ORCID Profile
0000-0001-8522-4983
Current Organisation
University of Manchester
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Publisher: Oxford University Press (OUP)
Date: 19-05-2020
Abstract: The discovery that at least some Fast Radio Bursts (FRBs) repeat has ruled out cataclysmic events as the progenitors of these particular bursts. FRB 121102 is the most well-studied repeating FRB but despite extensive monitoring of the source, no underlying pattern in the repetition has previously been identified. Here, we present the results from a radio monitoring c aign of FRB 121102 using the 76 m Lovell telescope. Using the pulses detected in the Lovell data along with pulses from the literature, we report a detection of periodic behaviour of the source over the span of 5 yr of data. We predict that the source is currently ‘off’ and that it should turn ‘on’ for the approximate MJD range 59002−59089 (2020 June 2 to 2020 August 28). This result, along with the recent detection of periodicity from another repeating FRB, highlights the need for long-term monitoring of repeating FRBs at a high cadence. Using simulations, we show that one needs at least 100 h of telescope time to follow-up repeating FRBs at a cadence of 0.5–3 d to detect periodicities in the range of 10–150 d. If the period is real, it shows that repeating FRBs can have a large range in their activity periods that might be difficult to reconcile with neutron star precession models.
Publisher: Oxford University Press (OUP)
Date: 02-11-2022
Abstract: Developing an effective automatic classifier to separate genuine sources from artifacts is essential for transient follow-ups in wide-field optical surveys. The identification of transient detections from the subtraction artifacts after the image differencing process is a key step in such classifiers, known as real-bogus classification problem. We apply a self-supervised machine learning model, the deep-embedded self-organizing map (DESOM) to this ‘real-bogus’ classification problem. DESOM combines an autoencoder and a self-organizing map to perform clustering in order to distinguish between real and bogus detections, based on their dimensionality-reduced representations. We use 32 × 32 normalized detection thumbnails as the input of DESOM. We demonstrate different model training approaches, and find that our best DESOM classifier shows a missed detection rate of $6.6{{\\ \\rm per\\,cent}}$ with a false-positive rate of $1.5{{\\ \\rm per\\,cent}}$. DESOM offers a more nuanced way to fine-tune the decision boundary identifying likely real detections when used in combination with other types of classifiers, e.g. built on neural networks or decision trees. We also discuss other potential usages of DESOM and its limitations.
Publisher: Oxford University Press (OUP)
Date: 14-04-2022
Abstract: We report four new pulsars discovered in the core-collapsed globular cluster (GC) NGC 6624 by the TRAPUM Large Survey Project with the MeerKAT telescope. All of the new pulsars found are isolated. PSR J1823−3021I and PSR J1823−3021K are millisecond pulsars with period of respectively 4.319 and 2.768 ms. PSR J1823−3021J is mildly recycled with a period of 20.899 ms, and PSR J1823−3022 is a long period pulsar with a period of 2.497 s. The pulsars J1823−3021I, J1823−3021J, and J1823−3021K have position and dispersion measure (DM) compatible with being members of the GC and are therefore associated with NGC 6624. Pulsar J1823−3022 is the only pulsar bright enough to be re-detected in archival observations of the cluster. This allowed the determination of a timing solution that spans over two decades. It is not possible at the moment to claim the association of pulsar J1823−3022 with the GC given the long period and large offset in position (∼3 arcmin) and DM (with a fractional difference of 11 percent compared the average of the pulsars in NGC 6624). The discoveries made use of the beamforming capability of the TRAPUM backend to generate multiple beams in the same field of view which allows sensitive searches to be performed over a few half-light radii from the cluster centre and can simultaneously localize the discoveries. The discoveries reflect the properties expected for pulsars in core-collapsed GCs.
