Spatial Distribution of Pulsars

The spatial distributions of pulsars with ages of less than 10⁶ years and more than 10⁶ years are shown to be quite different. The spatial distributions of pulsars with ages of less than 106 years and of the remnants of supernova outbursts are essentially the same. They lie near the galactic plane in a thin zone of width 400 pc. The overwhelming majority of pulsars with ages exceeding 106 years lie outside this zone. These facts suggest a genetic relationship between pulsars and supernova remnants. The spatial distribution of pulsars with different emission periods also supports this point.     

Braking index diagnostics of pulsars. I. alignment,Counteralignment and slowing-down noise

We show that the strong correlation observed between the braking indices (n) and the slowing-down ages (Τ) of pulsars is inconsistent with counteralignment between their rotation and magnetic axes, but that the data on pulsars with positive braking indices is consistent with alignment. Alternatively, slowing-down noise can quantitatively account for the data on all pulsars except the Crab and the Vela, and so for the apparent|n| ∼ Τ² correlation observed for the older pulsars.     

Interstellar electron density

We impose the requirement that the spatial distribution of pulsars deduced from their dispersion measures using a model of the galactic electron density (n _e ) should be consistent with cylindrical symmetry around the galactic centre (assumed to be 10 kpc from the Sun). Using a carefully selected subsample of the pulsars detected by the II Molonglo Survey (II MS), we test a number of simple models and conclude that (i) the effective mean 〈n_e〉) for the whole galaxy is 0.037_-0.012 ^+0.020 cm^-3, (ii) the scale height of electrons is greater than 300 pc and probably about 1 kpc or more, and (iii) there is little evidence for variation of n_e with galactic radius R_GC for R_Gc ≳ 5 kpc. Further, we make a detailed analysis of the contribution to n_e from H II regions. Combining the results of a number of relatively independent calculations, we propose a model for the galactic electron density of the formn _e (z) = 0.030 + 0.020 exp (- |z|/70) cm^-3 where z(pc) is the height above the galactic plane and the second term describes the contribution from H II regions. We believe the statistical uncertainties in the parameters of this model are quite small.     

基于双谱滤波的综合脉冲星时算法研究

毫秒脉冲星的自转频率非常稳定,提供了一种独立的基于遥远自然天体并能持续数百万乃至数十亿年的时间基准,具有稳定性强、运行时间长、服务范围广等特点.为了减弱毫秒脉冲星计时观测中各种高斯噪声对脉冲星时的影响,研究了一种基于双谱滤波的综合脉冲星时构建算法,处理分析了国际脉冲星计时阵(International Pul-sar Timing Array,IPTA)最新发布的4颗毫秒脉冲星(PSR J0437-4715、J0613-0200、J1713+0747和J1909-3744)的观测数据,分析了不同时间尺度综合脉冲星时的稳定性,并与构成国际原子时(International Atomic Time,TAI)的4家授时单位原子钟稳定性进行了比较.结果表明:双谱滤波算法能够较好地抑制观测噪声,提高综合脉冲星时的稳定性.相比于经典加权算法,综合脉冲星时1 yr、10 yr稳定度从7.77×10^-14、8.56×10^-16分别提高到1.50×10^-14、3.50×10^-16,单脉冲星时稳定性的提升也类似.同时发现,综合脉冲星时稳定性在5 yr及以上时间尺度上优于原子钟稳定性,可用于改善当前原子时的长期稳定性.     

Millisecond pulsars

In 1982 we discovered a pulsar with the phenomenal rotation rate of 642 Hz, 20 times faster than the spin rate of the Crab pulsar. The absence of supernova debris in the vicinity of the pulsar at any wavelength indicates an age of the neutron star greater than 10⁵ yr. The miniscule spindown rate of 1.1 × 10^-19 confirms the old age and indicates a surface magnetic field of 10⁹ G. A second millisecond pulsar was discovered by Boriakoff, Buccheri & Fauci (1983) in a 120-day orbit. These fast pulsars may have been spun-up by mass transfer in a close binary evolutionary stage. Arrival-time observations of the 642-Hz pulsar display remarkably low residuals over the first 14 months. The stability implied by these observations, 3 × 10^-14, suggests that millisecond pulsars will provide the most accurate basis for terrestrial dynamical time. If so, the pulsar data will lead to improvements in the planetary ephemeris and to new searches for light-year scale gravitational waves. Many new searches for fast pulsars are under way since previous sky surveys excluded pulsars with spins above 60 Hz.     

