arXiv subjects

G. Hobbs

Publications and source records attributed to G. Hobbs.

At least 73 records · Page 4Linked to original sources

arXiv2016

Radio detections of southern ultracool dwarfs

We report the results of a volume-limited survey using the Australia Telescope Compact Array to search for transient and quiescent radio emission from 15 southern hemisphere ultracool dwarfs. We detect radio emission from 2MASSW J0004348-404405 increasing the number of radio loud ultracool dwarfs to 22. We also observe radio emission from 2MASS J10481463-3956062 and 2MASSI J0339352-352544, two sources with previous radio detections. The radio emission from the three detected sources shows no variability or flare emission. Modelling this quiescent emission we find that it is consistent with optically thin gyrosynchrotron emission from a magnetosphere with an emitting region radius of (1 - 2)$R_*$, magnetic field inclination 20$^{\circ}$ - 80$^{\circ}$, field strength $\sim$10 - 200 G, and power-law electron density $\sim$10$^4$ - 10$^8$ cm$^{-3}$. Additionally, we place upper limits on four ultracool dwarfs with no previous radio observations. This increases the number of ultracool dwarfs studied at radio frequencies to 222. Analysing general trends of the radio emission for this sample of 15 sources, we find that the radio activity increases for later spectral types and more rapidly rotating objects. Furthermore, comparing the ratio of the radio to X-ray luminosities for these sources, we find 2MASS J10481463-3956062 and 2MASSI J0339352-352544 violate the Guedel-Benz relation by more than two orders of magnitude.

astro-ph.SR↗

arXiv2015

A pilot ASKAP survey of radio transient events in the region around the intermittent pulsar PSR J1107-5907

We use observations from the Boolardy Engineering Test Array (BETA) of the Australian Square Kilometre Array Pathfinder (ASKAP) telescope to search for transient radio sources in the field around the intermittent pulsar PSR J1107-5907. The pulsar is thought to switch between an "off" state in which no emission is detectable, a weak state and a strong state. We ran three independent transient detection pipelines on two-minute snapshot images from a 13 hour BETA observation in order to 1) study the emission from the pulsar, 2) search for other transient emission from elsewhere in the image and 3) to compare the results from the different transient detection pipelines. The pulsar was easily detected as a transient source and, over the course of the observations, it switched into the strong state three times giving a typical timescale between the strong emission states of 3.7 hours. After the first switch it remained in the strong state for almost 40 minutes. The other strong states lasted less than 4 minutes. The second state change was confirmed using observations with the Parkes radio telescope. No other transient events were found and we place constraints on the surface density of such events on these timescales. The high sensitivity Parkes observations enabled us to detect individual bright pulses during the weak state and to study the strong state over a wide observing band. We conclude by showing that future transient surveys with ASKAP will have the potential to probe the intermittent pulsar population.

astro-ph.SR↗

arXiv2015

Versatile Directional Searches for Gravitational Waves with Pulsar Timing Arrays

By regularly monitoring the most stable millisecond pulsars over many years, pulsar timing arrays (PTAs) are positioned to detect and study correlations in the timing behaviour of those pulsars. Gravitational waves (GWs) from supermassive black hole binaries (SMBHBs) are an exciting potentially detectable source of such correlations. We describe a straight-forward technique by which a PTA can be "phased-up" to form time series of the two polarisation modes of GWs coming from a particular direction of the sky. Our technique requires no assumptions regarding the time-domain behaviour of a GW signal. This method has already been used to place stringent bounds on GWs from individual SMBHBs in circular orbits. Here, we describe the methodology and demonstrate the versatility of the technique in searches for a wide variety of GW signals including bursts with unmodeled waveforms. Using the first six years of data from the Parkes Pulsar Timing Array, we conduct an all-sky search for a detectable excess of GW power from any direction. For the lines of sight to several nearby massive galaxy clusters, we carry out a more detailed search for GW bursts with memory, which are distinct signatures of SMBHB mergers. In all cases, we find that the data are consistent with noise.

