Accretion and jet formation
While much of what we say here is relevant also for neutron star X-ray binaries (NHXBs), but the following description is specific to black holes (BHs) in particular.
In a black hole X-ray binary (BHB), matter from a normal star is accreted via a stellar wind or Roche lobe overflow into the area of influence of the black hole. In the simplest representation, this accretion process results in the formation of three basic structures around the black hole (BH, see Fig. 1): Due to its angular momentum, the accreted matter forms an accretion disk in which it spirals slowly inwards due to viscosity-induced transfer of angular momentum outwards. The lost gravitational potential energy is thermalized and radiated as a blackbody like spectrum that is detected in the X-ray regime for stellar BHs and in the optical or UV for supermassive BHs. Close to the BH, for reasons which we still do not fully understand, a fraction of the accreted material is ejected outwards, usually at relativistic speeds. The bulk power in these jets can be comparable to the luminosity of the accretion disk, or in some cases even dominate the system luminosity. Finally, the hard X-ray spectra observed from many BHs indicate the need for a third component, the accretion disk corona, where the UV and soft X-rays from the disk or jet are Compton up-scattered in a hot (kT ∼100 keV in a binary BH) plasma to energies up to several 100 keV.
Fig.1: Bottom right: sketch of an X-ray binary with a jet, accretion disk, and compact inner corona. Left and Top right: Radio image (λ = 6 cm, courtesy VLA/NRAO) of the radio galaxy Cygnus A and sketch of an X-ray binary system. Both kinds of system exhibit the same components, however, the mass/size difference of these systems ranges typically over factors of 106-109.
Changes in the radio through γ-ray spectral and timing characteristics are observed on timescales of seconds to months. These changes are thought to result from an interplay of dominance of these components in time and are used to classify BHB behavior into so-called “accretion states” (see Fig. 2): During the “soft state”, the X-ray spectrum of the source is well described by an accretion disk spectrum, the source shows only weak X-ray variability, and radio emission from the jet is non-detectable. During the “hard state”, on the other hand, the X-ray spectrum is a non-thermal power-law with an exponential cut-off extending to >150 keV and strong radio emission is observed. During “intermediate states” between these states, the X-ray spectrum softens and intermittent radio flaring is observed. While some BHBs remain in one state for most of the time (e.g., Cygnus X-1, the first BH discovered, is mainly a hard state source), most BHBs are found in transient systems. These transients are typically quiescent at least 90% of the time, but have week to decades long outbursts during which they can reach luminosities as high as the Eddington luminosity – the luminosity at which the radiation pressure generated by the accretion process can disrupt the accretion flow itself.
Fig. 2: Hardness intensity diagram of the BHB GX 339−4 during a 525 day long outburst. Each dot is the average of a day’s observations. The main accretion states are noted. The schematics show roughly our best sense of the interplay between the accretion disk, accretion disk corona, and jet.
The behavior of these transient sources during the outburst illustrates well the interplay between the different components making up the multi-wavelength spectrum of BHBs. This behavior is best shown in terms of an hardness intensity diagram (HID) such as Fig. 2, in which the source luminosity is plotted against the X-ray spectral shape (“soft” meaning thermal, “hard” meaning a power law continuum). In such a diagram, sources start the outburst in the lower right corner and follow a counter-clockwise q-shaped path in the HID, transiting to different accretion states along the way. Jets are only detected in the hard and intermediate state. Hard state jets are less relativistic and more continuous, yet still dominate the power output compared to the accretion disk, while jets during the intermediate state are faster, can show apparent superluminal motion, and have discrete ejecta.
In order to properly reconstruct the physical mechanisms underlying the observed emission properties, however, one needs to have detailed physical models to compare against both spectral and timing data. Although available observations have already helped in forging a rough understanding of the interplay of the disk, accretion disk corona, and jets, our ability to derive the detailed physics of BH accretion is inhibited by our limited understanding of all of these components, as well as the exact nature of their relationship to each other.
The critical questions currently under debate are:
1. Fundamental Properties of the Accretion Flow: How does matter accrete through the accretion disk? What triggers outburst cycles, and how? What is the actual accretion rate, and how does that drive state changes? How do accretion flows form coronae and atmospheres, and what is the final geometry of the cooler/hotter phases of accretion? How do accretion disks radiate? Does accretion disk emission from a BH allow us to test GR in the strong field limit?
2. Fundamental Properties of Jet Outflows: How, why and under what conditions are jets formed? Are jets powered mainly via the accretion flow or BH spin? How are jets accelerated and collimated? Do jets contain mostly matter or electromagnetic fields, and is the matter mostly hadronic or leptonic? How are particles accelerated in the jets? What are the physical and formational differences between the two types of jets observed in galactic BHs? What are the neutrino production mechanisms in jets?
3. Black Holes and Other Compact Objects: What are the differences in the accretion flows onto black holes and neutron stars? Does the physics of accretion scale predictably from BHBs, via intermediate mass BHs, to the supermassive BHs in the centers of galaxies? How to accretion states in BHBs relate to the different classes observed in Active Galactic Nuclei? What is the impact of BHs on the larger environment?
Here at the API we are actively involved in pursuing answers for all of the above topics, and our groups include both observational and theoretical efforts.
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