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Register a new userAn evolving GeV spectrum from prompt to afterglow: the case of GRB 160509A
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We present the high-energy emission properties of GRB 160509A, from its prompt mission to late afterglow phase. GRB 160509A contains two emission episodes: 0-40s and 280-420s after the burst onset (t0). The relatively high fluence of GRB 160509A allows us to establish an evolving spectrum above 100 MeV. During the first emission episode, the >100 MeV spectrum is soft with Gamma=>3.0, which can be smoothly connected to keV energies with a Band function with a high-energy cutoff. The >100 MeV spectrum rapidly changes to a hard spectrum with Gamma<=1.5 after t0+40s. The existence of very energetic photons, e.g., a 52 GeV that arrives t0+77 seconds, and a 29 GeV that arrives t0+70 ks, is hard to reconcile by the synchrotron emission from forward-shock electrons, but likely due to inverse Compton mechanism (e.g., synchrotron self-Compton emission). A soft spectrum (Gamma~2) between 300s and 1000s after the burst onset is also found at a significance of about 2 standard deviations, which suggests a different emission mechanism at work for this short period of time. GRB 160509A represents the latest example where inverse Compton emission has to be taken into account in explaining the afterglow GeV emission, which had been suggested long before the launch of Fermi LAT.
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GRB 190114C is the first gamma-ray burst detected at Very High Energies (VHE, i.e. >300 GeV) by the MAGIC Cherenkov telescope. The analysis of the emission detected by the Fermi satellite at lower energies, in the 10 keV -- 100 GeV energy range, up to ~ 50 seconds (i.e. before the MAGIC detection) can hold valuable information. We analyze the spectral evolution of the emission of GRB 190114C as detected by the Fermi Gamma-Ray Burst Monitor (GBM) in the 10 keV -- 40 MeV energy range up to ~60 sec. The first 4 s of the burst feature a typical prompt emission spectrum, which can be fit by a smoothly broken power-law function with typical parameters. Starting on ~4 s post-trigger, we find an additional nonthermal component, which can be fit by a power law. This component rises and decays quickly. The 10 keV -- 40 MeV flux of the power-law component peaks at ~ 6 s; it reaches a value of 1.7e-5 erg cm-2 s-1. The time of the peak coincides with the emission peak detected by the Large Area Telescope (LAT) on board Fermi. The power-law spectral slope that we find in the GBM data is remarkably similar to that of the LAT spectrum, and the GBM+LAT spectral energy distribution seems to be consistent with a single component. This suggests that the LAT emission and the power-law component that we find in the GBM data belong to the same emission component, which we interpret as due to the afterglow of the burst. The onset time allows us to estimate the initial jet bulk Lorentz factor Gamma_0 is about 500, depending on the assumed circum-burst density.
The ultra-long Gamma Ray Burst GRB 111209A at redshift z=0.677, is so far the longest GRB ever observed, with rest frame prompt emission duration of ~4 hours. In order to explain the bursts exceptional longevity, a low metallicity blue supergiant progenitor has been invoked. In this work, we further investigate this peculiar burst by performing a multi-band temporal and spectral analysis of both the prompt and the afterglow emission. We use proprietary and publicly available data from Swift, Konus Wind, XMM-Newton, TAROT as well as from other ground based optical and radio telescopes. We find some peculiar properties that are possibly connected to the exceptional nature of this burst, namely: i) an unprecedented large optical delay of 410+/-50 s is measured between the peak epochs of a marked flare observed also in gamma-rays after about 2 ks from the first Swift/BAT trigger; ii) if the optical and X-ray/gamma-ray photons during the prompt emission share a common origin, as suggested by their similar temporal behavior, a certain amount of dust in the circumburst environment should be introduced, with rest frame visual dust extinction of AV=0.3-1.5 mag; iii) at the end of the X-ray steep decay phase and before the start of the X-ray afterglow, we detect the presence of a hard spectral extra power law component never revealed so far. On the contrary, the optical afterglow since the end of the prompt emission shows more common properties, with a flux power law decay with index alpha=1.6+/-0.1 and a late re-brightening feature at 1.1 day. We discuss our findings in the context of several possible interpretations given so far to the complex multi-band GRB phenomenology. We also attempt to exploit our results to further constrain the progenitor nature properties of this exceptionally long GRB, suggesting a binary channel formation for the proposed blue supergiant progenitor.
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 ($f_{ u} propto t^{-1.96 pm 0.07}$) is also inconsistent with the $f_{ u} propto t^{-p}$ 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 $f_{ u} propto t^{-p}$. 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 $f_{ u} propto t^{-1}$) 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.
The Supercritical Pile is a very economical GRB model that provides for the efficient conversion of the energy stored in the protons of a Relativistic Blast Wave (RBW) into radiation and at the same time produces - in the prompt GRB phase, even in the absence of any particle acceleration - a spectral peak at energy $sim 1$ MeV. We extend this model to include the evolution of the RBW Lorentz factor $Gamma$ and thus follow its spectral and temporal features into the early GRB afterglow stage. One of the novel features of the present treatment is the inclusion of the feedback of the GRB produced radiation on the evolution of $Gamma$ with radius. This feedback and the presence of kinematic and dynamic thresholds in the model are sources of potentially very rich time evolution which we have began to explore. In particular, one can this way obtain afterglow light curves with steep decays followed by the more conventional flatter afterglow slopes, while at the same time preserving the desirable features of the model, i.e. the well defined relativistic electron source and radiative processes that produce the proper peak in the $ u F_{ u}$ spectra. In this note we present the results of a specific set of parameters of this model with emphasis on the multiwavelength prompt emission and transition to the early afterglow.
Using a detailed model of the internal shock phase, we discuss the origin of the prompt emission in gamma-ray bursts. We focus on the identification of the dominant radiative process (Fermi-GBM range) and propose an explanation for some features observed by Fermi-LAT at high energy in some GRB lightcurves.
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