arXiv subjects

Annarita Ierardi

Publications and source records attributed to Annarita Ierardi.

7 recordsLinked to original sources

arXiv2026

X-ray Flares in Gamma-Ray Bursts at High and Very-High-Energies

A significant fraction of GRB early afterglows exhibit fast and bright X-ray flares, discovered by Swift X-ray Telescope. Their rapid temporal variability and spectral evolution have suggested an internal shock origin, analogous to prompt MeV emission. However, their physical origin and radiation mechanism remain debated. High-energy (> 100 MeV) and very-high-energy (> 30 GeV) gamma-ray observations provide a powerful probe of flare dissipation region, constraining its size, magnetic field strength, and particle acceleration under the synchrotron self-Compton scenario in optically thin relativistic jets. We present a systematic multi-wavelength study of 66 X-ray flares from 47 GRBs observed by X-ray Telescope over 17 years, all within field of view of Fermi Large Area Telescope. We investigate their GeV counterparts and find that only five flares exhibit significant high-energy emission (> 3 sigma). Broadband spectral modeling indicates that this GeV emission is consistent with standard forward shock afterglow. We further investigate correlations between flare spectral properties and energy fluxes at 1 keV, 10 keV, and 1 GeV. Using a synchrotron self-Compton model, we constrain the physical conditions of the emitting region, including magnetic field strength, bulk Lorentz factor, and emission radius. For the most stringent GeV upper limits, we find a magnetic-to-electron luminosity ratio greater than or equal to 1, implying a highly magnetized emitting region. We predict very-high-energy gamma-ray emission from X-ray flares at early (~500s) and late (~5000s) times and assess their detectability with Cherenkov Telescopes. We find that later X-ray flares provide the most promising targets for follow-up observations owing to improved observational accessibility, sensitivity, and reduced response-time constraints of facilities such as Cherenkov Telescope Array Observatory.

astro-ph.HE↗

arXiv2026

The ultra-fast afterglow of GRB 260226A

Long-duration gamma-ray bursts are typically powered by relativistic jets launched after the core collapse of some rapidly rotating massive stars. Internal dissipation releases part of the jet energy as highly variable MeV prompt emission, while the remaining kinetic energy drives an external shock into the surrounding medium and produces the so-called afterglow. During the first minutes of the afterglow, the unsteady jet transfers energy to the external shock. The early afterglow emission in MeV-GeV energies is rarely observed because the emergence of afterglow can be overshined by the prompt emission. Here we report exceptional observations of GRB 260226A with the Fermi Large Area Telescope, which recorded the largest number of photons above 100 MeV from a gamma-ray burst. These data allow us to reconstruct the evolution of the bolometric flux of the afterglow from its emergence during the prompt emission phase with unprecedented detail. The afterglow component peaks near 50 MeV and fades rapidly, first as t$^{-1.5}$ and then transiting to an ultra-fast t$^{-2.8}$ decay after about one minute. This behavior cannot be explained by standard synchrotron emission from a blast wave propagating into a cold medium. We interpret it as external inverse Compton radiation from freshly heated electrons cooling on prompt photons in a dense, pair-loaded stellar wind. GRB 260226A therefore shows that MeV-GeV observations can directly reveal the formation of the external shock and the massive-star environment is significantly reshaped by the prompt emission.

astro-ph.HE↗

arXiv2026

MeV absorption in gamma-ray bursts as a probe of their progenitor environments

A small fraction of X-ray photons from $γ$-ray bursts (GRBs), after escaping the relativistic jet, are scattered by electrons in the circumburst medium. Subsequent photon-photon absorption between the incoming MeV $γ$-rays and the back-scattered X-rays generate electron-positron pairs, enriching the surrounding medium with leptons. We investigate how these back-scattered photons modify the prompt GRB spectrum through $γ-γ$ absorption. In a dense and pair-loaded environment, the emerging spectra exhibit a broad absorption feature, whose profile is sensitive to the low-energy spectral index $α$. In particular, spectra with $α> -1$ develop a pronounced, saddle-shaped absorption between 1 and 100 MeV (rest frame). Such external MeV absorption could account for the spectral curvature seen in some bright GRBs, and may point to a dense circum-stellar medium (CSM) around their progenitor stars - consistent with early observations of core-collapse supernovae. In this scenario, the blastwave caused by the GRB is expected to start off with a relatively low Lorentz factor, and undergo an acceleration phase when traversing the large density drop at the interface between the dense CSM and the surrounding medium. The impact of these non-trivial dynamics on the afterglow emission is yet to be explored.

astro-ph.HE↗

arXiv2026

Thermal Electrons in an Ultra-Relativistic Shock Shape the Optical Afterglow of GRB 250702F

Observing early optical emission from gamma-ray bursts (GRBs) contemporaneous with the MeV prompt emission phase remains rare, requiring rapid-response robotic facilities. The Ondřejov D50 telescope detected the optical counterpart of GRB 250702F at z = 1.520 only 27.8 s after trigger, enabling high-cadence monitoring during the brightest prompt emission pulses. The optical light curve reveals two distinct flares. The first (30 - 100 s) is spectrally consistent with the MeV prompt emission. The second flare (100 - 1400 s) exhibits an unusual morphology (F_nu ~ t^-alpha): a rapid rise to a plateau, followed by a steep decay (alpha ~ 1.6) before transitioning to a standard power-law afterglow (alpha = 0.79). This steep decay phase cannot be explained by nonthermal electrons accelerated at the forward shock, and reverse-shock scenario is disfavored due to the long duration of the flare and the temporal offset from the underlying deceleration time. We interpret the steep decay as the synchrotron frequency of a thermal (Maxwellian) electron population sweeping through the optical band. Modeling yields a non-thermal energy fraction delta ~ 0.8 with the remaining energy heating electrons at characteristic Lorentz factor gamma_th ~ 900. These observations provide evidence for thermal electron signatures in GRB afterglows, consistent with predictions from particle-in-cell simulations of ultra-relativistic collisionless shocks.

