Optical Studies of Millisecond Pulsar Companions

Abstract

In observations of four binary millisecond pulsars with the Palomar 200 inch (5.1 m) telescope, we have detected white dwarf companions to PSR J1640+2224 and PSR J2145-0750 and placed upper limits of mr = 24 and mr = 25 on companions to PSR J0751+1807 and PSR J2229+2643. The temperature inferred for the companion to PSR J1640+2224 is T = 3700 +/- 300 K. The luminosity inferred for a star at a distance of 1.2 kpc is consistent with that expected from a (2 +/- 1) x 109 yr old helium white dwarf. For PSR J2145-0750, the measured colors are inconsistent, possibly due to a combination of emission lines in the spectrum and systematic effects introduced when subtracting a nearby star. The companion temperature derived lies in the range 4300--6800 K. For a carbon-oxygen white dwarf, the corresponding cooling age is 2--6 x 109 yr. The cooling ages of both systems with detected white dwarf companions are significantly less than the upper limits from the pulsar spin-down ages. For both pulsars, the inferred accretion rate for mass transfer during Roche lobe overflow is 2 orders of magnitude below Eddington accretion rates.