XMM–Newton observation of the persistent Be/NS X–ray binary pulsar RX J1037.5–5647 in a low luminosity state

The spectra of several X–ray binary pulsars display a clear soft excess, which in most cases can be described with a blackbody model, above the main power–law component. While in the high–luminosity sources it is usually characterized by low temperature (kT < 0.5 keV) and large emission radius (R > 100 km), in the two persistent and low–luminosity pulsars 4U 0352+309 and RX J0146.9+6121 this component has a high temperature (kT > 1 keV) and a smaller radius (R < 0.5 km), consistent with the estimated size of the neutron–star polar cap. Here we report on the timing and spectral analysis of RX J1037.5–5647, another low–luminosity persistent Be binary pulsar, based on the first XMM–Newton observation of this source. We have found a best–fit period P = 853.4 ± 0.2 s, that implies an average pulsar spin–up Ṗ ≃ −2 × 10 s s in the latest decade. The estimated source luminosity is LX ∼ 10 erg s, a value comparable to that of the other persistent Be binary pulsars and about one order of magnitude lower than in most of the previous measurements. The source spectrum can be described with a power law plus blackbody model, with kTBB = 1.26 −0.09 keV and RBB = 128 +13 −21 m, suggesting a polar–cap origin of this component. These results strengthen the hypothesis that, in addition to low luminosities and long periods, this class of sources is characterized also by common spectral properties.