arXiv subjects

Jonathan Granot

Publications and source records attributed to Jonathan Granot.

At least 91 records · Page 5Linked to original sources

arXiv2011

The missing link: Merging neutron stars naturally produce jet-like structures and can power short Gamma-Ray Bursts

Short Gamma-Ray Bursts (SGRBs) are among the most luminous explosions in the universe, releasing in less than one second the energy emitted by our Galaxy over one year. Despite decades of observations, the nature of their "central-engine" remains unknown. Considering a binary of magnetized neutron stars and solving Einstein equations, we show that their merger results in a rapidly spinning black hole surrounded by a hot and highly magnetized torus. Lasting over 35 ms and much longer than previous simulations, our study reveals that magnetohydrodynamical instabilities amplify an initially turbulent magnetic field of ~ 10^{12} G to produce an ordered poloidal field of ~ 10^{15} G along the black-hole spin-axis, within a half-opening angle of ~ 30 deg, which may naturally launch a relativistic jet. The broad consistency of our ab-initio calculations with SGRB observations shows that the merger of magnetized neutron stars can provide the basic physical conditions for the central-engine of SGRBs.

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arXiv2010

Jets and Gamma-Ray Burst Unification Schemes

There are several lines of evidence indicating that the ultra-relativistic outflows powering gamma-ray bursts (GRBs) are collimated into narrow jets. However, these are indirect, and the jet structure is rather poorly constrained. What is more, the jet dynamics have still not been investigated in detail. It has been suggested that the observed variety between different long duration events, ranging from bright spectrally hard GRBs, to dimmer and spectrally softer X-ray flashes (XRFs) may be largely due to different viewing angles (or lines of sight) relative to rather similar relativistic jets. Here we describe the current state of knowledge on these topics, explain some of the most relevant physics behind some of the basic principles, and discuss prospects for the future.

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arXiv2010

Late time observations of GRB080319B: jet break, host galaxy and accompanying supernova

The Swift discovered GRB080319B was by far the most distant source ever observed at naked eye brightness, reaching a peak apparent magnitude of 5.3 at a redshift of z=0.937. We present our late-time optical (HST, Gemini & VLT) and X-ray (Chandra) observations, which confirm that an achromatic break occurred in the power-law afterglow light curve at ~11 days post-burst. This most likely indicates that the gamma-ray burst (GRB) outflow was collimated, which for a uniform jet would imply a total energy in the jet E_{jet} \gsim 10^{52} erg. Our observations also show a late-time excess of red light, which is well explained if the GRB was accompanied by a supernova (SN), similar to those seen in some other long-duration GRBs. The latest observations are dominated by light from the host and show that the GRB took place in a faint dwarf galaxy (r(AB)\approx27.0, rest-frame M_B\approx-17.2). This galaxy is small even by the standards of other GRB hosts, which is suggestive of a low metallicity environment. Intriguingly, the properties of this extreme event - a small host and bright supernova - are entirely typical of the very low-luminosity bursts such as GRB980425 and GRB060218.

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arXiv2010

Impulsive Acceleration of Strongly Magnetized Relativistic Flows

The strong variability of magnetic central engines of AGN and GRBs may result in highly intermittent strongly magnetized relativistic outflows. We find a new magnetic acceleration mechanism for such impulsive flows that can be much more effective than the acceleration of steady-state flows. This impulsive acceleration results in kinetic-energy-dominated flows at astrophysically relevant distances from the central source. For a spherical flow, a discrete shell ejected from the source over a time t_0 with Lorentz factor Gamma~1 and initial magnetization sigma_0 = B_0^2/(4 pi rho_0 c^2) >> 1 quickly reaches a typical Lorentz factor Gamma ~ sigma_0^{1/3} and magnetization sigma ~ sigma_0^{2/3} at the distance R_0 ~ ct_0. At this point the magnetized shell of width Delta ~ R_0 in the lab frame loses causal contact with the source and continues to accelerate by spreading significantly in its own rest frame. The expansion is driven by the magnetic pressure gradient and leads to relativistic relative velocities between the front and back of the shell. While the expansion is roughly symmetric in the center of momentum frame, in the lab frame most of the energy and momentum remain in a region (or shell) of width Delta ~ R_0 at the head of the flow. This acceleration proceeds as Gamma ~ (sigma_0 R/R_0)^{1/3} and sigma ~ sigma_0^{2/3} (R/R_0)^{-1/3} until reaching a coasting radius R_c ~ R_0 sigma_0^2 where the kinetic energy becomes dominant: Gamma ~ sigma_0 and sigma ~ 1 at R_c. Then the shell starts coasting and spreading (radially) causing its magnetization to drop as sigma ~ R_c/R at R>R_c. Given the typical variability time-scales of AGN and GRBs, the magnetic acceleration in these sources is a combination of the quasi-steady-state collimation acceleration close to the source and the impulsive acceleration further out. The interaction with the external medium is briefly discussed.

