magnetized outflow. We investigate it using Bayesian inference and a sample of 8 GRBs with known redshifts which are observed simultaneously with $\textit{Fermi}$ GBM and $\textit{Swift}$ XRT. This provides us with an energy range of $0.3$~keV to $40$~MeV and much tighter parameter constraints. We analyze 32 spectra and find that 16 are well described by the model. We also find that the estimates of the bulk Lorentz factor, $\Gamma$, and the fireball luminosity, $L_{0,52}$, decrease while the fraction of dissipated energy, $\varepsilon_{\mathrm{d}}$, increase in the joint fits compared to GBM only fits. These changes are caused by a small excess of counts in the XRT data, relative to the model predictions from fits to GBM only data. The fact that our limited implementation of the physical scenario yields 50\% accepted spectra is promising, and we discuss possible model revisions in the light of the new data. Specifically, we argue that the inclusion of significant magnetization, as well as removing the assumption of internal shocks, will provide better fits at low energies., Comment: 20 pages, 11 figures

Being the most extreme explosions in the universe, gamma-ray bursts (GRBs) provide a unique laboratory to study various plasma physics phenomena. The complex lightcurve and broad-band, non-thermal spectra indicate a very complicated system on the one hand, but on the other hand provide a wealth of information to study it. In this chapter I focus on recent progress in some of the key unsolved physical problems. These include: (1) Particle acceleration and magnetic field generation in shock waves; (2) Possible role of strong magnetic fields in accelerating the plasmas, and accelerating particles via magnetic reconnection process; (3) Various radiative processes that shape the observed lightcurve and spectra, both during the prompt and the afterglow phases, and finally (4) GRB environments and their possible observational signature., Comment: Invited chapter for a special issue of "galaxies", dedicated to "Cosmic Plasmas and Electromagnetic phenomena"

It has been suggested that the prompt emission in gamma-ray bursts (GRBs) could be described by radiation from the photosphere in a hot fireball. Such models must be tested by directly fitting them to data. In this work we use data from the Fermi Gamma-ray Space Telescope and consider a specific photospheric model, in which kinetic energy of a low-magnetisation outflow is dissipated locally by internal shocks below the photosphere. We construct a table model with a physically motivated parameter space and fit it to time-resolved spectra of the 36 brightest Fermi GRBs with known redshift. We find that about two thirds of the examined spectra cannot be described by the model, as it typically under-predicts the observed flux. However, since the sample is strongly biased towards bright GRBs, we argue that this fraction will be significantly lowered when considering the full population. From the successful fits we find that the model can reproduce the full range of spectral slopes present in the sample. For these cases we also find that the dissipation consistently occurs at a radius of $\sim 10^{12}$ cm and that only a few percent efficiency is required. Furthermore, we find a positive correlation between the fireball luminosity and the Lorentz factor. Such a correlation has been previously reported by independent methods. We conclude that if GRB spectra are due to photospheric emission, the dissipation cannot only be the specific scenario we consider here., Comment: 25 pages, 21 figures, 1 supplementary table

The emission mechanism during the prompt phase in gamma-ray bursts (GRBs) can be investigated through correlations between spectral properties. Here, we revisit the correlation relating the instantaneous flux, $F$, and the photon index below the spectral break, $\alpha$, in individual emission pulses, by studying the 38 most prominent pulses in the Fermi/GBM GRB catalogue. First, we search for signatures of the bias in the determination of $\alpha$ due to the limited spectral coverage (window effect) expected in the synchrotron case. The absence of such a characteristic signature argues against the simplest synchrotron models. We instead find that the observed correlation between $F$ and $\alpha$ can, in general, be described by the relation $F(t) \propto {\rm e}^{k\,\alpha(t)}$, for which the median $k = 3$. We suggest that this correlation is a manifestation of subphotospheric heating in a flow with a varying entropy. Around the peak of the light curve, a large entropy causes the photosphere to approach the saturation radius, leading to an intense emission with a narrow spectrum. As the entropy decreases the photosphere secedes from the saturation radius, and weaker emission with a broader spectrum is expected. This simple scenario naturally leads to a correlated variation of the intensity and spectral shape, covering the observed range., Comment: 15 pages, 10 figures, 1 table, accepted for publication in MNRAS

