arXiv subjects

G. Hobbs

Publications and source records attributed to G. Hobbs.

At least 127 records · Page 7Linked to original sources

arXiv2007

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 millisecond 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 characterised 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.

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arXiv2006

The Parkes multibeam pulsar survey: VI. Discovery and timing of 142 pulsars and a Galactic population analysis

[ABRIDGED] 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 30000 +/- 1100 potentially detectable pulsars (i.e. those beaming towards us) having 1.4-GHz luminosities above 0.1 mJy kpc squared. 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 scale height 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 scale height of 330 pc.

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arXiv2006

TEMPO2, a new pulsar timing package. I: Overview

Contemporary pulsar timing experiments have reached a sensitivity level where systematic errors introduced by existing analysis procedures are limiting the achievable science. We have developed tempo2, a new pulsar timing package that contains propagation and other relevant effects implemented at the 1ns level of precision (a factor of ~100 more precise than previously obtainable). In contrast with earlier timing packages, tempo2 is compliant with the general relativistic framework of the IAU 1991 and 2000 resolutions and hence uses the International Celestial Reference System, Barycentric Coordinate Time and up-to-date precession, nutation and polar motion models. Tempo2 provides a generic and extensible set of tools to aid in the analysis and visualisation of pulsar timing data. We provide an overview of the timing model, its accuracy and differences relative to earlier work. We also present a new scheme for predictive use of the timing model that removes existing processing artifacts by properly modelling the frequency dependence of pulse phase.

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arXiv2005

Transient radio bursts from rotating neutron stars

The `radio sky' is relatively unexplored for transient signals, although the potential of radio-transient searches is high, as demonstrated recently by the discovery of a previously unknown type of source which varies on timescales of minutes to hours. Here we report a new large-scale search for radio sources varying on much shorter timescales. This has revealed 11 objects characterized by single, dispersed bursts having durations between 2 and 30 ms. The average time intervals between bursts range from 4 minutes to 3 hours, with radio emission typically detectable for < 1 s per day. From an analysis of the burst arrival times, we have identified periodicities in the range 0.4 - 7 s for ten of the 11 sources, suggesting a rotating neutron star origin. Despite the small number of sources presently detected, their ephemeral nature implies a total Galactic population which significantly exceeds that of the regularly pulsing radio pulsars. Five of the ten sources have periods greater than 4 s, and period derivatives have been measured for three of the sources, with one having a very high inferred magnetic field of 5e13 G, suggesting that this new population is related to other classes of isolated neutron stars observed at X-ray and gamma-ray wavelengths.

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arXiv2005

Evidence for alignment of the rotation and velocity vectors in pulsars

We present strong observational evidence for a relationship between the direction of a pulsar's motion and its rotation axis. We show carefully calibrated polarization data for 25 pulsars, 20 of which display linearly polarized emission from the pulse longitude at closest approach to the magnetic pole. Such data allow determination of the position angle of the linear polarisation which in turn reflects the position angle of the rotation axis. Of these 20 pulsars, 10 show an offset between the velocity vector and the polarisation position angle which is either less than 10\degr or more than 80\degr, a fraction which is very unlikely by random chance. We believe that the bimodal nature of the distribution arises from the presence of orthogonal polarisation modes in the pulsar radio emission. In some cases this orthogonal ambiguity is resolved by observations at other wavelengths so that we conclude that the velocity vector and the rotation axis are aligned at birth. Strengthening the case is the fact that 4 of the 5 pulsars with ages less than 3 Myr show this relationship, including the Vela pulsar. We discuss the implications of these findings in the context of the Spruit & Phinney (1998)\nocite{sp98} model of pulsar birth-kicks. We point out that, contrary to claims in the literature, observations of double neutron star systems do not rule out aligned kick models and describe a possible observational test involving the double pulsar system.

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arXiv2005

Timing measurements and proper motions of 74 pulsars using the Nanshan radio telescope

We have measured the positions of 74 pulsars from regular timing observations using the Nanshan radio telescope at Urumqi Observatory between 2000 January and 2004 August (MJD 51500 -- 53240). Proper motions were determined for these pulsars by comparing their current positions with positions given in pulsar catalogues. We compare our results to earlier measurements in the literature and show that, in general, the values agree. New or improved proper motions are obtained for 16 pulsars. The effect of period fluctuations and other timing noise on the determination of pulsar positions is investigated. For our sample, the mean and rms transverse velocities are 443 and 224 km/s respectively, agreeing with previous work even though we determine distances using the new NE2001 electron density model.

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arXiv2005

Supernova Remnants in the Magellanic Clouds. V. The Complex Interior Structure of the N206 SNR

The N206 supernova remnant (SNR) in the Large Magellanic Cloud (LMC) has long been considered a prototypical "mixed morphology" SNR. Recent observations, however, have added a new twist to this familiar plot: an elongated, radially-oriented radio feature seen in projection against the SNR face. Utilizing the high resolution and sensitivity available with the Hubble Space Telescope, Chandra, and XMM-Newton, we have obtained optical emission-line images and spatially resolved X-ray spectral maps for this intriguing SNR. Our findings present the SNR itself as a remnant in the mid to late stages of its evolution. X-ray emission associated with the radio "linear feature" strongly suggests it to be a pulsar-wind nebula (PWN). A small X-ray knot is discovered at the outer tip of this feature. The feature's elongated morphology and the surrounding wedge-shaped X-ray enhancement strongly suggest a bow-shock PWN structure.

