arXiv subjects

Asaf Pe'er

Publications and source records attributed to Asaf Pe'er.

At least 19 recordsLinked to original sources

arXiv2026

Radiation-Driven Magnetic Fields in Sub-Keplerian Accretion Flows: An Alternative to MRI

Large scale magnetic fields play a crucial role in shaping the dynamics and structure of the inner parts of accretion disks and the resulting jets from the vicinity of black holes and neutron stars. Currently, the primary mechanism considered for generating and amplifying large scale magnetic fields in accretion disks is the magneto-rotational instability (MRI). Here we show that non-conservative radiation fields provide a rapid and independent mechanism for generating and amplifying magnetic fields in accretion flows. A luminous, compact corona generates a radiation-driven poloidal field that is subsequently amplified by differential rotation, yielding quadratic magnetic-field growth and, over a broad range of coronal luminosities and sizes and MRI amplification locations, magnetization on timescales comparable to or shorter than those of the MRI. The mechanism therefore provides not merely an alternative to MRI, but an additional and, over a substantial region of parameter space, dominant channel for magnetic-field generation from initial conditions with zero magnetic field. Since the radiation-driven source arises directly from the non-conservative nature of the radiation field, such magnetic-field generation is an inevitable consequence of sufficiently strong and anisotropic radiation sources in black-hole accretion flows. More generally, the mechanism requires only a non-conservative radiation field and differential plasma motion, and is therefore expected to operate in a broad range of luminous astrophysical systems, including active galactic nuclei, gamma-ray bursts, and tidal disruption events.

astro-ph.HE↗

arXiv2026

Wind-confined jet collimation revealed by the acceleration-phase photosphere of GRB 220426A

We analyze the prompt emission of the exceptionally bright GRB 220426A observed by Fermi/GBM, whose time-resolved spectra are among the narrowest measured in any GRB. Here we show that observations during the first $\sim 5$s are consistent with the signal being emitted while the jet was still in the initial radiation-dominated acceleration phase. The time-resolved spectra allow the effective launch radius, $r_0$, to be inferred with unusual precision. We find $r_0 \sim \mathrm{few} \times 10^{10}\,\mathrm{cm}$, increasing linearly with time. These findings match the theoretical predictions of the recollimation shock, suggesting that this GRB shows the first clear evidence for the existence and evolution of a recollimation shock. Using this interpretation, the linear increase observed is sustained over a period longer than expected from a pure jet breakout. Therefore, the jet collimation must have persisted even after breakout. We suggest that the collimation was maintained by a finite, dense, wind-like circumburst medium, which would reproduce the observed behavior of the prompt emission. We conclude that late-stage progenitor mass-loss can shape the earliest prompt emission and that the photospheric emission provides a way to probe both the jet collimation and the innermost region of the circumburst medium (CBM) surrounding the progenitor, independently of the constraints from interacting supernovae.

astro-ph.HE↗

arXiv2026

Characterizing the Scale Height and Filamentary Structure of Radiatively Cooled MADs

Radiative cooling can strongly influence the structure and dynamics of black hole accretion disks. Here, we perform general relativistic magnetohydrodynamic (GR-MHD) simulations of magnetically arrested disks (MADs) around a non-spinning black hole. Radiative cooling is consistently included in the simulations and its intensity is scaled by the mass accretion rate ranging from $10^{-7}$ to $10^{-4} \dot{M}_{\mathrm{Edd}}$. Considering synchrotron and bremsstrahlung emission, we quantify how radiative losses modify the disk structure and the accretion dynamics. In the inner MAD disk regions, accumulation of magnetic field regulates gas accretion, enforcing the gas into a discrete interchange-driven filamentary structure. We identify, both analytically and numerically, a transition mass accretion rate above which radiative cooling becomes faster than the heating, which is assumed to occur via local coupling to the magnetic field. Above this mass accretion rate, cooling substantially reduces the gas thermal pressure, leading to considerably thinner and denser accretion filaments, and a substantial increase in radiative efficiency, relative to lower accretion rates. We show that under these conditions, conventional measures of the disk scale height become misleading in MAD flows. We therefore introduce an alternative definition based on the polar position of the density maximum, which more robustly characterizes the filamentary structure of the disks in the presence of strong magnetic fields and cooling.

