Soft X-rays from polar caps of the millisecond pulsar J0437-4715

We show that the soft X-ray spectra and light curves observed with the ROSAT and EUVE from the closest known millisecond pulsar J0437-4715 can be interpreted as thermal radiation from two hot polar caps whose emitting layers (atmospheres) are comprised of hydrogen. The simplest model yields a uniform temperature of (0.8 - 0.9) × 10⁶ K within a cap radius of 0.7 - 0.9 km. The spectral fits indicate that the temperature may be nonuniformly distributed along the cap surface. The distribution can be approximated by a central core heated up to (1 - 2) × 10⁶ K within a radius of 0.2 - 0.4 km, surrounded by a colder rim with temperatures (3 - 5) × 10⁵ K extending out to 2 - 6 km. The polar cap interpretation implies low column densities, (1 - 3) × 10¹⁹ cm^-2, and a high degree of ionization, > 20%, of the interstellar hydrogen towards the pulsar. The inferred bolometric luminosity of the polar caps, (1.0 - 1.6) × 10³⁰ erg s^-1, is in excellent agreement with the predictions of the slot-gap model of radio pulsars developed by Arons and his coworkers. Similar polar cap radiation should be emitted by other millisecond pulsars, although in some of them (e. g., PSR B1821-24) the soft X-ray flux is dominated by the nonthermal radiation from pulsar magnetospheres.