Sources of TeV emission

Enormous amounts of energy are needed to emit photons in the TeV range. Thus, all sources of TeV emission are linked to extreme physical processes, such as the ones found in explosions of stars (Supernovae and Supernovae Remnants), Gamma-ray bursts (GRBs), X-ray and Gamma-ray binaries and active supermassive black holes at the center of galaxies (Active Galactic Nuclei or AGNs).

Supernova remnants (SNRs)

The SNR 1006 observed in multiple wavelength: X-rays from Chandra in blue, optical from CTIO and DSS in yellow, orange and light blue, and radio from VLA/GBV in red. [Credits: NASA/CXC]

H.E.S.S. observation of SN 1006. [Credits: HESS Collaboration]

A common source of gamma-rays are Supernova remnants (SNRs). when a star collapses in a supernova, a shock wave is sent in the surrounding interstellar medium (ISM).

The shock fronts are very efficient particle accelerators. Models predict that more than 10% of the kinetic energy of a supernova is spent into the acceleration of cosmic rays. For this reasons, supernovae are believed to be responsible for the majority of the cosmic rays in the galaxy with energies from up to 10¹⁵ eV.

If synchrotron radiation is present, cosmic rays can produce gamma-rays through the inverse Compton-effect. It is therefore possible to observe SNRs with gamma-ray telescopes. Similar mechanisms exist in neutron stars (pulsars), which produce very high electric and magnetic fields and thus act as particle accelerators. Their particle wind (pulsar wind) also creates shock fronts in the ISM which emit hard gamma-rays.

Binary systems

Example of a gamma binary system composed of a pulsar and a blue giant. The pulsar gets so close to the blue giant during its orbit that it goes through its gas envelope. The pulsar wind interacts with the gas envelope in such a way that the gamma ray emission increases during the entry and exit of the pulsar while the radio and X-ray emission remains constant. [Credits: NASA/Goddard Space Flight Center/Francis Reddy]

Most stars that we can observe are not single systems but instead have a partner star which is gravitationally bound to them. In certain circumstances, these systems can exchange matter between them. The process in which matter is transferred from one star to another is called accretion.

Candidates for the emission of X-rays and Gamma-rays are binary systems which contain a compact objects, this means a neutron star or a black hole. When matter hits the surface of the neutron star, a hotspot is formed which emits X-rays or in some cases even Gamma rays. A similar scenario occurs when the compact object is a black hole surrounded by an accretion disk. In this case, X-ray and Gamma-ray emission is formed at the spot where the matter falls onto the accretion disk.

Active Galactic Nuclei (AGNs)

Standard model for AGNs. The supermassive black hole in the center of a galaxy is surrounded by an enormous accretion disk. [Credits: C.M Urry & P. Padovani]

Supermassive black holes at the center of active galaxies (Active Galactic Nuclei or AGNs) have up to billions of the mass of the Sun and emit relativistic jets from which matter streams out. Current theories state that these jets can accelerate electrons up to the TeV-regime and that this is the cause of the very strong emission in the Gamma-ray band (through Synchrotron emission and the Inverse Compton effect).

Data analysis and simulations

Top: observations and simulated spectrum for the different scenarios of the propagation of cosmic rays into the medium. Bottom: Simulated 1 TeV sky map in the slow diffusion scenario in the molecular clouds. The green contours show the H.E.S.S. observations and the green circles represent the two H.E.S.S. sources. [Credits: Cui et al. 2016]

The IAAT is involved in the data analysis of the TeV-observatory H.E.S.S. The main research topics are of Supernova Remnants and binary systems.

For example, in the last years, a group of researchers in Tübingen investigated whether or not two H.E.S.S. sources, which are close to each other are causally related (Cui et al. 2016). One of the sources, the SNR HESS J1731-347, produces strong Gamma and X-ray rays, while the bigger and still unidentified source HESS J1729-345 shows a much weaker emission. The second source is located at the same position as a known radio-bright molecular cloud. Researchers at the IAAT showed through simulation studies that the SNR is most likely still expanding and that the high energy cosmic rays which were produced by the SNR interact with the molecular cloud thus producing the second source.

Additional analyses of H.E.S.S. data at the IAAT have lead to a better understanding of the origin of the TeV emission (Capasso et al. 2016). The morphology of the radiation emission region was investigated with unprecedented precision. Through this study, the TeV emitting region could be compared to the gas distribution of the molecular cloud and further confirmed the scenario of the interaction between cosmic rays and molecular clouds.