The Gamma Ray Pulsar Population

We apply a likelihood analysis to pulsar detections, pulsar upper limits, and diffuse background measurements from the OSSE and EGRET instruments on the Compton Gamma Ray Observatory to constrain the luminosity law for gamma-ray pulsars and some properties of the gamma-ray pulsar population. We find that the dependence of luminosity on spin period and dipole magnetic field is much steeper at OSSE than at EGRET energies (50-200 keV and >100 MeV, respectively), suggesting that different emission mechanisms are responsible for low- and high-energy gamma-ray emission. Incorporating a spin-down model and assuming a pulsar spatial distribution, we estimate the fraction of the Galactic gamma-ray background due to unidentified pulsars and find that pulsars may be an important component of the OSSE diffuse flux, but are most likely not important at EGRET energies. Using measurements of the diffuse background flux from these instruments, we are able to place constraints on the braking index, initial spin period, and magnetic field of the Galactic pulsar population. We are also able to constrain the pulsar birthrate to be between 1/(25 yr) and 1/(500 yr). Our results are based on a large gamma-ray beam, but they do not scale in a simple way with beam size. With our assumed beam size, the implied gamma-ray efficiency for the EGRET detections is no more than 20%. We estimate that about 20 of the 169 unidentified EGRET sources are probably gamma-ray pulsars. We use our model to predict the pulsar population that will be seen by future gamma-ray instruments and estimate that GLAST will detect roughly 750 gamma-ray pulsars as steady sources, only 120 of which are currently known radio pulsars.Comment: 32 pages, including figures. submitted to Ap

Absolute Timing of the Crab Pulsar with RXTE

We have monitored the phase of the main X-ray pulse of the Crab pulsar with the Rossi X-ray Timing Explorer (RXTE) for almost eight years, since the start of the mission in January 1996. The absolute time of RXTE's clock is sufficiently accurate to allow this phase to be compared directly with the radio profile. Our monitoring observations of the pulsar took place bi-weekly (during the periods when it was at least 30 degrees from the Sun) and we correlated the data with radio timing ephemerides derived from observations made at Jodrell Bank. We have determined the phase of the X-ray main pulse for each observation with a typical error in the individual data points of 50 us. The total ensemble is consistent with a phase that is constant over the monitoring period, with the X-ray pulse leading the radio pulse by 0.0102+/-0.0012 period in phase, or 344+/-40 us in time. The error estimate is dominated by a systematic error of 40 us in the radio data, arising from uncertainties in the variable amount of pulse delay due to interstellar scattering and instrumental calibration. The statistical error is 0.00015 period, or 5 us. The separation of the main pulse and interpulse appears to be unchanging at time scales of a year or less, with an average value of 0.4001+/-0.0002 period. There is no apparent variation in these values with energy over the 2-30 keV range. The lag between the radio and X-ray pulses may be constant in phase (rotational) or constant in time (linear pathlength). We are not (yet) able to distinguish between these two interpretations.Comment: 11 pages, 2 figure

Searching for sub-millisecond pulsars from highly polarized radio sources

Pulsars are among the most highly polarized sources in the universe. The NVSS has catalogued 2 million radio sources with linear polarization measurements, from which we have selected 253 sources, with polarization percentage greater than 25%, as targets for pulsar searches. We believe that such a sample is not biased by selection effects against ultra-short spin or orbit periods. Using the Parkes 64m telescope, we conducted searches with sample intervals of 0.05 ms and 0.08 ms, sensitive to submillisecond pulsars. Unfortunately we did not find any new pulsars.Comment: 2 pages 1 figure. To appear in "Young Neutron Stars and Their Environments" (IAU Symposium 218, ASP Conference Proceedings), eds F. Camilo and B. M. Gaensle

Neutron star magnetic field evolution, crust movement and glitches

Spinning superfluid neutrons in the core of a neutron star interact strongly with co-existing superconducting protons. One consequence is that the outward(inward) motion of core superfluid neutron vortices during spin-down(up) of a neutron star may alter the core's magnetic field. Such core field changes are expected to result in movements of the stellar crust and changes in the star's surface magnetic field which reflect those in the core below. Observed magnitudes and evolution of the spin-down indices of canonical pulsars are understood as a consequence of such surface field changes. If the growing crustal strains caused by the changing core magnetic field configuration in canonical spinning-down pulsars are relaxed by large scale crust-cracking events, special properties are predicted for the resulting changes in spin-period. These agree with various glitch observations, including glitch activity, permanent shifts in spin-down rates after glitches in young pulsars, the intervals between glitches, families of glitches with different magnitudes in the same pulsar, the sharp drop in glitch intervals and magnitudes as pulsar spin-periods approach 0.7s, and the general absence of glitching beyond this period.Comment: LaTex, 28 pages, 8 figs, accepted for publication in Ap

