[vsnet-grb-info 27310] GRB 210204A / ZTF21aagwbjr: Swift-XRT observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29413
SUBJECT: GRB 210204A / ZTF21aagwbjr: Swift-XRT observations
DATE:    21/02/06 16:22:25 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), E. Ambrosi	  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M.
Perri (SSDC & INAF-OAR), A. Tohuvavohu (U. Toronto), B. Sbarufatti
(PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans
(U. Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of ZTF21aagwbjr (Kool et
al., 29405), which may be related to the Fermi/GBM / GECAM / IPN
detected burst GRB 210204A (GCN Circ., 29390, GCN Circ. 29392, GCN
Circ. 29408), collecting 2.9 ks of Photon Counting (PC) mode data
between T0+160.9 ks and T0+173.2 ks. 

An uncatalogued X-ray source is detected consistent with being within 1
arcsec of the ZTF position and is believed to be the afterglow, or at
least, the X-ray counterpart of the ZTF21aagwbjr. Using 2886 s of PC
mode data and 6 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 = 117.08071, +11.40951 which is equivalent to:
RA (J2000): 07h 48m 19.37s
Dec(J2000): +11d 24' 34.2"

with an uncertainty of 1.7 arcsec (radius, 90% confidence). This
position is 0.7 arcsec from the ZTF position.  The light curve is
consistent with a constant source of mean count rate 9.1e-02 ct/sec. 

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.73 (+0.31, -0.29). The
best-fitting absorption column is  1.7 (+1.3, -1.1) x 10^21 cm^-2, in
excess of the Galactic value of 3.5 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 4.0 x 10^-11 (4.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.7 (+1.3, -1.1) x 10^21 cm^-2
Galactic foreground: 3.5 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     1.73 (+0.31, -0.29)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00014055. The results of the full
analysis of the XRT observations are available at
https://www.swift.ac.uk/ToO_GRBs/00014055.
This circular is an official product of the Swift-XRT team.