arXiv subjects

O. S. Salafia

Publications and source records attributed to O. S. Salafia.

At least 19 recordsLinked to original sources

arXiv2026

Comparing the size evolution of GRB 221009A with afterglow models

We present very long baseline interferometry (VLBI) observations of GRB221009A obtained with the global-VLBI network at 5 GHz between 231 and 750 days after the burst. The quality of our observations allowed us to measure the sub-milliarcsecond full-width at half maximum of the source surface brightness distribution with a one-sigma equivalent error of 13% and 30% at 231 and 391 days post-burst, respectively. By fitting the temporal evolution of these new size measurements alongside those previously published in the literature at 5 and 8 GHz with a single power law $\propto t^a$, we find a slope of $a = 0.52^{+0.14}_{-0.12}$. Using the public code VegasAfterglow, we model the 5 GHz size evolution considering the afterglow emission of the forward shock produced by a jet decelerating in either a homogeneous interstellar medium (ISM) or into a surrounding medium with a wind-like density profile. The model constrains the collimation-corrected Sedov ratio, that is, the ratio of the jet kinetic energy $E_\mathrm{jet}$ to the density of the surrounding medium ($n$ in the ISM scenario, or $A_\star$ in the wind scenario), in the range $\log[(E_\mathrm{jet}/n)\,\mathrm{erg}^{-1}\,\mathrm{cm}^{-3}] = 54.2^{+0.4}_{-0.5}$ for the ISM scenario and $\log[(E_\mathrm{jet}/A_\star)\,\mathrm{erg}^{-1}] = 51.7^{+0.4}_{-0.2}$ for the wind scenario. Among the works in the literature that fit the light curves and spectra of GRB221009A based on the standard afterglow model, and which can be reproduced with VegasAfterglow, none has best-fitting parameters that are compatible with these constraints within $2σ$. Moreover, we show that their predicted size evolution is not compatible with the observed one. Our results highlight the importance of size measurements of GRB afterglows as an independent probe of their dynamics, structure, and emission mechanisms.

astro-ph.HE↗

arXiv2026

ENGRAVE follow-up of a type IIb supernova spatially coincident with the sub-threshold gravitational wave trigger S250818k

The candidate gravitational wave (GW) event S250818k was one of only three non-retracted LIGO-Virgo-KAGRA public alerts issued during the fourth observing run of the network (O4) with a binary neutron star (BNS) merger classification probability exceeding one percent. This triggered a prompt search for a potential electromagnetic (EM) counterpart in the large localisation error region (949 deg$^2$ projected in the sky at 90% credible level). The transient SN2025ulz, discovered by the Zwicky Transient Facility (ZTF) during the search, attracted a great deal of attention due to a potential spatial and temporal coincidence, and due to its initial fast decay and featureless spectrum. Here, we report on the follow up of this transient by the Electromagnetic counterparts of gravitational wave sources at the Very Large Telescope (ENGRAVE) Collaboration. We conducted an extensive multi-wavelength observational campaign, which led to the spectral classification of the transient as a type IIb supernova (SN), indicating that it is unrelated to the candidate GW event. In this article, we describe our observing strategies, data reduction, and interpretation. All of our results confirm and strengthen our classification of the source, and also show that shock cooling tails associated with type IIb SNe are one of the most prominent contaminants in kilonova searches.

astro-ph.HE↗

arXiv2026

Colour evolution in the radio afterglow of GRB 241025A

We present the observing campaign of the afterglow of GRB241025A, a gamma-ray burst (GRB) whose prompt emission has been simultaneously detected by Swift, Einstein Probe, Fermi/GBM, SVOM, Konus-Wind and VZLUSAT-2 3U CubeSat. Our multi-wavelength campaign comprises radio, near-infrared, Optical and X-ray observations. The afterglow was clearly detected in all bands. We performed a semi-empirical fit of the data, showing that the afterglow behaviour can be reasonably reproduced by a single component, i.e. an ultra-relativistic shock. However, the results from the semi-empirical fit are inconsistent with the predicted evolution from the standard afterglow model in the slow cooling regime. Specifically, we found that at early times the synchrotron self-absorption frequency $ν_a$ should be at higher frequencies with respect to the ones sampled by our campaign, in order to explain the observed colour evolution in radio, namely the spectral evolution in time. To reconcile the prediction from the standard model with the observed data set, we fit the observations with a semi-analytical model, including a multiplicative factor $τ_{enh}$ to the optical depth which, in turn, artificially increases $ν_a$. We found that the radio colour evolution, together with the near-infrared, optical and X-ray emission, can be described reasonably well by a forward shock from a structured jet, provided that the optical depth in the shocked material is enhanced by a factor $τ_{enh}=500$. We suggest that such enhancement in the optical depth can result from a population of cold electrons in the downstream material, i.e. electrons that were not accelerated by Fermi I process at the shock front, in agreement with the theoretical expectations previously reported in the literature. Overall, our work underscores the importance of systematic, multi-frequency, multi-epoch radio follow-ups of these extreme events.

