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

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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.

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