astro-ph.HE↗

arXiv2025

Early X-ray emission of short Gamma-Ray Bursts: insights into physics and multi-messenger prospects

Early X-ray emission of Gamma-Ray Bursts (GRBs) traces the transition between the prompt emission and the afterglow radiation, and its rapid flux decline is often interpreted as the tail of the prompt emission. As such, it can offer insights into the emission mechanisms active during the prompt emission and the physics of GRB jets. In this work, we focus on merger-driven GRBs, which are sources of gravitational waves (GWs) detectable by ground-based interferometers, such as LIGO, Virgo, and KAGRA. We present a systematic analysis of the early X-ray emission ($t < 10^3 \ \mathrm{s}$) of a sample of 16 merger-driven GRB candidates detected by the Neil Gehrels Swift Observatory (hereafter, Swift). We performed a time-resolved spectral analysis of soft and hard X-ray data (0.3-150 keV) by fitting two curved spectral models to the spectra: a physical synchrotron model and an empirical smoothly broken power law model. We characterized the evolution of the peak energy and bolometric flux, and derived the intrinsic properties of the 10 bursts with measured redshift. We discovered a tight correlation between the rest-frame peak energy of the spectra and the isotropic-equivalent luminosity. Specifically, we obtained $ν_{c,z} \propto L_{\rm iso}^{(0.64 \pm 0.03)}$ when adopting the synchrotron model, and $E_{p,z} \propto L_{\rm iso}^{(0.58 \pm 0.04)}$ when adopting the smoothly broken power law. Both relations were extrapolated to the typical prompt emission energies and well describe the properties of short GRBs detected in the MeV gamma-rays. These results suggest a common origin for the prompt and steep-decay emissions in merger-driven GRBs, and rule out high-latitude emission as the dominant process shaping the early X-ray tails. Finally, we assessed the detectability of these sources with the Wide-field X-ray Telescope onboard the Einstein Probe mission.

astro-ph.HE↗

arXiv2025

Ultra-long MeV transient from a relativistic jet: a tidal disruption event candidate

On July 2, 2025, the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray space telescope detected three short-duration MeV transients with overlapping sky locations. These events, named as GRB 250702D, B, and E (collectively referred to as DBE), triggered the detector with delays of approximately 1-2 hours between each burst. Follow-up observations of this unusually long MeV transient (lasting >3 hours) by the Neil Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array over a period of 10 days revealed a steep temporal decline in soft X-rays ($\propto t^{-1.9 \pm 0.1}$). The time-averaged spectra during the outbursts are well described by a single power law $dN_γ/dE \propto E^{-1.5}$, while upper limits above 100 MeV imply a spectral cutoff between 10 MeV and 100 MeV. Using standard gamma-ray transparency arguments, we derive a lower limit on the bulk Lorentz factor. Combined with the steep decline in X-rays, these constraints point to a relativistic jet origin. The properties of DBE are inconsistent with established GRB spectral-energy correlations, disfavoring classical long GRB progenitors. Instead, the basic characteristics of DBE resemble those of previously reported jetted tidal disruption events (TDEs), though alternative progenitor channels cannot be excluded. In the relativistic TDE scenario, DBE is the first one with detected MeV gamma-ray emission. We argue that the observed emission is most likely produced by synchrotron radiation from sub-TeV electrons.

astro-ph.HE↗

arXiv2025

Broadband Modelling of GRB 230812B Afterglow: Implications for VHE $γ$-ray Detection with IACTs

A significant fraction of the energy from the $γ$-ray burst (GRB) jets, after powering the keV-MeV emission, forms an ultra-relativistic shock propagating into the circumburst medium. The particles in the medium accelerate through the shock and produce afterglow emission. Recently, a few GRB afterglows have been observed in TeV $γ$-rays by Cherenkov Telescopes. This provides access to broadband spectra of GRB afterglows containing rich information about the microphysics of relativistic shocks and the profile of the circumburst medium. Since the transition from synchrotron to inverse Compton regime in afterglow spectra occurs between hard X-rays and the very-high-energy (VHE) $γ$-rays, detection in one of these bands is required to identify the two spectral components. The early afterglow data in the hard X-rays, along with the GeV emission, could accurately constrain the spectral shape and help in capturing the spectral turnover to distinguish the two components. We present the multiwavelength spectral and temporal study, focused on the keV-VHE domain, of GRB 230812B, one of the brightest GRBs detected by Fermi Gamma Ray Burst Monitor (GBM), along with the detection of a 72 GeV photon in Large Area Telescope (LAT) during the early afterglow phase. Through a detailed modelling of the emission within the afterglow external forward shock in a wind-like scenario, we predict optical to high-energy observations up to ~1 day. We emphasize the importance of following up poorly localised GRBs by demonstrating that even without prompt sub-degree localisation, such as in GRB 230812B, it is possible to recover the emission using imaging atmospheric Cherenkov telescopes, thanks to their relatively wider field of view. Moreover, we show that the low energy threshold of Large-Sized Telescope is essential in discovering the VHE component at much higher redshifts, typical of long GRBs.

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