GRB~250704B/EP250704a a Short Gamma-Ray Burst Powered by a Magnetar

TL;DR

The study analyzes GRB 250704B/EP250704a, a short GRB with a pronounced day-scale optical plateau and extended X-ray emission, using comprehensive multi-wavelength data from gamma-ray to radio. The authors develop a magnetar-centered energy-injection model that incorporates fallback accretion and time-evolving microphysical parameters to explain the complex light curves and spectra. Bayesian and frequentist fits yield a low-density environment, a compact jet with a few degree opening angle, and magnetar properties around B ~ a few×10^14 G and P ~ 1.1 ms, consistent with a long-lived magnetar remnant. The results show that extended X-ray emission arises from internal dissipation in the magnetar wind while the optical plateau and afterglow are governed by magnetar-driven energy injection and evolving microphysics, highlighting the magnetar scenario as a robust mechanism for this event and similar sGRBs.

Abstract

GRB~250704B/EP250704a, identified as a short gamma-ray burst (sGRB), exhibited prolonged X-ray emission following the prompt phase and, in optical and infrared (IR) bands, an unusual one-day plateau succeeded by a rapid decline. This sGRB was observed by multiple satellites and ground-based observatories across the electromagnetic spectrum. This study presents temporal and spectral analyses from radio to gamma-ray frequencies, spanning several observation periods beginning after the trigger and continuing for nearly 2 days. The results of the temporal and spectral analyses of the prompt episode, the extended X-ray component, and the afterglow phase are consistent with a millisecond magnetar undergoing accretion. The long-lasting X-ray emission is attributed to the internal energy dissipation of the magnetar spin-down power, governed by the magnetization parameter; the extended optical/IR plateau to synchrotron afterglow emission with energy injection; and the steep decay to changes in microphysical parameters during the post-jet break phase. The X-ray observations are consistent with the superposition of spin-down luminosity and synchrotron afterglow scenario. These findings suggest that the compact-object remnant is most likely a long-lived magnetar.

Figures (7)

• Figure 1: The upper-left panel presents the KW public light curves of GRB 250704B/EP250704a in the 18-70, 70-300, and 300-1160 keV channels, arranged from top to bottom. The orange region indicates the period used for spectral data collection. The upper-right panel displays the corresponding spectrum. The rate values across these three channels are averaged and fit to a Gaussian function. The least squares method fits the spectrum with a Band function. The lower panel shows the HXM2 ( CALET/CGBM) light curve on counts in five channels (from top to bottom): 7 - 10, 10 - 25, 25 - 50, 50 - 100, and 100 - 170 keV. The dotted lines in each channel correspond to the best-fit straight line.
• Figure 2: The upper panels show the X-ray light curve at 0.3-10 keV (left) and optical/IR light curves (right) in the J, z, i, r and g filters. The lines in both panels correspond to the best-fit PL segments. The lower panel displays the SED of EP250704a at $2.7\times 10^3\, {\rm s}$ and $1.5\times 10^3\, {\rm s}$ (lighter points). We combined binned X-ray data from the Swift/XRT repository and interpolated the optical flux densities corrected for Galactic extinction.
• Figure 3: The upper-left panel displays the X-ray observations, with the best-fit curve generated by the internal energy dissipation of the magnetar spin-down power with the respective magnetization parameter, and the HXM2 (CALET) light curve on counts in the 7 - 10 keV channel. The upper-right panel shows the evolution of the light cylinder, corotation and Alfven radii together with the evolution of the spin period. The lower panel shows the multi-wavelength afterglow observations with the best-fit curves obtained by the internal energy dissipation of the magnetar spin-down power and synchrotron FS model with continuous energy injection from the millisecond magnetar. The inset exhibits the X-ray data with the best-fit spin-down flux. All panels are plotted with a fallback accretion rate with just one characteristic timescale.
• Figure 4: Corner plot illustrating the median values of parameters (blue lines) obtained from fitting the X-ray, optical and radio observations of GRB 250704B requiring the internal energy dissipation of the magnetar spin-down power and synchrotron afterglow emission with energy injection and microphysical parameter evolution. The median values of the parameters found are listed in Table \ref{['tab5:fit']}.
• Figure 5: The same as Figure \ref{['fig:multi_fit_v1']}, but for a fallback accretion rate with two characteristic timescales.