GCN Circular 18629
Subject
GRB 151120A: Swift-XRT afterglow detection
Date
2015-11-20T20:16:45Z (8 years ago)
From
Sarah Gibson at U.of Leicester <slg44@leicester.ac.uk>
A.P. Beardmore (U. Leicester), A. Maselli (INAF-IASFPA), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N.
Burrows (PSU), T.G.R. Roegiers (PSU), S.L. Gibson (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 INTEGRAL-detected
burst GRB 151120A (Mereghetti et al. GCN Circ. 18623), collecting 3.9
ks of Photon Counting (PC) mode data between T0+16.4 ks and T0+23.6 ks.
An uncatalogued X-ray source is detected inside or close to the
INTEGRAL error region and is above the RASS limit, and is therefore
likely the GRB afterglow. The position of this source is RA,
Dec=157.2410, -32.5262 which is equivalent to:
RA (J2000): 10:28:57.83
Dec(J2000): -32:31:34.2
with an uncertainty of 3.6 arcsec (radius, 90% confidence). This
position is 29 arcsec from the INTEGRAL position and 1.6 arcsec from
the afterglow detected by GROND (GCN 18624).
The light curve can be modelled with a power-law decay with a decay
index of alpha=2.3 (+1.0, -0.9).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.00 (+0.31, -0.28). The
best-fitting absorption column is 2.7 (+1.3, -1.1) x 10^21 cm^-2, in
excess of the Galactic value of 9.0 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 3.7 x 10^-11 (5.3 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 2.7 (+1.3, -1.1) x 10^21 cm^-2
Galactic foreground: 9.0 x 10^20 cm^-2
Excess significance: 2.8 sigma
Photon index: 2.00 (+0.31, -0.28)
If the light curve continues to decay with a power-law decay index of
2.3, the count rate at T+24 hours will be 2.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.5 x
10^-14 (1.1 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020559/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020559.
This circular is an official product of the Swift-XRT team.