The furthest QSOs in the X-rays
Abstract
In the last 20 years, more than 200 accreting supermassive black holes (SMBHs), shining as quasi-stellar objects (QSOs), have been discovered at z>6, i.e., only <1 Gyr after the Big Bang. The very existence of these objects is a currently unsolved challenge to our theoretical knowledge of SMBH formation. X-ray observations provide a direct view into the very inner regions of the accretion disk/hot corona system, allowing us to study the SMBH accretion physics and possible evolution in the early universe. I will present the main results of recent X-ray observations of the first statistically significant samples of z>6 QSOs, focusing on the observables that we use to study the physics of SMBH accretion (e.g., the relations between Lx and LUV) and their possible evolution in the first Gyr of the universe. I will also present new follow-up observations of two high-redshift QSOs that showed noticeable properties. The first QSO, PSO167 at z=6.5, shows evidence of extremely weak X-ray emission, due to either uncommon accretion physics or heavy obscuration. The second QSO, J1641 at z=6.05, has been found to be an extremely variable (by a factor of >7) X-ray source on timescale of a few months in the rest frame.
Peculiar objects like these might provide unique information on the physics behind the fast growth of high-redshift SMBHs.
