X-Ray Studies of Two Neutron Stars in 47 Tucanae: Toward Constraints on the Equation of State

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States
The Astrophysical Journal (Impact Factor: 5.99). 01/2003; 588(1). DOI: 10.1086/374039
Source: arXiv


We report spectral and variability analysis of two quiescent low mass X-ray binaries (X5 and X7, previously detected with the ROSAT HRI) in a Chandra ACIS-I observation of the globular cluster 47 Tuc. X5 demonstrates sharp eclipses with an 8.666+-0.01 hr period, as well as dips showing an increased N_H column. The thermal spectra of X5 and X7 are well-modeled by unmagnetized hydrogen atmospheres of hot neutron stars. No hard power law component is required. A possible edge or absorption feature is identified near 0.64 keV, perhaps an OV edge from a hot wind. Spectral fits imply that X7 is significantly more massive than the canonical 1.4 \Msun neutron star mass, with M>1.8 \Msun for a radius range of 9-14 km, while X5's spectrum is consistent with a neutron star of mass 1.4 \Msun for the same radius range. Alternatively, if much of the X-ray luminosity is due to continuing accretion onto the neutron star surface, the feature may be the 0.87 keV rest-frame absorption complex (O VIII & other metal lines) intrinsic to the neutron star atmosphere, and a mass of 1.4 \Msun for X7 may be allowed. Comment: 16 pages, 7 figures, accepted by ApJ

Download full-text


Available from: C. O. Heinke, Sep 19, 2013

  • No preview · Article ·
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: After almost 2.5 years of actively accreting, the neutron star X-ray transient and eclipsing binary MXB 1659-29 returned to quiescence in 2001 September. We report on a Chandra observation of this source taken a little over a month after this transition. The source was detected at an unabsorbed 0.5-10 keV flux of only (2.7 - 3.6) x 10^{-13} ergs/s/cm/cm, which implies a 0.5-10 keV X-ray luminosity of approximately (3.2 - 4.3) x 10^{33} (d/10 kpc)^2 erg/s, with d the distance to the source in kpc. Its spectrum had a thermal shape and could be well fitted by either a blackbody with a temperature kT of 0.3 keV or a neutron star atmosphere model with a kT of ~0.1 keV. The luminosity and spectral shape of MXB 1659-29 are very similar to those observed of the other neutron star X-ray transients when they are in their quiescent state. The source was variable during our observation, exhibiting a complete eclipse of the inner part of the system by the companion star. Dipping behavior was observed before the eclipse, likely due to obscuration by an extended feature in the outer part of a residual accretion disk. We discuss our observation in the context of the cooling neutron star model proposed to explain the quiescent properties of neutron star X-ray transients. Comment: Accepted for publication in ApJ. It will be published in the 10 September 2003 issue
    Preview · Article · Jul 2002 · The Astrophysical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the effects of color superconductivity on the structure and formation of compact stars. We find that a huge amount of energy, of the order of 1053 erg, can be released in the conversion from a (metastable) hadronic star into a (stable) hybrid or quark star. If the conversion takes place immediately after the deleptonization of the proto‐neutron star, the released energy can help Supernovae to explode. If the conversion is delayed the energy released can power a Gamma Ray Burst. A delay between the Supernova and the subsequent Gamma Ray Burst is possible, in agreement with the delays proposed in recent analysis of astrophysical data.
    Full-text · Article · Apr 2003
Show more