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Astron. Astrophys. 354, 513-521 (2000)

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4. Discussion

At the end of 1996 and early 1997 the CGRO experiments EGRET and COMPTEL observed the well-known quasar 3C 273 at [FORMULA]-ray energies continuously for 7 weeks. The blazar was [FORMULA]-ray active and therefore it was significantly detected by both experiments. Assuming isotropic emission, H0=60 km/s/Mpc, and a q0=0.5 cosmology, we derive an average 7-week luminosity in the EGRET band (100 MeV - 10 GeV) of [FORMULA]1.7[FORMULA]1046erg/s, for the flaring period (period B in Table 2) [FORMULA]2.7[FORMULA]1046erg/s, and [FORMULA]0.9[FORMULA]1046erg/s for the periods outside the two-week flare. The 7-week average luminosity in the COMPTEL band (1-30 MeV) is derived to be [FORMULA]12[FORMULA]1046erg/s.

The blazar was simultaneously observed in the X-ray band by the BeppoSax satellite in a monitoring fashion covering a period of 1.5 weeks with four individual pointings. In X-rays the quasar was about 15% brighter in the first observation than in the last one (Haardt et al. 1998), which is consistent with the [FORMULA]-ray behaviour above 100 MeV. In addition, they note that during these monitoring observations the source was, on average, a factor of 2 brighter as observed half a year earlier. This suggests a correlation of the X- and [FORMULA]-ray behaviour of 3C 273.

Simultaneous X-ray and [FORMULA]-ray measurements provide the possibility of estimating some physical source parameters, if one assumes that both photon populations are generated co-spatially. This is not an unreasonable assumption, given the indication of correlated variability as mentioned above. It was observed also in other blazars like 3C 279 for example (Wehrle et al. 1998). Using the simultaneous X-ray spectra of the BeppoSax Medium Energy Concentrator Spectrometers (MECS) published by Haardt et al. (1998) we derive a flux of [FORMULA]16µJy at 1 keV for 3C 273. Applying the expression for the lower limit on the Doppler factor, [FORMULA], given by Dondi & Ghisellini (1995)and assuming H0=60 km/s/Mpc, we derive


where tvar is the variability time scale in days, E[FORMULA] the highest unabsorbed [FORMULA]-ray energy in GeV, and [FORMULA] the spectral index in X-rays. With a variability scale of one week as observed by EGRET at a significance level of 2.7[FORMULA], an E[FORMULA] of 1 GeV, and an [FORMULA] of 0.6 (energy index) as observed simultaneously by the BeppoSax MECS, we derive a lower limit on the Doppler factor of


A [FORMULA] 2.4 implies that the [FORMULA]-ray luminosities mentioned above overestimate the intrinsic luminosities at least by a factor of about 33 since [FORMULA].

Although EGRET observed time variability at energies above 100 MeV, the COMPTEL experiment between 1 and 30 MeV simultaneously measures a constant flux of 3C 273. In particular, COMPTEL observes no hints of increased [FORMULA]-ray emission in any of its energy bands during the two-week flaring period. This is a surprising result, and in some respects different to simultaneous COMPTEL/EGRET observations of other flaring blazars. For example, during the major outburst of 3C 279, the COMPTEL 10-30 MeV flux followed the flux trend as observed by EGRET at higher energies (Collmar et al. 1997a). Also, by analysing the COMPTEL data of the first 3.5 years on the blazar PKS 0528+134, Collmar et al. (1997b)found the trend that the COMPTEL upper energy band follows the EGRET light curve, while the emission in the COMPTEL 1-3 MeV band was independent of the EGRET-observed behaviour. According to the measurements presented here, the largest [FORMULA]-ray flux ([FORMULA]100 MeV) of 3C 273 is only a high-energy phenomenon, because it is (at least simultaneously) restricted to energies above [FORMULA]30 MeV. This is consistent with the hardening of the [FORMULA]-ray spectrum during the flare. This observation suggests either an additional spectral component which becomes important at energies above 100 MeV or a time-offset between the high- and low-energy [FORMULA]-rays is required.

A different behaviour in the MeV- and [FORMULA]100 MeV band had been observed in the so-called `MeV-blazars' GRO J0506-609 (Bloemen et al. 1995) and PKS 0208-512 (Blom et al. 1995), which - in contrast to the presented case - showed simultaneously strong MeV-emission compared to weak emission above 100 MeV. These sources indicated first that several emission components or mechanisms can be operating at [FORMULA]-ray energies.

