Discovery of two new persistent Be/X-ray pulsar systems

Abstract

We present RXTE observations of two recently identified massive X-ray binaries. RX J0440.9+4431/BSD 24-491 and RX J1037.5-564/LS 1698 are confirmed as accreting Be/X-ray systems following the discovery of X-ray pulsations, with barycentric pulse periods of 202.5$\pm$0.5 s and 860$\pm$2 s respectively. The X-ray spectral analysis shows that the energy spectra of the pulsars can be represented by a power-law, modified at low energy by an absorption component and at high energy by a cut-off. Very weak Fe lines may be present. Both sources appear to display a low cut-off energy when compared to typical X-ray pulsars, low X-ray variability (factor of < 10), and no dependence of the X-ray spectrum with energy. Given the similarity of these X-ray properties with those of the other persistent BeXRB pulsars, 4U0352+309/X Per and RX J0146.9+6121/LS I +61 235, we suggest that RX J0440.9+4431/BSD 24-491 and RX J1037.5-564/LS 1698 are also members of this subclass.