X‐Ray Emission from the Old Pulsar B0950+08

Abstract

We present the timing and spectral analyses of the XMM-Newton data on the 17 Myr old, nearby radio pulsar B0950+08. This observation revealed pulsations of the X-ray flux of PSR B0950+08 at its radio period, P ’ 253 ms. The pulse shape and pulsed fraction are apparently different at lower and higher energies of the observed 0.2‐10 keVenergy range, which suggests that the radiation cannot be explained by a single emission mechanism. The X-ray spectrum of the pulsar can be fitted with a power-law model with a photon index ¼1:75 � 0:15 and an (isotropic) luminosity LX ¼ (9:8 � 0:2) ; 10 29 ergs s � 1 in the 0.2‐10 keV range. Better fits are obtained with two-component, power-law plus thermal models with ¼1:30 � 0:10 and LX ¼ (9:7 � 0:1) ;10 29 ergs s � 1 for the power-law component, which presumably originates from the pulsar’s magnetosphere. The thermal component, dominating at EP0:7 keV, can be interpreted as radiation from heated polar caps on the neutron star surface covered with a hydrogen atmosphere. The inferred effective temperature, radius, and bolometric luminosity of the polar caps are TPC � 1M K,RPC � 250 m, and LPC � 3 ;1029 ergs s � 1 . Optical through X-ray nonthermal spectrum of the pulsar can be described as a single power law with ¼1:3 1:4 for the two-component X-ray fit. The ratio of the nonthermal X-ray (1‐10 keV) luminosity to the nonthermal optical (4000‐9000 8) luminosity, � 360, is within the range of 10 2 103 observed for younger pulsars, which suggests that the magnetospheric X-ray and optical emissions are powered by the same mechanism in all pulsars, independent of age and spin-down power. Assuming a standard neutron star radius, the upper limit on the temperature of the bulk of the neutron star surface, inferred from the optical and X-ray data, is about 0.15 MK. We also analyze X-ray observations of several other old pulsars, B2224+65, J2043+2740, B0628� 28, B1813� 36, B1929+10, and B0823+26, and compare their properties with those of PSR B0950+08.