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arXiv2010

Highlights from Fermi GRB observations

The Fermi Gamma-Ray Space Telescope has more than doubled the number of Gamma-Ray Bursts (GRBs) detected above 100 MeV within its first year of operation. Thanks to the very wide energy range covered by Fermi's Gamma-ray Burst Monitor (GBM; 8 keV to 40 MeV) and Large Area Telescope (LAT; 25 MeV to >300 GeV) it has measured the prompt GRB emission spectrum over an unprecedentedly large energy range (from ~8 keV to ~30 GeV). Here I briefly outline some highlights from Fermi GRB observations during its first ~1.5 yr of operation, focusing on the prompt emission phase. Interesting new observations are discussed along with some of their possible implications, including: (i) What can we learn from the Fermi-LAT GRB detection rate, (ii) A limit on the variation of the speed of light with photon energy (for the first time beyond the Planck scale for a linear energy dependence from direct time of arrival measurements), (iii) Lower-limits on the bulk Lorentz factor of the GRB outflow (of ~1000 for the brightest Fermi LAT GRBs), (iv) The detection (or in other cases, lack thereof) of a distinct spectral component at high (and sometimes also at low) energies, and possible implications for the prompt GRB emission mechanism, (v) The later onset (and longer duration) of the high-energy emission (>100 MeV), compared to the low-energy (< ~1 MeV) emission, that is seen in most Fermi-LAT GRBs.

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arXiv2010

The long rapid decay phase of the extended emission from the short GRB 080503

GRB080503 was classified as a short GRB with extended emission (Perley et al. 2009). The origin of such extended emission (found in about a quarter of Swift short GRBs) is still unclear and may provide some clues to the identity of the elusive progenitors of short GRBs. The extended emission from GRB 080503 is followed by a rapid decay phase (RDP) that is detected over an unusually large dynamical range (one decade in time and ~3.5 decades in flux). We model the broad envelope of extended emission and the subsequent RDP using a physical model (Genet & Granot 2009), in which the prompt emission (and its tail) is the sum of its individual pulses (and their tails). For GRB 080503, a single pulse fit is found to be unacceptable. The RDP displays very strong spectral evolution and shows some evidence for the presence of two spectral components with different temporal behaviour, likely arising from distinct physical regions. A two pulse fit provides a much better fit to the data. The shallow gamma-ray and steep hard X-ray decays are hard to account for simultaneously, and require the second pulse to deviate from the simplest version of the model we use. Therefore, while high latitude emission is a viable explanation for the RDP in GRB080503, it is quite plausible that another mechanism is at work here. Finally, we note that the properties of the RDP following the extended emission of short GRBs appear to have different properties than that following the prompt emission of long GRBs. However, a larger sample of short GRBs with extended emission is required before any strong conclusion can be drawn.

