Context. BL Lacertae objects are variable at all energy bands on time scales down to minutes. To ... more Context. BL Lacertae objects are variable at all energy bands on time scales down to minutes. To construct and interpret their spectral energy distribution (SED), simultaneous broad-band observations are mandatory. Up to now, the number of objects studied during such campaigns is very limited and biased towards high flux states. Aims. We present the results of a dedicated multi-wavelength study of the high-frequency peaked BL Lacertae (HBL) object and known TeV emitter 1ES 2344+514 by means of a pre-organised campaign. Methods. The observations were conducted during simultaneous visibility windows of MAGIC and AGILE in late 2008. The measurements were complemented by Metsähovi, RATAN-600, KVA+Tuorla, Swift and VLBA pointings. Additional coverage was provided by the ongoing long-term F-GAMMA and MOJAVE programs, the OVRO 40-m and CrAO telescopes as well as the Fermi satellite. The obtained SEDs are modelled using a one-zone as well as a self-consistent two-zone synchrotron self-Compton model. Results. 1ES 2344+514 was found at very low flux states in both X-rays and very high energy gamma rays. Variability was detected in the low frequency radio and X-ray bands only, where for the latter a small flare was observed. The X-ray flare was possibly caused by shock acceleration characterised by similar cooling and acceleration time scales. MOJAVE VLBA monitoring reveals a static jet whose components are stable over time scales of eleven years, contrary to previous findings. There appears to be no significant correlation between the 15 GHz and R-band monitoring light curves. The observations presented here constitute the first multi-wavelength campaign on 1ES 2344+514 from radio to VHE energies and one of the few simultaneous SEDs during low activity states. The quasi-simultaneous Fermi-LAT data poses some challenges for SED modelling, but in general the SEDs are described well by both applied models. The resulting parameters are typical for TeV emitting HBLs. Consequently it remains unclear whether a so-called quiescent state was found in this campaign.
We are conducting a long-term monitoring campaign of the optical and near-IR emission of gamma-ra... more We are conducting a long-term monitoring campaign of the optical and near-IR emission of gamma-ray bright blazars using the SMARTS 1.3m telescope and ANDICAM instrument. We report a continued increase in optical and IR brightness of the blazar PKS 0208-512 during the last 3 weeks at BVRJK bands. Observations carried out on UTC 2011 August 3 (JD 2455777.83480) show R=17.59 and J=15.95 and those on UTC 2011 August 23 (JD 2455797.84450) show R=15.62 and J=13.76.
... Laura Maraschi Osservatorio Astronomico di Brera, Via Brera 28, Milano Italy Rita M. Sambruna... more ... Laura Maraschi Osservatorio Astronomico di Brera, Via Brera 28, Milano Italy Rita M. Sambruna George Mason University, 4400 University Drive, Fairfax, VA, USA & C. Megan Urry Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, USA Abstract. ...
We are conducting a long-term monitoring campaign of the optical and near-IR magnitudes of gamma-... more We are conducting a long-term monitoring campaign of the optical and near-IR magnitudes of gamma-ray bright blazars using the SMARTS 1.3m telescope and ANDICAM instrument. We report a strong optical and IR flare in the blazar 3C 454.3 concurrent with the present extraordinarily high gamma-ray activity (ATel #2328, ATel #2326, ATel #2322), peaking on UT 2009 December 03 with B=14.98, V=14.39, R=13.83, J=11.94, K=10.08.
The Yale/SMARTS optical-near-IR monitoring program has followed the variations in emission of the... more The Yale/SMARTS optical-near-IR monitoring program has followed the variations in emission of the Fermi-LAT monitored blazars in the southern sky with closely spaced observations since 2008. We report the discovery of an optical-near-IR (OIR) outburst with no accompanying gamma-rays in the blazar PKS 0208-512, one of the targets of this program. While the source undergoes three outbursts of 1 mag or more at OIR wavelengths lasting for longer than 3 months during 2008-2011, only interval 1 and 3 have corresponding bright phases in GeV energies lasting longer than 1 month. The OIR outburst during interval 2 is comparable in brightness and temporal extent to the OIR flares during intervals 1 and 3 which do have gamma-ray counterparts. Gamma-ray and OIR variability are very well-correlated in most cases in the Fermi blazars and the lack of correlation in this case is anomalous. By analyzing the gamma-ray, OIR, and supporting multi-wavelength variability data in details, we speculate that the location of the outburst in the jet during interval 2 was closer to the black hole where the jet is more compact and the magnetic field strength is higher, and the bulk Lorentz factor of the material in the jet is smaller. These result in a much lower Compton dominance and no observable gamma-ray outburst during interval 2.
