65 research outputs found
Enabling pulsar and fast transient searches using coherent dedispersion
We present an implementation of the coherent dedispersion algorithm capable
of dedispersing high-time-resolution radio observations to many different
dispersion measures (DMs). This approach allows the removal of the dispersive
effects of the interstellar medium and enables searches for pulsed emission
from pulsars and other millisecond-duration transients at low observing
frequencies and/or high DMs where time broadening of the signal due to
dispersive smearing would otherwise severely reduce the sensitivity. The
implementation, called 'cdmt', for Coherent Dispersion Measure Trials, exploits
the parallel processing capability of general-purpose graphics processing units
to accelerate the computations. We describe the coherent dedispersion algorithm
and detail how cdmt implements the algorithm to efficiently compute many
coherent DM trials. We present the concept of a semi-coherent dedispersion
search, where coherently dedispersed trials at coarsely separated DMs are
subsequently incoherently dedispersed at finer steps in DM. The software is
used in an ongoing LOFAR pilot survey to test the feasibility of performing
semi-coherent dedispersion searches for millisecond pulsars at 135MHz. This
pilot survey has led to the discovery of a radio millisecond pulsar -- the
first at these low frequencies. This is the first time that such a broad and
comprehensive search in DM-space has been done using coherent dedispersion, and
we argue that future low-frequency pulsar searches using this approach are both
scientifically compelling and feasible. Finally, we compare the performance of
cdmt with other available alternatives.Comment: 8 pages, 7 figures, submitted to Astronomy and Computin
LOFAR discovery of the fastest-spinning millisecond pulsar in the Galactic field
We report the discovery of PSR J09520607, a 707-Hz binary millisecond
pulsar which is now the fastest-spinning neutron star known in the Galactic
field (i.e., outside of a globular cluster). PSR J09520607 was found using
LOFAR at a central observing frequency of 135 MHz, well below the 300 MHz to 3
GHz frequencies typically used in pulsar searches. The discovery is part of an
ongoing LOFAR survey targeting unassociated Fermi Large Area Telescope
-ray sources. PSR J09520607 is in a 6.42-hr orbit around a very
low-mass companion ( M) and we identify a
strongly variable optical source, modulated at the orbital period of the
pulsar, as the binary companion. The light curve of the companion varies by 1.6
mag from at maximum to , indicating that it is
irradiated by the pulsar wind. Swift observations place a 3- upper
limit on the keV X-ray luminosity of erg
s (using the 0.97 kpc distance inferred from the dispersion measure).
Though no eclipses of the radio pulsar are observed, the properties of the
system classify it as a black widow binary. The radio pulsed spectrum of PSR
J09520607, as determined through flux density measurements at 150 and 350
MHz, is extremely steep with (where ).
We discuss the growing evidence that the fastest-spinning radio pulsars have
exceptionally steep radio spectra, as well as the prospects for finding more
sources like PSR J09520607.Comment: 9 pages, 3 figures, 1 table, published in ApJ letter
Revealing the Dynamic Magneto-ionic Environments of Repeating Fast Radio Burst Sources through Multi-year Polarimetric Monitoring with CHIME/FRB
Fast radio bursts (FRBs) display a confounding variety of burst properties
and host galaxy associations. Repeating FRBs offer insight into the FRB
population by enabling spectral, temporal and polarimetric properties to be
tracked over time. Here, we report on the polarized observations of 12
repeating sources using multi-year monitoring with the Canadian Hydrogen
Intensity Mapping Experiment (CHIME) over 400-800 MHz. We observe significant
RM variations from many sources in our sample, including RM changes of several
hundred over month timescales from FRBs 20181119A,
20190303A and 20190417A, and more modest RM variability ( few tens rad m) from FRBs 20181030A, 20190208A, 20190213B and
20190117A over equivalent timescales. Several repeaters display a frequency
dependent degree of linear polarization that is consistent with depolarization
via scattering. Combining our measurements of RM variations with equivalent
constraints on DM variability, we estimate the average line-of-sight magnetic
field strength in the local environment of each repeater. In general, repeating
FRBs display RM variations that are more prevalent/extreme than those seen from
radio pulsars in the Milky Way and the Magellanic Clouds, suggesting repeating
FRBs and pulsars occupy distinct magneto-ionic environments
Testing afterglow models of FRB 200428 with early post-burst observations of SGR 1935 + 2154
High Energy Astrophysic
LOFAR Detection of 110-188 MHz Emission and Frequency-Dependent Activity from FRB 20180916B
FRB 20180916B is a well-studied repeating fast radio burst source. Its
proximity (~150 Mpc), along with detailed studies of the bursts, have revealed
many clues about its nature -- including a 16.3-day periodicity in its
activity. Here we report on the detection of 18 bursts using LOFAR at 110-188
MHz, by far the lowest-frequency detections of any FRB to date. Some bursts are
seen down to the lowest-observed frequency of 110 MHz, suggesting that their
spectra extend even lower. These observations provide an order-of-magnitude
stronger constraint on the optical depth due to free-free absorption in the
source's local environment. The absence of circular polarization and nearly
flat polarization angle curves are consistent with burst properties seen at
300-1700 MHz. Compared with higher frequencies, the larger burst widths
(~40-160 ms at 150 MHz) and lower linear polarization fractions are likely due
to scattering. We find ~2-3 rad/m^2 variations in the Faraday rotation measure
that may be correlated with the activity cycle of the source. We compare the
LOFAR burst arrival times to those of 38 previously published and 22 newly
detected bursts from the uGMRT (200-450 MHz) and CHIME/FRB (400-800 MHz).
Simultaneous observations show 5 CHIME/FRB bursts when no emission is detected
by LOFAR. We find that the burst activity is systematically delayed towards
lower frequencies by ~3 days from 600 MHz to 150 MHz. We discuss these results
in the context of a model in which FRB 20180916B is an interacting binary
system featuring a neutron star and high-mass stellar companion.Comment: Accepted for publication by ApJ
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