arXiv subjects

S. Oslowski

Publications and source records attributed to S. Oslowski.

At least 37 records · Page 2Linked to original sources

arXiv2017

The UTMOST: A hybrid digital signal processor transforms the MOST

The Molonglo Observatory Synthesis Telescope (MOST) is an 18,000 square meter radio telescope situated some 40 km from the city of Canberra, Australia. Its operating band (820-850 MHz) is now partly allocated to mobile phone communications, making radio astronomy challenging. We describe how the deployment of new digital receivers (RX boxes), Field Programmable Gate Array (FPGA) based filterbanks and server-class computers equipped with 43 GPUs (Graphics Processing Units) has transformed MOST into a versatile new instrument (the UTMOST) for studying the dynamic radio sky on millisecond timescales, ideal for work on pulsars and Fast Radio Bursts (FRBs). The filterbanks, servers and their high-speed, low-latency network form part of a hybrid solution to the observatory's signal processing requirements. The emphasis on software and commodity off-the-shelf hardware has enabled rapid deployment through the re-use of proven 'software backends' for its signal processing. The new receivers have ten times the bandwidth of the original MOST and double the sampling of the line feed, which doubles the field of view. The UTMOST can simultaneously excise interference, make maps, coherently dedisperse pulsars, and perform real-time searches of coherent fan beams for dispersed single pulses. Although system performance is still sub-optimal, a pulsar timing and FRB search programme has commenced and the first UTMOST maps have been made. The telescope operates as a robotic facility, deciding how to efficiently target pulsars and how long to stay on source, via feedback from real-time pulsar folding. The regular timing of over 300 pulsars has resulted in the discovery of 7 pulsar glitches and 3 FRBs. The UTMOST demonstrates that if sufficient signal processing can be applied to the voltage streams it is possible to perform innovative radio science in hostile radio frequency environments.

astro-ph.IM↗

arXiv2017

Limits on the Mass, Velocity and Orbit of PSR J1933$-$6211

We present a high-precision timing analysis of PSR J1933$-$6211, a millisecond pulsar (MSP) with a 3.5-ms spin period and a white dwarf (WD) companion, using data from the Parkes radio telescope. Since we have accurately measured the polarization properties of this pulsar we have applied the matrix template matching approach in which the times of arrival are measured using full polarimetric information. We achieved a weighted root-mean-square timing residuals (rms) of the timing residuals of 1.23 $\rm μs$, 15.5$\%$ improvement compared to the total intensity timing analysis. After studying the scintillation properties of this pulsar we put constraints on the inclination angle of the system. Based on these measurements and on $χ^2$ mapping we put a 2-$σ$ upper limit on the companion mass (0.44 M$_\odot$). Since this mass limit cannot reveal the nature of the companion we further investigate the possibility of the companion to be a He WD. Applying the orbital period-mass relation for such WDs, we conclude that the mass of a He WD companion would be about 0.26$\pm$0.01 M$_\odot$ which, combined with the measured mass function and orbital inclination limits, would lead to a light pulsar mass $\leqslant$ 1.0 M$_\odot$. This result seems unlikely based on current neutron star formation models and we therefore conclude that PSR J1933$-$6211 most likely has a CO WD companion, which allows for a solution with a more massive pulsar.

astro-ph.HE↗

arXiv2017

Robust Estimation of Scattering in Pulsar Timing Analysis

We present a robust approach to incorporating models for the time-variable broadening of the pulse profile due to scattering in the ionized interstellar medium into profile-domain pulsar timing analysis. We use this approach to simultaneously estimate temporal variations in both the dispersion measure (DM) and scattering, together with a model for the pulse profile that includes smooth evolution as a function of frequency, and the pulsar's timing model. We show that fixing the scattering timescales when forming time-of-arrival estimates, as has been suggested in the context of traditional pulsar timing analysis, can significantly underestimate the uncertainties in both DM, and the arrival time of the pulse, leading to bias in the timing parameters. We apply our method using a new, publicly available, GPU accelerated code, both to simulations, and observations of the millisecond pulsar PSR J1643$-$1224. This pulsar is known to exhibit significant scattering variability compared to typical millisecond pulsars, and we find including low-frequency ($< 1$ GHz) data without a model for these scattering variations leads to significant periodic structure in the DM, and also biases the astrometric parameters at the $4σ$ level, for example, changing proper motion in right ascension by $0.50 \pm 0.12$. If low frequency observations are to be included when significant scattering variations are present, we conclude it is necessary to not just model those variations, but also to sample the parameters that describe the variations simultaneously with all other parameters in the model, a task for which profile domain pulsar timing is ideally suited.

