Andrea Possenti

A double-pulsar system: A rare laboratory for relativistic gravity and plasma physics

The clocklike properties of pulsars moving in the gravitational fields of their unseen neutron-star companions have allowed unique tests of general relativity and provided evidence for gravitational radiation. We report here the detection of the 2.8-second pulsar J0737–3039B as the companion to the 23-millisecond pulsar J0737–3039A in a highly relativistic double neutron star system, allowing unprecedented tests of fundamental gravitational physics. We observed a short eclipse of J0737–3039A by J0737–3039B and orbital modulation of the flux density and the pulse shape of J0737–3039B, probably because of the influence of J0737–3039As energy flux on its magnetosphere. These effects will allow us to probe magneto-ionic properties of a pulsar magnetosphere.

Tests of general relativity from timing the double pulsar

The double pulsar system PSR J0737-3039A/B is unique in that both neutron stars are detectable as radio pulsars. They are also known to have much higher mean orbital velocities and accelerations than those of other binary pulsars. The system is therefore a good candidate for testing Einsteins theory of general relativity and alternative theories of gravity in the strong-field regime. We report on precision timing observations taken over the 2.5 years since its discovery and present four independent strong-field tests of general relativity. These tests use the theory-independent mass ratio of the two stars. By measuring relativistic corrections to the Keplerian description of the orbital motion, we find that the “post-Keplerian” parameter s agrees with the value predicted by general relativity within an uncertainty of 0.05%, the most precise test yet obtained. We also show that the transverse velocity of the systems center of mass is extremely small. Combined with the systems location near the Sun, this result suggests that future tests of gravitational theories with the double pulsar will supersede the best current solar system tests. It also implies that the second-born pulsar may not have formed through the core collapse of a helium star, as is usually assumed.

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 kpc 2 . 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.

Re-examining the X-ray versus spin-down luminosity correlation of rotation powered pulsars

The empirical relation between the X-ray luminosity (in the 2{10 keV band) and the rate of spin-down energy loss Lsd of a sample of 39 pulsars is re-examined considering recent data from ASCA, RXTE, BeppoSAX, Chandra ,a ndXMM-Newton and including statistical and systematic errors. The data show a signicant scatter around an average correlation between Lx;(2 10) and Lsd. By tting a dependence of Lx;(2 10) on the period P and period derivative _ P of the type Lx;(2 10)/ P a _

Placing Limits on the Stochastic Gravitational-Wave Background Using European Pulsar Timing Array Data

The paper ‘Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data’ was published in Mon. Not. R. Astron. Soc. 414, 3117–3128 (2011).

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 > 6 s and are therefore among the slowest rotating radio pulsars known. Conversely, with P = 1.8 ms, PSR J1843−1113 has the third-shortest period of pulsars currently known. This pulsar and PSR J1905+0400 (P = 3.8 ms) are both solitary. We also provide orbital parameters for a new binary system, PSR J1420−5625, which has P = 34 ms, an orbital period of 40 d and a minimum companion mass of 0.4 solar masses. The 10 ◦ -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 20 cm for the redetected pulsars.

A New Low Magnetic Field Magnetar: The 2011 Outburst of Swift J1822.3-1606

We report on the long-term X-ray monitoring withSwift,RXTE,Suzaku,Chandra, andXMM-Newton of the outburst of the newly discovered magnetar Swift J1822.3−1606 (SGR 1822−1606), from the first observations soon after the detection of the short X-ray bursts which led to its discovery, through the first stages of its outburst decay (covering the time span from 2011 July until the end of 2012 April). We also report on archival ROSAT observations which detected the source during its likely quiescent state, and on upper limits on Swift J1822.3−1606’s radiopulsed and optical emission during outburst, with the Green Bank Telescope and the Gran Telescopio Canarias, respectively. Our X-ray timing analysis finds the source rotating with a period of P = 8.43772016(2) s and a period derivative ˙ P = 8.3(2) × 10 −14 ss −1 , which implies an inferred dipolar surface magnetic field of B � 2.7 × 10 13 G at the equator. This measurement makes Swift J1822.3−1606 the second lowest magnetic field magnetar (after SGR 0418+5729). Following the flux and spectral evolution from the beginning of the outburst, we find that the flux decreased by about an order of magnitude, with a subtle softening of the spectrum, both typical of the outburst decay of magnetars. By modeling the secular thermal evolution of Swift J1822.3−1606, we find that the observed timing properties of the source, as well as its quiescent X-ray luminosity, can be reproduced if it was born with a poloidal and crustal toroidal fields of Bp ∼ 1.5 × 10 14 G and Btor ∼ 7 × 10 14 G, respectively, and if its current age

A Radio-loud Magnetar in X-ray Quiescence

As part of a survey for radio pulsars with the Parkes 64 m telescope, we have discovered PSR J1622-4950, a pulsar with a 4.3 s rotation period. Follow-up observations show that the pulsar has the highest inferred surface magnetic field of the known radio pulsars (B {approx}3 x 10{sup 14} G), and it exhibits significant timing noise and appears to have an inverted spectrum. Unlike the vast majority of the known pulsar population, PSR J1622-4950 appears to switch off for many hundreds of days and even in its on-state exhibits extreme variability in its flux density. Furthermore, the integrated pulse profile changes shape with epoch. All of these properties are remarkably similar to the only two magnetars previously known to emit radio pulsations. The position of PSR J1622-4950 is coincident with an X-ray source that, unlike the other radio pulsating magnetars, was found to be in quiescence. We conclude that our newly discovered pulsar is a magnetar-the first to be discovered via its radio emission.

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 a 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 data base. 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-h orbit that exhibits nulling behaviour, not previously observed in recycled or binary objects), PSR J1802-2124 (an intermediate mass binary pulsar) and PSR J1801-1417 (a solitary millisecond pulsar).

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.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 hr and an eccentricity of 0.18. They also revealed a significant advance of periastron, 2585 ± 0002 yr-1. Assuming this is entirely due to general relativity implies a total system mass (pulsar plus companion) of 2.574 ± 0.003 M☉. This mass and the significant orbital eccentricity suggest that this is a double neutron star system. Measurement of the gravitational redshift, γ, and an evaluation of the Shapiro delay shape, s, indicate a low companion mass of less than 1.25 M☉. 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.