TITLE:   GCN CIRCULAR
NUMBER:  33558
SUBJECT: GRB 230307A: Chandra late-time detection of the X-ray afterglow
DATE:    23/04/03 17:31:15 GMT
FROM:    Wen-fai Fong at Northwestern U  <wfong@northwestern.edu>

A. Rouco Escorial (ESA/ESAC), B. Gompertz (U. Birmingham), W. Fong (Northwestern), A. J. Levan (Radboud), J. Rastinejad (Northwestern), E. Berger (Harvard) report:

"The Chandra X-ray Observatory observed the extremely bright long-duration GRB 230307A (Fermi GBM Team, GCN 33405; Xiong et al., GCN 33406; Dalessi et al., GCN 33407, 33411; Burns et al., GCN 33414) starting on 2023 April 1st 21:30:20 UT, with a median observation time of ~25.4 days post-trigger. We obtained an ACIS-S observation under Director’s Discretionary Time (Proposal 24408902, ObsID 27778; PI: Fong), with an effective exposure time of ~16.8 ks.

At the XRT position (Evans et al. GCN 33429), we identify 4 net counts, compared to an expected background of ~0.2 counts. Using the method of Kraft, Burrows & Nousek (1991ApJ 374 444) for the confidence limits in the low counts regime this suggests the detection of a source at >3-sigma confidence. We infer a count rate of ~2.4E-4 counts/seconds. Performing a joint spectral fit with the Swift/XRT data we obtain an X-ray flux of FX ~ 1.1e-14 erg/s/cm2.

Combining this Chandra observation with earlier XRT data, we infer a power-law decline with a decay index (FX~t^alpha) of alpha~-1.3.

We thank Pat Slane, Harvey Tananbaum and the CXO staff for the rapid approval and planning of these observations."