Publisher: Oxford University Press (OUP)
Date: 16-06-2022
Abstract: We report on a search for persistent radio emission from the one-off fast radio burst (FRB) 20190714A, as well as from two repeating FRBs, 20190711A and 20171019A, using the MeerKAT radio telescope. For FRB 20171019A, we also conducted simultaneous observations with the High-Energy Stereoscopic System (H.E.S.S.) in very high-energy gamma rays and searched for signals in the ultraviolet, optical, and X-ray bands. For this FRB, we obtain a UV flux upper limit of $1.39 \\times 10^{-16}~{\\rm erg\\, cm^{-2}\\, s^{-1}}$Å−1, X-ray limit of $\\sim 6.6 \\times 10^{-14}~{\\rm erg\\, cm^{-2}\\, s^{-1}}$ and a limit on the very high energy gamma-ray flux $\\Phi (E\\gt 120\\, {\\rm GeV}) \\lt 1.7\\times 10^{-12}\\, \\mathrm{erg\\, cm^{-2}\\, s^{-1}}$. We obtain a radio upper limit of ∼15 $\\mu$Jy beam−1 for persistent emission at the locations of both FRBs 20190711A and 20171019A with MeerKAT. However, we detected an almost unresolved (ratio of integrated flux to peak flux is ∼1.7 beam) radio emission, where the synthesized beam size was ∼ 8 arcsec size with a peak brightness of $\\sim 53\\, \\mu$Jy beam−1 at MeerKAT and $\\sim 86\\, \\mu$Jy beam−1 at e-MERLIN, possibly associated with FRB 20190714A at z = 0.2365. This represents the first detection of persistent continuum radio emission potentially associated with a (as-yet) non-repeating FRB. If the association is confirmed, one of the strongest remaining distinction between repeaters and non-repeaters would no longer be applicable. A parallel search for repeat bursts from these FRBs revealed no new detections down to a fluence of 0.08 Jy ms for a 1 ms duration burst.
Publisher: American Astronomical Society
Date: 18-09-2019
Abstract: The Low-Frequency Array radio telescope discovered the 707 Hz binary millisecond pulsar (MSP) J0952−0607 in a targeted radio pulsation search of an unidentified Fermi gamma-ray source. This source shows a weak energy flux of F γ = 2.6 × 10 −12 erg cm −2 s −1 in the energy range between 100 MeV and 100 GeV. Here we report the detection of pulsed gamma-ray emission from PSR J0952−0607 in a very sensitive gamma-ray pulsation search. The pulsar’s rotational, binary, and astrometric properties are measured over 7 years of Fermi -Large Area Telescope data. For this we take into account the uncertainty on the shape of the gamma-ray pulse profile. We present an updated radio-timing solution now spanning more than 2 years and show results from optical modeling of the black-widow-type companion based on new multiband photometric data taken with HiPERCAM on the Gran Telescopio Canarias on La Palma and ULTRACAM on the New Technology Telescope at ESO La Silla (based on observations collected at the European Southern Observatory, Chile programme 0101.D-0925, PI: Clark, C. J.). PSR J0952−0607 is now the fastest-spinning pulsar for which the intrinsic spin-down rate has been reliably constrained ( ). The inferred surface magnetic field strength of is among the 10 lowest of all known pulsars. This discovery is another ex le of an extremely fast spinning black-widow pulsar hiding within an unidentified Fermi gamma-ray source. In the future such systems might help to pin down the maximum spin frequency and the minimum surface magnetic field strength of MSPs.
