The Fermi blazar sequence Ghisellini, G; Righi, C; Costamante, L ...
Monthly notices of the Royal Astronomical Society,
07/2017, Volume:
469, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract
We revisit the blazar sequence exploiting the complete, flux-limited sample of blazars with known redshift detected by the Fermi satellite after 4 yr of operations (the 3LAC sample). We ...divide the sources into γ-ray luminosity bins, collect all the archival data for all blazars, and construct their spectral energy distribution (SED). We describe the average SED of blazars in the same luminosity bin through a simple phenomenological function consisting of two broken power laws connecting with a power law describing the radio emission. We do that separately for BL Lacs and for flat spectrum radio quasars (FSRQs) and also for all blazars together. The main results are: (i) FSRQs display approximately the same SED as the luminosity increases, but the relative importance of the high-energy peak increases; (ii) as a consequence, the X-ray spectra of FSRQs become harder for larger luminosities; (iii) BL Lacs indeed form a sequence: they become redder (i.e. smaller peak frequencies) with increasing luminosities, with a softer γ-ray slope and a larger dominance of the high-energy peak; (iv) for all blazars (BL Lacs+FSRQs), these properties become more prominent, as the highest luminosity bin is populated mostly by FSRQs and the lowest luminosity bin mostly by BL Lacs. This agrees with the original blazar sequence, although BL Lacs never have an average γ-ray slope as hard as found in the original sequence. (v) At high luminosities, a large fraction of FSRQs show signs of thermal emission from the accretion disc, contributing to the optical–UV (ultraviolet).
Fermi/LAT broad emission line blazars Ghisellini, G; Tavecchio, F
Monthly notices of the Royal Astronomical Society,
04/2015, Volume:
448, Issue:
2
Journal Article
Peer reviewed
Open access
We study the broad emission line blazars detected in the γ-ray band by the Large Area Telescope onboard the Fermi satellite and with the optical spectrum studied by Shaw et al. The observed broad ...line strength provides a measure of the ionizing luminosity of the accretion disc, while the γ-luminosity is a proxy for the bolometric non-thermal beamed jet emission. The resulting sample, composed by 217 blazars, is the best suited to study the connection between accretion and jet properties. We compare the broad emission line properties of these blazars with those of radio-quiet and radio-loud quasars present in the Sloan Digital Sky Survey, to asses differences and similarities of the disc luminosity and the virial black hole mass. For most sources, we could derive the black hole mass by reproducing the IR–optical–UV data with a standard accretion disc spectrum, and we compared the black hole masses derived with the two methods. The distributions of the masses estimated in the two ways agree satisfactorily. We then apply a simple, one-zone, leptonic model to all the 217 objects of our sample. The knowledge of the black hole mass and disc luminosity helps to constrain the jet parameters. On average, they are similar to what found by previous studies of smaller samples of sources.
We recently proposed that structured (spine-sheath) jets associated with BL Lac objects could offer a suitable environment for the production of the extragalactic high-energy (E > 100 TeV) neutrino ...recently revealed by IceCube. Our previous analysis was limited to low-power BL Lac objects. We extend our preliminary study to the entire BL Lac population, assuming that the entire diffuse emission is accounted for by these sources. The neutrino output from a single source depends on a relatively large number of parameters. However, for several of them we have constraints coming from observations and previous application of the structured jet model to blazar and radiogalaxy emission. The observed neutrino spectrum then fixes the remaining free parameters. We assume that the power of cosmic rays as well as the radiative luminosity of the sheath depends linearly on the jet power. In turn, we assume that the latter is well traced by the γ-ray luminosity. We exploit the BL Lac γ-ray luminosity function and its cosmic evolution as recently inferred from Fermi-LAT data to derive the expected neutrino cumulative intensity from the entire BL Lac population. When considering only the low-power BL Lacs, a large cosmic ray power for each source is required to account for the neutrino flux. Instead, if BL Lacs of all powers produce neutrinos, the power demand decreases, and the required cosmic ray power becomes of the same order of the radiative jet power. In our scheme, the maximum energy of cosmic rays is constrained to be ≲ few PeV by the lack of events above few PeV. Although such a value is obtained through a fine-tuning with the data, we show that it could be possibly related to the equilibrium between cooling and acceleration processes for high-energy cosmic rays. We also discuss the prospects for the direct association of IceCube events with BL Lacs, providing an estimate of the expected counts for the most promising sources.
