Swift observation of 1RXS J173006.4+033813 and Association with USNO-B1.0 0936-0303814

ATel #2015; Andrew S. Shevchuk, Derek B. Fox (PSU), Monica Turner, Robert E. Rutledge (McGill)
on 16 Apr 2009; 21:31 UT
Credential Certification: R. E. Rutledge (rutledge@physics.mcgill.ca)

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In ATel #2014, a variable optical source USNO B1.0 0936-0303814 is reported in the field of 1RXS J173006.4+033813. This USNO-B1.0 source is cataloged with a B=17.1 magnitude.

The X-ray source was observed with Swift/XRT as part of our ongoing study of RASS/BSC X-ray sources without likely off-band counterparts. The X-ray source was detected in a Swift/XRT observation on 2006-02-09T16:56, with a total exposure of 1106 seconds at a position (J2000) R.A. (J2000) = 17h30m06.19s, dec. = +03d38m18.8s, with an astrometric uncertainty of 1.0 arcsec (statistical) and 3.4 arcsec (systematic), with a countrate of 22+/-5 c/ksec. Absorption is low in the direction of this source (N_H=0.07e22 cm-2). The best-fit photon power-law spectrum has a slope of 1.8+/-0.5, with a corresponding flux of 1.2e-12 ergs cm-2 s-1 (0.5-10 keV). Assuming the same spectrum observed during the RASS/BSC, the X-ray flux is lower by a factor of ~2.

A simultaneous UVOT image (filter M2) detects a UV source (M2=17.8+/-0.1 mag) consistent with this position, and with a position (corrected to the local 2MASS frame) of

  
R.A.(J2000) = 17h30m06.46s  
dec.= +03d38m19.6s  

with an astrometric uncertainty of 1 arcsec. Based on the local density of UVOT sources of the same or brighter magnitude, the probability of chance coincidence between the UVOT and XRT sources is 0.001. We therefore associate the UVOT and XRT sources, with 99.9% confidence. We therefore affirmatively associate 1RXS J173006.4+033813=XRT J173006.19+033818.8=UVOT J173006.46+033819.6. The 1 arcsec astrometry decreases the RASS uncertainty region (10 arcsec radius) by a factor of x100.

This SWIFT position is consistent with the position of USNO-B1.0 0936-0303814 reported in ATel #2014. The UVOT and USNO-B1 sources are offset by 0.5", which is within the astrometric errors. Given the local field density of B=17.1 mag objects, these objects are associated with 99.8% confidence.

The USNO-B1 source is not detected in the 2MASS survey, which implies a blue optical counterpart to this 1RXS source. An optical spectrum would be useful to classify this source.