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

Abstract

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 a 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, which implies an average pulsar spin-up dot P ≃ -2×10^-8 s s^-1 in the last decade. The estimated source luminosity is L_X 10³⁴ erg s^-1, a value comparable to 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 kT_BB = 1.26^+0.16_-0.09 keV and R_BB = 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 also characterized by common spectral properties.