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Swift-UVOT observations of X-ray transient Swift J1753.5-0127

ATel #553; Martin Still (NASA/GSFC), Pete Roming (PSU), Catherine Brocksopp (MSSL) and Craig B. Markwardt (UMD/NASA/GSFC) on behalf of the Swift-UVOT team
on 4 Jul 2005; 17:17 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

Subjects: Optical, Ultra-Violet, Binary, Black Hole, Transient

Referred to by ATel #: 555

The Swift Ultra-Violet/Optical Telescope (UVOT) began settled observations of Swift J1753.5-0127 (Palmer et al; ATel #546) on 2005 July 1 at 21:32:26 UT, 195s after the Swift-BAT trigger. A series of 10-100s exposures in six optical and UV filters were obtained up to 1237s after the trigger. The source position is RA = 17h 53m 28.48s, Dec = -1h 27' 04.3" (J2000), with a 0.5 arcsec uncertainty radius (1-sigma). There is no temporal variability on 10-1000s timescales above a 1-sigma threshold in any filter. Time-averaged instrumental magnitudes and model-folded fluxes are as follows:

Filter    lambda_c (A)  Texp (s)  Mag       Flux (mJy)  
UVW2      1800            78      16.1(3)   0.43(2)  
UVM2      2200           178      16.5(3)   0.16(1)  
UVW1      2500           112      16.0(2)   0.39(2)  
U         3500            78      15.3(2)   0.83(3)  
B         4400            78      16.2(1)   1.06(4)  
V         5300           368      15.9(1)   1.20(3)  

De-reddened data provide a poor fit to a single black body spectral model. Acceptable fits are found with either a powerlaw model of photon index gamma = 0.4(4) with E(B-V) = 0.42(8), or an accretion disk model of composite black bodies, where the inner disk temperature is limited to T > 116,000 K and E(B-V) = 0.36(2). For comparison, the mapped galactic extinction for the direction of the source yields a colour excess of E(B-V) = 0.45.