C. A. Jordan

A periodically active pulsar giving insight into magnetospheric physics

PSR B1931+24 (J1933+2421) behaves as an ordinary isolated radio pulsar during active phases that are 5 to 10 days long. However, when the radio emission ceases, it switches off in less than 10 seconds and remains undetectable for the next 25 to 35 days, then switches on again. This pattern repeats quasi-periodically. The origin of this behavior is unclear. Even more remarkably, the pulsar rotation slows down 50% faster when it is on than when it is off. This indicates a massive increase in magnetospheric currents when the pulsar switches on, proving that pulsar wind plays a substantial role in pulsar spin-down. This allows us, for the first time, to estimate the magnetospheric currents in a pulsar magnetosphere during the occurrence of radio emission.

The International Pulsar Timing Array project: using pulsars as a gravitational wave detector

The International Pulsar Timing Array project combines observations of pulsars from both northern and southern hemisphere observatories with the main aim of detecting ultra-low frequency (similar to 10(-9)-10(-8) Hz) gravitational waves. Here we introduce the project, review the methods used to search for gravitational waves emitted from coalescing supermassive binary black-hole systems in the centres of merging galaxies and discuss the status of the project.

High-precision timing of 42 millisecond pulsars with the European Pulsar Timing Array

We report on timing, flux density, and polarimetric observations of the transient magnetar and 5.54 s radio pulsar XTE J1810-197 using the GBT, Nancay, and Parkes radio telescopes beginning in early 2006, until its sudden disappearance as a radio source in late 2008. Repeated observations through 2016 have not detected radio pulsations again. The torque on the neutron star, as inferred from its rotation frequency derivative f-dot, decreased in an unsteady manner by a factor of 3 in the first year of radio monitoring. In contrast, during its final year as a detectable radio source, the torque decreased steadily by only 9%. The period-averaged flux density, after decreasing by a factor of 20 during the first 10 months of radio monitoring, remained steady in the next 22 months, at an average of 0.7+/-0.3 mJy at 1.4 GHz, while still showing day-to-day fluctuations by factors of a few. There is evidence that during this last phase of radio activity the magnetar had a steep radio spectrum, in contrast to earlier behavior. There was no secular decrease that presaged its radio demise. During this time the pulse profile continued to display large variations, and polarimetry indicates that the magnetic geometry remained consistent with that of earlier times. We supplement these results with X-ray timing of the pulsar from its outburst in 2003 up to 2014. For the first 4 years, XTE J1810-197 experienced non-monotonic excursions in f-dot by at least a factor of 8. But since 2007, its f-dot has remained relatively stable near its minimum observed value. The only apparent event in the X-ray record that is possibly contemporaneous with the radio shut-down is a decrease of ~20% in the hot-spot flux in 2008-2009, to a stable, minimum value. However, the permanence of the high-amplitude, thermal X-ray pulse, even after the radio demise, implies continuing magnetar activity.

A Dark Hydrogen Cloud in the Virgo Cluster

VIRGOHI 21 is an H I source detected in the Virgo Cluster survey of Davies et al. that has a neutral hydrogen mass of 108 M and a velocity width of ?V20 = 220 km s-1. From the Tully-Fisher relation, a galaxy with this velocity width would be expected to be 12 mag or brighter; however, deep CCD imaging has failed to turn up a counterpart down to a surface brightness level of 27.5 B mag arcsec-2. The H I observations show that it is extended over at least 16 kpc, which, if the system is bound, gives it a minimum dynamical mass of ~1011 M and a mass-to-light ratio of Mdyn/LB > 500 M/L. If it is tidal debris, then the putative parents have vanished; the remaining viable explanation is that VIRGOHI 21 is a dark halo that does not contain the expected bright galaxy. This object was found because of the low column density limit of our survey, a limit much lower than that achieved by all sky H I surveys such as those carried out at Parkes and Jodrell Bank. Further such sensitive surveys might turn up a significant number of the dark matter halos predicted by galaxy formation models.

The Proper Motion, Age, and Initial Spin Period of PSR J0538+2817 in S147

We present results of the timing observations of the 143 ms pulsar PSR J0538+2817 that provide a proper-motion measurement clearly showing an association of the pulsar with the supernova remnant S147. We measure a proper motion of 67 mas yr-1, implying a transverse velocity of v = 385 km s-1. We derive an age of the pulsar and S147 of only 30 ± 4 kyr, which is a factor of 20 times less than the pulsars characteristic age of τc = 620 kyr. This age implies an initial spin period of P0 = 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.

