THE AGILE CONTRIBUTION TO AGNs STUDIES

The AGILE γ-ray telescope (Tavani et al. 1998) will have a relatively uniform sensitivity over a very wide field of view. This allows to simultaneously monitor several AGN’s and to react quickly with multiwavelength observations in the case of flaring activity. The large cumulative sky coverage will also lead to a significant increase in the number of γ-ray emitting AGN’s known at the end of the mission. 1. The AGILE contribution to the study of Active Galactic Nuclei Contrary to previous γ-ray instruments based on gas spark chambers (like COS B and EGRET), the tracker of AGILE, using silicon strip detectors, does not need a separate triggering device. This allows to greatly increase the incidence angle of the accepted photons, resulting in a sensitivity that remains close to the on-axis value within a very large field of view. While EGRET at an angle of 30◦ has only ∼10% of the on-axis effective area (Thompson et. al. 1993), the AGILE sensitivity is almost uniform within a radius of ∼ 50-60◦. For example, a single pointing toward 3C 273 (Fig.1) allows to simultaneously observe and to look for significant flaring events in more than 20 sources (including 3C 279, Cen A, Mrk 421 and many other blazars). A γ-ray outburst similar to that observed from the radio-loud quasar PKS 0528+134 in March 1993 (Mukherjee et al. 1996) can easily be detected by AGILE with an integration time of only one day (Fig.2). We plan to routinely search for such kind of events with a quick look analysis of the data and to inform the scientific community in order to react as soon as possible with coordinated observations. Another advantage of the large field of view is that the total exposure factor (cm s) for each direction of the sky will be greater than that obtained by an instrument with a smaller field of view. We have estimated that, after a sequence of pointings equal to that carried out in the EGRET phases 1 and 2 (∼2 years of observations), the average (over the whole sky) exposure factor reached by AGILE would be greater by a factor ∼4. Since this reflects into a factor ∼2 lower limiting flux, we can roughly expect, assuming a γ-ray LogN-LogS with slope 3/2, about three times more AGN’s than the ∼70 discovered by EGRET. This is actually a lower limit since a further increase in sensitivity in confused regions at low galactic latitude is obtained thanks to the better angular resolution of AGILE (Morselli et al. 1998). Fig. 1. Comparison between the AGILE and EGRET fields of view for a pointing toward 3C 279 and 3C 273 (the two regions correspond to radii of 25◦ and 60◦). Fig. 2. Expected AGILE sensitivity (solid lines) for observations of 1 day and 1 week.