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

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6. Discussion

The interpretation of upper limits from BL Lacs is difficult, because at the present time there is no detailed model to predict TeV fluxes from the different classes of BL Lacs. Stecker et al. 1996 have attempted to predict TeV fluxes from a number of nearby XBLs by assuming that all XBLs have similar spectral characteristics to the known TeV emitter Mkn 421. Macomb et al. 1995 argue that the observed X-ray/TeV gamma-ray flux increases from Mkn 421 during flares indicate that the TeV gamma-rays are produced by a synchrotron self-Compton (SSC) mechanism. Stecker et al. 1996 note that the Compton emission from the electrons in the jet has a similar spectrum to the synchrotron component, but upshifted by the square of the maximum Lorentz factor (estimated to be [FORMULA] for Mkn 421) of the electrons. For Mkn 421 and PKS2155-304 (both XBLs detected by EGRET) the luminosities in the Compton and synchrotron regions are nearly equal. Assuming that other XBLs also have [FORMULA], Stecker et al. 1996 derive the following relationship between X-ray and TeV source fluxes :

[FORMULA]

where [FORMULA] is the frequency and F the flux in each energy band. Using an assumed [FORMULA] spectral index for all XBLs, and absorption of TeV gamma-rays in the intergalactic infra-red based on an average of Models 1 and 2 from Stecker & de Jager 1997, they estimate TeV fluxes above 1 TeV from a number of nearby XBLs based on EXOSAT X-ray flux measurements. It should be noted that a more recent paper (Stecker & de Jager 1998) concludes that the absorption of TeV gamma-rays in the infra-red background is overestimated in Stecker & de Jager 1997. Allowing for this, the predicted flux for PKS2005-489 above 1 TeV is F([FORMULA] photons [FORMULA]. Above 2 TeV (the threshold of CANGAROO in this analysis) this flux would be F([FORMULA] photons [FORMULA], below the flux sensitivity of the measurement made in this paper. The photon spectral index assumed in Stecker et al. 1996 is now incompatible with recent measurements of Mkn 421 and Mkn 501. It is also possible that the SSC mechanism is not primarily responsible for the TeV gamma-ray emission and a number of other mechanisms have been suggested (Mannheim 1993, Dermer & Schlickeiser 1994 and references therein).

Of the other predictions of Stecker et al. 1996, Kerrick et al. 1995 derive an upper limit to the [FORMULA]0.3 TeV emission for the XBL 1ES 1727+502 that is above the calculated flux. It is also worth noting that a deep exposure of the RBL BL Lacertae with the Whipple telescope did not yield a detection (Catanese et al. 1997b).

Current and future observations of a range of BL Lacs by ground based erenkov telescopes should help to clarify the mechanisms for the production of high energy photons in these sources.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998
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