Southern Pulsars Research Articles

Search for an Isotropic Gravitational-wave Background with the Parkes Pulsar Timing Array

Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A background of GWs modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial correlation. Here we describe a search for an isotropic stochastic gravitational-wave background (GWB) using observations of 30 millisecond pulsars from the third data release of the Parkes Pulsar Timing Array (PPTA), which spans 18 yr. Using current Bayesian inference techniques we recover and characterize a common-spectrum noise process. Represented as a strain spectrum , we measure and α = −0.45 ± 0.20, respectively (median and 68% credible interval). For a spectral index of α = −2/3, corresponding to an isotropic background of GWs radiated by inspiraling supermassive black hole binaries, we recover an amplitude of . However, we demonstrate that the apparent signal strength is time-dependent, as the first half of our data set can be used to place an upper limit on A that is in tension with the inferred common-spectrum amplitude using the complete data set. We search for spatial correlations in the observations by hierarchically analyzing individual pulsar pairs, which also allows for significance validation through randomizing pulsar positions on the sky. For a process with α = −2/3, we measure spatial correlations consistent with a GWB, with an estimated false-alarm probability of p ≲ 0.02 (approx. 2σ). The long timing baselines of the PPTA and the access to southern pulsars will continue to play an important role in the International Pulsar Timing Array.

The Parkes pulsar timing array second data release: timing analysis

ABSTRACT The main goal of pulsar timing array experiments is to detect correlated signals such as nanohertz-frequency gravitational waves. Pulsar timing data collected in dense monitoring campaigns can also be used to study the stars themselves, their binary companions, and the intervening ionized interstellar medium. Timing observations are extraordinarily sensitive to changes in path-length between the pulsar and the Earth, enabling precise measurements of the pulsar positions, distances and velocities, and the shapes of their orbits. Here we present a timing analysis of 25 pulsars observed as part of the Parkes Pulsar Timing Array (PPTA) project over time spans of up to 24 yr. The data are from the second data release of the PPTA, which we have extended by including legacy data. We make the first detection of Shapiro delay in four Southern pulsars (PSRs J1017−7156, J1125−6014, J1545−4550, and J1732−5049), and of parallax in six pulsars. The prominent Shapiro delay of PSR J1125−6014 implies a neutron star mass of Mp = 1.5 ± 0.2 M⊙ (68 per cent credibility interval). Measurements of both Shapiro delay and relativistic periastron advance in PSR J1600−3053 yield a large but uncertain pulsar mass of $M_p = 2.06^{+0.44}_{-0.41}$ M⊙ (68 per cent credibility interval). We measure the distance to PSR J1909−3744 to a precision of 10 lyr, indicating that for gravitational wave periods over a decade, the pulsar provides a coherent baseline for pulsar timing array experiments.

MWA tied-array processing II: Polarimetric verification and analysis of two bright southern pulsars

Abstract Polarimetric studies of pulsars at low radio frequencies provide important observational insights into the pulsar emission mechanism and beam models, and probe the properties of the magneto-ionic interstellar medium (ISM). Aperture arrays are the main form of next-generation low-frequency telescopes, including the Murchison Widefield Array (MWA). These require a distinctly different approach to data processing (e.g. calibration and beamforming) compared to traditional dish antennas. As the second paper of this series, we present a verification of the MWA’s pulsar polarimetry capability, using two bright southern pulsars, PSRs J0742–2822 and J1752–2806. Our observations simultaneously cover multiple frequencies (76–313 MHz) and were taken at multiple zenith angles (ZA) during a single night for each pulsar. We show that the MWA can be reliably calibrated for ZA ≲45° and frequencies ≲270 MHz. We present the polarimetric profiles for PSRs J0742–2822 and J1752–2806 at frequencies lower than 300 MHz for the first time, along with an analysis of the linear polarisation degree and pulse profile evolution with frequency. For PSR J0742–2822, the measured degree of linear polarisation shows a rapid decrease at low frequencies, in contrast with the generally expected trend, which can be attributed to depolarisation effects from small-scale, turbulent, magneto-ionic ISM components. This effect has not been widely explored for pulsars in general and will be further investigated in future work.

