Volume 573, January 2015
|Number of page(s)||9|
|Section||Stellar structure and evolution|
|Published online||17 December 2014|
The neutron star in HESS J1731−347: Central compact objects as laboratories to study the equation of state of superdense matter
Institut für Astronomie und Astrophysik, Universität Tübingen
2 Kazan (Volga region) Federal University, Kremlevskaya 18, 420008 Kazan, Russia
3 Ioffe Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia
Received: 25 July 2014
Accepted: 1 October 2014
Context. Central compact objects (CCOs) in supernova remnants are isolated thermally emitting neutron stars (NSs). They are most probably characterized by a magnetic field strength that is roughly two orders of magnitude lower than that of most of the radio and accreting pulsars. The thermal emission of CCOs can be modeled to obtain constraints on the physical parameters of the star such as its mass, radius, effective temperature, and chemical composition.
Aims. The CCO in HESS J1731−347 is one of the brightest objects in this class. Starting from 2007, it was observed several times with different X-ray satellites. Here we present our analysis of two new XMM-Newton observations of the source performed in 2013 which increase the total exposure time of the data available for spectral analysis by a factor of about five compared to the analyses presented before.
Methods. We use our numerical spectral models for carbon and hydrogen atmospheres to fit the spectrum of the CCO. From our fits, we derive constraints on the physical parameters of the emitting star such as its mass, radius, distance, and effective temperature. We also use the new data to derive new upper limits on the source pulsations and to confirm the absence of a long-term flux and spectral variability.
Results. The analysis shows that atmosphere models are clearly preferred by the fit over the blackbody spectral function. Under the assumption that the X-ray emission is uniformly produced by the entire star surface (supported by the lack of pulsations), hydrogen atmosphere models lead to uncomfortably large distances of the CCO, above 7–8 kpc. On the other hand, the carbon atmosphere model formally excludes distances above 5–6 kpc and is compatible with the source located in the Scutum-Crux (~3 kpc) or Norma-Cygnus (~4.5 kpc) Galactic spiral arm. We provide and discuss the corresponding confidence contours in the NS mass–radius plane. The measured effective temperature indicates that the NS is exceptionally hot for the estimated age of ~30 kyr. We discuss possible cooling scenarios to explain this property, as well as possible additional constraints on the star mass and radius from cooling theory.
Key words: stars: neutron / stars: atmospheres
© ESO, 2014
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.