ASASSN-18pe: A new cataclysmic variable on a slow rise to outburst?

ATel #11867; J. Strader (Michigan St.), A. Bahramian (Curtin), M. J. Darnley (LJMU), L. Chomiuk, A. Kawash, E. Aydi (Michigan St.), K. Stanek, C. Kochanek (OSU), B. Shappee (Hawaii), & J. L. Prieto (UDP/MAS)
on 18 Jul 2018; 22:11 UT
Credential Certification: Jay Strader (strader@pa.msu.edu)

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We report the discovery of ASASSN-18pe, a new Galactic transient at J2000 coordinates of R.A.=17:15:21.54, Dec. = +06:00:27.2 . The source first appeared on 2018 July 10 with V=15.8 and has been rising steadily since then, reaching V=14.3 on 2018 July 17. ASAS-SN shows no previous variable or transient source at this location. The PS1 photometry for the source (Chambers et al 2016, arXiv:1612.05560) implies a mean quiescent mag of V~18.2, so a rise of at least 3.9 mag from quiescence so far and a total rise time of > 1 week. A link to the current ASAS-SN Sky Patrol light curve can be found below.

We obtained a 300 s optical spectrum with SPRAT on the Liverpool Telescope on 2018 July 17.9. While of modest signal-to-noise in the lines, the R~350 spectrum shows a blue continuum, with Hbeta, Hgamma, and perhaps Hdelta in absorption at z~0. No Halpha absorption is seen, perhaps because it is filled in by emission. There may be HeII 4686 in broad (possibly double-peaked) emission, but it is only weakly detected.

We also observed the source with Swift/XRT for 1 ksec on 2018 July 17.7. ASASSN-18pe was clearly detected but faint, with 13 events. A basic spectral analysis gives an X-ray flux of 5(-3/+7)e-13 erg/s/cm^2 (0.5-10 keV) for a power-law fit with a photon index of 2.5(1.0) and N_H = 5.8e20 cm^-2. This flux gives an X-ray luminosity of 6e31(d/kpc)^2 erg/s. A second Swift observation is planned for July 22.

The optical spectrum and probable low X-ray luminosity would normally suggest that the most likely explanation for ASASSN-18pe is a dwarf nova rising to outburst. However, the slow rise is unusual for a dwarf nova (e.g., Szkody & Mattei 1984, PASP, 96, 988). The rare systems with slow rises, such as GK Per, are generally interpreted to have long orbital periods and evolved secondaries (e.g., Crampton et al. 1986, ApJ, 300, 788). The modest foreground extinction (likely no more than A_V ~ 0.5) and faint quiescent magnitude would imply a distance of at least a few kpc if the secondary is evolved, in turn implying an X-ray luminosity of ~> 5e32 erg/s.

We emphasize that, while several aspects of the transient resemble a dwarf nova, the slow rise time and possible high luminosity suggest that other, more exotic possibilities, such as an unusual X-ray binary outburst, should be considered. Additional observations, including higher-resolution optical spectroscopy, are encouraged.

ASAS-SN Sky Patrol light curve