M. Ali Alpar

On Young Neutron Stars as Propellers and Accretors with Conventional Magnetic Fields

The similarity of rotation periods of 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 spin-down phase, extending through the propeller and early accretion stages. The DTNs are interpreted as sources in the propeller stage. Their luminosities arise from frictional heating in the neutron star. If the 8.4 s rotation period of the DTN RX J0720.4-3125 is close to its rotational equilibrium period, the estimated propeller torque indicates a magnetic field in the 1012 G range. The mass inflow rate onto the propeller is on the order of the accretion rates of the AXPs. The limited range of rotation periods, taken to be close to equilibrium periods, and conventional magnetic fields in the range 5 × 1011 to 5 × 1012 G correspond to a range of mass inflow rates 3.2 × 1014 g s-1< < 4.2 × 1017 g s-1. Observed spin-down rates of the AXPs and SGRs also fit in with estimates for these magnetic fields and equilibrium periods. The source of the mass inflow is a remnant accretion disk formed as part of the fallback during the supernova explosion. These classes of sources thus represent the alternative pathways for those neutron stars that do not become radio pulsars. For the highest mass inflow rates the propeller action may support enough circumstellar material so that the optical thickness to electron scattering destroys the X-ray beaming, and the rotation period is not observable. These are the radio-quiet neutron stars at the centers of supernova remnants Cas A, Puppis A, RCW 103, and 296.5+10. The statistics and ages of DTNs suggest that sources in the propeller phase are quite common, maybe accounting for the majority of neutron stars formed in supernovae. AXPs are the rare cases whose history has allowed them to evolve rapidly to the post-propeller accretion phase. The different classes represent alternative pathways rather than consecutive phases of evolution. Thus, for example, the AXPs are not descendants of the DTNs. This model obviates the need to postulate magnetars for AXPs and DTNs. Frequently sampled timing observations of AXPs, SGRs, and DTNs can distinguish between this explanation and the magnetar model.

Postglitch relaxation of the vela pulsar after its first eight large glitches: A reevaluation with the vortex creep model

We present a comprehensive reevaluation of eight of the nine glitches observed to date from the Vela pulsar, and the postglitch relaxation following each glitch. All glitch data sets can be described in terms of three distinct components of short and intermediate time scale exponential relaxation, followed by a long-term recovery of the glitch-induced change in the spin-down rate that is linear in t, ΔΩ˙ c (t)∞t. We interpret the short and the intermediate time scale exponential relaxation, characterized by relaxation times of 10 hr, 3 d .2, and 32 d as the linear response of vortex creep in those regions of the pinned superfluid in the neutron star crust through which no sudden vortex motion occurred at the time of the glitch

Can Thin Disks Produce Anomalous X-Ray Pulsars?

We investigate whether young neutron stars with fallback disks can produce anomalous X-ray pulsars (AXPs) within timescales indicated by the ages of associated supernova remnants. The system passes through a propeller stage before emerging as an AXP or a radio pulsar. The evolution of the disk is described by a diffusion equation that has self-similar solutions with either angular momentum or total mass of the disk conserved. We associate these two types of solutions with accretor and propeller regimes, respectively. Our numerical calculations of thin-disk models with changing inner radius take into account the supercritical accretion at the early stages and electron scattering and bound-free opacities with rich metal content. Our results show that, assuming a fraction of the mass inflow is accreted onto the neutron star, the fallback disk scenario can produce AXPs for acceptable parameters.

Pulsar braking indices, glitches and energy dissipation in neutron stars

Almost all pulsars with anomalous positive ¨ � measurements (corresponding to anomalous braking indices in the range 5 10 −7 ) as well as post-glitch or interglitch ¨ � measurements, obey the scaling between ¨ � and glitch parameters originally noted in the Vela pulsar. Negative second derivative values can be understood in terms of glitches that were missed or remained unresolved. We discuss the glitch rates and a priori probabilities of positive and negative braking indices according to the model developed for the Vela pulsar. This behaviour supports the universal occurrence of a non-linear dynamical coupling between the neutron star crust and an interior superfluid component. The implied lower limit to dynamical energy dissipation in a neutron star with spindown rate ˙ � is ˙ Ediss > 1.7× 10 −6 ˙ Erot. Thermal luminosities and surface temperatures due to dynamical energy dissipation are estimated for old neutron stars which are spinning down as rotating magnetic dipoles beyond the pulsar death line.

DISKS SURVIVING THE RADIATION PRESSURE OF RADIO PULSARS

The radiation pressure of a radio pulsar does not necessarily disrupt a surrounding disk. The position of the inner radius of a thin disk around a neutron star, determined by the balance of stresses, can be estimated by comparing the electromagnetic energy density generated by the neutron star as a rotating magnetic dipole in vacuum with the kinetic energy density of the disk. Inside the light cylinder, the near zone electromagnetic field is essentially the dipole magnetic field, and the inner radius is the conventional Alfven radius. Far outside the light cylinder, in the radiation zone, = , and the electromagnetic energy density is S/c ∝ 1/r2, where S is the Poynting vector. Shvartsman argued that a stable equilibrium cannot be found in the radiative zone because the electromagnetic energy density dominates over the kinetic energy density, with the relative strength of the electromagnetic stresses increasing with radius. In order to check whether this is also true near the light cylinder, we employ the Deutsch global electromagnetic field solutions for rotating oblique magnetic dipoles. Near the light cylinder the electromagnetic energy density increases steeply enough with decreasing r to balance the kinetic energy density at a stable equilibrium. The transition from the near zone to the radiation zone is broad. The radiation pressure of the pulsar cannot disrupt the disk for values of the inner radius up to about twice the light cylinder radius if the rotation axis and the magnetic axis are orthogonal. This allowed range beyond the light cylinder extends much farther for small inclination angles. The mass flow rate in quiescent phases of accretion-driven millisecond pulsars can occasionally drop to values low enough that the inner radius of the disk goes beyond the light cylinder. The possibilities considered here may be relevant for the evolution of spun-up X-ray binaries into millisecond pulsars, for some transients, and for the evolution of young neutron stars if there is a fallback disk surrounding the neutron star.

