Klein–Nishina Effects on Optically Thin Synchrotron and Synchrotron Self-Compton Spectrum (2009)
Nakar, Ehud, Ando, Shin'ichiro, Sari, Re'em
We present analytic approximations to the optically thin synchrotron and synchrotron self-Compton (SSC) spectra when Klein-Nishina (KN) effects are important and pair production and external...
Piran, Tsvi, Shaviv, Nir J., Nakar, Ehud
Recent measurements of the positron/electron ratio in the cosmic ray (CR) flux exhibits an apparent anomaly, whereby this ratio increases between 10 and 100 GeV. In contrast, this ratio should...
Gamma-ray burst light curves in the relativistic turbulence and relativistic subjet models (2009)
Lazar, Ayah, Nakar, Ehud, Piran, Tsvi
Randomly oriented relativistic emitters in a relativistically expanding shell provide an alternative to internal shocks as a mechanism for producing gamma-ray bursts' variable light curves with...
Klein-Nishina Effects on Optically Thin Synchrotron and Synchrotron Self-Compton Spectrum (2009)
Nakar, Ehud, Ando, Shin'ichiro, Sari, Re'em
We present analytic approximations to the optically thin synchrotron and synchrotron self-Compton (SSC) spectra when Klein-Nishina (KN) effects are important and pair production and external...
Bloom, Joshua S., Holz, Daniel E., Hughes, Scott A., Menou, Kristen, Adams, Allan, Anderson, Scott F., ...
It is widely expected that the coming decade will witness the first direct detection of gravitational waves (GWs). The ground-based LIGO and Virgo GW observatories are being upgraded to advanced...
Shaviv, Nir J., Nakar, Ehud, Piran, Tsvi
Recent measurements of the positron/electron ratio in the cosmic ray (CR) flux exhibits an apparent anomaly, whereby this ratio increases with energy between 10 and 100 GeV. In contrast, this ratio...
GRB Light Curves in the Relativistic Turbulence Model (2009)
Lazar, Ayah, Nakar, Ehud, Piran, Tsvi
Randomly oriented relativistic emitters in a relativistically expanding shell provides an alternative to internal shocks as a mechanism for producing GRBs' variable light curves with efficient...
GeV emission from prompt and afterglow phases of gamma-ray bursts (2008)
Ando, Shin'ichiro, Nakar, Ehud, Sari, Re'em
We investigate the GeV emission from gamma-ray bursts (GRBs) using the results from the Energetic Gamma Ray Experimental Telescope (EGRET) and in view of the Gamma-Ray Large Area Space Telescope...
GeV emission from prompt and afterglow phases of gamma-ray bursts (2008)
Ando, Shin'ichiro, Nakar, Ehud, Sari, Re'em
We investigate the GeV emission from gamma-ray bursts (GRBs) using the results from the Energetic Gamma Ray Experimental Telescope (EGRET) and in view of the Gamma-Ray Large Area Space Telescope...
New imaging and spectroscopy of the locations of several short-hard gamma-ray bursts (2008)
Gal-Yam, Avishay, Nakar, Ehud, Ofek, Eran O., Cenko, S. B., Kulkarni, S. R., Soderberg, A. M., ...
The detection and characterization of the afterglow emission and host galaxies of short-hard gamma-ray bursts (SHBs) is one of the most exciting recent astronomical discoveries. In particular,...
Couch, Sean M., Milosavljevic, Milos, Nakar, Ehud
We investigate the interaction of nonthermal ions (protons and nuclei) accelerated in an ultrarelativistic blastwave with the pre-existing magnetic field of the medium into which the blastwave...
GeV Emission from Prompt and Afterglow Phases of Gamma-Ray Bursts (2008)
Ando, Shin'ichiro, Nakar, Ehud, Sari, Re'em
We investigate the GeV emission from gamma-ray bursts (GRBs), using the results from the Energetic Gamma Ray Experimental Telescope (EGRET), and in view of the Gamma-ray Large Area Space Telescope...
The Cluster-Merger Shock in 1E 0657-56 (2008)
Koda, Jun, Milosavljevic, Milos, Shapiro, Paul R., Nagai, Daisuke, Nakar, Ehud
The merging galaxy cluster 1E 0657-56, known as the "bullet cluster," is one of the hottest clusters known. The X-ray emitting plasma exhibits bow-shock-like temperature and density jumps. The...
Nakar, Ehud, Milosavljević, Miloš, Nagai, Daisuke
X-ray observations of galaxy cluster merger shocks can be used to constrain nonthermal processes in the intracluster medium (ICM). The presence of nonthermal pressure components in the ICM, as well...
Cenko, S. Bradley, Berger, Edo, Nakar, Ehud, Kasliwal, Mansi M., Cucchiara, Antonio, Kulkarni, Shri R., ...
We present optical spectra of the host galaxies of the short-duration gamma-ray burst GRB 070429B and the likely short-duration with extended emission GRB 070714B. In both cases, we find a single...
