23 research outputs found
Magnetic Fields In Relativistic Collisionless Shocks
No full textWe present a systematic study on magnetic fields in gamma-ray burst (GRB) external forward shocks (FSs). There are 60 (35) GRBs in our X-ray (optical) sample, mostly from Swift. We use two methods to study is an element of(B) (fraction of energy in magnetic field in the FS): (1) for the X-ray sample, we use the constraint that the observed flux at the end of the steep decline is >= X-ray FS flux; (2) for the optical sample, we use the condition that the observed flux arises from the FS (optical sample light curves decline as similar to t(-1), as expected for the FS). Making a reasonable assumption on E (jet isotropic equivalent kinetic energy), we converted these conditions into an upper limit (measurement) on is an element of(n2/(p+1))(B) for our X-ray (optical) sample, where n is the circumburst density and p is the electron index. Taking n = 1 cm(-3), the distribution of is an element of(B) measurements (upper limits) for our optical (X-ray) sample has a range of similar to 10 (8)-10 (3) (similar to 10 (6)-10 (3)) and median of similar to few x 10 (5) (similar to few x 10 (5)). To characterize how much amplification is needed, beyond shock compression of a seed magnetic field similar to 10 mu G, we expressed our results in terms of an amplification factor, AF, which is very weakly dependent on n (AF alpha n(0.21)). The range of AF measurements (upper limits) for our optical (X-ray) sample is similar to 1-1000 (similar to 10-300) with a median of similar to 50 (similar to 50). These results suggest that some amplification, in addition to shock compression, is needed to explain the afterglow observations.NSF ast-0909110ERC advanced grant (GRB)I-CORE Program of the PBCISF 1829/12Astronom
Parameterizing the gamma-ray emission of short GRB jets
No full textThe goal of this phase of our research project was to compare current competing models of the structure of the prompt gamma-ray emission that results from the merging of two neutron stars in a binary system, and to see how the differences in proposed parameters affect the detectability of these events from Earth at off-axis angles. A series of programs were written in Python to reproduce two models: a simulation based on the jointly detected GRB170817 by Kathirgamaraju et. al 2017, and a piecewise analytical approximation by Beniamini and Nakar 2018. The simulated model was tested singularly in how the properties of the luminosity profile changed as the parameter gammamin - the minimum allowed speed of the jet that defined its boundary - was raised and lowered, effectively widening and narrowing the jet structure and allowing for more or less successful detection events at large angles. We then attempted to find a broken-power-law approximation of this structure by visually fitting a modified equation from Beniamini and Nakar and generating a new luminosity profile from it. It was found that the modified equation-based structure was able to accurately produce profiles that matched the behavior of both the original simulation and the fully analytical model, though a discrepancy between the two becomes apparent in their comparison that the analytical model predicts significantly higher luminosities at large observation angles than the simulation. Continuing work with this project involves statistically testing the impact of this discrepancy on the rate of successful detection of events
From black hole to afterglow
No full textJets, launched by compact objects, collide with the interstellar medium and emit across the electromagnetic spectrum. We refer to this emission as the "afterglow". We use the HARMPI code to simulate a jet and extract the jet energy as a function of angle from the jet axis. We are currently writing a C++ code to calculate the afterglow as a function of arbitrary observer angle. The jet properties from HARMPI will be used as an input for the afterglow calculation in our C++ code and will allow us to predict several observable features. This project is motivated by the recent detection of the GW170817 afterglow and it is a necessary step towards being able to analyze future detections
A Millimeter Rebrightening in GRB 210702A
No full textWe present X-ray to radio frequency observations of the bright long gamma-ray burst GRB 210702A. Our Atacama Large Millimeter/submillimeter Array 97.5 GHz observations show a significant rebrightening by a factor of ≈2 beginning at 8.2 days post-burst and rising to peak brightness at 18.1 days before declining again. This is the first such rebrightening seen in a millimeter afterglow light curve. A standard forward shock model in a stellar wind circumburst medium can explain most of our X-ray, optical, and millimeter observations prior to the rebrightening, but significantly overpredicts the self-absorbed radio emission, and cannot explain the millimeter rebrightening. We investigate possible explanations for the millimeter rebrightening, and find that energy injection or a reverse shock from a late-time shell collision are plausible causes. Similar to other bursts, our radio data may require alternative scenarios such as a thermal electron population or a structured jet to explain the data. Our observations demonstrate that millimeter light curves can exhibit some of the rich features more commonly seen in optical and X-ray afterglow light curves, motivating further millimeter wavelength studies of GRB afterglows
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High-energy emission and recent afterglow studies of gamma-ray bursts
No full texttextGamma-ray Bursts (GRBs) are powerful explosions that emit
most of their energy, as their name suggests, in gamma-rays of typical energies
of about 1 MeV. This emission lasts for about two minutes or less and it is called the prompt emission. The isotropic energy radiated in GRBs is equivalent to the energy that the
Sun will radiate in its entire lifetime. After decades of studying
this cosmological phenomenon, we have come to learn that it involves
a collimated and relativistic jet. Also, we know that they
radiate energy in the X-ray, optical and radio bands for days, weeks and years, respectively, which is called the
afterglow. Recently, NASA's Fermi Satellite was launched and,
in addition to MeV photons, it detected GeV photons from these astrophysical sources.
