V488 Sge has become a super-soft X-ray source
ATel #18084; K. L. Page (U. Leicester) and G. J. M. Luna (UNAHUR/CONICET/Argentina)
on 1 Oct 2026; 19:09 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
V488 Sge (Nova Sagittae 2026) was discovered on 2026 Aug. 25.1278 UTC,
and classified as a classical Fe II-type nova (CBET #5729, ATel #18009,
#18008, ,#18012, #18014, #18021, #18022, #18033, #18053). An
observation by NuSTAR on 2026 Aug. 31 detected shock-powered X-rays
(ATel #18028).
Swift observations were requested when the ToO programme was re-opened,
with the first observation occurring on Sept. 5, showing a
faint, hard X-ray source with a count rate of 0.037 +0.004/-0.003
(0.3-10 keV). Further observations on Sept. 7, 12 and 21 showed a
similarly hard spectrum (kT = 7.7 +11.7/-3.3 keV jointly fitted between
all four observations), with evidence for changing absorption with
NH = (1.2 +0.6/-0.5)x1022 cm-2 on
Sept. 5 declining to (2.1 +1.4/-1.0)x1021 cm-2
on Sept. 11.
The most recent observation, obtained on Sept. 27/28, revealed a
significant increase in soft (<1 keV) counts, indicating the start of
the super-soft source (SSS) phase. This new component was not detected
in the first two snapshots collected on Sept. 27 between 05:03-11:13
UT, but became evident in the third during 20:10-20:22 UT, before
disappearing again before the fourth (Sept. 28 05:30-05:49 UT), indicating
variability during the switch on of the SSS as has been seen in a
number of other novae observed by Swift. The soft component can be
approximated by a BB model of kT = 44 +13/-11 eV and NH =
(1.6 +1.3/-0.8) x1021 cm-2; the observed flux of
this spectrum is 5.68 x 10-12 erg cm-2
s-1 (0.3-10 keV).
We thank the Swift PI, Brad Cenko, and the Swift planning and
operations teams, for their support over the past 22 years, and as the
mission sadly draws to a close.
