Identification and characterization of optical companions to the population of millisecond pulsars in the globular cluster M3

Abstract

The study of binary millisecond pulsars (MSPs) in globular clusters (GCs) is a key ingredient to study binary and stellar evolution under extreme conditions. In this context, an accurate analysis of the optical emission, which is mostly dominated by the companion star, is essential for a comprehensive characterization of these systems and their role within their environment. In this work, we present a multi-wavelength investigation of five binary MSPs in the Galactic GC M3 (NGC 5272) using archival Hubble Space Telescope (HST) data. Our analysis builds on the timing solutions obtained with the FAST radio Telescope by Li et al. (2024). For each MSP, we carry out precise astrometric cross-matching with the accurate radio positions to identify potential counterparts. When a match is found, we analyse its location in the colour-magnitude diagrams and compare the results with updated binary evolution models to infer the system properties. We confirm the identification of the optical companion to M3B, matching the source previously reported by Cadelano et al. (2019), and successfully identify and characterize the optical companions to M3D and M3F. All three are consistent with helium white dwarfs, as expected from the canonical formation scenario. For M3A and M3E, no reliable counterparts are found, but we place strong upper limits on the brightness and mass of the undetected companion. In the case of M3E, we detect a red object near the radio position in two F814W observations; however, astrometric measurements over a 15-year baseline reveal a significant proper motion inconsistent with cluster membership, identifying the source as a foreground contaminant. This study highlights the effectiveness of combining precise radio timing with deep, multi-band HST images to uncover and constrain the nature of MSP companions in GCs, offering insights into their formation and evolutionary histories.