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Astron. Astrophys. 364, 165-169 (2000)

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3. The TeV gamma ray signal strength

Our recent work on PKS 2155-304 (Chadwick et al. 1999a) has demonstrated a method of assessing the signal strength of gamma rays recorded by Cherenkov telescopes. It is suited to measurements made at different epochs and at different zenith angles when the telescope has different sensitivities and consequently a varying background cosmic ray detection rate. We have estimated the strength of TeV gamma ray emission by expressing it as a fraction of the cosmic ray background remaining after image shape and orientation selection (Fegan 1997). In so doing we make allowances for variations in sensitivity, in first order, due to changes in efficiency of the telescope and variations in telescope performance with zenith angle. It is also assumed that the slopes of the gamma ray and cosmic ray spectra are similar.

In the present study, the average gamma ray signal strength from Cen X-3, expressed as a percentage of the cosmic ray background remaining after shape and orientation selection is [FORMULA]%. The most straightforward, but not most powerful, test for constancy of emission is to repeat this process for the data recorded in each of the 5 dark periods as shown in Fig. 1a and then test for consistency between the values obtained using a [FORMULA] test. On the basis of this test we find no internal evidence for monthly variability of the VHE signal; the data treated this way are consistent with a constant signal strength [FORMULA], 4 df).

[FIGURE] Fig. 1. a The VHE gamma ray flux from Cen X-3 averaged over observing periods. b  The VHE gamma ray flux from Cen X-3 plotted on a day-by-day basis.

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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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