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GCN Circular 36558

Subject
GRB 240527B: Swift-XRT afterglow detection
Date
2024-05-29T05:53:44Z (2 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J. D. Gropp (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), T. Sbarrato
(INAF-OAB) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 240527B, collecting 5.2 ks of Photon
Counting (PC) mode data between T0+27.3 ks and T0+135.5 ks. 

Eight uncatalogued X-ray sources are detected, of which one ("Source
7") is above the RASS 3-sigma upper limit at this position and fading
with >3-sigma significance, and is therefore likely the GRB afterglow.
Using 1624 s of PC mode data and 3 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 = 128.15387, -14.43738 which is
equivalent to:

RA (J2000): 08h 32m 36.93s
Dec(J2000): -14d 26' 14.6"

with an uncertainty of 2.0 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=1.23 (+0.16, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.70 (+0.08, -0.06). The
best-fitting absorption column is  consistent with the Galactic value
of 5.7 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 3.8 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.7 (+/-1.9) x 10^20 cm^-2
Galactic foreground: 5.7 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.70 (+0.08, -0.06)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021694.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021694.

This circular is an official product of the Swift-XRT team.


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