Publisher: Oxford University Press (OUP)
Date: 06-01-2023
Abstract: More than 100 millisecond pulsars (MSPs) have been discovered in radio observations of gamma-ray sources detected by the Fermi Large Area Telescope (LAT), but hundreds of pulsar-like sources remain unidentified. Here, we present the first results from the targeted survey of Fermi-LAT sources being performed by the Transients and Pulsars with MeerKAT (TRAPUM) Large Survey Project. We observed 79 sources identified as possible gamma-ray pulsar candidates by a Random Forest classification of unassociated sources from the 4FGL catalogue. Each source was observed for 10 min on two separate epochs using MeerKAT’s L-band receiver (856–1712 MHz), with typical pulsed flux density sensitivities of $\\sim 100\\, \\mu$Jy. Nine new MSPs were discovered, eight of which are in binary systems, including two eclipsing redbacks and one system, PSR J1526−2744, that appears to have a white dwarf companion in an unusually compact 5 h orbit. We obtained phase-connected timing solutions for two of these MSPs, enabling the detection of gamma-ray pulsations in the Fermi-LAT data. A follow-up search for continuous gravitational waves from PSR J1526−2744 in Advanced LIGO data using the resulting Fermi-LAT timing ephemeris yielded no detection, but sets an upper limit on the neutron star ellipticity of 2.45 × 10−8. We also detected X-ray emission from the redback PSR J1803−6707 in data from the first eROSITA all-sky survey, likely due to emission from an intrabinary shock.
Publisher: Oxford University Press (OUP)
Date: 09-01-2023
Abstract: The most massive globular cluster in our Galaxy, Omega Centauri, is an interesting target for pulsar searches, because of its multiple stellar populations and the intriguing possibility that it was once the nucleus of a galaxy that was absorbed into the Milky Way. The recent discoveries of pulsars in this globular cluster and their association with known X-ray sources was a hint that, given the large number of known X-ray sources, there is a much larger undiscovered pulsar population. We used the superior sensitivity of the MeerKAT radio telescope to search for pulsars in Omega Centauri. In this paper, we present some of the first results of this survey, including the discovery of 13 new pulsars the total number of known pulsars in this cluster currently stands at 18. At least half of them are in binary systems and preliminary orbital constraints suggest that most of the binaries have light companions. We also discuss the ratio between isolated and binaries pulsars, and how they were formed in this cluster.
Publisher: EDP Sciences
Date: 11-2018
DOI: 10.1051/0004-6361/201833133
Abstract: Aims . The shape of low-frequency radio continuum spectra of normal galaxies is not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFAR Multifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large s le of nearby star-forming galaxies. Methods . Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. The spectra are explained through the use of a three-dimensional model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Results . Our galaxies’ spectra are generally flatter at lower compared to higher frequencies: the median spectral index α low measured between ≈50 MHz and 1.5 GHz is −0.57 ± 0.01 while the high-frequency one α high , calculated between 1.3 GHz and 5 GHz, is −0.77 ± 0.03. As there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we argue that the observed flattening is not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990, ApJ, 352, 30). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flattening due to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxies of our s le. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100–200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions . Our modelling of galaxy spectra suggests that the weak spectral flattening observed in the nearby galaxies studied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way’s foreground on the spectra of all external galaxies is also expected at very low frequencies.
Publisher: Elsevier BV
Date: 06-2015
Publisher: Oxford University Press (OUP)
Date: 09-10-2020
Abstract: The amount of observational data produced by time-domain astronomy is exponentially increasing. Human inspection alone is not an effective way to identify genuine transients from the data. An automatic real-bogus classifier is needed and machine learning techniques are commonly used to achieve this goal. Building a training set with a sufficiently large number of verified transients is challenging, due to the requirement of human verification. We present an approach for creating a training set by using all detections in the science images to be the s le of real detections and all detections in the difference images, which are generated by the process of difference imaging to detect transients, to be the s les of bogus detections. This strategy effectively minimizes the labour involved in the data labelling for supervised machine learning methods. We demonstrate the utility of the training set by using it to train several classifiers utilizing as the feature representation the normalized pixel values in 21 × 21 pixel st s centred at the detection position, observed with the Gravitational-wave Optical Transient Observer (GOTO) prototype. The real-bogus classifier trained with this strategy can provide up to $95{{\\ \\rm per\\ cent}}$ prediction accuracy on the real detections at a false alarm rate of $1{{\\ \\rm per\\ cent}}$.
