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

Are We Seeing Magnetic Axis Reorientation in the Crab and Vela Pulsars?

Variation in the angle $\alpha$ between a pulsar's rotational and magnetic axes would change the torque and spin-down rate. We show that sudden increases in $\alpha$, coincident with glitches, could be responsible for the persistent increases in spin-down rate that follow glitches in the Crab pulsar. Moreover, changes in $\alpha$ at a rate similar to that inferred for the Crab pulsar account naturally for the very low braking index of the Vela pulsar. If $\alpha$ increases with time, all pulsar ages obtained from the conventional braking model are underestimates. Decoupling of the neutron star liquid interior from the external torque cannot account for Vela's low braking index. Variations in the Crab's pulse profile due to changes in $\alpha$ might be measurable.Comment: 14 pages and one figure, Latex, uses aasms4.sty. Accepted to ApJ Letter

Long-term Observations of Three Nulling Pulsars

We present an analysis of approximately 200 hours of observations of the pulsars J1634$-$5107, J1717$-$4054 and J1853$+$0505, taken over the course of 14.7 yr. We show that all of these objects exhibit long term nulls and radio-emitting phases (i.e. minutes to many hours), as well as considerable nulling fractions (NFs) in the range $\sim67\,\% - 90\,\%$. PSR J1717$-$4054 is also found to exhibit short timescale nulls ($1 - 40~P$) and burst phases ($\lesssim 200~P$) during its radio-emitting phases. This behaviour acts to modulate the NF, and therefore the detection rate of the source, over timescales of minutes. Furthermore, PSR J1853$+$0505 is shown to exhibit a weak emission state, in addition to its strong and null states, after sufficient pulse integration. This further indicates that nulls may often only represent transitions to weaker emission states which are below the sensitivity thresholds of particular observing systems. In addition, we detected a peak-to-peak variation of $33\pm1\,\%$ in the spin-down rate of PSR J1717$-$4054, over timescales of hundreds of days. However, no long-term correlation with emission variation was found.Comment: 10 pages, 8 figures, accepted for publication in MNRA

Characterization of the Crab Pulsar's Timing Noise

We present a power spectral analysis of the Crab pulsar's timing noise, mainly using radio measurements from Jodrell Bank taken over the period 1982-1989. The power spectral analysis is complicated by nonuniform data sampling and the presence of a steep red power spectrum that can distort power spectra measurement by causing severe power ``leakage''. We develop a simple windowing method for computing red noise power spectra of uniformly sampled data sets and test it on Monte Carlo generated sample realizations of red power-law noise. We generalize time-domain methods of generating power-law red noise with even integer spectral indices to the case of noninteger spectral indices. The Jodrell Bank pulse phase residuals are dense and smooth enough that an interpolation onto a uniform time series is possible. A windowed power spectrum is computed revealing a periodic or nearly periodic component with a period of about 568 days and a 1/f^3 power-law noise component with a noise strength of 1.24 +/- 0.067 10^{-16} cycles^2/sec^2 over the analysis frequency range 0.003 - 0.1 cycles/day. This result deviates from past analyses which characterized the pulse phase timing residuals as either 1/f^4 power-law noise or a quasiperiodic process. The analysis was checked using the Deeter polynomial method of power spectrum estimation that was developed for the case of nonuniform sampling, but has lower spectral resolution. The timing noise is consistent with a torque noise spectrum rising with analysis frequency as f implying blue torque noise, a result not predicted by current models of pulsar timing noise. If the periodic or nearly periodic component is due to a binary companion, we find a companion mass > 3.2 Earth masses.Comment: 53 pages, 9 figures, submitted to MNRAS, abstract condense

