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GCN Circular 21103

Subject
GRB 170510A: Swift-XRT afterglow detection
Date
2017-05-18T15:59:27Z (7 years ago)
From
Antonino D'Ai at IASF-PA <antonino.dai@ifc.inaf.it>
A. Cholden-Brown (PSU), S. J. LaPorte (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), V.
D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), J.A.
Kennea (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 170510A (Bissaldi et al. GCN Circ. 21080).
The observations now extend from T0+24.7 ks to T0+713.5 ks, and are
entirely in Photon Counting (PC) mode. The source previously reported
by Melandri et al. (GCN Circ. 21086), "Source 1", is fading with
3-sigma significance, and is therefore likely the GRB afterglow.

Using 1264 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 = 160.15394, -39.21244 which is
equivalent to:

RA (J2000): 10h 40m 36.95s
Dec(J2000): -39d 12' 44.8"

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 13.2 arcmin from the Fermi/LAT position.  The source is
fading with alpha >0.9.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.8, -0.4). The
best-fitting absorption column is  consistent with the Galactic value
of 6.1 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 3.2 x 10^-11 (3.5 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6 (+/-24) x 10^20 cm^-2
Galactic foreground: 6.1 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.8 (+0.8, -0.4)

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/xrt_products/TILED_GRB00067/Source1.php.
The results of the full analysis of the tiled XRT observations are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00067.

Swift/UVOT observed "Source 1" at T0+696 ks. We did not find an optical
counterpart. We measured a 3 sigma upper limit >21 mag in the UVW1
filter, using an exposure of 1671 s.

This circular is an official product of the Swift-XRT and UVOT teams.
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