Merging supermassive black hole binaries produce low-frequency gravitational waves, which pulsar timing experiments are searching for. Much of the current theory is developed within the plane-wave formalism, and here we develop the more general Fresnel formalism. We show that Fresnel corrections to gravitational wave timing residual models allow novel measurements to be made, such as direct measurements of the source distance from the timing residual phase and frequency, as well as direct measurements of chirp mass from a monochromatic source. Probing the Fresnel corrections in these models will require future pulsar timing arrays with more distant pulsars across our Galaxy (ideally at the distance of the Magellanic Clouds), timed with precisions less than $100$ ns, with distance uncertainties reduced to the order of the gravitational wavelength. We find that sources with chirp mass of order $10^9 \ \mathrm{M}_\odot$ and orbital frequency $ω_0 > 10$ nHz are good candidates for probing Fresnel corrections. With these conditions met, the measured source distance uncertainty can be made less than 10 per cent of the distance to the source for sources out to $\sim 100$ Mpc, source sky localization can be reduced to sub-arcminute precision, and source volume localization can be made to less than $1 \ \text{Mpc}^3$ for sources out to 1-Gpc distances.
Pulsar timing experiments are currently searching for gravitational waves, and this dissertation focuses on the development and study of the pulsar timing residual models used for continuous wave searches. The first goal of this work is to re-present much of the fundamental physics and mathematics concepts behind the calculations and theory used in pulsar timing. While there exist many reference sources in the literature, I try to offer a fully self-contained explanation of the fundamentals of this research which I hope the reader will find helpful. The next goal broadly speaking has been to f…
The sensitivity of pulsar timing arrays to gravitational waves is, at some level, limited by timing noise. Red timing noise - the stochastic wandering of pulse arrival times with a red spectrum - is prevalent in slow-spinning pulsars and has been identified in many millisecond pulsars. Phenomenological models of timing noise, such as from superfluid turbulence, suggest that the timing noise spectrum plateaus below some critical frequency, $f_c$, potentially aiding the hunt for gravitational waves. We examine this effect for individual pulsars by calculating minimum observation times, $T_{\rm m…
A Realistic Pulsar -- Supermassive Black Hole Timing Model
Peking University（北京大学）；Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学科维理天文与天体物理研究所）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）
General relativity has predicted the existence of gravitational waves (GW), which are waves of the distortions of space-time with two degrees of polarization and the propagation speed of light. Alternative theories predict more polarizations, up to a maximum of six, and possible deviation of propagation speed from the light speed. The present paper reviews recent proposals to test the gravity theories in the radiation regime by observing GWs using pulsar timing arrays.
Pulsar Timing Arrays are a prime tool to study unexplored astrophysical regimes with gravitational waves. Here we show that the detection of gravitational radiation from individually resolvable super-massive black hole binary systems can yield direct information about the masses and spins of the black holes, provided that the gravitational-wave induced timing fluctuations both at the pulsar and at the Earth are detected. This in turn provides a map of the non-linear dynamics of the gravitational field and a new avenue to tackle open problems in astrophysics connected to the formation and evolu…
Complementary to ground-based laser interferometers, pulsar timing array experiments are being carried out to search for nanohertz gravitational waves. Using the world's most powerful radio telescopes, three major international collaborations have collected $\sim$10-year high precision timing data for tens of millisecond pulsars. In this paper we give an overview on pulsar timing experiments, gravitational wave detection in the nanohertz regime, and recent results obtained by various timing array projects.
Pulsars are remarkably precise "celestial clocks" that can be used to explore many different aspects of physics and astrophysics. In this article I give a brief summary of pulsar properties and describe some of the applications of pulsar timing, including tests of theories of gravitation, efforts to detect low-frequency gravitational waves using pulsar timing arrays and establishment a "pulsar timescale".
Millisecond pulsars are intrinsically very stable clocks and precise measurement of their observed pulse periods can be used to study a wide variety of astrophysical phenomena. In particular, observations of a large sample of millisecond pulsars at regular intervals, constituting a Pulsar Timing Array (PTA), can be used as a detector of low-frequency gravitational waves and to establish a standard of time independent of terrestrial atomic timescales. Three major timing array projects have been established: The European Pulsar Timing Array (EPTA), the North American pulsar timing array (NANOGra…
Highly precise pulsar timing is very important for understanding the nature of a neutron star, and it can even be used to detect gravitational waves. Unfortunately, the accuracy of the pulsar timing is seriously affected by the spin-down irregularities of pulsars.