astro-ph.IM↗

arXiv2015

Timing analysis for 20 millisecond pulsars in the Parkes Pulsar Timing Array

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, as well as the first significant measurements of some post-Keplerian orbital parameters in six binary pulsars, caused by kinematic effects. Improved Shapiro delay measurements have resulted in much improved pulsar mass measurements, particularly for PSRs J0437-4715 and J1909-3744 with $M_p=1.44\pm0.07$ $M_\odot$ and $M_p=1.47\pm0.03$ $M_\odot$ respectively. The improved orbital period-derivative measurement for PSR J0437-4715 results in a derived distance measurement at the 0.16% level of precision, $D=156.79\pm0.25$ pc, one of the most fractionally precise distance measurements of any star to date.

astro-ph.HE↗

arXiv2015

Gravitational waves from binary supermassive black holes missing in pulsar observations

Gravitational waves are expected to be radiated by supermassive black hole binaries formed during galaxy mergers. A stochastic superposition of gravitational waves from all such binary systems will modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrain the characteristic amplitude of this background, $A_{\rm c,yr}$, to be < $1.0\times10^{-15}$ with 95% confidence. This limit excludes predicted ranges for $A_{\rm c,yr}$ from current models with 91-99.7% probability. We conclude that binary evolution is either stalled or dramatically accelerated by galactic-center environments, and that higher-cadence and shorter-wavelength observations would result in an increased sensitivity to gravitational waves.

astro-ph.CO↗

arXiv2015

Pulsar Observations of Extreme Scattering Events

Extreme scattering events (ESEs) in the interstellar medium (ISM) were first observed in regular flux measurements of compact extragalactic sources. They are characterized by a flux variation over a period of weeks, suggesting the passage of a "diverging plasma lens" across the line of sight. Modeling the refraction of such a lens indicates that the structure size must be of order AU and the electron density of order 10s of cm^{-3}. Similar structures have been observed in measurements of pulsar intensity scintillation and group delay. Here we report observations of two ESEs showing increases in both intensity scintillation and dispersion made with the Parkes Pulsar Timing Array (PPTA). These allow us to make more complete models of the ESE, including an estimate of the "outer-scale" of the turbulence in the plasma lens. These observations show clearly that the ESE structure is fully turbulent on an AU scale. They provide some support for the idea that the structures are extended along the line of sight, such as would be the case for a scattering shell. The dispersion measurements also show a variety of AU scale structures which would not be called ESEs, yet involve electron density variations typical of ESEs and likely have the same origin.

astro-ph.SR↗

arXiv2015

The Parkes multibeam pulsar survey: VII. Timing of four millisecond pulsars and the underlying spin period distribution of the Galactic millisecond pulsar population

We present timing observations of four millisecond pulsars discovered in the Parkes 20-cm multibeam pulsar survey of the Galactic plane. PSRs J1552-4937 and J1843-1448 are isolated objects with spin periods of 6.28 and 5.47 ms respectively. PSR J1727-2946 is in a 40-day binary orbit and has a spin period of 27 ms. The 4.43-ms pulsar J1813-2621 is in a circular 8.16-day binary orbit around a low-mass companion star with a minimum companion mass of 0.2 solar masses. Combining these results with detections from five other Parkes multibeam surveys, gives a well-defined sample of 56 pulsars with spin periods below 20 ms. We develop a likelihood analysis to constrain the functional form which best describes the underlying distribution of spin periods for millisecond pulsars. The best results were obtained with a log-normal distribution. A gamma distribution is less favoured, but still compatible with the observations. Uniform, power-law and Gaussian distributions are found to be inconsistent with the data. Galactic millisecond pulsars being found by current surveys appear to be in agreement with a log-normal distribution which allows for the existence of pulsars with periods below 1.5 ms.

