TY - JOUR A1 - Wambsganß, Joachim A1 - Schmidt, Robert W. A1 - Colley, W. A1 - Kundic, T. A1 - Turner, E. L. T1 - Microlensing results from APO monitoring of the double quasar Q0957+561A,B between 1995 and 1998 N2 - If the halo of the lensing galaxy 0957+561 is made of massive compact objects (MACHOs), they must affect the lightcurves of the quasar images Q0957+561 A and B differently. We search for this microlensing effect in the double quasar by comparing monitoring data for the two images A and B - obtained with the 3.5m Apache Point Observatory from 1995 to 1998 - with intensive numerical simulations. This way we test whether the halo of the lensing galaxy can be made of MACHOs of various masses. We can exclude a halo entirely made out of MACHOs with masses between 10-6 Msun and 10-2 Msun for quasar sizes of less than 3x 1014 h60-1/2 cm, hereby extending previous limits upwards by one order of magnitude. Y1 - 2000 ER - TY - JOUR A1 - Schmidt, Robert W. A1 - Wambsganß, Joachim A1 - Pen, U.-L. A1 - Turner, E. L. T1 - APO monitoring of Q2237+0305 in 1995-97 : evidence for microlensing Y1 - 2001 SN - 1-583-81074-9 ER - TY - JOUR A1 - Schmidt, Robert W. A1 - Kundic, T. A1 - Pen, U.-L. A1 - Wambsganß, Joachim A1 - Bergeron, L. E. A1 - Colley, W. A1 - Corson, C. A1 - Hastings, N. G. A1 - Hoyes, T. A1 - Long, D. C. A1 - Loomis, K. A. A1 - Malhotra, S. A1 - Rhoads, J. E. A1 - Stanek, K. Z. T1 - Optical monitoring of the gravitationally lensed quasar Q2237+0305 from APO between June 1995 and January 1998 N2 - We present a data set of images of the gravitationally lensed quasar Q2237+0305, that was obtained at the Apache Point Observatory (APO) between June 1995 and January 1998. Although the images were taken under variable, often poor seeing conditions and with coarse pixel sampling, photometry is possible for the two brighter quasar images A and B with the help of exact quasar image positions from HST observations. We obtain a light curve with 73 data points for each of the images A and B. There is evidence for a long (ga 100 day) brightness peak in image A in 1996 with an amplitude of about 0.4 to 0.5 mag (relative to 1995), which indicates that microlensing has been taking place in the lensing galaxy. Image B does not vary much over the course of the observation period. The long, smooth variation of the light curve is similar to the results from the OGLE monitoring of the system (Wozniak et al. cite{Wozniak00}). Based on observations obtained with the Apache Point Observatory 3.5-meter telescope, which is owned and operated by the Astrophysical Research Consortium. Y1 - 2002 ER - TY - JOUR A1 - Schmidt, Robert W. A1 - Allen, S. W. A1 - Fabian, A. C. T1 - An improved approach to measuring H-0 using X-ray and SZ observations of galaxy clusters N2 - We present an improved method for predicting the Sunyaev-Zeldovich (SZ) effect in galaxy clusters from spatially resolved, spectroscopic X-ray data. Using the deprojected electron density and temperature profiles measured within a fraction of the virial radius, and assuming a Navarro-Frenk-White mass model, we show how the pressure profile of the X-ray gas can be extrapolated to large radii, allowing the Comptonization parameter profile for the cluster to be predicted precisely. We apply our method to Chandra observations of three X-ray-luminous, dynamically relaxed clusters with published SZ data: RX J1347.5-1145, Abell 1835 and Abell 478. Combining the predicted and observed SZ signals, we determine improved estimates for the Hubble constant from each cluster and obtain a weighted mean of H (0) = 69 +/- 8 km s(-1) Mpc(-1) for a cosmology with Omega(m) = 0.3 and Omega(Lambda) = 0.7. This result is in good agreement with independent findings from the Hubble Key Project and the combination of cosmic microwave background and galaxy cluster data Y1 - 2004 SN - 0035-8711 ER - TY - JOUR A1 - Allen, S. W. A1 - Schmidt, Robert W. A1 - Ebeling, H. A1 - Fabian, A. C. A1 - van Speybroeck, L. T1 - Constraints on dark energy from Chandra observations of the largest relaxed galaxy clusters N2 - We present constraints on the mean dark energy density, Omega(X) and dark energy equation of state parameter, w(X), based on Chandra measurements of the X-ray gas mass fraction in 26 X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.07 < z < 0.9. Under the assumption that the X-ray gas mass fraction measured within r(2500) is constant with redshift and using only weak priors on the Hubble constant and mean baryon density of the Universe, we obtain a clear detection of the effects of dark energy on the distances to the clusters, confirming (at comparable significance) previous results from Type la supernovae studies. For a standard Lambda cold dark matter (CDM) cosmology with the curvature Omega(K) included as a free parameter, we find Omega(Lambda) = 0.94(-0.23)(+0.21) (68 per cent confidence limits). We also examine extended XCDM dark energy models. Combining the Chandra data with independent constraints from cosmic microwave background experiments, we find Omega(X) = 0.75 +/- 0.04, Omega(m) = 0.26(- 0.04)(+0.06) and w(X) =-1.26 +/- 0.24. Imposing the prior constraint w(X) > -1, the same data require w(X) < -0.7 at 95 per cent confidence. Similar results on the mean matter density and dark energy equation of state parameter, &UOmega;(m) = 0.24 ± 0.04 and w(X) 1.20(-0.28)(+0.24), are obtained by replacing the cosmic microwave background data with standard priors on the Hubble constant and mean baryon density and assuming a flat geometry Y1 - 2004 SN - 0035-8711 ER -