GCN Circular 25251
Subject
GRB 190731A: Swift-XRT afterglow detection
Date
2019-08-01T21:24:27Z (5 years ago)
From
Andy Beardmore at U Leicester <ab271@leicester.ac.uk>
J.P. Osborne (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi
(INAF-IASFPA), M. Perri (ASDC), J. D. Gropp (PSU), A. Tohuvavohu (PSU),
J.A. Kennea (PSU), K.L. Page (U. Leicester) and P.A. Evans (U.
Leicester) report on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 190731A (Ohno et al. GCN Circ. 25244),
collecting 5.0 ks of Photon Counting (PC) mode data between T0+33.4 ks
and T0+62.9 ks.
Eleven uncatalogued X-ray sources are detected, of which one ("Source
1") is fading with 3-sigma significance, and is therefore likely the
GRB afterglow. Using 1953 s of PC mode data and 4 UVOT images, we find
an enhanced XRT position (using the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue): RA, Dec = 339.72015,
-76.57744 which is equivalent to:
RA (J2000): 22h 38m 52.84s
Dec(J2000): -76d 34' 38.8"
with an uncertainty of 2.2 arcsec (radius, 90% confidence). This
position is 4.0 arcmin from the Fermi/LAT position.
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.3 (+0.6, -0.7).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.5 (+/-0.5). The
best-fitting absorption column is 5.9 (+2.8, -2.2) x 10^21 cm^-2, in
excess of the Galactic value of 9.8 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 3.3 x 10^-11 (7.8 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 5.9 (+2.8, -2.2) x 10^21 cm^-2
Galactic foreground: 9.8 x 10^20 cm^-2
Excess significance: 3.6 sigma
Photon index: 2.5 (+/-0.5)
If the light curve continues to decay with a power-law decay index of
1.3, the count rate at T+24 hours will be 0.015 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.9 x
10^-13 (1.2 x 10^-12) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/ToO_GRBs/00020914/Source1.php.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00020914.
This circular is an official product of the Swift-XRT team.