A. Oscoz

The Influence of Gravitational Microlensing on the Broad Emission Lines of Quasars

We discuss the effects of microlensing on the broad emission lines (BELs) of QSOs in the light of recent determinations of the size of the broad-line region (BLR) and its scaling with QSO luminosity. Microlensing by star-sized objects can produce significant amplifications in the BEL of some multiple-imaged QSOs, and could be very relevant for high-ionization lines. We have identified a group of 10 gravitational lens systems (~30% of the selected sample) in which microlensing could be observed. Using standard kinematic models for active galactic nuclei, we have studied the changes induced in the line profile by a microlens located at different positions with respect to the center of the BLR. We found that microlensing could produce important effects such as the relative enhancement of different parts of the line profile or the displacement of the peak of the line. The study of BEL profiles of different ionization in a microlensed QSO image could be an alternative method for probing the BLR structure and size.

Detection of the 2175 Å Extinction Feature at z = 0.83

We determine the extinction curve in the z_l=0.83 lens galaxy of the gravitational lens SBS0909+532 from the wavelength dependence of the flux ratio between the lensed quasar images (z_s=1.38) from 3400 to 9200\AA. It is the first measurement of an extinction curve at a cosmological distance of comparable quality to those obtained within the Galaxy. The extinction curve has a strong 2175\AA feature, a noteworthy fact because it has been weak or non-existent in most estimates of extinction curves outside the Galaxy. The extinction curve is fitted well by a standard

The Very Red Afterglow of GRB 000418: Further Evidence for Dust Extinction in a Gamma-Ray Burst Host Galaxy*

R_V=2.1\pm0.9

QSO 2237+0305 VR Light Curves from Gravitational LensES International Time Project Optical Monitoring

Galactic extinction curve. If we assume standard Galactic extinction laws, the estimated dust redshift of

Time Delay in QSO 0957+561 From 1984-1999 Optical Data

z=0.88\pm0.02

The Optical/IR Counterpart of the 1998 July 3 Gamma-Ray Burst and Its Evolution

is in good agreement with the spectroscopic redshift of the lens galaxy. The widespread assumption that SMC extinction curves are more appropriate models for cosmological dust may be incorrect.

Around-the-Clock Observations of the Q0957+561A,B Gravitationally Lensed Quasar. II. Results for the Second Observing Season

We report near-infrared and optical follow-up observations of the afterglow of the GRB 000418 start- ing 2.5 days after the occurrence of the burst and extending over nearly 7 weeks. GRB 000418 represents the second case for which the afterglow was initially identi—ed by observations in the near-infrared. During the —rst 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times the temporal evolution of the afterglow —attens with respect to a simple power-law decay. Attributing this to an underlying host galaxy, we —nd its magnitude to be R \ 23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R(K B 4 mag. The observations can be explained by an adiabatic, spherical —reball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions. Subject heading: gamma rays: bursts

Two-dimensional Spectroscopy Reveals an Arc of Extended Emission in the Gravitational Lens System Q2237+0305

We present VR observations of QSO 2237+0305 conducted by the Gravitational Lensing International Time Project collaboration from 1999 October 1 to 2000 February 3. The observations were made with the 2.56 m Nordic Optical Telescope at Roque de los Muchachos Observatory, La Palma (Spain). The point-spread function (PSF) fitting method and an adapted version of the ISIS subtraction method have been used to derive the VR light curves of the four components (A-D) of the quasar. The mean errors range in the intervals 0.01-0.04 mag (PSF fitting) and 0.01-0.02 mag (ISIS subtraction), with the faintest component (D) having the largest uncertainties. We address the relatively good agreement between the A and D light curves derived using different filters, photometric techniques, and telescopes. The new VR light curves of component A extend the time coverage of a high-magnification microlensing peak, which was discovered by the OGLE team.

High-contrast optical imaging of companions: the case of the brown dwarf binary HD 130948BC

Photometric optical data of QSO 0957+561 covering the period 1984-1999 are analyzed to discern between the two values of the time delay (417 and 424 days) mostly accepted in the recent literature. The observations, performed by groups from three different institutions—Princeton University, Harvard-Smithsonian Center for Astrophysics, and Instituto de Astrofisica de Canarias—and including new unpublished 1998-1999 data from the IAC80 Telescope, were obtained in five filters (V, R, I, g, and r). The different light curves have been divided into observational seasons, and two restrictions have been applied to calculate the time delay better: (1) points with a strange photometric behavior have been removed, and (2) data sets without large gaps have been selected. Simulated data were generated to test several numerical methods intended to compute the time delay (ΔτAB). The methods giving the best results—the discrete correlation function, δ2, z-transformed discrete correlation function, and linear interpolation—were then applied to real data. A first analysis of the 23 different time delays derived from each technique shows that ΔτAB must be in the interval 420-424 days. From our statistical study, a most probable value of ΔτAB = 422.6 ± 0.6 days is inferred.

Recurrence of the blue wing enhancements in the high-ionization lines of SDSS 1004+4112A

We imaged the X-ray error box of GRB 980703, beginning 22.5 hr after the γ-ray event, in both the optical R and near-infrared H bands. A fading optical/IR object was detected within the X-ray error box, coincident with the variable radio source reported by Frail et al. in 1998, who also detected the optical transient independently of us. Further imagery revealed the gamma-ray burst (GRB) host galaxy, with R = 22.49±0.04 and H=20.5 ± 0.25, the brightest so far detected. When excluding its contribution to the total flux, both the R- and H-band light curves are well fit by a power-law decay with index α1.4. Our data suggest an intrinsic column density in the host galaxy of ~3.5 × 1021 cm-2, which indicates the existence of a dense and gas-rich medium in which the GRB occurred, thus supporting the hypernova model scenarios.