Swift-UVOT detection of XTEJ1817-330 in the UV

ATel #740; D. Steeghs, M. A.P. Torres (CfA), J. Miller (UMich), P. G.Jonker (SRON/CfA)
on 19 Feb 2006; 21:14 UT
Credential Certification: Danny Steeghs (dsteeghs@cfa.harvard.edu)

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The SWIFT Observatory performed a TOO observation of the active transient XTE J1817-330 (ATel #714) on Feb 18th 2006, covering 04:50-05:10UT. The Ultra-Violet/Optical Telescope (UVOT) collected a series of 160-190s exposures of the field surrounding the transient using its six broad-band filters.

Processed UVOT images reveal a well detected point source at the reported position of the transient (ATel #721,#724,#733) in all filters. The transient is particularly evident in the UV filters. After extracting the source counts, we used the latest UVOT calibration files and derived the following magnitudes and fluxes in the UVOT pass-bands;

  
FILTER   WAVELENGTH (A)     MAG (Vega)        FLUX (erg/s/cm^2/A) 
  
UW2      1800               14.88(3)          8.65e-15            
  
UM2      2200               14.91(3)          6.16e-15            
  
UW1      2500               14.58(3)          6.18e-15            
  
U        3500               14.38(8)          5.93e-15            
  
B        4400               16.1(2)           2.26e-15            
  
V        5300               15.39(5)          2.22e-15            
  

Given the sensitivity of the UV flux to interstellar extinction, these fluxes clearly demonstrate the low hydrogen column along the line of sight towards XTE J1817-330. Comparing the above fluxes with hot black-body models suggests a hydrogen column around 2-5 10^21. The ultra-violet constraints permitted thanks to such a modest column will greatly enhance ongoing efforts targeting this bright transient at X-ray wavelengths.

We thank the SWIFT team for their cooperation and encourage observations of this source at all wave-bands.