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arXiv2016

Wide-band Profile Domain Pulsar Timing Analysis

We extend profile domain pulsar timing to incorporate wide-band effects such as frequency-dependent profile evolution and broadband shape variation in the pulse profile. We also incorporate models for temporal variations in both pulse width and in the separation in phase of the main pulse and interpulse. We perform the analysis with both nested sampling and Hamiltonian Monte Carlo methods. In the latter case we introduce a new parameterisation of the posterior that is extremely efficient in the low signal-to-noise regime and can be readily applied to a wide range of scientific problems. We apply this methodology to a series of simulations, and to between seven and nine yr of observations for PSRs J1713$+$0747, J1744$-$1134, and J1909$-$3744 with frequency coverage that spans 700-3600MHz. We use a smooth model for profile evolution across the full frequency range, and compare smooth and piecewise models for the temporal variations in DM. We find the profile domain framework consistently results in improved timing precision compared to the standard analysis paradigm by as much as 40% for timing parameters. Incorporating smoothness in the DM variations into the model further improves timing precision by as much as 30%. For PSR J1713+0747 we also detect pulse shape variation uncorrelated between epochs, which we attribute to variation intrinsic to the pulsar at a level consistent with previously published analyses. Not accounting for this shape variation biases the measured arrival times at the level of $\sim$30ns, the same order of magnitude as the expected shift due to gravitational-waves in the pulsar timing band.

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arXiv2016

The disturbance of a millisecond pulsar magnetosphere

Pulsar timing has enabled some of the strongest tests of fundamental physics. Central to the technique is the assumption that the detected radio pulses can be used to accurately measure the rotation of the pulsar. Here we report on a broad-band variation in the pulse profile of the millisecond pulsar J1643-1224. A new component of emission suddenly appears in the pulse profile, decays over 4 months, and results in a permanently modified pulse shape. Profile variations such as these may be the origin of timing noise observed in other millisecond pulsars. The sensitivity of pulsar-timing observations to gravitational radiation can be increased by accounting for this variability.

astro-ph.HE↗

arXiv2016

A model for distortions of polarisation-angle curves in radio pulsars

Some radio pulsar profiles (in particular those of millisecond pulsars contain wide emission structures which cover large intervals of pulse phase. Local distortions of an average curve of polarisation angle (PA) can be identified in such profiles, and they are often found to be associated with absorption features or narrow emission components. The features may be interpreted as a convolution of a lateral profile of an emitter with a microscopic radiation pattern of a non-negligible angular extent. We study a model which assumes that such an extended microbeam of the X-mode curvature radiation is spreading the radiation polarised at a fixed position angle within an interval of pulse phase. The model is capable of interpreting the strongly dissimilar polarisation of double notches in PSR B1821-24A (for which we present new polarisation data from the Nancay Radio Telescope) and PSR J0437-4715. It also explains a step-like change in PA observed at the bifurcated trailing component in the profile of J0437-4715. A generic form of the modelled PA distortion is a zigzag-shaped wiggle, which in the presence of the second polarisation mode (O mode) can be magnified or transformed into a W- or U-shaped deflection of a total net PA. The model's efficiency in interpreting dissimilar polarisation effects provides further credence to the stream-based (fan-beam) geometry of pulsar emission. It also suggests that the microbeam width may not always be assumed negligible in comparison with the angular scale of emissivity gradients in the emission region.

astro-ph.HE↗

arXiv2016

A deep campaign to characterize the synchronous radio/X-ray mode switching of PSR B0943+10

We report on simultaneous X-ray and radio observations of the mode-switching pulsar PSR B0943+10 obtained with the XMM-Newton satellite and the LOFAR, LWA and Arecibo radio telescopes in November 2014. We confirm the synchronous X-ray/radio switching between a radio-bright (B) and a radio-quiet (Q) mode, in which the X-ray flux is a factor ~2.4 higher than in the B-mode. We discovered X-ray pulsations, with pulsed fraction of 38+/-5% (0.5-2 keV), during the B-mode, and confirm their presence in Q-mode, where the pulsed fraction increases with energy from ~20% up to ~65% at 2 keV. We found marginal evidence for an increase in the X-ray pulsed fraction during B-mode on a timescale of hours. The Q-mode X-ray spectrum requires a fit with a two-component model (either a power-law plus blackbody or the sum of two blackbodies), while the B-mode spectrum is well fit by a single blackbody (a single power-law is rejected). With a maximum likelihood analysis, we found that in Q-mode the pulsed emission has a thermal blackbody spectrum with temperature ~3.4x10^6 K and the unpulsed emission is a power-law with photon index ~2.5, while during B-mode both the pulsed and unpulsed emission can be fit by either a blackbody or a power law with similar values of temperature and photon index. A Chandra image shows no evidence for diffuse X-ray emission. These results support a scenario in which both unpulsed non-thermal emission, likely of magnetospheric origin, and pulsed thermal emission from a small polar cap (~1500 m^2) with a strong non-dipolar magnetic field (~10^{14} G), are present during both radio modes and vary in intensity in a correlated way. This is broadly consistent with the predictions of the partially screened gap model and does not necessarily imply global magnetospheric rearrangements to explain the mode switching.

