Astron. Astrophys. 337, 25-30 (1998)

5. Results

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.

Fig. 1. Scatter plot of night by night flux limits for each of the sources (indicated by crosses). Also shown are the flux limits for the total data set for each source (stars). The two dashed lines show the 2TeV fluxes from Mkn 421 (left) and Mkn 501 (right) (see text for details).

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.

Fig. 2. Distribution of the significances of night by night excesses for all sources.

Table 2. The 2 flux upper limits, F(2TeV) ( photons cm^-2 s^-1) and approximate MJD (-40000) for each on-source observation.

© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998
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