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Accuracy of Mass and Radius Determination for Neutron Stars in X-ray Bursters from Simulated LOFT Spectra

Majczyna A., Madej J., Różańska A., Należyty M.

Acta Astronomica

2017

Vol. 67, No. 2

171--180

We present a simulation of an X-ray spectrum of a hot neutron star, as would be seen by the LAD detector on board of LOFT satellite. We also compute a grid of theoretical spectra corresponding to a range of effective temperatures Teff and surface gravities log g with values corresponding to compact stars in Type I X-ray bursters. A neutron star with the mass M=1.64 ,M☉ and the radius R=11.95 km (which yields the surface gravity log g=14.30 [cgs] and the surface redshift z=0.30) is used in simulation. Accuracy of mass and radius determination by fitting theoretical spectra to the observed one is found to be M=1.64+0.16 -0.02 M☉ and R=11.95+1.57 -0.40 km (2σ). The confidence contours for these two variables are narrow but elongated, and therefore the resulting constraints on the EOS cannot be strong. Note, that in this paper we aim to discuss error contours of NS mass and radius, whereas discussion of EOS is beyond the scope of this work.

Mass and Radius Determination for the Neutron Star in X-ray Burst Source 4U/MXB 1728-34

Majczyna A., Madej J.

Acta Astronomica

2005

Vol. 55, No. 4

349--366

We analyzed archival X-ray spectra of MXB 1728-34 obtained in 1996-99 by the Proportional Counter Array on board of the RXTE satellite. X-ray spectra were fitted to our extensive grids of model atmosphere spectra to determine the effective temperature Teff on the neutron star surface, logarithm of surface gravity log g, and simultaneously the gravitational redshift z. We chose fitting of numerical model spectra plus broad Gaussian line, modified by interstellar absorption and the absorption on dust. We arbitrarily assumed either hydrogen-helium chemical composition of a model atmosphere, or H-He-Fe mixture in solar proportion. The statistically best values of log g and z were subsequently used to determine mass and radius of the neutron star. We obtained the best values of parameters for the neutron star in X-ray burst source MXB1728-34: mass either M=0.40 Msolar or 0.63 Msolar (for H-He or H-He-Fe models, respectively), radius R=4.6 km or 5.3 km, log g=14.6 or 14.6 and the gravitational redshift z=0.14 or 0.22. All the above parameters have very wide 1σ confidence limits. Their values strongly support the equation of state for strange matter in MXB 1728-34.

Detection of SGR 1900+14 Pulse on August 27, 1998 by RF-15I Soft/Hard Solar X-ray Photometer

Sylwester J., Kordylewski Z., Siarkowski M., Gburek S., Farnik F., Likin O.

Acta Astronomica

1998

Vol. 48, No. 4

819--824

We report detection of the supposed magnetar gamma pulse by the common Czech-Polish solar Soft/Hard X-ray Photometer aboard the INTERBALL-Tail satellite. Timing information presented may allow for better triangulation of the position of the gamma source.