The maximum frequency of gravitational waves (GWs) detectable with traditional pulsar timing methods is set by the Nyquist frequency ($f_{\rm{Ny}}$) of the observation. Beyond this frequency, GWs leave no temporal-correlated signals; instead, they appear as white noise in the timing residuals. The variance of the GW-induced white noise is a function of the position of the pulsars relative to the GW source. By observing this unique functional form in the timing data, we propose that we can detect GWs of frequency $>$ $f_{\rm{Ny}}$ (super-Nyquist frequency GWs; SNFGWs). We demonstrate the feasib…
We present an overview of precise pulsar timing using data from the Large Area Telescope (LAT) on Fermi. We describe the analysis techniques including a maximum likelihood method for determining pulse times of arrival from unbinned photon data. In addition to determining the spindown behavior of the pulsars and detecting glitches and timing noise, such timing analyses allow the precise determination of the pulsar position, thus enabling detailed multiwavelength follow up.
The pulsar timing residuals induced by gravitational waves from non-evolving single binary sources are affected by many parameters related to the relative positions of the pulsar and the gravitational wave sources. We will fully analyze the effects due to different parameters one by one. The standard deviations of the timing residuals will be calculated with a variable parameter fixing a set of other parameters. The orbits of the binary sources will be generally assumed to be elliptical. The influences of different eccentricities on the pulsar timing residuals will also studied in detail. We f…
Though pulsars spin regularly, the differences between the observed and predicted ToA (time of arrival), known as "timing noise", can still reach a few milliseconds or more. We try to understand the noise in this paper. As proposed by Xu & Qiao in 2001, both dipole radiation and particle emission would result in pulsar braking. Accordingly, possible fluctuation of particle current flow is suggested here to contribute significant ToA variation of pulsars. We find that the particle emission fluctuation could lead to timing noise which can't be eliminated in timing process, and that a longer peri…
We investigate the impact of intrinsic, kinematic and gravitational effects on high precision pulsar timing. We present an analytical derivation and a numerical computation of the impact of these effects on the first and second derivative of the pulsar spin frequency. In addition, in the presence of white noise, we derive an expression to determine the expected measurement uncertainty of a second derivative of the spin frequency for a given timing precision, observing cadence and timing baseline and find that it strongly depends on the latter ($\propto t^{-7/2}$). We show that for pulsars with…
Young pulsars deviate from a perfectly regular spin-down by two non-deterministic phenomena: impulsive glitches and timing noise. Both phenomena are interesting per se, and may provide insights into the superfluid properties of neutron stars, but they also act as a barrier to high-precision pulsar timing and gravitational wave experiments. We study a minimal stochastic model to describe the spin-down of a multicomponent neutron star, with fluctuations in both the internal and external torques. The power spectral density and timing noise strength of this kind of model can be obtained analytical…
We present a principal component analysis method which tracks and compensates for short-timescale variability in pulsar profiles, with a goal of improving pulsar timing precision. We couple this with a fast likelihood technique for determining pulse time of arrival, marginalizing over the principal component amplitudes. This allows accurate estimation of timing errors in the presence of pulsar variability. We apply the algorithm to the slow pulsar PSR J2139+0040 using an archived set of untargeted raster-scan observations at arbitrary epochs across four years, obtaining an improved timing solu…
A pulsar timing array (PTA) refers to a program of regular, high-precision timing observations of a widely distributed array of millisecond pulsars. Here we review the status of the three primary PTA projects and the joint International Pulsar Timing Array project. We discuss current results related to ultra-low-frequency gravitational wave searches and highlight opportunities for the near future.