The problem of accelerating cosmic rays is one of fundamental importance, particularly given the uncertainty in the conditions inside the acceleration sites. Here we examine Diffusive Shock Acceleration in arbitrary turbulent magnetic fields, constructing a new model that is capable of bridging the gap between the very weak ($\delta B/B_0 \ll 1$) and the strong turbulence regimes. To describe the diffusion we provide quantitative analytical description of the "Bohm exponent" in each regime. We show that our results converge to the well known quasi-linear theory in the weak turbulence regime. In the strong regime, we quantify the limitations of the Bohm-type models. Furthermore, our results account for the anomalous diffusive behaviour which has been noted previously. Finally, we discuss the implications of our model in the study of possible acceleration sites in different astronomical objects., Comment: Accepted for publication by ApJ

The acceleration site for ultra-high energy cosmic rays (UHECR) is still an open question despite extended research. In this paper, we reconsider the prompt phase of gamma-ray bursts (GRBs) as a possible candidate for this acceleration and constrain the maximum proton energy in optically thin synchrotron and photospheric models, using properties of the prompt photon spectra. We find that neither of the models favour acceleration of protons to $10^{20}$ eV in high-luminosity bursts. We repeat the calculations for low-luminosity GRBs (llGRBs) considering both protons and completely stripped iron and find that the highest obtainable energies are $< 10^{19}$ eV and $< 10^{20}$ eV for protons and iron respectively, regardless of the model. We conclude therefore that for our fiducial parameters, GRBs, including low-luminosity bursts, contribute little to none to the UHECR observed. We further constrain the conditions necessary for an association between UHECR and llGRBs and find that iron can be accelerated to $10^{20}$ eV in photospheric models, given very efficiency acceleration and/or a small fractional energy given to a small fraction of accelerated electrons. This will necessarily result in high prompt optical fluxes, and the detection of such a signal could therefore be an indication of successful UHECR acceleration at the source., Comment: Larger parameter space explored and some minor changes. Main results unchanged. Final version accepted for publication in ApJ

We extend our study of energetic radio loud and quiet gamma-ray bursts (GRBs), suggesting these GRBs potentially come from two separate progenitor systems. We expand the sample from our previous paper (Lloyd-Ronning & Fryer, 2017) and find our results are strengthened - radio quiet GRBs have significantly shorter intrinsic prompt duration, and are also less energetic on average. However, the tenuous correlation between isotropic energy and intrinsic duration in the radio dark sample remains tenuous and is slightly weakened by adding more bursts. Interestingly, we find an anti-correlation between the intrinsic duration and redshift in the radio bright sample but not the radio dark sample, further supporting that these two samples may come from separate progenitors. We also find that very high energy (0.1 - 100 GeV) extended emission is only present in the radio loud sample. There is no significant difference between the presence of X-ray/optical plateaus or the average jet opening angles between the two samples. We explore the interpretation of these results in the context of different progenitor models. The data are consistent with the radio loud GRBs coming from a Helium-merger system and the radio quiet GRBs coming from a collapsar system, but may also reflect other dichotomies in the inner engine such as a neutron star versus black hole core., Comment: Accepted to ApJ

The origin of Fast Radio Bursts (FRBs) is still mysterious. All FRBs to date show extremely high brightness temperatures, requiring a coherent emission mechanism. Using constraints derived from the physics of one of these mechanisms, the synchrotron maser, as well as observations, we show that accretion induced explosions of neutron stars with surface magnetic fields of $B_*\lesssim10^{11}$ G are favoured as FRB progenitors., Comment: 6 pages, 1 figure. Accepted for publication in ApJ Letters. Revised discussion of the cyclotron maser in highly magnetised plasma

