OGLE-LMC511.16.41797: A second transient supersoft X-ray source without a classical nova eruption

ATel #16373; P. Mroz (University of Warsaw), H. Szegedi (University of the Free State), K. L. Page (University of Leicester), P. A. Charles (University of Southampton), D. A.H. Buckley (South African Astronomical Observatory, University of Cape Town, University of the Free State), K. Krol, A. Udalski (University of Warsaw), P. J. Meintjes (University of the Free State)
on 8 Dec 2023; 19:29 UT
Credential Certification: Przemek Mroz (pmroz@astrouw.edu.pl)

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ASASSN-16oh was discovered as the first (and only) transient supersoft source without any signature of nova-like nuclear fusion taking place (Maccarone et al. 2019, Nature Astronomy 3, 173). Its unusual outburst light curve, narrow optical (Balmer and strong He II) emission lines, and X-ray spectrum led Maccarone et al. to propose that X-rays come from a spreading layer - a belt on the surface of the white dwarf near the inner edge of the accretion disk in which a large fraction of the total accretion energy is emitted. However, the nature of ASASSN-16oh remains controversial with an alternative interpretation being that of a non-ejecting nova outburst (Hillman et al. 2019, ApJL 879, 5; Kato et al. 2020, ApJ 892, 15).

We have carried out searches for objects showing similar optical outburst light curves in the Optical Gravitational Lensing Experiment (OGLE) survey archive. We identified ~30 new sources in the Large Magellanic Cloud (LMC). The full list will be published elsewhere (Mroz et al., in prep).

One of the objects, OGLE-LMC511.16.41797 (equatorial coordinates 04:59:56.68, -67:31:48.9; J2000) is currently in outburst. The OGLE data revealed five outbursts (in 2007, 2012, 2016, 2019, 2022); the quiescent magnitudes are I = 20.87 +/- 0.02 and V = 22.08 +/- 0.05. The current outburst started between Oct 15.3 UT and Oct 26.3 UT, 2023, gradually brightening by 2.6 mag to I = 18.271 +/- 0.021 on Dec 6.1 UT, 2023.

We obtained three low-resolution spectra of the object on Nov 22, Dec 2, and Dec 3, 2023 with the Robert Stobie Spectrograph (RSS) mounted on the South African Large Telescope (SALT), as part of the SALT Large Science Program on Transients. The spectra cover the range 3500-6700 A and reveal narrow (FWHM = 247 +/- 28 km/s) emission lines (Balmer lines, He II 4686 A). The Bowen blend (4640-4650 A) is also clearly detected from Dec 2 - 3, 2023. The emission lines are redshifted with a radial velocity of 307.9 +/- 7.9 km/s, close to that of the LMC. Both the light curve shape and the optical spectra do not match those of classical novae.

The object was also observed by the Neil Gehrels Swift Observatory on Dec 6, 2023 for about 1.6 ks. The Swift XRT detected a faint X-ray source, at 0.011 +0.003/-0.002 count s^-1 (0.3-10 keV), at the position of the transient. There are only 13 source photons detected, 8/13 photons are below 0.5 keV and 10/13 photons are below 1 keV. The X-ray spectrum can be fitted with a blackbody of kT = 54 +47/-33 eV absorbed by NH < 8.7e21 cm^-2. If we fix NH to the LMC value (1.3e21 cm^-2), then the temperature is slightly better constrained: kT = 54 +25/-18 eV. We caution that, given the small number of photons detected, the spectral fits are very uncertain. Assuming that the blackbody model describes the X-ray spectrum, the observed luminosity (0.3-10 keV) is approximately 1.1e35 erg s^-1, or 4.3e35 erg s^-1 unabsorbed, about an order of magnitude smaller than that of ASASSN-16oh.

The target was also observed by the Swift UVOT in the UVM2 filter. However, the nearest source detected in the UVOT images (UVM2 = 14.965 +/- 0.019 mag) is slightly offset from the target position by 4.0 arcsec and is likely a blend of the target and a nearby (5.4 arcsec) hot star.

Overall, the optical (light curve shape, narrow emission lines, strong He II emission line) and X-ray properties of OGLE-LMC511.16.41797 match well those of ASASSN-16oh.

Follow up observations in all bands are encouraged.

We thank the Neil Gehrels Swift Observatory for scheduling the observations.