We present the results of high time resolution optical photometry of five quiescent soft X-ray transients (SXTs): V404 Cyg, A0620-00, J0422+32, GS 2000+25, and Cen X-4. We detect fast optical variations superposed on the secondary star's double-humped ellipsoidal modulation. The variability resembles typical flare activity and has amplitudes

We study a recently observed gravitational microlensing peak in the V-band light curve of Q2237+0305A using a relatively simple physical model, but one that is highly consistent with the data (the best-fit reduced chi 2 is very close to 1). The source quasar is assumed to be a Newtonian geometrically-thin and optically-thick accretion disk. The

The determination of the fundamental cosmological parameters has achieved dramatic progress over the past few years. Using Cepheids and secondary distance indicators, the expansion rate of the Universe is constrained to H0 = 60-75 km s-1 Mpc-1. More stringent constraints can be expected from independent determinations based on the Sunyaev-Ze'dovich

By using the Synspec program with different LTE and NLTE atmospheric models of Alpha Lyrae (Vega, spectral type A0V), we obtain different emitted fluxes. Taking into account the distance from Hipparcos, it is obtained the spectrum that would be observed from Earth for each model. This spectrum is compared with UV calibrations from the IUE and UARS

During the GTC integration phase, the Commissioning Instrument (CI) will be a diagnostic tool for performance verification. The CI features four operation modes-imaging, pupil imaging, Curvature WFS, and high resolution Shack-Hartmann WFS. After the GTC Commissioning we also plan to install a Pyramid WFS. This instrument can therefore serve as a

On two occasions, we obtained nearly simultaneous ~=4 ks snapshot observations of the Galactic black hole and microquasar XTE J1550-564 with Chandra and RXTE near the peak of its 2000 May outburst. The low-energy sensitivity of Chandra and the resolution of the High Energy Transmission Grating Spectrometer (HETGS), coupled with the broad energy