Smooth light curves from a bumpy ride: relativistic blast wave encounters a density jump (2007)
In the standard forward shock model for gamma-ray burst (GRB) afterglow, the observed afterglow emission is synchrotron radiation from a quasi-spherical, adiabatic, self-similar, relativistic blast...
Milosavljevic, Milos, Nakar, Ehud, Zhang, Fan
Because conical segments of quasispherical ultrarelativistic blastwaves are causally disconnected on angular scales larger than the blastwave inverse Lorentz factor, astrophysical blastwaves can...
Nakar, Ehud, Milosavljevic, Milos, Nagai, Daisuke
X-ray observations of galaxy cluster merger shocks can be used to constrain nonthermal processes in the intracluster medium (ICM). The presence of nonthermal pressure components in the ICM, as well...
The Cluster-Merger Shock in 1E 0657-56: Faster than the Speeding Bullet? (2007)
Milosavljevic, Milos, Koda, Jun, Nagai, Daisuke, Nakar, Ehud, Shapiro, Paul R.
Shock waves driven in the intergalactic medium during the merging of galaxy clusters have been observed in X-ray imaging and spectroscopy. Fluid motions inferred from the shock strength and...
Some Theoretical Implications of Short-Hard Gamma-Ray Burst observations (2007)
Short-hard and long-soft gamma-ray bursts (GRBs) are two distinct phenomena, but their prompt and afterglow emission show many similarities. This suggests that two different progenitor systems lead...
Short-Hard Gamma-Ray Bursts (2007)
Two types of Gamma-ray bursts (GRBs) are observed: short duration hard spectrum GRBs and long duration soft spectrum GRBs. For many years long GRBs were the focus of intense research while the lack...
Milosavljević, Miloš, Nakar, Ehud
Collisionless shocks are commonly argued to be the sites of cosmic-ray (CR) acceleration. We study the influence of CRs on weakly magnetized relativistic collisionless shocks and apply our results to...
Smooth Light Curves from a Bumpy Ride: Relativistic Blast Wave Encounters a Density Jump (2006)
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...
Milosavljevic, Milos, Nakar, Ehud
Collisionless shocks are commonly argued to be the sites of cosmic ray (CR) acceleration. We study the influence of CRs on weakly magnetized relativistic collisionless shocks and apply our results to...
Nakar, Ehud, Gal-Yam, Avishay, Fox, Derek B.
The recent discovery of the first four afterglows of short-hard gamma-ray bursts (SHBs), suggests that they typically result from long-lived progenitor systems. The most popular progenitor model...
The Progenitors of Short-Hard Gamma-Ray Bursts from an Extended Sample of Events (2005)
Gal-Yam, Avishay, Nakar, Ehud, Ofek, Eran, Fox, D. B., Cenko, S. B., Kulkarni, S. R., ...
The detection of the afterglow emission and host galaxies of short-hard gamma-ray bursts (SHBs) is one of the most exciting recent astronomical discoveries. Indications that SHB progenitors belong to...
Gal-Yam, Avishay, Ofek, Eran O., Poznanski, Dovi, Levinson, Amir, Waxman, Eli, Frail, Dale A., ...
We had previously reported on a survey for radio transients, used to set an upper limit on the number of orphan gamma-ray burst (GRB) radio afterglows, and thus a lower limit on the typical GRB...
Milosavljevic, Milos, Nakar, Ehud
Models for the synchrotron emission of gamma-ray burst afterglows suggest that the magnetic field is generated in the shock wave that forms as relativistic ejecta plow through the circum-burst...
Milosavljevic, Milos, Nakar, Ehud, Spitkovsky, Anatoly
It is generally accepted that magnetic fields generated in the nonlinear development of the transverse Weibel instability provide effective collisionality in unmagnetized collisionless shocks....
In Nakar & Piran (2004) we have shown that a large fraction of BATSE bursts cannot satisfy the Amati relation between the isotropic energy - peak energy of GRBs. Our results have been corroborated by...
Early GRB afterglow from a reverse shock as a tracer of the prompt gamma-ray light curve (2005)
We discuss the optical and radio early afterglow emission of the reverse shock that crosses a baryonic ejecta as it interacts with the external interstellar medium (ISM). We show that the peak of the...
The distances of short-hard GRBs and the SGR connection (2005)
Nakar, Ehud, Gal-Yam, Avishay, Piran, Tsvi, Fox, Derek B.
We present a search for nearby (D
Pure and loaded fireballs in SGR giant flares (2005)
Nakar, Ehud, Piran, Tsvi, Sari, Re'em
On December 27, 2004, a giant flare from SGR 1806$-$20 was detected on earth. Its thermal spectrum and temperature suggest that the flare resulted from an energy release of about $10^{47}$ erg/sec...
Outliers to the Isotropic Energy - Peak Energy Relation in GRBs (2004)
The peak energy - isotropic energy (EpEi) relation is among the most intriguing recent discoveries concerning GRBs. It can have numerous implications on our understanding of the emission mechanism of...
The apparent size of GRB afterglows as a test of the Fireball model (2004)
Oren, Yonatan, Nakar, Ehud, Piran, Tsvi
Taylor et al. (2004) reported recently on the first direct measurement of the apparent size of a GRB afterglow. Here we confront these observations with the predictions of the fireball model. We use...
