A pinboard by
Sean Mooney

PhD candidate, University College Dublin


I am observing black holes using the LOFAR radio telescope

Some galaxies emit more light from the central region than from all of the stars in the galaxy combined. Such galaxies are said to have an active galactic nucleus. Many of these active regions emit jets of plasma which travel at near the speed of light. I study a subclass of active galactic nuclei known as blazars. A blazar has the jet of high-speed plasma pointed towards the Earth. As we are looking down these energetic jets, blazars appear very bright.

I am studying a blazar called 3C 273. It has been the focus of intense research for over fifty years due to its unusual properties. Despite this, relatively little is known about 3C 273. I am observing the radio emission that is coming from this blazar as our understanding of the radio portion of the spectrum is particularly poor. Recent technological developments in the field of radio astronomy will allow me to bridge the gap in our knowledge on 3C 273.

Specifically, I want to address two questions:

  • What causes the unique features seen in the radio spectrum?
  • How does 3C 273 compare to other blazars and other active galaxies?

My findings could lay the foundations for a wider study on blazars.

I use a state-of-the-art radio telescope called LOFAR to observe 3C 273 as it offers unprecedented resolution at radio frequencies. Unlike a traditional single-dish radio telescope, LOFAR is made up of many stations which are scattered across Europe. The signals from all of the stations can be combined to act as one giant telescope which is the size of the distance between the stations. This technique is known as interferometry.

LOFAR is a cutting-edge telescope as analysing the data requires advanced supercomputers. The data analysis alone is an area of active research. The volume of data generated also presents a challenge. LOFAR produces more than 1,600 GB of raw data per second and reducing an 8-hour observation of 3C 273 can take several weeks. In this regard, my research involves elements from the Big Data industry and such work would not have been possible even a decade ago.


Revisiting the radio interferometer measurement equation. I. A full-sky Jones formalism

Abstract: Since its formulation by Hamaker et al., the radio interferometer measurement equation (RIME) has provided a rigorous mathematical basis for the development of novel calibration methods and techniques, including various approaches to the problem of direction-dependent effects (DDEs). This series of papers aims to place recent developments in the treatment of DDEs into one RIME-based mathematical framework, and to demonstrate the ease with which the various effects can be described and understood. It also aims to show the benefits of a RIME-based approach to calibration. Paper I re-derives the RIME from first principles, extends the formalism to the full-sky case, and incorporates DDEs. Paper II then uses the formalism to describe self-calibration, both with a full RIME, and with the approximate equations of older software packages, and shows how this is affected by DDEs. It also gives an overview of real-life DDEs and proposed methods of dealing with them. Applying this to WSRT data (Paper III) results in a noise-limited image of the field around 3C 147 with a very high dynamic range (1.6 million), and none of the off-axis artifacts that plague regular selfcal. The resulting differential gain solutions contain significant information on DDEs, and can be used for iterative improvements of sky models. Perhaps most importantly, sources as faint as 2 mJy have been shown to yield meaningful differential gain solutions, and thus can be used as potential calibration beacons in other DDE-related schemes.

Pub.: 04 Feb '11, Pinned: 31 Jul '17

LOFAR: The LOw-Frequency ARray

Abstract: LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

Pub.: 19 May '13, Pinned: 31 Jul '17