A possible X-ray detection of the binary millisecond pulsar J1012+5307

A possible X-ray detection of the newly discovered binary millisecond radio pulsar PSR J1012+5307 was obtained from an archival ROSAT observation. The 80 +/- 24 photons detected correspond to a 0.1-2.4 keV luminosity of approximate to 2.5 x 10(30) ergs s(-1) at the nominal dispersion measure distance of 520 pc. This luminosity is a factor of 2 less than that of PSR J0437-4715, a near twin of PSR J1012+5307 in its spin parameters and energetics, and the only millisecond pulsar from which pulsed X-rays have definitely been detected. PSR J1012+5307 is also within 6 degrees of the ''H I hole'' in Ursa Major, providing a new estimate of the electron column density through this region, which confirms that the ionized column density is also low. The small neutral column density to PSR J1012+5307; N-H < 7.5 x 10(19) cm(-2), will facilitate future soft X-ray study, which will help to discriminate between thermal and nonthermal origins of the X-ray emission in millisecond pulsars.