Radio Pulsar Surveys Research Articles

The TRAPUM Large Magellanic Cloud pulsar survey with MeerKAT – I. Survey set-up and first seven pulsar discoveries

ABSTRACT The Large Magellanic Cloud (LMC) presents a unique environment for pulsar population studies due to its distinct star formation characteristics and proximity to the Milky Way. As part of the TRAPUM (TRAnsients and PUlsars with MeerKAT) Large Survey Project, we are using the core array of the MeerKAT radio telescope (MeerKAT) to conduct a targeted search of the LMC for radio pulsars at L-band frequencies, 856–1712 MHz. The excellent sensitivity of MeerKAT, coupled with a 2-h integration time, makes the survey 3 times more sensitive than previous LMC radio pulsar surveys. We report the results from the initial four survey pointings which have resulted in the discovery of seven new radio pulsars, increasing the LMC radio pulsar population by 30 per cent. The pulse periods of these new pulsars range from 278 to 1690 ms, and the highest dispersion measure is 254.20 $\mathrm{ pc} \, \mathrm{ cm}^{-3}$. We searched for, but did not find any significant pulsed radio emission in a beam centred on the SN 1987A remnant, establishing an upper limit of 6.3 $\mu \mathrm{ Jy}$ on its minimum flux density at 1400 MHz.

On the Initial Spin Period Distribution of Neutron Stars

We derive the initial spin period distribution of neutron stars by studying the population of young pulsars associated with supernova remnants. Our hierarchical Bayesian approach accounts for the measurement uncertainties of individual observations and selection effects. Without correcting for selection effects, as done in previous studies, we find that pulsar initial spin periods follow a Weibull distribution, peaking at 40 ms, which is favored against a lognormal distribution with a Bayes factor of 200. The known selection effects in radio pulsar surveys, including pulse broadening and period-dependent beaming fraction, have been quantitatively investigated. We show that, based on measurements of pulsar luminosity and spin period from the ATNF Pulsar Catalogue, the impact of pulse broadening on the inference of the pulsar period distribution is likely to be insignificant. Correcting for the beaming selection effect, the Weibull distribution remains the preferred model, while its peak slightly shifts to longer periods at 50 ms. Our method will prove useful in constraining the birth properties of neutron stars in the Square Kilometre Array era.

The TRAPUM Small Magellanic Cloud pulsar survey with MeerKAT – I. Discovery of seven new pulsars and two Pulsar Wind Nebula associations

ABSTRACT The sensitivity of the MeerKAT radio interferometer is an opportunity to probe deeper into the population of rare and faint extragalactic pulsars. The TRAPUM (TRAnsients and PUlsars with MeerKAT) collaboration has conducted a radio-domain search for accelerated pulsars and transients in the Small Magellanic Cloud (SMC). This partially targeted survey, performed at L band (856–1712 MHz) with the core array of the MeerKAT telescope in 2-h integrations, is twice as sensitive as the latest SMC radio pulsar survey. We report the discovery of seven new SMC pulsars, doubling this galaxy’s radio pulsar population and increasing the total extragalactic population by nearly a quarter. We also carried out a search for accelerated millisecond pulsars in the SMC globular cluster NGC 121 using the full array of MeerKAT. This improved the previous upper limit on pulsed radio emission from this cluster by a factor of 6. Our discoveries reveal the first radio pulsar-PWN systems in the SMC, with only one such system previously known outside our Galaxy (the ‘Crab pulsar twin’ in the Large Magellanic Cloud, PSR J0540−6919). We associate the 59 ms pulsar discovery PSR J0040−7337, now the fastest spinning radio pulsar in the SMC, with the bow-shock Pulsar Wind Nebula (PWN) of supernova remnant DEM S5. We also present a new young pulsar with a 79 ms period, PSR J0048−7317, in a PWN recently discovered in a MeerKAT radio continuum image. Using the multibeam capability of MeerKAT, we localized our pulsar discoveries, and two previous Murriyang discoveries, to a positional uncertainty of a few arcseconds.