astro-ph.HE↗

arXiv2026

A self-consistent explanation of the MeV line in GRB 221009A unveils a dense circum-stellar medium

GRB~221009A has been the brightest gamma-ray burst (GRB) observed to date, and its afterglow has been characterized with unprecedented detail at TeV energies by LHAASO. Quite puzzlingly, it is also the most energetic GRB known. Among the riddles posed by this mysterious source, however, the sheer energetics are hardly the most intriguing: an unprecedented emission line at around 10 MeV has been uncovered by a detailed spectral analysis of Fermi/GBM data immediately following the brightest peak in the GRB prompt emission and the peak of the TeV afterglow. The temporal evolution of the line properties can be explained as high-latitude emission from a geometrically thin, relativistically expanding shell where annihilation of a large number of electron-positron pairs took place. We show that this interpretation yields stringent constraints on the properties of such shell, that point to a process that happens at radii typical of external shocks. We then demonstrate that the shell could have been the blastwave associated with the GRB precursor, with the line arising after pair loading of such blastwave as it was illuminated by the bright and hard radiation of the GRB. The scenario, which also explains the abrupt initial rise of the LHAASO afterglow, requires the progenitor of the GRB to have been surrounded by a circum-stellar medium (CSM) extending out to a few 10^15 cm, with a density 10^8-10^9 cm-3 reminiscent of those found in Type IIn supernovae. The consequences of such a CSM on the dynamics and emission of the external shock are yet to be fully explored. If future, more detailed work will confirm the compatibility of the GRB 221009A afterglow with our scenario, this will provide a precious clue to the nature of the progenitor of this peculiar GRB, which could also be present in other bursts that feature a long quiescence followed by a bright emission episode with a hard spectrum.

astro-ph.HE↗

arXiv2025

A new long gamma-ray burst formation pathway at solar metallicity

Context. Long gamma-ray bursts (LGRBs) are generally observed in low-metallicity environments. However, 10 to 20 per cent of LGRBs at redshift $z<2$ are associated with near-solar to super-solar metallicity environments, remaining unexplained by traditional LGRB formation pathways that favour low metallicity progenitors. Aims. In this work, we propose a novel formation channel for LGRBs that is dominant at high metallicities. We explore how a stripped primary star in a binary can be spun up by a second, stable reverse-mass-transfer phase, initiated by the companion star. Methods. We use POSYDON, a state-of-the-art population synthesis code that incorporates detailed single- and binary-star mode grids, to investigate the metallicity dependence of the stable reverse-mass-transfer LGRB formation channel. We determine the available energy to power an LGRB from the rotational profile and internal structure of a collapsing star, and investigate how the predicted rate density of the proposed channel changes with different star formation histories and criteria for defining a successful LGRB. Results. Stable reverse mass transfer can produce rapidly rotating, stripped stars at collapse. These stars retain enough angular momentum to account for approximately 10-20% of the observed local LGRB rate density, under a reasonable assumption for the definition of a successful LGRB. However, the local rate density of LGRBs from stable reverse mass transfer can vary significantly, between 1 and 100 Gpc$^{-3}$ yr$^{-1}$, due to strong dependencies on cosmic star formation rate and metallicity evolution, as well as the assumed criteria for successful LGRBs.

