arXiv subjects

Jacqueline N. Hewitt

Publications and source records attributed to Jacqueline N. Hewitt.

68 records · Page 4Linked to original sources

arXiv2006

The Extragalactic Lens VLBI Imaging Survey (ELVIS). I. A Search for the Central Image in the Gravitational Lens PMN J1838-3427

The Extragalactic Lens VLBI Imaging Survey (ELVIS) searches for central images of lensed radio quasars, in order to measure the central density profiles of distant galaxies. Here we present sensitive multi-epoch Very Long Baseline Array (VLBA) observations of PMN J1838-3427 at 8 GHz, with a 1 sigma noise level of 38 microJy beam^(-1). Based on the absence of a central image of the background source at this level, we explore the possibilities for the central matter distribution in the lens galaxy. A power-law density profile, rho ~ r^(-gamma), must have gamma > 1.93. Thus the density profile is close to an isothermal profile (gamma = 2) or steeper. The upper limit on any constant-density core in an otherwise isothermal profile is ~< 5 parsecs. We also derive the constraints on models in which the density profile is isothermal on kiloparsec scales, but is allowed to have a different power law in the central ~100 parsecs. If the lens galaxy harbors a supermassive black hole, the galaxy profile is allowed to be shallower, but for the expected black hole mass the galaxy profile must still be close to isothermal or steeper.

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arXiv2005

A Search for Radio Gravitational Lenses, Using the Sloan Digital Sky Survey and the Very Large Array

We report on a novel search for radio gravitational lenses. Using the Very Large Array, we imaged ten candidates with both dual redshifts in Sloan Digital Sky Survey spectra and 1.4 GHz radio flux >2 mJy in the FIRST survey. The VLA maps show that in each case the radio emission is associated with the foreground galaxy rather than being lensed emission from the background galaxy, although at least four of our targets are strong lenses at optical wavelengths. These SDSS dual-redshift systems do not have lensed radio emission at the sensitivity of current radio surveys.

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arXiv2005

The Sensitivity of First Generation Epoch of Reionization Observatories and Their Potential for Differentiating Theoretical Power Spectra

Statistical observations of the epoch of reionization (EOR) power spectrum provide a rich data set for understanding the transition from the cosmic "dark ages" to the ionized universe we see today. EOR observations have become an active area of experimental cosmology, and three first generation observatories--MWA, PAST, and LOFAR--are currently under development. In this paper we provide the first quantitative calculation of the three dimensional power spectrum sensitivity, incorporating the design parameters of a planned array. This calculation is then used to explore the constraints these first generation observations can place on the EOR power spectrum. The results demonstrate the potential of upcoming power spectrum observations to constrain theories of structure formation and reionization.

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arXiv2004

Gravitational Lensing Signatures of Supermassive Black Holes in Future Radio Surveys

Observational measurements of the relationship between supermassive black holes (SMBHs) and the properties of their host galaxies are an important method for probing theoretical hierarchical growth models. Gravitational lensing is a unique mechanism for acquiring this information in systems at cosmologically significant redshifts. We review the calculations required to include SMBHs in two standard galactic lens models, a cored isothermal sphere and a broken power law. The presence of the SMBH produces two primary effects depending on the lens configuration, either blocking the ``core'' image that is usually predicted to form from a softened lens model, or adding an extra, highly demagnified, image to the predictions of the unaltered lens model. The magnitudes of these effects are very sensitive to galaxy core sizes and SMBH masses. Therefore, observations of these lenses would probe the properties of the inner regions of galaxies, including their SMBHs. Lensing cross-sections and optical depth calculations indicate that in order to fully observe these characteristic signatures, flux ratios of order 10^6 or more between the brightest and faintest images of the lens must be detectable, and thus the next generation of radio telescope technology offers the first opportunity for a serious observational campaign. Core images, however, are already detectable and with additional observations their statistics may be used to guide future SMBH searches.

