GCN Circular 32135
Subject
GRB 220527A: Swift-XRT afterglow detection
Date
2022-05-28T01:10:03Z (3 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
B. Sbarufatti (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E.
Ambrosi (INAF-IASFPA) , D.N. Burrows (PSU), J. D. Gropp (PSU) 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 220527A (Bissaldi et al. GCN Circ. 32131),
collecting 1.7 ks of Photon Counting (PC) mode data between T0+30.1 ks
and T0+31.9 ks.
Two uncatalogued X-ray sources are detected, of which one ("Source 2")
is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow. Using 1733 s of PC mode data and 1
UVOT image, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 323.52805, -14.97183 which is equivalent to:
RA (J2000): 21h 34m 06.73s
Dec(J2000): 14d 58' 18.6"
with an uncertainty of 2.6 arcsec (radius, 90% confidence). This
position is 4.9 arcmin from the Fermi/LAT position. The light curve is
consistent with a constant source of mean count rate 3.9e-01 ct/sec. A
power-law fit gives an index of 0.6 (+2.5, -1.1).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.83 (+0.21, -0.20). The
best-fitting absorption column is 9.8 (+6.1, -3.5) x 10^20 cm^-2,
consistent with the Galactic value of 6.2 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 3.6 x 10^-11 (4.2 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 9.8 (+6.1, -3.5) x 10^20 cm^-2
Galactic foreground: 6.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.83 (+0.21, -0.20)
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021503.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021503.
This circular is an official product of the Swift-XRT team.