TITLE:   GCN CIRCULAR
NUMBER:  26175
SUBJECT: GRB 191031D: Chandra upper limit 
DATE:    19/11/05 14:50:39 GMT
FROM:    Wen-fai Fong at Northwestern U  <wfong@northwestern.edu>

A. Rouco Escorial, W. Fong, (Northwestern), E. Berger (Harvard), and R. Margutti (Northwestern) report:

"We initiated observations of the location of the short-duration GRB191031D (Fermi-GBM et al., GCN 26111; D'Elia et al., GCN 26112; Mailyan et al., GCN 26118) with the Chandra X-ray Observatory on 2019 Nov 3.694 UT (mid-time of 16:39:42 UT), 2.8 days post-burst under Program 20500021 (ObsID 21302; PI: Berger). The effective exposure time is 19.8 ksec.

We do not detect an X-ray source within the enhanced XRT position (Osborne et al., GCN 26117), and calculate a 3-sigma limit of <4.5e-4 cps in the 0.5-10 keV energy range using the method described by Gehrels (1986). Applying the best-fit XRT spectral parameters, we calculate an unabsorbed flux limit of <6.0e-15 erg/s/cm^2 in the 0.3-10 keV energy range. 

Relative to the XRT light curve starting from t~900 sec post-burst, this constrains the late-time X-ray power-law decline to be alpha<-0.75 (where F~t^alpha).

We thank the Chandra staff for the rapid scheduling and execution of these observations."