Andre Fletcher | The University of Western Australia (original) (raw)

Papers by Andre Fletcher

This thesis is an investigation into the nature and evolution of two distinct classes of astrophy... more This thesis is an investigation into the nature and evolution of two distinct classes of astrophysical object, believed to be initially powered by gravitational accretion: extragalactic radio sources, and stars in young stellar clusters. Using the VLA interferometer, the MIT group has observed -10,000 radio sources in the past 15 years. One goal of this thesis is to extend the MIT gravitational lens search into the Southern Hemisphere via VLA imaging of 1750 Parkes-MIT-NRAO survey sources. I document the observations, reduction and results from 17 years of MIT radio & optical surveys. I have organized the data into an archive. I present new MIT-VLA lens candidates, a large sample of Compact Steep Spectrum close doubles, and observations of both an unusual radio ring galaxy and a binary quasar. From my extraction of the angular size distribution for 4741 MIT-VLA sources, I find that the mean jet advance speed is non-relativistic, given reasonable assumptions about jet ages. I conclud...

Journal of Astronomy and Space Sciences, 2001

We have recently compiled a database of the properties of 192 impact craters, which supercedes pr... more We have recently compiled a database of the properties of 192 impact craters, which supercedes previous compilations. Using our database, the impact structures found in North America, Europe and Australia have been examined; these cratonic areas have been relatively stable for considerably long geological periods, and thus have been best preserved. It is confirmed that there is a close correlation between the geological epoch boundaries, the epochs of mass extinctions, antral the "timing" of impacts. In addition, the terrestrial cumulative flux of objects >20km is found to be , over the last 120 Myr, which is much smaller than the published values in McEwen et al. (1997) and Shoemaker (1998) (. For terrestrial impact structures with D> 50 km, the apparent cumulative flux over the last 2450 Myr is ~50 times smaller than the corresponding value for the Moon. If we assume that the Earth and the Moon suffered the same level of bombardment over this time, this would mean ...

VizieR Online Data Catalog, Feb 1, 2001

American Astronomical Society Meeting Abstracts, Dec 1, 1994

The Parkes-MIT-NRAO (PMN) Sky Survey at 5 GHz radio frequency revealed 36,640 sources over an are... more The Parkes-MIT-NRAO (PMN) Sky Survey at 5 GHz radio frequency revealed 36,640 sources over an area of 4.51 steradians, down to a limiting flux density of typically 35 mJy. The estimated completeness is greater than 95%, and near the limiting flux the reliability is at least 90% (Griffith &amp; Wright, A.J. 105 (5) May 1993). We have made VLA X-Band

Large capital inflows are often associated with subsequent credit and investment booms, inflation... more Large capital inflows are often associated with subsequent credit and investment booms, inflation, overheating, real exchange rate misalignments, current account imbalances and financial sector weaknesses culminating in financial crisis, and long-lasting output losses. India too has received large and volatile capital flows since 1993-94, especially during 2004-09. Nonetheless, macroeconomic, price and financial stability has been maintained in an environment of high growth. What explains these desirable outcomes in India? An assessment of external sector and monetary management policies adopted by India since the early 1990s, undertaken in this paper, suggests that the outcomes can be attributed to a judicious use of menu of options. These have included: active management of the capital account, especially debt flows; within debt flows, tighter prudential restrictions on access of financial intermediaries to external borrowings visa -vis nonfinancial corporate entities; flexibility in exchange rate movements but with capacity to intervene in times of excessive volatility along with appropriate sterilisation of interventions; associated building up of adequate reserves; continuous development of financial markets in terms of participants and instruments; strengthening of the financial sector through prudential regulation while also enhancing competition; pre-emptive tightening of prudential norms in case of sectors witnessing very high credit growth; and refinements in the institutional framework for monetary policy. As a result of this approach, growth in monetary and credit aggregates could be contained consistent with the real economy undergoing growth, structural transformation and financial deepening. Inflation was contained even as growth accelerated. Overall, financial stability was maintained even as the global economic environment was characterised by a series of financial crises. The impossible trinity was managed by preferring middle solutions of open but managed capital account and flexible exchange rate but with management of volatility. Rather than relying on a single instrument, many instruments have been used in coordination. This was enabled by the fact that both monetary policy and regulation of banks and other financial institutions and key financial markets are under the jurisdiction of the Reserve Bank, which permitted smooth use of various policy instruments. Key lessons from the Indian experience are that monetary policy needs to move away from narrow price stability/inflation targeting objective. Given the volatility and the need to ensure broader stability of the financial system, central banks need multiple instruments. Capital account management has to be countercyclical, just as is the case monetary and fiscal policies. Judgements in capital account management are no more complex than those made in monetary management.

The Astronomical Journal, 1998

The Astrophysical Journal, 2001

We present radio and optical observations of MG0248+0641, which contains a kiloparsecscale ring-l... more We present radio and optical observations of MG0248+0641, which contains a kiloparsecscale ring-like structure in one of its radio lobes. The radio observations show a typical core-double morphology: a central core between two lobes, each of which has a hotspot. The western radio lobe appears as a nearly continuous ring, with linear polarization vectors which are oriented in a radial direction from the ring center. We consider several different interpretations for the nature of this ring, including gravitational lensing of a normal jet by a foreground galaxy. Even though simple lensing models can describe the ring morphology reasonably well, the high linear polarization seen around the ring cannot be easily explained and no lensing object has yet been found in deep optical and infrared searches within the extent of the ring. The chance interposition of a galactic supernova remnant, nova, planetary nebula, or HII region, has been ruled out. The highly polarized ring of MG0248+0641 is...

