Comprehensive X-Ray Observations of the Exceptional Ultralong X-Ray and Gamma-Ray Transient GRB 250702B with Swift, NuSTAR, and Chandra: Insights from the X-Ray Afterglow Properties
Brendan O’Connor, Ramandeep Gill, James DeLaunay, Jeremy Hare, Dheeraj R. Pasham, Eric R. Coughlin, Ananya Bandopadhyay, Akash Anumarlapudi, Paz Beniamini, Jonathan Granot, Igor Andreoni, Jonathan Carney, M. J. Moss, Ersin Göğüş, J. A. Kennea, Malte Busmann, S. Dichiara, James Freeburn, D. Gruen, Xander J. Hall, A. Palmese, Tyler Parsotan, S. Ronchini, A. Tohuvavohu, Maia A. Williams
Abstract
Abstract 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 ultralong gamma-ray bursts (GRBs). However, unlike any known GRB, the Einstein Probe detected soft-X-ray emission up to 24 hr 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 and 65 days (observer frame) after the initial high-energy trigger. The X-ray emission decays steeply as ∼ t −1.9 and shows short-timescale X-ray variability (Δ 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 out to ∼0.3 days and continued NuSTAR variability to ∼2 days imply sustained central engine activity; including the early Einstein Probe X-ray detections, the required engine duration is ≳3 days. Afterglow modeling favors the combination of forward- and reverse-shock emission in a windlike ( k ≈ 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 ultralong 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.