MHz VLA Observations Research Articles

The peculiar cluster MACS J0417.5-1154 in the C and X-bands

We present 5.5 and 9.0 GHz Australia Telescope Compact Array (ATCA) observations of the cluster MACSJ0417.5-1154, one of the most massive galaxy clusters and one of the brightest in X-ray in the Massive Cluster Survey (MACS). We estimate diffuse emission at 5.5 and 9.0 GHz from our ATCA observations, and compare the results with the 235 MHz and 610 MHz GMRT observations and 1575 MHz VLA observations. We also estimate the diffuse emission at low frequencies from existing GLEAM survey data (using the MWA telescope), and find that the steepening reported in earlier studies may have been an artefact of underestimates of diffuse emission at low frequencies. High-frequency radio observations of galaxy cluster mergers therefore provide an important complement to low-frequency observations, not only for a probing the `on' and `off' state of radio halos in these mergers, but also to constrain energetics of cluster mergers. We comment on the future directions that further studies of this cluster can take.

Deep 230–470 MHz VLA observations of the mini-halo in the Perseus cluster

We present a low-frequency view of the Perseus cluster with new observations from the Karl G. Jansky Very Large Array (JVLA) at 230-470 MHz. The data reveal a multitude of new structures associated with the mini-halo. The mini-halo seems to be influenced both by the AGN activity as well as by the sloshing motion of the cool core cluster's gas. In addition, it has a filamentary structure similar to that seen in radio relics found in merging clusters. We present a detailed description of the data reduction and imaging process of the dataset. The depth and resolution of the observations allow us to conduct for the first time a detailed comparison of the mini-halo structure with the X-ray structure as seen in the Chandra X-ray images. The resulting image shows very clearly that the mini-halo emission is mostly contained behind the cold fronts, similar to that predicted by simulations of gas sloshing in galaxy clusters. However, due to the proximity of the Perseus cluster, as well as the quality of the data at low radio frequencies and at X-ray wavelengths, we also find evidence of fine structure. This structure includes several radial radio filaments extending in different directions, a concave radio structure associated with the southern X-ray bay and sharp edges that correlate with X-ray edges. Mini-halos are therefore not simply diffuse, uniform radio sources, but are rather filled with a rich variety of complex structures. These results illustrate the high-quality images that can be obtained with the new JVLA at low radio-frequencies, as well as the necessity to obtain deeper, higher-fidelity radio images of mini-halos and halos in clusters to further understand their origin.

Detection of very-high-energyγ-ray emission from the vicinity of PSR B1706–44 and G 343.1–2.3 with H.E.S.S.

The gamma-ray pulsar PSR B1706-44 and the adjacent supernova remnant (SNR) candidate G343.1-2.3 were observed by H.E.S.S. during a dedicated observation campaign in 2007. As a result of this observation campaign, a new source of very-high-energy (VHE; E > 100 GeV) gamma-ray emission, HESS J1708-443, was detected with a statistical significance of 7 sigma, although no significant point-like emission was detected at the position of the energetic pulsar itself. In this paper, the morphological and spectral analyses of the newly-discovered TeV source are presented. The centroid of HESS J1708-443 is considerably offset from the pulsar and located near the apparent center of the SNR, at RA(J2000) = 17h08m11s +/- 17s and Dec(J2000) = -44d20' +/- 4'. The source is found to be significantly more extended than the H.E.S.S. point spread function (~0.1 deg), with an intrinsic Gaussian width of 0.29 deg +/- 0.04 deg. Its integral flux between 1 and 10 TeV is ~ 3.8 x 10^-12 ph cm^-2 s^-1, equivalent to 17% of the Crab Nebula flux in the same energy range. The measured energy spectrum is well-fit by a power law with a relatively hard photon index Gamma = 2.0 +/- 0.1 (stat) +/- 0.2 (sys). Additional multi-wavelength data, including 330 MHz VLA observations, were used to investigate the VHE gamma-ray source's possible associations with the pulsar wind nebula of PSR B1706-44 and/or with the complex radio structure of the partial shell-type SNR G343.1-2.3.

A New Radio Detection of the Transient Bursting Source GCRT J1745−3009

GCRT J1745-3009 is a transient bursting radio source located in the direction of the Galactic center, discovered by Hyman et al. in 330 MHz VLA observations from 2002 September 30-October 1. We have searched for bursting activity from GCRT J1745-3009 in nearly all of the available 330 MHz VLA observations of the Galactic center since 1989, as well as in 2003 Giant Metrewave Radio Telescope (GMRT) observations. We report a new radio detection of the source in 330 MHz GMRT data taken on 2003 September 28. Due to sparse sampling of the observation, only a single burst, already at or passed its peak, was detected. The maximum flux density detected was ~0.5 Jy, approximately 3 times weaker than the five bursts detected in 2002. We present tentative evidence suggesting that this burst is an isolated one, but we cannot rule out additional undetected bursts that may have occurred with the same ~77 minute periodicity observed in 2002 or with a different periodicity. Assuming the peak emission was detected, the decay time of the burst, ~2 minutes, is consistent with that determined for the 2002 bursts. Based on the total time for which we have observations, we estimate that the source exhibits bursting activity roughly 9% of the time.

