arXiv subjects

Ben Stappers

Publications and source records attributed to Ben Stappers.

51 records · Page 3Linked to original sources

arXiv2010

Early Pulsar Observations with LOFAR

This contribution to the proceedings of "A New Golden Age for Radio Astronomy" is simply intended to give some of the highlights from pulsar observations with LOFAR at the time of its official opening: June 12th, 2010. These observations illustrate that, though LOFAR is still under construction and astronomical commissioning, it is already starting to deliver on its promise to revolutionize radio astronomy in the low-frequency regime. These observations also demonstrate how LOFAR has many "next-generation" capabilities, such as wide-field multi-beaming, that will be vital to open a new Golden Age in radio astronomy through the Square Kilometer Array and its precursors.

astro-ph.IM↗

arXiv2010

A Radio-loud Magnetar in X-ray Quiescence

As part of a survey for radio pulsars with the Parkes 64-m telescope we have discovered PSR J1622-4950, a pulsar with a 4.3-s rotation period. Follow-up observations show that the pulsar has the highest inferred surface magnetic field of the known radio pulsars (B ~ 3e14 G), exhibits significant timing noise and appears to have an inverted spectrum. Unlike the vast majority of the known pulsar population, PSR J1622-4950 appears to switch off for many hundreds of days and even in its on-state exhibits extreme variability in its flux density. Furthermore, the integrated pulse profile changes shape with epoch. All of these properties are remarkably similar to the only two magnetars previously known to emit radio pulsations. The position of PSR J1622-4950 is coincident with an X-ray source that, unlike the other radio pulsating magnetars, was found to be in quiescence. We conclude that our newly discovered pulsar is a magnetar - the first to be discovered via its radio emission.

astro-ph.HE↗

arXiv2010

Switched magnetospheric regulation of pulsar spin-down

Pulsars are famed for their rotational clock-like stability and their highly-repeatable pulse shapes. However, it has long been known that there are unexplained deviations (often termed "timing noise") from the rate at which we predict these clocks should run. We show that timing behaviour often results from typically two different spin-down rates. Pulsars switch abruptly between these states, often quasi-periodically, leading to the observed spin-down patterns. We show that for six pulsars the timing noise is correlated with changes in the pulse shape. Many pulsar phenomena including mode-changing, nulling, intermittency, pulse shape variability and timing noise are therefore linked and caused by changes in the pulsar's magnetosphere. We consider the possibility that high-precision monitoring of pulse profiles could lead to the formation of highly-stable pulsar clocks.

astro-ph.GA↗

arXiv2010

The European Pulsar Timing Array: current efforts and a LEAP toward the future

The European Pulsar Timing Array (EPTA) is a multi-institutional, multi-telescope collaboration, with the goal of using high-precision pulsar timing to directly detect gravitational waves. In this article we discuss the EPTA member telescopes, current achieved timing precision, and near-future goals. We report a preliminary upper limit to the amplitude of a gravitational wave background. We also discuss the Large European Array for Pulsars, in which the five major European telescopes involved in pulsar timing will be combined to provide a coherent array that will give similar sensitivity to the Arecibo radio telescope, and larger sky coverage.

astro-ph.HE↗

arXiv2009

Finding pulsars with LOFAR

We investigate the number and type of pulsars that will be discovered with the low-frequency radio telescope LOFAR. We consider different search strategies for the Galaxy, for globular clusters and for other galaxies. We show that a 25-day all-sky Galactic survey can find approximately 900 new pulsars, probing the local pulsar population to a deep luminosity limit. For targets of smaller angular size such as globular clusters and galaxies many LOFAR stations can be combined coherently, to make use of the full sensitivity. Searches of nearby northern-sky globular clusters can find new low luminosity millisecond pulsars. Giant pulses from Crab-like extragalactic pulsars can be detected out to over a Mpc.

