OPTICAL SPECTROSCOPY AND PHOTOMETRY OF THE COMPANION OF THE BRIGHT MILLISECOND PULSAR J-0437-4715

Absolute astrometry on two Schmidt sky survey plates and relative astr ometry in a new CCD image identify a faint star as the optical counter part of the recently discovered nearby millisecond and low-mass binary pulsar PSR J 0437-4715. The star lies within 0.2+/-0.4 arcsec of the pulsar's radio position and has a proper motion of 0.11+/-0.04 arcsec yr-1. Spectra show the star to be a DC-type degenerate red star with a n approximate temperature of 4000 K. The absolute luminosity of such a star and its apparent V magnitude of 20.6 agree well with die radio d istance based on the dispersion measure. Depending on assumptions made about the luminosity, the most probable range for the cooling age of the companion is 0.9-3.7 Gyr, i.e. longer than the published spin-down age of 0.8 Gyr of the pulsar. If the latter age is correct, the diffe rence potentially raises questions about the spin-down mechanism of mi llisecond pulsars, which may not be dominated by the braking effect of the magnetic dipole. The relatively old age would also support previo us suggestions that the decay of pulsar magnetic fields asymptotically terminates at a level of 10(8)-10(9) G. It might furthermore reduce d ie discrepancy between the presently observed numbers of low-mass bina ry pulsars and low-mass X-ray binaries which are thought to be the pro genitors of the former. On the other hand, more recent radio observati ons suggest a dynamical age of the pulsar of 2 Gyr which agrees well w ith the cooling age of its companion.