A new luminous supersoft X - ray source in NGC 300

Context. We report the discovery of a new luminous supersoft source, XMMU J005455.0−374117, in the nearby spiral galaxy NGC 300, in XMM-Newton observations performed on 2005 May 22 and on 2005 November 25. The source is not present in ROSAT data nor in the previous XMM-Newton observations of 2000 December/2001 January. The unique luminous supersoft source, XMMU J005510.7−373855, detected in the 1992 May/June ROSAT data and in the 2000/2001 XMM-Newton data, fell below detectability. This source already appeared highly variable in ROSAT observations. Aims. We report on the temporal and spectral analysis of this new supersoft source (SSS) and compare its properties with the previous known SSS. Methods. We present the light curves of the SSS, model its spectrum and estimate the corresponding flux and luminosities. Results. The light curve of XMMU J005455.0−374117 does not show large fluctuations in any of the observations and its spectrum can be modelled with an absorbed blackbody with kT ∼ 60 eV. The corresponding bolometric luminosity is 8.1 +1.4 −4.5 × 10 38 erg s −1 in the first observation and drops to 2.2 +0.5 −1.4 × 10 38 erg s −1 six months later. No optical source brighter than m V ∼ 21.7 mag is found coincident with its position. Conclusions. The luminosity of these two SSSs is higher than what has been found for 'classical' SSSs. Their nature could be explained by beamed emission from steady nuclear burning of hydrogen onto white dwarfs, or accretion onto stellar-mass black hole with matter outflow or observed at high inclination angle. The presence of an intermediate-mass black hole seems unlikely in our case.