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arXiv2004

The Design of Radio Telescope Array Configurations using Multiobjective Optimization: Imaging Performance versus Cable Length

The next generation of radio telescope interferometric arrays requires careful design of the array configuration to optimize the performance of the overall system. We have developed a framework, based on a genetic algorithm, for rapid exploration and optimization of the objective space pertaining to multiple objectives. We have evaluated a large space of possible designs for 27-, 60-, 100-, and 160-station arrays. The 27-station optimizations can be compared to the well-known VLA case, and the larger array designs apply to arrays currently under design such as LOFAR, ATA, and the SKA. In the initial implementation of our framework we evaluate designs with respect to two metrics, array imaging performance and the length of cable necessary to connect the stations. Imaging performance is measured by the degree to which the sampling of the uv plane is uniform. For the larger arrays we find that well-known geometric designs perform well and occupy the Pareto front of optimum solutions. For the 27-element case we find designs, combining features of the well-known designs, that are more optimal as measured by these two metrics. The results obtained by the multiobjective genetic optimization are corroborated by simulated annealing, which also reveals the role of entropy in array optimization. Our framework is general, and may be applied to other design goals and issues, such as particular schemes for sampling the uv plane, array robustness, and phased deployment of arrays.

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arXiv2001

PMN J1632-0033: A new gravitationally lensed quasar

We report the discovery of a gravitationally lensed quasar resulting from our survey for lenses in the southern sky. Radio images of PMN J1632-0033 with the VLA and ATCA exhibit two compact, flat-spectrum components with separation 1.47" and flux density ratio 13.2. Images with the HST reveal the optical counterparts to the radio components and also the lens galaxy. An optical spectrum of the bright component, obtained with the first Magellan telescope, reveals quasar emission lines at redshift 3.42. Deeper radio images with MERLIN and the VLBA reveal a faint third radio component located near the center of the lens galaxy, which is either a third image of the background quasar or faint emission from the lens galaxy.

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arXiv2001

PMN J0134-0931: A gravitationally lensed quasar with an unusual radio morphology

The radio-loud quasar PMN 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 millarcseconds. 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.

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arXiv2000

A nearly symmetric double-image gravitational lens

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 1.13 arcseconds in VLA, MERLIN and VLBA images. The components have a flux ratio of 1:1 at radio frequencies ranging from 5 GHz to 22 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 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.

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arXiv2000

PMN J1838-3427: A new gravitationally lensed quasar

We report the discovery of a new double-image quasar that was found during a search for gravitational lenses in the southern sky. Radio source PMN J1838-3427 is composed of two flat-spectrum components with separation 1", flux density ratio 14:1 and matching spectral indices, in VLA and VLBA images. Ground-based BRI images show the optical counterpart (total I=18.6) is also double with the same separation and position angle as the radio components. An HST/WFPC2 image reveals the lens galaxy. The optical flux ratio (27:1) is higher than the radio value probably due to differential extinction of the components by the lens galaxy. An optical spectrum of the bright component contains quasar emission lines at z=2.78 and several absorption features, including prominent Ly-alpha absorption. The lens galaxy redshift could not be measured but is estimated to be z=0.36 +/- 0.08. The image configuration is consistent with the simplest plausible models for the lens potential. The flat radio spectrum and observed variability of PMN J1838-3427 suggest the time delay between flux variations of the components is measurable, and could thus provide an independent measurement of H_0.

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arXiv2000

A multipole-Taylor expansion for the potential of gravitational lens MG J0414+0534

We employ a multipole-Taylor expansion to investigate how tightly the gravitational potential of the quadruple-image lens MG J0414+0534 is constrained by recent VLBI observations. These observations revealed that each of the four images of the background radio source contains four distinct components, thereby providing more numerous and more precise constraints on the lens potential than were previously available. We expand the two-dimensional lens potential using multipoles for the angular coordinate and a modified Taylor series for the radial coordinate. After discussing the physical significance of each term, we compute models of MG J0414+0534 using only VLBI positions as constraints. The best-fit model has both interior and exterior quadrupole moments as well as exterior m=3 and m=4 multipole moments. The deflector centroid in the models matches the optical galaxy position, and the quadrupoles are aligned with the optical isophotes. The radial distribution of mass could not be well constrained. We discuss the implications of these models for the deflector mass distribution and for the predicted time delays between lensed components.

