ROSAT Observations of Cooling Neutron Stars

Werner Becker and Joachim Trümper

The thermal evolution of neutron stars depends on the physical characteristics of neutron star matter at supernuclear densities. The possibility of altered hadronic interactions, the existence of stable pions, kaons, hyperons or free quarks in the core of a neutron star and the influence of superfluid neutrons in the inner crust all yield different cooling rates and theoretical predictions, so that the neutron star surface temperature as a function of the neutron star age reflects the stellar composition. Comparing the neutron star temperatures and temperature upper limits measured by ROSAT with the theoretical predictions based on different equations of state thus provide the empirical basis essential for the verification of neutron star models and cooling theories.

Making use of the good spatial resolution of the ROSAT High Resolution Imager we have observed the Crab-pulsar emission (Becker & Aschenbach 1995) and PSR 1509-58 in MSH 15-52 (Becker & Trümper 1997), which are both surrounded by strong synchrotron nebulae. In both cases the pulsar radiation is dominated by magnetospheric emission. The DC component was used to derive upper limits for the photospheric emission (cf. Fig.1).

Four pulsars (Vela, PSR 0656+14, Geminga and PSR 1055-52) show a two component behaviour of their X-ray emission: a hard power-law component which can be attributed to magnetospheric emission and a soft phase shifted component which is interpreted as photospheric emission. The corresponding bolometric,photospheric luminosities tex2html_wrap_inline35 are shown in Figure 1.

In the supernova remnants RCW 103, Puppis A and PKS 1207-51 point sources have been found showing luminosities and temperatures compatible with cooling neutron stars. For comparison, the cooling curves for standard and accelerated neutron star cooling (FP-Model) including frictional heating (J1-J3) are shown (Umeda et al. 1993). The figure demonstrates that the thermal emission from the observed objects is largely consistent with the prediction of standard cooling for a neutron star with a medium stiff equation of state.

Becker,W., Aschenbach, B., in The Lives of Neutron Stars, eds. Alpar, Kiziuglu & van Paradijse, 1995.
Becker,W., Trümper, J., A&A, 1997, 326, 682-691
Umeda, H., et al., ApJ, 1993, 408, 286.