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Register a new userAccretion-Assisted Spindown of Young Radio Pulsars
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We present a model for the spindown of young radio pulsars in which the neutron star loses rotational energy not only by emitting magnetic dipole radiation but also by torquing a surrounding accretion disk produced by supernova fallback. The braking index predicted in our model is in general less than n=3 (the value for pure dipole magnetic radiation), in agreement with the reported values of n<3 for five young radio pulsars. With an accuracy of 30% or better, our model reproduces the age, braking index and third frequency derivative of the Crab pulsar for a disk accretion rate in the range 3 x 10^16 - 10^17 g/s.
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The observed braking index n_{obs} which had been determined for a few young pulsars, had been found to differ from the expected value for a rotating magnetic dipole model. Also, the observational jerk parameter, determined for two of these pulsars, disagree with the theoretical prediction m_{obs} = 15 in both cases. We propose a simple model able to account for these differences, based on a growth of the torque function K = -(dot Omega)/(Omega^{n}), under the constraint that n_{obs} is a constant. We show that there is observational evidence supporting the latter hypotesis, and derive initial values for several physical quantities for the four pulsars whose n_{obs} have been measured.
Recently, Parthsarathy et al. analysed long-term timing observations of 85 young radio pulsars. They found that 11 objects have braking indices ranging $sim 10-100$, far from the classical value $n=3$. They also noted a mild correlation between measured value of $n$ and characteristic age of a radio pulsar. In this article we systematically analyse possible physical origin of large braking indices. We find that a small fraction of these measurements could be caused by gravitational acceleration from an unseen ultra-wide companion of a pulsar or by precession. Remaining braking indices cannot be explained neither by pulsar obliquity angle evolution, nor by complex high-order multipole structure of the poloidal magnetic field. The most plausible explanation is a decay of the poloidal dipole magnetic field which operates on a time scale $sim 10^4-10^5$ years in some young objects, but has significantly longer time scale in other radio pulsars. This decay can explain both amplitude of measured $n$ and some correlation between $n$ and characteristic age. The decay can be caused by either enhanced crystal impurities in the crust of some isolated radio pulsars, or more likely, by enhanced resistivity related to electron scattering off phonons due to slow cooling of low-mass neutron stars. If this effect is indeed the main cause of the rapid magnetic field decay manifesting as large braking indices, we predict that pulsars with large braking indices are hotter in comparison to those with $napprox 3$.
The role of magnetic field decay in normal radio pulsars is still debated. In this paper we present results which demonstrate that an episode of magnetic field decay in hot young neutron stars can explain anomalous values of braking indices recently measured for more than a dozen of sources. It is enough to have few tens of per cent of such hot NSs in the total population to explain observables. Relatively rapid decay operates at ages $lesssim$~few~$times100$~kyrs with a characteristic timescale of a similar value. We speculate that this decay can be related to electron scattering off phonons in neutron star crusts. This type of decay saturates as a neutron star cools down. Later on, a much slower decay due to crustal impurities dominates. Finally, we demonstrate that this result is in agreement with our early studies.
Forty six gamma-ray pulsars were reported in the First Fermi Large Area Telescope (LAT) Catalog of Gamma-ray Pulsars. Over forty more have been seen since then. A simple but effective figure-of-merit for gamma-detectability is sqrt(Edot)/d^2, where Edot is the pulsar spindown power and d the distance. We are tracking down the best gamma-ray candidates not yet seen. We present the timing and spectral analysis results of some new high spindown power, nearby gamma-ray pulsars. We also update some population distribution plots in preparation for the 2nd Fermi LAT gamma-ray Pulsar Catalog.
We present results of Compton Gamma-Ray Observatory EGRET observations of the unidentified high-energy gamma-ray sources 2EG J1049-5847 (GEV J1047-5840, 3EG J1048-5840) and 2EG J1103-6106 (3EG J1102-6103). These sources are spatially coincident with the young, energetic radio pulsars PSRs B1046-58 and J1105-6107, respectively. We find evidence for an association between PSR B1046-58 and 2EG J1049-5847. The gamma-ray pulse profile, obtained by folding time-tagged photons having energies above 400 MeV using contemporaneous radio ephemerides, has probability of arising by chance of 1.2E-4 according to the binning-independent H-test. A spatial analysis of the on-pulse photons reveals a point source of equivalent significance 10.2 sigma. Off-pulse, the significance drops to 5.8 sigma. Archival ASCA data show that the only hard X-ray point source in the 95% confidence error box of the gamma-ray source is spatially coincident with the pulsar within the 1 uncertainty (Pivovaroff, Kaspi & Gotthelf 1999). The double peaked gamma-ray pulse morphology and leading radio pulse are similar to those seen for other gamma-ray pulsars and are well-explained in models in which the gamma-ray emission is produced in charge-depleted gaps in the outer magnetosphere. The inferred pulsed gamma-ray flux above 400 MeV, (2.5 +/- 0.6) x 10E-10 erg/cm^2/s, represents 0.011 +/- 0.003 of the pulsars spin-down luminosity, for a distance of 3 kpc and 1 sr beaming. For PSR J1105-6107, light curves obtained by folding EGRET photons using contemporaneous radio ephemerides show no significant features. We conclude that this pulsar converts less than 0.014 of its spin-down luminosity into E > 100 MeV gamma-rays beaming in our direction (99% confidence), assuming a distance of 7 kpc, 1 sr beaming and a duty cycle of 0.5.
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