Independent classification of transient AT2020iko as a CV with a superoutburst and re-brightenings

ATel #14132; Eleonora Caruso (NYU Shanghai), David M. Russell (NYU Abu Dhabi), Elme Breedt (IoA, University of Cambridge), Maria Cristina Baglio, Payaswini Saikia (NYU Abu Dhabi), Maryam Modjaz, Tyler Pritchard (CCPP, NYU), D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 31 Oct 2020; 14:04 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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We report on the independent classification of transient AT2020iko (ATLAS20leg; ZTF20aawbodq) discovered on April 26, 2020 by the public ZTF survey, at a magnitude g'=16.74+/-0.03, preceded by non-detections 3 days before. The transient was also detected by ATLAS on April 27 with 18.56 mag in the orange-ATLAS filter, which corresponds to a broad 'r+i' filter. The source then faded rapidly (~1 mag/day) to around g'=20 mag, then suddenly re-brightened to R~16.5 mag on May 2 (ATel #13706). The source was initially classified as possibly an extragalactic transient, but Soraisam et al. (ATel #13706) encouraged further observations of the anomalous transient.

We monitored AT2020iko in optical g' and R bands with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes, between April 26 (MJD 58965) and June 23 (MJD 59023) before a Sun constraint (the light curve is linked below). Our light curve also includes data points from the ANTARES alert-broker, and ATel #13857. For the analysis of the LCO data, we are making use of the new real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details). We report several re-brightenings of the source, with peaks on May 16 (MJD 58985) and June 9 (MJD 59009), with magnitudes R=17.28+/- 0.004 and R=17.17+/-0.04 respectively. There is a small reflare between these peaks, on May 27 (MJD 58996).

The morphology of our light curve resembles those of Cataclysmic Variables (CVs). Specifically, we classify a superoutburst lasting from May 2 (MJD 58971) to May 9 (MJD 58978) followed by two smaller outbursts separated by ~ 25 days. The plateau of the superoutburst has a decay rate of ~ 0.1 mag/day. For the two main re-brightenings, the decay rate is 0.81 mag/day and 0.70 mag/day, respectively. The superoutburst implies that the transient is a SU UMa-type CV. The transient is bluer when it is brighter, ranging from optical colors of g'-R = -0.25 near the brightest peak, to g'-R = +0.45 at fainter magnitudes of g'=20.58. The color change is consistent with a blackbody varying between ~8,000 K and ~30,000 K, typical of thermal emission from accretion discs in CVs.

Another interesting feature of the lightcurve is the short-duration peak on April 26th (MJD 58965). It appears that the CV underwent an outburst and then faded by ~3 mag before the observed superoutburst. This is reminiscent of the "double superoutbursts" which are a characteristic feature of the WZ Sge-type CVs (Fig.9; Kato, 2015). The peak magnitude of this outburst (g'=16.18) puts the source in the faint tail of the magnitude distribution of WZ Sge stars (Fig.2; Kato, 2015). Using the PanSTARRS non-detection of g'>23.3, the outburst amplitude Delta_m > 7.1 indeed bears greater similarity to known WZ Sge stars (Fig.3; Kato, 2015).

In line with what Soraisam et al. (2020) concluded in their recent paper (arXiv:2010.14679), our light curve as well as the analyzed features suggest that AT2020iko can be classified as a CV. We note that the first re-brightening after the superoutburst reported by Soraisam et al. was brighter than shown in their light curve, and there was a further rebrightening of similar magnitude ~25 days later. The number of re-brightenings of the source rules out any type of supernova or extragalactic fast optical transient. This work makes use of observations from the Las Cumbres Observatory (LCO). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

AT2020iko light curve 1