GRB 990123 revisited: Further Evidence for a Reverse Shock (2004)
Recently we have presented a new theoretical analysis of the reverse shock emission. We use this analysis here to revisit GRB 990123. We find new and compelling evidences that the optical flash and...
Early Afterglow Emission from a Reverse Shock as a Diagnostic Tool for GRB Outflows (2004)
The Gamma-Ray burst (GRB) - afterglow transition is one of the most interesting and least studied GRB phases. During this phase the relativistic ejecta begins interacting with the surrounding matter....
Survival probabilities in time-dependent random walks (2004)
We analyze the dynamics of random walks in which the jumping probabilities are periodic {\it time-dependent} functions. In particular, we determine the survival probability of biased walkers who are...
Evolutionary Minority Game: The Roles of Response Time and Mutation Threshold (2004)
In the evolutionary minority game, agents are allowed to evolve their strategies (``mutate'') based on past experience. We explore the dependence of the system's global behavior on the response time...
Ehud Nakar, Jonathan Granot, Dafne Guetta
The two leading models for the structure of gamma-ray burst (GRB) jets are the uniform jet model and the universal structured jet (USJ) model. In the latter, all GRB jets are intrinsically identical...
Some Recent Peculiarities of the Early Afterglow (2003)
Piran, Tsvi, Nakar, Ehud, Granot, Jonathan
We consider some recent developments in GRB/afterglow observations: (i) the appearance of a very hard prompt component in GRB 941017, and (ii) variability in the early afterglow light curves of GRB...
Testing the Predictions of the Universal Structured GRB Jet Model (2003)
Nakar, Ehud, Granot, Jonathan, Guetta, Dafne
The two leading models for the structure of GRB jets are the uniform jet model and the universal structured jet (USJ) model. In the latter, all GRB jets are intrinsically identical and the energy per...
Polarization and light curve variability: the "patchy shell" model (2003)
Recent advances in early detection and detailed monitoring of GRB afterglows have revealed variability in some afterglow light curves. One of the leading models for this behavior is the patchy shell...
Implications of the $\gamma$-ray Polarization of GRB 021206 (2003)
Nakar, Ehud, Piran, Tsvi, Waxman, Eli
We compare two possible scenarios for the producing of high level of polarization within the prompt emission of a GRB: synchrotron emission from a relativistic jet with a uniform (in space and time)...
The Variable Light Curve of GRB 030329: The Case for Refreshed Shocks (2003)
Granot, Jonathan, Nakar, Ehud, Piran, Tsvi
GRB 030329 is unique in many aspects. It has a very low redshift for a GRB, $z=0.1685$, and is therefore very bright and easy to monitor, making it the most well studied afterglow to date. It shows a...
Modeling Fluctuations in the GRB-Afterglow Light Curves (2003)
The fluctuations observed in the light curves of some GRB-afterglows (such as GRB 021004) provide a useful tool to probe the circum-burst density profile and to probe the variations in the energy of...
Strategy updating rules and strategy distributions in dynamical multiagent systems (2003)
In the evolutionary version of the minority game, agents update their strategies (gene-value $p$) in order to improve their performance. Motivated by recent intriguing results obtained for...
Variability in GRB Afterglows and GRB 021004 (2002)
Nakar, Ehud, Piran, Tsvi, Granot, Jonathan
We present general analytic expressions for GRB afterglow light curves arising from a variable external density profile and/or a variable energy in the blast wave. The former could arise from a...
On-Axis Orphan Afterglows (2002)
In many GRB inner engine models the highly relativistic GRB jets are engulfed by slower moving matter. This could result in different beaming for the prompt $\gamma$-ray emission and for the lower...
Temporal oscillations and phase transitions in the evolutionary minority game (2002)
The study of societies of adaptive agents seeking minority status is an active area of research. Recently, it has been demonstrated that such systems display an intriguing phase-transition: agents...
The Detectability of Orphan Afterglows (2002)
Nakar, Ehud, Piran, Tsvi, Granot, Jonathan
The realization that GRBs release a constant amount of energy implies that the post jet-break afterglow evolution would be largely universal. For a given redshift all afterglows should be detected up...
How to Construct a GRB Engine? (2002)
According to the fireball model GRBs are produced when a relativistic flow is dissipated by shocks. The questions how does a GRB's "inner engine" accelerates and collimated the relativistic flow is...
Self-Segregation vs. Clustering in the Evolutionary Minority Game (2002)
Complex adaptive systems have been the subject of much recent attention. It is by now well-established that members (`agents') tend to self-segregate into opposing groups characterized by extreme...
Time Scales in Long GRBs (2001)
We analyze a sample of bright long bursts and find that the pulses duration have a lognormal distribution while the intervals between pulses have an excess of long intervals (relative to lognormal...
Temporal properties of short GRBs (2001)
We analyze a sample of bright short bursts from the BATSE 4B-catalog and find that many short bursts are highly variable ($\delta t_{min}/T\ll 1$, where $\delta t_{min}$ is the shortest pulse...