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 +0.07 −0.24 M ⊙ and 11.3 +1.3 −1.0 km if we fit the spectrum over the 0.3 − 10 keV range, but 1.50 +0.4 −1.0 M ⊙ and 12.2 +0.8 −3.6 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.

Unexpected late-time temperature increase observed in the two neutron star crust-cooling sources XTE J1701−462 and EXO 0748−676

“…So far, cooling up to ∼1700 t q has been reported (e.g. Díaz Trigo et al 2011; Degenaar et al 2009Degenaar et al , 2011bDegenaar et al , 2014Cheng et al 2017). We report on two new observations, which increases the observed cooling baseline to ∼3500 t q .…”

Testing the Equation of State with Electromagnetic Observations

“…However, this analysis exposed a worrisome dependence on the energy range over which the fits were performed with a best-fitting mass and radius of M ≈ 2 M and R ≈ 11 km for 0.3-10 keV and M ≈ 1.5 M and R ≈ 12 km for 0.3-10 keV. This was ascribed to the strong energy-dependence of the applied neutron star atmosphere model (Cheng et al 2017). A similar type of analysis was recently attempted for the well-studied transient LMXB Aql X-1, leading to M ≈ 1.2 M and R ≈ 10.5 km (Li et al 2017).…”

“…These efforts have concentrated on sources that are relatively bright in quiescence. One of such sources is EXO 0748-676 (shown in Figure 10), which was recently studied by Cheng et al (2017) in an attempt to constrain its mass and radius. However, this analysis exposed a worrisome dependence on the energy range over which the fits were performed with a best-fitting mass and radius of M ≈ 2 M and R ≈ 11 km for 0.3-10 keV and M ≈ 1.5 M and R ≈ 12 km for 0.3-10 keV.…”

“…Energy range considered for spectral fitting: A study of the quiescent neutron star LMXB EXO 0748-676 recently highlighted that the energy range over which the thermal fits are performed (0.3-10 or 0.5-10 keV) can have a profound impact on the resulting mass and radius measurement (Cheng et al 2017).…”

Properties of rotating neutron star in density-dependent relativistic mean-field models

“…Figure 7 shows if it rotates at the Keplerian frequency, its frequency is in the range of [700, 850]Hz. Two different mass ranges of, 2.00 +0.07 −0.24 M ⊙ and 1.5 +0.4 −1.0 M ⊙ [70] are obtained for the low-mass X-ray binary neutron star, EXO 0748-676, with rotating frequency of 552 Hz [71], a polar redshift of Z P ≈ 0.35 [72]. If an approximate mass 2.00 M ⊙ is confirmed, none of the considered EOSs in this study is appropriate for describing this star, and stiffer EOSs should be used for this purpose.…”