Giovanni Fossati

A unifying view of the spectral energy distributions of blazars

ABSTRA C T We collect data at well-sampled frequencies from the radio to the g-ray range for the following three complete samples of blazars: the Slew survey, the 1-Jy samples of BL Lacs and the 2-Jy sample of flat-spectrum radio-loud quasars (FSRQs). The fraction of objects detected in g-rays (E * 100 MeV) is ,17, 26 and 40 per cent in the three samples respectively. Except for the Slew survey sample, g-ray detected sources do not differ either from other sources in each sample, or from all the g-ray detected sources, in terms of the distributions of redshift, radio and X-ray luminosities or of the broad-band spectral indices (radio to optical and radio to X-ray). We compute average spectral energy distributions (SEDs) from radio to g-rays for each complete sample and for groups of blazars binned according to radio luminosity, irrespective of the original classification as BL Lac or FSRQ. The resulting SEDs show a remarkable continuity in that (i) the first peak occurs in different frequency ranges for different samples/luminosity classes, with most luminous sources peaking at lower frequencies; (ii) the peak frequency of the g-ray component correlates with the peak frequency of the lower energy one; (iii) the luminosity ratio between the high and low frequency components increases with bolometric luminosity. The continuity of properties among different classes of sources and the systematic trends of the SEDs as a function of luminosity favour a unified view of the blazar phenomenon: a single parameter, related to luminosity, seems to govern the physical properties and radiation mechanisms in the relativistic jets present in BL Lac objects as well as in FSRQs. The general implications of this unified scheme are discussed while a detailed theoretical analysis, based on fitting continuum models to the individual spectra of most g-ray blazars, is presented in a separate paper.

A theoretical unifying scheme for gamma-ray bright blazars

ABSTRA C T The phenomenology of g-ray bright blazars can be accounted for by a sequence in the source power and intensity of the diffuse radiation field surrounding the relativistic jet. Correspondingly, the equilibrium particle distribution peaks at different energies. This leads to a trend in the observed properties: an increase of the observed power corresponds to: (i) a decrease in the frequencies of the synchrotron and inverse Compton peaks, and (ii) an increase in the ratio of the powers of the high- and low-energy spectral components. Objects along this sequence would be observationally classified respectively as high-frequency BL Lac objects, lowfrequency BL Lac objects, high-polarization quasars and low-polarization quasars. The proposed scheme is based on the correlations among the physical parameters derived in the present paper by applying to 51 g-ray loud blazars two of the most accepted scenarios for the broad-band emission of blazars, namely the synchrotron self-Compton and external Compton models. This also explains the observational trends presented by Fossati et al., dealing with the spectral energy distributions of all blazars. This gives us confidence that our scheme applies to all blazars as a class.

Hard X{ray properties of blazars ?

We have considered all blazars observed in the X{ray band and for which the slope of the X{ray spectrum is available. We have collected 421 spectra of 268 blazars, including 12 archival unpublished ASCA spectra of 7 blazars whose analysis is presented here. The X{ray spectra of blazars show trends as a function of their power, conrming that the blazar overall energy distribution can be parameterized on the basis of one parameter only, i.e. the bolometric luminosity. This is conrmed by the relatively new hard (2{10 keV) X{ray data. Our results conrm the idea that in low power objects the X{ray emission mechanism is the synchrotron process, dominating both the soft and the hard X{ray emissions. Low energy peaked BL Lac objects are intermediate, often showing harder spectra in the hard X{ray band, suggesting that the synchrotron process dominates in the soft band, with the inverse Compton process dominating at high energies. The most powerful objects have X{ray spectra that are flat both in the soft and in the hard band, consistent with a dominating inverse Compton component.

On the Soft X-Ray Spectra of γ-Loud Blazars

ROSAT observations of a large sample of bright gamma-ray (E > 100 MeV) blazars are presented. Results of a detailed spectral analysis in the soft

X-Ray Emission of Markarian 421: New Clues from Its Spectral Evolution. II. Spectral Analysis and Physical Constraints

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Theoretical Implications from the Spectral Evolution of Markarian 501 Observed with BeppoSAX

0.1-2.0 keV energy range are discussed in relation to the overall energy distribution with particular emphasis on the relation between X-ray and gamma-ray properties. A significant anti-correlation between X-ray and gamma-ray spectral shapes of flat radio spectrum quasars (FSRQ) and BL Lacs has been discovered. A different shape in the overall energy distributions from radio to gamma-ray energies between FSRQ and BL Lacs is also implied by the correlation of their broad-band spectral indices

Extreme synchrotron BL Lac objects - Stretching the blazar sequence

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SIMULTANEOUS X-RAY AND TeV OBSERVATIONS OF A RAPID FLARE FROM MARKARIAN 421

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Complex Spectral Variability from Intensive Multiwavelength Monitoring of Markarian 421 in 1998

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On the soft X-ray spectra of gamma-loud blazars

. Both the above correlations can be explained if both the IR to UV emission and the hard X-ray to gamma-ray emission originate from the same electron population, via, respectively, the synchrotron process and the inverse Compton mechanism. We suggest that a key parameter for understanding the overall energy distributions of both classes of objects is the energy at which the synchrotron emission peaks in a