Upper limit map of a background of gravitational waves

We searched for an anisotropic background of gravitational waves using data from the LIGO S4 science run and a method that is optimized for point sources. This is appropriate if, for example, the gravitational wave background is dominated by a small number of distinct astrophysical sources. No signal was seen. Upper limit maps were produced assuming two different power laws for the source strain power spectrum. For an f^-3 power law and using the 50 Hz to 1.8 kHz band the upper limits on the source strain power spectrum vary between 1.2e-48 Hz^-1 (100 Hz/f)^3 and 1.2e-47 Hz^-1 (100 Hz /f)^3, depending on the position in the sky. Similarly, in the case of constant strain power spectrum, the upper limits vary between 8.5e-49 Hz^-1 and 6.1e-48 Hz^-1. As a side product a limit on an isotropic background of gravitational waves was also obtained. All limits are at the 90% confidence level. Finally, as an application, we focused on the direction of Sco-X1, the closest low-mass X-ray binary. We compare the upper limit on strain amplitude obtained by this method to expectations based on the X-ray luminosity of Sco-X1.Comment: 11 pages, 9 figures, 2 table

Upper limit map of a background of gravitational waves

We searched for an anisotropic background of gravitational waves using data from the LIGO S4 science run and a method that is optimized for point sources. This is appropriate if, for example, the gravitational wave background is dominated by a small number of distinct astrophysical sources. No signal was seen. Upper limit maps were produced assuming two different power laws for the source strain power spectrum. For an f^-3 power law and using the 50 Hz to 1.8 kHz band the upper limits on the source strain power spectrum vary between 1.2e-48 Hz^-1 (100 Hz/f)^3 and 1.2e-47 Hz^-1 (100 Hz /f)^3, depending on the position in the sky. Similarly, in the case of constant strain power spectrum, the upper limits vary between 8.5e-49 Hz^-1 and 6.1e-48 Hz^-1. As a side product a limit on an isotropic background of gravitational waves was also obtained. All limits are at the 90% confidence level. Finally, as an application, we focused on the direction of Sco-X1, the closest low-mass X-ray binary. We compare the upper limit on strain amplitude obtained by this method to expectations based on the X-ray luminosity of Sco-X1.Comment: 11 pages, 9 figures, 2 table

Search for gravitational-lensing signatures in the full third observing run of the LIGO-Virgo network

Gravitational lensing by massive objects along the line of sight to the source causes distortions of gravitational wave-signals; such distortions may reveal information about fundamental physics, cosmology and astrophysics. In this work, we have extended the search for lensing signatures to all binary black hole events from the third observing run of the LIGO--Virgo network. We search for repeated signals from strong lensing by 1) performing targeted searches for subthreshold signals, 2) calculating the degree of overlap amongst the intrinsic parameters and sky location of pairs of signals, 3) comparing the similarities of the spectrograms amongst pairs of signals, and 4) performing dual-signal Bayesian analysis that takes into account selection effects and astrophysical knowledge. We also search for distortions to the gravitational waveform caused by 1) frequency-independent phase shifts in strongly lensed images, and 2) frequency-dependent modulation of the amplitude and phase due to point masses. None of these searches yields significant evidence for lensing. Finally, we use the non-detection of gravitational-wave lensing to constrain the lensing rate based on the latest merger-rate estimates and the fraction of dark matter composed of compact objects

Teaching the DiPasquale-Wheaton Model

The DiPasquale-Wheaton (1992) model graphically determines rental price, asset price, newly constructed stock, and total stock in a real estate market. Despite its frequent use in academic research, few textbooks exposit the model. I conjecture this is due in part to the difficulty of deriving its comparative static results. I derive a supply curve that simplifies graphical analysis and perform a complete graphical comparative static analysis. Although the main objective of this paper is to encourage pedagogical usage of the model, an Appendix provides a mathematical derivation of the comparative static results, which has not heretofore appeared in the literature

The Effect of Density and Trip-Chaining on the Interaction between Urban Form and Transit Demand

It is unclear whether policies designed to reduce auto and increase transit usage achieve their objective. Evidence is mixed because most empirical research on these policies use ad hoc specifications, whereas our models are drawn from economic theory. Three models of increasing generality show how endogenizing relevant variables changes results obtained by others. The theoretical hypotheses are empirically tested using a dataset that integrates travel and land use. Our main findings are (1) population density has a small impact on transit demand, which decreases when residential location is endogenous; (2) households living farther from work use less transit, a result of trip-chaining; and (3) reducing the spatial allocation of non-work activities, improving transit accessibility at and around subcenters, and increasing the presence of retail locations in proximity to transit-oriented households would increase transit demand

Integrating Transit and Urban Form

This study develops an integrated behavioral model of transit patronage and urban form. Although herein focused on transit, the framework can be easily generalized to study other forms of travel. Advanced econometric methods are used to test specific behavioral hypotheses developed in the theoretical models. Findings are then summarized in a succinct fashion showing relevance and magnitude of the impact of land use on transit demand. The empirical models also quantify these relationships in the form of point elasticity estimates that can be used as indicators of the relevance of transit supply measures. This work summarizes the study results, an exposition of the methodology and tables that lay out the findings in a readily accessible format

THE EFFECT OF TRANSPORTATION SUBSIDIES ON URBAN SPRAWL

This paper provides theoretical and empirical analyses of the effect of transportation subsidies on urban sprawl in a two-mode urban spatial model. Comparative static analysis shows, among other things, that the urban area contracts with a public transit subsidy but expands with an auto subsidy. The paper provides the first empirical test of these hypotheses and finds that the spatial size of the urbanized area shrinks with an increase in transit subsidies but increases at a decreasing rate with auto subsidies. Copyright (c) Blackwell Publishing, Inc. 2008