Exploring the optical properties of redback pulsars: The case of J1717+4308A in the globular cluster M92

Abstract

Binary millisecond pulsars (MSPs) in globular clusters (GCs) are key for binary and stellar evolution studies under extreme conditions. The identification of their optical companion stars is instrumental in order to characterise these systems and to constrain the possible recycling mechanisms. For this work, we searched for the optical counterpart to PSR J1717+4308A (hereafter M92A) in the GC M92. To this end, we exploited a multi-epoch, multi-wavelength dataset obtained with the Hubble Space Telescope. We constructed colour--magnitude diagrams, investigated proper motions to assess cluster membership, and modelled the observed light curves. We identified an object located at only 0.02 arcsec from the nominal radio position as the likely optical companion to M92A. The star is significantly bluer than the main sequence at the same luminosity level and exhibits clear photometric variability with a periodicity in agreement with the orbital motion of the binary. The light curve displays two maxima and two minima, indicative of strong tidal distortion and only mild irradiation. Such mild irradiation is consistent with the ratio of the pulsar spin-down to the companion flux ($f_\mathrm{sd}$), which for M92A lies close to the boundary between ellipsoidal- and irradiation-dominated regimes ($f_\mathrm{sd} \approx 2.71$). From the light curve modelling we inferred the main physical properties of the companion star. The best-fit model indicates a high-inclination system with a relatively low-mass companion and a massive neutron star. With a base temperature of $\sim7200$ K, the companion ranks among the hottest redbacks known to date. This object therefore adds additional pieces to the puzzle of MSP companion properties and contributes to outlining the characteristics of redbacks across the different classes.