The Parkes Southern Pulsar Survey — II. Final results and population analysis

A survey of the entire southern sky for millisecond and low-luminosity pulsars using the ATNF Parkes radio telescope has now been completed. The survey detected 298 pulsars, of which 101 were previously unknown. The new pulsars include 17 millisecond pulsars. This is the largest sample of both normal and millisecond pulsars detected in any survey. Combining our sample with other recent surveys in the Northern Hemisphere, we present a statistical study of the populations of both normal and millisecond pulsars. We find that the improved statistics allow us to estimate the number and birth-rate of both types of pulsar down to a 400-MHz luminosity limit of 1 mJy kpc². The local surface densities of potentially observable normal pulsars and millisecond pulsars are both about 30 kpc⁻², corresponding to ∼ 30000 potentially observable pulsars of each type in the Galaxy. Once beaming effects are taken into consideration we estimate that the active population of normal pulsars is ∼ 160000. Although there is evidence for flattening of the luminosity function of normal pulsars, this is not evident for millisecond pulsars which probably have a substantial population with luminosities below 1 mJy kpc². After correcting for beaming effects, we estimate that a normal pulsar is born with a luminosity greater than 1 mJy kpc² between once every 60 and 330 yr in the Galaxy. The birth-rate of millisecond pulsars is at least 3 × 10⁻⁶ yr⁻¹ above the same luminosity limit. Modelling the observed transverse speeds of millisecond pulsars using a dynamical simulation, we find their mean birth velocity to be 130 ± 30 km s⁻¹, significantly lower than that of the normal pulsars.     

Discovery of 10 pulsars in an Arecibo drift-scan survey

We present the results of a 430-MHz survey for pulsars conducted during the upgrade to the 305-m Arecibo radio telescope. Our survey covered a total of 1147 deg² of sky using a drift-scan technique. We detected 33 pulsars, 10 of which were not known prior to the survey observations. The highlight of the new discoveries is PSR J0407+1607, which has a spin period of 25.7 ms, a characteristic age of 1.5 Gyr and is in a 1.8-yr orbit about a low-mass  (>0.2 M_⊙)  companion. The long orbital period and small eccentricity  ( e = 0.0009)  make the binary system an important new addition to the ensemble of binary pulsars suitable to test for violations of the strong equivalence principle. We also report on our initially unsuccessful attempts to detect optically the companion to J0407+1607, which imply that its absolute visual magnitude is >12.1. If, as expected on evolutionary grounds, the companion is an He white dwarf, our non-detection implies a cooling age of least 1 Gyr.     

Drifting sub-pulses in two newly discovered pulsars

We have detected the rare phenomenon of stable, drifting sub-pulse behaviour in two pulsars discovered in the recent Swinburne intermediate latitude pulsar survey. The pulsars, PSR     and PSR J1919+0134, have approximate periods ( P ) of 1.873 and 1.6039 s respectively.
Both pulsars have multicomponent profiles, and distinct drifting is observed across them. We have identified a single drift mode in both pulsars: the drift rate for PSR     being 5.4(1) ms P ⁻¹ and 5.8(2) ms P ⁻¹ for PSR 1919+0134. The drifting is linear across the profile with no departure from linearity at the edges within the sensitivity of our observations.     

Precision timing measurements of PSR J1012+5307

We present results and applications of high-precision timing measurements of the binary millisecond pulsar J1012+5307. Combining our radio timing measurements with results based on optical observations, we derive complete 3D velocity information for this system. Correcting for Doppler effects, we derive the intrinsic spin parameters of this pulsar and a characteristic age of 8.6±1.9 Gyr . Our upper limit for the orbital eccentricity of only 8×10⁻⁷ (68 per cent confidence level) is the smallest ever measured for a binary system. We demonstrate that this makes the pulsar an ideal laboratory in which to test certain aspects of alternative theories of gravitation. Our precision measurements suggest deviations from a simple pulsar spin-down timing model, which are consistent with timing noise and the extrapolation of the known behaviour of slowly rotating pulsars.     

The Parkes Multibeam Pulsar Survey: PSR J1811−1736, a pulsar in a highly eccentric binary system

We are undertaking a high-frequency survey of the Galactic plane for radio pulsars, using the 13-element multibeam receiver on the 64-m Parkes radio telescope. We describe briefly the survey system and some of the initial results. PSR J1811−1736, one of the first pulsars discovered with this system, has a rotation period of 104 ms. Subsequent timing observations using the 76-m radio telescope at Jodrell Bank show that it is in an 18.8-d, highly eccentric binary orbit. We have measured the rate of advance of periastron which indicates a total system mass of 2.6±0.9 M_⊙, and the minimum companion mass is about 0.7 M_⊙. This, the high orbital eccentricity and the recycled nature of the pulsar suggest that this system is composed of two neutron stars, only the fourth or fifth such system known in the disc of the Galaxy.     