We have constructed timing solutions for 81 gamma-ray pulsars covering more than five years of Fermi data. The sample includes 37 radio-quiet or radio-faint pulsars which cannot be timed with other telescopes. These timing solutions and the corresponding pulse times of arrival are prerequisites for further study, e.g. phase-resolved spectroscopy or searches for mode switches. Many gamma-ray pulsars are strongly affected by timing noise, and we present a new method for characterizing the noise process and mitigating its effects on other facets of the timing model. We present an analysis of timi…
We examine timing noise in both magnetars and regular pulsars, and find that there exists a component of the timing noise with strong magnetic field dependence above B ~ 10^{12.5} G. The dependence of the timing noise floor on the magnetic field is also reflected in the smallest observable glitch size. We find that magnetospheric torque variation cannot explain this component of timing noise. We calculate the moment of inertia of the magnetic field outside of a neutron star and show that this timing noise component may be due to variation of this moment of inertia, and could be evidence of rap…
Gravitational waves provide a new probe of the Universe which can reveal a number of cosmological and astrophysical phenomena that cannot be observed by electromagnetic waves. Different frequencies of gravitational waves are detected by different means. Among them, precision measurements of pulsar timing provides a natural detector for gravitational waves with light-year scale wavelengths. In this review, first a basic framework to detect a stochastic gravitational wave background using pulsar timing array is introduced, and then possible interpretations of the latest observational result of 1…
Gravitational scattering events, in which the path of an interstellar object is deflected by a pulsar or the solar system, give rise to reflex motion which can potentially be detected using pulsar timing. We determine the form of the timing signal expected from a gravitational scattering event, which is ramp-like and resembles the signal produced by a glitch or a gravitational wave burst with memory (BWM), and investigate the prospects for detecting such a signal using a pulsar timing array. The level of timing precision currently achieved for some millisecond pulsars makes it possible to dete…
We have used the Nanshan 25-m Radio Telescope at Xinjiang Astronomical Observatory to obtain timing observations of 87 pulsars from 2002 July to 2014 March. Using the "Cholesky" timing analysis method we have determined positions and proper motions for 48 pulsars, 24 of which are improved positions compared to previously published values. We also present the first published proper motions for nine pulsars and improved proper motions for 21 pulsars using pulsar timing and position comparison method. The pulsar rotation parameters are derived and are more accurate than previously published value…
Radio timing observations of a millisecond pulsar in orbit around the Galactic centre black hole (BH) or a BH at the centre of globular clusters could answer foundational questions in astrophysics and fundamental physics. Pulsar radio astronomy typically employs the post-Keplerian approximation to determine the system parameters. However, in the strong gravitational field around the central BH, higher order relativistic effects may become important. We compare the pulsar timing delays given by the post-Keplerian approximation with those given by a relativistic timing model. We find significant…
To successfully detect gravitational waves with pulsar timing arrays, we need to have a comprehensive understanding of the physical origins and statistical characteristics of the noise in pulse arrival times and identify mitigation methods to reduce the noise. In this paper we will review radiometer noise, phase jitter noise and timing noise in the noise budget of pulsar timing and show various efforts used to reduce them. We will briefly discuss the results of an overall assessment of the components and physical causes of the timing residuals for millisecond pulsars in the North American Nano…
We aim to resolve a misunderstanding about whether the so-called "Earth term" in the pulsar timing response to a gravitational wave is in phase among a set of pulsars. We note that the misunderstanding has potentially arisen from the statements that the Earth term is "coherent" or "builds up coherently" among the pulsars. We clarify what authors mean by "coherent" in these statements, pointing out that "coherent" does not indicate that the Earth terms are in phase among the pulsars. Using the pulsar timing residuals induced by a continuous gravitational wave, we show that the Earth term does n…
Pulsar's position, proper motion and parallax are important parameters in timing equations. It is a really challenging work to fit astrometric parameters accurately through pulsar timing, especially for pulsars that show irregular timing properties. As the fast development of related techniques, it is possible to measure astrometric parameters of more and more pulsars in a model$\textrm{-}$independent manner with the Very Long Baseline Interferometry (VLBI). In this work, we select 16 normal pulsars, whose parallax and proper motion have not been successfully fitted with timing observations or…
