The nearby galaxy NGC 300 is hosting two luminous transient supersoft X-ray sources with bolometric luminosities above 3 \times 10 ^ { 38 } erg s ^ { -1 } , assuming simple black-body spectra with temperatures around 60–70 eV .
For one of these , SSS _ { 1 } , a periodic modulation of 5.4 h was observed in an XMM-Newton observation from 1st of January 2001 lasting 47 ks , but not visible 6 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 17 to 20 December 2016 lasting for 310 ks .
The luminosity was similar as in December 2000 , and the 0.2 - 2.0 keV light curve revealed again a periodic modulation , with a period of 4.68 \pm 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 \pm 1.1 h ) , the two values could be marginally compatible , and maybe associated with an orbital period , although the signal strength is highly variable .
Thanks to the new long exposures , an additional absorption feature is now visible in the spectra , that we modelled with an absorption edge .
This component decreases the bolometric luminosity below 3 \times 10 ^ { 38 } 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 8 years .
We discuss viable models involving white dwarfs , neutron stars or black holes .