The cooling, mass and radius of the neutron star in EXO 0748-676 in quiescence with XMM-Newton

Abstract

We analyse four XMM-Newton observations of the neutron-star low-mass X-ray binary EXO 0748$-$676 in quiescence. We fit the spectra with an absorbed neutron-star atmosphere model, without the need for a high-energy (power-law) component; with a 95 per cent confidence the power-law contributes less than 1 per cent to the total flux of the source in $0.5-10.0$ keV. The fits show significant residuals at around 0.5 keV which can be explained by either a hot gas component around the neutron star or a moderately broad emission line from a residual accretion disc. The temperature of the neutron-star has decreased significantly compared to the previous observation, from 124 eV to 105 eV, with the cooling curve being consistent with either an exponential decay plus a constant or a (broken) power law. The best-fitting neutron-star mass and radius can be better constrained if we extend the fits down to the lowest possible energy available. For an assumed distance of 7.1 kpc, the best-fitting neutron-star mass and radius are $2.00_{-0.24}^{+0.07}{M}_{\odot }$ and $11.3_{-1.0}^{+1.3}$ km if we fit the spectrum over the $0.3-10$ keV range, but $1.50_{-1.0}^{+0.4}{M}_{\odot }$ and $12.2_{-3.6}^{+0.8}$ km if we restrict the fits to the $0.5-10$ keV range. We finally discuss the effect of the assumed distance to the source upon the best-fitting neutron-star mass and radius. As systematic uncertainties in the deduced mass and radius depending on the distance are much larger than the statistical errors, it would be disingenuous to take these results at face value.

12 pages, 6 figures

References in corpus (12)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Models of crustal heating in accreting neutron stars
• The Burst Spectra of EXO 0748-676 during a Long 2003 XMM-Newton Observation
• Rapid Cooling of the Neutron Star in the Quiescent Super-Eddington Transient XTE J1701-462
• A strongly heated neutron star in the transient Z source MAXI J0556-332
• Biases for neutron-star mass, radius and distance measurements from Eddington-limited X-ray bursts
• Probing the Crust of the Neutron Star in EXO 0748-676
• Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748-676 back to quiescence
• Multiwavelength Observations of EXO 0748--676 -- II. Emission Line Behavior
• Properties of the ionised plasma in the vicinity of the neutron-star X-ray binary EXO 0748-676
• Dynamical constraints on the neutron star mass in EXO 0748-676
• Gemini/GMOS Spectroscopy of EXO 0748-676 (=UY Vol) in Outburst

Cited by in corpus (2)

• Unexpected late-time temperature increase observed in two neutron star crust cooling sources -- XTE~J1701-462 and EXO~0748-676
• Properties of Rotating Neutron Star in Density-dependent Relativistic Mean-field Models