We report on the XMM–Newton observation of the Be/neutron star X–ray binary system RX J0146.9+6121 , a long period ( \sim 23 m ) pulsar in the NGC 663 open cluster .
The X–ray luminosity decreased by a factor two compared to the last observation carried out in 1998 , reaching a level of \sim 1 \times 10 ^ { 34 } erg s ^ { -1 } , the lowest ever observed in this source .
The spectral analysis reveals the presence of a significant excess at low energies over the main power–law spectral component .
The soft excess can be described by a black–body spectrum with a temperature of about 1 keV and an emitting region with a radius of \sim 140 m. Although the current data do not permit to ascertain whether the soft excess is pulsed or not , its properties are consistent with emission from the neutron star polar cap .
This is the third detection of a soft excess in a low luminosity ( \sim 1 \times 10 ^ { 34 } erg s ^ { -1 } ) pulsar , the other being X Per and 3A 0535+262 , suggesting that such spectral component , observed up to date in higher luminosity systems , is a rather common feature of accreting X–ray pulsars .
The results on these three sources indicate that , in low luminosity systems , the soft excess tends to have a higher temperature and a smaller surface area than in the high luminosity ones .