Probing general relativistic effects during AGN X-ray eclipses

TL;DR

This paper discusses how X-ray eclipses in Active Galactic Nuclei can be used to detect relativistic effects near black holes, offering a new method to test extreme gravity and AGN structure.

Contribution

It proposes using X-ray eclipse observations to directly observe relativistic effects in AGNs, highlighting potential for future high-sensitivity measurements.

Findings

Relativistic effects may be detectable with current X-ray observations.

Eclipses allow probing of disc emission patterns affected by gravity.

Future observatories will enable tests on many AGNs.

Abstract

Long X-ray observations of bright Active Galactic Nuclei show that X-ray eclipses, with durations from a few hours to a few days, are rather common. This opens up a new window of opportunity in the search for signatures of relativistic effects in AGNs: an obscuring cloud covers/uncovers different parts of the accretion disc at different times, allowing a direct check of the expected pattern of disc emission. In particular, the combination of gravitational redshift and relativistic Doppler boosting should imply strong differences between the receding and approaching parts of an inclined thin disc. At present, these effects may be already detectable with a "lucky" XMM-Newton or Suzaku observation of a complete eclipse by a Compton-thick cloud (a rare, but not impossible-to-see event). In the future, higher sensitivity observatories will be able to perform these tests easily on tens of AGNs. This will provide a powerful and direct way to test extreme gravity, and to probe the structure of AGNs in the close vicinity of the central black holes.