Quasi-periodic oscillations (QPOs) at frequencies near 1000 Hz were recently discovered in several x-ray binaries containing neutron stars. Two sources show no correlation between QPO frequency and source count rate (Berger et al. 1996, Zhang et al. 1996). We suggest that the QPO frequency is determined by the Keplerian orbital frequency near the marginally stable orbit predicted by general rel...

We predict from a survey of equations of state and observations of X–ray pulsations from SAX J1808.4–3658, that the upper limit of the mass of the compact star is 2.27 M⊙ . The corresponding upper limit of the radius comes out to be 9.73 km. We also do a study to estimate the lower limit of the mass of the compact star. Such a limit is very useful to put constraints on equations of state. We al...

SS precursors are the underside reflections from the upper mantle discontinuities such as those at 410 and 660 km. They are important tools for studying the existence and characteristics of mantle discontinuities both on the global and regional scales (see Deuss, 2009 for a review). Due to their sensitivity at the midpoint between source and receiver, coverage for oceanic regions is achieved wh...

We study the broad band (3-100 keV) spectrum of Cygnus X–2 with INTEGRAL. We find that the spectrum is well fitted by a Comptonized component with a seed-photons temperature of ∼ 1 keV, an electron temperature of ∼ 3 keV and an optical depth τ ∼ 8. Assuming spherical geometry, the radius of the seed-photons emitting region is ∼ 17 km. The source shows no hard X-ray emission; it was detected onl...

We show that a survey of equations of state and observations of X–ray pulsations from SAX J1808.4– 3658 give 2.27 M⊙ as the upper limit of the compact star mass. The corresponding upper limit of the radius comes out to be 9.73 km. We also do a probabilistic study to estimate the lower limit of the mass of the compact star. Such a limit puts useful constraints on equations of state. We also disc...

We present a pulsar-atmosphere (PA) model for modulated thermal X-ray emission from cooling magnetized neutron stars. The model synthesizes the spectral properties of detailed stellar atmosphere calculations with the non-uniform surface properties anticipated for isolated, aging radio pulsars, and general relativistic effects on photon trajectories. We analyze the archival Chandra observations ...

Recent ROSAT studies of narrow-line Seyfert 1 galaxies revealed these objects (with FWHM Hβ ≤ 2000 km s−1) generally show steeper soft X-ray spectra than broad line Seyfert 1’s and that there are noAGNwith broad lines and steep X-ray spectra. We derive a simple theoretical model which explains this observed correlation between the line width and the spectral index for Seyfert 1 galaxies. Assumi...

Introduction: There has been a curiosity about the spheroid geometry for PET scanners developments since several years ago, therefore in this study, we are aiming to evaluate the performance of this geometry and compare its performance with cylindrical geometry using Monte Carlo simulation. Methods: We simulated a spheroid geometry with a radius of 199 mm...