A targeted radio pulsar survey of redback candidates with MeerKAT

ABSTRACT Redbacks are millisecond pulsar binaries with low-mass, irradiated companions. These systems have a rich phenomenology that can be used to probe binary evolution models, pulsar wind physics, and the neutron star mass distribution. A number of high-confidence redback candidates have been identified through searches for variable optical and X-ray sources within the localization regions of unidentified but pulsar-like Fermi-LAT gamma-ray sources. However, these candidates remain unconfirmed until pulsations are detected. As part of the TRAPUM project, we searched for radio pulsations from six of these redback candidates with MeerKAT. We discovered three new radio millisecond pulsars, PSRs J0838−2827, J0955−3947, and J2333−5526, confirming their redback nature. PSR J0838−2827 remained undetected for 2 yr after our discovery despite repeated observations, likely due to evaporated material absorbing the radio emission for long periods of time. While, to our knowledge, this system has not undergone a transition to an accreting state, the disappearance, likely caused by extreme eclipses, illustrates the transient nature of spider pulsars and the heavy selection bias in uncovering their radio population. Radio timing enabled the detection of gamma-ray pulsations from all three pulsars, from which we obtained 15-yr timing solutions. All of these sources exhibit complex orbital period variations consistent with gravitational quadrupole moment variations in the companion stars. These timing solutions also constrain the binary mass ratios, allowing us to narrow down the pulsar masses. We find that PSR J2333−5526 may have a neutron star mass in excess of 2 M⊙.

The LOFAR Tied-Array All-Sky Survey: Timing of 35 radio pulsars and an overview of the properties of the LOFAR pulsar discoveries

The LOFAR Tied-Array All-Sky Survey (LOTAAS) is the most sensitive untargeted radio pulsar survey performed at low radio frequencies (119−151 MHz) to date and has discovered 76 new radio pulsars, including the 23.5-s pulsar J0250+5854, which up until recently was the slowest spinning radio pulsar known. In this paper, we report on the timing solutions of 35 pulsars discovered by LOTAAS, which include a nulling pulsar and a mildly recycled pulsar, and thereby complete the full timing analysis of the LOTAAS pulsar discoveries. We give an overview of the findings from the full LOTAAS sample of 76 pulsars, discussing their pulse profiles, radio spectra, and timing parameters. We found that the pulse profiles of some of the pulsars show profile variations in time or frequency, and while some pulsars show signs of scattering, a large majority display no pulse broadening. The LOTAAS discoveries have on average steeper radio spectra and longer spin periods (1.4×), as well as lower spin-down rates (3.1×) compared to the known pulsar population. We discuss the cause of these differences and attribute them to a combination of selection effects of the LOTAAS survey as well as previous pulsar surveys, though we cannot rule out that older pulsars tend to have steeper radio spectra.

A dynamically discovered and characterized non-accreting neutron star–M dwarf binary candidate

Typically, neutron stars are discovered by observations at radio, X-ray or gamma-ray wavelengths. Unlike radio pulsar surveys and X-ray observations, optical time-domain surveys can unveil and characterize exciting but less explored non-accreting and/or non-beaming neutron stars in binaries. Here we report the discovery of such a neutron star candidate using the LAMOST spectroscopic survey. The candidate, designated LAMOST J112306.9 + 400736, is in a single-lined spectroscopic binary containing an optically visible M star. The star’s large radial velocity variation and ellipsoidal variations indicate a relatively massive unseen companion. Utilizing follow-up spectroscopy from the Palomar 200 in. telescope and high-precision photometry from the Transiting Exoplanet Survey Satellite, we measure a companion mass of \(1.2{4}_{-0.03}^{+0.03}\,{M}_{\odot }\). Main-sequence stars with this mass are ruled out, leaving a neutron star or a massive white dwarf. Although a massive white dwarf cannot be excluded, the lack of UV excess radiation from the companion supports the neutron star hypothesis. Deep radio observations with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) yielded no detections of either pulsed or persistent emission. J112306.9 + 400736 is not detected in numerous X-ray and gamma-ray surveys, suggesting that the neutron star candidate is not currently accreting and pulsing. Our work exemplifies the capability of discovering compact objects in non-accreting close binaries by synergizing optical time-domain spectroscopy and high-cadence photometry.

