arXiv subjects
Publications and source records attributed to M. Ali Alpar.
Interaction with possible fallback material, along with the magnetic fields and rotation rates at birth should determine the fates and categories of young neutron stars. This paper addresses some issues related to pure or hybrid accretion models for explaining the properties of young neutron stars.
We suggest a new scenario to explain the outburst light curves of black hole soft X-ray transients together with the secondary maximum and the bump seen on their decay phases. Our explanations are based on the disk instability models considering the effect of X-ray irradiation. The scenario is consistent with the observed X-ray delays by a few days with respect to the optical rise for both the main outburst and the later maxima. We test our scenario by numerically solving the disk diffusion equation. The obtained model curve fits well to the observed X-ray outburst photon flux curve of the black hole soft X-ray transient GS/GRS 1124-68, a typical representative of the four BH SXTs with very similar light curves.
Large pulsar glitches seem to be common to all radio pulsars and to exhibit a universal behaviour connecting the rate of occurrence, event size and interglitch relaxation that can be explained if the glitches are due to angular momentum exchange in the neutron star. This has implications for the energy dissipation rate. The large timing excursions observed in SGRs are not similar to the pulsar glitches. The one glitch observed in an AXP is very similar to pulsar glitches and demonstrates that typical neutron star glitch response occurs when the external torque is constant for an extended period of time. In the long run the timing variations of both the AXPs and the SGRs are much stronger than the timing noise of radio pulsars but comparable with the behaviour of accreting sources. For the DTNs, the thermal luminosities do not seem compatible with cooling ages and statistics if these sources are magnetars. The energy dissipation rates implied by the dynamical behaviour of glitching pulsars will provide the luminosities of the DTNs under propeller spindown torques supplied by fossil disks on neutron stars with conventional 10$^{12}$ G dipole magnetic fields.
Neutron stars may be surrounded by fall-back disks formed from supernova core-collapse. If the disk circumscribes the light-cylinder, the neutron star will be an active radio pulsar spinning down under the propeller spin-down torque applied by the disk as well as the usual magnetic dipole radiation torque. Evolution across the P-P_dot diagram is very rapid when pulsar spin-down is dominated by the propeller torque. This explains the distribution of pulsars in the P-P_dot diagram.
We report the results of analysis of observations of the Vela Pulsar by PCA on RXTE. Our data consists of two parts. The first part contains observations at 1, 4, and 9 days after the glitch in 1996 and has 27000 sec. total exposure time. The second part of observations were performed three months after this glitch and have a total exposure time of 93000 sec. We found pulsations in both sets. The observed spectrum is a power-law with no apparent change in flux or count rate. The theoretical expectations of increase in flux due to internal heating after a glitch are smaller than the uncertainty of the observations.
The similarity of rotation periods from three interesting classes of neutron stars, the anomalous X-ray pulsars (AXPs), the soft gamma ray repeaters (SGRs) and the dim isolated thermal neutron stars (DTNs) suggests a common mechanism with an asymptotic spindownphase, extending through the propeller and early accretion stages. The DTNs are interpreted as sources in the propeller stage. Their low luminosities arise from frictional heating in the neutron star. SGRs and AXPs are accreting at $\dot{M} \sim 10^{15} gm/s $. The limited range of near equilibrium periods corresponds to a limited range of mass inflow rates $\dot{M}$. For lower rates the source of mass inflow may be depleted before the asymptotic stage is reached, while sources with higher $\dot{M}$ or later ages possess circumstellar material that is optically thick to electron scattering, destroying the X-ray beaming and the modulation at the rotation period. The model works with conventional magnetic fields of 10$^{11}-10^{12}$ G, obviating the need to postulate magnetars. Frequently sampled timing observations of AXPs, SGRs and DTNs can distinguish between this explanation and the magnetar model.
We have investigated the stability of the pulse frequency second derivatives ($\ddot ν$) of PSR 0823+26, PSR 1706-16, PSR 1749-28, PSR 2021+51 which show significant quadratic trends in their pulse frequency histories in order to determine whether the observed second derivatives are secular or they arise as part of noise processes. We have used TOA data extending to more than three decades which are the longest time spans ever taken into account in pulse timing analyses. We investigated the stability of pulse frequency second derivative in the framework of low resolution noise power spectra (Deeter 1984) estimated from the residuals of pulse frequency and TOA data. We have found that the $\ddot ν$ terms of these sources arise from the red torque noise in the fluctuations of pulse frequency derivatives which may originate from the external torques from the magnetosphere of pulsar.
We note that the solution of accretion disk dynamics for an initial delta-function mass distribution gives a light curve that fits both the rise and the decay pattern of the outburst light curves of black-hole soft X-ray transients (BSXTs) until the onset of the first mini outburst quite well. The Green's function solution of Lynden-Bell & Pringle (1974) is employed for two differenttime-independent viscosity laws to calculate the expected count rates of X-ray photons in the Ginga energy bands as a function of time. For both models basic characteristics of the outburst light curves of two typical sources GS 2000+25 and GS/GRS 1124-68 are reproduced together with plausible values of the thin disk parameter $α$ and the recurrence times. This agreement with the outburst light curves and the source properties during quiescence support the idea of mass accumulation and the sporadic release of accumulated mass at the outer disk.