Pulsar magnetic alignment and the pulsewidth-age relation

Using pulsewidth data for 872 isolated radio pulsars we test the hypothesis that pulsars evolve through a progressive narrowing of the emission cone combined with progressive alignment of the spin and magnetic axes. The new data provide strong evidence for the alignment over a time-scale of about 1 Myr with a log standard deviation of around 0.8 across the observed population. This time-scale is shorter than the time-scale of about 10 Myr found by previous authors, but the log standard deviation is larger. The results are inconsistent with models based on magnetic field decay alone or monotonic counter-alignment to orthogonal rotation. The best fits are obtained for a braking index parameter n_gamma approximately equal to 2.3, consistent the mean of the six measured values, but based on a much larger sample of young pulsars. The least-squares fitted models are used to predict the mean inclination angle between the spin and magnetic axes as a function of log characteristic age. Comparing these predictions to existing estimates it is found that the model in which pulsars are born with a random angle of inclination gives the best fit to the data. Plots of the mean beaming fraction as a function of characteristic age are presented using the best-fitting model parameters.Comment: 13 pages, 11 figures, Accepted for publication in MNRA

Dissociation between the impact of evidence on eye movement target choice and confidence judgements

It has been suggested that the evidence used to support a decision to move our eyes and the confidence we have in that decision are derived from a common source. Alternatively, confidence may be based on further post-decisional processes. In three experiments we examined this. In Experiment 1, participants chose between two targets on the basis of varying levels of evidence (i.e., the direction of motion coherence in a Random-Dot-Kinematogram). They indicated this choice by making a saccade to one of two targets and then indicated their confidence. Saccade trajectory deviation was taken as a measure of the inhibition of the non-selected target. We found that as evidence increased so did confidence and deviations of saccade trajectory away from the non-selected target. However, a correlational analysis suggested they were not related. In Experiment 2 an option to opt-out of the choice was offered on some trials if choice proved too difficult. In this way we isolated trials on which confidence in target selection was high (i.e., when the option to opt-out was available but not taken). Again saccade trajectory deviations were found not to differ in relation to confidence. In Experiment 3 we directly manipulated confidence, such that participants had high or low task confidence. They showed no differences in saccade trajectory deviations. These results support post-decisional accounts of confidence: evidence supporting the decision to move the eyes is reflected in saccade control, but the confidence that we have in that choice is subject to further post-decisional processes

A Search for Pulsars in Quiescent Soft X-Ray Transients. I

We have carried out a deep search at 1.4 GHz for radio pulsed emission from six soft X-ray transient sources observed during their X-ray quiescent phase. The commonly accepted model for the formation of the millisecond radio pulsars predicts the presence of a rapidly rotating, weakly magnetized neutron star in the core of these systems. The sudden drop in accretion rate associated with the end of an X-ray outburst causes the Alfv\`en surface to move outside the light cylinder, allowing the pulsar emission process to operate. No pulsed signal was detected from the sources in our sample. We discuss several mechanisms that could hamper the detection and suggest that free-free absorption from material ejected from the system by the pulsar radiation pressure could explain our null result.Comment: accepted by Ap

Long term monitoring of mode switching for PSR B0329+54

The mode switching phenomenon of PSR B0329+54 is investigated based on the long-term monitoring from September 2003 to April 2009 made with the Urumqi 25m radio telescope at 1540 MHz. At that frequency, the change of relative intensity between the leading and trailing components is the predominant feature of mode switching. The intensity ratios between the leading and trailing components are measured for the individual profiles averaged over a few minutes. It is found that the ratios follow normal distributions, where the abnormal mode has a wider typical width than the normal mode, indicating that the abnormal mode is less stable than the normal mode. Our data show that 84.9% of the time for PSR B0329+54 was in the normal mode and 15.1% was in the abnormal mode. From the two passages of eight-day quasi-continuous observations in 2004, and supplemented by the daily data observed with 15 m telescope at 610 MHz at Jodrell Bank Observatory, the intrinsic distributions of mode timescales are constrained with the Bayesian inference method. It is found that the gamma distribution with the shape parameter slightly smaller than 1 is favored over the normal, lognormal and Pareto distributions. The optimal scale parameters of the gamma distribution is 31.5 minutes for the abnormal mode and 154 minutes for the normal mode. The shape parameters have very similar values, i.e. 0.75^{+0.22}_{-0.17} for the normal mode and 0.84^{+0.28}_{-0.22} for the abnormal mode, indicating the physical mechanisms in both modes may be the same. No long-term modulation of the relative intensity ratios was found for both the modes, suggesting that the mode switching was stable. The intrinsic timescale distributions, for the first time constrained for this pulsar, provide valuable information to understand the physics of mode switching.Comment: 31 pages,12 figures, Accepted by the Ap