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Abstract:

We present multiwavelength observations of the Swift short γ-ray burst GRB 231117A, localized to an underlying galaxy at redshift z = 0.257 at a small projected offset (∼2 kpc). We uncover long-lived X-ray Chandra X-ray Observatory and radio/millimeter (VLA, MeerKAT, and ALMA) afterglow emission, detected to ∼37 days and ∼20 days (rest frame), respectively. We measure a wide jet (∼10 .° 4) and relatively high circumburst density (∼0.07 cm−3) compared to the short GRB population. Our data cannot be easily fit with a standard forward shock model, but they are generally well fit with the incorporation of a refreshed forward shock and a reverse shock at <1 day. We incorporate GRB 231117A into a larger sample of 132 X-ray detected events, 71 of which were radio-observed (17 cm-band detections), for a systematic study of the distributions of redshifts, jet and afterglow properties, galactocentric offsets, and local environments of events with and without detected radio afterglows. Compared to the entire short GRB population, the majority of radio-detected GRBs are at relatively low redshifts (z < 0.6) and have high circumburst densities (>10−2 cm−3), consistent with their smaller (<8 kpc) projected galactocentric offsets. We additionally find that 70% of short GRBs with opening angle measurements were radio-detected, indicating the importance of radio afterglows in jet measurements, especially in the cases of wide (>10°) jets where observational evidence of collimation may only be detectable at radio wavelengths. Owing to improved observing strategies and the emergence of sensitive radio facilities, the number of radio-detected short GRBs has quadrupled in the past decade.

Abstract:

Despite being operational for only a short time, the Einstein Probe mission, with its large field of view and rapid localization capabilities, has already significantly advanced the study of rapid variability in the soft X-ray sky. We report the discovery of luminous and variable radio emission from the Einstein Probe fast X-ray transient EP240414a, the second such source with a radio counterpart. The radio emission at 3 GHz peaks at ∼30 days postexplosion and with a spectral luminosity ∼2 × 1030 erg s−1 Hz−1, similar to what is seen from long gamma-ray bursts, and distinct from other extragalactic transients including supernovae and tidal disruption events, although we cannot completely rule out emission from engine driven stellar explosions, e.g., the fast blue optical transients. An equipartition analysis of our radio data reveals that an outflow with at least a moderate bulk Lorentz factor (Γ ≳ 1.6) with a minimum energy of ∼1048 erg is required to explain our observations. The apparent lack of a reported gamma-ray counterpart to EP240414a could suggest that an off-axis or choked jet could be responsible for the radio emission, although a low-luminosity gamma-ray burst may have gone undetected. Our observations are consistent with the hypothesis that a significant fraction of extragalactic fast X-ray transients are associated with the deaths of massive stars.