astro-ph.HE↗

arXiv2016

A millisecond pulsar in an extremely wide binary system

We report on 22 yrs of radio timing observations of the millisecond pulsar J1024$-$0719 by the telescopes participating in the European Pulsar Timing Array (EPTA). These observations reveal a significant second derivative of the pulsar spin frequency and confirm the discrepancy between the parallax and Shklovskii distances that has been reported earlier. We also present optical astrometry, photometry and spectroscopy of 2MASS J10243869$-$0719190. We find that it is a low-metallicity main-sequence star (K7V spectral type, $\mathrm{[M/H]}=-1.0$, $T_\mathrm{eff}=4050\pm50$ K) and that its position, proper motion and distance are consistent with those of PSR J1024$-$0719. We conclude that PSR J1024$-$0719 and 2MASS J10243869$-$0719190 form a common proper motion pair and are gravitationally bound. The gravitational interaction between the main-sequence star and the pulsar accounts for the spin frequency derivatives, which in turn resolves the distance discrepancy. Our observations suggest that the pulsar and main-sequence star are in an extremely wide ($P_\mathrm{b}>200$ yr) orbit. Combining the radial velocity of the companion and proper motion of the pulsar, we find that the binary system has a high spatial velocity of $384\pm45$ km s$^{-1}$ with respect to the local standard of rest and has a Galactic orbit consistent with halo objects. Since the observed main-sequence companion star cannot have recycled the pulsar to millisecond spin periods, an exotic formation scenario is required. We demonstrate that this extremely wide-orbit binary could have evolved from a triple system that underwent an asymmetric supernova explosion, though find that significant fine-tuning during the explosion is required. Finally, we discuss the implications of the long period orbit on the timing stability of PSR J1024$-$0719 in light of its inclusion in pulsar timing arrays.

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arXiv2016

The International Pulsar Timing Array: First Data Release

The highly stable spin of neutron stars can be exploited for a variety of (astro-)physical investigations. In particular arrays of pulsars with rotational periods of the order of milliseconds can be used to detect correlated signals such as those caused by gravitational waves. Three such "Pulsar Timing Arrays" (PTAs) have been set up around the world over the past decades and collectively form the "International" PTA (IPTA). In this paper, we describe the first joint analysis of the data from the three regional PTAs, i.e. of the first IPTA data set. We describe the available PTA data, the approach presently followed for its combination and suggest improvements for future PTA research. Particular attention is paid to subtle details (such as underestimation of measurement uncertainty and long-period noise) that have often been ignored but which become important in this unprecedentedly large and inhomogeneous data set. We identify and describe in detail several factors that complicate IPTA research and provide recommendations for future pulsar timing efforts. The first IPTA data release presented here (and available online) is used to demonstrate the IPTA's potential of improving upon gravitational-wave limits placed by individual PTAs by a factor of ~2 and provides a 2-sigma limit on the dimensionless amplitude of a stochastic GWB of 1.7x10^{-15} at a frequency of 1 yr^{-1}. This is 1.7 times less constraining than the limit placed by (Shannon et al. 2015), due mostly to the more recent, high-quality data they used.

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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

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.