The parkes multibeam pulsar survey and interstellar scattering

The Parkes multibeam pulsar survey is a major survey for pulsars lying within a 10^°-wide strip along the southern Galactic plane, using the multibeam receiver on the Parkes 64-m radiotelescope. It is an international collaboration between groups at Jodrell Bank Observatory, Massachusetts Institute of Technology, Bologna Astronomical Observatory and the ATNF. The survey commenced in 1997 August, and has so far succeeded in finding more than 550 previously unknown pulsars. Many of these are distant, with some beyond the centre of the Galaxy according to current models of the interstellar electron density distribution. Interstellar scattering affects the pulse profile of many of the more distant pulsars even at 1374 MHz, the centre frequency of the survey. Preliminary results from the survey are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

The drift model of “magnetars”

It is shown that the drift waves near the light cylinder can cause the modulation of emission with periods of order several seconds. These periods explain the intervals between successive pulses observed in AXPs, SGRs and radio pulsars with long periods. The model under consideration gives the possibility to calculate real rotation periods P of host neutron stars. It is shown that P≤1 s for the investigated objects. The magnetic fields at the surface of the neutron star are of order 10¹¹–10¹³ G and equal to the fields usual for the known radio pulsars.      

Short-term intrinsic-intensity variations of pulsars

Pulsars show intensity variations over timescales ranging from a few microseconds to a few years. Short-term intensity variations,i.e. those having timescales of a few minutes to a few hours had been difficult to study as their timescales are similar to those due to interstellar scintillations. We present here a method to separate the autocorrelation function of the short-term broadband intensity variations from that of the interstellar scintillations and thus overcome the above difficulty. The method assumes that the intrinsic variations are correlated over a bandwidth much larger than the decorrelation bandwidth for scintillations. Hence the ratio of the power in the variations due to the two causes depends on the bandwidth used. By applying the method to the intensity variations of 24 pulsars, we show that the presence of short-term intrinsic variations is very common in the radiation of pulsars. Quasi-periodicities were detected in the intensity variations of many pulsars, but their origin is not clear.     

Can the known millisecond pulsars help in the detection of intermediate-mass black holes at the centers of globular clusters?

We consider the possibility of detecting intermediate-mass (10³–10⁴ M _⊙) black holes, whose existence at the centers of globular clusters is expected from optical and infrared observations, using precise pulse arrival timing for the millisecond pulsars in globular clusters known to date. For some of these pulsars closest to the cluster centers, we have calculated the expected delay times of pulses as they pass in the gravitational field of the central black hole. The detection of such a time delay by currently available instruments for the known pulsars is shown to be impossible at a black hole mass of 10³ M _⊙ and very problematic at a black hole mass of 10⁴ M _⊙. In addition, the signal delay will have a negligible effect on the pulsar periods and their first derivatives compared to the current accuracy of their measurements.     

Interstellar scintillation measurements of pulsars at 326.5 MHz

We have measured the decorrelation frequency (f _v ) and decorrelation time (t _v ) for 15 pulsars. We show by combining our data with those of others thatfv∫ DM^-1.79±0.14 andt _v ∫ DM^-0.80±0.15 up to a dispersion measure (DM) of about 60 cm³ pc. The combined data set does not form a complete sample, but the relations obtained from our measurements on 14 pulsars, which form almost a complete sample up to 41 cm³ pc, are consistent with the above relations, suggesting that these relations are not seriously affected by selection effects. The relations are broadly in agreement with those expected from a homogeneous interstellar medium and are in disagreement with earlier conclusions by others that these relations steepen even for low-DM pulsars. The agreement suggests that the local interstellar medium is homogeneous at least up to a distance of about 2 kpc.     

