Swift follow-up observations of AT2021afpi / MASTER OT J030227.28+191754.5
ATel #15115 (15 Dec 2021): V. V. Neustroev (U. Oulu), J. P. Osborne (U. Leicester), K. L. Page (U. Leicester)
We report the results of follow-up observations of AT2021afpi = MASTER OT J030227.28+191754.5 carried out with the Neil Gehrels Swift Observatory. The object was discovered on 2021 November 27 as a bright optical transient by MASTER (ATel #15067) as a result of follow-up observations of the IceCube neutrino alert IceCube-211125A (GCN #31126). However, it has been shown (ATel #15081) that AT2021afpi started brightening at least 8.5 hours before the IceCube-211125A trigger (November 24, 21:53 UT). Based on the large amplitude of the optical outburst (~10 mag), the transient was initially classified as a classical nova (ATel #15069). Nevertheless, the following optical spectroscopic and time-resolved photometric observations have confirmed that AT2021afpi is a very high amplitude WZ Sge-type dwarf nova (ATel #15072, #15074).
Swift started monitoring AT2021afpi 3.9 days after the transient discovery. Although initially bright, with an XRT count rate ~1.1 c/s (ATel #15073, #15087), AT2021afpi has rapidly faded for ~10 days before stabilizing at ~0.02 c/s on December 4-5. On December 11, the X-ray flux dropped to the level of about ~0.005 c/s, but returned back to the level of ~0.02 c/s by the time of the next observation on December 13. However, the optical and UV light curves show only a smooth slow decline and no response to this X-ray drop. So far, the X-ray and UV flux evolution of AT2021afpi is similar to that of the superoutbursts of SSS J122221.7-311525 and GW Lib (Neustroev+18 A&A 611 13).
An X-ray spectrum consisting of the data with the lowest XRT count rates < 0.03 c/s is also consistent with the outburst spectra of SSS J122221.7-311525 and GW Lib. It can be satisfactorily fitted with one optically thin emission component. However, the two first Swift/XRT observations show the presence below 0.8 keV of an additional strong soft component. Thus, 2 optically-thin components are needed for a satisfactory fit of these "higher-state" data (see also ATel #15087). We thank the Swift PI, Brad Cenko, for approving the observations, and the Swift planning and operations teams for their ongoing support.
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