In the context of Inverse Compton models for γ-ray production in blazars, the authors examine the... more In the context of Inverse Compton models for γ-ray production in blazars, the authors examine the role of different sources of soft photons which can be up-scattered by relativistic electrons: (1) the synchrotron photons produced within the jet (2) "ambient" photons from an accretion disk, possibly scattered back to the jet by the broad line clouds and/or other diffuse plasma surrounding the nucleus. It is suggested that different photon sources may be dominant in different objects. External photons may be important in determining the γ-ray luminosity of Highly Polarized Quasars (HPQs), but not in BL Lacs, which lack a broad line region.
The recent campaign on NGC 5548 has shown the continuum to vary simultaneously at ultraviolet and... more The recent campaign on NGC 5548 has shown the continuum to vary simultaneously at ultraviolet and optical frequencies, with an upper limit to the delay of less than 4 days. We examine this result in the context of accretion disk theory. In the first part the standard a-disk model is discussed. The effects connected with heating of the outer regions by radiation from the inner regions are also considered. Neither scenario lends itself to a simple explanation of the observed data. A more successful model involves a geometrically thin outer disk heated by a central extended source of X-ray radiation.
We present the time variability properties of a sample of six blazars, AO 0235+164, 3C 273, 3C 27... more We present the time variability properties of a sample of six blazars, AO 0235+164, 3C 273, 3C 279, PKS 1510-089, PKS 2155-304, and 3C 454.3, at optical-IR frequencies as well as γ-ray energies. These observations were carried out as a part of the Yale/SMARTS program during 2008-2010 that has followed the variations in emission of the bright Fermi Large Area Telescope monitored blazars in the southern sky with closely spaced observations at BVRJK bands. We find that the optical-near IR variability properties are remarkably similar to those at the γ-ray energies. The discrete auto-correlation functions of the variability of these six blazars at optical-IR and γ-ray energies do not show any periodicity or characteristic timescale. The power spectral density (PSD) functions of the R-band variability of all six blazars are fit well by simple power-law functions with negative slopes such that there is higher amplitude variability on longer timescales. No clear break is identified in the PSD of any of the sources. The average slope of the PSD of R-band variability of these blazars is similar to what was found by the Fermi team for the γ-ray variability of a larger sample of bright blazars. This is consistent with leptonic models where the optical-IR and γ-ray emission is generated by the same population of electrons through synchrotron and inverse Compton processes, respectively. The prominent flares present in the optical-IR as well as the γ-ray light curves of these blazars are predominantly symmetric, i.e., have similar rise and decay timescales, indicating that the long-term variability is dominated by the crossing time of radiation or a disturbance through the emission region rather than by the acceleration or energy-loss timescales of the radiating electrons. For the blazar 3C 454.3, which has the highest-quality light curves, the total energy output, the ratio of γ-ray to optical energy output, and the γ-ray versus optical flux relation differ in the six individual flares observed between 2009 August and December. The results are consistent with the location of a large γ-ray outburst in 3C 454.3 during 2009 December being in the jet at ~18 pc from the central engine. This poses strong constraints on the models of high-energy emission in the jets of blazars.
Observational data and theoretical models for the physical processes and constituents of active g... more Observational data and theoretical models for the physical processes and constituents of active galactic nuclei are discussed. A summary of data on the continuum is presented, together with calculations of the dimensions of the emitting regions and the photon optical depths. Energy transfer in Compton scattering is investigated, as are cooling processes, the necessity for local acceleration mechanisms, and the density and size of inhomogeneities. The possibility of accretion onto massive black holes is discussed, as are quasi-spherical dissipative accretion, a continuum radiation emitted by magnetized spherically accreting plasma, and the observed line emission. Processes of X ray heating are explored. Deficiencies in a current model which includes a chaotic infall of magnetized plasma onto a black hole are suggested to be due to a neglect of the dynamical process, particularly in the broad line region where the flow may reverse because of the heating effect of hard photons emitted from the inner region.