astro-ph.IM↗

arXiv2015

A Study of Multi-frequency Polarization Pulse Profiles of Millisecond Pulsars

We present high signal-to-noise ratio, multi-frequency polarization pulse profiles for 24 millisecond pulsars that are being observed as part of the Parkes Pulsar Timing Array (PPTA) project. The pulsars are observed in three bands, centred close to 730, 1400 and 3100 MHz, using a dual-band 10 cm/50 cm receiver and the central beam of the 20 cm multibeam receiver. Observations spanning approximately six years have been carefully calibrated and summed to produce high S/N profiles. This allows us to study the individual profile components and in particular how they evolve with frequency. We also identify previously undetected profile features. For many pulsars we show that pulsed emission extends across almost the entire pulse profile. The pulse component widths and component separations follow a complex evolution with frequency; in some cases these parameters increase and in other cases they decrease with increasing frequency. The evolution with frequency of the polarization properties of the profile is also non-trivial. We provide evidence that the pre- and post-cursors generally have higher fractional linear polarization than the main pulse. We have obtained the spectral index and rotation measure for each pulsar by fitting across all three observing bands. For the majority of pulsars, the spectra follow a single power-law and the position angles follow a $λ^2$ relation, as expected. However, clear deviations are seen for some pulsars. We also present phase-resolved measurements of the spectral index, fractional linear polarization and rotation measure. All these properties are shown to vary systematically over the pulse profile.

astro-ph.GA↗

arXiv2014

Gravitational wave astronomy with the SKA

On a time scale of years to decades, gravitational wave (GW) astronomy will become a reality. Low frequency (nanoHz) GWs are detectable through long-term timing observations of the most stable pulsars. Radio observatories worldwide are currently carrying out observing programmes to detect GWs, with data sets being shared through the International Pulsar Timing Array project. One of the most likely sources of low frequency GWs are supermassive black hole binaries (SMBHBs), detectable as a background due to a large number of binaries, or as continuous or burst emission from individual sources. No GW signal has yet been detected, but stringent constraints are already being placed on galaxy evolution models. The SKA will bring this research to fruition. In this chapter, we describe how timing observations using SKA1 will contribute to detecting GWs, or can confirm a detection if a first signal already has been identified when SKA1 commences observations. We describe how SKA observations will identify the source(s) of a GW signal, search for anisotropies in the background, improve models of galaxy evolution, test theories of gravity, and characterise the early inspiral phase of a SMBHB system. We describe the impact of the large number of millisecond pulsars to be discovered by the SKA; and the observing cadence, observation durations, and instrumentation required to reach the necessary sensitivity. We describe the noise processes that will influence the achievable precision with the SKA. We assume a long-term timing programme using the SKA1-MID array and consider the implications of modifications to the current design. We describe the possible benefits from observations using SKA1-LOW. Finally, we describe GW detection prospects with SKA1 and SKA2, and end with a description of the expectations of GW astronomy.

astro-ph.IM↗

arXiv2014

Prospects for gravitational-wave detection and supermassive black hole astrophysics with pulsar timing arrays

Large-area sky surveys show that massive galaxies undergo at least one major merger in a Hubble time. Ongoing pulsar timing array (PTA) experiments are aimed at measuring the gravitational wave (GW) emission from binary supermassive black holes (SMBHs) at the centres of galaxy merger remnants. In this paper, using the latest observational estimates for a range of galaxy properties and scaling relations, we predict the amplitude of the GW background generated by the binary SMBH population. We also predict the numbers of individual binary SMBH GW sources. We predict the characteristic strain amplitude of the GW background to lie in the range $5.1\times10^{-16}<A_{\rm yr}<2.4\times10^{-15}$ at a frequency of $(1\,{\rm yr})^{-1}$, with 95% confidence. Higher values within this range, which correspond to the more commonly preferred choice of galaxy merger timescale, will fall within the expected sensitivity ranges of existing PTA projects in the next few years. In contrast, we find that a PTA consisting of at least 100 pulsars observed with next-generation radio telescopes will be required to detect continuous-wave GWs from binary SMBHs. We further suggest that GW memory bursts from coalescing SMBH pairs are not viable sources for PTAs. Both the GW background and individual GW source counts are dominated by binaries formed in mergers between early-type galaxies of masses $\gtrsim5\times10^{10}M_{\odot}$ at redshifts $\lesssim 1.5$. Uncertainties in the galaxy merger timescale and the SMBH mass - galaxy bulge mass relation dominate the uncertainty in our predictions.

