Alok Patnaik - Academia.edu (original) (raw)

Papers by Alok Patnaik

Research paper thumbnail of The core-jet structure of the JVAS gravitational lens B1030+074

We present results from VLBA+Effelsberg 1.7 GHz, VLBA 8.4 GHz and VLBA 15 GHz observations of the... more We present results from VLBA+Effelsberg 1.7 GHz, VLBA 8.4 GHz and VLBA 15 GHz observations of the JVAS gravitational lens system B1030+074. The VLBA+Effelsber g 1.7 GHz data with 3 mas resolution reveal a detailed 60 mas jet structure in the strong A component of t he lens system and maybe a first hint of the corresponding jet structure in

Research paper thumbnail of A Nearly Symmetric Double-Image Gravitational Lens 1

We report the discovery of a new double-image gravitational lens, resulting from our search for l... more We report the discovery of a new double-image gravitational lens, resulting from our search for lenses in the southern sky. Radio source PMN J2004[1349 is composed of two compact components separated by in VLA, MERLIN, and VLBA images. The components have a Ñux ratio of 1 :1 at radio 1A.13 frequencies ranging from 5 to 22.5 GHz. The I-band optical counterpart is also an equal double, with roughly the same separation and position angle as the radio double. Upon subtraction of the components from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004[1349 is a gravitational lens, additional information will be necessary (such as the redshifts of the galaxy and quasar and precise astrometry and photometry of the lens galaxy) before constructing detailed mass models.

Research paper thumbnail of Interferometric Phase Calibration Sources in the Declination Range 0o to −30o

The Astrophysical Journal Supplement Series, 2003

We present a catalog of 321 compact radio sources in the declination range 0 • > δ > −30 •. The p... more We present a catalog of 321 compact radio sources in the declination range 0 • > δ > −30 •. The positions of these sources have been measured with a two-dimensional rms accuracy of 35 milliarcseconds using the NRAO a Very Large Array. Each source has a peak flux density > 50 mJy at 8.4 GHz. We intend for this catalog to be used mainly for selection of phase calibration sources for radio interferometers, although compact radio sources have other scientific uses.

Research paper thumbnail of Everything you ever wanted to know about the JVAS gravitational lens system B1030+074 but were afraid to ask

We present an overview of all the observations (radio - VLA, MERLIN, VLBA,EVN - and optical - WFP... more We present an overview of all the observations (radio - VLA, MERLIN, VLBA,EVN - and optical - WFPC2 and NICMOS -) that were initially used to confirm the gravitational lens nature of the double JVAS system B1030+074. Since the 1.56 arcsec system showed some first indication of variability it has been monitored with the VLA and MERLIN to confirm its

Research paper thumbnail of PMN J2004-1349: A new gravitationally lensed quasar

We report the discovery of a new double-image quasar resulting from our search for gravitational ... more We report the discovery of a new double-image quasar resulting from our search for gravitational lenses in the southern sky. Radio source PMN J2004-1349 is composed of two flat-spectrum components of approximately equal brightness separated by 1. ′′ 13 in VLA, MERLIN and VLBA images. The I-band optical counterpart is also double, with roughly the same separation, position angle, and flux ratio as the radio double. Upon subtraction of the quasars from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004-1349 is a gravitational lens, additional information (such as the redshifts of the quasar and lens galaxy) will be needed before many interesting astrophysical quantities (such as the mass of the galaxy, or the predicted time delay between lensed images) can be inferred.

Research paper thumbnail of Gravitationally lensed radio sources in the Jodrell Bank-VLA Astrometric Survey

Monthly Notices of the Royal Astronomical Society, 1999

We report the ®nal results of the search for gravitationally lensed¯at-spectrum radio sources in ... more We report the ®nal results of the search for gravitationally lensed¯at-spectrum radio sources in the 2384-source Jodrell Bank±VLA Astrometric Survey (JVAS). We describe the process of selecting candidate lensed sources from 8.4-GHz VLA snapshot maps. 66 sources were selected as lens candidates and were followed up with multifrequency VLA and/or highresolution MERLIN 5-GHz observations. To illustrate the candidate elimination process, we discuss examples of sources representative of particular morphologies that have been ruled out by the follow-up observations. There are six gravitational lens systems in the JVAS, ®ve of which are new discoveries. We summarize the properties of all the systems.