We report ongoing spectroscopic observations of 3C 454.3 using the Small and Moderate Aperture Re... more We report ongoing spectroscopic observations of 3C 454.3 using the Small and Moderate Aperture Research Telescope System SMARTS 1.5m telescope + RC Spectrograph located at Cerro-Tololo Inter-American Observatory (CTIO). Spectra have been obtained roughly every 14 days from August 2008 through January 2011, during which 3C 454.3 has undergone several prominent optical (and γ-ray) flares. We find that while the equivalent width (EW) of 3C 454.3 varies, the line flux remains constant. This result suggests that the Broad Line Region is photoionized by the slowly varying accretion disk rather than radiation from the relativistic jet. We find the strength of the MgII line to be consistent with a constant line flux of ˜ 2.0× 10-14 erg s-1 cm2 A-1. This constant line flux argues against the photoionization of the BLR from the relativistic jet, as this source has undergone a number of flares during the course of observation with no corresponding change in line flux. This finding also implies that accretion disk is non-variable on timescales of at least 2.5 years.
ABSTRACT The standard model of an accretion disk is considered. The temperature in the inner regi... more ABSTRACT The standard model of an accretion disk is considered. The temperature in the inner region is computed assuming that the radiated power derives from Comptonized photons, produced in a homogeneous single-temperature plasma, supported by radiation pressure. The photon production mechanisms are purely thermal, including ion-electron bremsstrahlung, bound-free and bound-bound processes, and e-e bremsstrahlung. Pair production is not included, which limits the validity of the treatment to kT less than 60 keV. Three different approximations for the effects of Comptonization on the energy loss are used, yielding temperatures which agree within 50 percent. The maximum temperature is very sensitive to the accretion rate and viscosity parameters, ranging, for a 10 to the 8th solar mass black hole, between 0.1 and 50 keV for m between 0.1 and 1 and alpha between 0.1 and 1 and, for a 10-solar-mass black hole, between 0.6 and 60 keV for m between 0.1 and 0.9 and alpha between 0.1 and 0.5. For high viscosity and accretion rates, the emission spectra show a flat component following a peak corresponding to the temperature of the innermost optically thick annulus.
Iau Colloq 163 Accretion Phenomena and Related Outflows, 1997
The current status of understanding of the X-ray emission from Seyfert galaxies involves Comptoni... more The current status of understanding of the X-ray emission from Seyfert galaxies involves Comptonization of soft photons by hot subrelativistic electrons. After briefly reviewing the early theoretical basis for the presence of hot optically thin plasma in or around accretion disks and the key observations that led to develop the presently popular model of an accretion disk with a hot corona, we summarize recent progress in accretion models that take into account energy dissipation and/or angular momentum transport in the corona. Finally, adopting the simple scheme of a homogeneous plane parallel corona, we discuss in detail the dependence of the X-ray spectrum on the coronal parameters. Despite the strong coupling between optical depth and temperature which determines a spectral shape insensitive to their precise values, moderate spectral changes are possible. The spectral variability patterns can be used as diagnostics for coronal physics and should allow to determine whether the optical depth of the corona is dominated by e+e- pairs.
Monthly Notices of the Royal Astronomical Society, May 12, 1997
We discuss alternative interpretations of the differences in the spectral energy distributions (S... more We discuss alternative interpretations of the differences in the spectral energy distributions (SEDs) of BLLacs found in complete radio or X-ray surveys. A large body of observations in different bands suggests that the SEDs of BL Lac objects appearing in X-ray surveys differ from those appearing in radio surveys mainly in having a (synchrotron) spectral cut-off (or break) at much higher frequency. In order to explain the different properties of radio- and X-ray-selected BLLacs, Giommi and Padovani proposed a model based on a common radio luminosity function. At each radio luminosity, objects with high-frequency spectral cut-offs are assumed to be a minority. Nevertheless, they dominate the X-ray-selected population due to the larger X-ray-to-radio flux ratios. An alternative model explored here (reminiscent of the orientation models previously proposed) is that the X-ray luminosity function is `primary', and that at each X-ray luminosity a minority of objects have larger radio-to-X-ray ratios. The predictions of the two scenarios, computed via a Monte Carlo technique, are compared with the observed properties of BLLacs in the two samples extracted, respectively, from the 1-Jy radio survey and the Einstein Slew Survey. We show that both models can explain some but not all of the observed features. We then propose a completely new approach, based on the idea that the physical parameter which governs the shape of the SEDs is (or is associated with) the bolometric luminosity. Assuming an empirical relation between spectral shape and luminosity, we show that the observational properties of the two surveys can be reproduced with at least the same accuracy as the two previous models.