astro-ph.HE↗

arXiv2025

The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151

Context: The extreme luminosity of gamma-ray bursts (GRBs) makes them powerful beacons for studies of the distant Universe. The most luminous bursts are typically detected at moderate/high redshift, where the volume for seeing such rare events is maximized and the star-formation activity is greater than at z = 0. For distant events, not all observations are feasible, such as at TeV energies. Aims: Here we present a spectroscopic redshift measurement for the exceptional GRB 221009A, the brightest GRB observed to date with emission extending well into the TeV regime. Methods: We used the X-shooter spectrograph at the ESO Very Large Telescope (VLT) to obtain simultaneous optical to near-IR spectroscopy of the burst afterglow 0.5 days after the explosion. Results: The spectra exhibit both absorption and emission lines from material in a host galaxy at z = 0.151. Thus GRB 221009A was a relatively nearby burst with a luminosity distance of 745 Mpc. Its host galaxy properties (star-formation rate and metallicity) are consistent with those of LGRB hosts at low redshift. This redshift measurement yields information on the energy of the burst. The inferred isotropic energy release, $E_{\rm iso} > 5 \times 10^{54}$ erg, lies at the high end of the distribution, making GRB 221009A one of the nearest and also most energetic GRBs observed to date. We estimate that such a combination (nearby as well as intrinsically bright) occurs between once every few decades to once per millennium.

astro-ph.HE↗

arXiv2024

The expansion of the GRB 221009A afterglow

We observed $γ$-ray burst (GRB) 221009A using very long baseline interferomety (VLBI) with the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA), over a period spanning from 40 to 262 days after the initial GRB. The high angular resolution (mas) of our observations allowed us, for the second time ever, after GRB 030329, to measure the projected size, $s$, of the relativistic shock caused by the expansion of the GRB ejecta into the surrounding medium. Our observations support the expansion of the shock with a $>4σ$-equivalent significance, and confirm its relativistic nature by revealing an apparently superluminal expansion rate. Fitting a power law expansion model, $s\propto t^a$, to the observed size evolution, we find a slope $a=0.69^{+0.13}_{-0.14}$. Fitting the data at each frequency separately, we find different expansion rates, pointing to a frequency-dependent behaviour. We show that the observed size evolution can be reconciled with a reverse shock plus forward shock, provided that the two shocks dominate the emission at different frequencies and, possibly, at different times.

astro-ph.HE↗

arXiv2023

JWST detection of heavy neutron capture elements in a compact object merger

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs), sources of high-frequency gravitational waves and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process). These heavy elements include some of great geophysical, biological and cultural importance, such as thorium, iodine and gold. Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW170817. We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe.

astro-ph.HE↗

arXiv2023

The short gamma-ray burst population in a quasi-universal jet scenario

We describe a model of the short gamma-ray burst (SGRB) population under a `quasi-universal jet' scenario in which jets can differ in their on-axis peak prompt emission luminosity $L_c$, but share a universal angular luminosity profile $\ell(θ_v)=L(θ_v)/L_c$ as a function of the viewing angle $θ_v$. The model is fitted, through a Bayesian hierarchical approach inspired by gravitational wave (GW) population analyses, to 3 observed SGRB samples simultaneously: the Fermi/GBM sample of SGRBs with spectral information in the catalogue (367 events); a flux-complete sample of 16 Swift/BAT SGRBs also detected by GBM, with a measured redshift; and a sample of SGRBs with a binary neutron star (BNS) merger counterpart, which only includes GRB~170817A at present. The results favour a narrow jet core with half-opening angle $θ_c=2.1_{-1.4}^{+2.4}$ deg (90\% credible intervals from our fiducial `full sample' analysis) whose on-axis peak luminosity is distributed as $p(L_c) \propto L_c^{-A}$ with $A=3.2_{-0.4}^{+0.7}$ above a minimum luminosity $L_c^\star = 5_{-2}^{+11}\times 10^{51}$ erg s$^{-1}$. For $θ_v>θ_c$, the luminosity scales as a power law $\ell\propto θ_v^{-α_L}$ with $α_L=4.7_{-1.4}^{+1.2}$, with no evidence for a break. While the model implies an intrinsic `Yonetoku' correlation between $L$ and the peak photon energy $E_p$, its slope is somewhat shallower $E_p\propto L^{0.4\pm 0.2}$ than the apparent one, and the normalization is offset towards larger $E_p$, due to selection effects. The implied local rate density of SGRBs is between about 100 up to several thousands of events per Gpc$^{3}$ yr, in line with the BNS merger rate density inferred from GW observations. Based on the model, we predict 0.2 to 1.3 joint GW+SGRB detections per year by the Advanced GW detector network and Fermi/GBM during the O4 observing run.