A Census of Southern Pulsars at 185 MHz

AbstractThe Murchison Widefield Array, and its recently developed Voltage Capture System, facilitates extending the low-frequency range of pulsar observations at high-time and -frequency resolution in the Southern Hemisphere, providing further information about pulsars and the ISM. We present the results of an initial time-resolved census of known pulsars using the Murchison Widefield Array. To significantly reduce the processing load, we incoherently sum the detected powers from the 128 Murchison Widefield Array tiles, which yields ~10% of the attainable sensitivity of the coherent sum. This preserves the large field-of-view (~450 deg2 at 185 MHz), allowing multiple pulsars to be observed simultaneously. We developed a WIde-field Pulsar Pipeline that processes the data from each observation and automatically folds every known pulsar located within the beam. We have detected 50 pulsars to date, 6 of which are millisecond pulsars. This is consistent with our expectation, given the telescope sensitivity and the sky coverage of the processed data (~17 000 deg2). For 10 pulsars, we present the lowest frequency detections published. For a subset of the pulsars, we present multi-frequency pulse profiles by combining our data with published profiles from other telescopes. Since the Murchison Widefield Array is a low-frequency precursor to the Square Kilometre Array, we use our census results to forecast that a survey using the low-frequency component of the Square Kilometre Array Phase 1 can potentially detect around 9 400 pulsars.

Detection of 107 glitches in 36 southern pulsars

Timing observations from the Parkes 64-m radio telescope for 165 pulsars between 1990 and 2011 have been searched for period glitches. Data spans for each pulsar ranged between 5.3 and 20.8 yr. From the total of 1911 yr of pulsar rotational history, 107 glitches were identified in 36 pulsars. Out of these glitches, 61 have previously been reported whereas 46 are new discoveries. Glitch parameters, both for the previously known and the new glitch detections, were measured by fitting the timing residual data. Observed relative glitch sizes Δνg/ν range between 10−10 and 10−5, where ν = 1/P is the pulse frequency. We confirm that the distribution of Δνg/ν is bimodal with peaks at approximately 10−9 and 10−6. Glitches are mostly observed in pulsars with characteristic ages between 103 and 105 yr, with large glitches mostly occurring in the younger pulsars. Exponential post-glitch recoveries were observed for 27 large glitches in 18 pulsars. The fraction Q of the glitch that recovers exponentially also has a bimodal distribution. Large glitches generally have low Q, typically just a few per cent, but large Q values are observed in both large and small glitches. Observed time constants for exponential recoveries ranged between 10 and 300 d with some tendency for longer time-scales in older pulsars. Shorter time-scale recoveries may exist but were not revealed by our data which typically have observation intervals of 2–4 weeks. For most of the 36 pulsars with observed glitches, there is a persistent linear increase in |$\dot{\nu }$| (i.e. decrease in the slow-down rate |$|\dot{\nu }|$|⁠) in the interglitch interval. Where an exponential recovery is also observed, the effects of this are superimposed on the linear increase in |$\dot{\nu }$|⁠. In some but not all cases, the slope of the linear recovery changes at the time of a glitch. The |$\ddot{\nu }$| values characterizing the linear changes in |$\dot{\nu }$| are almost always positive and, after subtracting the magnetospheric component of the braking, are approximately proportional to the ratio of |$|\dot{\nu }|$| and the interglitch interval, as predicted by vortex-creep models.

17- and 24-GHz observations of southern pulsars

We present observations of PSRs J0437-4715, J0738-4042, J0835-4510, J0908-4913, J1048-5832, J1622-4950, J1644-4559, J1721-3532 and J1740-3015 at 17 GHz using the Parkes radio telescope. All 9 were detected at 17 GHz, additionally, we detected PSR J0835-4510 and J1622-4950 at 24 GHz. Polarisation profiles of each pulsar and the variation with frequency are discussed. In general, we find that the highly polarised edge components of young pulsars continue to dominate their profiles at 17 GHz. Older pulsars (>10^5 years) appear to be almost completely depolarised. Our detection of PSR J0437-4715 is the highest frequency observation of a millisecond pulsar to date, and implies a luminosity at 17 GHz of 14 {\mu}Jy kpc^2, and a mean spectral index of 2.2. We find that the spectral index of the magnetar PSR J1622-4950 is flat between 1.4 and 24 GHz, similar to the other known radio magnetars XTE J1810-197 and 1E 1547.0-5408. The profile is similar to that at 3.1 GHz, and is highly linearly polarised. Analysis of the frequency evolution of the profile of PSR J0835-4510 show that the profile is made of four components that vary with frequency only in their amplitude. The width and separation of the components remains fixed and the spectral index of each component can be determined independently.

