Optimum estimate of delays and dispersive effects in low-frequency interferometric observations

Modern radio interferometers sensitive to low frequencies will make use of wide-band detectors. For such wide bandwidths, dispersive atmospheric effects introduce variations in the fringe delay which change through the band of the receivers. These undesired dispersive effects must be estimated and calibrated with the highest precision. We studied the achievable precision in the estimate of the ionospheric dispersion and the dynamic range of the correlated fringes for different distributions of sub-bands in low-frequency and wide-band interferometric observations. Our study is focused on the case of sub-bands with a bandwidth much narrower than that of the total covered spectrum (case of LOFAR). We computed the uncertainty of the ionospheric delay, the delay ambiguity, and the dynamic range of the fringes using four different kinds of sub-band distributions: constant spacing between sub-bands, random spacings, spacings based on a power-law distribution, and spacings based on Golomb rulers (sets of integers whose sets of differences have non-repeated elements). For a large number of sub-bands ($> 20$, depending on the delay window) spacings based on Golomb rulers give the most precise estimates of dispersive effects and the highest fringe dynamic ranges. Spacings based on the power-law distribution give similar results, although better than those with the Golomb rulers for smaller number of sub-bands. Random distributions result in large fringe dynamic ranges, but the estimate of dispersive effects is worse. A constant spacing of sub-bands results in very bad fringe dynamic ranges, but good estimates of ionospheric dispersion. Combining all the results, the power-law distribution gives the best compromise between homogeneity in the bandwidth sampling, precision in the estimate of ionospheric effects, dynamic range of the correlated fringes, and group-delay ambiguity.Comment: 8 pages, 7 figures. Accepted for publication in A&

A GMRT 150 MHz search for variables and transients in Stripe 82

We have carried out a dedicated transient survey of 300 deg2 of the SDSS Stripe 82 region using the Giant Metrewave Radio Telescope (GMRT) at 150 MHz. Our multi-epoch observations, together with the TGSS survey, allow us to probe variability and transient activity on four different time-scales, beginning with 4 h and up to 4 yr. Data calibration, RFI flagging, source finding, and transient search were carried out in a semi-automated pipeline incorporating the SPAM recipe. This has enabled us to produce superior-quality images and carry out reliable transient search over the entire survey region in under 48 h post-observation. Among the few thousand unique point sources found in our 5σ single-epoch catalogues (flux density thresholds of about 24, 20, 16, and 18 mJy on the respective time-scales), we find <0.08 per cent, 0.01 per cent, <0.06 per cent, and 0.05 per cent to be variable (beyond a significance of 4σ and fractional variability of 30 per cent) on time-scales of 4 h, 1 d, 1 month, and 4 yr, respectively. This is substantially lower than that in the GHz sky, where ∼1 per cent of the persistent point sources are found to be variable. Although our survey was designed to probe a superior part of the transient phase space, our transient search did not yield any significant candidates. The transient (preferentially extragalactic) rate at 150 MHz is therefore <0.005 on time-scales of 1 month and 4 yr, and <0.002 on time-scales of 1 d and 4 h, beyond 7σ detection threshold. We put these results in perspective with the previous studies and give recommendations for future low-frequency transient surveys

Reduced Ambiguity Calibration for LOFAR

Interferometric calibration always yields non unique solutions. It is therefore essential to remove these ambiguities before the solutions could be used in any further modeling of the sky, the instrument or propagation effects such as the ionosphere. We present a method for LOFAR calibration which does not yield a unitary ambiguity, especially under ionospheric distortions. We also present exact ambiguities we get in our solutions, in closed form. Casting this as an optimization problem, we also present conditions for this approach to work. The proposed method enables us to use the solutions obtained via calibration for further modeling of instrumental and propagation effects. We provide extensive simulation results on the performance of our method. Moreover, we also give cases where due to degeneracy, this method fails to perform as expected and in such cases, we suggest exploiting diversity in time, space and frequency.Comment: Draft version. Final version published on 10 April 201

The Curious Case of Abell 2256

Abell 2256 is a rich, nearby (z=0.0594) galaxy cluster that has significant evidence of merger activity. We present new radio and X-ray observations of this system. The low-frequency radio images trace the diffuse synchrotron emission of the Mpc-scale radio halo and relics as well as a number of recently discovered, more compact, steep spectrum sources. The spectral index across the relics steepens from the north-west toward the south-east. Analysis of the spectral index gradients between low and and high-frequencies shows spectral differences away from the north-west relic edge such that the low-frequency index is significantly flatter than the high frequency spectral index near the cluster core. This trend would be consistent with an outgoing merger shock as the origin of the relic emission. New X-ray data from XMM-Newton reveal interesting structures in the intracluster medium pressure, entropy and temperature maps. The pressure maps show an overall low pressure core co-incident with the radio halo emission, while the temperature maps reveal multiple regions of cool emission within the central regions of Abell 2256. The two cold fronts in Abell 2256 both appear to have motion in similar directions.Comment: 4 pages, 3 figures, proceedings of contributed talk at "Non-Thermal Phenomena in Colliding Galaxy Clusters", held in Nice, 15-18 November 2010. To be published in Mem. S.A.I