On the Detectability of Ultracompact Binary Pulsar Systems

Abstract Using neural networks, we integrate the ability to account for Doppler smearing due to a pulsar’s orbital motion with the pulsar population synthesis package psrpoppy to develop accurate modeling of the observed binary pulsar population. As a first application, we show that binary neutron star systems where the two components have highly unequal mass are, on average, easier to detect than systems that are symmetric in mass. We then investigate the population of ultracompact (1.5 minutes ≤ P b ≤ 15 minutes) neutron star–white dwarf (NS–WD) and double neutron star (DNS) systems, which are promising sources for the Laser Interferometer Space Antenna gravitational-wave detector. Given the nondetection of these systems in radio surveys thus far, we estimate a 95% confidence upper limit of ∼1450 and ∼1100 ultracompact NS–WD and DNS systems in the Milky Way that are beaming toward the Earth, respectively. We also show that using survey integration times in the range 20 s–200 s with time-domain resampling will maximize the signal-to-noise ratio as well as the probability of detection of these ultracompact binary systems. Among all the large-scale radio pulsar surveys, those that are currently being carried out using archival data collected with the Arecibo radio telescope have a ∼50%–80% chance of detecting at least one of these systems using current integration integration times and ∼80%–95% using optimal integration times in the next several years.

A multiwavelength search for black widow and redback counterparts of candidate γ-ray millisecond pulsars

ABSTRACT The wealth of detections of millisecond pulsars (MSPs) in γ-rays by Fermi has spurred searches for these objects among the several unidentified γ-ray sources. Interesting targets are a sub-class of binary MSPs, dubbed ‘black widows’ (BWs) and ‘redbacks’ (RBs), which are in orbit with low-mass non-degenerate companions fully or partially ablated by irradiation from the MSP wind. These systems can be easily missed in radio pulsar surveys owing to the eclipse of the radio signal by the intra-binary plasma from the ablated companion star photosphere, making them better targets for multiwavelength observations. We used optical and X-ray data from public data bases to carry out a systematic investigation of all the unidentified γ-ray sources from the Fermi Large Area Telescope Third Source Catalog, which have been pre-selected as likely MSP candidates according to a machine-learning technique analysis. We tested our procedure by recovering known binary BW/RB identifications and searched for new ones, finding two possible candidates. At the same time, we investigated previously proposed BW/RB identifications and we ruled out one of them based upon the updated γ-ray source coordinates.

High plasma cut-off frequency blocks the radio emission from accreting millisecond X-ray pulsar SAX J1808.4-3658

SAX J1808.4-3658 is the first discovered transient accreting millisecond X-ray pulsar (AMXP) with a spin frequency of 401 Hz. It has been proposed that its radio pulses should be detected during the low state of X-ray emission, however there is no any successful evidence for its occurring a radio pulsar state with the sensitive radio telescopes. In this paper, we present a calculation of the plasma cut-off oscillation frequency of the accreting plasma above the magnetosphere of SAX J1808.4-3658, and obtain a limiting value of about ∼ 5.9 GHz, corresponding to the lowest X-ray luminosity, which is higher than the favorite “search window” (e.g. 1.4 GHz) of radio pulsar surveys. Moreover, after comparing the pressure of the dipole electromagnetic radiation with that of the accreting matter, we find that the transformational condition from the accretion-powered to rotational-powered pulsar state implies a critical X-ray luminosity of $4.57\times10^{33} \mbox{ erg}\,\mbox{s}^{-1}$. We propose the observational suggestion for searching the radio pulsar state of SAX J1808.4-3658 is that the radio astronomers should use the telescope with high frequency receivers to search the transition from AMXP to radio MSP during the low X-ray luminosity phase (e.g. quiescence).