PRECISION SOUTHERN HEMISPHERE VLBI PULSAR ASTROMETRY. II. MEASUREMENT OF SEVEN PARALLAXES

Accurate measurement of pulsar distances via astrometry using very long baseline interferometry enables the improvement of Galactic electron density distribution models, improving distance estimates for the vast majority of pulsars for which parallax measurements are unavailable. However, pulsars at southern declinations have been under-represented in previous interferometric astrometry campaigns. In order to redress this imbalance, we have conducted a two-year astrometric campaign targeting eight southern pulsars with the Australian Long Baseline Array. The program summarized in this paper has resulted in the measurement of seven new pulsar parallaxes, with success on objects down to a mean flux density of 0.8 mJy at 1600 MHz. Our results highlight the substantial uncertainties that remain when utilizing free electron density models for individual pulsar distances. Until this study, PSR J0630-2834 was believed to convert 16% of its spin-down energy into x-rays, but our measured parallax distance of 332 (+52 -40) pc has revised this value to <1%. In contrast, PSR J0108-1431 was found to be almost a factor of two more distant than previously thought, making its conversion of spin-down energy to x-rays the most efficient known (>1%). The 8.5 second radio pulsar J2144-3933 was found to be closer than previously predicted, making its apparent 1400 MHz radio luminosity the lowest of any known pulsar (20 microJy kpc^2). We have examined the growing population of neutron stars with accurate parallaxes to determine the effect of distance errors on the underlying neutron star velocity distribution, and find that typical distance errors may be biasing the estimated mean pulsar velocity upwards by 5%, and are likely to exaggerate the distribution's high-velocity tail.

High-frequency observations of southern pulsars

We present polarization data for 32 mainly southern pulsars at 8.4 GHz. The observations show that the polarization fraction is low in most pulsars at this frequency except for the young, energetic pulsars which continue to show polarization fractions in excess of 60 per cent. All the pulsars in the sample show evidence for conal emission with only one third also showing core emission. Many profiles are asymmetric, with either the leading or the trailing part of cone not detectable. Somewhat surprisingly, the asymmetric profiles tend to be more polarized than the symmetrical profiles. Little or no pulse narrowing is seen between 1 and 8.4 GHz. The spectral behaviour of the orthogonal polarization modes and radius to frequency mapping can likely account for much of the observational phenomenology. Highly polarized components may orginate from higher in the magnetosphere than unpolarized components.

Pulsar Rotation Measures and the Large‐Scale Structure of the Galactic Magnetic Field

The large-scale magnetic field of our Galaxy can be probed in three dimensions using Faraday rotation of pulsar signals. We report on the determination of 223 rotation measures from polarization observations of relatively distant southern pulsars made using the Parkes radio telescope. Combined with previously published observations these data give clear evidence for large-scale counterclockwise fields (viewed from the north Galactic pole) in the spiral arms interior to the Sun and weaker evidence for a counterclockwise field in the Perseus arm. However, in interarm regions, including the Solar neighbourhood, we present evidence that suggests that large-scale fields are clockwise. We propose that the large-scale Galactic magnetic field has a bisymmetric structure with reversals on the boundaries of the spiral arms. Streaming motions associated with spiral density waves can directly generate such a structure from an initial inwardly directed radial field. Large-scale fields increase toward the Galactic Center, with a mean value of about 2~$\mu$G in the Solar neighbourhood and 4~$\mu$G at a Galactocentric radius of 3 kpc.

Absolute polarization position angle profiles of southern pulsars at 1.4 and 3.1 GHz

We present here a direct comparison of the polarisation position angle (PA) profiles of 17 pulsars, observed at 1.4 and 3.1 GHz. Absolute PAs are obtained at each frequency, permitting a measurement of the difference in the profiles. By doing this, we obtain more precise rotation measure (RM) values for some of the pulsars in the current catalogue. We find that, apart from RM corrections, there are small, pulse longitude dependent differences in PA with frequency. Such differences go beyond the interpretation of a geometrical origin. We describe in detail the PA evolution between the two frequencies and discuss possible causes, such as orthogonal and non-orthogonal polarisation modes of emission. We also use the PA and total power profiles to estimate the difference in emission height at which the two frequencies originate. In our data sample, there are changes in the relative strengths of different pulse components, especially overlapping linearly polarised components, which coincide with intrinsic changes of the PA profile, resulting in interesting PA differences between the two frequencies.

