Swift-XRT/UVOT observations of the new Galactic Transient Swift J185003.2-005627

ATel #3454; A. P. Beardmore (U. Leicester), S. Campana (INAF-OAB), J. A. Kennea (PSU) and C. A. Swenson (PSU
on 25 Jun 2011; 16:56 UT
Credential Certification: Andy Beardmore (apb@star.le.ac.uk)

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We have analysed 2.8 ks of Swift/XRT data from the new X-ray 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 10²² 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 4.85x10^-10 (6.76x10^-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).