RS Oph has entered the Supersoft phase
ATel #14894; K. L. Page (U. Leicester)
on 5 Sep 2021; 06:06 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
Following the announcement in ATel #14885 of weak soft X-ray emission beginning to appear in the Swift XRT spectra of RS Oph from 2021 August 30, we can now confirm that RS Oph has entered the supersoft source (SSS) phase.
From 2021 September 01, the cadence of Swift observations was increased to twice a day, in order to monitor the possible new component. While this soft emission remained weak for five days, observations on September 04 show a strong increase in counts below 0.7 keV, which further brightened between the two observations performed around 10 UT (day 25.9, taking T0 to be the optical peak of the eruption, estimated to be around 2021 August 09.5417 from the AAVSO light-curve) and 20 UT (day 26.3) that day, causing the 0.3-10 keV count rate to increase from ~8 count s-1 to ~11 count s-1.
This soft component can be simply characterised by a blackbody with BB kT ~ 40 eV (assuming a total column density of 4 x 1021 cm-2, approximated from a single optically-thin component fit to the spectra just before the first sign of the enhanced soft emission), with the corresponding observed 0.3-1 keV flux increasing from 1.6x10-11 to 9.8x10-11 erg cm-2 s-1 between the two snapshots of data on September 04.
We note that, during the 2006 eruption of RS Oph, the first strong SSS component was detected on day 29, following a gap in observations from day 26, when there was a similarly weak component detected as shown in the spectra this past week (Osborne et al., 2011, ApJ, 727, 124). Thus far, the timescales for the X-ray evolution between the 2006 and 2021 eruptions appear similar.
We are grateful to the Swift PI and his deputies, as well as the science operations team, for their help in obtaining these data. Swift will continue to monitor RS Oph as the SSS emission evolves, and we encourage follow-up with other instruments and at all wavelengths to add to the dataset already stretching from gamma-rays to radio (ATels #14834, #14838, #14840, #14844, #14845, #14846, #14848, #14849, #14850, #14852, #14855, #14857, #14858, #14860, #14863, #14864, #14866, #14868, #14872, #14873, #14881, #14882, #14883, #14885, #14886).