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Possible start of the supersoft source phase in RS Oph

ATel #14885; K. L. Page (U. Leicester)
on 30 Aug 2021; 12:35 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

Subjects: X-ray, Nova

Referred to by ATel #: 14894, 14895

The Neil Gehrels Swift Observatory has been observing RS Oph approximately daily since the announcement of the latest eruption on 2021 August 08 (K. Geary; vsnet-alert 26131). The nova has been widely followed and detected across the electromagnetic spectrum, from gamma-rays and X-rays (ATels #14834, #14844, #14845, #14846, #14850, #14855, #14857, #14864, #14872, #14873, #14882) to infrared and optical (ATels #14838, #14840, #14852, #14858, #14860, #14863, #14866, #14868, #14881, #14883) to radio (ATel #14849). IceCube also checked for coincident neutrinos, though none was found (ATel #14851).

Following the first two Swift observations between which the X-rays were seen to brighten (ATel #14848), the X-ray count rate continued to increase, reaching a maximum of ~16 count s-1 around six days after the optical peak (estimated to be 2021 August 09 from AAVSO data). Following a three day gap in Swift observations due to the Moon observing constraint, the count rate was found to have decreased, to ~14 count s-1. Since this time, the fading in X-rays has been very gradual, with the count rate currently (August 30; day 20) around 9-10 count s-1.

The two snapshots of data collected on 2021 August 30 both show a slight increase in the soft counts below 0.6 keV. While there has been a steady decrease in the absorbing column required to fit the X-ray data (as expected, as the shock expands through the red giant companion; Sokoloski et al., 2006 Nature, 422, 276; Bode et al., 2006, ApJ, 652, 629), leading to an overall softening, the shape of these latest spectra is different, suggesting the appearance of a new soft component. While this apparent soft excess is only weak, we suggest it may be the beginning of the supersoft source phase, which was seen to be highly variable following the 2006 eruption of RS Oph (Osborne et al., 2011, ApJ, 727, 124).

Regular monitoring by Swift will continue, and we encourage observations at other wavelengths.

We thank the Swift PI and his deputies for approving these observations, as well as the science operations team for implementing them.