CTAO Science
Micro-quasars & γ-ray binaries
Some VHE gamma-ray emitters in the Milky Way are in known binary systems , consisting of a compact object – a neutron star or black hole orbiting a conventional star. Only several such systems have been observed with the current generation of gamma-ray telescopes. CTAO will uncover an entire population of such systems. Why some binary systems are active at high energy while others are not? What powers particle accelerators in gamma-ray binaries? What is the mechanism of gamma-ray emission from these systems?
A large part of the stars in the Galaxy form binary systems where two companion stars orbit around each other. When one of the stars reaches the end of its life and gravitationally collapses, it is replaced by a compact object: a neutron star or a black hole. The compact object may become a bright X-ray or gamma- ray source and the exact mechanisms involved are quite enigmatic. In some cases, it is not even clear if the powering energy is from matter falling on the compact object. It is evident that these galactic binary systems offer unique laboratories to study stellar mass black holes and neutron stars. The physical conditions (gas density, radiation, and magnetic field) change periodically along the eccentric orbit of the compact object around the companion star, changing the source activity and its high-energy emission. Some galactic stellar-mass black holes in binaries can be surrounded by accretion disks and accelerate jets, they are called micro-quasars. Understanding the mechanisms leading to high-energy activity in these sources can thus help clarifying the mechanisms powering quasars and their jets in active galactic nuclei. Most of the compact objects in stellar binary systems do not appear as gamma-ray sources. It is not clear what triggers the VHE emission and how the acceleration efficiency evolves with the age of the system. CTAO sensitivity will enable the detection of weaker sources and statistical studies of the gamma-ray binary source population. It will also provide insights into the origin of high-energy activity through high-quality measurements of the spectral and timing properties of brighter sources.