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

## TL;DR

This paper reports the detection of a very cool, helium-atmosphere white dwarf companion to the millisecond pulsar J0740+6620, providing insights into its temperature, age, and relation to the pulsar.

## Contribution

First identification of an ultracool helium-atmosphere white dwarf companion to a millisecond pulsar, expanding understanding of pulsar companion diversity.

## Key findings

- Companion likely an ultracool helium-atmosphere white dwarf with T ≤ 3500 K.
- Cooling age of the white dwarf is ≥ 5 Gyr, consistent with pulsar age.
- Detected in r' and i' bands, making it the reddest known white dwarf companion.

## Abstract

We report the detection of the likely companion of the binary millisecond pulsar 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 1$\sigma$ uncertainty of $\approx$ 0.2 arcsec. Its magnitudes are $r'=26.51\pm0.17$ and $i'=25.49\pm0.15$. Comparing the data with the white dwarf cooling tracks suggests that it can be an ultracool helium-atmosphere white dwarf with the temperature $\lesssim$ 3500 K and cooling age $\gtrsim$ 5 Gyr. The age is consistent with the pulsar characteristic age corrected for kinematic effects. This is the reddest source among known white dwarf companions of millisecond pulsars. Detection of the source in other bands would be useful to clarify its properties and nature.