Non-linear Redundancy Calibration

For radio interferometric arrays with a sufficient number of redundant spacings the multiplicity of measurements of the same sky visibility can be used to determine both the antenna gains as well as the true visibilities. Many of the earlier approaches to this problem focused on linearized versions of the relation between the measured and the true visibilities. Here we propose to use a standard non-linear minimization algorithm to solve for both the antenna gains as well as the true visibilities. We show through simulations done in the context of the ongoing upgrade to the Ooty Radio Telescope that the non-linear minimization algorithm is fast compared to the earlier approaches. Further, unlike the most straightforward linearized approach, which works with the logarithms of the visibilities and the gains, the non-linear minimization algorithm leads to unbiased solutions. Finally we present error estimates for the estimated gains and visibilities. Monte-Carlo simulations establish that the estimator is indeed statistically efficient, achieving the Cramer-Rao bound.Comment: 9 pages, 5 figures. Accepted for publication in MNRAS. The definitive version will be available at http://mnras.oxfordjournals.or

A search for the 55 MHz OH line

The OH molecule, found abundantly in the Milky Way, has four transitions at the ground state rotational level(J = 3/2) at cm wavelengths. These are E1 transitions between the F+ and F- hyperfine levels of the Lambda doublet of the J=3/2 state. There are also forbidden M1 transitions between the hyperfine levels within each of the doublet states occuring at frequencies 53.171 MHz and 55.128 MHz. These are extremely weak and hence difficult to detect. However there is a possibility that the level populations giving rise to these lines are inverted under special conditions, in which case it may be possible to detect them through their maser emission. We describe the observational diagnostics for determining when the hyperfine levels are inverted, and identify a region near W44 where these conditions are satisfied. A high-velocity-resolution search for these hyperfine OH lines using the low frequency feeds on four antennas of the GMRT and the new GMRT Software Backend(GSB) was performed on this target near W44. We place a 3-sigma upper limit of ~17.3 Jy (at 1 km/s velocity resolution) for the 55 MHz line from this region. This corresponds to an upper limit of 3 X 10^8 for the amplification of the Galactic synchrotron emission providing the background.Comment: 5 pages, 4 figures, accepted for publication in MNRAS. The definitive version is available at www.blackwell-synergy.co

Disentangling interstellar plasma screens with pulsar VLBI: Combining auto- and cross-correlations

Pulsar scintillation allows a glimpse into small-scale plasma structures in the interstellar medium, if we can infer their properties from the scintillation pattern. With Very Long Baseline Interferometry and working in delay-delay rate space, where the contributions of pairs of images to the interference pattern become localized, the scattering geometry and distribution of scattered images on the sky can be determined if a single, highly-anisotropic scattering screen is responsible for the scintillation. However, many pulsars are subject to much more complex scattering environments where this method cannot be used. We present a novel technique to reconstruct the scattered flux of the pulsar and solve for the scattering geometry in these cases by combining interferometric visibilities with cross-correlations of single-station intensities. This takes advantage of the fact that, considering a single image pair in delay-delay rate space, the visibilities are sensitive to the sum of the image angular displacements, while the cross-correlated intensities are sensitive to the difference, so that their combination can be used to localize both images of the pair. We show that this technique is able to reconstruct the published scattering geometry of PSR B0834+06, then apply it to simulations of more complicated scattering systems, where we find that it can distinguish features from different scattering screens even when the presence of multiple screens is not obvious in the Fourier transform of the dynamic spectrum. This technique will allow us to both better understand the distribution of scattering within the interstellar medium and to apply current scintillometry techniques, such as modelling scintillation and constraining the location of pulsar emission, to sources for which a current lack of understanding of the scattering environment precludes the use of these techniques. (abridged)Comment: Submitted to MNRAS; comments welcom

Detection of 15 bursts from FRB 180916.J0158+65 with the uGMRT

We report the findings of a uGMRT observing campaign on FRB 180916.J0158+65, discovered recently to show a 16.35-day periodicity of its active cycle. We observed the source at 550-750 MHz for $\sim 2$ hours each during three successive cycles at the peak of its expected active period. We find 0, 12, and 3 bursts respectively, implying a highly variable bursting rate even within the active phase. We consistently detect faint bursts with spectral energies only an order of magnitude higher than the Galactic burst source SGR~1935+2154. The times of arrival of the detected bursts rule out many possible aliased solutions, strengthening the findings of the 16.35-day periodicity. A short-timescale periodicity search returned no highly significant candidates. Two of the beamformer-detected bursts were bright enough to be clearly detected in the imaging data, achieving sub-arcsecond localization, and proving as a proof-of-concept for FRB imaging with the GMRT. We provide a $3\sigma$ upper limit of the persistent radio flux density at 650 MHz of $66~\mu{\rm Jy}$ which, combined with the EVN and VLA limits at 1.6~GHz, further constrains any potential radio counterpart. These results demonstrate the power of uGMRT for targeted observations to detect and localize known repeating FRBs.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter