A team of international astronomers searched for the collective gamma-ray (γ-ray) emission from a sample of Ultra-fast outflows (UFOs) using a stacking technique. They found that, high-velocity winds launch by black holes can produce gamma-rays. Their study recently appeared in APS Physics.
Accreting super-massive black holes (SMBHs) at the centers of galaxies, often called active galactic nuclei (AGN), have been observed to launch and power winds. These winds, which are also called ultra-fast outflows (UFOs), are made of highly ionized gas and are likely launched from near the SMBH. Their wide solid angle, and fast velocity allow UFOs to transfer a significant amount of kinetic energy from the AGN to the host galaxy. Propagating through the galaxy, the wind should interact with the interstellar medium creating a strong shock, similar to those observed in supernovae explosions, which is able to accelerate cosmic rays (CRs).
“In order to search for the collective UFO emission, we adoped a stacking technique. Our sample consists of all radio quiet UFO, which gives 11 sources in total.”
By adopting a sensitive stacking analysis they were able to detect the average γ-ray emission from these galaxies. In order to confirm whether the γ-ray emission from these galaxies is truly related to the presence of UFOs or not, they also performed several tests and concluded that yes it is.
Finally, their observations of AGN winds have suggested that AGN transfer a small fraction (∼1–5 %) of their bolometric luminosity to the winds. Their analysis indicated that, a portion of this transferred luminosity in turn accelerates cosmic rays (CRs) and
produces gamma rays.
They found that AGN convert ≈ 3 × 10¯4 of their bolometric luminosity into
gamma (γ) rays. They also found that ≈ 4 × 10¯4 of the wind mechanical power is transferred to γ rays. For comparison, in the Milky Way galaxy, supernova explosions transfer ≈ 2 × 10¯4 of their mechanical energy to γ rays.
“This shows that AGN winds, if sustained for a few million years, can energize a large fraction of the CR population within a galaxy.”
For more: The Fermi-LAT Collaboration, “Gamma rays from Fast Black-Hole Winds”, APS Physics, 2021. Link to paper
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