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Identification of the X-ray transient Swift J061223.0+701243.9

ATel #775; D. Grupe (PSU), J. P. Osborne (U Leicester), K. L. Page (U Leicester), D. N. Burrows (PSU), J. A. Nousek (PSU), and D. P. Schneider (PSU)
on 27 Mar 2006; 16:13 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dirk Grupe (grupe@astro.psu.edu)

Subjects: Optical, Ultra-Violet, X-ray, Binary, Cataclysmic Variable, Transient

We report on the optical identification of the X-ray transient Swift J061223.0+701243.9 (Grupe et al., 2006, GCN #4672, GCN #4681) which is an uncatalogued object discovered serendipitously by the Swift X-ray Telescope (XRT) while observing the Swift Burst Alert Telescope (BAT) trigger 180274 (Grupe et al. 2006, GCN #4663) on 2006 February 04 starting 20:35 UT. Note that this X-ray transient was not the cause of the trigger. The X-ray position is 8 arcmin away from the BAT position, outside the BAT 3.6 arcmin error circle (Sakamoto et al. 2006, GCN #4679).
Swift-XRT observed Swift J0612+7012 again for ~8 ks on 2006 March 06, starting at 09:21 UT over a period of about 13 hours. The spectrum during this time was well modelled by an absorbed power-law, with Gamma = 1.61 ± 0.24 and NH = (1.8 ± 0.8) x 10^{21} cm^-2 (90% containment). The mean count-rate was approximately 0.053 counts s^-1, corresponding to an observed (unabsorbed) flux of 3.20 (3.92) x 10^-12 ergs cm^-2 s^-1 in the 0.3-10 keV band. During this observation the source count rate increased from 0.05 to 0.08 counts/s over 9.6 hours, then fell over 3.2 hours to 0.03 counts/s. The results of the spectral and temporal analysis are in good agreement with those of the discovery observation of 2006 February 04.
Swift J061223.0+701243.9 was observed on 2006 March 21 starting at 02:27 UT with the Low-Resolution Spectrograph (LRS) at the Hobby-Eberly-Telescope (HET) at McDonald Observatory for 3 times 15 minutes using grism #1 with a 2" slit. The spectral resolution was 17A. The optical spectrum clearly shows strong hydrogen and helium emission lines, identifying this source as an X-ray binary system, either a low-mass X-ray binary or a Cataclysmic Variable. H-beta and H-alpha are clearly resolved and we found FWHM=900 km/s, while H-gamma and He II (4686) are clearly detected but the line widths are not resolved. We also found He I at 4471A and 5876A. The line widths, however, are not well-determined. Apart from the emission lines, the continuum is essentially featureless.
The optical position measured from the Swift UV-Optical telescope (UVOT) coadded U image is:

RA-2000: 06 12 22.6 ± 0.5"
Dec-2000: +70 12 43.4 ± 0.8"

The UVOT determined the following observed magnitudes on 2006 March 06:

V = 19.36+/-0.21
B = 19.69+/-0.15
U = 18.67+/-0.07
UVW1 = 19.05+/-0.08
UVM2 = 19.61+/-0.08