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arXiv2009

Limits on Radioactive-Powered Emission Associated With a Short-Hard GRB 070724A in a Star-Forming Galaxy

We present results of an extensive observing campaign of the short duration, hard spectrum gamma-ray burst (GRB) 070724A, aimed at detecting the radioactively-powered emission that might follow from a binary merger or collapse involving compact objects. Our multi-band observations span the range in time over which this so-called Li-Paczynski mini-supernova could be active, beginning within 3 hours of the GRB trigger, and represent some of the deepest and most comprehensive searches for such emission. We find no evidence for such activity and place limits on the abundances and the lifetimes of the possible radioactive nuclides that could form in the rapid decompression of nuclear-density matter. Furthermore, our limits are significantly fainter than the peak magnitude of any previously detected broad-lined Type Ic supernova (SN) associated with other GRBs, effectively ruling out a long GRB-like SN for with this event. Given the unambiguous redshift of the host galaxy (z=0.456), GRB 070724A represents one of a small, but growing, number of short-hard GRBs for which firm physical/restframe quantities currently exist. The host of GRB 070724A is a moderately star-forming galaxy with an older stellar population component and a relatively high metallicity of 12+log(O/H)_KD02=9.1. We find no significant evidence for large amounts of extinction along the line of sight that could mask the presence of a SN explosion and estimate a small probability for chance alignment with the putative host. We discuss how our derived constraints fit into the evolving picture of short-hard GRBs, their potential progenitors, and the host environments in which they are thought to be produced.

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arXiv2009

GRB Theory in the Fermi Era

Before the launch of the Fermi Gamma-ray Space Telescope there were only a handful of gamma-ray bursts (GRBs) detected at high energies (above 100 MeV), while several different suggestions have been made for possible high-energy emission sites and mechanisms. Here I briefly review some of the theoretical expectations for high-energy emission from GRBs, outline some of the hopes for improving our understanding of GRB physics through Fermi observations of the prompt GRB emission or the early afterglow (first few hours after the GRB), and summarize what we have learned so far from the existing Fermi GRB observations (over its first half-year of operation). Highlights include the first detection of > GeV emission from a short GRB, as well as detailed temporal and spectral information for the first GRB with > GeV emission and a measured redshift, that has the highest measured apparent (isotropic equivalent) radiated energy output (for any GRB), the largest lower limit on the bulk Lorentz factor of the emitting region, and constrains possible Lorentz invariance violation by placing a robust lower limit on the quantum gravity mass.

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arXiv2008

Analytic Expressions for the Surface Brightness Profile of GRB Afterglow Images

The exact profile of a gamma-ray burst (GRB) afterglow image on the plane of the sky can provide important constraints on the underlying physics. In particular, it can test whether the magnetic field in the emitting shocked external medium decreases significantly with the distance behind the shock front, or remains roughly constant. Moreover, it enables more accurate measurements of the afterglow image size and the expected scintillation properties. In this work analytic expressions are derived for the afterglow image in power law segments (PLSs) of the afterglow synchrotron spectrum in which the emission originates from a very thin layer just behind the shock, while simple semi-analytic expressions are derived for the remaining PLSs in which the emission arises from the bulk of the shocked fluid. In all cases the expressions are for a general power law external density profile, and are convenient to use in afterglow studies.

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arXiv2008

Critical Review of Basic Afterglow Concepts

The long lived afterglow emission that follows gamma-ray bursts (GRBs) was predicted prior to its detection in 1997, in the X-rays, optical and radio. It is thought to arise from the shock that is driven into the external medium as the latter decelerates the relativistic outflow that drives the GRB, and persists well after most of the energy in the outflow is transferred to the shocked external medium. As the blast wave decelerates, the typical emission frequency shifts to longer wavelength. Recent observations following the launch of the Swift satellite challenge the traditional afterglow modeling and call into questions some of the basic underlying concepts. This brief review outlines some of the major strengths and weaknesses of the standard afterglow model, as well as some of the challenges that it faces in explaining recent data, and potential directions for future study that may eventually help overcome some of the current difficulties.