Research paper thumbnail of Interferometric Phase Calibration Sources in the Declination Range 0deg to -30deg

We present a catalog of 321 compact radio sources in the declination range 0deg > delta > -... more We present a catalog of 321 compact radio sources in the declination range 0deg > delta > -30deg. The positions of these sources have been measured with a two-dimensional rms accuracy of 35 milliarcseconds using the NRAO Very Large Array. Each source has a peak flux density >50 mJy at 8.4 GHz. We intend for this catalog to be used mainly for selection of phase calibration sources for radio interferometers, although compact radio sources have other scientific uses.

Research paper thumbnail of The Nature of Radio-Intermediate Quasars: What is Radio-Loud and what is Radio-Quiet?

We have performed quasi-simultaneous radio flux density measurements at 2.7 and 10 GHz for all PG... more We have performed quasi-simultaneous radio flux density measurements at 2.7 and 10 GHz for all PG quasars with radio flux densities between 4-200 mJy. We find that a large fraction of these sources are variable, flat-spectrum quasars. This brings the total fraction of flat-spectrum quasars with a ratio between radio and optical flux of R>10 - a value previously used to define a radio-loud quasar - to 40% in the PG quasar sample. We also find that the median R-parameter of these flat-spectrum quasars is lower than those of steep-spectrum radio-loud quasars. This contradicts the predictions of the unified scheme and the idea that all flat-spectrum, core-dominated quasars are relativistically boosted lobe-dominated quasars. We show that this discrepancy is due to a population of flat-spectrum radio-intermediate quasars with 25<R<250 which can neither be explained as relativistically boosted radio-loud quasars nor as normal radio-weak quasars. We point out that a natural explan...

Research paper thumbnail of EVN+MERLIN Observations of Radio-Intermediate Quasars: Evidence for Boosted Radio-Weak Quasars

We present VLBI (EVN+MERLIN) observations of a sample of three low-redshift radio-intermediate PG... more We present VLBI (EVN+MERLIN) observations of a sample of three low-redshift radio-intermediate PG quasars (RIQ) with flat and variable radio spectrum (III Zw 2, PG 1309+355, PG 2209+184). Their radio-to-optical flux ratio (R) is slightly lower than the average R for steep-spectrum quasars, but their radio spectral properties are those of core-dominated quasars. It was proposed previously that these sources might be relativistically boosted jets in radio-weak quasars. Our VLBI observations now indeed confirm the presence of a high brightness temperature core in all three of these objects --- two of them have lower limits on T_B well in excess of 10^10 Kelvin. Moreover, we find no ``missing-flux'' which means that basically all the flux of these quasars is concentrated in the compact radio core. As the total radio flux is already at the low end for radio-loud quasars, we can place a strong limit on the presence of any extended emission. This limit is consistent with the extend...

Research paper thumbnail of Frequency dependent radio structure of the gravitational lens system B0218+357

We present multi-frequency radio continuum VLBI observations of the gravitational lens system B02... more We present multi-frequency radio continuum VLBI observations of the gravitational lens system B0218+357 carried out using a global VLBI network and the VLBA. The source has been observed with resolutions from 0.2 milliarcsec to 5 milliarcsec and displays interesting structure. The spectral properties of various components show that the lensed object is a standard flat spectrum radio source which has many self-absorbed components. Based on the flux ratio of the lensed images as a function of frequency we propose a simple model for the background radio source.

