Emergence and disappearance of micro-arcsecond structure in the scintillating quasar J1819+3845

The 4.8 GHz lightcurves of the scintillating intra-day variable quasar J1819+3845 during 2004-5 exhibit sharp structure, down to a time scale of 15 minutes, that was absent from lightcurves taken prior to this period and from the 2006 lightcurves. Analysis of the lightcurve power spectra show that the variations must be due to the emergence of new structure in the source. The power spectra yield a scattering screen distance of 3.8 +/- 0.3 pc for a best-fit v_ISS=59 +/- 0.5 km/s or 2.0 +/- 0.3 pc for the scintillation velocity reported by Dennett-Thorpe & de Bruyn (2003). The turbulence is required to be exceptionally turbulent, with C_N^2 > 0.7 Delta L_pc^{-1} m^{-20/3} for scattering material of thickness Delta L_{pc} pc along the ray path. The 2004 power spectrum can be explained in terms of a double source with a component separation 240 +/- 15 microas in 2004.Comment: MNRAS Lett (accepted), version with high-resolution figures at http://www.astro.caltech.edu/~jpm/MdB1819.pd

Radio Interferometric Calibration Using The SAGE Algorithm

Radio Interferometry is an essential method for astronomical observations. Self-calibration techniques have increased the quality of the radio astronomical observations (and hence the science) by orders of magnitude. Recently, there is a drive towards sensor arrays built using inexpensive hardware and distributed over a wide area acting as radio interferometers. Calibration of such arrays poses new problems in terms of computational cost as well as in performance of existing calibration algorithms. We consider the application of the Space Alternating Generalized Expectation Maximization (SAGE) \cite{Fess94} algorithm for calibration of radio interferometric arrays. Application to real data shows that this is an improvement over existing calibration algorithms that are based on direct, deterministic non linear optimization. As presented in this paper, we can improve the computational cost as well as the quality of the calibration using this algorithm.Comment: 6 pages, 3 figures, Appearing in 13th IEEE DSP workshop (IEEE Signal Processing Society

The LOFAR Magnetism Key Science Project

Measuring radio waves at low frequencies offers a new window to study cosmic magnetism, and LOFAR is the ideal radio telescope to open this window widely. The LOFAR Magnetism Key Science Project (MKSP) draws together expertise from multiple fields of magnetism science and intends to use LOFAR to tackle fundamental questions on cosmic magnetism by exploiting a variety of observational techniques. Surveys will provide diffuse emission from the Milky Way and from nearby galaxies, tracking the propagation of long-lived cosmic-ray electrons through magnetic field structures, to search for radio halos around spiral and dwarf galaxies and for magnetic fields in intergalactic space. Targeted deep-field observations of selected nearby galaxies and suspected intergalactic filaments allow sensitive mapping of weak magnetic fields through Rotation Measure (RM) grids. High-resolution observations of protostellar jets and giant radio galaxies reveal structures on small physical scales and at high redshifts, whilst pulsar RMs map large-scale magnetic structures of the Galactic disk and halo in revolutionary detail. The MKSP is responsible for the development of polarization calibration and processing, thus widening the scientific power of LOFAR.Comment: Proceedings of "Magnetic Fields in the Universe: From Laboratory and Stars to Primordial Structures", 2011 Aug. 21-27 in Zakopane/Poland, eds. M. Soida et a

Hemispheric Imaging of Galactic Neutral Hydrogen with a Phased Array Antenna System

Abstract The thousand element array (THEA) system is a phased array system consisting of 1 m 2 tiles having 64 Vivaldi elements each arranged on a regular 8-by-8 grid, which has been developed as a demonstrator of technology and applicability for SKA. In this paper we present imaging results of galactic neutral hydrogen with THEA. Measurements have been taken using a dense 2-by-2 array of four tiles as a four tile adder. The results are compared with results from the Leiden-Dwingeloo Survey, showing qualitative agreement, but also indicating that further studies are needed on the instrumental characteristics

Episodic Star Formation Coupled to Reignition of Radio Activity in 3C 236

We present Hubble Space Telescope UV and optical imaging of the radio galaxy 3C 236, whose relic 4 Mpc radio jet lobes and inner 2 kpc CSS radio source are evidence of multiple epochs of AGN activity. Our data confirm the presence of four bright knots of FUV emission in an arc along the edge of the inner circumnuclear dust disk in the galaxy's nucleus, as well as FUV emission cospatial with the nucleus itself. We interpret these to be sites of recent or ongoing star formation. We present photometry of these knots, as well as an estimate for the internal extinction in the source using the Balmer decrement from SDSS spectroscopy. We estimate the ages of the knots by comparing our extinction-corrected photometry with stellar population synthesis models. We find the four knots cospatial with the dusty disk to be young, of order 10^7 yr old. The FUV emission in the nucleus is likely due to an episode of star formation triggered ~10^9 yr ago. We argue that the young 10^7 yr old knots stem from an episode of star formation that was roughly coeval with the event resulting in reignition of radio activity, creating the CSS source. The 10^9 yr old stars in the nucleus may be associated with the previous epoch of activity that generated the 4 Mpc relic source, before it was cut off by exhaustion or interruption. The ages of the knots, considered in context with the disturbed morphology of the nuclear dust and the double-double morphology of the "old" and "young" radio sources, present evidence for an episodic AGN/starburst connection. We suggest that the AGN fuel supply was interrupted for ~10^7 yr due to a minor merger event and has now been restored, and the resultant non-steady flow of gas toward the nucleus is likely responsible for both the new episode of infall-induced star formation and also the multiple epochs of radio activity.Comment: 15 pages, 7 figures. Accepted for publication in ApJ

A very brief description of LOFAR - the Low Frequency Array

LOFAR (Low Frequency Array) is an innovative radio telescope optimized for the frequency range 30-240 MHz. The telescope is realized as a phased aperture array without any moving parts. Digital beam forming allows the telescope to point to any part of the sky within a second. Transient buffering makes retrospective imaging of explosive short-term events possible. The scientific focus of LOFAR will initially be on four key science projects (KSPs): 1) detection of the formation of the very first stars and galaxies in the universe during the so-called epoch of reionization by measuring the power spectrum of the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2) low-frequency surveys of the sky with of order $10^8$ expected new sources; 3) all-sky monitoring and detection of transient radio sources such as gamma-ray bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003) allowing for the first time access to particles beyond 10^21 eV (Scholten et al. 2006). Apart from the KSPs open access for smaller projects is also planned. Here we give a brief description of the telescope.Comment: 2 pages, IAU GA 2006, Highlights of Astronomy, Volume 14, K.A. van der Hucht, e