A high magnetic field radio pulsar survey with Swift/XRT

We present the X-ray survey results of high-magnetic-field radio pulsars (high-B PSRs) with Swift/XRT. X-ray observations of the rotation-powered pulsars with the dipole magnetic field ${\it B_{\rm d}}$ near the quantum critical field ${\it B_{\rm q}}=4.4\times10^{13}$ G is of great importance for understanding the transition between the rotation-powered pulsars and the magnetars, because there are a few objects that have magnetar-like properties. Out of the 27 high-B PSRs that are in the ATNF pulsar catalogue but have not been reported or have no effective upper-limits in the X-ray bands, we analyze the Swift/XRT data for 21 objects, where 6 objects are newly observed and 15 objects are taken from the archival data. As a result, we have new $3\sigma$ upper-limits for all the 21 objects. Since the upper-limits are tight, we conclude that we do not find any magnetar-like high-B PSRs such as PSR J1819$-$1458. The probability of the high X-ray efficiency in the high-B PSRs is obtained to be $11\%-29\%$. Combining the previous observations, we discuss which parameter causes magnetar-like properties. It may be suggested that the magnetar-like properties appear only when ${\it B_{\rm d}}\gtrsim10^{13.5}$ G for the radio pulsar population. This is true even if the radio-quiet high-B RPP are included.

Interpretation of no radio pulsar inside supernova 1987A: The high plasma cut-off frequency by the remnant media

Three decades have passed since the supernova SN 1987A was observed in the Large Magellanic Cloud, inside which the product is most likely a neutron star (NS) formed in the core collapse explosion.Although lots of observations with sensitive radio telescopes have taken place, astronomers have not yet detected any evidence for a radio pulsar around the remnant of 1987A. To investigate pulsars inside the SN remnants, we calculate the cut-off oscillation frequency of the plasma around the presumed NS inside SN1987A, as shown to be about 33 GHz at present (2018 CE), which is much higher than the favorite “searching window” (e.g. L-band ∼ 1.4 GHz) of radio pulsar surveys that have been commonly exploited by astronomers. Since radio waves with frequencies lower than the plasma cut-off frequency cannot penetrate the SN remnant media, we suggest that astronomers use higher frequency bands to search for a pulsar in SN 1987A.Furthermore, with the expansion of SN remnant media, we find that the plasma cut-off frequency can decay to the L-band (1.4 GHz) in the future. The strategy of finding a pulsar of SN 1987A is that either the high frequency bands of radio telescopes, or the high energy detections at Gamma-ray and X-ray bands by space satellites are applied.

On-the-fly Mapping of New Pulsars

Abstract Current single-dish, low-frequency radio pulsar surveys provide efficient sky coverage, but poor localization of new discoveries. Here, we describe a practical technique for rapidly localizing pulsars discovered in these surveys with on-the-fly mapping and provide code to facilitate and formalize its implementation. As a proof of concept, we alter the positions of four test sources and use the Green Bank Telescope (GBT) 350 MHz receiver to recover source positions within ≈1′–3′ of their true values, compared to an 18′ error radius for new discoveries. Achieving similar precision with a traditional gridding strategy using the GBT requires 2–3 times as much telescope time (including overhead), multiple receivers, and relies on assumptions about the pulsars’ spectral indices. For one of our test sources (PSR J1400−1431), this method revealed a discrepancy with the initial, published position, prompting additional follow-up and an improved timing solution. Rapid localization is important for improving data quality and providing flexibility in choice of center frequency for future timing observations—both of which facilitate evaluating new millisecond pulsars for potential inclusion in pulsar timing arrays.

Single-pulse classifier for the LOFAR Tied-Array All-sky Survey

Searches for millisecond-duration, dispersed single pulses have become a standard tool used during radio pulsar surveys in the last decade. They have enabled the discovery of two new classes of sources: rotating radio transients and fast radio bursts. However, we are now in a regime where the sensitivity to single pulses in radio surveys is often limited more by the strong background of radio frequency interference (RFI, which can greatly increase the false-positive rate) than by the sensitivity of the telescope itself. To mitigate this problem, we introduce the Single-pulse Searcher (SpS). This is a new machine-learning classifier designed to identify astrophysical signals in a strong RFI environment, and optimized to process the large data volumes produced by the new generation of aperture array telescopes. It has been specifically developed for the LOFAR Tied-Array All-Sky Survey (LOTAAS), an ongoing survey for pulsars and fast radio transients in the northern hemisphere. During its development, SpS discovered 7 new pulsars and blindly identified ~80 known sources. The modular design of the software offers the possibility to easily adapt it to other studies with different instruments and characteristics. Indeed, SpS has already been used in other projects, e.g. to identify pulses from the fast radio burst source FRB 121102. The software development is complete and SpS is now being used to re-process all LOTAAS data collected to date.

