Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aps..aprn17100t&link_type=abstract
American Physical Society, April Meeting, Jointly Sponsored with the High Energy Astrophysics Division (HEAD) of the American As
Astronomy and Astrophysics
Astrophysics
Scientific paper
Determination of masses and radii of neutron stars is one of the most crucial tasks of contemporary observational astronomy as long as it can provide important information about the state of matter inside compact stars. Thermonuclear flashes at bottoms of neutron stars referred to as Type I X-ray bursts, can reveal characteristics of central object provided that appropriate observational data and analysis techniques are present. Analytic theory of expansion and contraction stages of X-ray burster has been developed which provided spectral shapes and hardening/softening factors for emerging X-ray radiation. Gravitational effects and photon diffusion are assumed along with Comptonization and emission-absorption processes are accounted for. Until the advent of Rossi X-ray Timing Explorer (RXTE) the X-ray data suffered from low counting statistics and insufficient time resolution to permit studying temporal evolution of X-ray bursters in details and putting close constraints on the mass-radius relationship. Increased spectral quality and outstanding time resolution of RXTE data coupled with accuracy of the analytical description allow us to narrow constraints on neutron star mass and radius. We present the further development of analytical theory and the results of applying it to X-ray bursts from number of sources, observed with RXTE. More than 10 Type I X-ray bursts were detected from Cygnus X-2, some of which show photospheric radius expansion. We investigate this set of data to obtain tight constraints on mass and radius of the neutron star. One important application of our results is to test the kilohertz QPO models. Interpretation of microsecond variability in Cygnus X-2 and other LMXBs as the frequency of the minimum stable orbit led to the conclusion that neutron stars may have masses well above 1.4 solar masses. Our spectral analysis of Type I X-ray bursts in Cyg X-2 with taking into account all radiative transfer and General Relativity effects does not confirm the the mass-radius estimates derived from the last stable orbit model. We obtained the mass constraints within 1.4 +/- 0.2 solar masses, radius within 10 +/- 0.5 km and we found that 10 kpc for the distance to the source is consistent with the observations.
Shaposhnikov Nickolai
Titarchuk Lev
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