arXiv subjects

D. Gondek

Publications and source records attributed to D. Gondek.

4 recordsLinked to original sources

arXiv2000

Protoneutron stars and constraints on maximum and minimum mass of neutron stars

Constraints on minimum and maximum mass of ordinary neutron stars are imposed by the consideration of their early evolution (protoneutron star stage). Calculations are performed for a realistic standard model of hot, dense matter (Lattimer & Swesty 1991) valid for both supranuclear and subnuclear densities. Various assumptions concerning protoneutron star interior (large trapped lepton number, no trapped lepton number, isentropic, isothermal) are taken into account.

astro-ph↗

arXiv1999

Avoided crossings in radial pulsations of neutron and strange stars

Radial pulsations of neutron stars and strange quark stars with nuclear crust are studied. The avoided crossing phenomenon occurring for the radial modes is found and discussed. Neutron star models are constructed using a realistic equation of state of dense matter and strange star models using a phenomenological bag model of quark matter. The eigenfrequencies of the three lowest modes of linear, adiabatic pulsations are calculated, using the relativistic equations for the radial oscillations.

astro-ph↗

arXiv1997

Radial pulsations and stability of protoneutron stars

Radial pulsations of newborn neutron stars (protoneutron stars) are studied for a range of internal temperatures and entropies per baryon predicted by the existing numerical simulations. Protoneutron star models are constructed using a realistic equation of state of hot dense matter, and under various assumptions concerning stellar interior (large trapped lepton number, zero trapped lepton number, isentropic, isothermal). Under prevailing conditions, linear oscillations of a protoneutron star can be treated as adiabatic, and evolutionary effects can be neglected on dynamic timescale. The dynamic behavior is governed by the adiabatic index, which in turn depends on the physical state of the stellar interior. The eigenfrequencies of the lowest radial modes of linear, adiabatic pulsations are calculated. Stability of protoneutron stars with respect to small radial perturbations is studied, and the validity of the static stability criteria is discussed.

astro-ph↗

arXiv1996

The average X-ray/gamma-ray spectrum of radio-quiet Seyfert 1s

We have obtained the average 1--500 keV spectrum of radio-quiet Seyfert 1s using data from EXOSAT, Ginga, HEAO, and GRO/OSSE. The spectral fit to the combined average EXOSAT and OSSE data is fully consistent with that for Ginga and OSSE, confirming results from an earlier Ginga/OSSE sample. The average spectrum is well-fitted by a power-law X-ray continuum with an energy spectral index of $α\simeq 0.9$ moderately absorbed by an ionized medium and with a Compton reflection component. A high-energy cutoff (or a break) in the the power-law component at a few hundred keV or more is required by the data. We also show that the corresponding average spectrum from HEAO A1 and A4 is fully compatible with that obtained from EXOSAT, Ginga and OSSE. These results confirm that the apparent discrepancy between the results of Ginga (with $α\simeq 0.9$) and the previous results of EXOSAT and HEAO (with $α\simeq 0.7$) is indeed due to ionized absorption and Compton reflection first taken into account for Ginga but not for the previous missions. Also, our results confirm that the Seyfert-1 spectra are on average cut off in gamma-rays at energies of at least a few hundred keV, not at $\sim 40$ keV (as suggested earlier by OSSE data alone). The average spectrum is compatible with emission from either an optically-thin relativistic thermal plasma in a disk corona, or with a nonthermal plasma with a power-law injection of relativistic electrons.

astro-ph↗