Recent fading of AT2020iko to near quiescent levels, and search for a high energy counterpart

ATel #14133; 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; 14:19 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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In ATel #14132 we reported our monitoring of the transient AT2020iko in optical g' and R bands with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes from April to June 2020. Our last observation in June before a Sun constraint showed the source at a magnitude of g' = 20.35+-0.02, R = 19.96+-0.02 on June 23 (MJD 59023). After the Sun constraint in July-September, we report recent magnitudes of g' = 21.51 +- 0.22; R = 20.88 +- 0.21 on October 17 (MJD 59139) and g' = 22.32 +- 0.19; R = 21.71+- 0.19 on October 23 (MJD 59145). This is the faintest the source has appeared since discovery in April. The latest magnitudes are in line with the baseline trend of decreasing flux between the peaks of the first outbursts.

The source was originally found to lie within 0.5" of the position of an object classified as a galaxy, SDSS J090326.23+473159.5, and so the transient was previously classified as possibly an extragalactic transient (ATel #13706). However, the source has since been classified as a CV (Soraisam et al. 2020; ATel #14132). The unusual light curve and colours from the first outburst could have favoured a fast blue optical transient (FBOT) if the source was extragalactic, or a local LBV-like outburst/"supernova imposter". However, the multiple re-brightenings, of amplitude ~ 3-4 mag are very unusual for an extragalactic transient, and hence is more likely to be a CV.

Positional calibration using XB-NEWS gives coordinates of the transient of RA = 09:03:26.23, Dec = +47:31:59.1 (J2000), which differs from the SDSS position by 0.44" and is consistent with the ZTF position and within 0.7" of a point source with g' = 23.27 seen in the DESI Legacy Imaging Surveys (Soraisam et al. 2020). We note that no source at the SDSS position or XB-NEWS position exists in Pan-STARRS, with a magnitude upper limit of g' > 23.3, which is fainter than the SDSS magnitude of g' = 21.95 +- 0.15. This suggests the transient may have been active but faint during the SDSS epoch, or there may have been confusion between the point source and the extended source in SDSS. Our latest magnitudes indicate the source has faded to fainter levels than the SDSS detection. This seems to support the possible subclassification of the source as a WZ Sge type CV due to its outburst amplitude, as suggested in ATel #14132 from the PanSTARRS magnitude limit, and in Soraisam et al. (2020).

In addition, we checked for high energy counterparts. A Neil Gehrels Swift Observatory observation was taken on May 13 (MJD 58982). We analyzed the XRT and UVOT observations, resulting in an X-ray flux upper limit of < 0.132 counts/s (0.3-10 keV, which translates to an upper limit of 6E-12 erg/cm2/s for a conservative power-law index of 1.5) and a UVOT uvm2 magnitude (AB system) of 20.47 +/- 0.18 (see also Soraisam et al. 2020). No FERMI gamma-ray counterpart was detected.

We encourage further observations of the source in case there is a new re-brightening. 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 2