astro-ph.CO↗

arXiv2014

Searching for gravitational wave memory bursts with the Parkes Pulsar Timing Array

Anisotropic bursts of gravitational radiation produced by events such as super-massive black hole mergers leave permanent imprints on space. Such gravitational wave "memory" (GWM) signals are, in principle, detectable through pulsar timing as sudden changes in the apparent pulse frequency of a pulsar. If an array of pulsars is monitored as a GWM signal passes over the Earth, the pulsars would simultaneously appear to change pulse frequency by an amount that varies with their sky position in a quadrupolar fashion. Here we describe a search algorithm for such events and apply the algorithm to approximately six years of data from the Parkes Pulsar Timing Array. We find no GWM events and set an upper bound on the rate for events which could have been detected. We show, using simple models of black hole coalescence rates, that this non-detection is not unexpected.

astro-ph.GA↗

arXiv2014

An all-sky search for continuous gravitational waves in the Parkes Pulsar Timing Array data set

We present results of an all-sky search in the Parkes Pulsar Timing Array (PPTA) Data Release 1 data set for continuous gravitational waves (GWs) in the frequency range from $5\times 10^{-9}$ to $2\times 10^{-7}$ Hz. Such signals could be produced by individual supermassive binary black hole systems in the early stage of coalescence. We phase up the pulsar timing array data set to form, for each position on the sky, two data streams that correspond to the two GW polarizations and then carry out an optimal search for GW signals on these data streams. Since no statistically significant GWs were detected, we place upper limits on the intrinsic GW strain amplitude $h_0$ for a range of GW frequencies. For example, at $10^{-8}$ Hz our analysis has excluded with $95\%$ confidence the presence of signals with $h_0\geqslant 1.7\times 10^{-14}$. Our new limits are about a factor of four more stringent than those of Yardley et al. (2010) based on an earlier PPTA data set and a factor of two better than those reported in the recent Arzoumanian et al. (2014) paper. We also present PPTA directional sensitivity curves and find that for the most sensitive region on the sky, the current data set is sensitive to GWs from circular supermassive binary black holes with chirp masses of $10^{9} M_{\odot}$ out to a luminosity distance of about 100 Mpc. Finally, we set an upper limit of $4 \times 10^{-3} {\rm{Mpc}}^{-3} {\rm{Gyr}}^{-1}$ at $95\%$ confidence on the coalescence rate of nearby ($z \lesssim 0.1$) supermassive binary black holes in circular orbits with chirp masses of $10^{10}M_{\odot}$.

astro-ph.GA↗

arXiv2014

The Role of FAST in Pulsar Timing Arrays

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-noise will be the limiting noise processes for FAST data sets. Jitter noise will limit the timing precision achievable over data spans of a few years while timing noise will limit the precision achievable over many years.

astro-ph.IM↗

arXiv2014

Limitations in timing precision due to single-pulse shape variability in millisecond pulsars

High-sensitivity radio-frequency observations of millisecond pulsars usually show stochastic, broadband, pulse-shape variations intrinsic to the pulsar emission process. These variations induce jitter noise in pulsar timing observations; understanding the properties of this noise is of particular importance for the effort to detect gravitational waves with pulsar timing arrays. We assess the short-term profile and timing stability of 22 millisecond pulsars that are part of the Parkes Pulsar Timing Array sample by examining intra-observation arrival time variability and single-pulse phenomenology. In 7 of the 22 pulsars, in the band centred at approximately 1400MHz, we find that the brightest observations are limited by intrinsic jitter. We find consistent results, either detections or upper limits, for jitter noise in other frequency bands. PSR J1909-3744 shows the lowest levels of jitter noise, which we estimate to contribute $\sim$10 ns root mean square error to the arrival times for hour-duration observations. Larger levels of jitter noise are found in pulsars with wider pulses and distributions of pulse intensities. The jitter noise in PSR J0437-4715 decorrelates over a bandwidth of $\sim$2 GHz. We show that the uncertainties associated with timing pulsar models can be improved by including physically motivated jitter uncertainties. Pulse-shape variations will limit the timing precision at future, more sensitive, telescopes; it is imperative to account for this noise when designing instrumentation and timing campaigns for these facilities.

