The total data set for each source has been tested for the presence of gamma-ray signals. The significance of gamma-ray excesses have been calculated using a method based on that of Li & Ma 1983:
where S is the statistical significance and is the ratio of events in the on-source observation to those in the off-source observation in the range (where alpha is the image parameter describing image orientation). The values of and are calculated from the gamma-ray selected (alpha ) data for the on and off-source observations respectively. This method tends to slightly overestimate the significances because it does not account for the statistical uncertainty in calculating the value of . Based on Monte Carlo simulations we estimate that this effect is small - and less than the typical systematic differences caused by the variations in parameter distributions between on and off-source observations. In the total data set for each source no significant excess is seen for those events in the gamma-ray domain. The calculated excesses are (PKS0521-365), (EXO 0423.4-0840), (PKS2005-489) and (PKS2316-423). Upper limits to steady emission have been calculated after Protheroe 1984 and are shown in the scatter plot in Fig. 1.
We have also searched our data set for gamma-ray emission on a night by night basis. In general our observations of a source consist of a long (several hours) on-source run, with a similar length off-source run, offset in RA to provide the same coverage of azimuth and zenith. The flare search has been performed by calculating the on-source excess for each pair of on/off observations each night. In cases where there is no matching off-source run, an equivalent off-source run from another nearby night is used. Fig. 2 shows the distribution of on-source significances for all three sources. There is no evidence for gamma-ray flares on the timescale of 1 night for any of the sources. The most significant nightly excess (from PKS0521-365) has a nominal significance of but after allowing for the number of searches performed this significance is reduced to less than . The upper limits to gamma-ray emission for these observations are shown in Fig. 1. We have also included, in Table 2, a list of upper limits to gamma-ray emission for each individual observation.
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998