Abstract:Diffuse, non-thermal emission in galaxy clusters is increasingly being detected in low-frequency radio surveys and images. We present a new diffuse, steep-spectrum, non-thermal radio source within the cluster Abell 1127 found in survey data from the Murchison Widefield Array (MWA). We perform follow-up observations with the ‘extended’ configuration MWA Phase II with improved resolution to better resolve the source and measure its low-frequency spectral properties. We use archival Very Large Array S-band data t… Show more
“…Observation details are presented in Table 1. The MWA data are processed following Duchesne et al (2020). Briefly, MWA data are observed in a 2-min snapshot observing mode, with each 2-min snapshot calibrated and imaged independently and stacked in the image plane at the end. Figure 2. Composite image of Abell 3404.…”
Section: Methodsmentioning
confidence: 99%
“…CLEANing more deeply). After initial primary beam correction using the most recent Full-Embedded Element model (Sokolowski et al, 2017), these CLEANed images are then corrected for ionosphere-related astrometric shifts using d (Hurley-Walker & Hancock, 2018) and finally corrected for residual primary beam errors and flux scale errors with e (Duchesne et al, 2020) to ensure a common flux scaling across snapshots. The snapshots are then co-added, weighted by the square of the primary beam response and local noise.…”
Section: Methodsmentioning
confidence: 99%
“…As there is only a single primary beam, mosaicking is not required; however, a primary beam correction is applied. Because the source lies to the edge of the beam, we compare the ASKAP flux densities of sources across the image to extrapolated flux densities derived from the ATCA subband images and the catalogue used for MWA-2 flux scaling (independently, see Duchesne et al, 2020 for details of the calibration catalogue). For comparison with the MWA catalogue, the data are first convolved to a common resolution.…”
We report on the detection of a giant radio halo in the cluster Abell 3404 as well as confirmation of the radio halo observed in Abell 141 (with linear extents
$\sim\!770$
and
$\sim\!850$
kpc, respectively). We use the Murchison Widefield Array, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to characterise the emission and intervening radio sources from
$\sim100$
to 1 000 MHz; power law models are fit to the spectral energy distributions with spectral indices
$\alpha_{88}^{1\,110} = -1.66 \pm 0.07$
and
$\alpha_{88}^{943} = -1.06 \pm 0.09$
for the radio halos in Abell 3404 and Abell 141, respectively. We find strong correlation between radio and X-ray surface brightness for Abell 3404 but little correlation for Abell 141. We note that each cluster has an atypical morphology for a radio-halo-hosting cluster, with Abell 141 having been previously reported to be in a pre-merging state, and Abell 3404 is largely relaxed with only minor evidence for a disturbed morphology. We find that the radio halo powers are consistent with the current radio halo sample and
$P_\nu$
–M scaling relations, but note that the radio halo in Abell 3404 is an ultra-steep–spectrum radio halo (USSRH) and, as with other USSRHs lies slightly below the best-fit
$P_{1.4}$
–M relation. We find that an updated scaling relation is consistent with previous results and shifting the frequency to 150 MHz does not significantly alter the best-fit relations with a sample of 86 radio halos. We suggest that the USSRH halo in Abell 3404 represents the faint class of radio halos that will be found in clusters undergoing weak mergers.
“…Observation details are presented in Table 1. The MWA data are processed following Duchesne et al (2020). Briefly, MWA data are observed in a 2-min snapshot observing mode, with each 2-min snapshot calibrated and imaged independently and stacked in the image plane at the end. Figure 2. Composite image of Abell 3404.…”
Section: Methodsmentioning
confidence: 99%
“…CLEANing more deeply). After initial primary beam correction using the most recent Full-Embedded Element model (Sokolowski et al, 2017), these CLEANed images are then corrected for ionosphere-related astrometric shifts using d (Hurley-Walker & Hancock, 2018) and finally corrected for residual primary beam errors and flux scale errors with e (Duchesne et al, 2020) to ensure a common flux scaling across snapshots. The snapshots are then co-added, weighted by the square of the primary beam response and local noise.…”
Section: Methodsmentioning
confidence: 99%
“…As there is only a single primary beam, mosaicking is not required; however, a primary beam correction is applied. Because the source lies to the edge of the beam, we compare the ASKAP flux densities of sources across the image to extrapolated flux densities derived from the ATCA subband images and the catalogue used for MWA-2 flux scaling (independently, see Duchesne et al, 2020 for details of the calibration catalogue). For comparison with the MWA catalogue, the data are first convolved to a common resolution.…”
We report on the detection of a giant radio halo in the cluster Abell 3404 as well as confirmation of the radio halo observed in Abell 141 (with linear extents
$\sim\!770$
and
$\sim\!850$
kpc, respectively). We use the Murchison Widefield Array, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to characterise the emission and intervening radio sources from
$\sim100$
to 1 000 MHz; power law models are fit to the spectral energy distributions with spectral indices
$\alpha_{88}^{1\,110} = -1.66 \pm 0.07$
and
$\alpha_{88}^{943} = -1.06 \pm 0.09$
for the radio halos in Abell 3404 and Abell 141, respectively. We find strong correlation between radio and X-ray surface brightness for Abell 3404 but little correlation for Abell 141. We note that each cluster has an atypical morphology for a radio-halo-hosting cluster, with Abell 141 having been previously reported to be in a pre-merging state, and Abell 3404 is largely relaxed with only minor evidence for a disturbed morphology. We find that the radio halo powers are consistent with the current radio halo sample and
$P_\nu$
–M scaling relations, but note that the radio halo in Abell 3404 is an ultra-steep–spectrum radio halo (USSRH) and, as with other USSRHs lies slightly below the best-fit
$P_{1.4}$
–M relation. We find that an updated scaling relation is consistent with previous results and shifting the frequency to 150 MHz does not significantly alter the best-fit relations with a sample of 86 radio halos. We suggest that the USSRH halo in Abell 3404 represents the faint class of radio halos that will be found in clusters undergoing weak mergers.
