Theoretical models for the production of relativistic jets from active galactic nuclei predict that jet power arises from the spin and mass of the central supermassive black hole, as well as from the ...magnetic field near the event horizon. The physical mechanism underlying the contribution from the magnetic field is the torque exerted on the rotating black hole by the field amplified by the accreting material. If the squared magnetic field is proportional to the accretion rate, then there will be a correlation between jet power and accretion luminosity. There is evidence for such a correlation, but inadequate knowledge of the accretion luminosity of the limited and inhomogeneous samples used prevented a firm conclusion. Here we report an analysis of archival observations of a sample of blazars (quasars whose jets point towards Earth) that overcomes previous limitations. We find a clear correlation between jet power, as measured through the γ-ray luminosity, and accretion luminosity, as measured by the broad emission lines, with the jet power dominating the disk luminosity, in agreement with numerical simulations. This implies that the magnetic field threading the black hole horizon reaches the maximum value sustainable by the accreting matter.
The Fermi blazars' divide Ghisellini, G.; Maraschi, L.; Tavecchio, F.
Monthly notices of the Royal Astronomical Society. Letters,
June 2009, Letnik:
396, Številka:
1
Journal Article
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Flat-spectrum radio quasars (FSRQs) and BL Lac objects detected in the first three months of the Fermi survey neatly separate in the γ-ray spectral index versus γ-ray luminosity plane. BL Lac objects ...are less luminous and have harder spectra than broad-line blazars. We suggest that this division has its origin in the different accretion regimes of the two classes of objects. Using the γ-ray luminosity as a proxy for the observed bolometric one, we show that the boundary between the two subclasses of blazars can be associated with the threshold between the regimes of optically thick accretion discs and of radiatively inefficient accretion flows, which lies at an accretion rate of the order of 10−2 the Eddington rate. The spectral separation in hard (BL Lacs) and soft (FSRQs) objects can then result from the different radiative cooling suffered by the relativistic electrons in jets propagating in different ambients. We argue that the bulk of the most luminous blazars already detected by Fermi should be characterized by large black hole masses, around 109 solar masses, and predict that lowering the γ-ray flux threshold the region of the αγ–Lγ plane corresponding to steep spectral indices and lower luminosities will be progressively populated by FSRQs with lower mass black holes, while the region of hard spectra and large luminosities will remain forbidden.
We studied all blazars of known redshift detected by the Fermi satellite during its first 3-month survey. For the majority of them, pointed Swift observations ensure a good multiwavelength coverage, ...enabling us to reliably construct their spectral energy distributions (SEDs). We model the SEDs using a one-zone leptonic model and study the distributions of the derived interesting physical parameters as a function of the observed γ-ray luminosity. We confirm previous findings concerning the relation of the physical parameters with source luminosity which are at the origin of the blazar sequence. The SEDs allow to estimate the luminosity of the accretion disc for the majority of broad emitting line blazars, while for the lineless BL Lac objects in the sample upper limits can be derived. We find a positive correlation between the jet power and the luminosity of the accretion disc in broad-line blazars. In these objects, we argue that the jet must be proton dominated, and that the total jet power is of the same order of (or slightly larger than) the disc luminosity. We discuss two alternative scenarios to explain this result.
We study the relation between the mass accretion rate, the jet power and the black hole mass of blazars. With this aim, we make use of the Sloan Digital Sky Survey and the 11-month catalogue of ...blazars detected at energies larger than 100 MeV by the Large Area Telescope onboard the Fermi satellite. This allows us to construct a relatively large sample of blazars with information about both the luminosity (or upper limits) of their emission lines (used as a proxy for the strength of the disc luminosity) and the luminosity of the high-energy emission (used as a proxy for the jet power). We find a good correlation between the luminosity of the broad lines and the γ-ray luminosities as detected by Fermi, both using the absolute values of the luminosities and normalizing them to the Eddington value. The data we have analysed confirm that the division of blazars into BL Lacertae objects (BL Lacs) and flat spectrum radio quasars (FSRQs) is controlled by the line luminosity in Eddington units. For small values of this ratio, the object is a BL Lac, while it is a FSRQ for large values. The transition appears to be smooth, but a much larger number of objects is needed to confirm this point.