A Search for Radio Transients at 0.33 GHz in the GC

We report on a search for transient and variable radio sources in the Galactic center using a number of 327 MHz VLA observations made during the 1990's, and a series of monthly VLA observations made during Spring and Summer 2002. A typical yield of compact sources in a given epoch is roughly 200. We have detected one new bright radio transient, GCRT J1746–2757, located only 1.1 degrees north of the Galactic center. We discuss our on-going transient monitoring program and the implications of this work for constraining the Galactic center population of transients.

The Radio Spectral Index of 3C58

We report on 74 MHz and 330 MHz VLA observations of the plerionic supernova remnant 3C58. The radio spectral index, α, is uniform across the remnant to within 0.18, indicating a single source for the accelerated electrons. We find no emission from any shell surrounding the remnant. Our 3σ brightness limit on shell emission surrounding 3C58 is < 1.3×10−21 W m−2 Hz−1 sr−1 at 330 MHz.

VLA Images of Two Extended Radio Galaxies

There is some evidence from earlier studies that the two sources 0235 — 197 and 1203 + 043 exhibit low frequency (< 1 GHz) variability. This work shows that both sources have linear polarizations, if any, below the detection limits at 320 MHz, so we cannot explain the variability as being due to instrumental polarization effects as has been suggested for 3C159. Refractive scintillation may be the cause of the variability in 0235—197. The radio source 1203+043 lacks any bright compact component thereby ruling out a refractive scintillation mechanism for its variability. Consequently, it is possible that claims of variability in this source are spurious. However, the 320 MHz VLA observations show that 1203+043 has an ‘X'-shaped radio structure.

Low‐Frequency VLA Observations of Abell 754: Evidence for a Cluster Radio Halo and Possible Radio Relics

We present 74 MHz and 330 MHz VLA observations of Abell 754. Diffuse, halo-like emission is detected from the center of the cluster at both frequencies. At 330 MHz the resolution of 90'' distinguishes this extended emission from previously known point sources. In addition to the halo and at a much lower level, outlying steep-spectrum emission regions straddle the cluster center and are seen only at 74 MHz. The location, morphology and spectrum of this emission are all highly suggestive of at least one, and possibly two cluster radio relics. Easily obtained higher resolution, higher sensitivity VLA observations at both frequencies are required to confirm the extended nature of the halo-like emission and the 74 MHz relic detections. However, since there is prior evidence that this cluster is or has recently been in the process of a major merger event, the possible discovery of relics in this system is of great interest in light of recent observational and theoretical evidence in favor of a merger-relic connection. We discuss the possible role the merger shock waves, which are seen in the X-ray emission, may have played in the formation of the halo and radio relics in A754.

VLA polarimetry of two extended radio galaxies

Multi-wavelength VLA observations of two extended radio galaxies, 0235-197 and 1203+043 are presented. There is some evidence from earlier studies that these two sources exhibit low frequency (<1 GHz) variability. This work shows that both sources have linear polarizations, if any, below the detection limits at 320 MHz, so we cannot explain the variability as being due to instrumental polarization effects as has been suggested for 3C159. Refractive scintillation may be the cause of the variability in 0235-197. This would require the existence of a bright, compact component in one of the hot spots seen in these observations. This is not implausible but the resolution of this observational program is insufficent to address that question. The radio source 1203+043 lacks any bright compact component thereby ruling out a refractive scintillation mechanism for its variability. Consequently, it is possible that claims of variability in this source are spurious. However, the 320 MHz VLA observations show that 1203+043 has an `X'-shaped radio structure. This is a rare morphology for the brightness distribution of a radio galaxy; the implications of this are examined.

High Rydberg State Carbon Recombination Lines toward Cassiopeia A: 332 MHz VLA Observations and Comparison with H i and Molecular Lines: Erratum

High Rydberg State Carbon Recombination Lines toward Cassiopeia A: 332 MHz VLA Observations and Comparison with H i and Molecular Lines: Erratum

High Rydberg state carbon recombination lines toward Cassiopeia A: 332 MHz VLA observations and comparison with H I and molecular lines

We present observations of the C270alpha carbon recombination line, 21 cm neutral hydrogen line and (CO)-C-12 (J = 1 --> 0) molecular line toward Cas A. A comparison of the distribution of recombination line optical depths over the face of Cas A with that of H I optical depths and molecular line emission favors the association of C270alpha regions with H I rather than molecular clouds. The association makes it possible to self-consistently determine several physical parameters of the clouds by combining the recombination line and 21 cm H I measurements.