We investigate the impact of noise processes on high-precision pulsar timing. Our analysis focuses on the measurability of the second spin frequency derivative $\ddotν$. This $\ddotν$ can be induced by several factors including the radial velocity of a pulsar. We use Bayesian methods to model the pulsar times-of-arrival in the presence of red timing noise and dispersion measure variations, modelling the noise processes as power laws. Using simulated times-of-arrival that both include red noise, dispersion measure variations and non-zero $\ddotν$ values, we find that we are able to recover the …
We study the timing stability of three black widow pulsars, both in terms of their long-term spin evolution and their shorter-term orbital stability. The erratic timing behaviour and radio eclipses of the first two black widow pulsar systems discovered (PSRs B1957+20 and J2051$-$0827) was assumed to be representative for this class of pulsars. With several new black widow systems added to this population in the last decade, there are now several systems known that do not show these typical orbital variations or radio eclipses. We present timing solutions using 7$-$8 yrs of observations from fo…
The smooth spin-down of young pulsars is perturbed by two non-deterministic phenomenon, glitches and timing noise. Although the timing noise provides insights into nuclear and plasma physics at extreme densities, it acts as a barrier to high-precision pulsar timing experiments. An improved methodology based on Bayesian inference is developed to simultaneously model the stochastic and deterministic parameters for a sample of 85 high-$\dot{E}$ radio pulsars observed for $\sim$ 10 years with the 64-m Parkes radio telescope. Timing noise is known to be a red process and we develop a parametrizatio…
While pulsars possess exceptional rotational stability, large scale timing studies have revealed at least two distinct types of irregularities in their rotation: red timing noise and glitches. Using modern Bayesian techniques, we investigated the timing noise properties of 300 bright southern-sky radio pulsars that have been observed over 1.0-4.8 years by the upgraded Molonglo Observatory Synthesis Telescope (MOST). We reanalysed the spin and spin-down changes associated with nine previously reported pulsar glitches, report the discovery of three new glitches and four unusual glitch-like event…
We investigate the capability of pulsar timing arrays (PTAs) as a probe of primordial black holes (PBHs), which might constitute the Galactic dark matter. A PBH passing nearby the Earth or a pulsar gives an impulse acceleration and induces residuals on otherwise orderly pulsar timing data. We show that the timing residuals induced at pulsars are optimal for searching heavier PBHs than those at the Earth, and the two probes are highly complemental. Future facilities like SKA could detect PBHs with masses around 10^(22-28) g even if only a small fraction (< 1%) of the Galactic dark matter consis…
The ~800 yr-old pulsar J1846-0258 is a unique transition object between rotation-powered pulsars and magnetars: though behaving like a rotation-powered pulsar most of the time, in 2006 it exhibited a distinctly magnetar-like outburst accompanied by a large glitch. Here we present X-ray timing observations taken with the Rossi X-ray Timing Explorer over a 2.2-yr period after the X-ray outburst and glitch had recovered. We observe that the braking index of the pulsar, previously measured to be n=2.65+/-0.01, is now n=2.16+/-0.13, a decrease of 18+/-5%. We also note a persistent increase in the t…
Pulsar timing arrays (PTAs) will be sensitive to a finite number of gravitational wave (GW) "point" sources (e.g. supermassive black hole binaries). N quiet pulsars with accurately known distances d_{pulsar} can characterize up to 2N/7 distant chirping sources per frequency bin Δf_{gw}=1/T, and localize them with "diffraction limited" precision δθ\gtrsim (1/SNR)(λ_{gw}/d_{pulsar}). Even if the pulsar distances are poorly known, a PTA with F frequency bins can still characterize up to (2N/7)[1-(1/2F)] sources per bin, and the quasi-singular pattern of timing residuals in the vicinity of a GW so…
The form of timing residuals due to errors in pulsar spin period $P$ and its derivative $\dot{P}$, in positions, as well as in proper motions, have been well presented for decades in the literature. However, the residual patterns due to errors in the pulsar acceleration have not been reported previously, while a pulsar in the galaxy or a globular cluster (GC) will be unavoidably accelerated. The coupling effect of the pulsar transverse acceleration and the R$\rm{\ddot{o}}$mer delay on timing residuals are simulated in this work. The results show that the residual due to the effect can be ident…
The double pulsar (PSR J0737-3039A/B) provides some of the most stringent tests of general relativity (GR) and its alternatives. The success of this system in tests of GR is largely due to the high-precision, long-term timing of its recycled-pulsar member, pulsar A. On the other hand, pulsar B is a young pulsar that exhibits significant short-term and long-term timing variations due to the electromagnetic-wind interaction with its companion and geodetic precession. Improving pulsar B's timing precision is a key step towards improving the precision in a number of GR tests with PSR J0737-3039A/B…
Radio pulsars are often used as clocks in a wide variety of experiments. Imperfections in the clock, known as timing noise, have the potential to reduce the significance of, or even thwart e.g. the attempt to find a stochastic gravitational wave (GW) background. We measure the timing noise in a group of 129 mostly middle-aged pulsars (i.e. characterstic ages near 1~Myr) observed with the Parkes radio telescope on a monthly basis since 2014. We examine four different metrics for timing noise, but it remains unclear which, if any, provides the best determination. In spite of this, it is evident …
