November 6
Insights from the X-ray afterglow of the Ultra-long GRB 250702B
Speaker: Brendan O'Connor, UMD/GSFC
Abstract: GRB 250702B is the longest duration gamma-ray burst (GRB) ever observed, exhibiting multiple episodes of luminous prompt gamma-ray emission spanning 25 ks (7 hours) and placing it firmly in the ultra-long GRB regime. Unlike any previously known GRB, a soft X-ray precursor was detected by Einstein Probe (EP) up to 24 hours before the gamma-ray triggering episodes. We present a comprehensive X-ray campaign tracking the transient’s afterglow with Neil Gehrels Swift Observatory, Nuclear Spectroscopic Telescope Array (NuSTAR), and the Chandra X-ray Observatory from 0.5 to 65 days (observer frame) after the initial
high-energy trigger.
The X-ray emission declines steeply and displays rapid variability on short timescales in both Swift and NuSTAR, indicative of X-ray flares superposed on an external shock continuum. Serendipitous detections by the Swift Burst Alert Telescope (BAT) out to 0.3 days, together with continued NuSTAR variability to 2 days, imply sustained central engine activity; when the EP precursor is included, the required engine duration is more than 3 days. Broadband afterglow modeling favors a combination of forward and reverse shock emission in a wind-like circumburst environment.
These properties, especially the extraordinarily long-lived engine and the day soft X-ray precursor, are difficult to reconcile with a standard collapsar origin and do not align cleanly with canonical ultra-long GRBs or relativistic tidal disruption events. We discuss two possible hybrid interpretations in which 1) a star is disrupted by a stellar-mass black hole (a micro-TDE) and 2) a field binary where a black hole falls into a stripped star (a helium merger). These models offer a compelling explanation, while a relativistic TDE powered by an intermediate-mass black hole remains viable. Future observations of similar events are required to aid in disentangling between these models and additional theoretical work is required.
Host: Jillian Rastinejad & Sara Starechecki