Research paper thumbnail of Optical Imaging of B1422+231 — Prospects for Determining the Hubble Constant

Astrophysical Applications of Gravitational Lensing, 1996

Research paper thumbnail of VLA observations of the wide-angle tailed radio source 1313+073

Monthly Notices of the Royal Astronomical Society, 1984

Research paper thumbnail of Determination of time delay from the gravitational lens B1422+231

Monthly Notices of the Royal Astronomical Society, 2001

We present the radio light curves of lensed images of the gravitational lens B1422+231. The obser... more We present the radio light curves of lensed images of the gravitational lens B1422+231. The observations have been carried out using the VLA at 8.4 and 15 GHz over a period of 197 days. We describe a method to estimate the time delay from the observed light curves. Using this method, our cross-correlation analysis shows that the time delay between images B and A is 1.5±1.4d, between A and C is 7.6±2.5d, between B and C is 8.2±2.0d. When applied to other lensed systems with measured time delays our new method gives comparable results.

Research paper thumbnail of B2114+022: a distant radio source gravitationally lensed by a starburst galaxy

Monthly Notices of the Royal Astronomical Society, 2001

We have discovered a radio source (B21141022) with a unique structure during the course of the JV... more We have discovered a radio source (B21141022) with a unique structure during the course of the JVAS gravitational lens survey. VLA, MERLIN, VLBA and MERLIN1EVN radio maps reveal four compact components, in a configuration unlike that of any known lens system, or, for that matter, any of the ,15 000 radio sources in the JVAS and CLASS surveys. Three of the components are within 0.3 arcsec of each other while the fourth is separated from the group by 2.4 arcsec. The widest separation pair of components have similar radio structures and spectra. The other pair also have similar properties. This latter pair have spectra which peak at ,5 GHz. Their surface brightnesses are much lower than expected for synchrotron self-absorbed components. Ground-based and Hubble Space Telescope optical observations show two galaxies ðz ¼ 0:3157 and 0.5883) separated by 1.25 arcsec. The lower redshift galaxy has a post-starburst spectrum and lies close to, but not coincident with, the compact group of three radio components. No optical or infrared emission is detected from any of the radio components down to I ¼ 25 and H ¼ 23. We argue that the most likely explanation of the B21141022 system is that the post-starburst galaxy, assisted by the second galaxy, lenses a distant radio source producing the two wide-separation images. The other two radio components are then associated with the post-starburst galaxy. The combination of the angular sizes of these components, their radio spectra and their location with respect to their host galaxy still remains puzzling.

Research paper thumbnail of PMN J0134–0931: A gravitationally lensed quasar with an unusual radio morphology

The radio-loud quasar J0134–0931 was discovered to have an unusual morphology during our search f... more The radio-loud quasar J0134–0931 was discovered to have an unusual morphology during our search for gravitational lenses. In VLA and MERLIN images, there are 5 compact components with maximum separation 681 mas. All of these components have the same spectral index from 5 GHz to 43 GHz. In a VLBA image at 1.7 GHz, a curved arc of extended emission joins two of the components in a manner suggestive of gravitational lensing. At least two of the radio components have near-infrared counterparts. We argue that this evidence implies that J0134–0931 is a gravitational lens, although we have not been able to devise a plausible model for the foreground gravitational potential. Like several other radio-loud lenses, the background source has an extraordinarily red optical counterpart. Subject headings: gravitational lensing, quasars: individual (J0134–0931) 1.

Research paper thumbnail of A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked

Research paper thumbnail of An Integrated PHM Approach for Gas Turbine Engines

The prognosis & health management (PHM) of aerospace components is a very complex system. A compl... more The prognosis & health management (PHM) of aerospace components is a very complex system. A complete PHM system involves sensors, signal processing, condition monitoring, health assessment, physical reasoning and decision- making. Such an integrated approach to prognostics involves physics-based modelling, experience-based modelling and modelling based on the statistical properties of the components by using neural network and fuzzy logic techniques. In this paper, we give an overview of the PHM system currently being developed. 1.