Publisher: EDP Sciences
Date: 09-2017
Publisher: Springer Science and Business Media LLC
Date: 26-01-2023
DOI: 10.1038/S41550-022-01874-X
Abstract: Reliable neutron star mass measurements are key to determining the equation of state of cold nuclear matter, but such measurements are rare. Black widows and redbacks are compact binaries consisting of millisecond pulsars and semi-degenerate companion stars. Spectroscopy of the optically bright companions can determine their radial velocities, providing inclination-dependent pulsar mass estimates. Although inclinations can be inferred from subtle features in optical light curves, such estimates may be systematically biased due to incomplete heating models and poorly understood variability. Using data from the Fermi Large Area Telescope, we have searched for gamma-ray eclipses from 49 spider systems, discovering significant eclipses in 7 systems, including the prototypical black widow PSR B1957+20. Gamma-ray eclipses require direct occultation of the pulsar by the companion, and so the detection, or significant exclusion, of a gamma-ray eclipse strictly limits the binary inclination angle, providing new robust, model-independent pulsar mass constraints. For PSR B1957+20, the eclipse implies a much lighter pulsar (1.81 ± 0.07 solar masses) than inferred from optical light curve modelling.
Publisher: Cambridge University Press (CUP)
Date: 2021
DOI: 10.1017/PASA.2020.45
Abstract: The past few decades have seen the burgeoning of wide-field, high-cadence surveys, the most formidable of which will be the Legacy Survey of Space and Time (LSST) to be conducted by the Vera C. Rubin Observatory. So new is the field of systematic time-domain survey astronomy however, that major scientific insights will continue to be obtained using smaller, more flexible systems than the LSST. One such ex le is the Gravitational-wave Optical Transient Observer (GOTO) whose primary science objective is the optical follow-up of gravitational wave events. The amount and rate of data production by GOTO and other wide-area, high-cadence surveys presents a significant challenge to data processing pipelines which need to operate in near-real time to fully exploit the time domain. In this study, we adapt the Rubin Observatory LSST Science Pipelines to process GOTO data, thereby exploring the feasibility of using this ‘off-the-shelf’ pipeline to process data from other wide-area, high-cadence surveys. In this paper, we describe how we use the LSST Science Pipelines to process raw GOTO frames to ultimately produce calibrated coadded images and photometric source catalogues. After comparing the measured astrometry and photometry to those of matched sources from PanSTARRS DR1, we find that measured source positions are typically accurate to subpixel levels, and that measured L -band photometries are accurate to $\\sim50$ mmag at $m_L\\sim16$ and $\\sim200$ mmag at $m_L\\sim18$ . These values compare favourably to those obtained using GOTO’s primary, in-house pipeline, gotophoto , in spite of both pipelines having undergone further development and improvement beyond the implementations used in this study. Finally, we release a generic ‘obs package’ that others can build upon, should they wish to use the LSST Science Pipelines to process data from other facilities.