The efficiency of gamma-ray emission from pulsars

We present a modified scenario of gamma-ray emission from pulsars within the framework of polar cap models. Our model incorporates the possible acceleration of electron–positron pairs created in magnetospheres, and their subsequent contribution to the gamma-ray luminosity L _γ. It also reproduces the empirical trend in L _γ for seven pulsars detected with Compton Gamma-Ray Observatory ( CGRO ) experiments. At the same time it avoids basic difficulties faced by theoretical models when confronted with observational constraints.   We show that the classical and millisecond pulsars form two distinct branches in the L _γ— L _sd diagram (where L _sd is the spin-down luminosity). In particular, we explain why the millisecond pulsar J0437−4715 has not been detected with any of the CGRO instruments despite its very high position in the ranking list of spin-down fluxes (i.e. L _sd/ D ², where D is a distance). The gamma-ray luminosity predicted for this particular object is about one order of magnitude below the upper limit set by EGRET.     

The Parkes Multibeam Pulsar Survey – VI. Discovery and timing of 142 pulsars and a Galactic population analysis

We present the discovery and follow-up observations of 142 pulsars found in the Parkes 20-cm multibeam pulsar survey of the Galactic plane. These new discoveries bring the total number of pulsars found by the survey to 742. In addition to tabulating spin and astrometric parameters, along with pulse width and flux density information, we present orbital characteristics for 13 binary pulsars which form part of the new sample. Combining these results from another recent Parkes multibeam survey at high Galactic latitudes, we have a sample of 1008 normal pulsars which we use to carry out a determination of their Galactic distribution and birth rate. We infer a total Galactic population of  30 000 ± 1100  potentially detectable pulsars (i.e. those beaming towards us) having 1.4-GHz luminosities above 0.1 mJy kpc². Adopting the Tauris & Manchester beaming model, this translates to a total of  155 000 ± 6000  active radio pulsars in the Galaxy above this luminosity limit. Using a pulsar current analysis, we derive the birth rate of this population to be  1.4 ± 0.2  pulsars per century. An important conclusion from our work is that the inferred radial density function of pulsars depends strongly on the assumed distribution of free electrons in the Galaxy. As a result, any analyses using the most recent electron model of Cordes & Lazio predict a dearth of pulsars in the inner Galaxy. We show that this model can also bias the inferred pulsar scaleheight with respect to the Galactic plane. Combining our results with other Parkes multibeam surveys we find that the population is best described by an exponential distribution with a scaleheight of 330 pc. Surveys underway at Parkes and Arecibo are expected to improve the knowledge of the radial distribution outside the solar circle, and to discover several hundred new pulsars in the inner Galaxy.     

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.     

A timing formula for main-sequence star binary pulsars

In binary radio pulsars with a main-sequence star companion, the spin-induced quadrupole moment of the companion gives rise to a precession of the binary orbit. As a first approximation one can model the secular evolution caused by this classical spin-orbit coupling by linear-in-time changes of the longitude of periastron and the projected semi-major axis of the pulsar orbit. This simple representation of the precession of the orbit neglects two important aspects of the orbital dynamics of a binary pulsar with an oblate companion. First, the quasiperiodic effects along the orbit, owing to the anisotropic 1/ r ³ nature of the quadrupole potential. Secondly, the long-term secular evolution of the binary orbit, which leads to an evolution of the longitude of periastron and the projected semi-major axis, which is non-linear in time.   In this paper a simple timing formula for binary radio pulsars with a main-sequence star companion is presented which models the short-term secular and most of the short-term periodic effects caused by the classical spin-orbit coupling. I also give extensions of the timing formula that account for long-term secular changes in the binary pulsar motion. It is shown that the short-term periodic effects are important for the timing observations of the binary pulsar PSR B1259–63. The long-term secular effects are likely to become important in the next few years of timing observations of the binary pulsar PSR J0045–7319. They could help to restrict or even determine the moments of inertia of the companion star and thus probe its internal structure.   Finally, I reinvestigate the spin-orbit precession of the binary pulsar PSR J0045–7319 since the analysis given in the literature is based on an incorrect expression for the precession of the longitude of periastron. A lower limit of 20° for the inclination of the B star with respect to the orbital plane is derived.     

Optimal filters for the construction of the ensemble pulsar time

An algorithm of the ensemble pulsar time based on the optimal Wiener filtration method has been constructed. This algorithm allows the separation of the contributions to the post-fit pulsar timing residuals of the atomic clock and the pulsar itself. Filters were designed using the cross- and auto-covariance functions of the timing residuals. The method has been applied to the timing data of millisecond pulsars PSR B1855+09 and B1937+21 and allowed the filtering out of the atomic-scale component from the pulsar data. Direct comparison of the terrestrial time TT(BIPM06) and the ensemble pulsar time PT_ens revealed that the fractional instability of TT(BIPM06)−PT_ens is equal to  σ _z = (0.8 ± 1.9) × 10⁻¹⁵  . Based on the  σ _z   statistics of TT(BIPM06)−PT_ens, a new limit of the energy density of the gravitational wave background was calculated to be equal to  Ω_g h ²∼ 3 × 10⁻⁹  .     

