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D. R. Lorimer

Publications and source records attributed to D. R. Lorimer.

196 records · Page 11Linked to original sources
arXiv2001

Timing the millisecond pulsars in 47 Tucanae

In the last 10 years 20 millisecond pulsars have been discovered in the globular cluster 47 Tucanae. Hitherto, only 3 of these had published timing solutions. Here we improve upon these 3 and present 12 new solutions. These measurements can be used to determine a variety of physical properties of the pulsars and of the cluster. The 15 pulsars have positions determined with typical uncertianties of only a few milliarcsec and they are all located within 1.2 arcmin of the cluster centre. We have also measured the proper motions of 5 of the pulsars, which are consistent with the proper motion of 47 Tuc based on Hipparcos data. The period derivatives measured for many of the pulsars are dominated by the dynamical effects of the cluster gravitational field, and are used to constrain the surface mass density of the cluster. All pulsars have characteristic ages T > 170 Myr and magnetic fields B < 2.4e9 Gauss, and the average T > 1 Gyr. We have measured the rate of advance of periastron for the binary pulsar J0024-7204H, implying a total system mass 1.4+-0.8 solar masses.

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arXiv2000

A 1400-MHz pilot search for young pulsars

We have used the Effelsberg 100-m radio telescope to conduct a 1400-MHz ($λ$ 21-cm) search for young and rapidly rotating radio pulsars along a 2 deg$^2$ strip of the northern Galactic plane defined by $28^{\circ} \leq l \leq 30^{\circ}$ and $|b|\leq0.5^{\circ}$. This region lies close to the Scutum spiral arm which is already known to contain a number of radio and X-ray pulsars. The search was nominally sensitive to pulsars with 1400-MHz flux densities above 0.3 mJy; this represents a threefold improvement in sensitivity over all previous searches of this region of the Galaxy. Four new long-period pulsars were discovered as a result of this survey. All three previously known pulsars in this region were also detected. The four new pulsars are relatively young ($<$ 0.5 Myr), weak ($<1$ mJy) sources with dispersion measures in the range 180--910 cm$^{-3}$ pc. None of the newly-discovered pulsars are associated with catalogued supernova remnants.

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arXiv2000

The Effelsberg Search for Pulsars in the Galactic Centre

We report the status of a search for pulsars in the Galactic Centre, using a completely revised and improved high-sensitivity double-horn system at 4.85-GHz. We also present calculations about the success rate of periodicity searches for such a survey, showing that in contrast to conclusions in recent literature pulsars can be indeed detected at the chosen search frequency.

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arXiv1999

Radio Pulsars - An Observer's Perspective

Pulsar astronomy is currently enjoying one of the most productive phases in its history. In this review, I outline some of the basic observational aspects and summarise some of the latest results of searches for pulsars in the disk of our Galaxy and its globular cluster system.

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arXiv1999

Neutron Star Birth Rates

A crucial test any proposed evolutionary scenario must pass is can the birth rate of the sources we see be sustained by the proposed progenitor population? In this review, I investigate the methods used to determine the birth rates of normal and millisecond radio pulsars and summarise recent results for these two distinct neutron star populations.

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arXiv1999

Observations of 20 millisecond pulsars in 47 Tucanae at 20 cm

We have used a new observing system on the Parkes radio telescope to carry out a series of pulsar observations of the globular cluster 47 Tucanae at 20-cm wavelength. We detected all 11 previously known pulsars, and have discovered nine others, all of which are millisecond pulsars in binary systems. We have searched the data for relatively short orbital period systems, and found one pulsar with an orbital period of 96 min, the shortest of any known radio pulsar. The increased rate of detections with the new system resulted in improved estimates of the flux density of the previously known pulsars, determination of the orbital parameters of one of them, and a coherent timing solution for another one. Five of the pulsars now known in 47 Tucanae have orbital periods of a few hours and implied companion masses of only ~ 0.03 Msun. Two of these are eclipsed at some orbital phases, while three are seen at all phases at 20 cm but not always at lower frequencies. Four and possibly six of the other binary systems have longer orbital periods and companion masses ~ 0.2 Msun, with at least two of them having relatively large orbital eccentricities. All 20 pulsars have rotation periods in the range 2-8 ms.

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arXiv1999

Slow pulsars from the STScI/NAIC drift scan search

The unique sensitivity of the Arecibo telescope at 430 MHz motivated a drift scan search of the visible sky divided up into eight declination strips. Based on an analysis of the data collected in the region assigned to us, eight new long-period pulsars have so far been discovered.

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arXiv1999

Timing of the young pulsar J1907+0918

We have extensively searched for periodic signals in the area of the soft gamma-ray repeater SGR 1900+14, at 430 and 1410 MHz with the Arecibo telescope. Our observations did not reveal the 5.16-s periodicity reported for the magnetar down to a 430-MHz flux density limit of 150 uJy. During the search we discovered a 226-ms radio pulsar, PSR J1907+0918. The period derivative implies the characteristic age for this pulsar is only 38 kyr. Independent evidence in support of this apparent youth are the unusually high degree of circular polarization and its relatively flat radio spectrum. The close proximity of PSR J1907+0918 to SGR 1900+14 suggests that they may have evolved from a massive binary system. Regardless of this possibility, it is presently not clear to us that SGR 1900+14 is associated with the supernova remnant G42.8+0.6.

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arXiv1999

Detection of new emission components in PSR B0329+-54

To study the structure of emission beam, we have analysed the single pulse data of PSR B0329+-54 at 325 and 606 MHz. In order to unambiguously detect the weak emission components, we have developed a new data analysis technique, which we term ``window-thresholding''. By applying this technique to the data, we have detected three new emission components, and also confirmed the presence of a component which was proposed earlier. Hence our analysis indicates that PSR B0329+-54 has nine components, which is among the highest of all the known pulsars. The symmetric distribution of pulse components about the pulse centre, indicates that the emission beam is conal.