Publisher: EDP Sciences
Date: 2016
Publisher: Oxford University Press (OUP)
Date: 16-09-2019
Abstract: In this paper we report on $\sim 10$ yr of observations of PSR J2051$-$0827, at radio frequencies in the range 110–4032 MHz. We investigate the eclipse phenomena of this black widow pulsar using model fits of increased dispersion and scattering of the pulsed radio emission as it traverses the eclipse medium. These model fits reveal variability in dispersion features on time-scales as short as the orbital period, and previously unknown trends on time-scales of months–years. No clear patterns are found between the low-frequency eclipse widths, orbital period variations, and trends in the intrabinary material density. Using polarization calibrated observations we present the first available limits on the strength of magnetic fields within the eclipse region of this system the average line of sight field is constrained to be $10^{-4}$ G $\lesssim B_{||} \lesssim 10^2$ G, while for the case of a field directed near-perpendicular to the line of sight we find $B_{\perp } \lesssim 0.3$ G. Depolarization of the linearly polarized pulses during the eclipse is detected and attributed to rapid rotation measure fluctuations of $\sigma _{\text{RM}} \gtrsim 100$ rad m$^{-2}$ along, or across, the line of sights averaged over during a subintegration. The results are considered in the context of eclipse mechanisms, and we find scattering and/or cyclotron absorption provide the most promising explanation, while dispersion smearing is conclusively ruled out. Finally, we estimate the mass-loss rate from the companion to be $\dot{M}_{\text{C}} \sim 10^{-12}\, \mathrm{M}_\odot$ yr$^{-1}$, suggesting that the companion will not be fully evaporated on any reasonable time-scale
Publisher: Oxford University Press (OUP)
Date: 10-10-2022
Abstract: The TRAPUM collaboration has used the MeerKAT telescope to conduct a search for pulsed radio emission from the young Small Magellanic Cloud pulsar J0058−7218 located in the supernova remnant IKT 16, following its discovery in X-rays with XMM–Newton. We report no significant detection of dispersed, pulsed radio emission from this source in three 2-h L-band observations using the core dishes of MeerKAT, setting an upper limit of 7.0 $\\mu$Jy on its mean flux density at 1284 MHz. This is nearly seven times deeper than previous radio searches for this pulsar in Parkes L-band observations. This suggests that the radio emission of PSR J0058−7218 is not beamed towards Earth or that PSR J0058−7218 is similar to a handful of Pulsar Wind Nebulae systems that have a very low radio efficiency, such as PSR B0540−6919, the Large Magellanic Cloud Crab pulsar analogue. We have also searched for bright, dispersed, single radio pulses and found no candidates above a fluence of 93 mJy ms at 1284 MHz.
Publisher: EDP Sciences
Date: 11-02-2016
Publisher: EDP Sciences
Date: 05-2022
DOI: 10.1051/0004-6361/202142636
Abstract: Discovering radio pulsars, particularly millisecond pulsars (MSPs), is important for a range of astrophysical applications, such as testing theories of gravity or probing the magneto-ionic interstellar medium. We aim to discover pulsars that may have been missed in previous pulsar searches by leveraging known pulsar observables (primarily polarisation) in the sensitive, low-frequency radio images from the Low-Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS), and have commenced the Targeted search, using LoTSS images, for polarised pulsars (TULIPP) survey. For this survey, we identified linearly and circularly polarised point sources with flux densities brighter than 2 mJy in LoTSS images at a centre frequency of 144 MHz with a 48 MHz bandwidth. Over 40 known pulsars, half of which are MSPs, were detected as polarised sources in the LoTSS images and excluded from the survey. We have obtained beam-formed LOFAR observations of 30 candidates, which were searched for pulsations using coherent de-dispersion. Here, we present the results of the first year of the TULIPP survey. We discovered two pulsars, PSRs J1049+5822 and J1602+3901, with rotational periods of P = 0.73 s and 3.7 ms, respectively. We also detected a further five known pulsars (two slowly-rotating pulsars and three MSPs) for which accurate sky positions were not available to allow a unique cross-match with LoTSS sources. This targeted survey presents a relatively efficient method by which pulsars, particularly MSPs, may be discovered using the flexible observing modes of sensitive radio telescopes such as the Square Kilometre Array and its pathfinders recursors, particularly since wide-area all-sky surveys using coherent de-dispersion are currently computationally infeasible.
Publisher: Cambridge University Press (CUP)
Date: 2015
DOI: 10.1017/PASA.2015.37
Abstract: The Evolutionary Map of the Universe (EMU) is a proposed radio continuum survey of the Southern Hemisphere up to declination + 30°, with the Australian Square Kilometre Array Pathfinder (ASKAP). EMU will use an automated source identification and measurement approach that is demonstrably optimal, to maximise the reliability and robustness of the resulting radio source catalogues. As a step toward this goal we conducted a “Data Challenge” to test a variety of source finders on simulated images. The aim is to quantify the accuracy and limitations of existing automated source finding and measurement approaches. The Challenge initiators also tested the current ASKAPsoft source-finding tool to establish how it could benefit from incorporating successful features of the other tools. As expected, most finders show completeness around 100% at ≈ 10σ dropping to about 10% by ≈ 5σ. Reliability is typically close to 100% at ≈ 10σ, with performance to lower sensitivities varying between finders. All finders show the expected trade-off, where a high completeness at low signal-to-noise gives a corresponding reduction in reliability, and vice versa. We conclude with a series of recommendations for improving the performance of the ASKAPsoft source-finding tool.