Vortex creep and the internal temperature of neutron stars Timing noise in pulsars

Vortex creep theory is used to construct model noise power spectra for three physically distinct types of events which might give rise to pulsar timing noise. These are pure events, in which vortex unpinning is the source of the initial frequency jump; mixed events, in which the initial frequency jump is produced by some physical process other than vortex unpinning but leads to the unpinning of some vortices; and external events, in which the initial frequency jumps responsible for noise do not involve any vortex unpinning. For the first two types of events, it is found that relaxation processes in the region responsible for the noise will give rise to structure in the observed power spectra, while for external events, the resulting noise spectra will not be influenced by vortex creep. The theoretical results are compared with observed power spectra for 25 pulsars. The absence of structure in the observed power spectra of the Crab and Vela pulsars within the range of time scales which characterize their postglitch behavior indicates that the pinning regions which play a role in postglitch behavior do not experience the small unpinning events leading to timing noise. 24 references.

On the Rotational Dynamics of Magnetically Threaded Disks around Neutron Stars

We investigate the rotational dynamics of disk accretion around a strongly magnetized neutron star with an aligned dipole field. The magnetospheric field is assumed to thread the disk plasma both inside and outside the corotation radius. As a result of disk-star interaction, the magnetic torque on the disk affects the structure of accretion flow to yield the observed spin-up or spin-down rates for a source of given fastness, magnetic field strength, and mass accretion rate. Within the model we obtain a prescription for the dynamical viscosity of such magnetically modified solutions for a Keplerian disk. We then use this prescription to find a model solution for the rotation rate profile throughout the entire disk, including the non-Keplerian inner disk. We find that the non-Keplerian angular velocity transition region is not necessarily narrow for a source of given spin state. The boundary layer approximation, as in the standard magnetically threaded disk model, holds only in the case of dynamical viscosity decreasing all the way to the innermost edge of the disk. These results are applied to several observed disk-fed X-ray pulsars that have exhibited quasi-periodic oscillations (QPOs). The QPO frequencies provide a constraint on the fastness parameter and enable one to determine uniquely the width of the angular velocity transition zone for each source within model assumptions. We discuss the implications of these results on the value of the critical fastness parameter for a magnetized star in spin equilibrium. Applications of our model are also made with relevant parameters from recent numerical simulations of quasi-stationary disk-magnetized star interactions.

The timing noise of PSR 0823+26, 1706-16, 1749-28, 2021+51 and the anomalous braking indices

We have investigated the stability of the pulse frequency second derivatives (ν) of PSR 0823+26, 1706-16, 1749-28 and 2021+51 that show significant quadratic trends in their pulse-frequency histories in order to determine whether the observed second derivatives are secular or if they arise as part of noise processes. We have used time-of-arrival (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 the pulse-frequency second derivative in the framework of low-resolution noise power spectra estimated from the residuals of pulse frequency and TOA data. We have found that the 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 the pulsar.

VORTEX CREEP AGAINST TOROIDAL FLUX LINES, CRUSTAL ENTRAINMENT, AND PULSAR GLITCHES

A region of toroidally oriented quantized flux lines must exist in the proton superconductor in the core of the neutron star. This region will be a site of vortex pinning and creep. Entrainment of the neutron superfluid with the crustal lattice leads to a requirement of superfluid moment of inertia associated with vortex creep in excess of the available crustal moment of inertia. This will bring about constraints on the equation of state. The toroidal flux region provides the moment of inertia necessary to complement the crust superfluid with postglitch relaxation behavior fitting the observations.

The steady spin-down rate of 4U 1907+09

Using X-ray data from the Rossi X-ray Timing Explorer, we report the pulse timing results of the accretion-powered, high-mass X-ray binary pulsar 4UU 1907+09, covering a time-span of almost two years. We measured three new pulse periods in addition to the previously measured four pulse periods. We are able to connect pulse arrival times in phase for more than a year. The source has been spinning down almost at a constant rate, with a spin-down rate of v = (- 3.54 +/- 0.02) x 10(exp -14) Hz s(exp -1) for more than 15 yr. Residuals of pulse arrival times yield a very low level of random-walk noise, with a strength of approximately 2 x 10(exp -20) rad(exp 2) s(exp -3) on a time-scale of 383 d, which is 40 times lower than that of the high-mass X-ray binary pulsar Vela X-1. The noise strength is only a factor of five greater than that of the low-mass X-ray binary pulsar 4U 1626-67. The low level of the timing noise and the very stable spin-down rate of 4U 1907+09 make this source unique among the high-mass X-ray binary pulsars, providing another example, in addition to 4U 1626-67, of long-term quiet spin down from an accruing source. These examples show that the extended quiet spin-down episodes observed in the anomalous X-ray pulsars 1RXS J170849.0-400910 and 1E 2259+586 do not necessarily imply that these sources are not accreting pulsars.