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arXiv2015

Limits on Fast Radio Bursts at 145 MHz with ARTEMIS, a real-time software backend

Fast Radio Bursts (FRBs), are millisecond radio signals that exhibit dispersion larger than what the Galactic electron density can account for. We have conducted a 1446 hour survey for Fast Radio Bursts (FRBs) at 145~MHz, covering a total of 4193 sq. deg on the sky. We used the UK station of the LOFAR radio telescope -- the Rawlings Array -- , accompanied for a majority of the time by the LOFAR station at Nançay, observing the same fields at the same frequency. Our real-time search backend, ARTEMIS, utilizes graphics processing units to search for pulses with dispersion measures up to 320 cm$^{-3}$ pc. Previous derived FRB rates from surveys around 1.4~GHz, and favoured FRB interpretations, motivated this survey, despite all previous detections occurring at higher dispersion measures. We detected no new FRBs above a signal-to-noise threshold of 10, leading to the most stringent upper limit yet on the FRB event rate at these frequencies: 29 sky$^{-1}$ day$^{-1}$ for 5~ms-duration pulses above 62~Jy. The non-detection could be due to scatter-broadening, limitations on the volume and time searched, or the shape of FRB flux density spectra. Assuming the latter and that FRBs are standard candles, the non-detection is compatible with the published FRB sky rate, if their spectra follow a power law with frequency ($\propto ν^α$), with $α\gtrsim+0.1$, demonstrating a marked difference from pulsar spectra. Our results suggest that surveys at higher frequencies, including the low frequency component of the Square Kilometre Array, will have better chances to detect, estimate rates and understand the origin and properties of FRBs.

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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.

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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.

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arXiv2012

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

Signals from radio pulsars show a wavelength-dependent delay due to dispersion in the interstellar plasma. At a typical observing wavelength, this delay can vary by tens of microseconds on five-year time scales, far in excess of signals of interest to pulsar timing arrays, such as that induced by a gravitational-wave background. Measurement of these delay variations is not only crucial for the detection of such signals, but also provides an unparallelled measurement of the turbulent interstellar plasma at au scales. In this paper we demonstrate that without consideration of wavelength- independent red-noise, 'simple' algorithms to correct for interstellar dispersion can attenuate signals of interest to pulsar timing arrays. We present a robust method for this correction, which we validate through simulations, and apply it to observations from the Parkes Pulsar Timing Array. Correction for dispersion variations comes at a cost of increased band-limited white noise. We discuss scheduling to minimise this additional noise, and factors, such as scintillation, that can exacerbate the problem. Comparison with scintillation measurements confirms previous results that the spectral exponent of electron density variations in the interstellar medium often appears steeper than expected. We also find a discrete change in dispersion measure of PSR J1603-7202 of ~2x10^{-3} cm^{-3}pc for about 250 days. We speculate that this has a similar origin to the 'extreme scattering events' seen in other sources. In addition, we find that four pulsars show a wavelength-dependent annual variation, indicating a persistent gradient of electron density on an au spatial scale, which has not been reported previously.

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arXiv2012

The Parkes Pulsar Timing Array Project

A "pulsar timing array" (PTA), in which observations of a large sample of pulsars spread across the celestial sphere are combined, allows investigation of "global" phenomena such as a background of gravitational waves or instabilities in atomic timescales that produce correlated timing residuals in the pulsars of the array. The Parkes Pulsar Timing Array (PPTA) is an implementation of the PTA concept based on observations with the Parkes 64-m radio telescope. A sample of 20 millisecond pulsars is being observed at three radio-frequency bands, 50cm (~700 MHz), 20cm (~1400 MHz) and 10cm (~3100 MHz), with observations at intervals of 2 - 3 weeks. Regular observations commenced in early 2005. This paper describes the systems used for the PPTA observations and data processing, including calibration and timing analysis. The strategy behind the choice of pulsars, observing parameters and analysis methods is discussed. Results are presented for PPTA data in the three bands taken between 2005 March and 2011 March. For ten of the 20 pulsars, rms timing residuals are less than 1 microsec for the best band after fitting for pulse frequency and its first time derivative. Significant "red" timing noise is detected in about half of the sample. We discuss the implications of these results on future projects including the International Pulsar Timing Array (IPTA) and a PTA based on the Square Kilometre Array. We also present an "extended PPTA" data set that combines PPTA data with earlier Parkes timing data for these pulsars.

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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↗

arXiv2011

Population synthesis of double neutron stars

Using the StarTrack binary population synthesis code we model the population of double neutron stars in the Galaxy. We include a detailed treatment of the spin evolution of each pulsar due to processes such as spin-down and spin-up during accretion events as well as magnetic field decay. We also model the spatial distribution of double neutron stars by including their natal kicks and subsequent propagation in the Galactic gravitational potential. This synthetic pulsar population is compared to the observed sample of double neutron stars taking into account the selection effects of detection in the radio band, to determine the most likely evolutionary parameters. With these parameters we determine the properties of the double neutron star binaries detectable in gravitational waves by the high frequency interferometers LIGO and VIRGO. In particular, we discuss the distributions of chirp masses and mass ratios in samples selected by their radio or gravitational wave emission.

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