The ultracool helium-atmosphere white dwarf companion of PSR J0740+6620?

Abstract

ABSTRACT We report detection of the likely companion of the binary millisecond pulsar (MSP) J0740+6620 with the Gran Telescopio Canarias in the r′ and i′ bands. The position of the detected starlike source coincides with the pulsar coordinates within the 1σ uncertainty of ≈0.2 arcsec. Its magnitudes are r′ = 26.51 ± 0.17 and i′ = 25.49 ± 0.15. Comparing the data with the white dwarf (WD) cooling tracks suggests that it can be an ultracool helium-atmosphere WD with the temperature ≲3500 K and cooling age ≳5 Gyr. The age is consistent with the pulsar characteristic age corrected for kinematic effects. This is the reddest source among known WD companions of MSPs. Detection of the source in other bands would be useful to clarify its properties and nature.