An occultation of the active galaxy 3C 273 by the Moon was recorded on May 31, 2001, at the Calar Alto Observatory's 3.5m telescope using the Omega-Cass camera. More details on the use of this camera for lunar occultations, using a fast read-out mode in a subarray, was given in the case of the occultation of SAO 95190, a first-time case for such an array. Other infrared camers used for lunar occultation work include the ARNICA instrument at the TIRGO telescope.

The occultation reappearance of 3C 273 is interesting in two respects. Firstly, it is one of the rare cases of extragalactic objects observed by this technique for high angular resolution purposes. Secondly, this occultation was part of an ongoing series which takes place, two Saros cycles later, of the famous occultation which in 1962 allowed to establish the quasi-stellar nature of this radio source. At that time, radioastronomy had the disadvantage of having very poor angular resolution, and it was only thanks to such an occultation (measured in the radio domain), that it was possible to reduce the uncertainty on the position of 3C 273 from several arcminutes to few arcseconds.

With the present occultation, we take ideally the process one step further, and we can now establish an upper limit to the size of the central regions in this active galaxy. The angular resolution offered by our observation is about 50 times better than what HST can achieve.

This movie (284Kb) shows a sequence of 350 frames. Each frame covers 32 x 32 pixels corresponding to 9.6 x 9.6 arcseconds on the sky. They were obtained at the wavelength of 2.2 microns, with 8 milliseconds integration time and a frame rate of 62Hz. A very preliminary fit to the lightcurve obtained from these data is available here . We are currently awaiting the possibility to observe more occultations in this series, before publishing the results.

Added on 05-Mar-02: a report to appear in the Calar Alto Newsletter.

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