Ulf Borgeest

Variability Characteristics of Blazar OJ 287

Blazar OJ 287 is one of the best observed extragalactic objects. It’s historical light curve goes back to 1890’s. Based on the historical behaviour Sillanpaa et al. (1988) showed that OJ 287 displays large periodic outbursts, with a period of 11.7 years. We have monitored OJ 287 intensively for two years, during the OJ-94 project. This project was created for monitoring OJ 287 during its predicted new outburst in 1994. In the data archive we have over 7000 observations on OJ 287, in the radio, infrared and optical bands. This data archive contains the best ever obtained light curves for any extragalactic object. The optical light curve shows continuous variability down to time scales of tens of minutes. The variability observed in OJ 287 can be broken down to (at least) four different categories: 1. The large outbursts that occur every 11.7 years. The last one of these happened during November 1994, almost at the predicted time. These outbursts can be due to the binary black hole model proposed by Sillanpaa et al. (1988).

The Hamburg quasar monitoring program at Calar Alto

We carry out an optically broad-band photometric monitoring program of quasars using the MPIA 1.23 m telescope equipped with a CCD camera. The frames are reduced automatically within a few minutes after each exposure. More than 70 quasars, selected as good candidates to show gravitational microlensing variability in their lightcurves, are observed regularly with time spacings depending on their variability time scales. To optimize the efficiency of our program, we have added ≈- 200 quasars of lower priority. In this contribution, we concentrate on the technical aspects and show a few lightcurves of special interest. Further discussion can be found in Borgeest & Schramm (this proc., hereafter B&S).

Searching for microlensing in optical lightcurves of quasars

In this contribution, we treat the following aspects of gravitational microlensing: 1. the idea of Ostriker & Vietri that some BL Lac objects might be microlensed blazars, 2. the proposal that the variations of some optically violent variables (OVVs) might be the result of microlensing, and 3. the question if fore-ground galaxies of close angular separation will cause microlensing variability of quasars in a statistically significant manner. Indications for microlensing are only found concerning the third point.