Optical spectral observations of three binary millisecond pulsars

TL;DR

This study presents optical spectroscopic observations of stellar companions to three binary millisecond pulsars, identifying their spectral types, temperatures, and distances, which enhances understanding of their evolutionary states.

Contribution

First optical spectral analysis of companions to these pulsars, confirming their white dwarf nature and providing temperature and distance estimates.

Findings

Companions to J0621+2514 and J2317+1439 are DA-type white dwarfs.

Temperatures of companions are approximately 8600K and 9600K.

Estimated distances and cooling ages constrain their evolutionary history.

Abstract

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