When two plasmas collide, their interaction can be mediated by collisionless plasma instabilities or binary collisions between particles of each shell. By comparing the maximum growth rate of the collisionless instabilities with the collision frequency between particles of the shells, we determine the critical density separating the collisionless formation from the collisional formation of the resulting shock waves. This critical density is also the density beyond which the shock downstream is field free, as plasma instabilities do not have time to develop electromagnetic patterns. We further determine the conditions on the shells initial density and velocity for the downstream to be collisional. If these quantities fulfill the determined conditions, the collisionality of the downstream also prevents the shock from accelerating particles or generating strong magnetic fields. We compare the speed of sound with the relative speed of collision between the two shells, thus determining the portion of the parameters space where strong shock formation is possible for both classical and degenerate plasmas. Finally, we discuss the observational consequences in several astrophysical settings., Comment: To appearin Journal of Plasma Physics - 35 pages, 11 figures

It is widely believed that relativistic jets in X-ray binaries and active-galactic nuclei are powered by the rotational energy of black holes. This idea is supported by general-relativistic magnetohydrodynamic (GRMHD) simulations of accreting black holes, which demonstrate efficient energy extraction via the Blandford-Znajek mechanism. However, due to uncertainties in the physics of mass-loading, and the failure of GRMHD numerical schemes in the highly-magnetized funnel region, the matter content of the jet remains poorly constrained. We investigate the observational signatures of mass-loading in the funnel by performing general-relativistic radiative transfer calculations on a range of 3D GRMHD simulations of accreting black holes. We find significant observational differences between cases in which the funnel is empty and cases where the funnel is filled with plasma, particularly in the optical and X-ray bands. In the context of Sgr A*, current spectral data constrains the jet filling only if the black hole is rapidly rotating with $a\gtrsim0.9$. In this case, the limits on the infrared flux disfavour a strong contribution from material in the funnel. We comment on the implications of our models for interpreting future Event Horizon Telescope observations. We also scale our models to stellar-mass black holes, and discuss their applicability to the low-luminosity state in X-ray binaries., Comment: 13 pages, 11 figures, extended and clarified aspects of the discussion section, accepted for publication in ApJ

In this study we investigate jet interaction at a microscopic level in a cosmological environment, which responds to a key open question in the study of relativistic jets. Using small simulation systems during prior research, we initially studied the evolution of both electron-proton and electron-positron relativistic jets containing helical magnetic fields, by focusing on their interactions with an ambient plasma. Here, using larger jet radii, we have performed simulations of global jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). We found that the evolution of global jets strongly depends on the size of the jet radius. For example, phase bunching of jet electrons, in particular in the electron-proton jet, is mixed with larger jet radius due to the more complicated structures of magnetic fields with excited kinetic instabilities. In our simulation study these kinetic instabilities lead to new types of instabilities in global jets. In the electron-proton jet simulation a modified recollimation occurs and jet electrons are strongly perturbed. In the electron-positron jet simulation mixed kinetic instabilities occur at early times followed by a turbulence-like structure. Simulations using much larger (and longer) systems are further required in order to thoroughly investigate the evolution of global jets containing helical magnetic fields., Comment: 7 pages, 4 figures, submitted to special issue "Polarised Emission from Astrophysical Jets", June 12-16, 2017, Ierapetra, Greece. arXiv admin note: substantial text overlap with arXiv:1609.09363

Blazars show variability on timescales ranging from minutes to years, the former being comparable to and in some cases even shorter than the light-crossing time of the central black hole. The observed gamma-ray lightcurves can be described by a power-law power density spectrum (PDS), with a similar index for both BL Lacs and flat-spectrum radio quasars. We show that this variability can be produced by turbulence in relativistic jets launched by magnetically arrested accretion flows (MADs). We perform radiative transport calculations on the turbulent, highly-magnetized jet launching region of a MAD with a rapidly rotating supermassive black hole. The resulting synchrotron and synchrotron self-Compton emission, originating from close to the black hole horizon, is highly variable. This variability is characterized by PDS which is remarkably similar to the observed power-law spectrum at frequencies less than a few per day. Furthermore, turbulence in the jet launching region naturally produces fluctuations in the plasma on scales much smaller than the horizon radius. We speculate that similar turbulent processes, operating in the jet at large radii (and therefore high bulk Lorentz factor), are responsible for blazar variability over many decades in frequency, including on minute timescales., Comment: 8 pages, 5 figures, removed section from appendix, references added, accepted for publication in ApJ

