Many X–ray accreting pulsars have a soft excess below 10 keV .
This feature has been detected also in faint sources and at low luminosity levels , suggesting that it is an ubiquitous phenomenon .
In the case of the high luminosity pulsars ( L _ { X } > 10 ^ { 36 } erg s ^ { -1 } ) , the fit of this component with thermal emission models usually provides low temperatures ( kT < 0.5 keV ) and large emission regions ( R \geq a few hundred km ) ; for this reason , it is referred to as a ‘ soft ’ excess .
On the other hand , we recently found that in persistent , low–luminosity ( L _ { X } \sim 10 ^ { 34 } erg s ^ { -1 } ) and long–period ( P > 100 s ) Be accreting pulsars the observed excess can be modeled with a rather hot ( kT _ { BB } > 1 keV ) blackbody component of small area ( R _ { BB } < 0.5 km ) , which can be interpreted as emission from the NS polar caps .
In this paper we present the results of a recent XMM–Newton observation of the Galactic Be pulsar RX J0440.9+4431 , which is a poorly studied member of this class of sources .
We have found a best–fit period P = 204.96 \pm 0.02 s , which implies an average pulsar spin–down during the last 13 years , with \dot { P } \simeq 6 \times 10 ^ { -9 } s s ^ { -1 } .
The estimated source luminosity is L _ { X } \sim 8 \times 10 ^ { 34 } erg s ^ { -1 } : this value is higher by a factor < 10 compared to those obtained in the first source observations , but almost two orders of magnitude lower than those measured during a few outbursts detected in the latest years .
The source spectrum can be described with a power law plus blackbody model , with kT _ { BB } = 1.34 \pm 0.04 keV and R _ { BB } = 273 \pm 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 blackbody spectral component is a common property of this class of sources .