Polarization profiles of southern pulsars at 3.1 GHz

We present polarization profiles for 48 southern pulsars observed with the new 10-cm receiver at the Parkes telescope. We have exploited the low system temperature and high bandwidth of the receiver to obtain profiles which have good signal-to-noise for most of our sample at this relatively high frequency. Although, as expected, a number of profiles are less linearly polarized at 3.1 GHz than at lower frequencies, we identify some pulsars and particular components of profiles in other pulsars which have increased linear polarization at this frequency. We discuss the dependence of linear polarization with frequency in the context of a model in which emission consists of the superposition of two, orthogonally polarized modes. We show that a simple model, in which the orthogonal modes have different spectral indices, can explain many of the observed properties of the frequency evolution of both the linear polarization and the total power, such as the high degree of linear polarization seen at all frequencies in some high spin-down, young pulsars. Nearly all the position angle profiles show deviations from the rotating vector model; this appears to be a general feature of high-frequency polarization observations.

Polarization Observations of Nine Southern Millisecond Pulsars

Mean pulse profiles and polarization properties are presented for nine southern pulsars. The observations were made using the Parkes radio telescope at frequencies near 1330 MHz; three of the nine pulsars were also observed at 660 MHz. A very high degree of circular polarization was observed in PSR J1603-7202. Complex position angle variations that are not well described by the rotating vector model were observed in PSR J2124-3358 and PSR J2145-0750, both of which have very wide profiles. Rotation measures were obtained for all nine pulsars, with two implying relatively strong interstellar magnetic fields.

Multi-epoch H I line measurements of four southern pulsars

We have measured 21-cm absorption spectra in the direction of three southern pulsars, PSRs B0736-40, B1451-68 and B1641-45, in three separate epochs spread over 2.5 yr. We see no evidence for any changes in the absorption spectra over this time span in spite of good velocity resolution and sensitivity. Towards PSR B1641-45 we place an upper limit of 10 1 9 cm - 2 on the change in the column density of the cold, neutral gas. In addition, we observed PSR B 1557-50 and compared its HI absorption spectrum with spectra taken in 1980 and 1994. A prominent deep absorption feature seen at -110 km s - 1 in 1994 is weaker in 2000 and likely was not present at all in 1980. Using the standard interpretation which links the distance traversed by the pulsar to a physical cloud size, this results in a cloud size of ∼1000 au and density of 2 x 10 4 cm - 3 , parameters typical of those seen in other observations. These results are also consistent with an alternative model by Deshpande who expects the largest variations in optical depth to be seen against the longest time intervals.

Polarization Properties of Nine Southern Radio Pulsars

We report on radio polarimetry observations for nine southern pulsars. Six of the nine in the sample are young, with characteristic ages under 100 kyr and high spin-down luminosities. All six show a significant degree of linear polarization. We also confirm a previously noticed trend in which the degree of linear polarization increases with spin-down luminosity. Where possible, we have used the rotating-vector model of the pulsar emission geometry to fit the observed position angle data. Our fit for PSR J1513-5908 (B1509-58) in particular is useful for directly testing the magnetospheric model of Melatos in combination with further timing observations. For this pulsar we find that a magnetic inclination angle greater than or equal to 60° is excluded at the 3 σ level and that the geometry suggested by the morphology of an apparent bipolar X-ray outflow is marginally inconsistent with the Melatos model. We also report on the polarimetry of three older pulsars: PSR J0045-7319, PSR J1627-4850, and PSR J1316-6232 (whose discovery we also report). Of these, only PSR J0045-7319 shows significant polarization.