Context. BL Lacertae objects are variable at all energy bands on time scales down to minutes. To ... more Context. BL Lacertae objects are variable at all energy bands on time scales down to minutes. To construct and interpret their spectral energy distribution (SED), simultaneous broad-band observations are mandatory. Up to now, the number of objects studied during such campaigns is very limited and biased towards high flux states. Aims. We present the results of a dedicated multi-wavelength study of the high-frequency peaked BL Lacertae (HBL) object and known TeV emitter 1ES 2344+514 by means of a pre-organised campaign. Methods. The observations were conducted during simultaneous visibility windows of MAGIC and AGILE in late 2008. The measurements were complemented by Metsähovi, RATAN-600, KVA+Tuorla, Swift and VLBA pointings. Additional coverage was provided by the ongoing long-term F-GAMMA and MOJAVE programs, the OVRO 40-m and CrAO telescopes as well as the Fermi satellite. The obtained SEDs are modelled using a one-zone as well as a self-consistent two-zone synchrotron self-Compton model. Results. 1ES 2344+514 was found at very low flux states in both X-rays and very high energy gamma rays. Variability was detected in the low frequency radio and X-ray bands only, where for the latter a small flare was observed. The X-ray flare was possibly caused by shock acceleration characterised by similar cooling and acceleration time scales. MOJAVE VLBA monitoring reveals a static jet whose components are stable over time scales of eleven years, contrary to previous findings. There appears to be no significant correlation between the 15 GHz and R-band monitoring light curves. The observations presented here constitute the first multi-wavelength campaign on 1ES 2344+514 from radio to VHE energies and one of the few simultaneous SEDs during low activity states. The quasi-simultaneous Fermi-LAT data poses some challenges for SED modelling, but in general the SEDs are described well by both applied models. The resulting parameters are typical for TeV emitting HBLs. Consequently it remains unclear whether a so-called quiescent state was found in this campaign.
We are conducting a long-term monitoring campaign of the optical and near-IR emission of gamma-ra... more We are conducting a long-term monitoring campaign of the optical and near-IR emission of gamma-ray bright blazars using the SMARTS 1.3m telescope and ANDICAM instrument. We report a continued increase in optical and IR brightness of the blazar PKS 0208-512 during the last 3 weeks at BVRJK bands. Observations carried out on UTC 2011 August 3 (JD 2455777.83480) show R=17.59 and J=15.95 and those on UTC 2011 August 23 (JD 2455797.84450) show R=15.62 and J=13.76.
... Laura Maraschi Osservatorio Astronomico di Brera, Via Brera 28, Milano Italy Rita M. Sambruna... more ... Laura Maraschi Osservatorio Astronomico di Brera, Via Brera 28, Milano Italy Rita M. Sambruna George Mason University, 4400 University Drive, Fairfax, VA, USA & C. Megan Urry Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, USA Abstract. ...
We are conducting a long-term monitoring campaign of the optical and near-IR magnitudes of gamma-... more We are conducting a long-term monitoring campaign of the optical and near-IR magnitudes of gamma-ray bright blazars using the SMARTS 1.3m telescope and ANDICAM instrument. We report a strong optical and IR flare in the blazar 3C 454.3 concurrent with the present extraordinarily high gamma-ray activity (ATel #2328, ATel #2326, ATel #2322), peaking on UT 2009 December 03 with B=14.98, V=14.39, R=13.83, J=11.94, K=10.08.
The Yale/SMARTS optical-near-IR monitoring program has followed the variations in emission of the... more The Yale/SMARTS optical-near-IR monitoring program has followed the variations in emission of the Fermi-LAT monitored blazars in the southern sky with closely spaced observations since 2008. We report the discovery of an optical-near-IR (OIR) outburst with no accompanying gamma-rays in the blazar PKS 0208-512, one of the targets of this program. While the source undergoes three outbursts of 1 mag or more at OIR wavelengths lasting for longer than 3 months during 2008-2011, only interval 1 and 3 have corresponding bright phases in GeV energies lasting longer than 1 month. The OIR outburst during interval 2 is comparable in brightness and temporal extent to the OIR flares during intervals 1 and 3 which do have gamma-ray counterparts. Gamma-ray and OIR variability are very well-correlated in most cases in the Fermi blazars and the lack of correlation in this case is anomalous. By analyzing the gamma-ray, OIR, and supporting multi-wavelength variability data in details, we speculate that the location of the outburst in the jet during interval 2 was closer to the black hole where the jet is more compact and the magnetic field strength is higher, and the bulk Lorentz factor of the material in the jet is smaller. These result in a much lower Compton dominance and no observable gamma-ray outburst during interval 2.
