TITLE: GCN CIRCULAR NUMBER: 41309 SUBJECT: GRB 250702B: Chandra X-ray Detection DATE: 25/08/09 23:30:50 GMT FROM: Brendan O'Connor at Carnegie Mellon University boconno2@andrew.cmu.edu

B. O’Connor (CMU), D. Pasham (Eureka/George Washington), I. Andreoni (UNC), J. Hare (Catholic/GSFC), X. Hall (CMU), J. Carney (UNC), A. Palmese (CMU), M. Busmann (LMU), D. Gruen (LMU), report on behalf of a larger collaboration:

We observed the position of the ultra-long X-ray and gamma-ray transient GRB 250702B (GCN #40883, #40886, #40890), also known as EP250702a (GCN #40906), with the Chandra X-ray Observatory (CXO) through Director's Discretionary Time (PI: O'Connor). Observations were carried out with ACIS-S starting on 2025-08-09 at 05:49:28 UT, corresponding to ~38 d after the initial Fermi trigger, for a total of 27.7 ks.

We detect a clear X-ray source at the position of the infrared and radio counterparts of GRB 250702B (GCNs #40924, #41053). Adopting the best-fit spectrum from Swift/XRT (GCN #40919), we derive an unabsorbed 0.3-10 keV flux of ~3.3e-14 erg/cm^2/s. This is consistent with the extrapolation of the decay observed by Swift/XRT at earlier times (<10 days), and favors a powerlaw with temporal slope ~-1.9 as measured from the initial Fermi trigger time (GRB 250702D; GCN #40886).

Further analysis is underway.

We thank Pat Slane for approving our DDT request, and the entire staff of the Chandra X-ray Observatory for scheduling the observations.

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