October 5, 2025Open Access

Comprehensive X-ray Observations of the Exceptional Ultra-long X-ray and Gamma-ray Transient GRB 250702B with Swift, NuSTAR, and Chandra: Insights from the X-ray Afterglow Properties

BOBrendan O’ConnorCarnegie Mellon University RGRamandeep GillUniversidad Nacional Autónoma de México JDJames DeLaunayPennsylvania State University

Key Points

X-ray emission from GRB 250702B decayed steeply at rate of approximately t^-1.9, indicating strong afterglow.
Detection of soft X-ray emissions prior to gamma-ray triggers suggests significant early activity from the central engine.
Observations across 0.5 to 65 days point to a hybrid progenitor scenario involving a black hole or a relativistic tidal disruption event.
Late-time X-ray observations are crucial for distinguishing the true progenitor of this unique gamma-ray burst.

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 ultra-long gamma-ray bursts (GRBs). However, unlike any known GRB, the Einstein 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 t^-1. 9, and shows short timescale X-ray variability (ΔT/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 0. 3 days and continued NuSTAR variability to 2 days imply sustained central engine activity; including the precursor, the required engine duration is 3 days. Afterglow modeling favors the combination of forward and reverse shock emission in a wind-like (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 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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