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

¹ Department of Physics and Astronomy “Augusto Righi”, University of Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
² INAF – Astrophysics and Space Science Observatory of Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121, Bonn, Germany
⁴ Faculty of Physics, University of Bielefeld, Universitätsstraße 4, 33615 Bielefeld, Germany

^★ Corresponding author: greta.ettorre@inaf.it

Received: 21 August 2025
Accepted: 3 November 2025

Abstract

Context. The study of binary millisecond pulsars (MSPs) in globular clusters (GCs) is a key element to study binary and stellar evolution under extreme conditions, and it provides interesting insights into the physical properties of their host stellar systems.

Aims. 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 multiwavelength investigation of five binary MSPs in the Galactic GC M3 (NGC 5272) using archival Hubble Space Telescope (HST) data. Our analysis builds on phase-connected timing solutions recently obtained with the Five-hundred-meter Aperture Spherical radio Telescope.

Methods. Using deep HST images, we investigated the optical counterparts of MSPs M3A, M3B, M3D, M3E, and M3F. For each MSP, we carried out precise astrometric crossmatching with the highly accurate radio positions to identify potential counterparts. When a match was found, we analyzed its location in the color-magnitude diagrams and compared the results with updated binary evolution models to infer the system properties.

Results. We confirm the identification of the optical companion to M3B, consistent with the source previously reported in an earlier work, 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 were found, but we placed strong upper limits on the brightness and mass of the undetected companion. In the case of M3E, we detected a red object near the radio position in two F814W observations; however, astrometric measurements over a 15 year baseline revealed a significant proper motion inconsistent with cluster membership, identifying the source as a foreground contaminant.

Conclusions. This study highlights the effectiveness of combining precise radio timing with deep multiband HST images to uncover and constrain the nature of MSP companions in GCs, and thus the ability of this approach to offer insights into their formation and evolutionary histories.