John M. Sarkissian

Transient pulsed radio emission from a magnetar

Anomalous X-ray pulsars (AXPs) are slowly rotating neutron stars with very bright and highly variable X-ray emission that are believed to be powered by ultra-strong magnetic fields of >1014 G, according to the ‘magnetar’ model. The radio pulsations that have been observed from more than 1,700 neutron stars with weaker magnetic fields have never been detected from any of the dozen known magnetars. The X-ray pulsar XTE J1810 - 197 was revealed (in 2003) as the first AXP with transient emission when its luminosity increased 100-fold from the quiescent level; a coincident radio source of unknown origin was detected one year later. Here we show that XTE J1810 - 197 emits bright, narrow, highly linearly polarized radio pulses, observed at every rotation, thereby establishing that magnetars can be radio pulsars. There is no evidence of radio emission before the 2003 X-ray outburst (unlike ordinary pulsars, which emit radio pulses all the time), and the flux varies from day to day. The flux at all radio frequencies is approximately equal—and at >20 GHz XTE J1810 - 197 is currently the brightest neutron star known. These observations link magnetars to ordinary radio pulsars, rule out alternative accretion models for AXPs, and provide a new window into the coronae of magnetars.

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.

Upper Bounds on the Low-Frequency Stochastic Gravitational Wave Background from Pulsar Timing Observations: Current Limits and Future Prospects

Using a statistically rigorous analysis method, we place limits on the existence of an isotropic stochastic gravitational wave background using pulsar timing observations. We consider backgrounds whose characteristic strain spectra may be described as a power-law dependence with frequency. Such backgrounds include an astrophysical background produced by coalescing supermassive black-hole binary systems and cosmological backgrounds due to relic gravitational waves and cosmic strings. Using the best available data, we obtain an upper limit on the energy density per unit logarithmic frequency interval of Ω h2 ≤ 1.9 × 10-8 for an astrophysical background that is 5 times more stringent than the earlier limit of 1.1 × 10-7 found by Kaspi and colleagues. We also provide limits on a background due to relic gravitational waves and cosmic strings of Ω h2 ≤ 2.0 × 10-8 and Ω h2 ≤ 1.9 × 10-8, respectively. All of the quoted upper limits correspond to a 0.1% false alarm rate together with a 95% detection rate. We discuss the physical implications of these results and highlight the future possibilities of the Parkes Pulsar Timing Array project. We find that our current results can (1) constrain the merger rate of supermassive binary black hole systems at high redshift, (2) rule out some relationships between the black hole mass and the galactic halo mass, (3) constrain the rate of expansion in the inflationary era, and (4) provide an upper bound on the dimensionless tension of a cosmic string background.

Precision Timing of PSR J0437?4715: An Accurate Pulsar Distance, a High Pulsar Mass, and a Limit on the Variation of Newton's Gravitational Constant

Analysis of 10 years of high-precision timing data on the millisecond pulsar PSR J0437–4715 has resulted in a model-independent kinematic distance based on an apparent orbital period derivative, P_b, determined at the 1.5% level of precision (D_k = 157.0 ± 2.4 pc), making it one of the most accurate stellar distance estimates published to date. The discrepancy between this measurement and a previously published parallax distance estimate is attributed to errors in the DE200 solar system ephemerides. The precise measurement of P_b allows a limit on the variation of Newtons gravitational constant, |G/G| ≤ 23 × 10^−12 yr^−1. We also constrain any anomalous acceleration along the line of sight to the pulsar to |a⊙/c| ≤ 1.5 × 10^−18 s^−1 at 95% confidence, and derive a pulsar mass, m_(psr) = 1.76 ± 0.20 M⊙, one of the highest estimates so far obtained.

A test of general relativity from the three-dimensional orbital geometry of a binary pulsar

Binary pulsars provide an excellent system for testing general relativity because of their intrinsic rotational stability and the precision with which radio observations can be used to determine their orbital dynamics. Measurements of the rate of orbital decay of two pulsars have been shown to be consistent with the emission of gravitational waves as predicted by general relativity, but independent verification was not possible. Such verification can in principle be obtained by determining the orbital inclination in a binary pulsar system using only classical geometrical constraints. This would permit a measurement of the expected retardation of the pulse signal arising from the general relativistic curvature of space-time in the vicinity of the companion object (the ‘Shapiro delay’). Here we report high-precision radio observations of the binary millisecond pulsar PSR J0437-4715, which establish the three-dimensional structure of its orbit. We see the Shapiro delay predicted by general relativity, and we determine the mass of the neutron star and its white dwarf companion. The determination of such masses is necessary in order to understand the origin and evolution of neutron stars.

Detection of 107 glitches in 36 southern pulsars

Timing observations from the Parkes 64-m radio telescope for 165 pulsarsbetween 1990 and 2011 have been searched for period glitches. Data spansfor each pulsar ranged between 5.3 and 20.8 yr. From the total of 1911yr of pulsar rotational history, 107 glitches were identified in 36pulsars. Out of these glitches, 61 have previously been reported whereas46 are new discoveries. Glitch parameters, both for the previously knownand the new glitch detections, were measured by fitting the timingresidual data. Observed relative glitch sizes{

Measurement and correction of variations in interstellar dispersion in high-precision pulsar timing

\Delta

Dispersion measure variations and their effect on precision pulsar timing

}{

AN ECLIPSING MILLISECOND PULSAR WITH A POSSIBLE MAIN-SEQUENCE COMPANION IN NGC 6397

\nu

Development of a pulsar-based time-scale

}