Kavli Affiliate: Dheeraj Pasham
| First 5 Authors: Brendan O’Connor, Brendan O’Connor, , ,
| Summary:
GRB 250702B is an exceptional transient that produced multiple episodes of
luminous gamma-ray radiation lasting for $>25$ ks, placing it among the class
of ultra-long gamma-ray bursts (GRBs). However, unlike any known GRB, the
textitEinstein Probe detected soft X-ray emission up to 24 hours before the
gamma-ray triggers. We present comprehensive X-ray observations of the
transient’s afterglow obtained with the Neil Gehrels Swift Observatory, the
Nuclear Spectroscopic Telescope Array, and the Chandra X-ray Observatory
between 0.5 to 65 days (observer frame) after the initial high-energy trigger.
The X-ray emission decays steeply as $sim t^-1.9$, and shows short timescale
X-ray variability ($Delta T/T < 0.03$) in both Swift and NuSTAR, consistent
with flares superposed on an external shock continuum. Serendipitous detections
by the Swift Burst Alert Telescope (BAT) out to $sim$0.3 days and continued
NuSTAR variability to $sim$2 days imply sustained central engine activity;
including the precursor, the required engine duration is $gtrsim 3$ days.
Afterglow modeling favors the combination of forward and reverse shock emission
in a wind-like ($k approx 2$) environment. These properties, especially the
long-lived engine and early soft X-ray emission, are difficult to reconcile
with a collapsar origin, and GRB 250702B does not fit neatly with canonical
ultra-long GRBs or relativistic tidal disruption events (TDEs). A hybrid
scenario in which a star is disrupted by a stellar-mass black hole (a
micro-TDE) provides a plausible explanation, although a relativistic TDE from
an intermediate-mass black hole remains viable. Decisive discrimination between
progenitors will require sensitive late-time X-ray observations.
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