Abstract
Using deep Chandra observations of the globular cluster M28, we study the quiescent X-ray emission of a neutron star in a low-mass X-ray binary in order to constrain the chemical composition of the neutron star atmosphere and the equation of state of dense matter. We fit the spectrum with different neutron star atmosphere models composed of hydrogen, helium or carbon. The parameter values obtained with the carbon model are unphysical and such a model can be ruled out. Hydrogen and helium models give realistic parameter values for a neutron star, and the derived mass and radius are clearly distinct depending on the composition of the atmosphere. The hydrogen model gives masses/radii consistent with the canonical values of 1.4 Msun and 10 km, and would allow for the presence of exotic matter inside neutron stars. On the other hand, the helium model provides solutions with higher masses/radii, consistent with the stiffest equations of state. Measurements of neutron star masses/radii by spectral fitting should consider the possibility of heavier element atmospheres, which produce larger masses/radii for the same data, unless the composition of the accretor is known independently.
Highlights
Neutron stars (NS) are composed of the densest form of matter known to exist in our Universe, providing us with a unique laboratory to study cold matter at supra-nuclear density
Using deep Chandra observations of the globular cluster M28, we study the quiescent X-ray emission of a neutron star in a low-mass X-ray binary in order to constrain the chemical composition of the neutron star atmosphere and the equation of state of dense matter
Such qLMXBs can be X-ray transients with episodes of higher accretion that would heat up the NS star surface to the current observed temperatures (Brown et al 1998)
Summary
Neutron stars (NS) are composed of the densest form of matter known to exist in our Universe, providing us with a unique laboratory to study cold matter at supra-nuclear density. Becker et al (2003) have previously reported on a set of ∼40 ks Chandra X-ray Observatory ACIS-S observations of M28 (ObsIds 2683, 2684, 2685). They suggested that the luminous, soft Chandra source numbered 26 in their work (IAU-approved source name CXOGlb J182432.8-245208) is a transiently accreting NS in a LMXB in quiescence (qLMXB). We employed the IDL script ACIS Extract (Broos et al 2010) to confirm the validity of the source detections and refine the source positions This lead to the detection of 101 sources inside the half mass radius of M28, and 35 inside the core radius, with a minimum of 4 counts per source. We discarded bins flagged as bad, and bins below 0.3 keV where the response of the instrument is not calibrated (calibration down to 0.3 keV with ACIS-S3 has been clearly improved in CALDB 4.41)
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