astro-ph.HE↗

arXiv2026

Radiation-Driven Origin of Super-Equipartition Magnetic Fields in Accretion Discs and Outflows

Magnetic fields play a central role in accretion physics around black holes, yet their physical origin within accretion flows remains an open problem. In this work, we investigate the generation and subsequent evolution of magnetic fields triggered by anisotropic radiation fields in black hole accretion discs with compact rotating inner corona. We self-consistently evolve the magnetic field using the generalized field evolution MHD equation, including advection, shear-driven induction, and Hall effects. The radiation field acts as a primary field generator, while azimuthal rotation in the magnetized plasma provides rapid amplification. We find that radiation-generated fields efficiently reach a dominant toroidal component by Keplerian rotation, leading to magnetic field strengths of order $\sim 10^{8}\,\mathrm{G}$ in the vicinity of a 10 solar mass black hole and accretion disc-corona emitting at luminosity equivalent to the Eddington unit. These magnetic fields are achieved within viscous timescales and reach or exceed local equipartition estimates based on gas pressure. When vertical outflows are included, the amplified magnetic fields are advected into the corona, magnetizing disc-launched winds and jet precursors with field strengths of similar order. Our results demonstrate that radiation is not merely a passive component of accretion flows, but provides a robust and unavoidable trigger for the generation of dynamically significant magnetic fields. Our results provide a physically grounded explanation for the origin of large-scale, structured magnetic fields in and around accretion discs. This mechanism offers a pathway for magnetizing accretion discs and their outflows without invoking externally supplied magnetic flux, with broad implications for X-ray binaries, active galactic nuclei and other transients such as gamma-ray bursts (GRBs).

astro-ph.HE↗

arXiv2025

Impact of Spectral Coverage on Parameter recovery in Blazar Modeling

Understanding the impact of spectral coverage on parameter recovery is critical for accurate interpretation of blazar spectra. In this study, we examine how the data coverage influences the reliability of parameter estimation within the one-zone synchrotron self-Compton (SSC) framework. Using OJ 287, TXS 0506+056, and Mrk 421 as representative of the low-, intermediate- and high synchrotron peak classes (LSP, ISP and HSP), respectively, we generate synthetic SEDs based on their best-fit models and perform 1,000 fits for each of the 21 observational configurations per source type. Our analysis quantifies the coverage probability for all model parameters, such has the magnetic field strength and the electron luminosity, and reveals that different blazar subclasses exhibit distinct sensitivities to spectral gaps. For LSPs, a minimal dataset comprising optical/UV, X-ray, and GeV $γ$-ray bands is sufficient for robust parameter inference. In contrast, ISPs and HSPs require broader spectral coverage to constrain the physical parameters. For ISP, we find that reliable parameter recovery can be achieved with two different minimal band combinations: \textit{(i)} X-ray, high energy $γ$-ray, and very high energy $γ$-ray data, or \textit{(ii)} optical/UV, X-ray, and high energy $γ$-ray data. For HSPs, the minimal configuration enabling reliable parameter recovery includes the optical/UV, X-ray, and very high energy $γ$-ray bands. We discuss the role of very high energy $γ$-ray observations, showing that they significantly enhance parameter recovery for HSPs. Our results provide practical guidelines for designing optimized multi-wavelength observation campaigns and for assessing the robustness of SSC model inferences under incomplete spectral coverage.

astro-ph.HE↗

arXiv2025

Fast Radio Bursts from non-resonant Alfvén waves and synchrotron maser emission in the magnetar wind

Non-resonant interactions between Alfvén waves and a relativistic plasma result in the formation of the population inversions necessary for synchrotron maser emission (SME) across a wide range of magnetisations and temperatures. We calculate the peak frequencies of the SME resulting from this interaction and show that the characteristic frequencies and energetics of fast radio bursts (FRBs) can be produced in the relativistic wind of a magnetar using this mechanism. Wind Lorentz factors of $γ_w\gtrsim310$ are shown to be necessary to explain observed FRBs. Emission is possible at temperatures of $θ= k_bT/mc^2\lesssim 0.02$. We further examine the periods and magnetic fields of the central magnetar and demonstrate that the optimal values of these properties align with the observed magnetar population provided that the magnetosphere is disturbed by the flaring activity. These results allow the properties of the environment such as temperature and magnetisation to be probed from the observed FRB frequency and luminosity.