Publisher: EDP Sciences
Date: 12-2020
DOI: 10.1051/0004-6361/202039517
Abstract: Context. Radio pulses from pulsars are affected by plasma dispersion, which results in a frequency-dependent propagation delay. Variations in the magnitude of this effect lead to an additional source of red noise in pulsar timing experiments, including pulsar timing arrays (PTAs) that aim to detect nanohertz gravitational waves. Aims. We aim to quantify the time-variable dispersion with much improved precision and characterise the spectrum of these variations. Methods. We use the pulsar timing technique to obtain highly precise dispersion measure (DM) time series. Our dataset consists of observations of 36 millisecond pulsars, which were observed for up to 7.1 yr with the LOw Frequency ARray (LOFAR) telescope at a centre frequency of ~150 MHz. Seventeen of these sources were observed with a weekly cadence, while the rest were observed at monthly cadence. Results. We achieve a median DM precision of the order of 10 −5 cm −3 pc for a significant fraction of our sources. We detect significant variations of the DM in all pulsars with a median DM uncertainty of less than 2 × 10 −4 cm −3 pc. The noise contribution to pulsar timing experiments at higher frequencies is calculated to be at a level of 0.1–10 μ s at 1.4 GHz over a timespan of a few years, which is in many cases larger than the typical timing precision of 1 μ s or better that PTAs aim for. We found no evidence for a dependence of DM on radio frequency for any of the sources in our s le. Conclusions. The DM time series we obtained using LOFAR could in principle be used to correct higher-frequency data for the variations of the dispersive delay. However, there is currently the practical restriction that pulsars tend to provide either highly precise times of arrival (ToAs) at 1.4 GHz or a high DM precision at low frequencies, but not both, due to spectral properties. Combining the higher-frequency ToAs with those from LOFAR to measure the infinite-frequency ToA and DM would improve the result.
Publisher: American Astronomical Society
Date: 28-03-2013
Publisher: EDP Sciences
Date: 08-2022
DOI: 10.1051/0004-6361/202143006
Abstract: We report the discovery of 13 new pulsars in the globular cluster NGC 1851 by the TRAPUM Large Survey Project using the MeerKAT radio telescope. The discoveries consist of six isolated millisecond pulsars (MSPs) and seven binary pulsars, of which six are MSPs and one is mildly recycled. For all the pulsars, we present the basic kinematic, astrometric, and orbital parameters, where applicable, as well as their polarimetric properties, when these are measurable. Two of the binary MSPs (PSR J0514−4002D and PSR J0514−4002E) are in wide and extremely eccentric ( e 0.7) orbits with a heavy white dwarf and a neutron star as their companion, respectively. With these discoveries, NGC 1851 is now tied with M28 as the cluster with the third largest number of known pulsars (14). Its pulsar population shows remarkable similarities with that of M28, Terzan 5, and other clusters with comparable structural parameters. The newly found pulsars are all located in the innermost regions of NGC 1851 and will likely enable, among other things, detailed studies of the cluster structure and dynamics.
