11 research outputs found
Ensemble candidate classification for the LOTAAS pulsar survey
One of the biggest challenges arising from modern large-scale pulsar surveys is the number of candidates generated. Here, we implemented several improvements to the machine learning (ML) classifier previously used by the LOFAR Tied-Array All-Sky Survey (LOTAAS) to look for new pulsars via filtering the candidates obtained during periodicity searches. To assist the ML algorithm, we have introduced new features which capture the frequency and time evolution of the signal and improved the signal-to-noise calculation accounting for broad profiles. We enhanced the ML classifier by including a third class characterizing RFI instances, allowing candidates arising from RFI to be isolated, reducing the false positive return rate. We also introduced a new training data set used by the ML algorithm that includes a large sample of pulsars misclassified by the previous classifier. Lastly, we developed an ensemble classifier comprised of five different Decision Trees. Taken together these updates improve the pulsar recall rate by 2.5 per cent, while also improving the ability to identify pulsars with wide pulse profiles, often misclassified by the previous classifier. The new ensemble classifier is also able to reduce the percentage of false positive candidates identified from each LOTAAS pointing from 2.5 per cent (∼500 candidates) to 1.1 per cent (∼220 candidates)
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 seven 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
A LOFAR radio search for single and periodic pulses from M31
Bright, short radio bursts are emitted by sources at a large range of
distances: from the nearby Crab pulsar to remote Fast Radio Bursts (FRBs). FRBs
are likely to originate from distant neutron stars, but our knowledge of the
radio pulsar population has been limited to the Galaxy and the Magellanic
Clouds. In an attempt to increase our understanding of extragalactic pulsar
populations, and its giant-pulse emission, we employed the low-frequency radio
telescope LOFAR to search the Andromeda Galaxy (M31) for radio bursts emitted
by young, Crab-like pulsars. For direct comparison we also present a LOFAR
study on the low-frequency giant pulses from the Crab pulsar; their fluence
distribution follows a power law with slope 3.04(3). A number of candidate
signals were detected from M31 but none proved persistent. FRBs are sometimes
thought of as Crab-like pulsars with exceedingly bright giant pulses -- given
our sensitivity, we can rule out that M31 hosts pulsars more than an order of
magnitude brighter than the Crab pulsar, assuming their pulse scattering
follows that of the known FRBs.Comment: Accepted for publication in A&A. 6 pages with 4 nice figure
Quasi-simultaneous Radio/X-Ray Observations of the Candidate Transitional Millisecond Pulsar 3FGL J1544.6-1125 during its Low-luminosity Accretion-disk State
3FGL J1544.6-1125 is a candidate transitional millisecond pulsar (tMSP). Similar to the well-established tMSPs - PSR J1023+0038, IGR J18245-2452, and XSS J12270-4859 - 3FGL J1544.6-1125 shows γ-ray emission and discrete X-ray "low"and "high"modes during its low-luminosity accretion-disk state. Coordinated radio/X-ray observations of PSR J1023+0038 in its current low-luminosity accretion-disk state showed rapidly variable radio continuum emission - possibly originating from a compact, self-absorbed jet, the "propellering"of accretion material, and/or pulsar moding. 3FGL J1544.6-1125 is currently the only other (candidate) tMSP system in this state, and can be studied to see whether tMSPs are typically radio-loud compared to other neutron star binaries. In this work, we present a quasi-simultaneous Very Large Array and Swift radio/X-ray campaign on 3FGL J1544.6-1125. We detect 10 GHz radio emission varying in flux density from 47.7 ± 6.0 μJy down to ≲15 μJy (3σ upper limit) at four epochs spanning three weeks. At the brightest epoch, the radio luminosity is L 5 GHz = (2.17 ± 0.17) × 1027 erg s-1 for a quasi-simultaneous X-ray luminosity L 2-10 keV = (4.32 ± 0.23) × 1033 erg s-1 (for an assumed distance of 3.8 kpc). These luminosities are close to those of PSR J1023+0038, and the results strengthen the case that 3FGL J1544.6-1125 is a tMSP showing similar phenomenology to PSR J1023+0038.A.J. and J.W.T.H. acknowledge funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement nr. 337062 (DRAGNET). A.J. also acknowledges support from the NuSTAR mission. A.P. acknowledges support from an NWO Vidi Fellowship. J.C.A.M.-J. is the recipient of an Australian Research Council Future Fellowship (FT 140101082). S.B. was supported in part by NASA Swift Guest Investigator Cycle 12 program grant NNX16AN79G awarded through Columbia University
FRB 20210405I: a nearby Fast Radio Burst localised to sub-arcsecond precision with MeerKAT
We present the first sub-arcsecond localised Fast Radio Burst (FRB) detected
using MeerKAT. FRB 20210405I was detected in the incoherent beam using the
MeerTRAP pipeline on 2021 April 05 with a signal to noise ratio of 140.8 and a
dispersion measure of 565.17 pc cm. It was detected while MeerTRAP was
observing commensally with the ThunderKAT large survey project, and was
sufficiently bright that we could use the ThunderKAT 8s images to localise the
FRB. Two different models of the dispersion measure in the Milky Way and halo
suggest that the source is either right at the edge of the Galaxy, or outside.