astro-ph.HE↗

arXiv2023

Panning for gold, but finding helium: discovery of the ultra-stripped supernova SN2019wxt from gravitational-wave follow-up observations

We present the results from multi-wavelength observations of a transient discovered during the follow-up of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a possible binary neutron star merger in a low latency search. This search yielded SN2019wxt, a young transient in a galaxy whose sky position (in the 80\% GW contour) and distance ($\sim$150\,Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transient's tightly constrained age, its relatively faint peak magnitude ($M_i \sim -16.7$\,mag) and the $r-$band decline rate of $\sim 1$\,mag per 5\,days appeared suggestive of a compact binary merger. However, SN2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of $\sim 0.1\,M_\odot$, with $^{56}$Ni comprising $\sim 20\%$ of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitors that could give rise to the observed properties of SN2019wxt, and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling electromagnetic counterparts to GW events from transients such as SN2019wxt is challenging: in a bid to characterise the level of contamination, we estimated the rate of events with properties comparable to those of SN2019wxt and found that $\sim 1$ such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500\,Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns.

astro-ph.HE↗

arXiv2022

Multi-messenger observations of binary neutron star mergers in the O4 run

We present realistic expectations for the number and properties of neutron star binary mergers to be detected as multi-messenger sources during the upcoming fourth observing run (O4) of the LIGO-Virgo-KAGRA gravitational wave (GW) detectors, with the aim of providing guidance for the optimization of observing strategies. Our predictions are based on a population synthesis model which includes the GW signal-to-noise ratio, the kilonova (KN) optical and near-infrared light curves, the relativistic jet gamma-ray burst (GRB) prompt emission peak photon flux, and the afterglow light curves in radio, optical and X-rays. Within our assumptions, the rate of GW events to be confidently detected during O4 is $7.7^{+11.9}_{-5.7}$ yr$^{-1}$ (calendar year), 78% of which will produce a KN, and a lower 52% will also produce a relativistic jet. The typical depth of current optical electromagnetic search and follow up strategies is still sufficient to detect most of the KNe in O4, but only for the first night or two. The prospects for detecting relativistic jet emission are not promising. While closer events (within z<0.02) will likely still have a detectable cocoon shock breakout, most events will have their GRB emission (both prompt and afterglow) missed unless seen under a favorably small viewing angle. This reduces the fraction of events with detectable jets to 2% (prompt emission, serendipitous) and 10% (afterglow, deep radio monitoring), corresponding to detection rates of $0.17^{+0.26}_{-0.13}$ and $0.78^{+1.21}_{-0.58}$ yr$^{-1}$, respectively. When considering a GW sub-threshold search triggered by a GRB detection, our predicted rate of GW+GRB prompt emission detections increases up to a more promising $0.75^{+1.16}_{-0.55}$ yr$^{-1}$.

astro-ph.HE↗

arXiv2022

Constraints on the merging binary neutron star mass distribution and equation of state based on the incidence of jets in the population

A relativistic jet has been produced in the single well-localised binary neutron star (BNS) merger detected to date in gravitational waves (GWs), and the local rates of BNS mergers and short gamma-ray bursts are of the same order of magnitude. This suggests that jet formation is not a rare outcome for BNS mergers, and we show that this intuition can be turned into a quantitative constraint: at least about $1/3$ of GW-detected BNS mergers, and at least about $1/5$ of all BNS mergers, should produce a successful jet (90\% credible level). Whether a jet is launched depends on the properties of the merger remnant and of the surrounding accretion disc, which in turn are a function of the progenitor binary masses and equation of state (EoS). The incidence of jets in the population therefore carries information about the binary component mass distribution and EoS. Under the assumption that a jet can only be produced by a black hole remnant surrounded by a non-negligible accretion disc, we show how the jet incidence can be used to place a joint constraint on the space of BNS component mass distributions and EoS. The result points to a broad mass distribution, with particularly strong support for masses in the $1.3-1.6\,\mathrm{M_\odot}$ range. The constraints on the EoS are shallow, but we show how they will tighten as the knowledge on the jet incidence improves. We also discuss how to extend the method to include future BNS mergers, with possibly uncertain jet associations.