Publisher: Oxford University Press (OUP)
Date: 12-01-2018
DOI: 10.1093/MNRAS/STY081
Publisher: Oxford University Press (OUP)
Date: 03-03-2020
Abstract: We present a comparative study of the low-frequency eclipses of spider (compact and irradiating binary) PSRs B1957+20 and J1816+4510. Combining these data with those of three other eclipsing systems we study the frequency dependence of the eclipse duration. PSRs B1957+20 and J1816+4510 have similar orbital properties, but the companions to the pulsars have masses that differ by an order of magnitude. A dedicated c aign to simultaneously observe the pulsed and imaged continuum flux densities throughout the eclipses reveals many similarities between the excess material within the two binaries, irrespective of the companion star properties. The observations show that the pulsar fluxes are removed from the line of sight throughout the main body of the eclipses. For PSR J1816 + 4510, we present the first direct evidence of an eclipse mechanism that transitions from one that removes the pulsar flux from the line of sight to one that merely smears out pulsations, and claim that this is a consequence of scattering in a tail of material flowing behind the companion. Inferred mass-loss rates from the companion stars are found to be $\\dot{M}_{\\text{C}} \\sim 10^{-12}$ and $\\sim 2 \\times 10^{-13}\\,\\mathrm{M}_\\odot$ yr−1 for PSR B1957+20 and PSR J1816 + 4510, respectively seemingly too low to evaporate the stars within Hubble time. Measurements of eclipse durations over a wide range of radio frequencies show a significant dependence of eclipse duration on frequency for all pulsars, with wider eclipses at lower frequencies. These results provide a marked improvement in the observational constraints available for theoretical studies of the eclipse mechanisms.
Publisher: American Astronomical Society
Date: 12-07-2019
Publisher: Oxford University Press (OUP)
Date: 2023
Abstract: Multi-element interferometers such as MeerKAT, which observe with high time resolution and have a wide field of view, provide an ideal opportunity to perform real-time, untargeted transient and pulsar searches. However, because of data storage limitations, it is not always feasible to store the baseband data required to image the field of a discovered transient or pulsar. This limits the ability of surveys to effectively localize their discoveries and may restrict opportunities for follow-up science, especially of one-off events like some fast radio bursts. Here, we present a novel maximum-likelihood estimation approach to localizing transients and pulsars detected in multiple MeerKAT tied-array beams at once, which we call tied-array beam localization, as well as a Python implementation of the method named SeeKAT. We provide real-world ex les of SeeKAT’s use as well as a Monte Carlo analysis to show that it is capable of localizing single pulses detected in beamformed MeerKAT data to (sub)arcsec precision.
Publisher: Oxford University Press (OUP)
Date: 13-06-2015
Publisher: EDP Sciences
Date: 10-2015
Publisher: American Astronomical Society
Date: 04-2020
Publisher: EDP Sciences
Date: 2019
DOI: 10.1051/0004-6361/201732515
Abstract: Context . In the previous decade, two new classes of fast radio transients were detected: the Galactic, rotating radio transients (RRATs) and the extragalactic fast radio bursts (FRBs). If the detectable emission of these objects extends to lower radio frequencies, the LOw Frequency ARray (LOFAR) is ideally suited to seek and localize these transients at frequencies of 10–250 MHz. This is due to LOFAR’s sensitivity, erse beamform capabilities, and transient buffers for the in idual elements that allow post-event imaging of events, potentially at arcsecond resolution. Aims . Our aim is to identify and localize pulses at frequencies below 250 MHz and, in the case of nondetections, derive upper limits on the sky and volume rates of FRBs. Methods . A real-time search program for fast radio transients is installed on the LOFAR systems which runs commensally with other observations, and uses the wide incoherent LOFAR beam (11.25 deg 2 at 150 MHz). Buffered data from hundreds of dipoles are used to reconstruct the direction and polarization information of the event, and to distinguish between celestial, terrestrial, and instrumental origins. Results . Observations were taken covering either the frequency range 119–151 MHz or in four frequency bands, each of 2 MHz in width, centered at 124, 149, 156, and 185 MHz. A first pilot survey covered a range of dispersion measures (DM) below 120 pc cm −3 , focusing on Galactic sources, and resulted in an upper limit on the transient rate at LOFAR frequencies of less than 1500 events per sky per day above a fluency of 1.6 kJy ms for an 8-ms pulse. A second pilot survey covered a range of DMs below 500 pc cm −3 , focusing on extragalactic sources to about 1 Gpc, and resulted in an upper limit of 1400 events per sky per day above a fluency of 6.0 kJy ms for an 8-ms pulse. Using a model for the distance-DM relationship, this equates to an upper limit of 134 events per Gpc 3 per day.