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arXiv2008

LOFAR Transients and the Radio Sky Monitor

The study of transient and variable low-frequency radio sources is a key goal for LOFAR, with an extremely broad science case ranging from relativistic jets sources to pulsars, exoplanets, radio bursts at cosmological distances, the identification of gravitational wave sources and even SETI. In this paper we will very briefly summarize the science of the LOFAR Transients key science project, will outline the capabilities of LOFAR for transient studies, and introduce the LOFAR Radio Sky Monitor, a proposed mode in which LOFAR regularly scans 2 pi radians of sky.

astro-ph↗

arXiv2008

The Subpulse Modulation Properties of Pulsars and its Frequency Dependence

A large sample of about two hundred pulsars have been observed to study their subpulse modulation at an observing wavelength of (when achievable) both 21 and 92 cm using the Westerbork Synthesis Radio Telescope. For 57 pulsars drifting subpulses are discovered for the first time and are confirmed for many others. This leads to the conclusion that it could well be that the drifting subpulse mechanism is an intrinsic property of the emission mechanism itself, although for some pulsars it is difficult or impossible to detect. It appears that the youngest pulsars have the most disordered subpulses and the subpulses become more and more organized into drifting subpulses as the pulsar ages. Drifting subpulses are in general found at both frequencies and the measured values of P3 at the two frequencies are highly correlated, showing the broadband nature of this phenomenon. Also the modulation indices measured at the two frequencies are clearly correlated, although at 92 cm they are on average possibly higher. The correlations with the modulation indices are argued to be consistent with the picture in which the radio emission is composed out of a drifting subpulse signal plus a quasi-steady signal which becomes, on average, stronger at high observing frequencies. There is no obvious correlation found between P3 and the pulsar age (or any other pulsar parameter) contrary to reports in the past.

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arXiv2008

Searching for the signatures of jet-ISM interactions in X-ray binaries

Jets from X-ray binaries are continuously injecting matter and energy into the surrounding interstellar medium (ISM). However, there exist to date relatively few cases where jet-ISM interactions have been directly observed. We review the current examples, and go on to present new data on the proposed hotspots of GRS1915+105, finding no concrete evidence for any association between the hotspots and the central source, in agreement with previous findings in the literature. We also present preliminary results on radio and H-alpha searches for jet-ISM interactions around known X-ray binaries, and discuss strategies for future searches.

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arXiv2008

PuMaII: A wide band pulsar machine for the WSRT

The Pulsar Machine II (PuMa II) is the new flexible pulsar processing backend system at the Westerbork Synthesis Radio Telescope (WSRT), specifically designed to take advantage of the upgraded WSRT. The instrument is based on a computer cluster running the Linux operating system, with minimal custom hardware. A maximum of 160 MHz analogue bandwidth sampled as 8X20 MHz subbands with 8-bit resolution can be recorded on disks attached to separate computer nodes. Processing of the data is done in the additional 32-nodes allowing near real time coherent dedispersion for most pulsars observed at the WSRT. This has doubled the bandwidth for pulsar observations in general, and has enabled the use of coherent dedispersion over a bandwidth eight times larger than was previously possible at the WSRT. PuMa II is one of the widest bandwidth coherent dedispersion machines currently in use and has a maximum time resolution of 50ns. The system is now routinely used for high precision pulsar timing studies, polarization studies, single pulse work and a variety of other observational work.

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arXiv2007

Finding pulsars with LOFAR

We investigate the number and type of pulsars that will be discovered with the low-frequency radio telescope LOFAR. We consider different search strategies for the Galaxy, for globular clusters and for galaxies other than our own. We show an all-sky Galactic survey can be optimally carried out by incoherently combining the LOFAR stations. In a 60-day all-sky Galactic survey LOFAR can find over a thousand pulsars, probing the local pulsar population to a very deep luminosity limit. For targets of smaller angular size, globular clusters and galaxies, the LOFAR stations can be combined coherently, making use of the full sensitivity. Searches of nearby northern-sky globular clusters can find large numbers of low luminosity millisecond pulsars (eg. over 10 new millisecond pulsars in a 10-hour observation of M15). If the pulsar population in nearby galaxies is similar to that of the Milky Way, a 10-hour observation could find the 10 brightest pulsars in M33, or pulsars in other galaxies out to a distance of 1.2Mpc.

