Swift detection of the start of the supersoft source phase of U Scorpii (2022)
ATel #15438; K. L. Page, P. A. Evans and A. P. Beardmore (U. Leicester)
on 16 Jun 2022; 12:54 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
The Neil Gehrels Swift Observatory started observing U Scorpii on 2022
June 07, 11.4 hr after the discovery by M. Moriyama on 2022 June 6.720
(vsnet alert 26798), or 1.7 hr after the optical peak of on June 7.125
(ATel #15423). Multiple daily observations were obtained until U Sco entered Swift's Moon proximity observing constraint on June 11, with observations
starting again on June 15.
On June 16, in a snapshot starting at 05:04 UT (9.5 days after discovery; 9.1
days after optical peak), an X-ray source was
detected at a count rate of 0.022 +/- 0.004 count s-1. Of
the 17 photons detected in a 10 pixel radius circle, 15 are below 0.5
keV, indicating the source is in the supersoft phase. A fit to this
spectrum suggests a blackbody temperature of ~50 eV; given the small number
of photons, we cannot be more precise at this time. Using this fit,
the count rate corresponds to an observed 0.3-10 keV flux of 9.4 x
10-13 erg cm-2 s-1.
Combining all the data (~14 ks) before June 16, we found a 3-sigma
upper limit of 0.0018 count s-1 (equivalent to 1.5 x
10-13 erg cm-2 s-1, assuming a
non-supersoft spectrum).
ATel #15436 reports a NICER detection of 2 x 10-12 erg
cm-2 s-1 (0.22-10 keV) in data starting on 2022
June 8, with most of this flux above 1 keV. Combining the ~6 ks of
Swift-XRT data collected between June 8 and 15, we find no X-ray
detection down to a 3-sigma upper limit of 0.0028 count s-1
(2.3 x 10-13 erg cm-2 s-1, again assuming a non-supersoft spectrum) over 0.3-10 keV.
Swift will continue to monitor U Sco several times a day. We thank the
Swift PI and his deputies for approving these ongoing observations,
and the science operations team for their work in scheduling them.