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arXiv1999

A southern VLA-based gravitational lens search

We present a status report on our search for multiple-image QSOs among southern radio sources. Our goal is to identify new lenses for use in studies of cosmology and galaxy structure. To date we have examined 3300 radio sources, identified one probable lens and one probable binary QSO along with about 50 candidates deserving further follow-up.

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arXiv1998

The Radio Wavelength Time Delay of Gravitational Lens 0957+561

The gravitational lens 0957+561 was monitored with the Very Large Array from 1979 to 1997. The 6 cm light curve data from 1995-1997 and the 4 cm data from 1990-1997 are reported here. At 4 cm, the intrinsic source variations occur earlier and are twice as large as the corresponding variations at 6 cm. The VLBI core and jet components have different magnification factors, leading to different flux ratios for the varying and non-varying portions of the VLA light curves. Using both the PRHQ and Dispersion statistical techniques, we determined the time delay, core flux ratio, and excess non-varying B image flux density. The fits were performed for the 4 cm and 6 cm light curves, both individually and jointly, and we used Gaussian Monte Carlo data to estimate 68% statistical confidence levels. The delay estimates from each individual wavelength were inconsistent given the formal uncertainties, suggesting that there are unmodeled systematic errors in the analysis. We roughly estimate the systematic uncertainty in the joint result from the difference between the 6 cm and 4 cm results, giving 409+-30 days for the PRHQ statistic and 397+-20 days for the Dispersion statistic. These results are consistent with the current optical time delay of 417+-3 days, reconciling the long-standing difference between the optical and radio light curves and between different statistical analyses. The unmodeled systematic effects may also corrupt light curves for other lenses, and we caution that multiple events at multiple wavelengths may be necessary to determine an accurate delay in any lens system. Now that consensus has been reached regarding the time delay in the 0957+561 system, the most pressing issue remaining for determining H_0 is a full understanding of the mass distribution in the lens.

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arXiv1997

15 GHz Monitoring of the Gravitational Lens MG 0414+0534

We report the results of monitoring the four images of the gravitational lens MG 0414+0534 at 15 GHz. In 35 VLA maps spanning 180 days, we measure root-mean-square variations in the image light curves of ~3.5% mostly due to variations in the flux density calibration. The flux ratios, which are independent of flux density calibration variations, show root-mean-square variability of 1-3%. Extensive simulations of the data analysis process show that the observed variations in the flux ratios are likely to be due entirely to errors in the deconvolution process. It is possible that some of the observed variation is due to the source; however, the signal-to-noise ratio is too small to make a time delay determination using a data set of this size.

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arXiv1996

The 6 cm Light Curves of B0957+561, 1979-1994: New Features and Implications for the Time Delay

We report on 15 years of VLA monitoring of the gravitational lens B0957+561 at 6 cm. Since our last report in 1992, there have been 32 additional observations, in which both images have returned to their quiescent flux density levels and the A image has brightened again. We estimate the time delay from the light curves using three different techniques: the chi-squared analysis of Press, Rybicki, & Hewitt (1992a,b), the dispersion analysis of Pelt et al. (1994, 1996), and the locally normalized discrete correlation function of Lehár et al. (1992). Confidence intervals for these time delay estimates are found using Monte Carlo techniques. With the addition of the new observations, it has become obvious that five observations from Spring 1990 are not consistent with the statistical properties of the rest of the light curves, so we analyze the light curves with those points removed, as well as the complete light curves. The three statistical techniques applied to the two versions of the data set result in time delay values in the range 398 to 461 days (or 1.09 to 1.26 years, A leading B), each with ~5% formal uncertainty. The corresponding flux ratios (B/A) are in the range 0.698 to 0.704. Thus, the new features in the light curve show that the time delay is less than 500 days, in contrast with analysis of earlier versions of the radio light curves. The large range in the time delay estimates is primarily due to unfortunate coincidences of observing gaps with flux variations.

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