astro-ph.SR↗

arXiv2014

Timing, polarimetry and physics of the bright, nearby millisecond pulsar PSR J0437-4715 - a single-pulse perspective

Single pulses from radio pulsars contain a wealth of information about emission and propagation in the magnetosphere and insight into their timing properties. It was recently demonstrated that single-pulse emission is responsible for limiting the timing stability of the brightest of millisecond pulsars. We report on an analysis of more than a million single-pulses from PSR J0437-4715 and present various statistical properties such as the signal-to-noise ratio (S/N) distribution, timing and polarimetry of average profiles integrated from subpulses with chosen S/N cut-offs, modulation properties of the emission, phase-resolved statistics of the S/N, and two dimensional spherical histograms of the polarization vector orientation. The last of these indicates the presence of orthogonally polarised modes (OPMs). Combined with the dependence of the polarisation fraction on the S/N and polarimetry of the brightest pulses, the existence of OPMs constrains pulsar emission mechanisms and models for the plasma physics in the magnetosphere.

astro-ph.SR↗

arXiv2014

Binary super-massive black hole environments diminish the gravitational-wave signal in the pulsar timing band

We assess the effects of super-massive black hole (SMBH) environments on the gravitational-wave (GW) signal from binary SMBHs. To date, searches with pulsar timing arrays for GWs from binary SMBHs, in the frequency band $\sim1-100$\,nHz, include the assumptions that all binaries are circular and evolve only through GW emission. However, dynamical studies have shown that the only way that binary SMBH orbits can decay to separations where GW emission dominates the evolution is through interactions with their environments. We augment an existing galaxy and SMBH formation and evolution model with calculations of binary SMBH evolution in stellar environments, accounting for non-zero binary eccentricities. We find that coupling between binaries and their environments causes the expected GW spectral energy distribution to be reduced with respect to the standard assumption of circular, GW-driven binaries, for frequencies up to $\sim20$\,nHz. Larger eccentricities at binary formation further reduce the signal in this regime. We also find that GW bursts from individual eccentric binary SMBHs are unlikely to be detectable with current pulsar timing arrays. The uncertainties in these predictions are large, owing to observational uncertainty in SMBH-galaxy scaling relations and the galaxy stellar mass function, uncertainty in the nature of binary-environment coupling, and uncertainty in the numbers of the most massive binary SMBHs. We conclude, however, that low-frequency GWs from binary SMBHs may be more difficult to detect with pulsar timing arrays than currently thought.

astro-ph.CO↗

arXiv2013

Gravitational-wave Limits from Pulsar Timing Constrain Supermassive Black Hole Evolution

The formation and growth processes of supermassive black holes (SMBHs) are not well constrained. SMBH population models, however, provide specific predictions for the properties of the gravitational-wave background (GWB) from binary SMBHs in merging galaxies throughout the Universe. Using observations from the Parkes Pulsar Timing Array, we constrain the fractional GWB energy density with 95% confidence to be $Ω_{GW}(H_0/73 {\rm km} {\rm s}^{-1} {\rm Mpc}^{-1})^2 < 1.3 \times 10^{-9}$ at a frequency of 2.8 nHz, which is approximately a factor of six more stringent than previous limits. We compare our limit to models of the SMBH population and find inconsistencies at confidence levels between 46% and 91%. For example, the standard galaxy formation model implemented in the Millennium simulations is inconsistent with our limit with 50% probability.

astro-ph.CO↗

arXiv2013

Interplanetary spacecraft navigation using pulsars

We demonstrate how observations of pulsars can be used to help navigate a spacecraft travelling in the solar system. We make use of archival observations of millisecond pulsars from the Parkes radio telescope in order to demonstrate the effectiveness of the method and highlight issues, such as pulsar spin irregularities, which need to be accounted for. We show that observations of four millisecond pulsars every seven days using a realistic X-ray telescope on the spacecraft throughout a journey from Earth to Mars can lead to position determinations better than approx. 20km and velocity measurements with a precision of approx. 0.1m/s.

astro-ph.IM↗