arXiv subjects

J. W. T. Hessels

Publications and source records attributed to J. W. T. Hessels.

At least 145 records · Page 8Linked to original sources

arXiv2015

Timing of five millisecond pulsars discovered in the PALFA survey

We present the discovery of five millisecond pulsars (MSPs) from the PALFA Galactic plane survey using Arecibo. Four of these (PSRs J0557+1551, J1850+0244, J1902+0300, and J1943+2210) are binary pulsars whose companions are likely white dwarfs, and one (PSR J1905+0453) is isolated. Phase-coherent timing solutions, ranging from $\sim$1 to $\sim$3 years in length, and based on observations from the Jodrell Bank and Arecibo telescopes, provide precise determinations of spin, orbital, and astrometric parameters. All five pulsars have large dispersion measures ($>100$ pc cm$^{-3}$, within the top 20% of all known Galactic field MSPs) and are faint (1.4 GHz flux density < 0.1 mJy, within the faintest 5% of all known Galactic field MSPs), illustrating PALFA's ability to find increasingly faint, distant MSPs in the Galactic plane. In particular, PSR J1850+0244 has a dispersion measure of 540 pc cm$^{-3}$, the highest of all known MSPs. Such distant, faint MSPs are important input for accurately modeling the total Galactic MSP population.

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arXiv2015

Pulsar polarisation below 200 MHz: Average profiles and propagation effects

We present the highest-quality polarisation profiles to date of 16 non-recycled pulsars and four millisecond pulsars, observed below 200 MHz with the LOFAR high-band antennas. Based on the observed profiles, we perform an initial investigation of expected observational effects resulting from the propagation of polarised emission in the pulsar magnetosphere and the interstellar medium. The predictions of magnetospheric birefringence in pulsars have been tested using spectra of the pulse width and fractional polarisation from multifrequency data. The derived spectra offer only partial support for the expected effects of birefringence on the polarisation properties, with only about half of our sample being consistent with the model's predictions. It is noted that for some pulsars these measurements are contaminated by the effects of interstellar scattering. For a number of pulsars in our sample, we have observed significant variations in the amount of Faraday rotation as a function of pulse phase, which is possibly an artefact of scattering. These variations are typically two orders of magnitude smaller than that observed at 1400 MHz by Noutsos et al. (2009), for a different sample of southern pulsars. In this paper we present a possible explanation for the difference in magnitude of this effect between the two frequencies, based on scattering. Finally, we have estimated the magnetospheric emission heights of low-frequency radiation from four pulsars, based on the phase lags between the flux-density and the PA profiles, and the theoretical framework of Blaskiewicz, Cordes & Wasserman (1991). These estimates yielded heights of a few hundred km; at least for PSR B1133+16, this is consistent with emission heights derived based on radius-to-frequency mapping, but is up to a few times larger than the recent upper limit based on pulsar timing.

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arXiv2015

Pulsar Wind Nebulae in the SKA era

Neutron stars lose the bulk of their rotational energy in the form of a pulsar wind: an ultra-relativistic outflow of predominantly electrons and positrons. This pulsar wind significantly impacts the environment and possible binary companion of the neutron star, and studying the resultant pulsar wind nebulae is critical for understanding the formation of neutron stars and millisecond pulsars, the physics of the neutron star magnetosphere, the acceleration of leptons up to PeV energies, and how these particles impact the interstellar medium. With the SKA1 and the SKA2, it could be possible to study literally hundreds of PWNe in detail, critical for understanding the many open questions in the topics listed above.