We show that these GeV photons are produced when the GRB jet interacts with the medium
that surrounds it: the external forward shock model. We arrive at this conclusion
not only by studying the GeV emission, but also by studying the afterglow observations
(Chapter 2). We corroborate this model by studying the electron acceleration
in the external forward shock model and find that electrons can radiate at the maximum observed
energy of ~ 10 GeV (Chapter 3). We also provide an extensive analysis of the most recent
afterglow observations of GRB 090902B within the same
framework of an external forward shock origin. We find
that the data for this burst requires a small deviation from the
traditionally used power-law electron energy distribution, however,
our previous results remain unchanged (Chapter 4). To conclude,
we use the end of the prompt emission phase, which exhibits a
steep X-ray temporal decay, to constrain the behavior of the
central engine responsible for launching the relativistic jet (Chapter 5).Physic
Energies of GRB blast waves and prompt efficiencies as implied by modelling of X-ray and GeV afterglows
No full textWe thank Pawan Kumar and Rodolfo Santana for helpful comments. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. The work was supported by the ERC grant GRBs, by a grant from the Israel ISF–China NSF collaboration, by a grant from the Israel Space Agency, and by the I-Core Center of Excellence in Astrophysics. LN was supported by a Marie Curie Intra-European Fellowship of the European Community's 7th Framework Programme (PIEF-GA-2013- 627715).We consider a sample of 10 gamma-ray bursts with long-lasting ( ≳ 102 s) emission detected by Fermi/Large Area Telescope and for which X-ray data around 1 d are also available. We assume that both the X-rays and the GeV emission are produced by electrons accelerated at the external forward shock, and show that the X-ray and the GeV fluxes lead to very different estimates of the initial kinetic energy of the blast wave. The energy estimated from GeV is on average ̃50 times larger than the one estimated from X-rays. We model the data (accounting also for optical detections around 1 d, if available) to unveil the reason for this discrepancy and find that good modelling within the forward shock model is always possible and leads to two possibilities: (i) either the X-ray emitting electrons (unlike the GeV emitting electrons) are in the slow-cooling regime or (ii) the X-ray synchrotron flux is strongly suppressed by Compton cooling, whereas, due to the Klein-Nishina suppression, this effect is much smaller at GeV energies. In both cases the X-ray flux is no longer a robust proxy for the blast wave kinetic energy. On average, both cases require weak magnetic fields and relatively large isotropic kinetic blast wave energies corresponding to large lower limits on the collimated energies, in the range for an ISM (interstellar medium) environment with n ̃ 1 cm-3 and 10^{52} erg* ̃ 1. These energies are larger than those estimated from the X-ray flux alone, and imply smaller inferred values of the prompt efficiency mechanism, reducing the efficiency requirements on the still uncertain mechanism responsible for prompt emission
The GRB Prompt Emission: An Unsolved Puzzle
No full textThe recent multi-messenger and multi-wavelength observations of gamma-ray bursts (GRBs) have encouraged renewed interest in these energetic events. In spite of the substantial amount of data accumulated during the past few decades, the nature of the prompt emission remains an unsolved puzzle. We present an overview of the leading models for their prompt emission phase, focusing on the perspective opened by future missions
