Browsing by Author "Hjorth, J"

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    An optical time-delay for the lensed BAL quasar HE 2149-2745
    (2002) Burud, I; Courbin, F; Magain, P; Lidman, C; Hutsemékers, D; Kneib, JP; Hjorth, J; Brewer, J; Pompei, E; Germany, L; Pritchard, J; Jaunsen, AO; Letawe, G; Meylan, G
    We present optical V and i-band light curves of the gravitationally lensed BAL quasar HE 2149-2745. The data, obtained with the 1.5 m Danish Telescope (ESO-La Silla) between October 1998 and December 2000, are the first from a long-term project aimed at monitoring selected lensed quasars in the Southern Hemisphere. A time delay of 103+/-12 days is determined from the light curves. In addition, VLT/FORS1 spectra of HE 2149 2745 are deconvolved in order to obtain the spectrum of the faint lensing galaxy, free of any contamination by the bright nearby two quasar images. By cross-correlating the spectrum with galaxy-templates we obtain a tentative redshift estimate of z = 0.495+/-0:01. Adopting this redshift, a Omega = 0.3, Lambda = 0.7 cosmology, and a chosen analytical lens model, our time-delay measurement yields a Hubble constant of H-0 = 66+/-8 km s(-1) Mpc(-1) (1sigma error) with an estimated systematic error of +/-3 km s(-1) Mpc(-1). Using non-parametric models yields H-0 = 65+/-8 km s(-1) Mpc(-1) (1sigma error) and confirms that the lens exhibits a very dense/concentrated mass profile. Finally, we note, as in other cases, that the flux ratio between the two quasar components is wavelength dependent. While the flux ratio in the broad emission lines-equal to 3.7-remains constant with wavelength, the continuum of the brighter component is bluer. Although the data do not rule out extinction of one quasar image relative to the other as a possible explanation, the effect could also be produced by differential microlensing by stars in the lensing galaxy.
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    The late afterglow and host galaxy of GRB 990712
    (2000) Hjorth, J; Holland, S; Courbin, F; Dar, A; Olsen, LF; Scodeggio, M
    We present deep Hubble Space Telescope (HST) imaging, as well as ground-based imaging and spectroscopy, of the optical afterglow associated with the long-duration gamma-ray burst GRB 990712 and its host galaxy. The data were obtained 48-123 days after the burst occurred. The magnitudes of the host (R = 21.9, V = 22.5) and optical afterglow (R = 25.4, V= 25.8, 47.7 days after the burst) favor a scenario in which the optical light follows a pure power-law decay with an index of alpha similar to -1.0. We find no evidence for a contribution from a supernova like SN 1998bw. This suggests that either there are multiple classes of long-duration gamma-ray bursts or that the peak luminosity of the supernova was more than 1.5 mag fainter than SN 1998bw. The HST images and EFOSC2 spectra indicate that the gamma-ray burst was located in a bright, extended feature (possibly a star-forming region) 1.4 kpe from the nucleus of a 0.2L*(B) galaxy at z = 0.434, possibly a Seyfert 2 galaxy. The late-time afterglow and host galaxy of GRB 990712 bear some resemblance to those of GRB 970508.
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    Time delay and lens redshift for the doubly imaged BAL quasar SBS 1520+530
    (2002) Burud, I; Hjorth, J; Courbin, F; Cohen, JG; Magain, P; Jaunsen, AO; Kaas, AA; Faure, C; Letawe, G
    We present optical R-band light curves of the gravitationally lensed quasar SBS 1520+530 derived from data obtained at the Nordic Optical Telescope. A time delay of 130 +/- 3 days (1sigma) is determined from the light curves. In addition, spectra of SBS 1520+530 obtained at the Keck Observatory are spatially deconvolved in order to extract the spectrum of the faint lensing galaxy, free of any contamination by the light from the bright quasar images. This spectrum indicates a lens redshift z = 0.717, in agreement with one of the absorption systems found in the quasar spectra. The best mass model of the system includes a second nearby galaxy and a cluster of galaxies in addition to the main lensing galaxy. Adopting this model and an Omega = 0.3, Lambda = 0.7 cosmology, our time-delay measurement yields a Hubble constant of H-0 = 51 +/- 9 km s(-1) Mpc(-1) (1sigma error).