Research paper thumbnail of A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked spectrum

III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical o... more III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical of a Seyfert galaxy, but also with an extremely variable, blazar-like radio core. We have now discovered a new radio flare where the source has brightened more than twenty-fold within less than two years. A broad-band radio spectrum between 1.4 and 666 GHz shows a textbook-like synchrotron spectrum peaking at 43 GHz, with a selfabsorbed synchrotron spectral index +2:5 at frequencies below 43 GHz and an optically thin spectral index \Gamma0:75 at frequencies above 43 GHz. The outburst spectrum can be well fitted by two homogenous, spherical components with equipartition sizes of 0.1 and 0.2 pc at 43 and 15 GHz, and with magnetic fields of 0.4 and 1 Gauss. VLBA observations at 43 GHz confirm this double structure and these sizes. Time scale arguments suggest that the emitting regions are shocks which are continuously accelerating particles. This could be explained by a frustrated jet scenar...

Research paper thumbnail of PMN J2004–1349: A new gravitationally lensed quasar 1

We report the discovery of a new double-image quasar resulting from our search for gravitational ... more We report the discovery of a new double-image quasar resulting from our search for gravitational lenses in the southern sky. Radio source PMN J2004-1349 is composed of two flat-spectrum components of approximately equal brightness separated by 1. ′′ 13 in VLA, MERLIN and VLBA images. The I-band optical counterpart is also double, with roughly the same separation, position angle, and flux ratio as the radio double. Upon subtraction of the quasars from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004-1349 is a gravitational lens, additional information (such as the redshifts of the quasar and lens galaxy) will be needed before many interesting astrophysical quantities (such as the mass of the galaxy, or the predicted time delay between lensed images) can be inferred.

Research paper thumbnail of A nearly symmetric double-image gravitational lens 1

We report the discovery of a new double-image gravitational lens, resulting from our search for l... more We report the discovery of a new double-image gravitational lens, resulting from our search for lenses in the southern sky. Radio source PMN J2004[1349 is composed of two compact components separated by in VLA, MERLIN, and VLBA images. The components have a Ñux ratio of 1 : 1 at radio 1A .13 frequencies ranging from 5 to 22.5 GHz. The I-band optical counterpart is also an equal double, with roughly the same separation and position angle as the radio double. Upon subtraction of the components from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004[1349 is a gravitational lens, additional information will be necessary (such as the redshifts of the galaxy and quasar and precise astrometry and photometry of the lens galaxy) before constructing detailed mass models.

Research paper thumbnail of The core-jet structure of the JVAS gravitational lens B1030+074

We present results from VLBA+Effelsberg 1.7 GHz, VLBA 8.4 GHz and VLBA 15 GHz observations of the... more We present results from VLBA+Effelsberg 1.7 GHz, VLBA 8.4 GHz and VLBA 15 GHz observations of the JVAS gravitational lens system B1030+074. The VLBA+Effelsber g 1.7 GHz data with 3 mas resolution reveal a detailed 60 mas jet structure in the strong A component of t he lens system and maybe a first hint of the corresponding jet structure in

Research paper thumbnail of A Nearly Symmetric Double-Image Gravitational Lens 1

We report the discovery of a new double-image gravitational lens, resulting from our search for l... more We report the discovery of a new double-image gravitational lens, resulting from our search for lenses in the southern sky. Radio source PMN J2004[1349 is composed of two compact components separated by in VLA, MERLIN, and VLBA images. The components have a Ñux ratio of 1 :1 at radio 1A.13 frequencies ranging from 5 to 22.5 GHz. The I-band optical counterpart is also an equal double, with roughly the same separation and position angle as the radio double. Upon subtraction of the components from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004[1349 is a gravitational lens, additional information will be necessary (such as the redshifts of the galaxy and quasar and precise astrometry and photometry of the lens galaxy) before constructing detailed mass models.

Research paper thumbnail of Interferometric Phase Calibration Sources in the Declination Range 0o to −30o

The Astrophysical Journal Supplement Series, 2003

We present a catalog of 321 compact radio sources in the declination range 0 • > δ > −30 •. The p... more We present a catalog of 321 compact radio sources in the declination range 0 • > δ > −30 •. The positions of these sources have been measured with a two-dimensional rms accuracy of 35 milliarcseconds using the NRAO a Very Large Array. Each source has a peak flux density > 50 mJy at 8.4 GHz. We intend for this catalog to be used mainly for selection of phase calibration sources for radio interferometers, although compact radio sources have other scientific uses.

