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Eclipse mapping of EXO 0748-676: evidence for a massive neutron star

Eclipse mapping of EXO 0748-676: evidence for a massive neutron star
Eclipse mapping of EXO 0748-676: evidence for a massive neutron star
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, utilizing 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+0.07−0.08 and an inclination i=76.5±1.41.1⁠. Combining these with the previously measured radial velocity of 410 ± 5 km s−1, derived from Doppler mapping analysis of H α emission during quiescence, returns an NS mass of ∼2 M⊙ 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 ∼1.4 M⊙ canonical NS mass is possible when considering radial velocity values derived from other emission lines observed both during outburst and quiescence.
1365-2966
4736–4756
Knight, Amy H.
394b1ae4-038f-43d2-bb70-96d030ef2c6e
Ingram, Adam
01a02529-ad9f-4936-af5d-c200f88d4e53
Middleton, Matthew
f91b89d9-fd2e-42ec-aa99-1249f08a52ad
Drake, Jeremy J.
8c6fab1e-a517-4a97-ad90-904e79cda61a
Knight, Amy H.
394b1ae4-038f-43d2-bb70-96d030ef2c6e
Ingram, Adam
01a02529-ad9f-4936-af5d-c200f88d4e53
Middleton, Matthew
f91b89d9-fd2e-42ec-aa99-1249f08a52ad
Drake, Jeremy J.
8c6fab1e-a517-4a97-ad90-904e79cda61a

Knight, Amy H., Ingram, Adam, Middleton, Matthew and Drake, Jeremy J. (2021) Eclipse mapping of EXO 0748-676: evidence for a massive neutron star. Monthly Notices of the Royal Astronomical Society, 510 (4), 4736–4756. (doi:10.1093/mnras/stab3722).

Record type: Article

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, utilizing 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+0.07−0.08 and an inclination i=76.5±1.41.1⁠. Combining these with the previously measured radial velocity of 410 ± 5 km s−1, derived from Doppler mapping analysis of H α emission during quiescence, returns an NS mass of ∼2 M⊙ 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 ∼1.4 M⊙ canonical NS mass is possible when considering radial velocity values derived from other emission lines observed both during outburst and quiescence.

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2201.02188 - Accepted Manuscript
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Accepted/In Press date: 16 December 2021
e-pub ahead of print date: 23 December 2021

Identifiers

Local EPrints ID: 456037
URI: http://eprints.soton.ac.uk/id/eprint/456037
ISSN: 1365-2966
PURE UUID: b2fdb96b-e557-4df5-a408-97fb257b8b7f

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Date deposited: 12 Apr 2022 16:44
Last modified: 16 Mar 2024 16:29

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Contributors

Author: Amy H. Knight
Author: Adam Ingram
Author: Jeremy J. Drake

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