We present the results of a recent re-reduction of the data from the Very Large Array (VLA) Low-frequency Sky Survey (VLSS). We used the VLSS catalogue as a sky model to correct the ionospheric ...distortions in the data and create a new set of sky maps and corresponding catalogue at 73.8 MHz. The VLSS Redux (VLSSr) has a resolution of 75 arcsec, and an average map rms noise level of σ ∼ 0.1 Jy beam−1. The clean bias is 0.66 × σ and the theoretical largest angular size is 36 arcmin. Six previously unimaged fields are included in the VLSSr, which has an unbroken sky coverage over 9.3 sr above an irregular southern boundary. The final catalogue includes 92 964 sources. The VLSSr improves upon the original VLSS in a number of areas including imaging of large sources, image sensitivity, and clean bias; however the most critical improvement is the replacement of an inaccurate primary beam correction which caused source flux errors which vary as a function of radius to nearest pointing centre in the VLSS.
A radio counterpart to a neutron star merger Hallinan, G.; Corsi, A.; Mooley, K. P. ...
Science (American Association for the Advancement of Science),
12/2017, Letnik:
358, Številka:
6370
Journal Article
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Gravitational waves have been detected from a binary neutron star merger event, GW170817. The detection of electromagnetic radiation from the same source has shown that the merger occurred in the ...outskirts of the galaxy NGC 4993, at a distance of 40 megaparsecs from Earth. We report the detection of a counterpart radio source that appears 16 days after the event, allowing us to diagnose the energetics and environment of the merger. The observed radio emission can be explained by either a collimated ultrarelativistic jet, viewed off-axis, or a cocoon of mildly relativistic ejecta. Within 100 days of the merger, the radio light curves will enable observers to distinguish between these models, and the angular velocity and geometry of the debris will be directly measurable by very long baseline interferometry.
The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution ( 2 5), sensitivity (a 1 goal of 70 Jy/beam in the coadded data), ...full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2-4 GHz). The first observations began in 2017 September, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hr of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (decl. > −40°), a total of 33 885 deg 2 . The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an "on the fly" interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations.
Abstract
We present radio and X-ray studies of A3444 and MS1455.0+2232, two galaxy clusters with radio minihalos in their cool cores. A3444 is imaged using the Giant Metrewave Radio Telescope (GMRT) ...at 333, 607, and 1300 MHz and the Very Large Array at 1435 MHz. Most of the minihalo is contained within
r
< 120 kpc, but a fainter extension, stretching out to 380 kpc southwest of the center, is detected at 607 MHz. Using Chandra, we detect four X-ray sloshing cold fronts: three in the cool core at
r
= 60, 120, and 230 kpc, and a fourth one at
r
= 400 kpc—in the region of the southwestern radio extension—suggesting that the intracluster medium (ICM) is sloshing on a cluster-wide scale. The radio emission is contained within the envelope defined by these fronts. We also analyzed archival 383 MHz GMRT and Chandra observations of MS 1455.0+2232, which exhibits a known minihalo with its bright part delineated by cold fronts inside the cool core, but with a faint extension beyond the core. Similarly to A3444, we find a cold front at
r
∼ 425 kpc, containing the radio emission. Thus the entire diffuse radio emission seen in these clusters appears to be related to large-scale sloshing of the ICM. The radio spectrum of the A3444 minihalo is a power law with a steep index
α
= 1.0 ± 0.1. The spectrum steepens with increasing distance from the center, as expected if the minihalo originates from reacceleration of relativistic particles by the sloshing-induced turbulence in the ICM.
Cluster mergers leave distinct signatures in the intracluster medium (ICM) in the form of shocks and diffuse cluster radio sources that provide evidence for the acceleration of relativistic ...particles. However, the physics of particle acceleration in the ICM is still not fully understood. Here we present new 1-4 GHz Jansky Very Large Array (VLA) and archival Chandra observations of the HST Frontier Fields Cluster Abell 2744. In our new VLA images, we detect the previously known ∼2.1 Mpc radio halo and ∼1.5 Mpc radio relic. We carry out a radio spectral analysis from which we determine the relic's injection spectral index to be . This corresponds to a shock Mach number of under the assumption of diffusive shock acceleration. We also find evidence for spectral steepening in the post-shock region. We do not find evidence for a significant correlation between the radio halo's spectral index and ICM temperature. In addition, we observe three new polarized diffuse sources and determine two of these to be newly discovered giant radio relics. These two relics are located in the southeastern and northwestern outskirts of the cluster. The corresponding integrated spectral indices measure −1.81 0.26 and −0.63 0.21 for the SE and NW relics, respectively. From an X-ray surface brightness profile we also detect a possible density jump of co-located with the newly discovered SE relic. This density jump would correspond to a shock front Mach number of .
