arXiv subjects

R. M. Shannon

Publications and source records attributed to R. M. Shannon.

151 records · Page 9Linked to original sources

arXiv2012

Development of a pulsar-based timescale

Using observations of pulsars from the Parkes Pulsar Timing Array (PPTA) project we develop the first pulsar-based timescale that has a precision comparable to the uncertainties in international atomic timescales. Our ensemble of pulsars provides an Ensemble Pulsar Scale (EPS) analogous to the free atomic timescale Echelle Atomique Libre (EAL). The EPS can be used to detect fluctuations in atomic timescales and therefore can lead to a new realisation of Terrestrial Time, TT(PPTA11). We successfully follow features known to affect the frequency of the International Atomic Timescale (TAI) and we find marginally significant differences between TT(PPTA11) and TT(BIPM11). We discuss the various phenomena that lead to a correlated signal in the pulsar timing residuals and therefore limit the stability of the pulsar timescale.

astro-ph.IM↗

arXiv2012

Optimal Interpolation and Prediction in Pulsar Timing

For pulsar projects it is often necessary to predict the pulse phase in advance, for example, when preparing for new observations. Interpolation of the pulse phase between existing measurements is also often required, for example, when folding X-ray or gamma-ray observations according to the radio pulse phase. Until now these procedures have been done using various ad hoc methods. The purpose of this paper is to show how to interpolate or predict the pulse phase optimally using statistical models of the various noise processes and the phase measurement uncertainty.

astro-ph.SR↗

arXiv2012

Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves

We present an analysis of high-precision pulsar timing data taken as part of the North American Nanohertz Observatory for Gravitational waves (NANOGrav) project. We have observed 17 pulsars for a span of roughly five years using the Green Bank and Arecibo radio telescopes. We analyze these data using standard pulsar timing models, with the addition of time-variable dispersion measure and frequency-variable pulse shape terms. Sub-microsecond timing residuals are obtained in nearly all cases, and the best root-mean-square timing residuals in this set are ~30-50 ns. We present methods for analyzing post-fit timing residuals for the presence of a gravitational wave signal with a specified spectral shape. These optimally take into account the timing fluctuation power removed by the model fit, and can be applied to either data from a single pulsar, or to a set of pulsars to detect a correlated signal. We apply these methods to our dataset to set an upper limit on the strength of the nHz-frequency stochastic supermassive black hole gravitational wave background of h_c (1 yr^-1) < 7x10^-15 (95%). This result is dominated by the timing of the two best pulsars in the set, PSRs J1713+0747 and J1909-3744.

astro-ph.CO↗

arXiv2011

Minimum Requirements for Detecting a Stochastic Gravitational Wave Background Using Pulsars

We assess the detectability of a nanohertz gravitational wave (GW) background with respect to additive red and white noise in the timing of millisecond pulsars. We develop detection criteria based on the cross-correlation function summed over pulsar pairs in a pulsar timing array. The distribution of correlation amplitudes is found to be non-Gaussian and highly skewed, which significantly influences detection and false-alarm probabilities. When only white noise and GWs contribute, our detection results are consistent with those found by others. Red noise, however, drastically alters the results. We discuss methods to meet the challenge of GW detection ("climbing mount significance") by distinguishing between GW-dominated and red or white-noise limited regimes. We characterize detection regimes by evaluating the number of millisecond pulsars that must be monitored in a high-cadence, 5-year timing program for a GW background spectrum $h_c(f) = A f^{-2/3}$ with $A = 10^{-15}$ yr$^{-2/3}$. Unless a sample of 20 super-stable millisecond pulsars can be found --- those with timing residuals from red-noise contributions $σ_r \lesssim 20$ ns --- a much larger timing program on $\gtrsim 50 - 100$ MSPs will be needed. For other values of $A$, the constraint is $σ_r \lesssim 20 {\rm ns} (A/10^{-15} {\rm yr}^{-2/3})$. Identification of suitable MSPs itself requires an aggressive survey campaign followed by characterization of the level of spin noise in the timing residuals of each object. The search and timing programs will likely require substantial fractions of time on new array telescopes in the southern hemisphere as well as on existing ones.