Sizing up the population of gamma-ray binaries

Context.Gamma-ray binaries are thought to be composed of a young pulsar in orbit around a massive O or Be star with their gamma-ray emission powered by pulsar spin-down. The number of such systems in our Galaxy is not known.Aims.We aim to estimate the total number of gamma-ray binaries in our Galaxy and to evaluate the prospects for new detections in the GeV and TeV energy range, taking into account that their gamma-ray emission is modulated on the orbital period.Methods.We modelled the population of gamma-ray binaries and evaluated the fraction of detected systems in surveys with theFermi-LAT (GeV), H.E.S.S., HAWC and CTA (TeV) using observation-based and synthetic template light curves.Results.The detected fraction depends more on the orbit-average flux than on the light-curve shape. Our best estimate for the number of gamma-ray binaries is 101\hbox{$_{-52}^{+89}$} systems. A handful of discoveries are expected by pursuing theFermi-LAT survey. Discoveries in TeV surveys are less likely. However, this depends on the relative amounts of power emitted in GeV and TeV domains. There could be as many as ≈ 200 HESS J0632+057-like systems with a high ratio of TeV to GeV emission compared to other gamma-ray binaries. Statistics allow for as many as three discoveries in five years of HAWC observations and five discoveries in the first two years of the CTA Galactic Plane survey.Conclusions.We favour continuedFermi-LAT observations over ground-based TeV surveys to find new gamma-ray binaries. Gamma-ray observations are most sensitive to short orbital period systems with a high spin-down pulsar power. Radio pulsar surveys (SKA) are likely to be more efficient in detecting long orbital period systems, providing a complementary probe into the gamma-ray binary population.

RADIO DETECTION PROSPECTS FOR A BULGE POPULATION OF MILLISECOND PULSARS AS SUGGESTED BY FERMI-LAT OBSERVATIONS OF THE INNER GALAXY

ABSTRACT The dense stellar environment of the Galactic center has been proposed to host a large population of as-yet undetected millisecond pulsars (MSPs). Recently, this hypothesis has found support in an analysis of gamma-rays detected using the Large Area Telescope onboard the Fermi satellite, which revealed an excess of diffuse GeV photons in the inner 15 deg about the Galactic center. The excess can be interpreted as the collective emission of thousands of MSPs in the Galactic bulge, with a spherical distribution strongly peaked toward the Galactic center. In order to fully establish the MSP interpretation, it is essential to find corroborating evidence in multi-wavelength searches, most notably through the detection of radio pulsations from individual bulge MSPs. Based on globular cluster observations and gamma-ray emission from the inner Galaxy, we investigate the prospects for detecting MSPs in the Galactic bulge. While previous pulsar surveys failed to identify this population, we demonstrate that upcoming large-area surveys of this region should lead to the detection of dozens of bulge MSPs. Additionally, we show that deep targeted searches of unassociated Fermi sources should be able to detect the first few MSPs in the bulge. The prospects for these deep searches are enhanced by a tentative gamma-ray/radio correlation that we infer from high-latitude gamma-ray MSPs. Such detections would constitute the first clear discoveries of field MSPs in the Galactic bulge, with far-reaching implications for gamma-ray observations, the formation history of the central Milky Way, and strategy optimization for future deep radio pulsar surveys.

A MILLISECOND INTERFEROMETRIC SEARCH FOR FAST RADIO BURSTS WITH THE VERY LARGE ARRAY

A MILLISECOND INTERFEROMETRIC SEARCH FOR FAST RADIO BURSTS WITH THE VERY LARGE ARRAY

The Galactic Centre pulsar population

Abstract The recent discovery of a magnetar in the Galactic Centre region has allowed Spitler et al. to characterize the interstellar scattering in that direction. They find that the temporal broadening of the pulse profile of the magnetar is substantially less than that predicted by models of the electron density of that region. This raises the question of what the plausible limits for the number of potentially observable pulsars – i.e. the number of pulsars beaming towards the Earth – in the Galactic Centre are. In this Letter, using reasonable assumptions – namely (i) the luminosity function of pulsars in the Galactic Centre region is the same as that in the field, (ii) the region has had a constant pulsar formation rate, (iii) the spin and luminosity evolution of magnetars and pulsars are similar and (iv) the scattering in the direction of the Galactic Centre magnetar is representative of the entire inner parsec – we show that the potentially observable population of pulsars in the inner parsec has a conservative upper limit of ∼200 and that it is premature to conclude that the number of pulsars in this region is small. We also show that the observational results so far are consistent with this number and make predictions for future radio pulsar surveys of the Galactic Centre.

