29 research outputs found
LOFAR observations of gravitational wave merger events: O3 results and O4 strategy
The electromagnetic counterparts to gravitational wave (GW) merger events
hold immense scientific value, but are difficult to detect due to the typically
large localisation errors associated with GW events. The Low-Frequency Array
(LOFAR) is an attractive GW follow-up instrument owing to its high sensitivity,
large instantaneous field of view, and ability to automatically trigger on
events to probe potential prompt emission within minutes. Here, we report on
144-MHz LOFAR radio observations of three GW merger events containing at least
one neutron star that were detected during the third GW observing run.
Specifically, we probe 9 and 16 per cent of the location probability density
maps of S190426c and S200213t, respectively, and place limits at the location
of an interesting optical transient (PS19hgw/AT2019wxt) found within the
localisation map of S191213g. While these GW events are not particularly
significant, we use multi-epoch LOFAR data to devise a sensitive wide-field GW
follow-up strategy to be used in future GW observing runs. In particular, we
improve on our previously published strategy by implementing direction
dependent calibration and mosaicing, resulting in nearly an order of magnitude
increase in sensitivity and more uniform coverage. We achieve a uniform
sensitivity of Jy across a single instantaneous LOFAR
pointing's 21 deg core, and a median sensitivity of 1.1 mJy when
including the full 89 deg hexagonal beam pattern. We also place the
deepest transient surface density limits yet on of order month timescales for
surveys between 60--340 MHz (0.017 deg above mJy and 0.073
deg above mJy).Comment: Replaced with accepted version for publication in MNRA
Long-term study of extreme giant pulses from PSR B0950+08 with AARTFAAC
We report on the detection of extreme giant pulses (GPs) from one of the
oldest-known pulsars, the highly variable PSR B0950+08, with the
Amsterdam-ASTRON Radio Transient Facility And Analysis Centre (AARTFAAC), a
parallel transient detection instrument operating as a subsystem of the LOw
Frequency ARray (LOFAR). During processing of our Northern Hemisphere survey
for low-frequency radio transients, a sample of 275 pulses with fluences
ranging from 42 to 177 kJy ms were detected in one-second snapshot images. The
brightest pulses are an order of magnitude brighter than those previously
reported at 42 and 74 MHz, on par with the levels observed in a previous
long-term study at 103 MHz. Both their rate and fluence distribution differ
between and within the various studies done to date. The GP rate is highly
variable, from 0 to 30 per hour, with only two three-hour observations
accounting for nearly half of the pulses detected in the 96 h surveyed. It does
not vary significantly within a few-hour observation, but can vary strongly one
from day to the next. The spectra appear strongly and variably structured, with
emission sometimes confined to a single 195.3 kHz subband, and the pulse
spectra changing on a timescale of order 10 min.Comment: 10 pages, 9 figures, 1 table. Submitted to MNRA
Feedback from low-luminosity radio galaxies:B2 0258+35
Low-luminosity radio-loud active galactic nuclei (AGN) are of importance in
studies concerning feedback from radio AGN since a dominant fraction of AGN
belong to this class. We report high-resolution Very Large Array (VLA) and
European VLBI Network (EVN) observations of HI-21cm absorption from a young,
compact steep-spectrum radio source, B2 0258+35, nested in the early-type
galaxy NGC 1167, which contains a 160 kpc HI disc. Our VLA and EVN HI
absorption observations, modelling, and comparison with molecular gas data
suggest that the cold gas in the centre of NGC 1167 is very turbulent (with a
velocity dispersion of ~ 90 km/s) and that this turbulence is induced by the
interaction of the jets with the interstellar medium (ISM). Furthermore, the
ionised gas in the galaxy shows evidence of shock heating at a few kpc from the
radio source. These findings support the results from numerical simulations of
radio jets expanding into a clumpy gas disc, which predict that the radio jets
in this case percolate through the gas disc and drive shocks into the ISM at
distances much larger than their physical extent. These results expand the
number of low-luminosity radio sources found to impact the surrounding medium,
thereby highlighting the possible relevance of these AGN for feedback.Comment: 11 pages, 9 figures; Accepted for publication in Astronomy &
Astrophysic
The redshift evolution of extragalactic magnetic fields
Faraday rotation studies of distant radio sources can constrain the evolution
and the origin of cosmic magnetism. We use data from the LOFAR Two Metre Sky
Survey: Data Release 2 (LoTSS DR2) to study the dependence of the Faraday
rotation measure (RM) on redshift. By focusing on radio sources that are close
in terms of their projection on the sky, but physically unrelated (random
pairs), we measure the RM difference, RM, between the two sources.