The main goal of pulsar timing array experiments is to detect correlated signals such as nanohertz-frequency gravitational waves. Pulsar timing data collected in dense monitoring campaigns can also be used to study the stars themselves, their binary companions, and the intervening ionised interstellar medium. Timing observations are extraordinarily sensitive to changes in path length between the pulsar and the Earth, enabling precise measurements of the pulsar positions, distances and velocities, and the shapes of their orbits. Here we present a timing analysis of 25 pulsars observed as part o…
Pulsar Timing Arrays use a set of millisecond pulsars in an attempt to directly detect nanohertz gravitational waves. For this purpose, high precision timing of the pulsars is essential and ultimately a precision of the order of ~100 ns is required. Propagation effects in the interstellar medium cause the radio emission from a pulsar to be dispersed and scattered, introducing time variable delays of the pulses on their way to Earth. If these delays are not properly corrected for, they may cause significant errors in the timing analysis of a pulsar. These proceedings will review the effects of …
The LOFAR Tied-Array All-Sky Survey (LOTAAS) is the most sensitive untargeted radio pulsar survey performed at low radio frequencies (119--151\,MHz) to date and has discovered 76 new radio pulsars, among which the 23.5-s pulsar J0250+5854, up until recently the slowest-spinning radio pulsar known. Here, we report on the timing solutions of 35 pulsars discovered by LOTAAS, which include a nulling pulsar and a mildly recycled pulsar, and thereby complete the full timing analysis of the LOTAAS pulsar discoveries. We give an overview of the findings from the full LOTAAS sample of 76 pulsars, discu…
In this paper we present a coherent timing analysis of the 401 Hz pulsations of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst. Using observations collected with the Neutron Star Interior Composition Explorer (NICER), we establish the pulsar spin frequency and orbital phase during its latest epoch. We find that the 2019 outburst shows a pronounced evolution in pulse phase over the course of the outburst. These phase shifts are found to correlate with the source flux, and are interpreted in terms of hot-spot drift on the stellar surface, driven by changes in th…
Efforts are underway to use high-precision timing of pulsars in order to detect low-frequency gravitational waves. A limit to this technique is the timing noise generated by dispersion in the plasma along the line of sight to the pulsar, including the solar wind. The effects due to the solar wind vary with time, influenced by the change in solar activity on different time scales, ranging up to $\sim 11$ years for a solar cycle. The solar wind contribution depends strongly on the angle between the pulsar line of sight and the solar disk, and is a dominant effect at small separations. Although s…
We present timing models for 20 millisecond pulsars in the Parkes Pulsar Timing Array. The precision of the parameter measurements in these models has been improved over earlier results by using longer data sets and modelling the non-stationary noise. We describe a new noise modelling procedure and demonstrate its effectiveness using simulated data. Our methodology includes the addition of annual dispersion measure (DM) variations to the timing models of some pulsars. We present the first significant parallax measurements for PSRs J1024-0719, J1045-4509, J1600-3053, J1603-7202, and J1730-2304,…
The SUrvey for Pulsars and Extragalactic Radio Bursts ran from 2014 April to 2019 August, covering a large fraction of the southern hemisphere at mid- to high-galactic latitudes, and consisting of 9-minute pointings taken with the 20-cm multibeam receiver on the Parkes Radio Telescope. Data up to 2017 September 21 have been searched using standard Fourier techniques, single-pulse searches, and Fast Folding Algorithm searches. We present 19 new discoveries, bringing the total to 27 discoveries in the programme, and we report the results of follow-up timing observations at Parkes for 26 of these…
Precision astrometry is an integral component of successful pulsar timing campaigns. Astrometric parameters are commonly derived by fitting them as parameters of a timing model to a series of pulse times of arrival (TOAs). TOAs measured to microsecond precision over several-year spans can yield position measurements with sub-milliarcsecond precision. However, timing-based astrometry can become biased if a pulsar displays any red spin noise, which can be compared to the red noise signal produced by the stochastic gravitational wave background. We investigate how noise of different spectral type…
There are two types of timing irregularities seen in pulsars: glitches and timing noise. Both of these phenomena can help us probe the interior of such exotic objects. This article presents a brief overview of the observational and theoretical aspects of pulsar timing irregularities and the main results from the investigations of these phenomena in India. The relevance of such Indian programs for monitoring of young pulsars with the Square Kilometer Array (SKA) is presented, highlighting possible contributions of the Indian neutron star community to the upcoming SKA endeavour.