Signatures of multiple episodes of AGN activity in the core of Abell 1795

In this paper we analyze AGN activity signatures in the rich nearby galaxy cluster Abell 1795 aiming to confirm and characterize the long-term feedback history in the system. We combine radio observations at 610 and 235 MHz from the Giant Metrewave Radio Telescope (GMRT) with 3.4 Msec X-ray data from the Chandra Observatory. Extracting radial temperature profiles, as well as X-ray and radio surface brightness profiles in three directions showing major morphological disturbances, we highlight the signatures of activity in the system. For the first time we observe radio emission corresponding to the NW X-ray depression, which provides evidence in favor of the classification of the depression as a cavity. We identify two other X-ray cavities situated NW and SW of the AGN. While the central radio emission corresponding to the inner cavities shows a flatter spectral index, the radio extensions associated with the farthest X-ray cavities consist of aged plasma. All observed signatures both in radio and X-ray are consistent with several consecutive episodes of AGN activity, which gave rise to the observed morphology NW and SW from the core. In the southern region, we confirm the cooling wake hypothesis for the origin of the long tail. The deep X-ray data also allows us to distinguish significant distortions in the tail's inner parts, which we attribute to the activity of the AGN

Pulsar candidates towards Fermi unassociated sources

We report on a search for steep spectrum radio sources within the 95 per cent confidence error ellipses of the Fermi unassociated sources from the Large Area Telescope (LAT). Using existing catalogues and the newly released Giant Metrewave Radio Telescope all-sky survey at 150 MHz, we identify compact radio sources that are bright at MHz frequencies but faint or absent at GHz frequencies. Such steep spectrum radio sources are rare and constitute a sample of pulsar candidates, selected independently of period, dispersion measure, interstellar scattering and orbital parameters. We find point-like, steep spectrum candidates towards 11 Fermi sources. Based on the gamma-ray/radio positional coincidence, the rarity of such radio sources, and the properties of the 3FGL sources themselves, we argue that many of these sources could be pulsars. They may have been missed by previous radio periodicity searches due to interstellar propagation effects or because they lie in an unusually tight binary. If this hypothesis is correct, then renewed gamma-ray and radio periodicity searches at the positions of the steep spectrum radio sources may reveal pulsations

Low-frequency radio study of MACS clusters at 610 and 235 MHz using the GMRT

Studies have shown that mergers of massive galaxy clusters produce shocks and turbulence in the intra-cluster medium, the possible event that creates radio relics, as well as the radio halos. Here we present GMRT dual-band (235 and 610~MHz) radio observations of four such clusters from the MAssive Cluster Survey (MACS) catalogue. We report the discovery of a very faint, diffuse, elongated radio source with a projected size of about 0.5~Mpc in cluster MACSJ0152.5-2852. We also confirm the presence of a radio relic-like source (about 0.4~Mpc, previously reported at 325~MHz) in MACSJ0025.4-1222 cluster. Proposed relics in both these clusters are found apparently inside the virial radius instead of their usual peripheral location, while no radio halos are detected. These high-redshift clusters (z=0.584 and 0.413) are among the earliest merging systems detected with cluster radio emissions. In MACSJ1931-2635 cluster, we found a radio mini-halo and an interesting highly bent pair of radio jets. Further, we present here a maiden study of low frequency (GMRT 235&610~MHz) spectral and morphological signatures of a previously known radio cluster MACSJ0014.3-3022 (Abell~2744). This cluster hosts a relatively flat spectrum (α610235∼−1.15), giant (∼1.6~Mpc each) halo-relic structure and a close-by high-speed (1769±148359~km~s−1) merger-shock (M=2.02±0.170.41) originated from a possible second merger in the cluster

A sharp view on the low-frequency radio sky

The study of cosmic large-scale structure formation benefits from radio observations, because it provides an unbiased view on the early Universe. Distant radio galaxies and diffuse cluster sources generally have a steep spectrum, which implies an increased brightness towards lower frequencies (below 300 MHz). The quality of low-frequency radio observations is compromised by the propagation effects on cosmic radio waves passing through the ionosphere. In this thesis, we present a calibration method for low-frequency radio interferometric observations. This method significantly improves the quality of radio maps from archival observations as compared to other existing calibration methods. The method was used to produce one of the deepest high-resolution surveys at 153 MHz to date, including the detection of 16 candidate distant radio galaxies. Furthermore, the method was used in a study of the diffuse radio sources in the merging galaxy cluster Abell 2256. These observations support the theory of revival of old radio sources through cluster merger shock compression. Finally, we present a study of the cosmic large-scale structure near a radio galaxy in the early universe by using an optical selection technique for galaxies. The projected galaxy distribution appears to trace the cosmic structure during the assembly of galaxy clusters.UBL - phd migration 201