astro-ph.HE↗

arXiv2022

Multi-wavelength view of the close-by GRB 190829A sheds light on gamma-ray burst physics

Gamma-ray bursts are produced as a result of cataclysmic events such as the collapse of a massive star or the merger of two neutron stars. We monitored the position of the close-by gamma-ray burst GRB~190829A, which originated from a massive star collapse, through very long baseline interferometry (VLBI) observations with the EVN and the VLBA, involving a total of 30 telescopes across 4 continents. We carried out a total of 9 observations between 9 and 117 days after the gamma-ray burst at 5 and 15 GHz, with a typical resolution of few milliarcseconds (mas). We obtained limits on the source size and expansion rate. The limits are in agreement with the size evolution entailed by a detailed modelling of the multi-wavelength light curves with a forward plus reverse shock model, which agrees with the observations across almost 18 orders of magnitude in frequency (including the High Energy Stereoscopic System data at TeV photon energies) and more than 4 orders of magnitude in time. Thanks to the broad, high-cadence coverage of the afterglow, afterglow degeneracies are broken to a large extent, allowing us to capture some unique physical insights: we find a low prompt emission efficiency $\lesssim 10^{-3}$; we constrain the fraction of electrons that are accelerated to relativistic speeds in the forward shock to be $χ_e<13\%$ at the 90\% credible level; we find that the magnetic field energy density in the reverse shock downstream must decay rapidly after the shock crossing. While our model assumes an on-axis jet, our VLBI astrometric measurements alone are not sufficiently tight as to exclude any off-axis viewing angle. On the other hand, we can firmly exclude the line of sight to have been more than $2\,\mathrm{deg}$ away from the border of the region that produced the prompt gamma-ray emission based on compactness arguments.

astro-ph.HE↗

arXiv2020

Distinguishing the nature of ''ambiguous'' merging systems hosting a neutron star: GW190425 in low-latency

GW190425 is the newly discovered gravitational wave (GW) source consistent with a neutron star-neutron star merger with chirp mass of $1.44\pm0.02M_\odot.$ This value falls in the $ambiguous$ interval as from the GW signal alone we can not rule out the presence of a black hole in the binary. In this case, the system would host a neutron star and a very light stellar black hole, with mass close to the maximum value for neutron stars, filling the $mass$ $gap$. No electromagnetic counterpart is firmly associated with this event, due to the poorly informative sky localisation and larger distance, compared to GW/GRB170817. We construct here kilonova light curve models for GW190425, for both double neutron star and black hole-neutron star systems, considering two equations of state consistent with current constraints from the signals of GW170817/GW190425 and the NICER results, including black hole spin effects and assuming a new formula for the mass of the ejecta. The putative presence of a light black hole in GW190425 would have produced a brighter kilonova emission compared to the double neutron star case, letting us to distinguish the nature of the companion to the neutron star. Concerning candidate counterparts of GW190425, classified later on as supernovae, our models could have discarded two transients detected in their early $r$-band evolution. Combining the chirp mass and luminosity distance information from the GW signal with a library of kilonova light curves helps identifying the electromagnetic counterpart early on. We remark that the release in low latency of the chirp mass in this interval of $ambiguous$ values appears to be vital for successful electromagnetic follow-ups.

astro-ph.HE↗

arXiv2020

Proton-synchrotron as the radiation mechanism of the prompt emission of GRBs?

We discuss the new surprising observational results that indicate quite convincingly that the prompt emission of Gamma-Ray Bursts (GRBs) is due to synchrotron radiation produced by a particle distribution that has a low energy cut-off. The evidence of this is provided by the low energy part of the spectrum of the prompt emission, that shows the characteristic F(nu) \propto nu^(1/3) shape followed by F(nu) \propto nu^(-1/2) up to the peak frequency. This implies that although the emitting particles are in fast cooling, they do not cool completely. This poses a severe challenge to the basic ideas about how and where the emission is produced, because the incomplete cooling requires a small value of the magnetic field, to limit synchrotron cooling, and a large emitting region, to limit the self-Compton cooling, even considering Klein-Nishina scattering effects. Some new and fundamental ingredient is required for understanding the GRBs prompt emission. We propose proton-synchrotron as a promising mechanism to solve the incomplete cooling puzzle.

