TITLE:   GCN CIRCULAR
NUMBER:  31851
SUBJECT: Chandra upper limit at the location of GRB 220408A
DATE:    22/04/08 10:23:23 GMT
FROM:    Sergio Campana at INAF-OAB  <sergio.campana@brera.inaf.it>

S. Campana (INAF - Osservatorio astronomico di Brera) reports

that Chandra serendipitously observed the location of GRB 220408A 
(Caputo et al. 2022, GCN 31848; Fermi team 2022, GCN 31847).
The UVOT position (Caputo et al. 2022) falls at the edge of the S4 chip 
of ACIS-S. The obsid 23474 observation lasted 36.6 ks and was taken on 
Dec 21, 2020.
No source is visible at this position.
We run srcflux (under CIAO 4.13 and CALDB 4.9.7) and derive a 3-sigma 
upper limit of 0.000206 c/s (2.38E-05,0.000432) on the 0.5-7 keV count 
rate. Assuming a power law photon index of 2 and a column density of 
2x10^20 cm^-2, we derive an upper limit on the 0.5-7 keV unabsorbed flux 
of 2.1e-14 erg/cm2/s.
If the GRB were at the M51 distance (8.58 Mpc), this translates into an 
upper limit on the 0.5-7 keV luminosity of 1.8e38 erg/s.