Eclipse Mapping of EXO 0748-676: Evidence for a Massive Neutron Star

Abstract

Determining the maximum possible neutron star (NS) mass places limits on the equation of state (EoS) of ultra-dense matter. The mass of NSs in low mass X-ray binaries can be determined from the binary mass function, providing independent constraints are placed on both the binary inclination and mass ratio. In eclipsing systems, they relate via the totality duration. EXO 0748-676 is an eclipsing NS low mass X-ray binary with a binary mass function estimated using stellar emission lines from the irradiated face of the companion. The NS mass is thus known as a function of mass ratio. Here we model the X-ray eclipses in several energy bands, utilising archival XMM-Newton data. We find a narrow region of absorbing material surrounding the companion star is required to explain the energy-dependent eclipses. Therefore, we suggest the companion may be experiencing ablation of its outer layers and that the system could transition into a redback millisecond pulsar. Our fit returns a mass ratio of $q=0.222_{-0.08}^{+0.07}$ and an inclination $i=76.5{\pm }_{1.1}^{1.4}$. Combining these with the previously measured radial velocity of $410\pm 5$ km/s, derived from Doppler mapping analysis of H${}_{\alpha }$ emission during quiescence, returns a NS mass of $\sim 2M_{\odot }$ even if the line originates as far from the NS as physically possible, favouring hard EoS. The inferred mass increases for a more realistic emission point. However, a $\sim 1.4M_{\odot }$ canonical NS mass is possible when considering radial velocity values derived from other emission lines observed both during outburst and quiescence.

arXiv comment21 pages, 13 Figures, accepted to MNRAS

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Cited by in corpus (8)

• The Nearest Neutron Star Candidate in a Binary Revealed by Optical Time-domain Surveys
• Mass Distribution and Maximum Mass of Neutron Stars: Effects of Orbital Inclination Angle
• The $\text{False Widow}$ Link Between Neutron Star X-ray Binaries and Spider Pulsars
• X-ray Eclipse Mapping Constrains the Binary Inclination and Mass Ratio of Swift J1858.6-0814
• Evaluating the efficacy of sonification for signal detection in univariate, evenly sampled light curves using astronify
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• An apparent positive relation between spin and orbital angular momentum in X-ray binaries
• An ultraviolet burst oscillation candidate from the low-mass X-ray binary EXO 0748-676