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arXiv2007

Opacity Build-up in Impulsive Relativistic Sources

Opacity effects in relativistic sources of high-energy gamma-rays, such as gamma-ray bursts (GRBs) or Blazars, can probe the Lorentz factor of the outflow and the distance of the emission site from the source, and thus help constrain the composition of the outflow (protons, pairs, magnetic field) and the emission mechanism. Most previous works consider the opacity in steady state. Here we study time dependent effects of the opacity to pair production ($γγ\to e^+e^-$) in impulsive relativistic sources. We present a simple, yet rich, semi-analytic model for the time and energy dependence of the optical depth, $τ$, where an ultra-relativistic thin spherical shell emits isotropically in its own rest frame over a finite range of radii, $R_0 < R < R_0 + ΔR$. This is very relevant for GRB internal shocks. We find that in an impulsive source ($ΔR <~ R_0$), while the instantaneous spectrum has an exponential cutoff above the photon energy $ε_1(T)$ where $τ= 1$, the time integrated spectrum has a power-law high-energy tail above a photon energy $ε_{1*} \sim ε_1(ΔT)$ where $ΔT$ is the duration of the emission episode. Furthermore, photons with $ε> ε_{1*}$ will arrive mainly near the onset of the spike or flare from to the short emission episode, as in impulsive sources it takes time to build-up the (target) photon field, and thus $τ(ε)$ initially increases with time and $ε_1(T)$ decreases with time, so that photons of energy $ε> ε_{1*}$ are able to escape the source mainly very early on while $ε_1(T) > ε$. As the source approaches a quasi-steady state ($ΔR >> R_0$), the time integrated spectrum develops an exponential cutoff, while the power-law tail becomes increasingly suppressed.

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arXiv2006

The flat decay phase in the early X-ray afterglows of Swift GRBs

Many Swift GRBs show an early phase of shallow decay in their X-ray afterglows, lasting from $t \sim 10^{2.5} $s to $\sim 10^4 $s after the GRB, where the flux decays as $\sim t^{-0.2}-t^{-0.8}$. This is perhaps the most mysterious of the new features discovered by Swift in the early X-ray afterglow, since it is still not clear what causes it. I discuss different possible explanations for this surprising new discovery, as well as their potential implications for the gamma-ray efficiency, the afterglow kinetic energy, and perhaps even for the physics of collisionless relativistic shocks.

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arXiv2006

The Structure and Dynamics of GRB Jets

There are several lines of evidence which suggest that the relativistic outflows in gamma-ray bursts (GRBs) are collimated into narrow jets. The jet structure has important implications for the true energy release and the event rate of GRBs, and can constrain the mechanism responsible for the acceleration and collimation of the jet. Nevertheless, the jet structure and its dynamics as it sweeps up the external medium and decelerates, are not well understood. In this review I discuss our current understanding of GRB jets, stressing their structure and dynamics.

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arXiv2006

A Simple Test of the External Shock Model for the Prompt Emission in Gamma-Ray Bursts

It is demonstrated here that if the prompt GRB emission is produced by the simplest version of the external shock model, a specific relation should prevail between the observed duration, isotropic equivalent energy, and photon peak energy. In essence, this relation arises because both the burst duration and the typical energy of the emitted synchrotron photons depend on the same combination of the, usually poorly constrained, external density at the deceleration radius, $n_{dec}$, and initial bulk Lorentz factor, $Γ_0$. This has the fortunate consequence of making the relation independent of both $Γ_0$ and $n_{dec}$. Unless the efficiency of electron acceleration is very low, synchrotron gamma-rays from the external shock would fail to meet the current observational constraints for the vast majority of GRBs, including those with a smooth, single peak temporal profile. This argues either against an external shock origin for the prompt emission in GRBs or for changes in our understanding of the microphysical and radiation processes occurring within the shocked region.

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arXiv2006

Smooth Light Curves from a Bumpy Ride: Relativistic Blast Wave Encounters a Density Jump

Some GRB afterglow light curves show significant variability, which often includes episodes of rebrightening. This was attributed in several cases to large fluctuations in the external density. Here we examine the effect of a sharp increase in the external density on the light curve by considering, for the first time, a full treatment of both the hydrodynamic evolution and the radiation in this scenario. To this end we develop a semi-analytic model for the light curve and carry out several elaborate numerical simulations using a 1D hydrodynamic code together with a synchrotron radiation code.Two spherically symmetric cases are explored in detail: a density jump in a uniform external medium, and a wind termination shock. Contrary to previous works, we find that even a sharp (step function) and large (by a factor of $a>>1$) increase in the external density does not produce sharp features in the light curve, and cannot account for significant temporal variability. For a wind termination shock, the light curve smoothly transitions between the asymptotic power-laws over about one decade in time, and there is no rebrightening in the optical or X-rays to serve as a clear observational signature. For a sharp jump in a uniform density we find that the maximal deviation $Δα_max$ of the temporal decay index $α$ from its asymptotic value, is bounded (e.g., $Δα_max <0.4$ for $a=10$); $Δα_max$ slowly increases with $a$, converging to $Δα_{max} \approx 1$ at very large $a$ values. Therefore, no optical rebrightening is expected here as well. In the X-rays, while the asymptotic flux is unaffected by the density jump, the fluctuations in $α$ are found to be comparable to those in the optical. Finally, we discuss the implications of our results for the origin of the fluctuations in several afterglows.