Weakly radiating radio pulsars

We analyze the statistical distribution of weakly radiating pulsars, i.e., radio pulsars that have passed to the stage of an orthogonal rotator during the evolution of the inclination angle X. We discuss in detail the factors that lead to a significant reduction in the energy losses for this class of objects. We have determined the number of weakly radiating radio pulsars and their distribution in spin period P. The predictions of a theory based on the model of current losses are shown to be consistent with observational data.     

Discovery of 28 pulsars using new techniques for sorting pulsar candidates

Modern pulsar surveys produce many millions of candidate pulsars, far more than can be individually inspected. Traditional methods for filtering these candidates, based upon the signal-to-noise ratio of the detection, cannot easily distinguish between interference signals and pulsars. We have developed a new method of scoring candidates using a series of heuristics which test for pulsar-like properties of the signal. This significantly increases the sensitivity to weak pulsars and pulsars with periods close to interference signals. By applying this and other techniques for ranking candidates from a previous processing of the Parkes Multi-beam Pulsar Survey, 28 previously unknown pulsars have been discovered. These include an eccentric binary system and a young pulsar which is spatially coincident with a known supernova remnant.     

Analysis of the Precision of Pulsar Time Clock Modeltwo

Millisecond pulsars have a very high rotation stability, which can be applied to many research fields, such as the establishment of the pulsar time standard, the detection of gravitational wave, the spacecraft navigation by using X-ray pulsars and so on. In this paper, we employ two millisecond pulsars PSR J0437-4715 and J1713+0743, which are observed by the International Pulsar Timing Array (IPTA), to analyze the precision of pulsar clock parameter and the prediction accuracy of pulse time of arrival (TOA). It is found that the uncertainty of spin frequency is 10^?15 Hz, the uncertainty of the first derivative of spin frequency is 10^?23 s^?2, and the precision of measured rotational parameters increases by one order of magnitude with the accumulated observational data every 4～5 years. In addition, the errors of TOAs within 4.8 yr which are predicted by the clock model established by the 10 yr data of J0437-4715 are less than 1 μs. Therefore, one can use the pulsar time standard to calibrate the atomic clock, and make the atomic time deviate from the TT (Terrestrial Time) less than 1 μs within 4.8 yr.     

Scintillation parameters for 49 pulsars

We report on multi-epoch, multifrequency observations of 64 pulsars with high spectral and time resolution. Scintillation parameters were obtained for 49 pulsars, including 13 millisecond pulsars. Scintillation speeds were derived for all 49, which doubles the number of pulsars with speeds measured in this way. There is excellent agreement between the scintillation speed and proper motion for the millisecond pulsars in our sample using the simple assumption of a mid-placed scattering screen. This indicates that the scaleheight of scattering electrons is similar to that of the dispersing electrons. In addition, we present observations of the Vela pulsar at 14 and 23 GHz, and show that the scintillation bandwidth scales as ν^3.93 over a factor of 100 in observing frequency. We show that for PSR J0742−2822, and perhaps PSR J0837−4135, the Gum nebula is responsible for the high level of turbulence along their lines of sight, contrary to previous indications. There is a significant correlation between the scintillation speeds and the product of the pulsar's period and period derivative for the 'normal' pulsars. However, we believe this to be caused by selection effects both in pulsar detection experiments and in the choice of pulsars used in scintillation studies.     

On the supernova remnants produced by pulsars

We conclude that pulsar-driven supernova remnants (SNRs) are extremely rare objects. Indeed an analysis of the known sample of plerions suggests a very low birthrate ∼ 1 in 240 years. Long-lived and bright plerions like the Crab nebula are likely to be produced only when the pulsar has an initial period ∼ 10–20 milliseconds and a field ∼ 10¹² G. Such pulsars inside rapidly expanding shell remnants should also produce detectable plerions. The extreme rarity of SNRs with such hybrid morphology leads us to conclude that these pulsars must have been born with an initial period larger than ∼ 35–70 milliseconds. Joint Astronomy Program, Department of Physics, Indian Institute of Science, Bangalore 560012.     