“…Source position offsets caused by ionospheric effects and the errors in the absolute flux density calibration were corrected using FITS_WARP b ) and FLUX_WARP c (Duchesne et al 2020), which apply corrections to MWA images b https://github.com/nhurleywalker/fits_warp. c https://gitlab.com/Sunmish/flux_warp.…”
Many short gamma-ray bursts (GRBs) originate from binary neutron star mergers, and there are several theories that predict the production of coherent, prompt radio signals either prior, during, or shortly following the merger, as well as persistent pulsar-like emission from the spin-down of a magnetar remnant. Here we present a low frequency (170–200 MHz) search for coherent radio emission associated with nine short GRBs detected by the Swift and/or Fermi satellites using the Murchison Widefield Array (MWA) rapid-response observing mode. The MWA began observing these events within 30–60 s of their high-energy detection, enabling us to capture any dispersion delayed signals emitted by short GRBs for a typical range of redshifts. We conducted transient searches at the GRB positions on timescales of 5 s, 30 s, and 2 min, resulting in the most constraining flux density limits on any associated transient of 0.42, 0.29, and 0.084 Jy, respectively. We also searched for dispersed signals at a temporal and spectral resolution of 0.5 s and 1.28 MHz, but none were detected. However, the fluence limit of 80–100 Jy ms derived for GRB 190627A is the most stringent to date for a short GRB. Assuming the formation of a stable magnetar for this GRB, we compared the fluence and persistent emission limits to short GRB coherent emission models, placing constraints on key parameters including the radio emission efficiency of the nearly merged neutron stars (
$\epsilon_r\lesssim10^{-4}$
), the fraction of magnetic energy in the GRB jet (
$\epsilon_B\lesssim2\times10^{-4}$
), and the radio emission efficiency of the magnetar remnant (
$\epsilon_r\lesssim10^{-3}$
). Comparing the limits derived for our full GRB sample (along with those in the literature) to the same emission models, we demonstrate that our fluence limits only place weak constraints on the prompt emission predicted from the interaction between the relativistic GRB jet and the interstellar medium for a subset of magnetar parameters. However, the 30-min flux density limits were sensitive enough to theoretically detect the persistent radio emission from magnetar remnants up to a redshift of
$z\sim0.6$
. Our non-detection of this emission could imply that some GRBs in the sample were not genuinely short or did not result from a binary neutron star merger, the GRBs were at high redshifts, these mergers formed atypical magnetars, the radiation beams of the magnetar remnants were pointing away from Earth, or the majority did not form magnetars but rather collapse directly into black holes.
“…These sources are referred to as 'phoenices' in the literature and, despite the first discovery two decades ago (Slee et al 2001), they are only now starting to be unveiled in larger numbers thanks to radio observations below a few hundred MHz (e.g. Nuza et al 2017;Kale et al 2018;Duchesne et al 2020;Mandal et al 2020).…”
Jets from active galactic nuclei (AGN) are known to recurrently enrich their surrounding medium with mildly-relativistic particles and magnetic fields. Here, we present a detailed multi-frequency analysis of the nearby (z=0.01646) galaxy group NGC 507. In particular, we present new high-sensitivity and high spatial resolution radio images in the frequency range 144-675 MHz obtained using LOFAR and uGMRT observations. These reveal the presence of previously undetected diffuse radio emission with complex, filamentary morphology, likely related to a previous outburst of the central galaxy. Based on spectral ageing considerations, we derived that the plasma was first injected by the AGN 240-380 Myr ago and is now cooling. Our analysis of deep archival XMM-Newton data confirms that the system is dynamically disturbed, as previously suggested. We detect two discontinuities in the X-ray surface brightness distribution (in East and South direction) tracing a spiral pattern, which we interpret as cold fronts produced by sloshing motions. The remarkable spatial coincidence observed between the newly-detected arc-like radio filament and the southern concave X-ray discontinuity strongly suggests that the remnant plasma has been displaced by the sloshing motions on large scales. Overall, NGC 507 represents one of the clearest examples known to date in which a direct interaction between old AGN remnant plasma and the external medium is observed in a galaxy group. Our results are consistent with simulations, which suggest that filamentary emission can be created by the cluster/group weather disrupting AGN lobes and spreading their relativistic content into the surrounding medium.
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