N. La Palombara, L. Sidoli, P. Esposito, A. Tiengo, S. Mereghetti
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 (Lx > 10^36 erg/s), the fit of this component with thermal
emission models usually provides low temperatures (kT < 0.5 keV) and large
emission regions (R > 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 (Lx ~ 10^34 erg/s) and long-period (P > 100 s) Be accreting
pulsars the observed excess can be modeled with a rather hot (kT > 1 keV)
blackbody component of small area (R < 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(+/-0.02) s, which implies an average pulsar spin-down during
the last 13 years, with dP/dt ~ 6x10^(-9) s/s. The estimated source luminosity
is Lx ~ 8x10^(34) erg/s: 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 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 blackbody spectral component is a common property of this class of
sources.
View original:
http://arxiv.org/abs/1112.5341
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