arXiv subjects

V. Ravi

Publications and source records attributed to V. Ravi.

64 records · Page 4Linked to original sources

arXiv2012

Detection of 107 glitches in 36 southern pulsars

Timing observations from the Parkes 64-m radio telescope for 165 pulsars between 1990 and 2011 have been searched for period glitches. A total of 107 glitches were identified in 36 pulsars, where 61 have previously been reported and 46 are new discoveries. Glitch parameters were measured by fitting the timing residual data. Observed relative glitch sizes Δν_g/νrange between 10^-10 and 10^-5, where ν= 1/P is the pulse frequency. We confirm that the distribution of Δν_g/νis bimodal with peaks at approximately 10^-9 and 10^-6. Glitches are mostly observed in pulsars with characteristic ages between 10^3 and 10^5 years, with large glitches mostly occurring in the younger pulsars. Exponential post-glitch recoveries were observed for 27 large glitches in 18 pulsars. The fraction Q of the glitch that recovers exponentially also has a bimodal distribution. Large glitches generally have low Q, typically a few per cent, but large Q values are observed in both large and small glitches. Observed time constants for exponential recoveries ranged between 10 and 300 days with some tendency for longer timescales in older pulsars. Shorter timescale recoveries may exist but were not revealed by our data which typically have observation intervals of 2 - 4 weeks. For most of the 36 pulsars with observed glitches, there is a persistent linear increase in \dotνin the inter-glitch interval. Where an exponential recovery is also observed, the effects of this are superimposed on the linear increase in \dotν. In some cases, the slope of the linear recovery changes at the time of a glitch. The \ddotνvalues characterising the linear changes in \dotνare almost always positive and, after subtracting the magnetospheric component of the braking, are approximately proportional to the ratio of |\dotν| and the inter-glitch interval, as predicted by vortex-creep models.

astro-ph.HE↗

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.

astro-ph.IM↗

arXiv2012

Does a 'stochastic' background of gravitational waves exist in the pulsar timing band?

We investigate the effects of gravitational waves (GWs) from a simulated population of binary super-massive black holes (SMBHs) on pulsar timing array datasets. We construct a distribution describing the binary SMBH population from an existing semi-analytic galaxy formation model. Using realizations of the binary SMBH population generated from this distribution, we simulate pulsar timing datasets with GW-induced variations. We find that the statistics of these variations do not correspond to an isotropic, stochastic GW background. The "Hellings & Downs" correlations between simulated datasets for different pulsars are recovered on average, though the scatter of the correlation estimates is greater than expected for an isotropic, stochastic GW background. These results are attributable to the fact that just a few GW sources dominate the GW-induced variations in every Fourier frequency bin of a 5-year dataset. Current constraints on the amplitude of the GW signal from binary SMBHs will be biased. Individual binary systems are likely to be detectable in 5-year pulsar timing array datasets where the noise is dominated by GW-induced variations. Searches for GWs in pulsar timing array data therefore need to account for the effects of individual sources of GWs.

astro-ph.CO↗

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

Observations and modelling of pulsed radio emission from CU Virginis

We present 13 cm and 20 cm radio observations of the magnetic chemically peculiar star CU Virginis taken with the Australia Telescope Compact Array. We detect two circularly polarised radio pulses every rotation period which confirm previous detections. In the first pulse, the lower frequency emission arrives before the higher frequency emission and the ordering reverses in the second pulse. In order to explain the frequency dependence of the time between the two pulses, we construct a geometric model of the magnetosphere of CU Virginis, and consider various emission angles relative to the magnetic field lines. A simple electron cyclotron maser emission model, in which the emission is perpendicular to the magnetic field lines, is not consistent with our data. A model in which the emission is refracted through cold plasma in the magnetosphere is shown to have the correct pulse arrival time frequency dependence.

astro-ph.SR↗

arXiv2011

The non-uniform, dynamic atmosphere of Betelgeuse observed at mid-infrared wavelengths

We present an interferometric study of the continuum surface of the red supergiant star Betelgeuse at 11.15 microns wavelength, using data obtained with the Berkeley Infrared Spatial Interferometer each year between 2006 and 2010. These data allow an investigation of an optically thick layer within 1.4 stellar radii of the photosphere. The layer has an optical depth of ~1 at 11.15 microns, and varies in temperature between 1900 K and 2800 K and in outer radius between 1.16 and 1.36 stellar radii. Electron-hydrogen atom collisions contribute significantly to the opacity of the layer. The layer has a non-uniform intensity distribution that changes between observing epochs. These results indicate that large-scale surface convective activity strongly influences the dynamics of the inner atmosphere of Betelgeuse, and mass-loss processes.