Research paper thumbnail of Everything you ever wanted to know about the JVAS gravitational lens system B1030+074 but were afraid to ask

We present an overview of all the observations (radio - VLA, MERLIN, VLBA,EVN - and optical - WFP... more We present an overview of all the observations (radio - VLA, MERLIN, VLBA,EVN - and optical - WFPC2 and NICMOS -) that were initially used to confirm the gravitational lens nature of the double JVAS system B1030+074. Since the 1.56 arcsec system showed some first indication of variability it has been monitored with the VLA and MERLIN to confirm its

Research paper thumbnail of PMN J2004-1349: A new gravitationally lensed quasar

We report the discovery of a new double-image quasar resulting from our search for gravitational ... more We report the discovery of a new double-image quasar resulting from our search for gravitational lenses in the southern sky. Radio source PMN J2004-1349 is composed of two flat-spectrum components of approximately equal brightness separated by 1. ′′ 13 in VLA, MERLIN and VLBA images. The I-band optical counterpart is also double, with roughly the same separation, position angle, and flux ratio as the radio double. Upon subtraction of the quasars from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004-1349 is a gravitational lens, additional information (such as the redshifts of the quasar and lens galaxy) will be needed before many interesting astrophysical quantities (such as the mass of the galaxy, or the predicted time delay between lensed images) can be inferred.

Research paper thumbnail of Gravitationally lensed radio sources in the Jodrell Bank-VLA Astrometric Survey

Monthly Notices of the Royal Astronomical Society, 1999

We report the ®nal results of the search for gravitationally lensed¯at-spectrum radio sources in ... more We report the ®nal results of the search for gravitationally lensed¯at-spectrum radio sources in the 2384-source Jodrell Bank±VLA Astrometric Survey (JVAS). We describe the process of selecting candidate lensed sources from 8.4-GHz VLA snapshot maps. 66 sources were selected as lens candidates and were followed up with multifrequency VLA and/or highresolution MERLIN 5-GHz observations. To illustrate the candidate elimination process, we discuss examples of sources representative of particular morphologies that have been ruled out by the follow-up observations. There are six gravitational lens systems in the JVAS, ®ve of which are new discoveries. We summarize the properties of all the systems.

Research paper thumbnail of Interferometric Phase Calibration Sources in the Declination Range 0deg to -30deg

We present a catalog of 321 compact radio sources in the declination range 0deg > delta > -... more We present a catalog of 321 compact radio sources in the declination range 0deg > delta > -30deg. The positions of these sources have been measured with a two-dimensional rms accuracy of 35 milliarcseconds using the NRAO Very Large Array. Each source has a peak flux density >50 mJy at 8.4 GHz. We intend for this catalog to be used mainly for selection of phase calibration sources for radio interferometers, although compact radio sources have other scientific uses.

Research paper thumbnail of The Nature of Radio-Intermediate Quasars: What is Radio-Loud and what is Radio-Quiet?

We have performed quasi-simultaneous radio flux density measurements at 2.7 and 10 GHz for all PG... more We have performed quasi-simultaneous radio flux density measurements at 2.7 and 10 GHz for all PG quasars with radio flux densities between 4-200 mJy. We find that a large fraction of these sources are variable, flat-spectrum quasars. This brings the total fraction of flat-spectrum quasars with a ratio between radio and optical flux of R>10 - a value previously used to define a radio-loud quasar - to 40% in the PG quasar sample. We also find that the median R-parameter of these flat-spectrum quasars is lower than those of steep-spectrum radio-loud quasars. This contradicts the predictions of the unified scheme and the idea that all flat-spectrum, core-dominated quasars are relativistically boosted lobe-dominated quasars. We show that this discrepancy is due to a population of flat-spectrum radio-intermediate quasars with 25<R<250 which can neither be explained as relativistically boosted radio-loud quasars nor as normal radio-weak quasars. We point out that a natural explan...

