Spectroscopy of the Optical Transient SSS130101:122222-311525

ATel #4704; Tom Marsh (Warwick), Christian Knigge (Southampton), Retha Pretorius (ESO & Southampton), James Miller-Jones (Curtin), Elmar Koerding (Nijmegen), Gregory Sivakoff (Alberta), Patrick Woudt (Capetown), Brian Warner (Capetown)
on 5 Jan 2013; 20:53 UT
Credential Certification: Christian Knigge (christian@astro.soton.ac.uk)

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We have obtained optical spectroscopy of the bright optical transient SSS130101:122222-311525 reported in ATel 4699 and 4700) with the 4.2m William Herschel Telescope on La Palma. The observations were taken on Jan 3, 2013 with the dual arm spectrograph ISIS, covering both blue and red optical wavelengths. The total exposure time was 29 mins, centered on 06:33 UT. The data were obtained at high airmass in poor seeing, and no flux calibration was attempted. The spectra exhibit very broad (FWZI ~ 7000 km/s) and double-peaked (peak-to-peak separation ~ 1000 km/s) Balmer emission lines. There are also hints of broad He I emission lines (4471 A, 5875 A, 6678 A) and of a broad feature around 4700 A that could be associated with the Bowen blend, He II 4686 and/or He I 4713. The existence of broad, double-peaked emission lines confirms that the transient is powered by an accreting compact object. On the whole, the appearance of our spectra is consistent with the source being a cataclysmic variable, i.e. an accreting white dwarf binary system. If so, the large outburst amplitude suggests that the transient is a dwarf nova of the SU UMa class caught in superoutburst. The absence of any absorption features and the weakness of high-ionization lines may be slightly unusual in this case, but not unprecedented (e.g. V455 And; Nogami et al. 2009, ASP Conf. Ser 404; p52). However, the possible association of the optical transient with a radio source remains intriguing. Even though radio emission from a putative jet has been reported in the proto-typical dwarf nova SS Cyg (Koerding et al. 2008, Science, 320, 1318), it is more commonly observed in compact binary systems containing accreting neutron stars and black holes. We therefore note that some such systems (the so-called soft X-ray transients or X-ray novae) can undergo optical outbursts of similar amplitude to the superoutbursts of SU UMa stars (e.g. GRO J0422+32; Kuulkers, Howell & van Paradijs 1996, ApJL, 462, L87). Considering also the fairly extreme width of the Balmer emission lines in our data, the nature of the compact object in SSS130101:122222-311525 is still not definitively established. Multi-wavelength follow-up observations of the transient are strongly encouraged.

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