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

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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Publication Details

Journal
The Astrophysical Journal Letters
Published
2025-11-14
DOI
https://doi.org/10.3847/2041-8213/ae1741
Citations
6
Primary Topic
Gamma-ray bursts and supernovae
Type
article
Field-Weighted Citation Impact
6.93
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article

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

Maia A. Williams, Jonathan Granot, James DeLaunay, Xander J. Hall et al.
6 citations
The Astrophysical Journal Letters
Gamma-ray bursts and supernovae
6.93
article

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

Maia A. Williams, Jonathan Granot, James DeLaunay, Xander J. Hall, M. J. Moss, Jonathan Carney, D. Gruen, Akash Anumarlapudi, Tyler Parsotan, Ananya Bandopadhyay, Ramandeep Gill, James Freeburn, Ersin Göğüş, A. Palmese, Brendan O’Connor, S. Dichiara, Jeremy Hare, A. Tohuvavohu, Igor Andreoni, Eric R. Coughlin, J. A. Kennea, Dheeraj R. Pasham, Paz Beniamini, Malte Busmann, S. Ronchini
article en
6 citations

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.

The Astrophysical Journal LettersVol. 994(1)
Northwestern University (US), University of North Carolina at Chapel Hill (US), California Institute of Technology (US), Goddard Space Flight Center (US), Pennsylvania State University (US), Open University of Israel (IL), University of America (US), George Washington University (US), Excellence Cluster Origins (DE), Türkisch-Deutsche Universität (TR), Eureka Scientific (US), Gran Sasso Science Institute (IT), Deleted Institution, Syracuse University (US), Carnegie Mellon University (US), Ludwig-Maximilians-Universität München (DE), Catholic University of America (US), Universidad Nacional Autónoma de México (MX)
Openalex Percentile: Top 3%
Gamma-ray bursts and supernovae
6.93
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