Canonical high-power blazars Ghisellini, G.; Tavecchio, F.
Monthly notices of the Royal Astronomical Society,
August 2009, Volume:
397, Issue:
2
Journal Article
Peer reviewed
Open access
The jets of powerful blazars propagate within relatively dense regions of radiation produced externally to the jet. This radiation is a key ingredient to understand the origin of the high-energy ...emission of blazars, from the X-ray to the γ-ray energy band. The main components contributing to the external radiation field are the accretion disc emission, including its X-ray corona, the broad-line region, the infrared emitting torus and the cosmic background radiation. Their importance changes as a function of the distance from the black hole and of the value of the bulk Lorentz factor of the jet. These external radiation fields control the amount of the inverse Compton radiation with respect to the synchrotron flux. Therefore, the predicted spectral energy distribution (SED) will depend on where the jet dissipates part of its energy to produce the observed radiation. We investigate in detail how the SED changes as a function of the location of the jet dissipation region by assuming rather ‘standard’ (i.e. ‘canonical’) prescriptions for the accretion disc and its X-ray corona, the profile of the jet magnetic field and the external radiation. We confirm that most of the dissipation, if producing the γ-ray flux we see, must occur at hundreds of Schwarzschild radii from the black hole, to avoid the γ−γ→e± process, and the consequent re-emission by the produced pairs. The magnetic energy density of a ‘canonical’ jet almost never dominates the radiative cooling of the emitting electrons, and consequently the inverse Compton flux almost always dominates the bolometric output. This is more so for large black hole masses. Dissipation taking place beyond the broad-line region is particularly interesting, since it accounts in a simple way for the largest inverse Compton to synchrotron flux ratios accompanied by an extremely hard X-ray spectrum. Furthermore, it makes the high-power blazars at high redshift useful tools to study the optical to UV cosmic backgrounds.
On-axis view of GRB 170817A Salafia, O. S.; Ghirlanda, G.; Ascenzi, S. ...
Astronomy and astrophysics (Berlin),
08/2019, Volume:
628
Journal Article
Peer reviewed
Open access
The peculiar short gamma-ray burst (SGRB) GRB 170817A has been firmly associated to the gravitational wave event GW170817, which has been unanimously interpreted as due to the coalescence of a double ...neutron star binary. The unprecedented behaviour of the non-thermal afterglow led to a debate over its nature, which was eventually settled by high-resolution VLBI observations that strongly support the off-axis structured jet scenario. Using information on the jet structure derived from multi-wavelength fitting of the afterglow emission and of the apparent VLBI image centroid motion, we compute the appearance of a GRB 170817A-like jet as seen by an on-axis observer and compare it to the previously observed population of SGRB afterglows and prompt emission events. We find that the intrinsic properties of the GRB 170817A jet are representative of a typical event in the SGRB population, hinting at a quasi-universal jet structure. The diversity in the SGRB afterglow population could therefore be ascribed in large part to extrinsic (redshift, density of the surrounding medium, viewing angle) rather than intrinsic properties. Although more uncertain, the comparison can be extended to the prompt emission properties, leading to similar conclusions.
ABSTRACT
The matter content of extragalactic relativistic jets is still an unsolved issue. There are strong arguments against pure electron–positron pair jets, but pairs could outnumber the electrons ...associated with protons by a factor of 10–20. This impacts on the estimate of the jet kinetic power, by reducing it by the same factor, and on the total energy delivered to leptons by the particle acceleration mechanism. Pairs cannot be created in the same jet zone responsible for the high‐energy γ‐ray emission we see in blazars, because the reprocessing of the created pairs would overproduce the X‐ray flux. Copious pair creation could occur in the inner zone of the still accelerating jet, where the bulk Lorentz factor is small. It is found that the inner zone can produce a sufficient number of pairs to replenish the zone of the jet where most of the luminosity is emitted, but only if the γ‐ray luminosity of the inner jet is above 1044 erg s−1 at ∼1 MeV. Since the beaming is modest, this emission can be observed at large viewing angles, and detected in radio galaxies and lobe‐dominated quasars at the flux level of 10−12–10−11 erg cm−2 s−1 for a source at a redshift z= 0.1.
