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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.

Spurious Frequencies in the Kepler Short Cadence Data

Baran A. S.

Acta Astronomica

2013

Vol. 63, No. 2

203--224

We present our search for artifacts in the Kepler short cadence data using a commonly known Fourier technique. We analyzed data on a monthly basis searching for a possible correlation between artifacts and the events attributed to the spacecraft as potential sources of the spurious frequencies. We defined a peak to be an artifact if it shows in at least two, yet preferentially most of the stars, during a given month. Besides the commonly known "long cadence comb" we found a periodic appearance of another two combs, one single artifact and very strange wide artifacts roaming between 10 c/d and 35 c/d. These artifacts evolve on a yearly basis (four of Kepler's rolls) and we may only speculate that their sources are in the reaction wheels since they are the only moving parts, or temperature variation. The orientation of the spacecraft is likely excluded from the possible sources. More resources are needed to provide a definite explanation of the artifacts.