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

Abstract

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 $(0.8-0.9)\times 10^{6}K$ within a cap radius of $0.7-0.9km$. 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 $(1-2)\times 10^{6}K$ within a radius of $0.2-0.4km$, surrounded by a colder rim with temperatures $(3-5)\times 10^{5}K$ extending out to $2-6km$. The polar cap interpretation implies low column densities, $(1-3)\times 10^{19}c{m}^{-2}$, and a high degree of ionization, $>20$%, of the interstellar hydrogen towards the pulsar. The inferred bolometric luminosity of the polar caps, $(1.0-1.6)\times 10^{30}erg{s}^{-1}$, 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.

17 pages with 11 figures l-aa.sty, accepted for publication in Astronomy and Astrophysics

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