Timing of pulsars found in a deep Parkes multibeam survey

We have carried out a sensitive radio pulsar survey along the northern Galactic plane (50° < l < 60° and |b| ≲ 2°) using the Parkes 20-cm multibeam system. We observed each position for 70 min on two separate epochs. Our analyses to date have so far resulted in the detection of 32 pulsars, of which 17 were previously unknown. Here, we summarize the observations and analysis and present the timing observations of 11 pulsars and discovery parameters for a further six pulsars. We also present a timing solution for the 166-ms bursting pulsar, PSR J1938+2213, previously discovered during an Arecibo drift-scan survey. Our survey data for this pulsar show that the emission can be described by a steady pulse component with bursting emission, which lasts for typically 20–25 pulse periods, superposed. Other new discoveries are the young 80.1-ms pulsar PSR J1935+2025 which exhibits a significant amount of unmodelled low-frequency noise in its timing residuals and the 4.2-ms pulsar PSR J1935+1726 which is in a low-mass binary system with a 90.7-d circular orbit.

Peace: pulsar evaluation algorithm for candidate extraction – a software package for post-analysis processing of pulsar survey candidates

Modern radio pulsar surveys produce a large volume of prospective candidates, the majority of which are polluted by human-created radio frequency interference or other forms of noise. Typically, large numbers of candidates need to be visually inspected in order to determine if they are real pulsars. This process can be labor intensive. In this paper, we introduce an algorithm called PEACE (Pulsar Evaluation Algorithm for Candidate Extraction) which improves the efficiency of identifying pulsar signals. The algorithm ranks the candidates based on a score function. Unlike popular machine-learning based algorithms, no prior training data sets are required. This algorithm has been applied to data from several large-scale radio pulsar surveys. Using the human-based ranking results generated by students in the Arecibo Remote Command enter programme, the statistical performance of PEACE was evaluated. It was found that PEACE ranked 68% of the student-identified pulsars within the top 0.17% of sorted candidates, 95% within the top 0.34%, and 100% within the top 3.7%. This clearly demonstrates that PEACE significantly increases the pulsar identification rate by a factor of about 50 to 1000. To date, PEACE has been directly responsible for the discovery of 47 new pulsars, 5 of which are millisecond pulsars that may be useful for pulsar timing based gravitational-wave detection projects.

Pulsar searches of Fermi unassociated sources with the Effelsberg telescope

Using the 100-m Effelsberg radio telescope operating at 1.36 GHz, we have performed a targeted radio pulsar survey of 289 unassociated γ-ray sources discovered by the Large Area Telescope (LAT) aboard the Fermi satellite and published in the 1FGL catalogue (Abdo et al. 2010a). This survey resulted in the discovery of millisecond pulsar J1745+1017, which resides in a short-period binary system with a low-mass companion, |$M_{c,\text{min}} \sim 0.0137\,\mathrm{M}_{\odot }$|⁠, indicative of ‘black widow’ type systems. A 2-yr timing campaign has produced a refined radio ephemeris, accurate enough to allow for phase-folding of the LAT photons, resulting in the detection of a dual-peaked γ-ray light curve, proving that PSR J1745+1017 is the source responsible for the γ-ray emission seen in 1FGL J1745.5+1018 (2FGL J1745.6+1015; Nolan et al. 2012). We find the γ-ray spectrum of PSR J1745+1017 to be well modelled by an exponentially cut-off power law with cut-off energy 3.2 GeV and photon index 1.6. The observed sources are known to contain a further 10 newly discovered pulsars which were undetected in this survey. Our radio observations of these sources are discussed and in all cases limiting flux densities are calculated. The reasons behind the seemingly low yield of discoveries are also discussed.