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 c an be interpreted as thermal radiation from two hot polar caps whose e mitting layers (atmospheres) are comprised of hydrogen. The simplest m odel yields a uniform temperature of (0.8 - 0.9) x 10(6) K within a ca p radius of 0.7 - 0.9 km. The spectral fits indicate that the temperat ure may be nonuniformly distributed along the cap surface. The distrib ution can be approximated by a central core heated up to (1 - 2) x 10( 6) K within a radius of 0.2 - 0.4 km, surrounded by a colder rim with temperatures (3 - 5) x 10(5) K extending out to 2 - 6 km. The polar ca p interpretation implies low column densities, (1 - 3) x 10(19) cm(-2) , and a high degree of ionization, > 20%, of the interstellar hydrogen towards the pulsar. The inferred bolometric luminosity of the polar c aps, (1.0 - 1.6) x 10(30) erg s(-1), is in excellent agreement with th e predictions of the slot-gap model of radio pulsars developed by Aron s 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.