Pulsar timing is a technique that uses the highly stable spin periods of neutron stars to investigate a wide range of topics in physics and astrophysics. Pulsar timing arrays (PTAs) use sets of extremely well-timed pulsars as a Galaxy-scale detector with arms extending between Earth and each pulsar in the array. These challenging experiments look for correlated deviations in the pulsars' timing that are caused by low-frequency gravitational waves (GWs) traversing our Galaxy. PTAs are particularly sensitive to GWs at nanohertz frequencies, which makes them complementary to other space- and grou…
The Five-hundred-meter Aperture Spherical Telescope (FAST) will become one of the world-leading telescopes for pulsar timing array (PTA) research. The primary goals for PTAs are to detect (and subsequently study) ultra-low-frequency gravitational waves, to develop a pulsar-based time standard and to improve solar system planetary ephemerides. FAST will have the sensitivity to observe known pulsars with significantly improved signal-to-noise ratios and will discover a large number of currently unknown pulsars. We describe how FAST will contribute to PTA research and show that jitter- and timing…
At the highest levels of pulsar timing precision achieved to date, experiments are limited by noise intrinsic to the pulsar. This stochastic wideband impulse modulated self-noise (SWIMS) limits pulsar timing precision by randomly biasing the measured times of arrival and thus increasing the root mean square (rms) timing residual. We discuss an improved methodology of removing this bias in the measured times of arrival by including information about polarized radiation. Observations of J0437-4715 made over a one-week interval at the Parkes Observatory are used to demonstrate a nearly 40 per cen…
Gravitational wave bursts produced by supermassive binary black hole mergers will leave a persistent imprint on the space-time metric. Such gravitational wave memory signals are detectable by pulsar timing arrays as a glitch event that would seem to occur simultaneously for all pulsars. In this paper, we describe an initial algorithm which can be used to search for gravitational wave memory signals. We apply this algorithm to the Parkes Pulsar Timing Array data set. No significant gravitational wave memory signal is founded in the data set.
Modeling of frequency-dependent effects, contributed by the turbulence in the free electron density of interstellar plasma, is required to enable the detection of the expected imprints from the stochastic gravitational-wave (GW) background in pulsar timing data. In this work, we present an investigation of temporal variations of interstellar medium for a set of millisecond pulsars (MSPs) with the upgraded GMRT aided by large fractional bandwidth at lower observing frequencies. Contrary to the conventional narrow-band analysis using a frequency invariant template profile, we applied $PulsePortr…
Pulsar timing provides a sensitive probe of small-scale structure. Gravitational perturbations arising from an inhomogeneous environment could manifest as detectable perturbations in the pulsation phase. Consequently, pulsar timing arrays have been proposed as a probe of dark matter substructure on mass scales as small as $10^{-11} M_\odot$. Since the small-scale mass distribution is connected to early-Universe physics, pulsar timing can therefore constrain the thermal history prior to Big Bang nucleosynthesis (BBN), a period that remains largely unprobed. We explore here the prospects for pul…
Pulsar Timing Array (PTA) experiments exploit the clock-like behaviour of an array of millisecond pulsars, with the goal of detecting low-frequency gravitational waves. PTA experiments have been in operation over the last decade, led by groups in Europe, Australia, and North America. These experiments use the most sensitive radio telescopes in the world, extremely precise pulsar timing models and sophisticated detection algorithms to increase the sensitivity of PTAs. No detection of gravitational waves has been made to date with this technique, but PTA upper limits already contributed to rule …
