Minutes-long soft X-ray prompt emission from a compact object merger

Abstract

Compact object mergers are multi-messenger sources and progenitors of some gamma-ray bursts (GRBs), primarily understood by gamma-ray observations, while poorly constrained in the prompt low-energy phase. A long-lasting X-ray emission was discussed as afterglows following several short-duration ($\lesssim$2 s) bursts, yet this prompt X-ray component was not directly observed or confirmed. Here we report the discovery of a minutes-long ($\sim$560 s) flash of soft X-rays immediately following the short ($\sim$0.4 s) GRB 250704B. The long-soft bump points to a distinct phase of prompt emission in X-rays detected by Einstein Probe in an event that otherwise appear as an ordinary short GRB, showing that long-lasting X-ray emission is likely a common feature of merger-driven bursts and a promising electromagnetic counterpart to gravitational-wave sources.

Figures (10)

• Figure 1: Detection images and light curves for EP250704a/GRB 250704B in different bands.(A) The image generated from the clean event data of EP-WXT, the GRB source is recognized with the red box. (B) The image generated from the clean event data of EP-FXT, the GRB source is recognized with the red box. (C) The image from VLT captured by FORS2, the optical afterglow of the burst is highlighted in the red circle. (D) The image from VLA captured in 10 GHz, the radio afterglow of the burst is highlighted in the red circle. (E) The light curves and spectral evolution of EP250704a/GRB 250704B. The blue and grey curves show the light curves of EP250704a in the energy range of 0.5--4.0 keV. The purple curve represents the light curve of GRB 250704B in the energy range of 15--5000 keV. The yellow and red points denote the low-energy spectral indices derived from the best-fit parameters of PL and CSBPL models, respectively. All error bars on data points indicate their 1 $\sigma$ confidence level.
• Figure 2: Temporal and statistical analyses of EP250704a.(A)$T_{90}$ calculation of the EP/WXT detection of EP250704a. (B)$T_{90}$ calculation of the initial spike of EP250704a detected by EP/WXT. In (A) and (B), the black curves represent the light curve of EP/WXT in the energy range of 0.5--4 keV and the accumulated counts. The purple dashed vertical lines denote the $T_{90}$ interval. The error bars mark the 1 $\sigma$ confidence level. (C) Minimum variability timescale distribution of EP-detected FXTs. The red dashed vertical line denotes the location of EP250704a on the plot. (D) The minimum variability timescale versus $T_{90}$ diagram. Type I and type II GRBs are represented by grey and cadet blue solid circles, respectively. EP250704a (Episode I) and GRB 250704B are highlighted by red and black stars, respectively. (E) The scaled light curves of the well-sampled EFXTs detected by EP during 1.5 years of operation, shown with the bin size of 2 s.
• Figure 3: Temporal and spectral consistency of the second pulse in the initial short spike.(A) Multiwavelength light curves of EP250704a/GRB 250704B. The WXT light curve is binned at 0.1 s, and the GRM light curves at 0.03 s. The blue and yellow curves are the best-fit Gaussian models for GRM light curves and an empirical Gaussian profile for the WXT light curve, respectively. (B) The FWHMs derived from the Gaussian models as a function of energy. (C) The observed and modeled photon spectrum of the second pulse. All error bars mark the 1 $\sigma$ confidence level.
• Figure 4: Comparison between GRB 250704B and other short GRB afterglows.(A) A comparison of the X-ray luminosity light curves between GRB 250704B and a sample of merger-type GRBs with extended emission (EE) observed by Swift. (B) Photon index $\Gamma_X$ and decay-index comparison for the plateau phase ($\alpha_1$, small symbols) and the subsequent decay phase ($\alpha_2$, large symbols) for the sample of short GRBs with shallow decay. Grey dots show the normal shallow decay and colored dots show bursts with internal plateau.
• Figure 5: Multi-band afterglow light curves of EP250704a/GRB 250704B. Energy flux at different frequencies, from radio to X-rays, versus time since the GRB trigger. The temporal evolution can be described by multiple power-law segments (dashed lines). Error bars are at the 1 $\sigma$ confidence level (c.l.). Downward triangles are upper limits at the 3 $\sigma$ c.l. from VLT, SVOM/VT, GTC, OSN and GRANDMA. Observations are compared to the optical lightcurves of known GRB-SNe: SN2025kgLi_2025, SN1998bwClocchiatti_2011 and SN2006ajSollerman_2006.