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)
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.