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arXiv2005

A statistical study of 233 pulsar proper motions

We present and analyse a catalogue of 233 pulsars with proper motion measurements. The sample contains a wide variety of pulsars including recycled objects and those associated with globular clusters or supernova remnants. After taking the most precise proper motions for those pulsars for which multiple measurements are available, the majority of the proper motions (58%) are derived from pulsar timing methods, 41% using interferometers and the remaining 1% using optical telescopes. Many of the 1-D and 2-D speeds (referring to speeds measured in one coordinate only and the magnitudes of the transverse velocities respectively) derived from these measurements are somewhat lower than earlier estimates because of the use of the most recent electron density model in determining pulsar distances. The mean 1-D speeds for the normal and recycled pulsars are 152(10) and 54(6) km/s respectively. The corresponding mean 2-D speeds are 246(22) and 87(13) km/s. PSRs B2011+38 and B2224+64 have the highest inferred 2-D speeds of ~1600 km/s. We study the mean speeds for different subsamples and find that, in general, they agree with previous results. Applying a novel deconvolution technique to the sample of 73 pulsars with characteristic ages less than 3 Myr, we find the mean 3-D pulsar birth velocity to be 400(40) km/s. The distribution of velocities is well described by a Maxwellian distribution with 1-D rms sigma=265 km/s. There is no evidence for a bimodal velocity distribution. The proper motions for PSRs B1830-08 and B2334+61 are consistent with their proposed associations with the supernova remnants W41 and G114.3+0.3 respectively.

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arXiv2004

Adaptive Filters Revisited - RFI Mitigation in pulsar observations

Pulsar detection and timing experiments are applications where adaptive filters seem eminently suitable tools for radio-frequency-interference (RFI) mitigation. We describe a novel variant which works well in field trials of pulsar observations centred on an observing frequency of 675 MHz, a bandwidth of 64 MHz and with 2-bit sampling. Adaptive filters have generally received bad press for RFI mitigation in radio astronomical observations with their most serious drawback being a spectral echo of the RFI embedded in the filtered signals. Pulsar observations are intrinsically less sensitive to this as they operate in the (pulsar period) time domain. The field trials have allowed us to identify those issues which limit the effectiveness of the adaptive filter. We conclude that adaptive filters can significantly improve pulsar observations in the presence of RFI.

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arXiv2004

PSR J1756-2251: a new relativistic double neutron star system

We report the discovery during the Parkes Multibeam Pulsar Survey of PSR J1756-2251, a 28.5 ms pulsar in a relativistic binary system. Subsequent timing observations showed the pulsar to have an orbital period of 7.67 hrs and an eccentricity of 0.18. They also revealed a significant advance of periastron, 2.585+/-0.002 deg./yr. Assuming this is entirely due to general relativity implies a total system mass (pulsar plus companion) of 2.574+/-0.003 solar mass. This mass and the significant orbital eccentricity suggest that this is a double neutron star system. Measurement of the gravitational redshift, gamma, and an evaluation of the Shapiro delay shape, s, indicate a low companion mass of <1.25 solar mass. The expected coalescence time due to emission of gravitational waves is only ~1.7 Gyr substantially less than a Hubble time. We note an apparent correlation between spin period and eccentricity for normally evolving double neutron star systems.

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arXiv2004

The Parkes Multibeam Pulsar Survey: V. Finding binary and millisecond pulsars

The Parkes Multibeam Pulsar Survey is the most successful survey of the Galactic plane ever performed, finding over 600 pulsars in the initial processing. We report on reprocessing of all 40,000 beams with a number of algorithms, including conventional frequency-domain searches and an acceleration search for fast binary pulsars. The very large volume of results coupled with the need to distinguish new candidates from known pulsars and their many harmonics, often with multiple detections from different search algorithms, necessitated the development of a new graphical selection tool tightly linked to a web-based results database. We discuss and demonstrate the benefits of these software systems which are specifically designed for large survey projects. The results of this processing have been encouraging; we have discovered 128 new pulsars including 11 binary and 15 millisecond pulsars, in addition to those previously found in the survey, we have thus far discovered 737 pulsars. In this paper we discuss the discoveries of PSR J1744-3922, a 172 ms mildly recycled pulsar in a 4.6 hr orbit that exhibits nulling behaviour, not previously observed in recycled or binary objects; PSR J1802-2124, an intermediate mass binary pulsar (IMBP) and PSR J1801-1417, a solitary millisecond pulsar.

