S-CUBED Detects Supersoft X-ray Counterpart to the SMC Nova ASASSN-26dt

ATel #18087; Thomas M. Gaudin (PSU), Jamie A. Kennea (Schmidt Sciences), Malcolm J. Coe (Southampton University), Kim L. Page (University of Leicester), Itumaleng Monageng (UCT/SAAO), David A. H. Buckley (SAAO/UCT), Phil A. Evans (University of Leicester)
on 3 Oct 2026; 12:18 UT
Credential Certification: Thomas Gaudin (tmg6006@psu.edu)

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An X-ray counterpart to the Small Magellanic Cloud (SMC) nova ASASSN-26dt, also known as AT 2026oyp, has been detected for the first time by an observation of the Swift SMC Survey (S-CUBED). The first detection occurred during a regularly-scheduled shallow monitoring observation that was conducted at 10:33 UTC on 22 September 2026. During this observation, the source was detected at a 0.3-10 keV count rate of 1.84 counts/s by the Swift X-ray Telescope (XRT). ASASSN-26dt was then detected at a count rate of 1.43 counts/s by S-CUBED during an observation conducted at 22:50 UTC on 29 September 2026. Based on these two detections, the S-CUBED position for the source is RA (J2000): 00h 50m 45.06s (12.68774), Dec (J2000): -71° 38' 32.2" (-71.6423) with a 90% confidence error region of 5.8". The only source inside the XRT error region is Gaia 4689290962169940224 which was noted as the unusual progenitor star of the nova as reported in ATEL #17878.

A follow-up Swift observation was conducted with XRT in Window Timing (WT) mode and the Ultraviolet/Optical Telescope (UVOT) in 0x30ed (six filter, blue-weighted imaging) mode on 2 October 2026. We detect ASASSN-26dt as a supersoft source with the vast majority of the counts being produced by photons with energies < 0.8 keV. The supersoft spectrum is best fit by an absorbed thermal blackbody spectrum. The count rate of the source during this observation was 1.96 counts/s which corresponds to a 0.3 - 1 keV luminosity of Lx = 2.95 (+2.86, -1.23) x 10^38 erg/s assuming a standard distance of 62.44 kpc (Graczyk et al. 2020) to the SMC. The best-fitting effective stellar temperature of this blackbody is Teff = 394000 (+/- 37000) K. Swift UVOT detects the source in all bands with decreasing brightness from UVW2-band to V-band. The magnitude of the source in each filter is: UVW2=14.01, UVM2=14.14, UVW1=14.38, U=14.99, B=16.34, V=16.53.

Based on this spectrum, it is clear that this nova has entered the supersoft X-ray source (SSS) phase. This is the first detection of an X-ray counterpart to the nova, but it is unclear when the SSS phase began. The 22 September detection of ASASSN-26dt occurred during the first weekly run of S-CUBED since Swift entered drag reduction mode in early February. Thus, no X-ray monitoring exists for the period between the onset of the nova in June and the first S-CUBED detection. Our best constraint for the onset is <106d after the optical peak of the nova. An X-ray luminosity of ~3 x 10^38 erg/s roughly corresponds to the maximum luminosity of the SSS phase that was found by Soraisam et al. (2016) in an analysis of extragalactic novae.

More follow-up observations have been requested from the Swift Observatory and the SALT telescope. We encourage further observations at all wavelengths to improve our understanding of this unusual nova.