According to magnetohydrodynamics (MHD), the encounter of two collisional magnetized plasmas at high velocity gives rise to shock waves. Investigations conducted so far have found that the same conclusion still holds in the case of collisionless plasmas. For the case of a flow-aligned field, MHD stipulates that the field and the fluid are disconnected, so that the shock produced is independent of the field. We present a violation of this MHD prediction when considering the encounter of two cold pair plasmas along a flow-aligned magnetic field. As the guiding magnetic field grows, isotropization is progressively suppressed, resulting in a strong influence of the field on the resulting structure. A micro-physics analysis allows to understand the mechanisms at work. Particle-in-cell simulations also support our conclusions and show that the results are not restricted to a strictly parallel field., Comment: To appear in Journal of Plasma Physics

We study the interaction of relativistic jets with their environment, using 3-dimensional relativistic particle-in-cell simulations for two cases of jet composition: (i) electron-proton ($e^{-}-p^{+}$) and (ii) electron-positron ($e^{\pm}$) plasmas containing helical magnetic fields. We have performed simulations of "global" jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability and the Mushroom instability. We have found that these kinetic instabilities are suppressed and new types of instabilities can grow. For the $e^{-}-p^{+}$ jet, a recollimation-like instability occurs and jet electrons are strongly perturbed, whereas for the $e^{\pm}$ jet, a recollimation-like instability occurs at early times followed by kinetic instability and the general structure is similar to a simulation without a helical magnetic field. We plan to perform further simulations using much larger systems to confirm these new findings., Comment: 4 pages, 1 figure, New Frontiers in Black Hole Astrophysics, Proceedings of the IAU Symposium No. 324, A. Gomboc, ed

Internal shocks between propagating plasma shells, originally ejected at different times with different velocities are believed to play a major role in dissipating the kinetic energy, thereby explaining the observed lightcurve and spectra in a large range of transient objects. Even if initially the colliding plasmas are cold, following the first collision the plasma shells are substantially heated, implying that in a scenario of multiple collisions, most collisions take place between plasmas of non-zero temperatures. Here, we calculate the dynamical properties of plasmas resulting from collision between arbitrarily hot plasma shells, moving at arbitrary speeds. We provide simple analytical expressions valid for both the ultra-relativistic and Newtonian velocities, for both hot and cold plasmas. We derive the minimum criteria required for the formation of the two-shock wave system, and show that in the relativistic limit, the minimum Lorentz factor is proportional to the square root of the ratio of the initial plasmas enthalpies. We provide basic scaling laws of synchrotron emission from both the forward and reverse shock waves, and show how these can be used to deduce the properties of the colliding shells. Finally, we discuss the implications of these results in the study of several astronomical transients, such as x-ray binaries, radio load quasars and gamma-ray bursts., Comment: 10 pages, 3 figures. Minor improvement of explanations; Added discussion on alternative models; Ap.J., in press

In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the initial evolution of both electron$-$proton ($e^{-}-p^{+}$) and electron$-$positron ($e^{\pm}$) relativistic jets containing helical magnetic fields, focusing on their interaction with an ambient plasma. We have performed simulations of "global" jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). In our initial simulation study these kinetic instabilities are suppressed and new types of instabilities can grow. In the $e^{-}-p^{+}$ jet simulation a recollimation-like instability occurs and jet electrons are strongly perturbed. In the $e^{\pm}$ jet simulation a recollimation-like instability occurs at early times followed by a kinetic instability and the general structure is similar to a simulation without helical magnetic field. Simulations using much larger systems are required in order to thoroughly follow the evolution of global jets containing helical magnetic fields., Comment: 9 pages, 6 figures, Blazars through Sharp Multi-Wavelength Eyes. Malaga (Spain). 30 May - 3 June 2016, movie is available at http://www.mpdi.com/link, Video S1