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arXiv1999

Puzzling Pulsars and Supernova Remnants

The fact that the majority of the youngest radio pulsars are surrounded by expanding supernova remnants is strong evidence that neutron stars are produced in the supernovae of massive stars. In many cases, the pulsar appears significantly offset from the geometric centre of the supernova remnant, indicating that the neutron star has moved away from the site of the explosion with a substantial space velocity since birth. Here we show that the these offsets show an overwhelming preference for one sign in terms of Galactic longitude, a result that has important implications for the number of genuine associations. The origin of this statistically significant effect may lie in a differential Galactic rotational velocity between stars and gas in the interstellar medium.

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arXiv1999

The characteristics of millisecond pulsar emission: III.From low to high frequencies

In this paper we present the first observations of a large sample of millisecond pulsars at frequencies of 2.7 GHz and 4.9 GHz. Our new flux measurements extend the known spectra for millisecond pulsars to the highest frequencies to date. The coverage of more than a decade of radio spectrum allows us for the first time to search for spectral breaks as so often observed for normal pulsars around 1 GHz. We align the observed millisecond pulsar profiles with data from lower frequencies. Deviations from a dipolar magnetic field structure are not evident and absolute timing across the wide frequency range with a single dispersion measure is possible. We seem to observe mainly unfilled emission beams, which must originate from a very compact region. A compact emission region is also suggested by a remarkably constant profile width or component separation over a very wide frequency range. For a few sources, polarization data at 2.7 and 4.9 GHz could also be obtained which indicate that millisecond pulsars are weakly polarized or even unpolarized at frequencies above 3 GHz. The simultaneous decrease in degree of polarization and the constant profile width thus question proposals which link de-polarization and decreasing profile width for normal pulsars to the same propagation effect (i.e. birefringence). Comparing the properties of core and conal like profile components to those of normal pulsars, we find less significant patterns in their spectral evolution for the population of millisecond pulsars. Hence, we suggest that core and conal emission may be created by the same emission process. In general, MSP emission properties tend to resemble those of normal pulsars only shifted towards higher frequencies.

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arXiv1999

Profile instabilities of the millisecond pulsar PSR J1022+1001

We present evidence that the integrated profiles of some millisecond pulsars exhibit severe changes that are inconsistent with the moding phenomenon as known from slowly rotating pulsars. We study these profile instabilities in particular for PSR J1022+1001 and show that they occur smoothly, exhibiting longer time constants than those associated with moding. In addition, the profile changes of this pulsar seem to be associated with a relatively narrow-band variation of the pulse shape. Only parts of the integrated profile participate in this process which suggests that the origin of this phenomenon is intrinsic to the pulsar magnetosphere and unrelated to the interstellar medium. A polarization study rules out profile changes due to geometrical effects produced by any sort of precession. However, changes are observed in the circularly polarized radiation component. In total we identify four recycled pulsars which also exhibit instabilities in the total power or polarization profiles due to an unknown phenomenon (PSRs J1022+1001, J1730-2304, B1821-24, J2145-0750). The consequences for high precision pulsar timing are discussed in view of the standard assumption that the integrated profiles of millisecond pulsars are stable. As a result we present a new method to determine pulse times-of-arrival that involves an adjustment of relative component amplitudes of the template profile. Applying this method to PSR J1022+1001, we obtain an improved timing solution with a proper motion measurement of -17 \pm 2 mas/yr in ecliptic longitude. Assuming a distance to the pulsar as inferred from the dispersion measure this corresponds to an one-dimensional space velocity of 50 km/s.

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arXiv1998

Pulsar Searches - Tricks of the Trade

The study of pulsars has come a long way since their accidental discovery as ``scruff'' on pen chart recordings of the radio sky at 81 MHz by Jocelyn Bell and Anthony Hewish at Cambridge some thirty years ago. The present sample of almost 800 objects is the result of an immense amount of observing time at the largest radio telescopes around the world. This article highlights the most exciting finds, and summarises the most commonly used, yet relatively simple, techniques which continue to be most efficient at finding weak periodic signals hidden in noisy time series.

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arXiv1998

A search for pulsars in supernova remnants

We have carried out a sensitive search for young pulsars associated with supernova remnants using the 76-m Lovell radio telescope at Jodrell Bank. The observations were made at 606 MHz using a system with a bandwidth of 8 MHz and a set noise temperature on cold sky of about 50 K. The survey targeted 33 remnants in the northern hemisphere and achieved a nominal sensitivity of ~1 mJy in most cases. Two pulsars were discovered in the course of this survey and the known pulsar PSR B1952+29 was detected. The new pulsars, J0215+6218 and J1957+2831, were found during searches of the supernova remnants G132.7+1.3 and G65.1+0.6 respectively. Based on a statistical analysis of the present sample of proposed pulsar-supernova remnant pairs, we conclude that at most 17 associations are likely to be real. We find no strong evidence for a genuine association between either of the two newly discovered pulsars and their target supernova remnants.

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arXiv1998

A flexible format for exchanging pulsar data

We describe a data format currently in use amongst European institutions for exchanging and archiving pulsar data. The format is designed to be as flexible as possible with regard to present and future compatibility with different operating systems. One application of the common format is simultaneous multi-frequency observations of single pulses. A data archive containing over 2500 pulse profiles stored in this format is now available via the Internet (see http://www.mpifr-bonn.mpg.de/pulsar/data), together with a small suite of computer programs that can read, write and display the data.

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