Research paper thumbnail of EVN+MERLIN Observations of Radio-Intermediate Quasars: Evidence for Boosted Radio-Weak Quasars

We present VLBI (EVN+MERLIN) observations of a sample of three low-redshift radio-intermediate PG... more We present VLBI (EVN+MERLIN) observations of a sample of three low-redshift radio-intermediate PG quasars (RIQ) with flat and variable radio spectrum (III Zw 2, PG 1309+355, PG 2209+184). Their radio-to-optical flux ratio (R) is slightly lower than the average R for steep-spectrum quasars, but their radio spectral properties are those of core-dominated quasars. It was proposed previously that these sources might be relativistically boosted jets in radio-weak quasars. Our VLBI observations now indeed confirm the presence of a high brightness temperature core in all three of these objects --- two of them have lower limits on T_B well in excess of 10^10 Kelvin. Moreover, we find no ``missing-flux'' which means that basically all the flux of these quasars is concentrated in the compact radio core. As the total radio flux is already at the low end for radio-loud quasars, we can place a strong limit on the presence of any extended emission. This limit is consistent with the extend...

Research paper thumbnail of Frequency dependent radio structure of the gravitational lens system B0218+357

We present multi-frequency radio continuum VLBI observations of the gravitational lens system B02... more We present multi-frequency radio continuum VLBI observations of the gravitational lens system B0218+357 carried out using a global VLBI network and the VLBA. The source has been observed with resolutions from 0.2 milliarcsec to 5 milliarcsec and displays interesting structure. The spectral properties of various components show that the lensed object is a standard flat spectrum radio source which has many self-absorbed components. Based on the flux ratio of the lensed images as a function of frequency we propose a simple model for the background radio source.

Research paper thumbnail of Optical Imaging of B1422+231 — Prospects for Determining the Hubble Constant

Astrophysical Applications of Gravitational Lensing, 1996

Research paper thumbnail of VLA observations of the wide-angle tailed radio source 1313+073

Monthly Notices of the Royal Astronomical Society, 1984

Research paper thumbnail of Determination of time delay from the gravitational lens B1422+231

Monthly Notices of the Royal Astronomical Society, 2001

We present the radio light curves of lensed images of the gravitational lens B1422+231. The obser... more We present the radio light curves of lensed images of the gravitational lens B1422+231. The observations have been carried out using the VLA at 8.4 and 15 GHz over a period of 197 days. We describe a method to estimate the time delay from the observed light curves. Using this method, our cross-correlation analysis shows that the time delay between images B and A is 1.5±1.4d, between A and C is 7.6±2.5d, between B and C is 8.2±2.0d. When applied to other lensed systems with measured time delays our new method gives comparable results.

Research paper thumbnail of B2114+022: a distant radio source gravitationally lensed by a starburst galaxy

Monthly Notices of the Royal Astronomical Society, 2001

We have discovered a radio source (B21141022) with a unique structure during the course of the JV... more We have discovered a radio source (B21141022) with a unique structure during the course of the JVAS gravitational lens survey. VLA, MERLIN, VLBA and MERLIN1EVN radio maps reveal four compact components, in a configuration unlike that of any known lens system, or, for that matter, any of the ,15 000 radio sources in the JVAS and CLASS surveys. Three of the components are within 0.3 arcsec of each other while the fourth is separated from the group by 2.4 arcsec. The widest separation pair of components have similar radio structures and spectra. The other pair also have similar properties. This latter pair have spectra which peak at ,5 GHz. Their surface brightnesses are much lower than expected for synchrotron self-absorbed components. Ground-based and Hubble Space Telescope optical observations show two galaxies ðz ¼ 0:3157 and 0.5883) separated by 1.25 arcsec. The lower redshift galaxy has a post-starburst spectrum and lies close to, but not coincident with, the compact group of three radio components. No optical or infrared emission is detected from any of the radio components down to I ¼ 25 and H ¼ 23. We argue that the most likely explanation of the B21141022 system is that the post-starburst galaxy, assisted by the second galaxy, lenses a distant radio source producing the two wide-separation images. The other two radio components are then associated with the post-starburst galaxy. The combination of the angular sizes of these components, their radio spectra and their location with respect to their host galaxy still remains puzzling.

