The pulsed X - ray light curves of the isolated neutron star RBS 1223 ⋆

We present multi-epoch spectral and timing analysis of the isolated neutron star RBS1223. New XMM-Newton data obtained in January 2004 confirm the spin period to be twice as long as previously thought, P spin = 10.31 s. The combined ROSAT, Chandra, XMM-Newton data (6 epochs) give, contrary to earlier findings, no clear indication of a spin evolution of the neutron star. The X-ray light curves are double-humped with pronounced hardness ratio variations suggesting an inhomo-geneous surface temperature with two spots separated by about ∼160 •. The sharpness of the two humps suggests a mildly relativistic star with a ratio between R ns , the neutron star radius at source, and r S , the Schwarzschild-radius, of R ns /r S > 3.5. Assuming Planckian energy distributions as local radiation sources, light curves were synthesized which were found to be in overall qualitative agreement with observed light curves in two different energy bands. The temperature distribution used was based on the crustal field models by Geppert et al. (2004) for a central temperature of T c = 10 8 K and an external dipolar field of B ∼ 10 13 G. This gives a mean atmospheric temperature of 55 eV. A much simpler model with two homogeneous spots with T ∞ = 92 eV, and 84 eV, and a cold rest star, T star,∞ < 45 eV, invisible at X-ray wavelengths, was found to be similarly successful. The new temperature determination and the new ˙ P spin suggest that the star is older than previously thought, T ≃ 10 5...6 yrs. The model-dependent distance to RBS1223 is estimated between 76 pc and 380 pc (for R ns = 12 km).