TeV BL Lac objects at the dawn of the Fermi era Tavecchio, F.; Ghisellini, G.; Ghirlanda, G. ...
Monthly notices of the Royal Astronomical Society,
01/2010, Letnik:
401, Številka:
3
Journal Article
Recenzirano
We reconsider the emission properties of BL Lac objects emitting in the high-energy γ-ray band, by exploiting the information in the MeV–GeV band obtained by the Large Area Telescope (LAT) on board ...the Fermi Gamma-Ray Space Telescope in its first three months of operation. To this aim we construct the spectral energy distribution of all the BL Lacs revealed by LAT and of the known TeV BL Lacs not detected by LAT, also including data from the Swift satellite, and model them with a simple one-zone leptonic model. The analysis shows that the BL Lacs detected by LAT (whether or not already detected in the TeV band) share similar physical parameters. While some of the TeV BL Lacs not revealed by LAT have spectral energy distributions and physical parameters very similar to the LAT BL Lacs, one group of objects displays peculiar properties (larger electron energies and smaller magnetic fields), suggesting different physical conditions in the emission region. Finally, we discuss possible criteria to select in an effective manner good new candidates for the Cherenkov telescopes among the LAT sources, presenting a list of predicted fluxes in the very high-energy band calculated including the effects of absorption by extragalactic background light.
We have conducted a multiwavelength survey of 42 radio loud narrow-1ine Seyfert 1 galaxies (RLNLS1s), selected by searching among all the known sources of this type and omitting those with steep ...radio spectra. We analyse data from radio frequencies to X-rays, and supplement these with information available from online catalogues and the literature in order to cover the full electromagnetic spectrum. This is the largest known multiwavelength survey for this type of source. We detected 90% of the sources in X-rays and found 17% at γ rays. Extreme variability at high energies was also found, down to timescales as short as hours. In some sources, dramatic spectral and flux changes suggest interplay between a relativistic jet and the accretion disk. The estimated masses of the central black holes are in the range ~106−8 M⊙, lower than those of blazars, while the accretion luminosities span a range from ~0.01 to ~0.49 times the Eddington limit, with an outlier at 0.003, similar to those of quasars. The distribution of the calculated jet power spans a range from ~1042.6 to ~1045.6 erg s-1, generally lower than quasars and BL Lac objects, but partially overlapping with the latter. Once normalised by the mass of the central black holes, the jet power of the three types of active galactic nuclei are consistent with each other, indicating that the jets are similar and the observational differences are due to scaling factors. Despite the observational differences, the central engine of RLNLS1s is apparently quite similar to that of blazars. The historical difficulties in finding radio-loud narrow-line Seyfert 1 galaxies might be due to their low power and to intermittent jetactivity.
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 gamma -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 gamma -ray energies. The discrete auto-correlation functions of the variability of these six blazars at optical-IR and gamma -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 gamma -ray variability of a larger sample of bright blazars. This is consistent with leptonic models where the optical-IR and gamma -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 gamma -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 gamma -ray to optical energy output, and the gamma -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 gamma -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.
