Swift discovery of a new X-ray and UV Transient.

ATel #4631; K. L. Page (U. Leicester), P. A. Evans (U. Leicester) and A. P. Beardmore (U. Leicester)
on 9 Dec 2012; 16:54 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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During a Target of Opportunity observation of Nova Mon 2012 on December 08, a new serendipitous X-ray source was detected in the Swift-XRT field of view. Using 992 s of XRT Photon Counting mode data and 3 UVOT images, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) of RA, Dec = 99.89012, 5.82154 which is equivalent to

RA(J2000) = 06^h 39^m 33.63^s
Dec(J2000) = +05^o 49' 17.6"

with an uncertainty of 2.5 arcsec (radius, 90% confidence).

This X-ray source has a count rate of 0.071 +0.008/-0.007 count s^-1 over 0.3-10 keV between 12:45 and 13:01 on December 08 (ObsID 00032529045). The previous observation of this field from 06:20 to 06:36 on the same date showed no sign of this source, to a 3-sigma upper limit of 0.013 count s^-1. Similarly, the following observation (19:11 to 19:28 on December 08) provides an upper limit of 0.017 count s^-1.

The source was also undetected in the ROSAT All-Sky Survey; the count rate of our Swift detection is a factor of three higher than the RASS 3-sigma upper limit at this location on the sky.

The spectrum can be modelled either as an absorbed power-law, with Gamma = 1.69 +0.66/-0.40 and N_H < 1.8 x 10²¹ cm^-2 (90% upper limit) or an absorbed optically thin thermal component, with kT = 7.0 +32.4/-3.6 keV and a column of N_H < 4.0 x 10²⁰ cm^-2. Both fits are statistically as good as each other. The observed flux over 0.3-10 keV is 3.1 x 10^-12 erg cm^-2 s^-1.

This new source is also detected in the three UVOT UV filters, with magnitudes of uvw2 (central wavelength = 1928 angstrom) = 15.04 +/- 0.03, uvm2 (2246 angstrom) = 15.78 +/- 0.05 and uvw1 (2600 angstrom) = 16.01 +/- 0.04. The upper limits before and after the detection were uvw2 > 19.8, uvm2 > 19.6 and uvw1 > 19.8.

Vizier reports no catalogued object within 8 arcsec of the X-ray position. Multi-wavelength observations are encouraged to determine the nature of this source.