Spectral analysis of the quiescent low-mass X-ray binary in the globular cluster M30

Abstract

We present a recent Chandra observation of the quiescent low-mass X-ray binary containing a neutron star, located in the globular cluster M30. We fit the thermal emission from the neutron star to extract its mass and radius. We find no evidence of flux variability between the two observations taken in 2001 and 2017, nor between individual 2017 observations, so we analyse them together to increase the signal to noise. We perform simultaneous spectral fits using standard light-element composition atmosphere models (hydrogen or helium), including absorption by the interstellar medium, correction for pile-up of X-ray photons on the detector, and a power-law for count excesses at high photon energy. Using a Markov-chain Monte Carlo approach, we extract mass and radius credible intervals for both chemical compositions of the atmosphere: $R_{\text{NS}}=7.94_{-1.21}^{+0.76}$ km and $M_{\text{NS}}<1.19$ M${}_{\odot }$ assuming pure hydrogen, and $R_{\text{NS}}=10.50_{-2.03}^{+2.88}$ km and $M_{\text{NS}}<1.78$ M${}_{\odot }$ for helium, where the uncertainties represent the 90% credible regions. For H, the small radius is difficult to reconcile with most current nuclear physics models (especially for nucleonic equations of state) and with other measurements of neutron star radii, with recent preferred values generally in the 11-14 km range. Whereas for He, the measured radius is consistent with this range. We discuss possible sources of systematic uncertainty that may result in an underestimation of the radius, identifying the presence of surface temperature inhomogeneities as the most relevant bias. According to this, we conclude that either the atmosphere is composed of He, or it is a H atmosphere with a significant contribution of hot spots to the observed radiation.

11 pages, 4 figures, accepted for publication in MNRAS

References in corpus (20)

• Shapiro delay measurement of a two solar mass neutron star
• A Massive Pulsar in a Compact Relativistic Binary
• PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter
• A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation
• A NICER view of PSR J0030+0451: Implications for the dense matter equation of state
• Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
• Improved Mass and Radius Constraints for Quiescent Neutron Stars in Omega Cen and NGC 6397
• The relativistic pulsar-white dwarf binary PSR J1738+0333 I. Mass determination and evolutionary history
• Rejecting proposed dense-matter equations of state with quiescent low-mass X-ray binaries
• Faint X-ray Sources in the Globular Cluster Terzan 5
• Constraining the equation of state of supra-nuclear dense matter from XMM-Newton observations of neutron stars in globular clusters
• Cooling of Accretion-Heated Neutron Stars
• Universality of the Acceleration Due to Gravity on the Surface of a Rapidly Rotating Neutron Star
• Chandra X-ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)
• Globular clusters with Gaia
• Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808
• An extremely low-mass He white dwarf orbiting the millisecond pulsar J1342+2822B in the globular cluster M3
• X-ray Spectral Identification of Three Candidate Quiescent Low-Mass X-ray Binaries in the Globular Cluster NGC 6304
• New possible class of neutron stars: hot and fast non-accreting rotators
• Investigating Variability of Quiescent Neutron Stars in the Globular Clusters NGC 6440 and Terzan 5

Cited by in corpus (3)

• Dense Matter in Neutron Stars with eXTP
• A deep Chandra survey for faint X-ray sources in the Galactic globular cluster M30, and searches for optical and radio counterparts
• Far-ultraviolet investigation into the galactic globular cluster M30 (NGC 7099): II. Potential X-ray counterparts and variable sources