Black holes and their emissions
Black holes are one of the most fascinating objects in our universe – from their formation in extreme, cataclysmic events to their interactions with the surrounding environment they contain an unending well of mysteries waiting to be solved. We know them as objects of extreme masses from which no information can escape. The extreme gravity of black holes causes matter that surrounds them to fall into them. This happens to everything that finds itself in the vicinity of a black hole, from tiniest particles to entire stars. The stuff that a black hole devours doesn’t just fall directly into it – it is a process more interesting and complex. The matter collects around the black hole in a process known as accretion. This leads to numerous interactions of the particles that constitute the matter and emission of electromagnetic radiation in all wavelengths. We can then detect that radiation with telescopes.
Fundamental radiation mechanisms
Accreting black holes are highly energetic systems in which particles accelerate and de-accelerate, radiating electromagnetic radiation at all wavelengths. To understand how we model emissions from an accreting black hole, we first need to understand two fundamental processes in which electromagnetic radiation can be produced Synchrotron and inverse Compton scattering.
Energy is released via Synchrotron process when a particle moving at light speeds encounters a region of a strong magnetic field. As electrons are forced to follow a curved path of the magnetic field they lose some of their kinetic energy – this energy is emitted as electromagnetic radiation.
A second process in which a lot of the emission is produced is inverse Compton scattering. In this process a photon is scattered by a highly energetic electron. Some of the electron’s energy will be transferred to the photon. The photon gains energy from the electron, which increases its wavelength.
These processes occur around different parts of an accreting black hole and produce emissions of different wavelengths.
Anatomy of a black hole
When matter approaches a black hole it first needs to lose its angular momentum – instead of falling directly onto the central object it follows a spiral path around it. Then by friction the momentum is transported outward, which allows the particles to move inwards. This forms an accretion disk around a black hole. In the process particles become deflected by the strong magnetic fields which leads to a release of energy. This energy can be detected by our telescopes in the form of electromagnetic radiation – light at different frequencies such as radio or x-ray.
The magnetic fields are also responsible for pushing some of the matter out. This creates a corona – a turbulent and extremely hot cloud located above the black hole. This region, due to its high temperatures emits even higher energy radiation.
The last component, one which is of special interest in our group, are the highly energetic jets. These emissions, launched from the opposing poles of the black hole carry ionised matter for up to hundreds of parsecs. The exact mechanism under which the jets are launched is still a mystery, but it is believed that the strong magnetic fields play a role. Due to their unstable nature jets are the perfect environment for many types of particle acceleration processes and emit so much energy at so many wavelengths that they very often dominate the entire spectrum of an object.
Miller-Jones, J. using software by Hynes, R. (2013). Components of an accreting stellar-mass black hole in a hard state. ICAR. https://www.icrar.org/4161-2/
Multi-wavelength spectra
Spectra observed in an astrophysical object can be plotted showing how the intensity of electromagnetic radiation varies with its frequency. Different properties of the spectrum give us information of the processes in which the radiation is produced in a specific object. Many objects, including accreting black holes, produce radiation that covers a wide range of wavelengths. A combined observation that spans the entire range of the electromagnetic radiation is known as a multi-wavelength spectrum.
There are many ways one could model such a complex system. Our group’s own code BHJet divides the emission based on the two basic types of radiative processes (Synchrotron and inverse Compton) and the general area in which the emission occurs – before or after so called dissipation region (z_diss). Dissipation region is a region away from the black hole in which the thermal equilibrium of particles is disrupted. It treats the emission from the accretion disk as a separate, additional component.
In the plot below you can see an example of a multi-wavelength spectra of an accreting stellar mass black hole. It consists of multiple components: the accretion disk (in orange), synchrotron radiation pre-dissipation region (in turquoise) and further away from the black hole (in blue). The last two components are inverse Compton similarly divided into two separate emission regions: pre-dissipation (in pink) and post-dissipation (in purple). The spectrum is summed up to then produce a total emission (in black).
Spectral energy distribution (SED) for a microquasar MAXI J1820+070.
Modelling multi-wavelength spectra allows us to understand the mechanisms that produce emissions in various regions of an accreting black hole through understanding how much energy is emitted at different frequency ranges. The models allow us to fit the spectra to real data and understand the physics of accreting black holes.