The impact of element diffusion on the formation and evolution of helium white dwarf stars

The aim of this work is to investigate the effect of element diffusion on the evolution of helium white dwarfs. To this end, we couple the multicomponent flow equations that describe gravitational settling, chemical and thermal diffusion to an evolutionary code. We compute the evolution of a set of helium white dwarf models with masses ranging from 0.169 to 0.406 M_⊙. In particular, several low-mass white dwarfs have been found in binary systems as companion to millisecond pulsars. In these systems, pulsar emission is activated by mass transfer episodes so that, if we place the zero-age point at the end of such mass transfer, then the pulsar and the white dwarf ages should be equal. Interestingly enough, available models of helium white dwarfs neglect element diffusion. Using such models, good agreement has been found between the ages of the components of the PSR J1012+5307 system. However, recent observations of the PSR B1855+09 system cast doubts on the correctness of such models, which predict a white dwarf age twice as long as the spin-down age of the pulsar. In this work, we find that element diffusion induces thermonuclear hydrogen shell flashes for models in the mass interval 0.18≲ M /M_⊙ ≲ 0.41 . We show, in particular, that the occurrence of these diffusion-induced flashes eventually leads to white dwarf models with hydrogen envelope masses too small to support any further nuclear burning, thus implying much shorter cooling ages than in the case when diffusion is neglected. In particular, excellent agreement is found between the ages of PSR B1855+09 system components, solving the age discrepancy from first principles.     

The mass of the neutron star in the binary millisecond pulsar PSR J 1012 + 5307

We have measured the radial velocity variation of the white dwarf secondary in the binary system containing the millisecond pulsar PSR J 1012 + 5307. Combined with the orbital parameters of the radio pulsar, we infer a mass ratio q (≡ M ₁/ M ₂) = 10.5 ± 0.5. Our optical spectroscopy has also allowed us to determine the mass of the white dwarf companion by fitting the spectrum to a grid of DA model atmospheres: we estimate M ₂ = 0.16 ± 0.02 M⊙, and hence the mass of the neutron star is 1.64 ± 0.22 M⊙, where the error is dominated by that of M ₂. The orbital inclination is 52 ± 4°. For an initial neutron star mass of ∼ 1.4 M⊙, only a few tenths of a solar mass at most has been successfully accreted over the lifetime of the progenitor low-mass X-ray binary. If the initial mass of the secondary was ∼ 1 M⊙, our result suggests that the mass transfer may have been non-conservative.     

A deep search for pulsar wind nebulae using pulsar gating

Using the Australia Telescope Compact Array (ATCA) we have imaged the fields around five promising pulsar candidates to search for radio pulsar wind nebulae (PWNe). We have used the ATCA in its pulsar-gating mode; this enables an image to be formed containing only off-pulse visibilities, thereby dramatically improving the sensitivity to any underlying PWN. Data from the Molonglo Observatory Synthesis Telescope were also used to provide sensitivity on larger spatial scales. This survey found a faint new PWN around PSR B0906−49; here we report on non-detections of PWNe towards PSRs B1046−58, B1055−52, B1610−50 and J1105−6107. Our radio observations of the field around PSR B1055−52 argue against previous claims of an extended X-ray and radio PWN associated with the pulsar. If these pulsars power unseen, compact radio PWNe, upper limits on the radio flux indicate that a fraction of less than 10⁻⁶ of their spin-down energy is used to power this emission. Alternatively, PSRs B1046−58 and B1610−50 may have relativistic winds similar to other young pulsars and the unseen PWN may be resolved and fainter than our surface brightness sensitivity threshold. We can then determine upper limits on the local interstellar medium (ISM) density of 2.2×10⁻³ and 1×10⁻² cm⁻³, respectively. Furthermore, we derive the spatial velocities of these pulsars to be ∼450 km s⁻¹ and thus rule out the association of PSR B1610−50 with supernova remnant (SNR) G332.4+00.1 (Kes 32). Strong limits on the ratio of unpulsed to pulsed emission are also determined for three pulsars.     

The micro-glitch in PSR B1821−24: a case for a strange pulsar?