Publisher: American Astronomical Society
Date: 25-11-2019
Publisher: Oxford University Press (OUP)
Date: 31-12-2015
Publisher: Oxford University Press (OUP)
Date: 14-07-2015
Publisher: American Astronomical Society
Date: 10-2020
Abstract: We report the discovery of 1.97 ms period gamma-ray pulsations from the 75 minute orbital-period binary pulsar now named PSR J1653−0158. The associated Fermi Large Area Telescope gamma-ray source 4FGL J1653.6−0158 has long been expected to harbor a binary millisecond pulsar. Despite the pulsar-like gamma-ray spectrum and candidate optical/X-ray associations—whose periodic brightness modulations suggested an orbit—no radio pulsations had been found in many searches. The pulsar was discovered by directly searching the gamma-ray data using the GPU-accelerated Einstein@Home distributed volunteer computing system. The multidimensional parameter space was bounded by positional and orbital constraints obtained from the optical counterpart. More sensitive analyses of archival and new radio data using knowledge of the pulsar timing solution yield very stringent upper limits on radio emission. Any radio emission is thus either exceptionally weak, or eclipsed for a large fraction of the time. The pulsar has one of the three lowest inferred surface magnetic-field strengths of any known pulsar with B surf ≈ 4 × 10 7 G. The resulting mass function, combined with models of the companion star’s optical light curve and spectra, suggests a pulsar mass ≳2 M ⊙ . The companion is lightweight with mass ∼0.01 M ⊙ , and the orbital period is the shortest known for any rotation-powered binary pulsar. This discovery demonstrates the Fermi Large Area Telescope's potential to discover extreme pulsars that would otherwise remain undetected.
Publisher: EDP Sciences
Date: 29-01-2016
Publisher: Oxford University Press (OUP)
Date: 09-02-2018
DOI: 10.1093/MNRAS/STY349
Publisher: EDP Sciences
Date: 30-06-2016
Publisher: American Astronomical Society
Date: 13-09-2017
Publisher: EDP Sciences
Date: 03-2021
DOI: 10.1051/0004-6361/201936108
Abstract: Context. Most neutron stars are expected to be born in supernovae, but only about half of supernova remnants (SNRs) are associated with a compact object. In many cases, a supernova progenitor may have resulted in a black hole. However, there are several possible reasons why true pulsar-SNR associations may have been missed in previous surveys: The pulsar’s radio beam may not be oriented towards us the pulsar may be too faint to be detectable or there may be an offset in the pulsar position caused by a kick. Aims. Our goal is to find new pulsars in SNRs and explore their possible association with the remnant. The search and selection of the remnants presented in this paper was inspired by the non-detection of any X-ray bright compact objects in these remnants when previously studied. Methods. Five SNRs were searched for radio pulsars with the Green Bank Telescope at 820 MHz with multiple pointings to cover the full spatial extent of the remnants. A periodicity search plus an acceleration search up to 500 m s −2 and a single pulse search were performed for each pointing in order to detect potential isolated binary pulsars and single pulses, respectively. Results. No new pulsars were detected in the survey. However, we were able to re-detect a known pulsar, PSR J2047+5029, near SNR G89.0+4.7. We were unable to detect the radio-quiet gamma-ray pulsar PSR J2021+4026, but we do find a flux density limit of 0.08 mJy. Our flux density limits make our survey two to 16 times more sensitive than previous surveys, while also covering the whole spatial extent of the same remnants. Conclusions. We discuss potential explanations for the non-detection of a pulsar in the studied SNRs and conclude that sensitivity is still the most likely factor responsible for the lack of pulsars in some remnants.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Start Date: 2018
End Date: 2019
Funder: Science and Technology Facilities Council
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