Research paper thumbnail of PMN J0134–0931: A gravitationally lensed quasar with an unusual radio morphology

The radio-loud quasar J0134–0931 was discovered to have an unusual morphology during our search f... more The radio-loud quasar J0134–0931 was discovered to have an unusual morphology during our search for gravitational lenses. In VLA and MERLIN images, there are 5 compact components with maximum separation 681 mas. All of these components have the same spectral index from 5 GHz to 43 GHz. In a VLBA image at 1.7 GHz, a curved arc of extended emission joins two of the components in a manner suggestive of gravitational lensing. At least two of the radio components have near-infrared counterparts. We argue that this evidence implies that J0134–0931 is a gravitational lens, although we have not been able to devise a plausible model for the foreground gravitational potential. Like several other radio-loud lenses, the background source has an extraordinarily red optical counterpart. Subject headings: gravitational lensing, quasars: individual (J0134–0931) 1.

Research paper thumbnail of A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked

Research paper thumbnail of An Integrated PHM Approach for Gas Turbine Engines

The prognosis & health management (PHM) of aerospace components is a very complex system. A compl... more The prognosis & health management (PHM) of aerospace components is a very complex system. A complete PHM system involves sensors, signal processing, condition monitoring, health assessment, physical reasoning and decision- making. Such an integrated approach to prognostics involves physics-based modelling, experience-based modelling and modelling based on the statistical properties of the components by using neural network and fuzzy logic techniques. In this paper, we give an overview of the PHM system currently being developed. 1.

Research paper thumbnail of A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked spectrum

III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical o... more III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical of a Seyfert galaxy, but also with an extremely variable, blazar-like radio core. We have now discovered a new radio flare where the source has brightened more than twenty-fold within less than two years. A broad-band radio spectrum between 1.4 and 666 GHz shows a textbook-like synchrotron spectrum peaking at 43 GHz, with a selfabsorbed synchrotron spectral index +2:5 at frequencies below 43 GHz and an optically thin spectral index \Gamma0:75 at frequencies above 43 GHz. The outburst spectrum can be well fitted by two homogenous, spherical components with equipartition sizes of 0.1 and 0.2 pc at 43 and 15 GHz, and with magnetic fields of 0.4 and 1 Gauss. VLBA observations at 43 GHz confirm this double structure and these sizes. Time scale arguments suggest that the emitting regions are shocks which are continuously accelerating particles. This could be explained by a frustrated jet scenar...

Research paper thumbnail of PMN J2004–1349: A new gravitationally lensed quasar 1

We report the discovery of a new double-image quasar resulting from our search for gravitational ... more We report the discovery of a new double-image quasar resulting from our search for gravitational lenses in the southern sky. Radio source PMN J2004-1349 is composed of two flat-spectrum components of approximately equal brightness separated by 1. ′′ 13 in VLA, MERLIN and VLBA images. The I-band optical counterpart is also double, with roughly the same separation, position angle, and flux ratio as the radio double. Upon subtraction of the quasars from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004-1349 is a gravitational lens, additional information (such as the redshifts of the quasar and lens galaxy) will be needed before many interesting astrophysical quantities (such as the mass of the galaxy, or the predicted time delay between lensed images) can be inferred.

Research paper thumbnail of A nearly symmetric double-image gravitational lens 1

We report the discovery of a new double-image gravitational lens, resulting from our search for l... more We report the discovery of a new double-image gravitational lens, resulting from our search for lenses in the southern sky. Radio source PMN J2004[1349 is composed of two compact components separated by in VLA, MERLIN, and VLBA images. The components have a Ñux ratio of 1 : 1 at radio 1A .13 frequencies ranging from 5 to 22.5 GHz. The I-band optical counterpart is also an equal double, with roughly the same separation and position angle as the radio double. Upon subtraction of the components from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004[1349 is a gravitational lens, additional information will be necessary (such as the redshifts of the galaxy and quasar and precise astrometry and photometry of the lens galaxy) before constructing detailed mass models.