The single glitch observed in PSR B1821−24, a millisecond pulsar in M28, is unusual on two counts. First, the magnitude of this glitch is at least an order of magnitude smaller  (Δν/ν∼ 10⁻¹¹)  than the smallest glitch observed to date. Secondly, all other glitching pulsars have strong magnetic fields with   B ≳ 10¹¹ G  and are young, whereas PSR B1821−24 is an old recycled pulsar with a field strength of  2.25 × 10⁹ G  . We have earlier suggested that some of the recycled pulsars could actually be strange quark stars. In this work, we argue that the crustal properties of such a strange pulsar are just right to give rise to a glitch of this magnitude, explaining the scarcity of larger glitches in millisecond pulsars.     

Limits on radio emission from pulsar wind nebulae

We report on a sensitive survey for radio pulsar wind nebulae (PWN) towards 27 energetic and/or high-velocity pulsars. Observations were carried out at 1.4 GHz using the Very Large Array and the Australia Telescope Compact Array and utilized pulsar-gating to search for off-pulse emission. These observing parameters resulted in a considerably more sensitive search than previous surveys and could detect PWN over a much wider range of spatial scales (and hence ambient densities and pulsar velocities). However, no emission clearly corresponding to a PWN was discovered. Based on these non-detections we argue that the young and energetic pulsars in our sample have winds which are typical of young pulsars, but produce unobservable PWN because they reside in low-density ( n ∼0.003 cm⁻³) regions of the interstellar medium. However, non-detection of PWN around older and less energetic pulsars can only be explained if the radio luminosity of their winds is less than 10⁻⁵ of their spin-down luminosity, implying an efficiency at least an order of magnitude smaller than that seen for young pulsars.     

Radio pulsars in Terzan 5

We report on searches of the globular cluster Terzan 5 for low-luminosity and accelerated radio pulsars using the 64-m Parkes radio telescope. One new millisecond pulsar, designated PSR J1748−2446C, was discovered, having a period of 8.44 ms. Timing measurements using the 76-m Lovell radio telescope at Jodrell Bank show that it is a solitary pulsar and lies close to the core of the cluster. We also present the results of timing measurements which show that the longer period pulsar PSR J1748−2444 (formerly known as PSR B1744−24B) lies 10 arcmin from the core of the cluster and is unlikely to be associated with the cluster. We conclude that there are further pulsars to be detected in the cluster.     

The possible companions of young radio pulsars

We discuss the formation of pulsars with massive companions in eccentric orbits. We demonstrate that the probability for a non-recycled radio pulsar to have a white dwarf as a companion is comparable to that of having an old neutron star as a companion. Special emphasis is given to PSR B1820−11 and PSR B2303+46. Based on population synthesis calculations we argue that PSR B1820−11 and PSR B2303+46 could very well be accompanied by white dwarfs with mass ≳1.1 M_⊙. For PSR B1820−11, however, we cannot exclude the possibility that its companion is a main-sequence star with a mass between ∼0.7 M_⊙ and ∼5 M_⊙.     

Glitch monitoring in PSRs B1046−58 and B1737−30

Large glitches were recently observed in the spin rates of two pulsars, B1046−58 and B1737−30. The glitches were characterized by fractional increases in rotation rate of 0.77 and  1.44×10⁻⁶  respectively. PSR B1737−30 is the most frequently glitching pulsar and this is the largest glitch so far observed from it. Most of the jump in the spin-down rate accompanying these glitches decayed away on short time-scales of a few days. For PSR B1737−30, there appears to be a cumulative shift in spin-down rate resulting from its frequent glitches. This probably accounts for its braking index of  −4±2  suggested by the available data, while a value of  2.1±0.2  is obtained for B1046−58.     

PSR J1012+5307: younger than it looks?

Lorimer et al. have recently reported that the spin-down age (∼7 × 10⁹ yr) of the low-mass binary pulsar PSR J1012+5307 is much higher than the cooling age (3 × 10⁸ yr) of its white dwarf companion. The proposed solutions for this discrepancy are outlined and discussed. In particular, the revised cooling age estimate proposed by Alberts et al. agrees with data from other low-mass binary pulsar systems if a transition to the 'classical' cooling regime occurs between ∼0.14 and ∼0.28 M_⊙. If this transition is excluded, PSR J1012+5307 seems to have finished its accretion phase far from the spin-up line.     

射电脉冲星周期跃变研究的进展

射电脉冲星周期跃变被认为是研究中子星内部结构和状态的极好探针。脉冲星高频巡天发现了一批年青脉冲星,脉冲星周期跃变的观测研究也有了飞快进展。至少发现了２５颗有跃变现象的脉冲星（简称跃变脉冲星）和７６次跃变事件。ＰＳＲＪ０８３５－４５１０是目前已有跃变脉冲星活动参数最高的,ＰＳＲＪ１３４１－６２２２０的跃变活动最频繁,而ＰＳＲＪ１６１４－５０４７在１９９５年发生的跃变是规模最大的,不同脉冲星的跃变事件