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GCN Circular 27589

GRB 200411A: Chandra observations indicative of temporal steepening
2020-04-15T21:20:48Z (4 years ago)
Wen-fai Fong at Northwestern U <>
A. Rouco Escorial, W. Fong (Northwestern), and E. Berger (Harvard) report:

���We initiated observations with the Chandra X-ray Observatory of the position of the short-duration GRB 200411A (Fermi-GBM et al., GCN 27535; Tohuvavohu et al., GCN 27536; Bissaldi et al., GCN 27543) starting on 2020 April 15 00:16:54 UT, or at a mid-time of 3.95 days after the GRB trigger. ACIS-S observations were taken under Proposal 21500089 (ObsID 22427; PI: Berger), with a total exposure time of 19.8 ksec.
We do not detect an X-ray source within or around the enhanced Swift-XRT position (90% confidence; Osborne et al., GCN 27538). Centered on the XRT position, we calculate a 3-sigma limit of <4.5e-4 cts/s in the 0.5-8 keV energy range following the method described by Gehrels (1986) for small numbers of events. Applying the best-fit PC mode XRT spectral parameters (Gamma=1.7, intrinsic NH=1.5e21 cm^-2) and fixing the Galactic NH=1.55e20 cm^-2 (Willingale et al. 2013), we calculate a 3-sigma unabsorbed flux limit of FX<8.4e-15 erg/s/cm^2 in the 0.3-10 keV energy range. 
The XRT afterglow starting from 200 sec post-burst exhibits a power-law decline with decay index alpha=-0.84 (+/- 0.2) (where FX~t^alpha), while the Chandra upper limit indicates a potential steepening to alpha<-1.3. If this steepening is interpreted as a jet break at ~0.5-2 days post-burst, using median values for the explosion properties and redshift based on properties of short GRBs gives an opening angle of ~4-7 degrees.
We thank the Chandra staff for the rapid planning and scheduling of these observations.���
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