We know that the light emitted as matter falls and disappears into a supermassive black hole is highly variable with time. Is this twinkling a random process or does it contain information on the physical properties of the system, such as the mass of the black hole or the rate at which matter falls onto it? An international team of astronomers from Greece, Germany and Italy addresses this question in a new study recently released to the scientific community.
It is currently believed that nearly all massive galaxies in the Universe host in their nuclear regions black holes that are million or even billion times more massive than our Sun. A prime example is our own Milky Way at the nucleus of which lurks a black hole that is 4.3 million times heavier than the Sun. This mass is similar to that of dwarf galaxies we observe in the Universe but is concentrated in a region of space smaller than our solar system. These extremely compact and heavy objects are thought to grow their masses over long periods of time by swallowing gaseous matter from their immediate environments. During this process an accretion disk is formed that funnels matter onto the supermassive black hole, thereby feeding it and increasing its mass. The infall of matter onto the compact object via the accretion disk is also accompanied by the release of huge amounts of energy that can be observed as light in different parts of the electromagnetic spectrum, from X-rays and ultraviolet to radio wavelengths. Galaxies with active supermassive black holes at their central regions that produce radiation as a result of accretion are referred to as Active Galactic Nuclei (AGN).
Understanding in detail the structure and formation of accretion disks remains a major challenge in current astrophysical research. One of the difficulties in addressing these points is that the accretion process is by nature highly dynamic, i.e. ever changing with time. Observationally, this is manifested as a “flickering” of the emitted flux, i.e. it is continuously varying with time around a mean value (see Figures 1, 2). It has long been known that this flickering, although stochastic in nature, is not completely random: it is not possible to precisely predict the flux that is emitted by the accretion process at any given time, but the probability of a particular flux can be mathematically quantified. Providing a comprehensive picture of the statistical properties of the AGN flickering is therefore the stepping stone toward accretion models and theories.
Researchers at the National Observatory of Athens in collaboration with the German eROSITA team and the University of Naples in Italy have been looking into the variability of AGN at X-ray wavelengths. They showed in a recent study that the amplitude and frequency of the X-ray flux variations depend on fundamental physical properties of the accreting system, such as the mass of the black hole and the rate at which matter falls onto it. “More massive black holes or black holes that are fed with material at a high rate appear to emit X-ray radiation with a low level of variability. Instead lighter black holes or those that accrete material at a low rate exhibit larger variations in their X-ray flux”, explains Antonis Georgakakis, staff researcher at the Institute of Astronomy Astrophysics Space Applications and Remote Sensing (IAASARS) of the National Observatory of Athens and main author of the study. This trend is demonstrated in Figure 2 showing realistic simulations of the X-ray flickering of AGN with different black hole masses and accretion rates.
Evidence for this behaviour has long been debated by the astronomy community. However, the recent work of the international team led by IAASARS researchers has been able to demonstrate these trends at a high level of confidence thanks to the quality and volume of the data they used. They combined observations from two European X-ray telescopes, the XMM-Newton of the European Space Agency and the eROSITA operated by a consortium of German astronomy institutes, to study the X-ray flux variations of a large sample of known active supermassive black holes (nearly 16,000) spread across the sky (see Figures 1, 3). “XMM-Newton and eROSITA have observed each of these 16,000 AGN at least twice over a period of 20 years. By measuring how much the X-ray flux between repeat observations of individual sources has changed and analysing all these flux differences together it is possible to learn about the statistical properties of the variability of the population”, clarifies Angel Ruiz, postdoctoral researcher at the National Observatory of Athens and co-author of the study. Figures 1, 3 show the X-ray images of one of the 16,000 AGN used in the analysis. This particular AGN has been observed by the XMM-Newton telescope several times between 2004 and 2019.
In addition to the quality of data, a central role in the results played the adopted methodology for analysing and interpreting the observations. A novel method has been developed to analyse the flux differences of the population that enables maximal information gain from the observations. “Often the flux of a source, as it varies, becomes too faint for the sensitivity limits of either the XMM-Newton or the eROSITA detectors. In this case the source remains hidden underneath the noise levels of these instruments and a direct measurement of the source’s flux is not possible. Instead only an upper limit can be estimated that represents the highest flux that the source could have. Traditional analysis methods often ignore such upper limits. However, they do contain useful information that our new statistical approach is designed to exploit to improve constraints on the variability properties of the population”, reveals Maurizio Paolillo, professor at the University of Naples Federico II and co-author of the study.
The results from this work are important to develop and refine theories of the accretion process onto supermassive black holes. Any such model should be able to explain the observed variability patterns and amplitudes revealed by the recent work of the international team led bhttps://www.mpe.mpg.de/7982604/news231208y IAASARS researchers. It is currently proposed that accretion disk instabilities or variations with time of the physical conditions and the amount of X-ray emitting plasma are responsible for the observed variability, although a complete and satisfactory theory is still under development.
This work coincides with the first world-wide release of the new X-ray observations carried out by the eROSITA instrument. Since its launch in 2019 eROSITA has been scanning the entire sky every 6 months with the goal of providing the most sensitive X-ray observations to date over the full sky. The first data release corresponds to the first 6 months of the eROSITA operations, i.e. the first all sky survey. The observations that are made public will be a unique resource to the astronomical community for a wide range of research applications from the study of compact objects like supermassive black holes to the formation of clusters of galaxies.
The eROSITA instrument has been developed by a team of German research groups led by the Max-Planck Institute for Extraterrestrial Physics. The National Observatory of Athens and IAASARS participates in the German consortium that operates the eROSITA instrument by contributing software tools for the calibration of the X-ray optics and the analysis of the observations.
IAASARS researchers involved in this work are Drs. Antonis Georgakakis, Angel Ruiz and Athanassios Akylas. The research leading to these results is funded from the Hellenic Foundation for Research and Innovation (HFRI) project “4MOVE-U” grant agreement 2688, which is part of the programme “2nd Call for HFRI Research Projects to support Faculty Members and Researchers”.