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Astron. Astrophys. 320, L5-L8 (1997)

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5. Variability

With this Mkn 501 dataset collected over an exceptionally long period (approx. 6 months) during which no significant change was made to the telescope and data processing stream, an investigation of the stability of our signal over time is of interest. In fact, flux variability is a common characteristic of EGRET blazar measurements (von Montigny et al. 1995 ). The Whipple group finds the VHE flux of Mkn 501's sister object Mkn 421 to be highly variable on timescales of months to days (Macomb et al. 1995 ) even observing a flare event in which the flux rose to [FORMULA] 50 times its quiescent level (Gaidos et al. 1996 ). During the 1995 Whipple Mkn 501 observations the flux from this source appeared approximately constant (Quinn et al. 1996 ) except on one day (of 58) when the measured rate lay 5 standard deviations above the mean. Figure 3 shows the rate of excess [FORMULA] -ray candidate events recorded by CT1 on nights when Mkn 501 was observed for more than 20 minutes at [FORMULA] [FORMULA]. Above our threshold of 1.5 TeV we have seen no significant "burst behaviour".

[FIGURE] Fig. 3. Daily excess [FORMULA] -ray event rates from Mkn 501. A constant fit (dashed line) to 70 nights of CT1 data (the 70 nights on which Mkn 501 was observed at [FORMULA] [FORMULA] for at least 20 minutes) leads to a rate of 1.88 [FORMULA] 0.26 h-1, consistent within our errors with the mean rate obtained from the total [FORMULA] [FORMULA] dataset of 2.39 [FORMULA] 0.46 h-1. The peak rate of 7.9 [FORMULA] 2.2 h-1, recorded during a 3.5 h observation centred on MJD 50253.97, lies 2.5 standard deviations from the mean.
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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998