astro-ph.HE↗

arXiv2020

Gamma-ray burst jet propagation, development of angular structure, and the luminosity function

The fate and observable properties of gamma-ray burst jets depend crucially on their interaction with the progenitor material that surrounds the central engine. We present a semi-analytical model of such interaction, which builds upon several previous analytical and numerical works, aimed at predicting the angular distribution of jet and cocoon energy and Lorentz factor after breakout, given the properties of the ambient material and of the jet at launch. Using this model, we construct synthetic populations of structured jets, assuming either a collapsar (for long gamma-ray bursts -- LGRBs) or a binary neutron star merger (for short gamma-ray bursts -- SGRBs) as progenitor. We assume all progenitors to be identical, and we allow little variability in the jet properties at launch: our populations therefore feature a quasi-universal structure. These populations are able to reproduce the main features of the observed LGRB and SGRB luminosity functions, although several uncertainties and caveats remain to be addressed.

astro-ph.HE↗

arXiv2019

Electromagnetic counterparts of black hole-neutron star mergers: dependence on the neutron star properties

Detections of gravitational waves (GWs) may soon uncover the signal from the coalescence of a black hole - neutron star (BHNS) binary, that is expected to be accompanied by an electromagnetic (EM) signal. In this paper, we present a composite semi-analytical model to predict the properties of the expected EM counterpart from BHNS mergers, focusing on the kilonova emission and on the gamma-ray burst afterglow. Four main parameters rule the properties of the EM emission: the NS mass $M_\mathrm{NS}$, its tidal deformability $Λ_\mathrm{NS}$, the BH mass and spin. Only for certain combinations of these parameters an EM counterpart is produced. Here we explore the parameter space, and construct light curves, analysing the dependence of the EM emission on the NS mass and tidal deformability. Exploring the NS parameter space limiting to $M_\mathrm{NS}-Λ_\mathrm{NS}$ pairs described by a physically motivated equations of state (EoS), we find that the brightest EM counterparts are produced in binaries with low mass NSs (fixing the BH properties and the EoS). Using constraints on the NS EoS from GW170817, our modeling shows that the emission falls in a narrow range of absolute magnitudes. Within the range of explored parameters, light curves and peak times are not dissimilar to those from NSNS mergers, except in the B band. The lack of an hyper/supra-massive NS in BHNS coalescences causes a dimming of the blue kilonova emission in absence of the neutrino interaction with the ejecta.

astro-ph.HE↗

arXiv2019

The rise and fall of the high-energy afterglow emission of GRB 180720B

The Gamma Ray Burst (GRB) 180720B is one of the brightest events detected by the Fermi satellite and the first GRB detected by the H.E.S.S. telescope above 100 GeV. We analyse the Fermi (GBM and LAT) and Swift (XRT and BAT) data and describe the evolution of the burst spectral energy distribution in the 0.5 keV - 10 GeV energy range over the first 500 seconds of emission. We reveal a smooth transition from the prompt phase, dominated by synchrotron emission in a moderately fast cooling regime, to the afterglow phase whose emission has been observed from the radio to the GeV energy range. The LAT (0.1 - 100 GeV) light curve initially rises ($F_{\rm LAT}\propto t^{2.4}$), peaks at $\sim$78 s, and falls steeply ($F_{\rm LAT}\propto t^{-2.2}$) afterwards. The peak, which we interpret as the onset of the fireball deceleration, allows us to estimate the bulk Lorentz factor $Γ_{0}\sim 150 \ (300)$ under the assumption of a wind-like (homogeneous) circum-burst medium density. We derive a flux upper limit in the LAT energy range at the time of H.E.S.S. detection, but this does not allow us to unveil the nature of the high energy component observed by H.E.S.S. We fit the prompt spectrum with a physical model of synchrotron emission from a non-thermal population of electrons. The 0 - 35 s spectrum after its $E F(E)$ peak (at 1 - 2 MeV) is a steep power law extending to hundreds of MeV. We derive a steep slope of the injected electron energy distribution $N(γ)\propto γ^{-5}$. Our fit parameters point towards a very low magnetic field ($B'\sim 1 $ G) in the emission region.

astro-ph.HE↗