The flat-spectrum radio quasar PKS 1222+216 (4C+21.35, z = 0.432) was detected in the very high energy γ-ray band by MAGIC during a highly active γ-ray phase following an alert by the Large Area ...Telescope (LAT) onboard Fermi. Its relatively hard spectrum (70–400 GeV photon index Γ = 2.7 ± 0.3) without a cut off, together with its observed variability on a timescale of ~10 min challenges standard emission models. In particular, if the emission originates in a portion of the relativistic jet located inside the broad line region (BLR), severe absorption of γ rays above a few tens of GeV is expected to be caused by the γγ → e ± process. These observations therefore imply that there is a very compact (Rb ~ 5 × 1014 cm) and rapidly moving blob located far beyond the BLR radius (to avoid the gamma-ray absorption through pair production) that is responsible for the rapidly varying high energy flux. However, the long-term (day-week) coherent evolution of the GeV flux recorded by LAT indicates that there could also be a substantial contribution from another, larger emission region. We model the spectral energy distribution of PKS 1222+216 during the epoch of the MAGIC detection assuming three different scenarios, namely: (1) a one-zone model considering only the emission from a compact blob outside the BLR; (2) a two-zone model consisting of a compact blob plus an emitting region encompassing the whole jet cross-section located outside the BLR; and (3) a two-zone model with the jet emitting region inside the BLR. In all cases we find that the high-energy emission from the compact blob is dominated by the inverse Compton scattering of the infrared thermal radiation of the dusty torus. Furthermore, both regions are matter-dominated, with the Poynting flux providing a negligible contribution to the total jet power. These results do not support models in which the compact blob is the result of reconnection events inside the jet or “needles” of high-energy electrons accelerated close to the BH. The observational framework and our radiative models might instead be compatible with scenarios in which the jet is re-collimated and focussed at large distances from the central BH.
We present the SEDs of a hard X-ray selected sample containing 136 sources with FSUB2-10keV/SUB>10SUP-14/SUP erg cmSUP-2/SUP sSUP-1/SUP 132 are AGNs. The sources are detected in a 1 degSUP2/SUP area ...of the XMM-Newton Medium Deep Survey where optical data from the VVDS and CFHTLS and infrared data from the SWIRE survey are available. Based on a SED fitting technique we derive photometric redshifts with sigma(1+z)=0.11 and 6% of outliers and identify AGN signatures in 83% of the objects. This fraction is higher than derived when a spectroscopic classification is available. The remaining 17SUP+9/SUPSUB-6/SUB% of AGNs show star-forming galaxy SEDs (SF class). The sources with AGN signatures are divided in two classes, AGN1 (33SUP+6/SUPSUB-1/SUB%) and AGN2 (50SUP+6/SUPSUB-11/SUB%). The AGN1 and AGN2 classes include sources whose SEDs are fitted by type 1 and type 2 AGN templates, respectively. On average, AGN1s show soft X-ray spectra, consistent with being unabsorbed, while AGN2s and SFs show hard X-ray spectra, consistent with being absorbed. The analysis of the average SEDs as a function of X-ray luminosity shows a reddening of the infrared SEDs, consistent with a decreasing contribution from the host galaxy at higher luminosities. The AGNs in the SF classes are likely obscured in the mid-infrared, as suggested by their low LSUB3-20mum/SUB/LSUPcorr/SUPSUB0.5-10keV/SUB ratios. We confirm the previously found correlation for AGNs between the radio luminosity and the X-ray and the mid-infrared luminosities. The X-ray-radio correlation can be used to identify heavily absorbed AGNs. However, the estimated radio fluxes for the missing AGN population responsible for the bulk of the background at E>10 keV are too faint to be detected even in the deepest current radio surveys.
Blue Fermi flat spectrum radio quasars Ghisellini, G; Tavecchio, F; Foschini, L ...
Monthly notices of the Royal Astronomical Society,
11 September 2012, Letnik:
425, Številka:
2
Journal Article
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Abstract
Many blazars detected by the Fermi satellite, observed spectroscopically in the optical, are line-less, and have been classified as BL Lac objects. Optical-ultraviolet (UV) photometry of ...nearly 100 of them allowed us to determine the redshift for a handful of objects and redshift upper limits in the great majority. A few of these are candidates to be 'blue quasars', namely flat spectrum radio quasars whose broad emission lines are hidden by an overwhelming synchrotron emission peaking in the UV. This implies that the emitting electrons have high energies. In turn, this requires relatively weak radiative cooling, a condition that can be met if the main radiative dissipation of the jet power occurs outside the broad-line region. We confirm this hypothesis by studying and modelling the spectral energy distributions of the four 'blue quasars' recently discovered. Furthermore, we discuss the distribution of Fermi blazars in the γ-ray spectral index-γ-ray luminosity plane, and argue that 'blue quasars' objects are a minority within the blazar populations.
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