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Astron. Astrophys. 353, 847-852 (2000)

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8. Observational strategy

It is widely believed, that TeV gamma-rays of BL Lac objects are produced by ultra-relativistic electrons, which emit synchrotron radiation in the keV band and produce the TeV photons due to Inverse Compton scattering of soft target photons (e.g., Ghisellini et al. 1998). The X-ray activity of a source gives crucial information about the presence of high energy electrons which could also produce TeV radiation. A major uncertainty arises since the keV photons could either be synchrotron photons of very high energy electrons (the actual energy of the electrons certainly depends on the value of the magnetic field) or could be inverse Compton photons produced by electrons with moderate energies. In the latter case, TeV emission of the source would be less probable.

The ASM/RXTE data provide a unique tool for monitoring the light curve of many X-BL Lac objects, but with a moderate sensitivity. In the case of Mkn 501, the state of increased activity lasted several months (Aharonian et al. 1999a). The ASM data could be used to detect such a state by averaging the data over several days or weeks. Such a large bining time does not allow us to search for short-term variability but is dictated by the low sensitivity of the detector. Moreover, as for Mkn 421, the range 2-12 keV can be located around the "pivot point" of the synchrotron spectrum (the hardening starts at the energy of the pivot point): it is not a completely safe indicator of activity, but it is certainly the best one available. Whenever such a state was detected, observations could be initiated. Such a strategy could be complemented with an alert-system which allows us to react to a flare detection in any chosen wavelength within several hours.

Fig. 7 shows the observation time required to achieve a detection for sources with redshifts ranging from 0. to 0.2. The intrinsic emission is assumed to be 10% to 100% of [FORMULA] (assuming a slope of 2.6) or of [FORMULA] (with a slope of 3.6, see Sect. 3). It can be recognized that, depending on the redshift of the source, observation times of several hours could suffice for a significant detection.

[FIGURE] Fig. 7. Expected observation time for achieving a 3-[FORMULA] detection for a flux (before absorption by the CIB) of 10%, 20%, 50% or 100% of [FORMULA] (full line) or of [FORMULA] (dashed line). Features of the HEGRA 4 CT-system and an energy range of 0.5-3 TeV have been assumed for the calculations; the results are valid for observations with zenith angles below [FORMULA]20o.

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© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000
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