We present a multiwavelength study of the massive galaxy cluster Abell 1763 at redshift z = 0.231. Image analysis of a 19.6 ks Chandra archival observation reveals a cluster-wide spiral of enhanced ...surface brightness in the intracluster medium (ICM). While such spirals are understood to form in clusters with sloshing strong cool cores (SCCs), the gas comprising the spiral's apex is of intermediate entropy (∼110 keV cm2) and cooling time (∼6.8 Gyr), indicating core disruption is occurring throughout the spiral formation process. Two subclusters dominated by the second- and third-ranked galaxies in the system lie along a line parallel to the elongation axis of the primary cluster's ICM. Both subsystems appear to have fallen in along a previously discovered intercluster filament and are each considered candidates as the perturber responsible for initiating disruptive core sloshing. Dynamical analysis indicates infall is occurring with a relative radial velocity of ∼1800 km s−1. The brightest cluster galaxy of Abell 1763 possesses a high line-of-sight peculiar velocity (vpec ∼ 650 km s−1) and hosts a powerful (P1.4 ∼ 1026 W Hz−1) bent double-lobed radio source, likely shaped by the relative bulk ICM flow induced in the merger. The cluster merger model of SCC destruction invokes low impact parameter infall as the condition required for core transformation. In contrast to this, the high angular momentum event occurring in Abell 1763 suggests that off-axis mergers play a greater role in establishing the non-cool core cluster population than previously assumed.
ABSTRACT We present results from a very deep (650 ks) Chandra X-ray observation of the galaxy group NGC 5813, the deepest Chandra observation of a galaxy group to date. This system uniquely shows ...three pairs of collinear cavities, with each pair associated with an unambiguous active galactic nucleus (AGN) outburst shock front. The implied mean kinetic power is roughly the same for each outburst, demonstrating that the average AGN kinetic luminosity can remain stable over long timescales (∼50 Myr). The two older outbursts have larger, roughly equal total energies as compared with the youngest outburst, implying that the youngest outburst is ongoing. We find that the gas radiative cooling rate and mean shock heating rate are well balanced at each shock front, suggesting that shock heating alone is sufficient to offset cooling and establish AGN/intracluster medium (ICM) feedback within at least the central 30 kpc. This heating takes place roughly isotropically and most strongly at small radii, as is required for feedback to operate. We suggest that shock heating may play a significant role in AGN feedback at smaller radii in other systems, where weak shocks are more difficult to detect. We find non-zero shock front widths that are too large to be explained by particle diffusion. Instead, all measured widths are consistent with shock broadening due to propagation through a turbulent ICM with a mean turbulent speed of ∼70 km s−1. Finally, we place lower limits on the temperature of any volume-filling thermal gas within the cavities that would balance the internal cavity pressure with the external ICM.
We report the results of a multiwavelength study of the nearby galaxy group, Abell 3581 (z = 0.0218). This system hosts the most luminous cool core of any nearby group and exhibits active radio mode ...feedback from the supermassive black hole in its brightest group galaxy, IC 4374. The brightest galaxy has suffered multiple active galactic nucleus outbursts, blowing bubbles into the surrounding hot gas, which have resulted in the uplift of cool ionized gas into the surrounding hot intragroup medium. High velocities, indicative of an outflow, are observed close to the nucleus and coincident with the radio jet. Thin dusty filaments accompany the uplifted, ionized gas. No extended star formation is observed; however, a young cluster is detected just north of the nucleus. The direction of rise of the bubbles has changed between outbursts. This directional change is likely due to sloshing motions of the intragroup medium. These sloshing motions also appear to be actively stripping the X-ray cool core, as indicated by a spiralling cold front of high-metallicity, low-temperature, low entropy gas.
Abstract
The study of stripped-envelope core-collapse supernovae (SNe), with evidence for strong interaction of SN ejecta with the circumstellar medium (CSM), provides insights into the pre-supernova ...progenitor, and a fast-forwarded view of the progenitor mass-loss history. In this context, we present late-time radio observations of SN 2004dk, a Type Ibc supernova located in the galaxy NGC 6118, at a distance of
d
L
≈ 23 Mpc. About 10 yr after explosion, SN 2004dk has shown evidence for H
α
emission, possibly linked to the SN ejecta interacting with a H-rich CSM. Using data from the VLA Low Band Ionosphere and Transient Experiment (VLITE), we confirm the presence of a late-time radio rebrightening accompanying the observed H
α
emission. We model the SN 2004dk radio light curves within the (spherically symmetric) synchrotron-self-absorption (SSA) model. Within this model, our VLITE observations combined with previously collected VLA data favor an interpretation of SN 2004dk as a strongly CSM-interacting radio SN going through a complex environment shaped by nonsteady mass loss from the SN progenitor.
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