Discovery of three new pulsars in a 610-MHz pulsar survey with the GMRT

We report on the discovery of three new pulsars in the first blind survey of the north Galactic plane  (45° < l < 135°; | b | < 1°)  with the Giant Meterwave Radio telescope (GMRT) at an intermediate frequency of 610 MHz. The survey covered 106 deg² with a sensitivity of roughly 1 mJy to long-period pulsars (pulsars with period longer than 1 s). The three new pulsars have periods of 318, 933 and 1056 ms. Their timing parameters and flux densities, obtained in follow-up observations with the Lovell Telescope at Jodrell Bank and the GMRT, are presented. We also report on pulse nulling behaviour in one of the newly discovered pulsars, PSR J2208+5500.     

Dispersion measure variations and their effect on precision pulsar timing

We present an analysis of the variations seen in the dispersion measures (DMs) of 20-ms pulsars observed as part of the Parkes Pulsar Timing Array project. We carry out a statistically rigorous structure function analysis for each pulsar and show that the variations seen for most pulsars are consistent with those expected for an interstellar medium characterized by a Kolmogorov turbulence spectrum. The structure functions for PSRs J1045−4509 and J1909−3744 provide the first clear evidence for a large inner scale, possibly due to ion–neutral damping. We also show the effect of the solar wind on the DMs and show that the simple models presently implemented into pulsar timing packages cannot reliably correct for this effect. For the first time we clearly show how DM variations affect pulsar timing residuals and how they can be corrected in order to obtain the highest possible timing precision. Even with our presently limited data span, the residuals (and all parameters derived from the timing) for six of our pulsars have been significantly improved by correcting for the DM variations.     

Statistics of neutron stars at the stage of a supersonic propeller

We analyze the statistical distribution of neutron stars at the stage of a supersonic propeller. An important point of our analysis is allowance for the evolution of the angle of inclination of the magnetic axis to the spin axis of the neutron star for the boundary of the transition to the supersonic propeller stage. We have determined the spin period distributions of pulsars at the propeller stage for two models: the model with hindered particle escape from the stellar surface and the model with free particle escape. As a result, we have shown that consistent allowance for the evolution of the inclination angle in the region of extinct radio pulsars for the two models leads to an increase in the total number of neutron stars at the supersonic propeller stage. This increase stems from the fact that when allowing for the evolution of the inclination angle χ for neutron stars in the region of extinct radio pulsars and, hence, for the boundary of the transition to the propeller stage, this transition is possible at shorter spin periods (P ～ 5–10 s) than assumed in the standard model.     

Pulsar braking indices revisited

Using the standard equation for the slowdown of a neutron star, we derive a formula for the braking index via integration rather than the conventional differentiation. The new formula negates the need to measure the second time derivative of the rotation frequency, ν¨ . We show that the method gives similar braking indices for PSR B1509−58 and the Crab pulsar to those already in the literature. We point out that our method is useful for obtaining the braking indices of moderate-aged pulsars without the need for long, phase-connected timing solutions. We applied the method to 20 pulsars and discuss the implications of the results. We find that virtually all the derived braking indices are dominated by the effects of (unseen) glitches, the recovery from which corrupts the value of ν˙ . However, any real, large, positive braking index has implications for magnetic field decay and offers support to recent models of pulsar evolution.     

How rapidly do neutron stars spin at birth? Constraints from archival X-ray observations of extragalactic supernovae

Traditionally, studies aimed at inferring the distribution of birth periods of neutron stars are based on radio surveys. Here we propose an independent method to constrain the pulsar spin periods at birth based on their X-ray luminosities. In particular, the observed luminosity distribution of supernovae (SNe) poses a constraint on the initial rotational energy of the embedded pulsars, via the     correlation found for radio pulsars, and under the assumption that this relation continues to hold beyond the observed range. We have extracted X-ray luminosities (or limits) for a large sample of historical SNe observed with Chandra , XMM and Swift , which have been firmly classified as core-collapse SNe. We have then compared these observational limits with the results of Monte Carlo simulations of the pulsar X-ray luminosity distribution for a range of values of the birth parameters. We find that a pulsar population dominated by millisecond periods at birth is ruled out by the data.