astro-ph.HE↗

arXiv2025

Direct Solution of the Time-Dependent Covariant Radiative Transfer Equation and its Coupling to General Relativistic Magnetohydrodynamics with cuHARM

In this paper we present a major update to the general relativistic magnetohydrodynamics (GRMHD) code cuHARM, which adds fully covariant treatment of radiation transport and the subsequent radiation backreaction on the dynamics of the fluid. For the radiative calculations, we discretize and solve the radiation transfer equation on a geodesic grid, in order to resolve the angular distribution of the radiation field everywhere in space. This allows for detailed treatment of non-isotropic radiation fields, which is crucial for accurately resolving regions of intermediate optical depth. We present the equations solved, the numerical methods used, and standard tests used to verify the different aspects of a radiation hydrodynamics code, in particular radiation transport and radiation-fluid interaction. We present an application of the code to the case of black hole radiative accretion. This new radiation module is fully GPU-accelerated and represents a major advance in the capabilities of cuHARM.

astro-ph.HE↗

arXiv2025

Bridging the Gap between Collisional and Collisionless Plasma Shocks: A Simulation Study using OSIRIS

Shock waves in plasmas can be characterized by the mechanisms behind their formation. When binary collisions are frequent, dissipation is collision-driven and the shock width is a few mean free paths. In contrast, collisionless shocks rely on collective plasma processes to establish dissipation on scales well below the mean free path. We bridge these regimes with particle-in-cell simulations using OSIRIS with a Coulomb-collision module, varying parameters that control collisionality. We find a smooth transition of the shock width in the intermediate region where the ion plasma parameter $N_D \approx 1$. Our results recover the asymptotic predictions: a collisional-regime width consistent with the Mott-Smith ansatz with a BGK operator, and the collisionless limit consistent with Tidman's classical formalism. The ion plasma parameter thus serves as a practical metric for identifying when shocks shift from fluid-like, mean-free-path scales to collisionless, sub-mean-free-path scales. We discuss implications for astrophysical environments, where shock breakout changes the shock width and marks the onset of efficient particle acceleration.

physics.plasm-ph↗

arXiv2025

Formation and Evolution of Pair-Alfvén Shocks with PIC Simulations

We consider the recently discovered \emph{pair-Alfvén shock wave} occurring in collisionless electron-positron plasmas. We perform a series of Particle-In-Cell studies in one and two dimensions in order to determine the stability conditions for such a shock and the mechanisms which sustain its growth. Building on our previous simulations, which established that these shocks are initially mediated by the Weibel instability before becoming Alfvénic, we demonstrate that the shock is sustained by self-generated Alfvén waves overtaking the shock in the upstream plasma. As a result growth is only possible when the guiding magnetic field strength, and hence Alfvén speed, is sufficiently small, $ω_c \lesssim 0.4$ (in normalized units). Furthermore the production of the waves in the upstream is dependent on a resonance between the Alfvén wave mode and the thermal noise in the plasma, which is inhibited at high magnetization. This explains the conditional absence of this type of shock identified previously.

astro-ph.HE↗

arXiv2025

Generation of magnetic fields around black hole accretion discs due to non conservative radiation fields

We investigate the generation of magnetic fields above black hole accretion discs due to the non-zero curl of the disc radiation field. By self consistently computing the components of the radiation flux and their curl, we show that the rotational nature of the radiation field induces charge separation, leading to magnetic field generation in the plasma above the disc. Solving the magnetohydrodynamic equations, we derive the time evolution of these fields and demonstrate that they grow over astrophysically relevant timescales. For a standard Keplerian accretion disc, the produced magnetic fields remain weak, on the order of a few Gauss, consistent with previous predictions. However, when a luminous corona is present in the inner disc region ($r_d < 3-10 r_g$), the generated fields reach dynamically significant strengths of up to $10^5$ Gauss where the magnetic energy density approaching few percentage of equipartition with the gas pressure. These fields develop within realistic growth timescales (such as viscous timescale) and can be dynamically significant in governing disc and jet evolution. Our findings suggest that radiation-driven magnetic fields play a crucial role in accretion flow magnetization, influencing both disc dynamics and observational signatures. The predicted field strengths could affect the thermal emission, synchrotron radiation, and polarization properties of black hole accretion systems, with implications for X-ray binaries, AGN, and jet formation. Future numerical simulations and high-resolution polarimetric observations, such as those from IXPE, eXTP, and EHT, may provide observational confirmation of our findings.

