D. Rafferty

LOFAR, VLA, and Chandra observations of the Toothbrush galaxy cluster

We present deep LOFAR observations between 120 and 181 MHz of the Toothbrush (RX J0603.3+ 4214), a cluster that contains one of the brightest radio relic sources known. Our LOFAR observations exploit a new and novel calibration scheme to probe 10 times deeper than any previous study in this relatively unexplored part of the spectrum. The LOFAR observations, when combined with VLA, GMRT, and Chandra X-ray data, provide new information about the nature of cluster merger shocks and their role in re-accelerating relativistic particles. We derive a spectral index of alpha = -0.8 +/- 0.1 at the northern edge of the main radio relic, steepening toward the south to alpha approximate to-2. The spectral index of the radio halo is remarkably uniform (alpha = -1.16, with an intrinsic scatter of <= 0.04). The observed radio relic spectral index gives a Mach number of M = 2.8(-0.3)(+0.5), assuming diffusive shock acceleration. However, the gas density jump at the northern edge of the large radio relic implies a much weaker shock (M approximate to 1.2, with an upper limit of M approximate to 1.5). The discrepancy between the Mach numbers calculated from the radio and X-rays can be explained if either (i) the relic traces a complex shock surface along the line of sight, or (ii) if the radio relic emission is produced by a re-accelerated population of fossil particles from a radio galaxy. Our results highlight the need for additional theoretical work and numerical simulations of particle acceleration and re-acceleration at cluster merger shocks.

The LOFAR Two-metre Sky Survey - I. Survey description and preliminary data release

The LOFAR Two-metre Sky Survey (LoTSS) is a deep 120-168 MHz imaging survey that will eventually cover the entire northern sky. Each of the 3170 pointings will be observed for 8 h, which, at most declinations, is sufficient to produce ~5? resolution images with a sensitivity of ~100 ?Jy/beam and accomplish the main scientific aims of the survey, which are to explore the formation and evolution of massive black holes, galaxies, clusters of galaxies and large-scale structure. Owing to the compact core and long baselines of LOFAR, the images provide excellent sensitivity to both highly extended and compact emission. For legacy value, the data are archived at high spectral and time resolution to facilitate subarcsecond imaging and spectral line studies. In this paper we provide an overview of the LoTSS. We outline the survey strategy, the observational status, the current calibration techniques, a preliminary data release, and the anticipated scientific impact. The preliminary images that we have released were created using a fully automated but direction-independent calibration strategy and are significantly more sensitive than those produced by any existing large-Area low-frequency survey. In excess of 44 000 sources are detected in the images that have a resolution of 25?, typical noise levels of less than 0.5 mJy/beam, and cover an area of over 350 square degrees in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00?00? to 57°00?00?).

LOFAR: Early imaging results from commissioning for Cygnus A

The Low Frequency Array (LOFAR) will operate between 10 and 250 MHz, and will observe the low frequency Universe to an unprecedented sensitivity and angular resolution. The construction and commissioning of LOFAR is well underway, with over 27 of the Dutch stations and five International stations routinely performing both single-station and interferometric observations over the frequency range that LOFAR is anticipated to operate at. Here, we summarize the capabilities of LOFAR and report on some of the early commissioning imaging of Cygnus A.

LOFAR discovery of an ultra-steep radio halo and giant head–tail radio galaxy in Abell 1132

Low-Frequency Array (LOFAR) observations at 144 MHz have revealed large-scale radio sources in the unrelaxed galaxy cluster Abell 1132. The cluster hosts diffuse radio emission on scales of similar to 650 kpc near the cluster centre and a head-tail (HT) radio galaxy, extending up to 1 Mpc, south of the cluster centre. The central diffuse radio emission is not seen in NRAO VLA FIRST Survey, Westerbork Northern Sky Survey, nor in C & D array VLA observations at 1.4 GHz, but is detected in our follow-up Giant Meterwave Radio Telescope (GMRT) observations at 325 MHz. Using LOFAR and GMRT data, we determine the spectral index of the central diffuse emission to be alpha = -1.75 +/- 0.19 (S alpha nu(alpha) ). We classify this emission as an ultra-steep spectrum radio halo and discuss the possible implications for the physical origin of radio haloes. The HT radio galaxy shows narrow, collimated emission extending up to 1 Mpc and another 300 kpc of more diffuse, disturbed emission, giving a full projected linear size of 1.3 Mpc - classifying it as a giant radio galaxy (GRG) and making it the longest HT found to date. The head of the GRG coincides with an elliptical galaxy (SDSS J105851.01+564308.5) belonging to Abell 1132. In our LOFAR image, there appears to be a connection between the radio halo and the GRG. The turbulence that may have produced the halo may have also affected the tail of the GRG. In turn, the GRG may have provided seed electrons for the radio halo.

Studying the late evolution of a radio-loud AGN in a galaxy group with LOFAR

Feedback by radio-loud active galactic nuclei (AGNs) in galaxy groups is not fully understood. Open questions include the duty cycle of the AGN, the spatial extent of the radio lobes, the effect they have on the intragroup medium and the fate of the cosmic rays. We present the discovery of a 650 kpc-radio galaxy embedded in steep diffuse emission at z=(0.18793 +/- 5) x 10(-5) located at the centre of the galaxy group MaxBCG J199.31832+51.72503 using an observation from the LOFAR Two-metre Sky Survey (LoTSS) at the central frequency of 144 MHz. Subsequently, we performed a Giant Meterwave Radio Telescope observation at the central frequency of 607 MHz to study the spectral properties of the source. The observations reveal a radio galaxy with a total radio power P-tot,P- 1.4 similar to 2.1 x 10(24) W Hz(-1), exhibiting two asymmetrical jets and lobes. The derived spectral index map shows a steepening towards the inner regions and a steep-spectrum core region. We model the integrated radio spectrum, providing two possible interpretations: the radio source is evolved but still active or it is just at the end of its active phase. Finally, in the same field of view we have discovered Mpc-sized emission surrounding a close pair of AGN located at a redshift z = (0.0587 +/- 2) x 10(-4) (SDSS J131544.56+521213.2 and SDSS J131543.99+521055.7) which could be a radio remnant source.

The Cocoon Shocks of Cygnus A: Pressures and Their Implications for the Jets and Lobes

We use 2.0 Msec of Chandra observations to investigate the cocoon shocks of Cygnus A and some implications for its lobes and jet. Measured shock Mach numbers vary in the range 1.18–1.66 around the cocoon. We estimate a total outburst energy of

LOFAR discovery of radio emission in MACS J0717.5+3745

simeq 4.7times {10}^{60},mathrm{erg}

LOFAR MSSS: The scaling relation between AGN cavity power and radio luminosity at low radio frequencies

, with an age of

The X-ray Ribs Within the Cocoon Shock of Cygnus A

simeq 2times {10}^{7},mathrm{years}

Search for low-frequency diffuse radio emission around a shock in the massive galaxy cluster MACS J0744.9+3927

. The average postshock pressure is found to be