# An extremely low-mass He white dwarf orbiting the millisecond pulsar J1342+2822B in the globular cluster M3

## TL;DR

This study identifies and characterizes the optical companion to a millisecond pulsar in globular cluster M3, revealing a low-mass helium white dwarf with specific physical properties and implications for the system's inclination and neutron star mass.

## Contribution

First optical identification and detailed characterization of the companion to the millisecond pulsar J1342+2822B in M3 using Hubble observations, including mass, temperature, and age estimates.

## Key findings

- Companion is a 0.19 Msun helium white dwarf.
- System observed almost edge-on, neutron star mass ~1.1 Msun.
- Candidate counterpart to another pulsar system identified, but not confirmed.

## Abstract

We report on the discovery of the companion star to the millisecond pulsar J1342+2822B in the globular cluster M3. We exploited a combination of near-ultraviolet and optical observations acquired with the Hubble Space Telescope in order to search for the optical counterparts to the known millisecond pulsars in this cluster. At a position in excellent agreement with that of the radio pulsar J1342+2822B (M3B), we have identified a blue and faint object (mF275W \approx 22.45) that, in the color-magnitude diagram of the cluster, is located in the region of He core white dwarfs. From the comparison of the observed magnitudes with theoretical cooling tracks we have estimated the physical properties of the companion star: it has a mass of only 0.19 \pm 0.02 Msun, a surface temperature of 12 \pm 1 x 10^3 K and a cooling age of 1.0\pm0.2 Gyr. Its progenitor was likely a ~ 0.84 M star and the bulk of the mass-transfer activity occurred during the sub-giant branch phase. The companion mass, combined with the pulsar mass function, implies that this system is observed almost edge-on and that the neutron star has a mass of 1.1 \pm 0.3 Msun, in agreement with the typical values measured for recycled neutron stars in these compact binary systems. We have also identified a candidate counterpart to the wide and eccentric binary millisecond pulsar J1342+2822D. It is another white dwarf with a He core and a mass of 0.22 \pm 0.2 Msun, implying that the system is observed at a high inclination angle and hosts a typical NS with a mass of 1.3 \pm 0.3 Msun. At the moment, the large uncertainty on the radio position of this millisecond pulsar prevents us from robustly concluding that the detected star is its optical counterpart.