On the magnetization of BL Lac jets Tavecchio, F; Ghisellini, G
Monthly notices of the Royal Astronomical Society,
03/2016, Volume:
456, Issue:
3
Journal Article
Peer reviewed
Open access
The current paradigm foresees that relativistic jets are launched as magnetically dominated flows, whose magnetic power is progressively converted to kinetic power of the matter of the jet, until ...equipartition is reached. Therefore, at the end of the acceleration phase, the jet should still carry a substantial fraction (...half) of its power in the form of a Poynting flux. It has been also argued that, in these conditions, the best candidate particle acceleration mechanism is efficient reconnection of magnetic field lines, for which it is predicted that magnetic field and accelerated relativistic electron energy densities are in equipartition. Through the modelling of the jet non-thermal emission, we explore if equipartition is indeed possible in BL Lac objects, i.e. low-power blazars with weak or absent broad emission lines. We find that one-zone models (for which only one region is involved in the production of the radiation we observe) the particle energy density is largely dominating (by 1-2 orders of magnitude) over the magnetic one. As a consequence, the jet kinetic power largely exceeds the magnetic power. Instead, if the jet is structured (i.e. made by a fast spine surrounded by a slower layer), the amplification of the inverse Compton emission due to the radiative interplay between the two components allows us to reproduce the emission in equipartition conditions. (ProQuest: ... denotes formulae/symbols omitted.)
We study the BL Lac objects detected in the 1-year all-sky survey of the Fermi satellite, with an energy spectral slope αγ in the 0.1-100 GeV band greater than 1.2. In the αγ versus γ-ray luminosity ...plane, these BL Lacs occupy the region populated by flat spectrum radio quasars (FSRQs). Studying the properties of their spectral energy distributions (SEDs) and of their emitting lines, we find that several of these BL Lacs have an SED similar to FSRQs and that they do have broad lines of large equivalent width (EW), and should be reclassified as FSRQs even adopting the current phenomenological definition (i.e. EW of the emitting line greater than 5 Å). In other cases, even if the EW is small, the emitting lines can be as luminous as in quasars, and again their SED is similar to the SED of FSRQs. Sources classified as BL Lacs with an SED appearing as intermediate between BL Lacs and FSRQs also have relatively weak broad emission lines and small EW, and can be considered as transition sources. These properties are confirmed also by model fitting that allows us to derive the relevant intrinsic jet parameters and the jet power. This study leads us to propose a physical distinction between the two classes of blazars, based on the luminosity of the broad-line region measured in Eddington units. The dividing line is of the order of L
BLR/L
Edd∼ 5 × 10−4, in good agreement with the idea that the presence of strong emitting lines is related to a transition in the accretion regime, becoming radiatively inefficient below a disc luminosity of the order of 1 per cent of the Eddington one.
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.
We report our observation of the short gamma-ray burst (GRB) GRB 170817A, associated to the binary neutron star merger gravitational wave (GW) event GW 170817, performed in the X-ray band with ...XMM-Newton 135 d after the event (on 29 December, 2017). We find evidence for a flattening of the X-ray light curve with respect to the previously observed brightening. This is also supported by a nearly simultaneous optical Hubble Space Telescope observation and successive X-ray Chandra and low-frequency radio observations recently reported in the literature. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the change in the temporal evolution of the light curve is due to the passage of the cooling frequency: its origin must be geometric or dynamical. We interpret all the existing afterglow data with two models: i) a structured jet and ii) a jet-less isotropic fireball with some stratification in its radial velocity structure. Both models fit the data and predict that the radio flux must decrease simultaneously with the optical and X-ray emission, making it difficult to distinguish between them at the present stage. Polarimetric measurements and the rate of short GRB-GW associations in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios.
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