astro-ph.HE↗

arXiv2025

A parametric study of population inversions in relativistic plasmas through nonresonant interactions with Alfvén waves and their applications to Fast Radio Bursts

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 Alfvén waves and a relativistic plasma result in the formation of population inversions across a wide range of magnetizations, $σ\gtrsim10^{-4}$, 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 $f_{inv}\gtrsim10^{-2}$ are achieved for high magnetizations $σ>1$. 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.

astro-ph.HE↗

arXiv2025

Radiative cooling changes the dynamics of magnetically arrested disks

We studied magnetically arrested disks (MAD) around rotating black holes (BH), under the influence of radiative cooling. We introduce a critical value of the mass accretion rate $\dot M_{\rm crit}$ for which the cooling by the synchrotron process efficiently radiates the thermal energy of the disk. We find $\dot M_{\rm crit} \approx 10^{-5.5} \dot M_{\rm Edd}$, where $\dot M_{\rm Edd}$ is the Eddington mass accretion rate. The normalization constant depends on the saturated magnetic flux and on the ratio of electron to proton temperatures, but not on the BH mass. We verify our analytical estimate using a suite of general relativistic magnetohydrodynamic (GRMHD) simulations for a range of black hole spin parameters $a \in \{ -0.94, -0.5, 0, 0.5, 0.94 \}$ and mass accretion rates ranging from $10^{-7}\dot M_{\rm Edd}$ to $10^{-4}\dot M_{\rm Edd}$. We numerically observe that the MAD parameter and the jet efficiency vary by a factor of $\approx 2$ as the mass accretion rate increases above $\dot M_{\rm crit}$, which confirms our analytical result. We further detail how the forces satisfying the quasi-equilibrium of the disk change, with the magnetic contribution increasing as the thermal contribution decreases.

astro-ph.HE↗

arXiv2024

Gamma-ray bursts: what do we know today that we did not know 10 years ago?

I discuss here the progress made in the last decade on few of the key open problems in GRB physics. These include: (1) the nature of GRB progenitors, and the outliers found to the collapsar/merger scenarios; (2) Jet structures, whose existence became evident following GRB/GW170817; (3) the great progress made in understanding the GRB jet launching mechanisms, enabled by general-relativistic magneto-hydrodynamic (GR-MHD) codes; (4) recent studies of magnetic reconnection as a valid energy dissipation mechanism; (5) the early afterglow, which may be highly affected by a wind bubble, as well as recent indication that in many GRBs, the Lorentz factor is only a few tens, rather than few hundreds. I highlight some recent observational progress, including major breakthrough in detecting TeV photons and the on-going debate about their origin, polarization measurements, as well as the pair annihilation line recently detected in GRB 221009A, and its implications on the prompt emission physics. I point into some open questions that I anticipate would be at the forefront of GRB research in the next decade.

astro-ph.HE↗

arXiv2024

A mechanism that could stop the acceleration process within a collisionless shock

Collisionless shocks are complex nonlinear structures that are not yet fully understood. In particular, the interaction between these shocks and the particles they accelerate remains elusive. Based on an instability analysis that relates the shock width to the spectrum of the accelerated particle and the shock density ratio, we find that the acceleration process could come to an end when the fraction of accelerated upstream particles reaches about 30\%. Only unmagnetized shocks are considered.

physics.plasm-ph↗

arXiv2024

Physical conditions that lead to the detection of the pair annihilation line in the BOAT GRB221009A

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 $Γ\approx 600$ 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.

astro-ph.HE↗

arXiv2024

Theory of photon scattering in shearing plasma: Applications to GRBs

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 ($β$) at high energies.

astro-ph.HE↗

arXiv2024

Unified theory of negative and positive spectral lags in GRB prompt phase due to shear Comptonization from a structured jet

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.

astro-ph.HE↗

arXiv2024

Gamma-ray burst interaction with the circumburst medium: The CBM phase of GRBs

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.

astro-ph.HE↗