In the context of Inverse Compton models for γ-ray production in blazars, the authors examine the... more In the context of Inverse Compton models for γ-ray production in blazars, the authors examine the role of different sources of soft photons which can be up-scattered by relativistic electrons: (1) the synchrotron photons produced within the jet (2) "ambient" photons from an accretion disk, possibly scattered back to the jet by the broad line clouds and/or other diffuse plasma surrounding the nucleus. It is suggested that different photon sources may be dominant in different objects. External photons may be important in determining the γ-ray luminosity of Highly Polarized Quasars (HPQs), but not in BL Lacs, which lack a broad line region.
The recent campaign on NGC 5548 has shown the continuum to vary simultaneously at ultraviolet and... more The recent campaign on NGC 5548 has shown the continuum to vary simultaneously at ultraviolet and optical frequencies, with an upper limit to the delay of less than 4 days. We examine this result in the context of accretion disk theory. In the first part the standard a-disk model is discussed. The effects connected with heating of the outer regions by radiation from the inner regions are also considered. Neither scenario lends itself to a simple explanation of the observed data. A more successful model involves a geometrically thin outer disk heated by a central extended source of X-ray radiation.
We present the time variability properties of a sample of six blazars, AO 0235+164, 3C 273, 3C 27... more We present the time variability properties of a sample of six blazars, AO 0235+164, 3C 273, 3C 279, PKS 1510-089, PKS 2155-304, and 3C 454.3, at optical-IR frequencies as well as γ-ray energies. These observations were carried out as a part of the Yale/SMARTS program during 2008-2010 that has followed the variations in emission of the bright Fermi Large Area Telescope monitored blazars in the southern sky with closely spaced observations at BVRJK bands. We find that the optical-near IR variability properties are remarkably similar to those at the γ-ray energies. The discrete auto-correlation functions of the variability of these six blazars at optical-IR and γ-ray energies do not show any periodicity or characteristic timescale. The power spectral density (PSD) functions of the R-band variability of all six blazars are fit well by simple power-law functions with negative slopes such that there is higher amplitude variability on longer timescales. No clear break is identified in the PSD of any of the sources. The average slope of the PSD of R-band variability of these blazars is similar to what was found by the Fermi team for the γ-ray variability of a larger sample of bright blazars. This is consistent with leptonic models where the optical-IR and γ-ray emission is generated by the same population of electrons through synchrotron and inverse Compton processes, respectively. The prominent flares present in the optical-IR as well as the γ-ray light curves of these blazars are predominantly symmetric, i.e., have similar rise and decay timescales, indicating that the long-term variability is dominated by the crossing time of radiation or a disturbance through the emission region rather than by the acceleration or energy-loss timescales of the radiating electrons. For the blazar 3C 454.3, which has the highest-quality light curves, the total energy output, the ratio of γ-ray to optical energy output, and the γ-ray versus optical flux relation differ in the six individual flares observed between 2009 August and December. The results are consistent with the location of a large γ-ray outburst in 3C 454.3 during 2009 December being in the jet at ~18 pc from the central engine. This poses strong constraints on the models of high-energy emission in the jets of blazars.
Observational data and theoretical models for the physical processes and constituents of active g... more Observational data and theoretical models for the physical processes and constituents of active galactic nuclei are discussed. A summary of data on the continuum is presented, together with calculations of the dimensions of the emitting regions and the photon optical depths. Energy transfer in Compton scattering is investigated, as are cooling processes, the necessity for local acceleration mechanisms, and the density and size of inhomogeneities. The possibility of accretion onto massive black holes is discussed, as are quasi-spherical dissipative accretion, a continuum radiation emitted by magnetized spherically accreting plasma, and the observed line emission. Processes of X ray heating are explored. Deficiencies in a current model which includes a chaotic infall of magnetized plasma onto a black hole are suggested to be due to a neglect of the dynamical process, particularly in the broad line region where the flow may reverse because of the heating effect of hard photons emitted from the inner region.
