XMM-Newton Observations during the supersoft phase of nova V488 Sge

ATel #18135; Natalia Edith Nuñez (ICATE-UNSJ), Gerardo Juan Manuel Luna (UNAHUR-CONICET), Jan-Uwe Ness (ESAC), Marina Orio (U. Wisconsin - INAF) and Kim Page (U. Leicester)
on 31 Oct 2026; 13:49 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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We report X-ray and ultraviolet observations of the recent nova V488 Sge obtained with the XMM-Newton satellite on October 21 2026, 57 days after optical maximum (ObsID: 0992191201). The EPIC instruments (MOS1 and MOS2) and the RGS grating spectrometers were operated in Small Window mode with the Medium Filter, while the Optical Monitor (OM) operated in fast and image modes using the UVW2 ultraviolet filter.

V488 Sge was discovered on August 25, 2026 (Barnhart et al. 2026, ATel #18009). NuSTAR detected shock-powered X-ray emission later that month (Sokolovsky et al. 2026, ATel#1028). Subsequent Swift observations in September revealed a hard X-ray source with declining absorption, culminating in the detection of the onset of the super-soft source (SSS) phase between September 27 and 28, which exhibited variability during its turn-on phase (Page & Luna, 2026 Atel #18084). The MOS1, MOS2, RGS1, and RGS2 light curves each show the same variability patterns with MOS1/2 varying between 5-15 counts per second (cps), RGS1/2 between 0.5-3.5 cps and no evident periodicities during the total exposure time of about 26 ks. The high-resolution spectrum obtained with the RGS reveals almost no continuum and shows several strong, broad, emission lines, such as the N VII line at ~25A. In spite of the complex nature of the RGS spectrum, the MOS spectra present themselves as smooth spectra with blackbody shape and can in fact be fitted with an absorbed blackbody model yielding kT~40eV and NH~4x10^21 cm^-2, similar to what Ness et al (2026, arXiv:2610.00762) have recently found for the early SSS spectra of RS Oph. We conclude that the RGS spectrum of V488 Sge may again be understood as an intermediate case between SSe and SSa (Ness et al. 2013, A&A, 559, A50).

Simultaneous light curves obtained with the OM show a mean magnitude in the UVW2 filter of ~12.9, with no indication of periodicities or large-amplitude flickering.

We thank the XMM-Newton Project Scientist, Dr. Erik Kuulkers, and his advisors for approving the time request, as well as the XMM-Newton planning and operations teams for their continuing support.