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

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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 [FORMULA] is the ratio of events in the on-source observation to those in the off-source observation in the range [FORMULA] (where alpha is the image parameter describing image orientation). The values of [FORMULA] and [FORMULA] are calculated from the gamma-ray selected (alpha [FORMULA]) 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 [FORMULA]. 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 [FORMULA] (PKS0521-365), [FORMULA] (EXO 0423.4-0840), [FORMULA] (PKS2005-489) and [FORMULA] (PKS2316-423). Upper limits to steady emission have been calculated after Protheroe 1984 and are shown in the scatter plot in Fig. 1.

[FIGURE] Fig. 1. Scatter plot of night by night [FORMULA] flux limits for each of the sources (indicated by crosses). Also shown are the [FORMULA] flux limits for the total data set for each source (stars). The two dashed lines show the [FORMULA]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 [FORMULA] 1 night for any of the sources. The most significant nightly excess (from PKS0521-365) has a nominal significance of [FORMULA] but after allowing for the number of searches performed this significance is reduced to less than [FORMULA]. 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.

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


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

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998