XMM-Newton observation of the persistent Be/NS X-ray binary pulsar RX J0440.9+4431

Many X-ray accreting pulsars have a soft excess below 10 keV. This feature was also detected in faint sources and at low luminosity levels, suggesting that it is an ubiquitous phenomenon. For the high luminosity pulsars (LX > 1036 erg s-1), the fit to this component with thermal emission models usually provides low temperatures (kT < 0.5 keV) and large emission regions (R ≥ a few hundred km), hence it is referred to as a "soft" excess. Nevertheless, we previously found that in persistent, low-luminosity (LX ~ 1034 erg s-1) and long-period (P > 100 s) Be accreting pulsars the observed excess can be modeled with a rather hot (kTBB > 1 keV) black-body component of small area (RBB < 0.5 km), which can be interpreted as emission from the NS polar caps. In this paper, we present an analysis of an XMM-Newton observation of the Galactic Be pulsar RX J0440.9+4431, which is a poorly studied member of this class of sources. We find a best-fit period of P = 204.96 ± 0.02 s, which implies an average pulsar spin-down over the past 13 years of Ṗ ≃ 6 × 10-9 s s-1. The estimated source luminosity is LX ~ 8 × 1034 erg s-1, which is higher by a factor of less than ten compared to those obtained in the first source observations, but almost two orders of magnitude lower than those measured during the few outbursts that have been detected most recently. The source spectrum can be described with a power-law and black-body model, with kTBB = 1.34 ± 0.04 keV and RBB = 273 ± 16 m, suggesting a polar-cap origin of this component. Our results support the classification of RX J0440.9+4431 as a persistent Be/NS pulsar, and confirm that the hot black-body spectral component is a common property of this class of sources.