In the standard models, the [FORMULA]-ray emission is generated within a jet, where blobs, filled with relativistic leptons, are moving at relativistic speeds along the jet axis. The [FORMULA]-ray emission is generated by inverse-Compton interactions of these blob leptons with soft photons which either are provided by the environment (e.g. accretion disk) of the jet or are self-generated synchrotron photons. In such a picture, the observed behaviour of 3C 273 could be qualitatively explained by a change in the energy distribution of the blob leptons, by a change of the energy distribution of the soft target photons or by both. If the [FORMULA]-ray flare is triggered by a change in the energy distribution of the blob leptons, the observation would require that only the high-energy end of the distribution, responsible for the [FORMULA]-rays in the EGRET band, is increased in energy as well as in number density, while at lower energies the distribution has to remain constant to keep the MeV-emission unchanged. This case, if applicable, might provide hints on the particle acceleration mechanism. If a variation of the soft photon distribution is responsible for the [FORMULA]-ray flare, a flare in a certain wavelength band, e.g. UV flare of accretion disk photons, could trigger the event.

An apparently natural explanation of this uncorrelated EGRET-COMPTEL behaviour would be if the MeV- and [FORMULA]100 MeV-emissions emanate from spatially different regions. However, we consider this explanation unlikely because - as mentioned above - simultaneous [FORMULA]-ray variations have been observed by EGRET and COMPTEL in several blazars. Also during the observational period presented here, both experiments found 3C 273 in an active [FORMULA]-ray state, which suggests a common region of photon generation.

In our opinition the most plausible scenario appears to be that the MeV and the GeV emissions are dominated by different radiation mechanisms with different flaring amplitudes, but emitted co-spatially by the same population of relativistic electrons which are also responsible at least for the high-frequency part of the synchrotron component. Such a two-component scenario for spectral variability during high-energy flares has first been suggested for PKS 0528+134 by Collmar et al. (1997b); see also B"ottcher & Collmar (1998)and Mukherjee et al. (1999). Given typical parameter values for the relativistic electron distribution in AGN jets, and scaling [FORMULA] in units of 3 (due to the results of Eq. 4), the synchrotron emission peaks at


where [FORMULA] is the dimensionless photon energy, [FORMULA] is the co-moving magnetic field in Gauss, and [FORMULA] is the average Lorentz factor of the electrons in units of [FORMULA]. With [FORMULA] and [FORMULA] being of order unity, the synchrotron peak is at [FORMULA] Hz, as generally observed for 3C 273. We will discuss two possible radiation mechanisms for the observed high-energy flare: a) Comptonization of accretion disk radiation which is reprocessed by broad-line region clouds (ECC for External Comptonization of radiation from Clouds; Sikora et al. 1994), and b) Comptonization of synchrotron radiation from the jet, reprocessed by broad-line region clouds (RSy for synchrotron reflection) as proposed by Ghisellini & Madau (1996). The ECC spectrum, which assumes the accretion disk (`blue bump') photons to be the soft target photons, is expected to have a rather narrow spectral distribution, peaking around


where [FORMULA] ([FORMULA]/10-4) - being of order unity - is the dimensionless [FORMULA] peak energy of the accretion disk spectrum and [FORMULA] is the bulk Lorentz factor in units of 10, which is also assumed to be of order unity. Consequently, this peak is expected to be at [FORMULA] GeV. In contrast, the RSy spectrum is expected to be much broader (similar to the synchrotron self-Compton spectrum) and peaks around


typically at MeV energies. The factors [FORMULA] and [FORMULA] in Eqs. (6) and (7) arise from the Lorentz boost of the external radiation field into the comoving rest frame of the blob and of the jet synchrotron radiation into the stationary frame of the BLR.

The estimates derived from Eqs. (6) and (7) indicate that a flare in the EGRET energy range without a significant variation of the MeV emission is more likely to be caused by the ECC mechanism than by the RSy scenario. We point out that these interpretations are based on time-variability of EGRET which is signifcant at 3.1[FORMULA].

Recently McHardy et al. (1999) reported simultaneous mm, infrared (IR), and X-ray (3-20 keV) observations of 3C 273 which cover in part the same time period as the [FORMULA]-ray observations reported here. In particular they observed a simultaneous IR and X-ray flare lasting for about 10 days, which in fact is simultaneous to the high-energy [FORMULA]-ray flare observed by EGRET. These simultaneous flares add important information in light of the two-component hypothesis for the [FORMULA]-ray spectrum proposed here and provide additional constraints for the modelling of the emission processes in this quasar. A discussion and interpretation of the peculiar variability pattern of 3C 273 in IR, X-rays and [FORMULA]-rays as observed in early 1997 will be given by Böttcher & Collmar (in preparation).

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Online publication: February 9, 2000