astro-ph.SR↗

arXiv2011

The magnetosphere of the ultracool dwarf DENIS 1048-3956

Ultracool dwarfs, the least-massive contributors to the stellar mass function, exhibit striking magnetic properties that are inconsistent with trends for more massive stars. Here, we present the widest-band radio observations to date of an ultracool dwarf, DENIS-P J104814.9-395604, in four 2 GHz bandwidths between wavelengths of 1 cm and 10 cm. These data were obtained with the Australia Telescope Compact Array using the new Compact Array Broadband Backend instrument. We detected a stable negatively-sloped power-law spectrum in total intensity, with spectral index alpha=1.71+-0.09. Circular polarization fractions between 0.25 and 0.4 were found at the low-frequency end of our detection band. We interpret these results as indicative of gyrosynchrotron emission. We suggest that the radio emission originates from beyond the co-rotation radius, R_C, of the star. Adopting this model, we find R_C between 1.2-2.9 R_*, and a non-thermal electron density and magnetic field strength between 10^(5)-10^(7.2) cm^(-3) and 70-260 G respectively at R_C. The model accounts for the violation of the Guedel-Benz relation between X-ray and radio luminosities of low-mass stars by DENIS-P J104814.9-395604.

astro-ph.SR↗

arXiv2010

The many faces of Betelgeuse

The dynamics of the surface and inner atmosphere of the red supergiant star Betelgeuse are the subject of numerous high angular resolution and spectroscopic studies. Here, we present three-telescope interferometric data obtained at 11.15 microns wavelength with the Berkeley Infrared Spatial Interferometer (ISI), that probe the stellar surface continuum. We find striking variability in the size, effective temperature, and degree of asymmetry of the star over the years 2006-2009. These results may indicate an evolving shell of optically thick material close to the stellar photosphere.

astro-ph.SR↗

arXiv2010

Observations of radio pulses from CU Virginis

The magnetic chemically peculiar star CU Virginis is a unique astrophysical laboratory for stellar magnetospheres and coherent emission processes. It is the only known main sequence star to emit a radio pulse every rotation period. Here we report on new observations of the CU Virginis pulse profile in the 13 and 20\,cm radio bands. The profile is known to be characterised by two peaks of 100$\%$ circularly polarised emission that are thought to arise in an electron-cyclotron maser mechanism. We find that the trailing peak is stable at both 13 and 20\,cm, whereas the leading peak is intermittent at 13\,cm. Our measured pulse arrival times confirm the discrepancy previously reported between the putative stellar rotation rates measured with optical data and with radio observations. We suggest that this period discrepancy might be caused by an unknown companion or by instabilities in the emission region. Regular long-term pulse timing and simultaneous multi-wavelength observations are essential to clarify the behaviour of this emerging class of transient radio source.

astro-ph.SR↗

arXiv2010

Wide radio beams from gamma-ray pulsars

We investigate the radio and gamma-ray beaming properties of normal and millisecond pulsars by selecting two samples from the known populations. The first, Sample G, contains pulsars which are detectable in blind searches of gamma-ray data from the Fermi Large Area Telescope. The second, Sample R, contains pulsars detectable in blind radio searches which have spin-down luminosities Edot > 10^{34} erg/s. We analyse the fraction of the gamma-ray-selected Sample G which have detectable radio pulses and the fraction of the radio-selected Sample R which have detectable gamma-ray pulses. Twenty of our 35 Sample G pulsars have already observed radio pulses. This rules out low-altitude polar-cap beaming models if, as is currently believed, gamma-ray beams are generated in the outer magnetosphere and are very wide. We further find that, for the highest-Edot pulsars, the radio and gamma-ray beams have comparable beaming factors, i.e., the beams cover similar regions of the sky as the star rotates. For lower-Edot gamma-ray emitting pulsars, the radio beams have about half of the gamma-ray sky coverage. These results suggest that, for high-Edot young and millisecond pulsars, the radio emission originates in wide beams from regions high in the pulsar magnetosphere, probably close to the null-charge surface and to the gamma-ray emitting regions. Furthermore, it suggests that for these high-Edot pulsars, as in the gamma-ray case, features in the radio profile represent caustics in the emission beam pattern.

astro-ph.HE↗