ABSTRACT We propose 3-mm ATCA monitoring for intraday variability (IDV) in Sagittarius A* (Sgr A*... more ABSTRACT We propose 3-mm ATCA monitoring for intraday variability (IDV) in Sagittarius A* (Sgr A*), the supermassive black hole at the Galactic Center. Our 3- & 2-mm Nobeyama Millimeter Array (NMA) monitoring of Sgr A* has revealed flares lasting from hours to months, including an IDV detection. ATCA would provide higher angular resolution, a longer observing window, and better calibrations, for tracking these flares. We did a 10 hr `pilot' 3-mm ATCA observation on 18 Oct 2005. The large flux variation in the 3-mm light-curve shows probably the IDV of Sgr A*, this result has demonstrated that the ATCA is very useful in searching the IDV profile. We propose to use our approved 3-mm VLBA time (Shen et al.) for a joint observation with ATCA to look for correlated structural variations during flares; our combined efforts should supply complementary constraints on Sgr A* radio source emission models. Moreover, the coordinated ATCA-NMA observing of Sgr A* would also provide cross-system 3-mm calibration checks, and confirmations of IDV flares.

Journal of the Korean Physical Society, 2004

There is now abundant evidence in favor of black holes (BHs) as the central engines in many diffe... more There is now abundant evidence in favor of black holes (BHs) as the central engines in many different types of astrophysical systems: active galactic nuclei (AGNs), X-ray binaries (XRBs), microquasars, and possibly gamma-ray bursts (GRBs). The history and status of analytic theories for BH plasma magnetospheres are reviewed, and new results are presented for a nonstationary version of the 1982 Macdonald & Thorne theory. A physical picture describing these new results is presented, and some of their possible applications to future work suggested.

ABSTRACT The new Korean VLBI Network construction project started in 2001. Currently funds have b... more ABSTRACT The new Korean VLBI Network construction project started in 2001. Currently funds have been allocated until 2007. Three new 20m diameter fast-slewing high-precision radio antennae will be built in South Korea at Seoul Tamna and Ulsan. These will be exclusively for VLBI astronomy astrometry and geodesy with a focus on developing millimeter-wave VLBI. First 2/8 22 43 and 86 GHz HEMT receivers will be used; 100/150 GHz SIS receivers will be installed later. The KVN Data Acquisition System (DAS) is being designed to realize a multi-band dual-circular polarization wide-bandwidth system. Very-high-speed digital samplers will generate data streams at rates of at least 1 Gigabit/s and a digital filter system will select the recorded frequency channels according to several observation modes one or more of which would be compatible with the VLBA and VERA DASs. The data will be transmitted by optical fibers to the new MIT Mark 5B hard-disk recorders. The KVN correlator will be designed with new technological concepts in mind. The observational capabilities and science goals of KVN are outlined. International scientific and technical collaborations with VLBI institutes will be essential for the ultimate success of the KVN project

Astrophysics and Space Science Library, 1998

The structure of the radio source RC B1146+052 of the "Cold"catalogue is investigated by data of ... more The structure of the radio source RC B1146+052 of the "Cold"catalogue is investigated by data of the MIT-GB-VLA survey at 4850 MHz. This source belongs to the steep spectrum radio sources subsample of the RC catalogue. Its spectral index is α =-1.04. The optical image of this source obtained with 6m telescope is analysed. The possible explanations of the complex structure of radio components are considered.

We present radio and optical observations of MG0248+0641, which contains a kiloparsec-scale ring-... more We present radio and optical observations of MG0248+0641, which contains a kiloparsec-scale ring-like structure in one of its radio lobes. The radio observations show a typical core-double morphology: a central core between two lobes, each of which has a hotspot. The western radio lobe appears as a nearly continuous ring, with linear polarization electric field vectors which are oriented in a radial direction from the ring center. We consider several different interpretations for the nature of this ring, including gravitational lensing of a normal jet by a foreground galaxy. Even though simple lensing models can describe the ring morphology reasonably well, the high linear polarization seen around the ring cannot be easily explained. The chance interposition of a galactic supernova remnant, nova, planetary nebula, or H II region, has been ruled out. The highly polarized ring of MG0248+0641 is much like the prominent ring seen in 3C219, and the multiple ones in 3C310 and Hercules A, ...

Journal of The Korean Astronomical Society

Active galactic nuclei (AGNs) are distant, powerful sources of radiation over the entire electrom... more Active galactic nuclei (AGNs) are distant, powerful sources of radiation over the entire electromagnetic spectrum, from radio waves to gamma-rays. There is much evidence that they are driven by gravitational accretion of stars, dust, and gas, onto central massive black holes (MBHs) imprisoning anywhere from 1 to 10,000 million solar masses; such objects may naturally form in the centers of galaxies during their normal dynamical evolution. A small fraction of AGNs, of the radio-loud type (RLAGNs), are somehow able to generate powerful synchrotron-emitting structures (cores, jets, lobes) with sizes ranging from pc to Mpc. A brief summary of AGN observations and theories is given, with an emphasis on RLAGNs. Preliminary results from the imaging of 10000 extragalactic radio sources observed in the MITVLA snapshot survey, and from a new analytic theory of the time-variable power output from Kerr black hole magnetospheres, are presented. To better understand the complex physical processes within the central engines of AGNs, it is important to confront the observations with theories, from the viewpoint of analyzing the time-variable behaviours of AGNs-which have been recorded over both`short' human (10 0 ?10 9 s) and`long' cosmic (10 13 ? 10 17 s) timescales. Some key ingredients of a basic mathematical formalism are outlined, which may help in building detailed Monte-Carlo models of evolving AGN populations; such numerical calculations should be potentially important tools for useful interpretation of the large amounts of statistical data now publicly available for both AGNs and RLAGNs.