[vsnet-grb-info 10660] Swift J185003.2-005627: Swift-XRT/UVOT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12087
SUBJECT: Swift J185003.2-005627: Swift-XRT/UVOT refined analysis
DATE:    11/06/25 16:51:02 GMT
FROM:    Andy Beardmore at U Leicester  <apb at star.le.ac.uk>

A. P. Beardmore (U. Leicester), S. Campana (INAF-OAB), J. A. Kennea
(PSU) and C. A. Swenson (PSU) report on behalf of the Swift/XRT
and UVOT teams:

We have analysed 2.8 ks of Swift/XRT data from the transient Swift
J185003.2-005627 (Beardmore, et al., GCN Circ. 12083), from 88 s to
17.5 ks after the BAT trigger. The data comprise 421 s in Windowed
Timing (WT) mode (the first 5 s were taken while Swift was slewing),
with the remainder in Photon Counting (PC) mode.

The astrometrically corrected X-ray position for this source (using
the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue), derived from 1660 s of XRT Photon Counting mode data and 2
UVOT images, is RA, Dec = 282.51390, -0.93980 which is equivalent to:

RA (J2000):  18 50 3.33
Dec (J2000): -00 56 23.3

with an uncertainty of 1.7 arcsec (radius, 90% confidence). This
position may be improved as more data are received. The position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and
Evans et al. (2009, MNRAS, 397, 1177).

The X-ray light curve initially decays like a powerlaw, with an index
of 1.158 (+0.102, -0.125), before flattening to a slope of 0.161
(+0.035, -0.072) at T +365 (+109, -40) s after the trigger.

A spectrum formed from the WT mode data from T+101 to T+429 s after
the trigger is best fit by an absorbed blackbody, with a temperature
of 0.83 +/- 0.03 keV and column density of 1.71 +/- 0.19 x 10^22
cm^-2, consistent with the Galactic column in the direction of the
source (Kalberla et al.  2005).  The observed (unabsorbed) 0.3-10 keV
flux for this spectrum is 4.85e-10 (6.76e-10) ergs cm^-2 s^-1.

The XRT spectrum, together with the soft BAT spectrum reported by
Markwardt et al. (GCN Circ. 12086) and the location of the source in
the Galactic plane, is suggestive of emission from a type I burst.

Swift/UVOT observations of the field were also performed but no
optical counterpart is detected at the XRT position.  Preliminary
3-sigma upper limits, using the UVOT photometric system (Poole et
al. 2008, MNRAS, 383, 627), are:

Filter       T_start(s)   T_stop(s)    Exp(s)     Mag

---------------------------------------------------------------------------

white          104          5305        628       >18.9
v              593          1763        136       >17.2
b              519          1863        136       >18.9
u              263          1837        382       >19.9
w1             642          1813        130       >19.7
w2             742          5340        125       >20.3

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 16.09 in the direction of the transient
(Schlegel et al. 1998).