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

TL;DR

This study systematically analyzes the early X-ray emission ($t<10^3$ s) of 16 merger-driven GRB candidates with Swift-XRT/BAT, fitting both a physical synchrotron model and an empirical smoothly broken power law to time-resolved, jointly analyzed spectra. It uncovers tight rest-frame peak-energy–luminosity correlations, $\nu_{c,z}$–$L_{iso}$ with $m=0.64\pm0.03$ and $E_{p,z}$–$L_{iso}$ with $m=0.58\pm0.04$, robust to model choice and extendable to prompt-energy scales observed by GBM. The results imply a common physical origin for prompt, extended emission, and steep-decay X-ray tails in merger-driven GRBs and argue against high-latitude emission as the dominant mechanism, instead pointing to processes like adiabatic cooling in the jet. The paper also assesses the detectability of these X-ray transients with wide-field monitors like EP-WXT, deriving practical implications for GW-triggered searches and highlighting the potential of rapid X-ray follow-up to improve localization and multi-messenger science.

Abstract

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.

Figures (13)

• Figure 1: Light curves of short GRBs detected by Swift-XRT in the 0.3-10 keV energy range. The colored lines represent the GRBs selected in our sample, while the grey lines all the other short GRBs.
• Figure 2: X-ray light curve (a) and time-resolved spectral evolution (b) of GRB 211211A.
• Figure 3: Left column: evolution of the flux in 0.3-150 keV energy band (top), evolution of $\nu_c$ (middle), and $\nu_c$ as a function of the bolometric flux (bottom), assuming a synchrotron spectral model. Right column: evolution of the flux in 0.3-150 keV energy band (top), evolution of $E_p$ (middle), and $E_p$ as a function of the bolometric flux (bottom), assuming a sBPL spectral model. Each color refers to a different GRB. Error bars represent 1 sigma uncertainties, while arrows represent 68% upper limits.
• Figure 4: $\nu_{c,z}$ - $L_{iso}$ relation fitted with the X-ray data of the bursts in our sample, represented by colored circles. The straight blue line represents the best-fit line from the linear fit, while the blue-shaded area shows the 3$\sigma_{sc}$ scatter region of the relation. The relation has been extrapolated to higher energies, where GBM short GRB data are represented with green squares. GRB 211211A data are taken from 2025AA...693A.156M and are represented with yellow stars.
• Figure 5: $E_{p,z}$ - $L_{iso}$ relation fitted with the X-ray data of the bursts in our sample, represented by colored circles. We assumed a sBPL spectral model. The straight green line represents the best-fit line from the linear fit, while the green-shaded area shows the 3$\sigma_{sc}$ scatter region of the relation. The relation has been extrapolated to higher energies, where GBM short GRB data are represented with green squares.