14 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
Proposed host galaxies of repeating fast radio burst sources detected by CHIME/FRB
We present a search for host galaxy associations for the third set of
repeating fast radio burst (FRB) sources discovered by the CHIME/FRB
Collaboration. Using the ~1 arcmin CHIME/FRB baseband localizations and
probabilistic methods. We identify potential host galaxies of two FRBs,
20200223B and 20190110C at redshifts of 0.06024(2) and 0.12244(6),
respectively. We also discuss the properties of a third marginal candidate host
galaxy association for FRB 20191106C with a host redshift of 0.10775(1). The
three putative host galaxies are all relatively massive, fall on the standard
mass-metallicity relationship for nearby galaxies, and show evidence of ongoing
star formation. They also all show signatures of being in a transitional
regime, falling in the "green valley" which is between the bulk of star-forming
and quiescent galaxies. The plausible host galaxies identified by our analysis
are consistent with the overall population of repeating and non-repeating FRB
hosts while increasing the fraction of massive and bright galaxies. Coupled
with these previous host associations, we identify a possible excess of FRB
repeaters whose host galaxies have M_u - M_r colors redder than the bulk of
star-forming galaxies. Additional precise localizations are required to confirm
this trend.Comment: 11 pages, submitted to AAS journal
Comprehensive Bayesian analysis of FRB-like bursts from SGR 1935+2154 observed by CHIME/FRB
The bright millisecond-duration radio burst from the Galactic magnetar SGR
1935+2154 in 2020 April was a landmark event, demonstrating that at least some
fast radio burst (FRB) sources could be magnetars. The two-component burst was
temporally coincident with peaks observed within a contemporaneous short X-ray
burst envelope, marking the first instance where FRB-like bursts were observed
to coincide with X-ray counterparts. In this study, we detail five new radio
burst detections from SGR 1935+2154, observed by the CHIME/FRB instrument
between October 2020 and December 2022. We develop a fast and efficient
Bayesian inference pipeline that incorporates state-of-the-art Markov chain
Monte Carlo techniques and use it to model the intensity data of these bursts
under a flexible burst model. We revisit the 2020 April burst and corroborate
that both the radio sub-components lead the corresponding peaks in their
high-energy counterparts. For a burst observed in 2022 October, we find that
our estimated radio pulse arrival time is contemporaneous with a short X-ray
burst detected by GECAM and HEBS, and Konus-Wind and is consistent with the
arrival time of a radio burst detected by GBT. We present flux and fluence
estimates for all five bursts, employing an improved estimator for bursts
detected in the side-lobes. We also present upper limits on radio emission for
X-ray emission sources which were within CHIME/FRB's field-of-view at trigger
time. Finally, we present our exposure and sensitivity analysis and estimate
the Poisson rate for FRB-like events from SGR 1935+2154 to be
events/day above a fluence of
during the interval from 28 August 2018 to 1 December 2022, although we note
this was measured during a time of great X-ray activity from the source.Comment: 22 pages, 6 figures, 4 tables. To be submitted to Ap
Multiwavelength Constraints on the Origin of a Nearby Repeating Fast Radio Burst Source in a Globular Cluster
Since fast radio bursts (FRBs) were discovered, their precise origins have
remained a mystery. Multiwavelength observations of nearby FRB sources provide
one of the best ways to make rapid progress in our understanding of the
enigmatic FRB phenomenon. We present results from a sensitive, broadband
multiwavelength X-ray and radio observational campaign of FRB 20200120E, the
closest known extragalactic repeating FRB source. At a distance of 3.63 Mpc,
FRB 20200120E resides in an exceptional location, within a ~10 Gyr-old globular
cluster in the M81 galactic system. We place deep limits on both the persistent
X-ray luminosity and prompt X-ray emission at the time of radio bursts from FRB
20200120E, which we use to constrain possible progenitors for the source. We
compare our results to various classes of X-ray sources and transients. In
particular, we find that FRB 20200120E is unlikely to be associated with:
ultraluminous X-ray bursts (ULXBs), similar to those observed from objects of
unknown origin in other extragalactic globular clusters; giant flares, like
those observed from Galactic and extragalactic magnetars; or most intermediate
flares and very bright short X-ray bursts, similar to those seen from magnetars
in the Milky Way. We show that FRB 20200120E is also unlikely to be powered by
a persistent or transient ultraluminous X-ray (ULX) source or a young,
extragalactic pulsar embedded in a Crab-like nebula. We also provide new
constraints on the compatibility of FRB 20200120E with accretion-based FRB
models involving X-ray binaries and models that require a synchrotron maser
process from relativistic shocks to generate FRB emission. These results
highlight the power that multiwavelength observations of nearby FRBs can
provide for discriminating between potential FRB progenitor models.Comment: 58 pages, 10 figures, 7 tables, submitte
A fast radio burst localized at detection to a galactic disk using very long baseline interferometry
Fast radio bursts (FRBs) are millisecond-duration, luminous radio transients
of extragalactic origin. These events have been used to trace the baryonic
structure of the Universe using their dispersion measure (DM) assuming that the
contribution from host galaxies can be reliably estimated. However,
contributions from the immediate environment of an FRB may dominate the
observed DM, thus making redshift estimates challenging without a robust host
galaxy association. Furthermore, while at least one Galactic burst has been
associated with a magnetar, other localized FRBs argue against magnetars as the
sole progenitor model. Precise localization within the host galaxy can
discriminate between progenitor models, a major goal of the field. Until now,
localizations on this spatial scale have only been carried out in follow-up
observations of repeating sources. Here we demonstrate the localization of FRB
20210603A with very long baseline interferometry (VLBI) on two baselines, using
data collected only at the time of detection. We localize the burst to SDSS
J004105.82+211331.9, an edge-on galaxy at , and detect recent
star formation in the kiloparsec-scale vicinity of the burst. The edge-on
inclination of the host galaxy allows for a unique comparison between the line
of sight towards the FRB and lines of sight towards known Galactic pulsars. The
DM, Faraday rotation measure (RM), and scattering suggest a progenitor
coincident with the host galactic plane, strengthening the link between the
environment of FRB 20210603A and the disk of its host galaxy. Single-pulse VLBI
localizations of FRBs to within their host galaxies, following the one
presented here, will further constrain the origins and host environments of
one-off FRBs.Comment: 40 pages, 13 figures, submitted. Fixed typo in abstrac
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