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ASASSN-17oz shows short-term photometric activity

ATel #11027; Christopher Lloyd (Univ. of Sussex), David Boyd, Ian Miller, Roger Pickard (BAA VSS)
on 1 Dec 2017; 12:43 UT
Credential Certification: Christopher Lloyd (c.lloyd@sussex.ac.uk)

Subjects: Optical, Cataclysmic Variable, Transient

We report time-series photometry of the unusual transient ASASSN-17oz (Jayasinghe et al., 2017, ATel #10991) taken with 35-cm class telescopes at three locations in the UK. The observations were made from UT 2017-11-25.83 to -26.08 (JD = 2458083.33 - 2458083.58) in some cases under less than ideal conditions. The magnitudes are unfiltered (Clear) reduced relative to the V-magnitudes of an ensemble of comparison star drawn from the APASS catalogue and give a mean magnitude C(V) = 15.65(1) at UT 2017-11-26.0 (JD = 2458083.5). At this time the star was fading rapidly with a mean rate of 0.38(3) magnitudes/day and the latest light curve from the ASAS-SN project show that as of 2017-11-29 the star had returned to its pre-outburst level of V ~ 16.2.

The best run also shows a clear approximately sinusoidal variation superimposed on the general decline with a full amplitude of 0.03 magnitudes. The best period is 0.072(2) days but the second part of the run does not fully align so the data are better fitted with a period of about twice this value giving a double humped light curve. Unfortunately the difference is at the limit of detectability but the longer period of 0.134(5) days is preferred.

The other feature of the light curve is that it shows low but significant flickering as determined using the procedures of Fritz & Bruch (1998, A&A, 332, 586). Flickering is usually a diagnostic of an accretion process which in this case suggests that the star is a cataclysmic variable of some type. The blue continuum with the Balmer lines and He 5875 in emission also suggest that the star is a cataclysmic variable (ATel #10991). If that is the case then the light curve variation could be interpreted as orbital with either of the two periods depending on the relative brightness of the hot spot and the cool component.

Superficially the result of McCollum & Rottler (2017, ATel #11011) that the precursor object of ASASSN-17oz was spectral type K1-K2 III/IV or K3-K4 IV/V appears at odds with this interpretation. However, the spectral type is consistent with the cool components of CVs, and it is quite possible for the accretion to turn off at quiescence, and the system to go dark.