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arXiv2004

The Very Young Radio Pulsar J1357-6429

We report the discovery of a radio pulsar with a characteristic age of 7300 years, making it one of the 10 apparently youngest Galactic pulsars known. PSR J1357-6429, with a spin period of P = 166 ms and spin-down luminosity of 3.1e36 ergs/s, was detected during the Parkes multibeam survey of the Galactic plane. We have measured a large rotational glitch in this pulsar, with Delta P/P = -2.4e-6, similar in magnitude to those experienced occasionally by the Vela pulsar. At a nominal distance of only ~ 2.5 kpc, based on the measured free electron column density of 127 pc/cc and the electron distribution model of Cordes & Lazio, this may be, after the Crab, the nearest very young pulsar known. The pulsar is located near the radio supernova remnant candidate G309.8-2.6.

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arXiv2004

The Parkes multibeam pulsar survey: IV. Discovery of 180 pulsars and parameters for 281 previously known pulsars

The Parkes multibeam pulsar survey has led to the discovery of more than 700 pulsars. In this paper, we provide timing solutions, flux densities and pulse profiles for 180 of these new discoveries. Two pulsars, PSRs J1736-2843 and J1847-0130 have rotational periods P > 6s and are therefore among the slowest rotating radio pulsars known. Conversely, with P = 1.8ms, PSR J1843-1113 has the third shortest period of pulsars currently known. This pulsar and PSR J1905+0400 (P = 3.8ms) are both solitary. We also provide orbital parameters for a new binary system, PSR J1420-5625, which has P = 34ms, an orbital period of 40 days and a minimum companion mass of 0.4 solar masses. The 10 degree-wide strip along the Galactic plane that was surveyed is known to contain 264 radio pulsars that were discovered prior to the multibeam pulsar survey. We have redetected almost all of these pulsars and provide new dispersion measure values and flux densities at 20cm for the redetected pulsars.

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arXiv2003

Two Radio Pulsars with Magnetar Fields

PSRs J1847-0130 and J1718-37184 have inferred surface dipole magnetic fields greater than those of any other known pulsars and well above the ``quantum critical field'' above which some models predict radio emission should not occur.These fields are similar to those of the anomalous X-ray pulsars (AXPs), which growing evidence suggests are ``magnetars''. The lack of AXP-like X-ray emission from these radio pulsars (and the non-detection of radio emission from the AXPs) creates new challenges for understanding pulsar emission physics and the relationship between these classes of apparently young neutron stars.

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arXiv2003

Long-Term Timing of 374 Pulsars

We have analysed the arrival times for 374 pulsars that have been observed for more than six years using the 76-m Lovell telescope at Jodrell Bank Observatory. Here we present a qualitative analysis of structures seen in the timing residuals.

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arXiv2003

The ATNF Pulsar Catalogue

The number of known pulsars has significantly increased over the previous few years. We have searched the literature to find papers announcing the discovery of pulsars or giving improved parameters for them. Data from the papers have been entered into a new pulsar catalogue that can be accessed via a web interface or from the command line (on Solaris or Linux machines). The user may request over 120 different parameters, select pulsars of interest, generate custom variables and choose between different ways of displaying or tabulating the data. Full bibliographic references are available on all observed parameters.

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arXiv2003

Proper motion, age and initial spin period of PSR J0538+2817 in S147

We present results of timing observations of the 143-ms pulsar J0538+2817 that provide a proper motion measurement which clearly associates the pulsar with the supernova remnant S147. We measure a proper motion of 67$_{-22}^{+48}$ mas yr$^{-1}$, implying a transverse velocity of $v= 385^{+260}_{-130}$ km s$^{-1}$. We derive an age of the pulsar and S147 of only $30\pm4$ kyr which is a factor of 20 times less than the pulsar's characteristic age of $τ_c = 620$ kyr. This age implies an initial spin period of $P_0=139$ ms, close to the present pulse period and a factor of several larger than what is usually inferred for birth periods. Implications for recent X-ray detections of this pulsar are discussed.

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arXiv2003

PSR J1847-0130: A Radio Pulsar with Magnetar Spin Characteristics

We report the discovery of PSR J1847-0130, a radio pulsar with a 6.7-s spin period, in the Parkes multibeam survey of the Galactic plane. The slowdown rate for the pulsar, 1.3x10^{-12} s/s, is high and implies a surface dipole magnetic field strength of 9.4x10^{13} G. This inferred dipolar magnetic field strength is the highest by far among all known radio pulsars and over twice the ``quantum critical field'' above which some models predict radio emission should not occur. The inferred dipolar magnetic field strength and period of this pulsar are in the same range as those of the anomalous X-ray pulsars, which have been identified as being "magnetars" whose luminous X-ray emission is powered by their large magnetic fields. We have examined archival ASCA data and place an upper limit on the X-ray luminosity of J1847-0130 which is lower than the luminosities of all but one AXP. The properties of this pulsar prove that inferred dipolar magnetic field strength and period cannot alone be responsible for the unusual high-energy properties of the magnetars and creates new challenges for understanding the possible relationship between these two manifestations of young neutron stars.

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