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arXiv2014

A Cosmic Census of Radio Pulsars with the SKA

The Square Kilometre Array (SKA) will make ground breaking discoveries in pulsar science. In this chapter we outline the SKA surveys for new pulsars, as well as how we will perform the necessary follow-up timing observations. The SKA's wide field-of-view, high sensitivity, multi-beaming and sub-arraying capabilities, coupled with advanced pulsar search backends, will result in the discovery of a large population of pulsars. These will enable the SKA's pulsar science goals (tests of General Relativity with pulsar binary systems, investigating black hole theorems with pulsar-black hole binaries, and direct detection of gravitational waves in a pulsar timing array). Using SKA1-MID and SKA1-LOW we will survey the Milky Way to unprecedented depth, increasing the number of known pulsars by more than an order of magnitude. SKA2 will potentially find all the Galactic radio-emitting pulsars in the SKA sky which are beamed in our direction. This will give a clear picture of the birth properties of pulsars and of the gravitational potential, magnetic field structure and interstellar matter content of the Galaxy. Targeted searches will enable detection of exotic systems, such as the ~1000 pulsars we infer to be closely orbiting Sgr A*, the supermassive black hole in the Galactic Centre. In addition, the SKA's sensitivity will be sufficient to detect pulsars in local group galaxies. To derive the spin characteristics of the discoveries we will perform live searches, and use sub-arraying and dynamic scheduling to time pulsars as soon as they are discovered, while simultaneously continuing survey observations. The large projected number of discoveries suggests that we will uncover currently unknown rare systems that can be exploited to push the boundaries of our understanding of astrophysics and provide tools for testing physics, as has been done by the pulsar community in the past.

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arXiv2014

Pulsars in Globular Clusters with the SKA

Globular clusters are highly efficient radio pulsar factories. These pulsars can be used as precision probes of the clusters' structure, gas content, magnetic field, and formation history; some of them are also highly interesting in their own right because they probe exotic stellar evolution scenarios as well as the physics of dense matter, accretion, and gravity. Deep searches with SKA1-MID and SKA1-LOW will plausibly double to triple the known population. Such searches will only require one to a few tied-array beams, and can be done during early commissioning of the telescope - before an all-sky pulsar survey using hundreds to thousands of tied-array beams is feasible. With SKA2 it will be possible to observe most of the active radio pulsars within a large fraction of the Galactic globular clusters, an estimated population of 600 - 3700 observable pulsars (those beamed towards us). This rivals the total population of millisecond pulsars that can be found in the Galactic field; fully characterizing it will provide the best-possible physical laboratories as well as a rich dynamical history of the Galactic globular cluster system.

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arXiv2014

A 24-Hour Global Campaign To Assess Precision Timing of the Millisecond Pulsar J1713+0747

The radio millisecond pulsar J1713+0747 is regarded as one of the highest-precision clocks in the sky, and is regularly timed for the purpose of detecting gravitational waves. The International Pulsar Timing Array collaboration undertook a 24-hour global observation of PSR J1713+0747 in an effort to better quantify sources of timing noise in this pulsar, particularly on intermediate (1 - 24 hr) timescales. We observed the pulsar continuously over 24 hr with the Arecibo, Effelsberg, GMRT, Green Bank, LOFAR, Lovell, Nancay, Parkes, and WSRT radio telescopes. The combined pulse times-of-arrival presented here provide an estimate of what sources of timing noise, excluding DM variations, would be present as compared to an idealized root-N improvement in timing precision, where N is the number of pulses analyzed. In the case of this particular pulsar, we find that intrinsic pulse phase jitter dominates arrival time precision when the S/N of single pulses exceeds unity, as measured using the eight telescopes that observed at L-band/1.4 GHz. We present first results of specific phenomena probed on the unusually long timescale (for a single continuous observing session) of tens of hours, in particular interstellar scintillation, and discuss the degree to which scintillation and profile evolution affect precision timing. This paper presents the data set as a basis for future, deeper studies.

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arXiv2014

Radio timing and optical photometry of the black widow system PSR J1518+0204C in the globular cluster M5

We report on the determination of astrometric, spin and orbital parameters for PSR J1518+0204C, a "black widow" binary millisecond pulsar in the globular cluster M5. The accurate position and orbital parameters obtained from radio timing allowed us to search for the optical companion. By using WFC3/HST images we identified a very faint variable star (m_F390W > 24.8, m_F606W > 24.3, m_F814W > 23.1) located at only 0.25" from the pulsar's timing position. Due to its strong variability, this star is visible only in a sub-sample of images. However, the light curve obtained folding the available data with the orbital parameters of the pulsar shows a maximum at the pulsar inferior conjunction and a possible minimum at the pulsar superior conjunction. Furthermore, the shape of the optical modulation indicates a heating process possibly due to the pulsar wind. This is the first identification of an optical companion to a black widow pulsar in the dense stellar environment of a globular cluster.