We report ongoing spectroscopic observations of 3C 454.3 using the Small and Moderate Aperture Re... more We report ongoing spectroscopic observations of 3C 454.3 using the Small and Moderate Aperture Research Telescope System SMARTS 1.5m telescope + RC Spectrograph located at Cerro-Tololo Inter-American Observatory (CTIO). Spectra have been obtained roughly every 14 days from August 2008 through January 2011, during which 3C 454.3 has undergone several prominent optical (and γ-ray) flares. We find that while the equivalent width (EW) of 3C 454.3 varies, the line flux remains constant. This result suggests that the Broad Line Region is photoionized by the slowly varying accretion disk rather than radiation from the relativistic jet. We find the strength of the MgII line to be consistent with a constant line flux of ˜ 2.0× 10-14 erg s-1 cm2 A-1. This constant line flux argues against the photoionization of the BLR from the relativistic jet, as this source has undergone a number of flares during the course of observation with no corresponding change in line flux. This finding also implies that accretion disk is non-variable on timescales of at least 2.5 years.
ABSTRACT The standard model of an accretion disk is considered. The temperature in the inner regi... more ABSTRACT The standard model of an accretion disk is considered. The temperature in the inner region is computed assuming that the radiated power derives from Comptonized photons, produced in a homogeneous single-temperature plasma, supported by radiation pressure. The photon production mechanisms are purely thermal, including ion-electron bremsstrahlung, bound-free and bound-bound processes, and e-e bremsstrahlung. Pair production is not included, which limits the validity of the treatment to kT less than 60 keV. Three different approximations for the effects of Comptonization on the energy loss are used, yielding temperatures which agree within 50 percent. The maximum temperature is very sensitive to the accretion rate and viscosity parameters, ranging, for a 10 to the 8th solar mass black hole, between 0.1 and 50 keV for m between 0.1 and 1 and alpha between 0.1 and 1 and, for a 10-solar-mass black hole, between 0.6 and 60 keV for m between 0.1 and 0.9 and alpha between 0.1 and 0.5. For high viscosity and accretion rates, the emission spectra show a flat component following a peak corresponding to the temperature of the innermost optically thick annulus.
Iau Colloq 163 Accretion Phenomena and Related Outflows, 1997
The current status of understanding of the X-ray emission from Seyfert galaxies involves Comptoni... more The current status of understanding of the X-ray emission from Seyfert galaxies involves Comptonization of soft photons by hot subrelativistic electrons. After briefly reviewing the early theoretical basis for the presence of hot optically thin plasma in or around accretion disks and the key observations that led to develop the presently popular model of an accretion disk with a hot corona, we summarize recent progress in accretion models that take into account energy dissipation and/or angular momentum transport in the corona. Finally, adopting the simple scheme of a homogeneous plane parallel corona, we discuss in detail the dependence of the X-ray spectrum on the coronal parameters. Despite the strong coupling between optical depth and temperature which determines a spectral shape insensitive to their precise values, moderate spectral changes are possible. The spectral variability patterns can be used as diagnostics for coronal physics and should allow to determine whether the optical depth of the corona is dominated by e+e- pairs.
Monthly Notices of the Royal Astronomical Society, May 12, 1997
We discuss alternative interpretations of the differences in the spectral energy distributions (S... more We discuss alternative interpretations of the differences in the spectral energy distributions (SEDs) of BLLacs found in complete radio or X-ray surveys. A large body of observations in different bands suggests that the SEDs of BL Lac objects appearing in X-ray surveys differ from those appearing in radio surveys mainly in having a (synchrotron) spectral cut-off (or break) at much higher frequency. In order to explain the different properties of radio- and X-ray-selected BLLacs, Giommi and Padovani proposed a model based on a common radio luminosity function. At each radio luminosity, objects with high-frequency spectral cut-offs are assumed to be a minority. Nevertheless, they dominate the X-ray-selected population due to the larger X-ray-to-radio flux ratios. An alternative model explored here (reminiscent of the orientation models previously proposed) is that the X-ray luminosity function is `primary', and that at each X-ray luminosity a minority of objects have larger radio-to-X-ray ratios. The predictions of the two scenarios, computed via a Monte Carlo technique, are compared with the observed properties of BLLacs in the two samples extracted, respectively, from the 1-Jy radio survey and the Einstein Slew Survey. We show that both models can explain some but not all of the observed features. We then propose a completely new approach, based on the idea that the physical parameter which governs the shape of the SEDs is (or is associated with) the bolometric luminosity. Assuming an empirical relation between spectral shape and luminosity, we show that the observational properties of the two surveys can be reproduced with at least the same accuracy as the two previous models.
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Papers by Laura Maraschi