astro-ph↗

arXiv2007

A highly polarised radio jet during the 1998 outburst of the black hole transient XTE J1748-288

XTE J1748-288 is a black hole X-ray transient which went into outburst in 1998 June. The X-ray lightcurves showed canonical morphologies, with minor variations on the ``Fast Rise Exponential Decay'' profile. The radio source, however, reached an unusually high flux density of over 600 mJy. This high radio flux was accompanied by an exceptional (>20%) fractional linear polarisation, the variability of which was anti-correlated with the flux density. We use this variability to discuss possible depolarisation mechanisms and to predict the underlying behaviour of the (unresolved) core/jet components.

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arXiv2006

The LOFAR Transients Key Project

LOFAR, the Low Frequency Array, is a new radio telescope under construction in the Netherlands, designed to operate between 30 and 240 MHz. The Transients Key Project is one of the four Key Science Projects which comprise the core LOFAR science case. The remit of the Transients Key Project is to study variable and transient radio sources detected by LOFAR, on timescales from milliseconds to years. This will be achieved via both regular snapshot monitoring of historical and newly-discovered radio variables and, most radically, the development of a `Radio Sky Monitor' which will survey a large fraction of the northern sky on a daily basis.

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arXiv2003

Pulsar bow shocks as probes of warm neutral gas

Pulsars have mean space velocities >~500 km/s. The consequent ram pressure results in tight confinement of the star's energetic wind, driving a bow shock into the surrounding medium. Pulsar bow shocks have long been regarded as a curiosity, but new optical and X-ray observations are both rapidly expanding the sample of such sources, and are offering new ways to probe the interaction between pulsars and their environments. Here we discuss some of these new results, and explain how these data can be used to probe the density and structure of neutral gas in the interstellar medium.

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arXiv2002

An optical bow shock around the nearby millisecond pulsar J2124-3358

We report the discovery of an H-alpha-emitting bow-shock nebula powered by the nearby millisecond pulsar J2124-3358. The bow shock is very broad, and is highly asymmetric about the pulsar's velocity vector. This shape is not consistent with that expected for the case of an isotropic wind interacting with a homogeneous ambient medium. Models which invoke an anisotropy in the pulsar wind, a bulk flow of the surrounding gas, or a density gradient in the ambient medium either perpendicular or parallel to the pulsar's direction of motion also fail to reproduce the observed morphology. However, we find an ensemble of good fits to the nebular morphology when we consider a combination of these effects. In all such cases, we find that the pulsar is propagating through an ambient medium of mean density 0.8-1.3 cm^(-3) and bulk flow velocity ~15-25 km/s and that the star has recently encountered an increase in density by 1-10 cm^(-3) over a scale ~<0.02 pc. The wide variety of models which fit the data demonstrate that in general there is no unique set of parameters which can be inferred from the morphology of a bow-shock nebula.

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arXiv2000

Non-detection of a pulsar-powered nebula in Puppis A, and implications for the nature of the radio-quiet neutron star RX J0822-4300

We report on a deep radio search for a pulsar wind nebula associated with the radio-quiet neutron star RX J0822-4300 in the supernova remnant Puppis A. The well-determined properties of Puppis A allow us to constrain the size of any nebula to less than 30 arcsec; however we find no evidence for such a source on any spatial scale up to 30 arcmin. These non-detections result in an upper limit on the radio luminosity of any pulsar-powered nebula which is three orders of magnitude below what would be expected if RX J0822-4300 was an energetic young radio pulsar beaming away from us, and cast doubt on a recent claim of X-ray pulsations from this source. The lack of a radio nebula leads us to conclude that RX J0822-4300 has properties very different from most young radio pulsars, and that it represents a distinct population which may be as numerous, or even more so, than radio pulsars.

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