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arXiv2006

Implications of the Early X-Ray Afterglow Light Curves of Swift GRBs

According to current models, gamma-ray bursts (GRBs) are produced when the energy carried by a relativistic outflow is dissipated and converted into radiation. The efficiency of this process, $ε_γ$, is one of the critical factors in any GRB model. The X-ray afterglow light curves of Swift GRBs show an early stage of flat decay. This has been interpreted as reflecting energy injection. When combined with previous estimates, which have concluded that the kinetic energy of the late ($\gtrsim 10$ hr) afterglow is comparable to the energy emitted in gamma-rays, this interpretation implies very high values of $ε_γ\gtrsim 0.9$. Such a high efficiency is hard to reconcile with most models, including in particular the popular internal-shocks model. We re-analyze the derivation of the kinetic energy from the afterglow X-ray flux and re-examine the resulting estimates of the efficiency. We confirm that, if the flatt decay arises from energy injection and the pre-Swift broad-band estimates of the kinetic energy are correct, then $ε_γ\gtrsim 0.9$. We point out, however, that another interpretation of the flattish decay - a variable X-ray afterglow efficiency (e.g., due to a time dependence of afterglow shock microphysical parameters) - is possible. We also show that direct estimates of the kinetic energy from the late X-ray afterglow flux are sensitive to the assumed values of the shock microphysical parameters and suggest that broad-band afterglow fits might have underestimated the kinetic energy (e.g., by overestimating the fraction of electrons that are accelerated to relativistic energies). Either one of these possibilities implies a lower gamma-ray efficiency, and their joint effect could conceivably reduce the estimate of the typical $ε_γ$ to $\sim 0.1-0.5$.

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arXiv2005

The Case for Anisotropic Afterglow Efficiency within Gamma-Ray Burst Jets

Early X-ray afterglows recently detected by {\it Swift} frequently show a phase of very shallow flux decay lasting from $\sim 10^{2.5} $s up to $\sim 10^4 $s, followed by a steeper, more familiar decay. We suggest that the flat early part of the light curve may be a combination of the decaying tail of the prompt emission and the delayed onset of the afterglow emission observed from viewing angles slightly outside the edge of the region within the jet with prominent afterglow emission, as predicted previously. This would imply that a significant fraction of viewers get very little external shock energy along their line of sight, and, therefore, see a very high $γ$-ray to kinetic energy ratio at early times. The early flat phase in the afterglow light curve implies, in a rather robust and model independent manner, a very large $γ$-ray efficiency, typically $\gtrsim 90%$, which is very difficult to extract from baryons by internal shocks.

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arXiv2005

Distribution of Gamma-ray Burst Ejecta Energy with Lorentz Factor

The early X-ray afterglow for a significant number of gamma-ray bursts detected by the Swift satellite is observed to have a phase of very slow flux decline with time ($F_ν\propto t^{-α}$ with $0.2 \lesssim α\lesssim 0.8$) for $10^{2.5} s \lesssim t \lesssim 10^4$ s, while the subsequent decline is the usual $1 \lesssim α_3 \lesssim 1.5$ behavior, that was seen in the pre-Swift era. We show that this behavior is a natural consequence of a small spread in the Lorentz factor of the ejecta, by a factor of $\sim 2-4$, where the slower ejecta gradually catch-up with the shocked external medium, thus increasing the energy of forward shock and delaying its deceleration. The end of the ``shallow'' flux decay stage marks the beginning of the Blandford-McKee self similar external shock evolution. This suggests that most of the energy in the relativistic outflow is in material with a Lorentz factor of $\sim 30-50$.

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