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arXiv2014

LOFAR tied-array imaging of Type III solar radio bursts

The Sun is an active source of radio emission which is often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), the Sun has not been imaged extensively because of the instrumental limitations of previous radio telescopes. Here, the combined high spatial, spectral and temporal resolution of the Low Frequency Array (LOFAR) was used to study solar Type III radio bursts at 30-90 MHz and their association with CMEs. The Sun was imaged with 126 simultaneous tied-array beams within 5 solar radii of the solar centre. This method offers benefits over standard interferometric imaging since each beam produces high temporal (83 ms) and spectral resolution (12.5 kHz) dynamic spectra at an array of spatial locations centred on the Sun. LOFAR's standard interferometric output is currently limited to one image per second. Over a period of 30 minutes, multiple Type III radio bursts were observed, a number of which were found to be located at high altitudes (4 solar radii from the solar center at 30 MHz) and to have non-radial trajectories. These bursts occurred at altitudes in excess of values predicted by 1D radial electron density models. The non-radial high altitude Type III bursts were found to be associated with the expanding flank of a CME. The CME may have compressed neighbouring streamer plasma producing larger electron densities at high altitudes, while the non-radial burst trajectories can be explained by the deflection of radial magnetic fields as the CME expanded in the low corona.

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arXiv2014

Initial LOFAR observations of Epoch of Reionization windows: II. Diffuse polarized emission in the ELAIS-N1 field

This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications for the extraction of the cosmological 21-cm signal from the Low-Frequency Array - Epoch of Reionization (LOFAR-EoR) data. We use the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes. The brightness temperature of the detected Galactic emission is on average 4 K in polarized intensity and covers the range from -10 to +13rad m^-2 in Faraday depth. The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology, at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to 1.5%. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies. The wide frequency range, good angular resolution and good sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of 1-2 rad m^-2 in Faraday depth. The different polarised patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarised foregrounds is a serious complication for Epoch of Reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%.

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arXiv2014

The Green Bank Northern Celestial Cap Pulsar Survey - I: Survey Description, Data Analysis, and Initial Results

We describe an ongoing search for pulsars and dispersed pulses of radio emission, such as those from rotating radio transients (RRATs) and fast radio bursts (FRBs), at 350 MHz using the Green Bank Telescope. With the Green Bank Ultimate Pulsar Processing Instrument, we record 100 MHz of bandwidth divided into 4,096 channels every 81.92 $μs$. This survey will cover the entire sky visible to the Green Bank Telescope ($δ> -40^\circ$, or 82% of the sky) and outside of the Galactic Plane will be sensitive enough to detect slow pulsars and low dispersion measure ($<$30 $\mathrm{pc\,cm^{-3}}$) millisecond pulsars (MSPs) with a 0.08 duty cycle down to 1.1 mJy. For pulsars with a spectral index of $-$1.6, we will be 2.5 times more sensitive than previous and ongoing surveys over much of our survey region. Here we describe the survey, the data analysis pipeline, initial discovery parameters for 62 pulsars, and timing solutions for 5 new pulsars. PSR J0214$+$5222 is an MSP in a long-period (512 days) orbit and has an optical counterpart identified in archival data. PSR J0636$+$5129 is an MSP in a very short-period (96 minutes) orbit with a very low mass companion (8 $M_\mathrm{J}$). PSR J0645$+$5158 is an isolated MSP with a timing residual RMS of 500 ns and has been added to pulsar timing array experiments. PSR J1434$+$7257 is an isolated, intermediate-period pulsar that has been partially recycled. PSR J1816$+$4510 is an eclipsing MSP in a short-period orbit (8.7 hours) and may have recently completed its spin-up phase.