This highlights the uncertainty in the Milky Way dispersion measure models,
particularly in the Galactic Plane, and the uncertainty of Milky Way halo
models. Further investigation and modelling of these uncertainties will be
facilitated by future detections and localisations of nearby FRBs. We use the
combined localisation, dispersion measure, scattering, specific luminosity and
chance coincidence probability information to find that the origin is most
likely extra-galactic and identify the likely host galaxy of the FRB: 2MASS
J17012494932475. Using SALT spectroscopy and archival observations of the
field, we find that the host is a disk/spiral galaxy at a redshift of
.Comment: 15 pages, 4 tables, 10 figures. Accepted to MNRA
Pulsar observations with European telescopes for testing gravity and detecting gravitational waves
A background of nanohertz gravitational waves from supermassive black hole binaries could soon be detected by pulsar timing arrays, which measure the times-of-arrival of radio pulses from millisecond pulsars with very high precision. The European Pulsar Timing Array uses five large European radio telescopes to monitor high-precision millisecond pulsars, imposing in this way strong constraints on a gravitational wave background. To achieve the necessary precision needed to detect gravitational waves, the Large European Array for Pulsars (LEAP) performs simultaneous observations of pulsars with all five telescopes, which allows us to coherently add the radio pulses, maximize the signal-to-noise of pulsar signals and increase the precision of times-of-arrival. We report on the progress made and results obtained by the LEAP collaboration, and in particular on the addition of the Sardinia Radio Telescope to the LEAP observations during its scientific validation phase. In addition, we discuss how LEAP can be used to monitor strong-gravity systems such as double neutron star systems and impose strong constraints on post-keplerian parameters
Probing the gravitational wave background from cosmic strings with LISA
International audienc
FRB 20210405I: the first Fast Radio Burst sub-arcsecond localised with MeerKAT
International audienceWe present the first sub-arcsecond localised Fast Radio Burst (FRB) detected using MeerKAT. The FRB, FRB 20210405I, was detected in the incoherent beam using the MeerTRAP pipeline on 2021 April 05 with a signal to noise ratio of 140.8 and a dispersion measure of 565.17 pc cm. It was detected while MeerTRAP was observing commensally with the ThunderKAT large survey project, and was sufficiently bright that we could use the ThunderKAT 8s images to localise the FRB. Two different models of the dispersion measure in the Milky Way and halo suggest that the source is either right at the edge of the Galaxy, or outside. However, we use the combined localisation, dispersion measure, scattering, specific luminosity and chance coincidence probability information to find that the origin is most likely extragalactic and identify the likely host galaxy of the FRB: 2MASS J17012494932475. Using SALT spectroscopy and archival observations of the field, we find that the host is a disk/spiral galaxy at a redshift of
Discovery of a radio emitting neutron star with an ultra-long spin period of 76 seconds
The radio-emitting neutron star population encompasses objects with spin
periods ranging from milliseconds to tens of seconds. As they age and spin more
slowly, their radio emission is expected to cease. We present the discovery of
an ultra-long period radio-emitting neutron star, J0901-4046, with spin
properties distinct from the known spin and magnetic-decay powered neutron
stars. With a spin-period of 75.88 s, a characteristic age of 5.3 Myr, and a
narrow pulse duty-cycle, it is uncertain how radio emission is generated and
challenges our current understanding of how these systems evolve. The radio
emission has unique spectro-temporal properties such as quasi-periodicity and
partial nulling that provide important clues to the emission mechanism.
Detecting similar sources is observationally challenging, which implies a
larger undetected population. Our discovery establishes the existence of
ultra-long period neutron stars, suggesting a possible connection to the
evolution of highly magnetized neutron stars, ultra-long period magnetars, and
fast radio burstsComment: Published in Nature Astronomy -
https://www.nature.com/articles/s41550-022-01688-
European Pulsar Timing Array Limits on Continuous Gravitational Waves from Individual Supermassive Black Hole Binaries
We have searched for continuous gravitational wave (CGW) signals produced by
individually resolvable, circular supermassive black hole binaries (SMBHBs) in
the latest EPTA dataset, which consists of ultra-precise timing data on 41
millisecond pulsars. We develop frequentist and Bayesian detection algorithms
to search both for monochromatic and frequency-evolving systems. None of the
adopted algorithms show evidence for the presence of such a CGW signal,
indicating that the data are best described by pulsar and radiometer noise
only. Depending on the adopted detection algorithm, the 95\% upper limit on the
sky-averaged strain amplitude lies in the range at . This limit varies
by a factor of five, depending on the assumed source position, and the most
constraining limit is achieved towards the positions of the most sensitive
pulsars in the timing array. The most robust upper limit -- obtained via a full
Bayesian analysis searching simultaneously over the signal and pulsar noise on
the subset of ours six best pulsars -- is . These limits, the
most stringent to date at , exclude the presence of
sub-centiparsec binaries with chirp mass M out to a
distance of about 25Mpc, and with M out to a
distance of about 1Gpc (). We show that state-of-the-art SMBHB
population models predict probability of detecting a CGW with the
current EPTA dataset, consistent with the reported non-detection. We stress,
however, that PTA limits on individual CGW have improved by almost an order of
magnitude in the last five years. The continuing advances in pulsar timing data
acquisition and analysis techniques will allow for strong astrophysical
constraints on the population of nearby SMBHBs in the coming years.Comment: 16 pages, 11 figures, accepted for publication in MNRA