Optical spectral observations of three binary millisecond pulsars

We present the results of optical spectroscopy of stellar companions to three binary millisecond pulsars, PSRs J0621+2514, J2317+1439, and J2302+4442, obtained with the Gran Telescopio Canarias. The spectrum of the J0621+2514 companion shows a blue continuum and prominent Balmer absorption lines. The latter are also resolved in the spectrum of the J2317+1439 companion, showing that both are DA-type white dwarfs. No spectral features are detected for the J2302+4442 companion; however, its broad-band magnitudes and the spectral shape of the continuum emission imply that this is also a DA-type white dwarf. Based on the spectral analyses, we conclude that the companions of J0621+2514 and J2317+1439 are relatively hot, with effective temperatures T-eff = 8600 +/- 200 and 9600 +/- 2000 K, respectively, while the J2302+4442 companion is significantly cooler, T eff <6000 K. We also estimated the distance to J0621+2514 of 1.1 +/- 0.3 kpc and argue that its companion and the companion of J2317+1439 are He-core white dwarfs providing constraints on their cooling ages of <= 2 Gyr.

Automated summary

This article presents the optical spectroscopy results of three binary millisecond pulsars, showing that the companions of J0621+2514 and J2317+1439 are DA-type white dwarfs with relatively high temperatures of 8600 +/- 200 and 9600 +/- 2000 K, respectively, while the companion of J2302+4442 is a cooler DA-type white dwarf with a temperature below 6000 K.