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Register a new userThe power of relativistic jets is larger than the luminosity of their accretion disks
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Added by
Gabriele Ghisellini
Publication date
2014
fields
Physics
and research's language is
English
Authors
G. Ghisellini
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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 black hole, as well as the magnetic field near the event horizon. The physical mechanism 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 used samples 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 gamma-ray luminosity, and accretion luminosity as measured by the broad emission lines, with the jet power dominating over 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.
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There are several methods to calculate the radiative and kinetic power of relativistic jets, but their results can differ by one or two orders of magnitude. Therefore, it is necessary to perform a calibration of the jet power, to understand the reasons for these differences (whether wrong hypotheses or intrinsic source variability), and if it is possible to converge to a reliable measurement of this physical quantity. We present preliminary results of a project aimed at calibrating the power of relativistic jets in active galactic nuclei (AGN) and X-ray binaries (XRB). We started by selecting all the AGN associations with known redshift in the Fourth Fermi LAT Gamma-Ray Catalog (4FGL). We then calculated the radiative and/or kinetic powers from available data or we extracted this information from literature. We compare the values obtained for overlapping samples and highlight early conclusions.
When an accretion disk falls prey to the runaway instability, a large portion of its mass is devoured by the black hole within a few dynamical times. Despite decades of effort, it is still unclear under what conditions such an instability can occur. The technically most advanced relativistic simulations to date were unable to find a clear sign for the onset of the instability. In this work, we present three-dimensional relativistic hydrodynamics simulations of accretion disks around black holes in dynamical space-time. We focus on the configurations that are expected to be particularly prone to the development of this instability. We demonstrate, for the first time, that the fully self-consistent general relativistic evolution does indeed produce a runaway instability.
We have calculated the relativistic reflection component of the X-ray spectra of accretion disks in active galactic nuclei (AGN). Our calculations have shown that the spectra can be significantly modified by the motion of the accretion flow and the gravity and rotation of the central black hole. The absorption edges in the spectra suffer severe energy shifts and smearing, and the degree of distortion depends on the system parameters, in particular, the inner radius of the accretion disk and the disk viewing inclination angles. The effects are significant. Fluorescent X-ray emission lines from the inner accretion disk could be powerful diagnostic of space-time distortion and dynamical relativistic effects near the event horizons of accreting black holes. However, improper treatment of the reflection component in fitting the X-ray continuum could give rise to spurious line-like features. These features mimic the true fluorescent emission lines and may mask their relativistic signatures. Fully relativistic models for reflection continua together with the emission lines are needed in order to extract black-hole parameters from the AGN X-ray spectra.
Several active galactic nuclei and microquasars are observed to eject plasmoids that move at relativistic speeds. We envisage the plasmoids as pre-existing current carrying magnetic flux ropes that were initially anchored in the accretion disk-corona. The plasmoids are ejected outwards via a mechanism called the toroidal instability (TI). The TI, which was originally explored in the context of laboratory tokamak plasmas, has been very successful in explaining coronal mass ejections from the Sun. Our model predictions for plasmoid trajectories compare favorably with a representative set of multi-epoch observations of radio emitting knots from the radio galaxy 3C120, which were preceded by dips in Xray intensity.
A number of neutron star low-mass X-ray binaries have recently been discovered to show broad, asymmetric Fe K emission lines in their X-ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating in the inner part of the accretion disk where strong relativistic effects can broaden emission lines. We present a comprehensive, systematic analysis of Suzaku and XMM-Newton spectra of 10 neutron star low-mass X-ray binaries, all of which display broad Fe K emission lines. Of the 10 sources, 4 are Z sources, 4 are atolls and 2 are accreting millisecond X-ray pulsars (also atolls). The Fe K lines are well fit by a relativistic line model for a Schwarzschild metric, and imply a narrow range of inner disk radii (6 - 15 GM/c^2) in most cases. This implies that the accretion disk extends close to the neutron star surface over a range of luminosities. Continuum modeling shows that for the majority of observations, a blackbody component (plausibly associated with the boundary layer) dominates the X-ray emission from 8 - 20 keV. Thus it appears likely that this spectral component produces the majority of the ionizing flux that illuminates the accretion disk. Therefore, we also fit the spectra with a blurred reflection model, wherein a blackbody component illuminates the disk. This model fits well in most cases, supporting the idea that the boundary layer is illuminating a geometrically thin disk.
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