Three 6-Magnitude Brightenings of the Candidate Dwarf Nova / Polar AT2018bby / ZTF20aammhxq / ATLAS18nuj

ATel #14843; Marco C. Lam (TAU), Eamonn Harvey (LJMU), Megan Newsome (UCSB/Las Cumbres), Yael Dgany (TAU), Matthew Green (TAU), Iair Arcavi (TAU), Paul Ross Mcwhirter (LJMU)
on 10 Aug 2021; 11:46 UT
Credential Certification: Iair Arcavi (arcavi@gmail.com)

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We report a candidate dwarf nova/polar/intermediate polar system (under the transient designations: AT2018bby, ZTF20aammhxq, and ATLAS18nuj) with three detected outbursts at an amplitude of ~6 magnitudes. Low-state durations are ~600 days. The object was reported as a candidate hot white dwarf in Bianchi et al. (2011) with Galex FUV=22.56 and NUV=22.95, and SDSS u=22.53, g=22.98, r=22.88, i=21.85, and z=22.83.

The first outburst was reported as supernova candidate ATLAS18nuj, reaching a maximum brightness of o=17.33 at UT 2018-04-26 11:39:50. The second and third outbursts were observed by ZTF reaching r=16.955 at UT 2020-02-09 12:08:54 and r=17.391 at UT 2021-08-04 05:29:02 at maximum. It has not been studied for variability in its low state.

We obtained a low-resolution spectrum with the FLOYDS spectrograph mounted on the Las Cumbres 2-meter Faulkes Telescope North on Haleakala, Hawaii, at UT 2021-08-07 06:22:38. The spectrum shows weak H-alpha, some He emission lines, and from H-beta to H-zeta, there are either emission lines on top of the absorption profiles, or Zeeman splitting features, suggesting a polar or intermediate polar system. H-alpha is seen weakly in emission. The spectrum is publicly available on the Transient Name Server. The emission is expected to be mainly from the accretion disk and white dwarf, as it is not typical of a black body.

The spectrum also shows several lines in emission, possibly associated with Fe I and He I, whereas He II at 4686A is not observed. The spectrum resembles the outburst spectra of U Gem, AR And and RZ LMi. Similarly to the candidate presented here, AR And lacks He II 4686A, but has He I in emission and is of the SS Cygni type dwarf nova subcategory.

Follow-up observations are encouraged at other wavelengths, especially radio and X-ray. High-resolution spectroscopy in the optical blue regime and polarimetric measurements are also encouraged. Photometry and spectroscopy of the object in its low-state are recommended in order to classify the system within the dwarf nova population.

Image of the FLOYDS Spectrum