WHT observations of Gaia16ara (=PS16ccm): A WZ Sge-type dwarf nova?

ATel #9226; F. Onori, D. Lena, Z. Kostrzewa-Rutkowska, M. A. P. Torres and P. G. Jonker (SRON, Nijmegen RU), M. Fraser (IoA), T. Wevers (Nijmegen RU).
on 8 Jul 2016; 18:06 UT
Credential Certification: Francesca Onori (f.onori@sron.nl)

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We present spectroscopic and photometric observations of a transient object discovered by Gaia on 2016 May 30 UT at G=18.4 called Gaia16ara. This variable source was previously identified by Pan-STARRS1 at r=17.4 on 2016 Apr 27 UT and it is not detected in pre-discovery SDSS.

In order to classify the optical transient, we observed Gaia16ara on 2016 Jul 4 UT with the Intermediate dispersion Spectrograph and Imaging System (ISIS) on the 4.2m William Herschel Telescope (WHT). Three 1200s exposures were taken with the R600B and R600R gratings, covering the 3700-4750 AA and 5600-7000 AA spectral range. A 0".8 slit width was used, providing a resolving power R = 2500 and R = 4000 for the blue and red ISIS arms, respectively. In addition, five 60s r-band images of the field containing Gaia16ara were obtained during the same night with the auxiliary-port camera (ACAM) at the WHT. These data sets were reduced with standard IRAF tasks. The offset in the wavelength calibration zero point is < 0.3 AA.

The red-arm spectra show Halpha in emission, centered at ~6559 AA with a FWHM of ~ 370 km/s and EW of ~ 23 AA. The line profile is well fitted with a 2-Gaussian component model with a separation of ~200 km/s. In addition, the emission appears to be contained in a shallow and broad absorption component. Weaker (EW < 1 AA) HeI 5875,6678 emission lines are present. The blue-arm spectra show clearly broad absorption from the Balmer series (Hgamma up to Heta) with FWZI ranging 3280 to 6150 km/s. Furthermore they show apparent emission components filling the absorption line cores. There is marginal evidence for weak HeII 4686 in emission. These spectra are reminiscent of those observed in the WZ Sge-type dwarf nova GW Lib in outburst. (see e.g., van Spaandonk et al. ,2010,MNRAS,401,1857).

Differential photometry with respect to SDSS objects in the ACAM field of view delivers r=18.805 +/- 0.008 for Gaia16ara. This magnitude is well above the detection limit in the pre-discovery SDSS images (r ~ 22), indicating that Gaia16ara is still active. The duration of the transient event is > 2 months and the outburst amplitude > 5 magnitudes.

The spectroscopic characteristics, together with both the duration and amplitude of the optical outburst could be consistent with Gaia16ara being a WZ Sge-type dwarf nova in superoutburst. Alternatively, the long outburst duration could be compatible with a black hole X-ray transient in a (faint) low-hard state (see e.g. . the light curves for XTE J1118+480, Uemura et al. 2001,PASJ,52,15; Zurita et al.,2007,ApJ,644,342)

We encourage X-ray observations to investigate the X-ray properties of this source and time-resolved photometry to measure the orbital and superhump periods.

We acknowledge ESA Gaia (http://cosmos.esa.int/gaia), and the DPAC Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/alerts/alertsindex) (Rixon et al, 2014, ATel #6593).