Thus, we isolate the extragalactic contribution to RM from other
contributions. We present a statistical analysis of the resulting sample of
random pairs and find a median absolute RM difference |RM| rad/m , with |RM| uncorrelated both with respect to
the redshift difference of the pair and the redshift of the nearer source, and
a median excess of random pairs over physical pairs of
rad/m. We seek to reproduce this result with Monte Carlo simulations
assuming a non vanishing seed cosmological magnetic field and a redshift
evolution of the comoving magnetic field strength that varies as . We find the best fitting results nG and that we
conservatively quote as upper limits due to an unmodelled but non vanishing
contribution of local environments to the RM difference. A comparison with
cosmological simulations shows our results to be incompatible with primordial
magnetogenesis scenarios with uniform seed fields of order nG
Progress with the LOFAR Imaging Pipeline
One of the science drivers of the new Low Frequency Array (LOFAR) is
large-area surveys of the low-frequency radio sky. Realizing this goal requires
automated processing of the interferometric data, such that fully calibrated
images are produced by the system during survey operations. The LOFAR Imaging
Pipeline is the tool intended for this purpose, and is now undergoing
significant commissioning work. The pipeline is now functional as an automated
processing chain. Here we present several recent LOFAR images that have been
produced during the still ongoing commissioning period. These early LOFAR
images are representative of some of the science goals of the commissioning
team members.Comment: 11 pages, 6 figures. Accepted for publication in proceedings of
"ISKAF2010 Science Meeting", PoS(ISKAF2010)05
FR II radio galaxies at low frequencies I : morphology, magnetic field strength and energetics
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. ©: 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.Due to their steep spectra, low-frequency observations of FR II radio galaxies potentially provide key insights in to the morphology, energetics and spectrum of these powerful radio sources. However, limitations imposed by the previous generation of radio interferometers at metre wavelengths has meant that this region of parameter space remains largely unexplored. In this paper, the first in a series examining FR IIs at low frequencies, we use LOFAR observations between 50 and 160 MHz, along with complementary archival radio and X-ray data, to explore the properties of two FR II sources, 3C452 and 3C223. We find that the morphology of 3C452 is that of a standard FR II rather than of a double-double radio galaxy as had previously been suggested, with no remnant emission being observed beyond the active lobes. We find that the low-frequency integrated spectra of both sources are much steeper than expected based on traditional assumptions and, using synchrotron/inverse-Compton model fitting, show that the total energy content of the lobes is greater than previous estimates by a factor of around 5 for 3C452 and 2 for 3C223. We go on to discuss possible causes of these steeper than expected spectra and provide revised estimates of the internal pressures and magnetic field strengths for the intrinsically steep case. We find that the ratio between the equipartition magnetic field strengths and those derived through synchrotron/inverse-Compton model fitting remains consistent with previous findings and show that the observed departure from equipartition may in some cases provide a solution to the spectral versus dynamical age disparity.Peer reviewe
The discovery of a radio galaxy of at least 5 Mpc
We discover what is in projection the largest known structure of galactic
origin: a giant radio galaxy with a projected proper length of $4.99 \pm 0.04\
\mathrm{Mpc}45 \pm 3\%25 \pm 9 \%23
\pm 11 \%5 \cdot
10^{-16}\ \mathrm{Pa}$, the pressures in the lobes are the lowest hitherto
found, and Alcyoneus therefore represents one of the most promising radio
galaxies yet to probe the warm-hot intergalactic medium.Comment: 18 pages, 14 figures, 3 tables, accepted for publication in Astronomy
& Astrophysic