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arXiv2014

Arecibo Pulsar Survey Using ALFA. III. Precursor Survey and Population Synthesis

The Pulsar Arecibo L-band Feed Array (PALFA) Survey uses the ALFA 7-beam receiver to search both inner and outer Galactic sectors visible from Arecibo ($32^{\circ}\lesssim \ell \lesssim 77^{\circ}$ and $168^{\circ}\lesssim \ell \lesssim 214^{\circ}$) close to the Galactic plane ($|b|\lesssim5^{\circ}$) for pulsars. In this paper we detail a precursor survey of this region with PALFA, which observed a subset of the full region (slightly more restrictive in $\ell$ and $|b|\lesssim1^{\circ}$) and detected 45 pulsars. For both Galactic millisecond and normal pulsar populations, we compare the survey's detections with simulations to model these populations and, in particular, to estimate the number of observable pulsars in the Galaxy. We place 95\% confidence intervals of 82,000 to 143,000 on the number of detectable normal pulsars and 9,000 to 100,000 on the number of detectable millisecond pulsars in the Galactic disk. These are consistent with previous estimates. Given the most likely population size in each case (107,000 and 15,000 for normal and millisecond pulsars, respectively) we extend survey detection simulations to predict that, when complete, the full PALFA survey should have detected $1,000\substack{+330 \\ -230}$ normal pulsars and $30\substack{+200 \\ -20}$ millisecond pulsars. Identical estimation techniques predict that $490\substack{+160 \\ -115}$ normal pulsars and $12\substack{+70 \\ -5}$ millisecond pulsars would be detected by the beginning of 2014; at the time, the PALFA survey had detected 283 normal pulsars and 31 millisecond pulsars, respectively.

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arXiv2014

Fast Radio Burst Discovered in the Arecibo Pulsar ALFA Survey

Recent work has exploited pulsar survey data to identify temporally isolated, millisecond-duration radio bursts with large dispersion measures (DMs). These bursts have been interpreted as arising from a population of extragalactic sources, in which case they would provide unprecedented opportunities for probing the intergalactic medium; they may also be linked to new source classes. Until now, however, all so-called fast radio bursts (FRBs) have been detected with the Parkes radio telescope and its 13-beam receiver, casting some concern about the astrophysical nature of these signals. Here we present FRB 121102, the first FRB discovery from a geographic location other than Parkes. FRB 121102 was found in the Galactic anti-center region in the 1.4-GHz Pulsar ALFA survey with the Arecibo Observatory with a DM = 557.4 $\pm$ 3 pc cm$^{-3}$, pulse width of $3\; \pm 0.5$ ms, and no evidence of interstellar scattering. The observed delay of the signal arrival time with frequency agrees precisely with the expectation of dispersion through an ionized medium. Despite its low Galactic latitude ($b = -0.2^{\circ}$), the burst has three times the maximum Galactic DM expected along this particular line-of-sight, suggesting an extragalactic origin. A peculiar aspect of the signal is an inverted spectrum; we interpret this as a consequence of being detected in a sidelobe of the ALFA receiver. FRB 121102's brightness, duration, and the inferred event rate are all consistent with the properties of the previously detected Parkes bursts.

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arXiv2014

Swings between rotation and accretion power in a millisecond binary pulsar

It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star's rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales.

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arXiv2014

A state change in the low-mass X-ray binary XSS J12270-4859

Millisecond radio pulsars acquire their rapid rotation rates through mass and angular momentum transfer in a low-mass X-ray binary system. Recent studies of PSR J1824-2452I and PSR J1023+0038 have observationally demonstrated this link, and they have also shown that such systems can repeatedly transition back-and-forth between the radio millisecond pulsar and low-mass X-ray binary states. This also suggests that a fraction of such systems are not newly born radio millisecond pulsars but are rather suspended in a back-and-forth state switching phase, perhaps for giga-years. XSS J12270-4859 has been previously suggested to be a low-mass X-ray binary, and until recently the only such system to be seen at MeV-GeV energies. We present radio, optical and X-ray observations that offer compelling evidence that XSS J12270-4859 is a low-mass X-ray binary which transitioned to a radio millisecond pulsar state between 2012 November 14 and 2012 December 21. Though radio pulsations remain to be detected, we use optical and X-ray photometry/spectroscopy to show that the system has undergone a sudden dimming and no longer shows evidence for an accretion disk. The optical observations constrain the orbital period to 6.913+-0.002 hr.