Glitches in southern pulsars

Timing observations of 40 mostly young pulsars using the ATNF Parkes radio telescope between 1990 January and 1998 December are reported. In total, 20 previously unreported glitches and ten other glitches were detected in 11 pulsars. These included 12 glitches in PSR J1341$- $6220, corresponding to a glitch rate of 1.5 glitches per year. We also detected the largest known glitch, in PSR J1614$-$5047, with $\Delta\nu_g/\nu \approx 6.5 \times 10^{-6}$ where $\nu = 1/P$ is the pulse frequency. Glitch parameters were determined both by extrapolating timing solutions to inter-glitch intervals and by phase-coherent timing fits across the glitch(es). Analysis of glitch parameters, both from this work and from previously published results, shows that most glitches have a fractional amplitude $\Delta\nu_g/\nu$ of between $10^{-8}$ and $10^{-6}$. There is no consistent relationship between glitch amplitude and the time since the previous glitch or the time to the following glitch, either for the ensemble or for individual pulsars. As previously recognised, the largest glitch activity is seen in pulsars with ages of order 10$^4$ years, but for about 30 per cent of such pulsars, no glitches were detected in the 8-year data span. There is some evidence for a new type of timing irregularity in which there is a significant increase in pulse frequency over a few days, accompanied by a decrease in the magnitude of the slowdown rate. Fits of an exponential recovery to post-glitch data show that for most older pulsars, only a small fraction of the glitch decays. In some younger pulsars, a large fraction of the glitch decays, but in others, there is very little decay.

H i line measurements of pulsars towards the Galactic Centre and the electron density in the inner Galaxy

We have measured 21-cm absorption and emission spectra in the direction of a further seven southern pulsars with the Parkes telescope to derive their kinematic distances and to study the interstellar medium. For the first time we have successfully obtained H i absorption measurements for PSRs J1602–5100,J1740–3015 and J1745–3040. We have also significantly improved the sensitivity and resolution on PSRs J1600–5044,J1752–2806 and J1825–0935, the spectra of which have previously been measured, and have corrected an error in the published distance to PSR J1824–1945. Since the Frail & Weisberg summary of pulsar distances in 1990, a further 23 pulsars now have measured H i distances, mainly through the efforts of the current group. We discuss the Taylor & Cordes electron density model in light of these new measurements and find that, although the model towards the Galactic Centre appears good, the line of sight through the Carina spiral arm is poorly fitted by the model.

Pulse Properties of a few Southern Pulsars at 150 MHz

AbstractWe present 150 MHz observations of the bright millisecond pulsar J0437-4715, and pulsars J1453-6413 and J1752-2806, made with the Mauritius Radio Telescope (MRT). We use single-pulse sequences to derive some preliminary results on the pulse morphologies.

Radio Polarimetry Results for Young Southern Pulsars

AbstractWe present radio polarimetry results for nine Southern pulsars. Six of the nine are young, with characteristic ages less than 100 kyr and high spin-down luminosities. All six show significant linear polarization, and we confirm a previously noticed trend in which the degree of linear polarization increases with spin-down luminosity. We have used the rotating vector model to fit the observed position angle data for PSR J1513–5908 (B1509–58). We find that a magnetic inclination angleα&amp;gt; 60° is excluded at the 3σlevel in the fit, and that the geometry suggested by the morphology of an apparent bipolar X-ray outflow is marginally inconsistent with a recent model of the pulsar magnetosphere.

Glitches in Southern Pulsars

AbstractParkes timing observations of 31 mostly young pulsars over nearly nine years are described. A total of 29 glitches were detected, of which 19 are previously unreported. Twelve glitches were seen in PSR J1341–6220, making this the most frequently glitching pulsar known, and the largest known glitch was detected in PSR J1614–5047. Distributions of glitch parameters were investigated.

Pulsar rotation measures and the magnetic structure of our Galaxy

We have obtained 63 rotation measures (RMs) from polarization observations of southern pulsars, of which 54 are new measurements and three are varied from previous values. The new pulsar RM data at high Galactic latitudes are mostly consistent with the antisymmetric RM distribution found previously. For the Galactic disc, evidence for a field reversal near the Perseus arm, and possibly another beyond it, is presented. Inside the Solar Circle, in addition to the two known field reversals in or near the Carina—Sagittarius arm and the Crux—Scutum arm, a further reversal in the Norma arm is tentatively identified. These reversals, together with the pitch angle derived from pulsar RM and stellar polarization distributions, are consistent with bisymmetric spiral models for the large-scale magnetic field structure in the disc of our Galaxy. However, discrimination between models is complicated by the presence of smaller scale irregularities in the magnetic field, as well as uncertainties in the theoretical modelling.