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 \sim 60 eV .
The corresponding bolometric luminosity is 8.1 ^ { +1.4 } _ { -4.5 } \times 10 ^ { 38 } \text { erg } \text { s } ^ { -1 } in the first observation and drops to 2.2 ^ { +0.5 } _ { -1.4 } \times 10 ^ { 38 } \text { erg } \text { s } ^ { -1 } six months later .
No optical source brighter than m _ { \text { V } } \sim 21.7 \text { 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 .