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arXiv2014

Discovery of Carbon Radio Recombination Lines in absorption towards Cygnus~A

We present the first detection of carbon radio recombination line absorption along the line of sight to Cygnus A. The observations were carried out with the LOw Frequency ARray in the 33 to 57 MHz range. These low frequency radio observations provide us with a new line of sight to study the diffuse, neutral gas in our Galaxy. To our knowledge this is the first time that foreground Milky Way recombination line absorption has been observed against a bright extragalactic background source. By stacking 48 carbon $α$ lines in the observed frequency range we detect carbon absorption with a signal-to-noise ratio of about 5. The average carbon absorption has a peak optical depth of 2$\times$10$^{-4}$, a line width of 10 km s$^{-1}$ and a velocity of +4 km s$^{-1}$ with respect to the local standard of rest. The associated gas is found to have an electron temperature $T_{e}\sim$ 110 K and density $n_{e}\sim$ 0.06 cm$^{-3}$. These properties imply that the observed carbon $α$ absorption likely arises in the cold neutral medium of the Orion arm of the Milky Way. Hydrogen and helium lines were not detected to a 3$σ$ peak optical depth limit of 1.5$\times$10$^{-4}$ for a 4 km s$^{-1}$ channel width. Radio recombination lines associated with Cygnus A itself were also searched for, but are not detected. We set a 3$σ$ upper limit of 1.5$\times$10$^{-4}$ for the peak optical depth of these lines for a 4 km s$^{-1}$ channel width.

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arXiv2014

A millisecond pulsar in a stellar triple system

Gravitationally bound three-body systems have been studied for hundreds of years and are common in our Galaxy. They show complex orbital interactions, which can constrain the compositions, masses, and interior structures of the bodies and test theories of gravity, if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, B1620-26 (with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multi-wavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar (1.4378(13) Msun, where Msun is the solar mass and the parentheses contain the uncertainty in the final decimal places) and the two white dwarf companions (0.19751(15) Msun and 0.4101(3) Msun), as well as the inclinations of the orbits (both approximately 39.2 degrees). The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory in which to test the strong equivalence principle of general relativity.

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arXiv2013

Searching for pulsars using image pattern recognition

In this paper, we present a novel artificial intelligence (AI) program that identifies pulsars from recent surveys using image pattern recognition with deep neural nets---the PICS (Pulsar Image-based Classification System) AI. The AI mimics human experts and distinguishes pulsars from noise and interferences by looking for patterns from candidate. The information from each pulsar candidate is synthesized in four diagnostic plots, which consist of up to thousands pixel of image data. The AI takes these data from each candidate as its input and uses thousands of such candidates to train its ~9000 neurons. Different from other pulsar selection programs which use pre-designed patterns, the PICS AI teaches itself the salient features of different pulsars from a set of human-labeled candidates through machine learning. The deep neural networks in this AI system grant it superior ability in recognizing various types of pulsars as well as their harmonic signals. The trained AI's performance has been validated with a large set of candidates different from the training set. In this completely independent test, PICS ranked 264 out of 277 pulsar-related candidates, including all 56 previously known pulsars, to the top 961 (1%) of 90008 test candidates, missing only 13 harmonics. The first non-pulsar candidate appears at rank 187, following 45 pulsars and 141 harmonics. In other words, 100% of the pulsars were ranked in the top 1% of all candidates, while 80% were ranked higher than any noise or interference. The performance of this system can be improved over time as more training data are accumulated. This AI system has been integrated into the PALFA survey pipeline and has discovered six new pulsars to date.

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arXiv2013

A state change in the missing link binary pulsar system PSR J1023+0038

We present radio, X-ray, and gamma-ray observations which reveal that the binary millisecond pulsar / low-mass X-ray binary transition system PSR J1023+0038 has undergone a transformation in state. Whereas until recently the system harbored a bright millisecond radio pulsar, the radio pulsations at frequencies between 300 to 5000 MHz have now become undetectable. Concurrent with this radio disappearance, the gamma-ray flux of the system has quintupled. We conclude that, though the radio pulsar is currently not detectable, the pulsar mechanism is still active and the pulsar wind, as well as a newly formed accretion disk, are together providing the necessary conditions to create the gamma-ray increase. The system is the first example of a transient, compact, low-mass gamma-ray binary and will continue to provide an exceptional test bed for better understanding the formation of millisecond pulsars as well as accretion onto neutron stars in general.

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