22 research outputs found
Shock location and CME 3D reconstruction of a solar type II radio burst with LOFAR
Context. Type II radio bursts are evidence of shocks in the solar atmosphere and inner heliosphere that emit radio waves ranging from sub-meter to kilometer lengths. These shocks may be associated with coronal mass ejections (CMEs) and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20â90 MHz) is now possible with the Low Frequency Array (LOFAR), opening a new radio window in which to study coronal shocks that leave the inner solar corona and enter the interplanetary medium and to understand their association with CMEs.
Aims. To this end, we study a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon.
Methods. Thetype II shock source-positions and spectra were obtained using 91 simultaneous tied-array beams of LOFAR, and the CME was observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) and by the COR2A coronagraph of the SECCHI instruments on board the Solar Terrestrial Relation Observatory(STEREO). The 3D structure was inferred using triangulation of the coronographic observations. Coronal magnetic fields were obtained from a 3D magnetohydrodynamics (MHD) polytropic model using the photospheric fields measured by the Heliospheric Imager (HMI) on board the Solar Dynamic Observatory (SDO) as lower boundary.
Results. The type II radio source of the coronal shock observed between 50 and 70 MHz was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with ΞBn ~ 70°. The type II radio burst showed first and second harmonic emission; the second harmonic source was cospatial with the first harmonic source to within the observational uncertainty. This suggests that radio wave propagation does not alter the apparent location of the harmonic source. The sources of the two split bands were also found to be cospatial within the observational uncertainty, in agreement with the interpretation that split bands are simultaneous radio emission from upstream and downstream of the shock front. The fast magnetosonic Mach number derived from this interpretation was found to lie in the range 1.3â1.5. The fast magnetosonic Mach numbers derived from modelling the CME and the coronal magnetic field around the type II source were found to lie in the range 1.4â1.6
Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies
Context. The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (< 100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. Aims. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15\u2033 with a high dynamic range and good image fidelity. Methods. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna system. We reduced the datasets and obtained an image for each A-team source. Results. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further
LOFAR 144-MHz follow-up observations of GW170817
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 4, June 2020, Pages 5110â5117, ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We present low-radio-frequency follow-up observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGO-Virgo. These data, with a central frequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. The maximum elevation of the target is just 13.7 degrees when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On time-scales of 130-138 and 371-374 days after the merger event, we obtain 3 upper limits for the afterglow component of 6.6 and 19.5 mJy beam, respectively. Using our best upper limit and previously published, contemporaneous higher-frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144 MHz: the two-point spectral index . We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.Peer reviewe
Low-frequency radio absorption in Cassiopeia A
Cassiopeia A is one of the best-studied supernova remnants. Its shocked
ejecta emits brightly in radio and X-rays. Its unshocked ejecta can be studied
through infrared emission, the radio-active decay of Ti, and low
frequency free-free absorption due to cold gas internal to the shell. Free-free
absorption is affected by the mass, geometry, temperature, and ionisation
conditions in the absorbing gas. Observations at the lowest radio frequencies
constrain a combination of these properties. We use LOFAR LBA observations at
30-77 MHz and L-band VLA observations to compare -matched images with a
common resolution of 17". We simultaneously fit, per pixel, for the emission
measure and the ratio of the emission from the unabsorbed front of the shell
versus the absorbed back of the shell. We explore the effects that low
temperatures and a high degree of clumping can have on the derived physical
properties, such as mass and density. We also compile published radio flux
measurements, fit for the absorption processes that occur in the radio band,
and consider how they affect the secular decline of the source. We find a mass
in the unshocked ejecta of for an assumed gas
temperature of K. This estimate is reduced for colder gas temperatures
and if the ejecta are clumped. We measure the reverse shock to have a radius of
" 6". We also find that a decrease in the amount of mass in the
unshocked ejecta (as more and more material meets the reverse shock and heats
up) cannot account for the observed low frequency behaviour of the secular
decline rate. To reconcile our low frequency absorption measurements with
models that predict little mass in the unshocked ejecta we need the ejecta to
be very clumped, or the temperature in the cold gas to be low ( K).
Both conditions can jointly contribute to the high absorption.Comment: Accepted for publication in A&A v2: including the DOI, language edit
LOFAR 144-MHz follow-up observations of GW170817
ABSTRACT
We present low-radio-frequency follow-up observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGOâVirgo. These data, with a central frequency of 144âMHz, were obtained with LOFAR, the Low-Frequency Array. The maximum elevation of the target is just 137 when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On time-scales of 130â138 and 371â374âd after the merger event, we obtain 3Ï upper limits for the afterglow component of 6.6 and 19.5âmJyâbeamâ1, respectively. Using our best upper limit and previously published, contemporaneous higher frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144âMHz: the two-point spectral index  â2.5. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.</jats:p
Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies
de Gasperin F, Vink J, McKean JP, et al. Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies. ASTRONOMY & ASTROPHYSICS. 2020;635: A150.Context. The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (<100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. Aims. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 1500 with a high dynamic range and good image fidelity. Methods. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna system. We reduced the datasets and obtained an image for each A-team source. Results. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further