New outburst from the luminous supersoft source SSS1 in NGC 300 with periodic modulation

The nearby galaxy NGC 300 is hosting two luminous transient supersoft X-ray sources with bolometric luminosities above 3 × 10³⁸ erg s^-1, assuming simple blackbody spectra with temperatures around 60-70 eV. For one of these, SSS₁, a periodic modulation of 5.4 h was observed in an XMM-Newton observation from 2001 January 1 lasting 47 ks, but not visible six days earlier when the luminosity was higher. We report here the detection of a new outburst from this source, which occurred during two more recent XMM-Newton observations performed on 2016 December 17-20 lasting for 310 ks. The luminosity was similar as in 2000 December, and the 0.2-2.0 keV light curve revealed again a periodic modulation, with a period of 4.68 ± 0.26 h, significant only in the first of the two observations. Taking into account the large uncertainties (the 2001 period was re-estimated at 5.7 ± 1.1 h), the two values could be marginally compatible, and maybe associated with an orbital period, although the signal strength is highly variable. Due to the new long exposures, an additional absorption feature is now visible in the spectra, which we modelled with an absorption edge. This component decreases the bolometric luminosity below 3 × 10³⁸ erg s^-1 and would therefore allow the presence of a white dwarf with a mass close to the Chandrasekhar limit. The system was found in outburst in 1992, 2000, 2008, and 2016 suggesting a possible recurrence period of about eight years. We discuss viable models involving white dwarfs, neutron stars, or black holes.