First Results from the HI Jodrell All Sky Survey: inclination-dependent selection effects in a 21-cm blind survey

Details are presented of the H i Jodrell All Sky Survey (HIJASS). HIJASS is a blind neutral hydrogen (H I ) survey of the northern sky (δ > 22°), being conducted using the multibeam receiver on the Lovell Telescope (full width at half-maximum beamwidth 12 arcmin) at Jodrell Bank. HIJASS covers the velocity range -3500 to 10 000 km s - 1 , with a velocity resolution of 18.1 km s - 1 and spatial positional accuracy of ∼2.5 arcmin. Thus far about 1115 deg 2 of sky have been surveyed. The average rms noise during the early part of the survey was around 16 mJy beam - 1 . Following the first phase of the Lovell Telescope upgrade (in 2001), the rms noise is now around 13 mJy beam - 1 . We describe the methods of detecting galaxies within the HIJASS data and of measuring their H I parameters. The properties of the resulting H i-selected sample of galaxies are described. Of the 222 sources so far confirmed, 170 (77 per cent) are clearly associated with a previously catalogued galaxy. A further 23 sources (10 per cent) lie close (within 6 arcmin) to a previously catalogued galaxy for which no previous redshift exists. A further 29 sources (13 per cent) do not appear to be associated with any previously catalogued galaxy. The distributions of peak flux, integrated flux, H I mass and cz are discussed. We show, using the HIJASS data, that H I self-absorption is a significant, but often overlooked, effect in galaxies with large inclination angles to the line of sight. Properly accounting for it could increase the derived H I mass density of the local Universe by at least 25 per cent. The effect that this will have on the shape of the H I mass function will depend on how self-absorption affects galaxies of different morphological types and H i masses. We also show that galaxies with small inclinations to the line of sight may also be excluded from H I-selected samples, since many such galaxies will have observed velocity widths that are too narrow for them to be distinguished from narrow-band radio-frequency interference. This effect will become progressively more serious for galaxies with smaller intrinsic velocity widths. If, as we might expect, galaxies with smaller intrinsic velocity widths have smaller HI masses, then compensating for this effect could significantly steepen the faint-end slope of the derived HI mass function.

The European Pulsar Timing Array:current efforts and a LEAP toward the future

The European Pulsar Timing Array (EPTA) is a multi-institutional, multi-telescope collaboration, with the goal of using high-precision pulsar timing to directly detect gravitational waves. In this paper we discuss the EPTA member telescopes, current achieved timing precision and near-future goals. We report a preliminary upper limit to the amplitude of a gravitational wave background. We also discuss the Large European Array for Pulsars, in which the five major European telescopes involved in pulsar timing will be combined to provide a coherent array that will give similar sensitivity to the Arecibo radio telescope, and larger sky coverage.

Generic tests of the existence of the gravitational dipole radiation and the variation of the gravitational constant

We present results from the high precision timing analysis of the pulsar-white dwarf (WD) binary PSR J1012+5307 using 15 years of multi-telescope data. Observations were performed regularly by the European Pulsar Timing Array (EPTA) network, consisting of Effelsberg, Jodrell Bank, Westerbork and Nancay. All the timing param- eters have been improved from the previously published values, most by an order of magnitude. In addition, a parallax measurement of π = 1.2(3)mas is obtained for the first time for PSR J1012+5307, being consistent with the optical estimation from the WD companion. Combining improved 3D velocity information and models for the Galactic potential the complete evolutionary Galactic path of the system is obtained. A new intrinsic eccentricity upper limit of e < 8.4× 10 7 is acquired, one of the smallest calculated for a binary system and a measurement of the variation of the projected semi-major axis also constrains the systems orbital orientation for the first time. It is shown that PSR J1012+5307 is an ideal laboratory for testing alternative theories of gravity. The measurement of the change of the orbital period of the system of u Pb = 5(1)× 10 14 is used to set an upper limit on the dipole gravitational wave emission that is valid for a wide class of alternative theories of gravity. Moreover, it is shown that in combination with other binary pulsars PSR J1012+5307 is an ideal system to provide self-consistent, generic limits, based only on millisecond pulsar data, for the dipole radiation and the variation of the gravitational constant u G.

Polarized radio emission from a magnetar

We present polarization observations of the radio emitting magnetar AXP J1810-197. Using simultaneous multi-frequency observations performed at 1.4, 4.9 and 8.4 GHz, we obtained polarization information for single pulses and the average pulse profile at several epochs. We find that in several respects this magnetar source shows similarities to the emission properties of normal radio pulsars while simultaneously showing striking differences. The emission is nearly 80-95% polarized, often with a low but significant degree of circular polarization at all frequencies which can be much greater in selected single pulses. The position angle swing has a low average slope of only 1 deg/deg, deviating significantly from an S-like swing as often seen in radio pulsars which is usually interpreted in terms of a rotating vector model and a dipolar magnetic field. The observed position angle is consistent at all frequencies while showing significant secular variations. On average the interpulse is less linearly polarized but shows a higher degree of circular polarization. Some epochs reveal the existence of non-orthogonal emission modes in the main pulse and systematic wiggles in the PA swing, while the interpulse shows a large variety of position angle values. We interprete many of the emission properties as propagation effects in a non-dipolar magnetic field configuration where emission from different multipole components is observed.

21 cm Synthesis Observations of VIRGOHI 21?A Possible Dark Galaxy in the Virgo Cluster

Many observations indicate that dark matter dominates the extragalactic universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously, we have suggested that VIRGOHI 21, a 21 cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line width (~200 km s(-1)) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ~10(8) yr ago with a perturber of ~10(11) M☉. This we take as additional evidence for the massive nature of VIRGOHI 21, as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the H I and find none down to an I-band surface brightness limit of 31.1 ± 0.2 mag arcsec(-2).