astro-ph.GA↗

arXiv2010

A Measurement Model for Precision Pulsar Timing

This paper describes a comprehensive measurement model for the error budget of pulse arrival times with emphasis on intrinsic pulse jitterand plasma propagation effects (particularly interstellar scattering), which are stochastic in time and have diverse dependences on radio frequency. To reduce their contribution, timing measurements can be made over a range of frequencies that depends on a variety of pulsar and instrumentation-dependent factors that we identify. A salient trend for high signal-to-noise measurements of millisecond pulsars is that time-of-arrival precision is limited either by irreducible interstellar scattering or by pulse-phase jitter caused by variable emission within pulsar magnetospheres. A cap on timing errors implies that pulsars must be confined to low dispersion measures (DMs) and observed at high frequencies. Use of wider bandwidths that increase signal-to-noise ratios will degrade timing precision if nondispersive chromatic effects are not mitigated. The allowable region in the DM-frequency plane depends on how chromatic timing perturbations are addressed. Without mitigation, observations at 1.4~GHz or 5~GHz are restricted to $\DM\lesssim 30$ and $\lesssim 100~\DMu$, respectively. With aggressive mitigation of interstellar scattering and use of large telescopes to provide adequate sensitivity at high frequencies (e.g. Arecibo, FAST, phase 1 of the SKA, and the SKA), pulsars with DMs up to 500~$\DMu$ can be used in precision timing applications. We analyze methods that fit arrival times vs. frequency at a given epoch prior to multi-epoch fitting. While the terms of greatest astrophysical interest are achromatic (e.g. orbital and gravitational wave perturbations), measurements may ultimately be limited by similarly achromatic stochasticity in a pulsar's spin rate.

astro-ph.IM↗

arXiv2006

Rocking the Lighthouse: Circumpulsar Asteroids and Radio Intermittency

We propose that neutral, circumpulsar debris entering the light cylinder can account for many time-dependent pulsar phenomena that are otherwise difficult to explain. Neutral material avoids propeller ejection and injects sufficient charges -- after heating, evaporation, and ionization -- to alter current flows and pair-production and thus trigger, detune, or extinguish coherent emission. Relevant phenomena, with time scales from seconds to months, include nulls, rotating radio transients (RRATs), rapid changes in pulse profile (``mode changes''), variable subpulse drift rates, quasi-periodic bursts from B1931+24, and torque variations. Over the 10 Myr lifetime of a canonical pulsar with trillion-gauss surface magnetic field, less than a millionth of an Earth mass of material is needed to modulate the Goldreich-Julian current by 100%. Circumpulsar material originates from metal-rich, supernova fallback gas that aggregates into asteroids. Debris disks can inject sufficient material on time scales of interest, yet be too tenuous to form large planets detectable in pulse timing data. Asteroid migration results from collisions and the radiation-driven Yarkovsky and Poynting-Robertson effects. For B1931+24, an asteroid in a $\sim 40$~day elliptical orbit pollutes the magnetosphere stochastically through collisions with other debris. Injection is less likely for hot, young and highly magnetized pulsars or millisecond pulsars that pre-ionize any debris material well outside their small magnetospheres. Injection effects will therefore be most prominent in long-period, cooler pulsars, consistent with the distribution of relevant objects in perid and period derivative. A pulsar's spin history and its radiation-beam orientation may influence whether it displays nulling, RRATs and other effects.

astro-ph↗

arXiv2006

Magnetospheric Birefringence Induces Unique Polarization Signatures in Neutron-Star Spectra

We study the propagation of polarization light through the magnetosphere of neutron stars. At intermediate frequencies (the optical through the infrared), both the birefringence induced by the plasma and by quantumelectrodynamics influence the observation polarization of radiation from the surface of the neutron star. Because these two processes compete in this regime, we find that polarization observations can constrain the properties of the neutron-star magnetosphere, specifically the total charge density. We calculate both the phase-resolved and the phase-averaged polarization signatures induced by magnetospheric birefringence.

astro-ph↗