TITLE:   GCN CIRCULAR
NUMBER:  42642
SUBJECT: GRB 251013C: Chandra X-ray observations and jet break detection
DATE:    25/11/11 10:50:45 GMT
FROM:    Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>

A. Martin-Carrillo (UCD), B. P. Gompertz (Birmingham), D. B. Malesani (DAWN/NBI and Radboud), report on behalf of a larger collaboration:

We observed the afterglow of GRB 251013C (Fermi GBM team, GCN 42221; Rakotondrainibe et al., GCN 42222) with the Chandra X-ray Observatory starting on 2025-11-09 at 03:09:42 UT (i.e., T0+27.396 d, observer frame), for a total of 40 ks exposure, in response to our DDT request (PI A. Martin-Carrillo, ObsID 31983). 

A faint X-ray source is detected at a location consistent with that of the afterglow (Palmerio et al., GCN 42223; Laskar et al., GCN 42243). Assuming a spectrum model consistent with the one measured by Swift/XRT (Evans et al., GCN 42232; https://www.swift.ac.uk/xrt_spectra/03000137/), we infer a flux in the 0.3-10 keV energy range of ~2.5e-15 erg s^-1 cm^-2.

This late Chandra observation implies that the source has undergone significant steepening in its X-ray flux decay when compared with the earlier Swift/XRT and EP/FXT observations (Evans et al., GCN 42232; Wang et al., GCN 42247), suggesting a jet break sometime between 6 and 10 days since trigger in the observer frame.

We thank the entire CXO team for the rapid evaluation and scheduling of our observations.