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

On the Apparent Nulls and Extreme Variability of PSR J1107-5907

We present an analysis of the emission behaviour of PSR J1107-5907, a source known to exhibit separate modes of emission, using observations obtained over approximately 10 yr. We find that the object exhibits two distinct modes of emission; a strong mode with a broad profile and a weak mode with a narrow profile. During the strong mode of emission, the pulsar typically radiates very energetic emission over sequences of ~200-6000 pulses (~60 s-24 min), with apparent nulls over time-scales of up to a few pulses at a time. Emission during the weak mode is observed outside of these strong-mode sequences and manifests as occasional bursts of up to a few clearly detectable pulses at a time, as well as low-level underlying emission which is only detected through profile integration. This implies that the previously described null mode may in fact be representative of the bottom-end of the pulse intensity distribution for the source. This is supported by the dramatic pulse-to-pulse intensity modulation and rarity of exceptionally bright pulses observed during both modes of emission. Coupled with the fact that the source could be interpreted as a rotating radio transient (RRAT)-like object for the vast majority of the time, if placed at a further distance, we advance that this object likely represents a bridge between RRATs and extreme moding pulsars. Further to these emission properties, we also show that the source is consistent with being a near-aligned rotator and that it does not exhibit any measurable spin-down rate variation. These results suggest that nulls observed in other intermittent objects may in fact be representative of very weak emission without the need for complete cessation. As such, we argue that longer (> 1 h) observations of pulsars are required to discern their true modulation properties.Comment: 15 pages, 10 figures, accepted for publication in MNRA

Long-term Radio Observations of the Intermittent Pulsar B1931+24

We present an analysis of approximately 13-yr of observations of the intermittent pulsar B1931+24 to further elucidate its behaviour. We find that while the source exhibits a wide range of nulling (~4-39 d) and radio-emitting (~1-19 d) timescales, it cycles between its different emission phases over an average timescale of approximately 38 d, which is remarkably stable over many years. On average, the neutron star is found to be radio emitting for 26 +- 6 % of the time. No evidence is obtained to suggest that the pulsar undergoes any systematic, intrinsic variations in pulse intensity during the radio-emitting phases. In addition, we find no evidence for any correlation between the length of consecutive emission phases. An analysis of the rotational behaviour of the source shows that it consistently assumes the same spin-down rates, i.e. nudot = -16 +- 1 x 10^-15 s^-2 when emitting and nudot = -10.8 +- 0.4 x 10^-15 s^-2 when not emitting, over the entire observation span. Coupled with the stable switching timescale, this implies that the pulsar retains a high degree of magnetospheric memory, and stability, in spite of comparatively rapid (~ms) dynamical plasma timescales. While this provides further evidence to suggest that the behaviour of the neutron star is governed by magnetospheric-state switching, the underlying trigger mechanism remains illusive. This should be elucidated by future surveys with next generation telescopes such as LOFAR, MeerKAT and the SKA, which should detect similar sources and provide more clues to how their radio emission is regulated.Comment: 12 pages, 12 figures, accepted for publication in MNRA

Spin frequency evolution and pulse profile variations of the recently re-activated radio magnetar XTE J1810-197

After spending almost a decade in a radio-quiet state, the Anomalous X-ray Pulsar XTE J1810-197 turned back on in early December 2018. We have observed this radio magnetar at 1.5 GHz with ~daily cadence since the first detection of radio re-activation on 8 December 2018. In this paper, we report on the current timing properties of XTE J1810-197 and find that the magnitude of the spin frequency derivative has increased by a factor of 2.6 over our 48-day data set. We compare our results with the spin-down evolution reported during its previous active phase in the radio band. We also present total intensity pulse profiles at five different observing frequencies between 1.5 and 8.4 GHz, collected with the Lovell and the Effelsberg telescopes. The profile evolution in our data set is less erratic than what was reported during the previous active phase, and can be seen varying smoothly between observations. Profiles observed immediately after the outburst show the presence of at least five cycles of a very stable ~50-ms periodicity in the main pulse component that lasts for at least tens of days. This remarkable structure is seen across the full range of observing frequencies.Comment: 9 pages, 7 figures, updated with additional analysis of the 50-ms oscillation, accepted for publication in MNRA

Determination of the orbital parameters of binary pulsars

We present a simple novel method for determining the orbital parameters of binary pulsars. This method works with any sort of orbital sampling, no matter how sparse, provided that information on the period derivatives is available with each measurement of the rotational period of the pulsar, and it is applicable to binary systems with nearly circular orbits. We use the technique to precisely estimate the hitherto unknown orbital parameters of two binary millisecond pulsars in the globular cluster 47 Tucanae, 47 Tuc S and T. The method can also be used more generally to make first-order estimates of the orbital parameters of binary systems using a minimal amount of data.Comment: 13 pages, 6 figures, accepted for publication in MNRA