Search

We present intensive optical, infrared, and radio monitoring observations of the BL Lac object OJ 287, taken between the years 1993-1998. Two large optical outbursts were detected at the predicted times in November 1994 and December 1995. The detection of these outbursts supports the binary black hole model for OJ 287. Optical and radio polarisation observations show large variability in the degree of polarisation and position angle, very similar to those observed during the 1983/84 outburst in OJ 287. The polarisation position angles show very similar behaviour during these observations, indicating that, at least, the magnetic field orientations in radio and optical bands are related in OJ 287. Optical and infrared light curves show continuous variability in time scales ranging from tens of minutes to years. In the radio bands we have observed some of the lowest ever measured flux levels. During the first optical outburst in November 1994 the observed radio flux was very low, but during the second optical outburst radio bands also showed high flux levels. This is a puzzling observation, which can hopefully be used for discriminating between different outburst models. On top of the large outbursts OJ 287 has displayed flaring activity in time scales from days to weeks and shorter time scale flickering.

In May 1989 we started an extensive monitoring program in order to find and analyse micro-lensing events in the light curves of a selected sample of quasars. We use the MaxPlanck-Institute 1.23 m telescope at the German-Spanish Astronomical Centre on Calar Alto equipped with a CCD camera. For our program, 15-30% of the observation time at this telescope are available in the next five years. Fluxes are measured through different standard broad-band filters relative to comparision stars in the CCD frame. A 0.01 mug accuracy (in relative photometry) can therefore also be reached for non-photometric conditions. Standard CCD reduction, identification of the objects and photometry are carried out in about 5 rain immediately after each observation by using an automatic routine (on a MicroVAX 3200 workstation) developed by us. We want to obtain up to 30 data points for each objects annual observing period with a maximum spacing of two to three weeks. It is therefore possible to recognize a micro-lensing high amplification event (HAE) at its steep wing, assuming that the transverse velocity of the foreground stars relative to the line of sight to the quasar does not exceed 1000 km/s. Since it is supposed that some 'blazars' have radiation components with relativistic transverse velocities, some members of this class will be monitored with correspondingly higher time resolution during shorter periods. When a HAE or another interesting feature will be detected, the fast reduction allows us to follow the light curve with higher accuracy and by using more different filters.

We compare galaxy counts in deep R-band exposures of the fields of 36 highly luminous, high redshift QSOs to those in control fields at a distance of 1 deg. We find indication for a weak overdensity of galaxies in the foreground of QSOs on scales of arcminutes on a low significance level. Counts inside rings around the quasars, stars in the quasar fields and stars in the control fields show evidence for an excess of galaxies on scales of several arcseconds around the quasars as well as for a stronger clustering of galaxies in the QSO fields than in the control fields. We interpret. this in terms of an amplification bias by gravitational lensing.

We present results from a two year intensive monitoring of BL Lac object 3C 66A (PKS 0219+428). This object was observed in outburst during these two years. It reached the brightest ever observed magnitude on V =13.59 (1.2.1995) and on K =10.59 (15.2.1994). The optical and infrared light curves are characterised by randomly distributed fast flares, lasting a few days and well defined outbursts lasting a week or two. On top of these flares we can occasionally see small amplitude microvariability. No clear correlation can be found between the spectral behaviour and the occurrence of these flares. In the radio bands 3C 66A was quite faint and very stable compared to the optical variations. The light curves will be presented with preliminary analysis and discussions on the possible causes for the observed variations.

A fading counterpart to a gamma-ray burst (GRB) would appear as a point source inside a GRB error region soon after the burst which dims on a timescale from minutes to days. The favorable circumstances of the burst GRB 930131 allowed for an international campaign to search for fading counterparts starting 6.8 hr after the burst. We report observations from many optical sites, two radio telescopes, and archival ROSAT data, including deep Schmidt exposures 35, 44, and 64 hr after the burst. No fading counterparts were detected with our observations.