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Astron. Astrophys. 327, L5-L8 (1997)

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4. Time variability

To investigate time variability, data were grouped in different time bins, ranging from about 5 min. to entire nights. We required that at least two telescopes triggered, and applied a loose angular cut at [FORMULA] ([FORMULA] as well as the loose selection based on the widths of the images. Only observations at zenith angles below [FORMULA] with good weather conditions are considered. Fig. 3 (a) shows the detection rate on a night by night basis for the whole data set. While the rate decreases by about 60 % during the first 9 nights, it increases by a factor of 3.3 during April 9 and again by a factor of 4.3 during April 12. Fig. 3 (b) gives a closer view on the period from April 12 to April 14 in 5 min. intervals. Data are statistically consistent with a constant flux within each of the 3 nights shown.

[FIGURE] Fig. 3. Detection rate of Mkn 501 on a night by night basis (a) for the whole data set and in 5 min. intervals (b) for the last 3 nights. The dashed lines indicate the average per night, the dotted line shows the Crab detection rate. Only observations with a zenith angle below [FORMULA] are considered. On March 17 (open circle) the trigger rate was reduced by about 15% due to slight overcast. Errors are statistical only.
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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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