A new study indicates that supermassive black holes at the center of active galaxies do not behave too differently than their stellar mass analogues in X-ray binaries, at least in terms of how surrounding matter accretes and the emission that it follows. We interviewed one of the two co-authors, Juan Fernández Ontiveros of INAF from Rome
The known black holes in the universe occur mainly in two classes, with masses extremely different from each other. On the one hand there are the “featherweights”, or black holes of stellar mass – we are talking about several times the mass of the Sun. Some of these are found in binary systems , in which the black hole orbits together with a star which regularly gases to grow their mass, which are called ‘binary X-rays’ as this process results in a strong emission in the X-rays.
On the other hand we find the heavyweights, supermassive black holes , with masses equal to millions or even billions of times that of the Sun. These giants live in the center of large galaxies, they are generally in a dormant state but when they “activate” they begin to devour the surrounding gas at a sustained rate and consequently emit radiation over the entire electromagnetic spectrum, giving rise to what astronomers call active galactic nuclei (Agn).
While the accretion processes of stellar-mass black holes are well known, those of their supermassive counterparts are less so, making a comparative analysis of the physical mechanisms at work in the two types of systems difficult. A new approach to the problem comes from the collaboration between Teo Muñoz Darias of the Canary Institute of Astrophysics (Iac), which deals with binaries X, and Juan A. Fernández Ontiveros of INAF, expert in active galaxies and supermassive black holes, the whose results recently appeared in the Monthly Notices of the Royal Astronomical Society .
In this study, the two Spanish researchers surveyed 167 active galaxies, looking for similarities between the accretion modes of supermassive black holes at their center and those of their stellar mass analogues in binary X. To learn more, Media Inaf interviewed Juan A. Fernández Ontiveros. Originally from Granada, he studied physics at the University of Granada and astrophysics at the University of La Laguna in Tenerife, Canary Islands, where he earned a doctorate on the subject of Agn, followed by post-doc at the Max Planck Institute of Radio Astronomy in Bonn , at the IAC, at the University of Athens and at the INAF Iaps in Rome.
Dr. Fernández Ontiveros, how did this study come about?
“It’s a collaboration with a longtime friend, Teo Muñoz Darias. We did the doctorate together in La Laguna: he works on X-ray binaries, which are systems that have a stellar-mass black hole , therefore small, with a companion star that transfers material onto the compact object. Instead, I work on supermassive black holes at the center of galaxies ».
What are these two types of systems different?
“X-ray binaries were discovered as explosions in X-rays, and from the point of view of accretion they are the systems we know best because they evolve on human time scales. They spend most of their lives in a ‘quiet’ state in which the companion star transfers material that accumulates in the accretion disk around the black hole. At some point the disc becomes unstable and a period of activity lasting months or years begins, during which these systems become very bright in X-rays, and eventually fade back to their initial state. Supermassive black holes at the center of galaxies, on the other hand, have much longer evolution times, of the order of millions of years, so we cannot follow their evolution on an individual level “.
These are very different timelines… how did you get around this problem?
‘We took a fairly large sample of data statistically, nearly 170 galaxies observed with infrared spectroscopy, and with this statistic we identified several accretion states in supermassive black holes. We cannot follow one during its evolution, but we can identify several that are in different states ”.
How do you practically observe gas falling on black holes?
«The accretion disk [through which the gas falls on the black hole, ed] in binary X-rays emits in X-rays and therefore we can measure this emission directly. In the case of Agn, the disc’s emission is mainly in the ultraviolet but it is absorbed a lot by the hydrogen in the Agn host galaxy – and also in our own galaxy, the Milky Way – so we cannot recover it in most cases. The technique we used, however, allows us to do this: the gas around the black hole absorbs ultraviolet radiation and processes it into emission lines [an emission line corresponds to light emitted by atoms, ions or molecules in a precise frequency, ed.] and we have measured these lines ».
What lines have you observed?
«The lines associated with the different chemical elements and their ions are excited by radiation coming from different parts of the electromagnetic spectrum: if you look at some of these lines you can reconstruct the shape of the original spectrum before it was absorbed. We used the oxygen and neon lines in the mid-infrared frequencies, using data from the Spitzer satellite . ‘
And what did you find?
“We used the lines to identify the presence or absence of the accretion disk in the various Agn, and we built a diagram to identify the different states of accretion.”
Can you explain what we see in the diagram?
«The diagram we built is called Led: luminosity-excitation diagram . Shows the brightness of the system relative to the brightness of the disc. The horizontal axis tells us the importance of the disk, the vertical axis the total brightness of the active nucleus; the latter, normalized with respect to the mass of the black hole, is linked to the system’s accretion state. There are two main accretion states: one bright, top left, dominated by the disk, and the other right, in which the disk is weak and dominates the emission from the jet and the corona around the black hole. The first is called soft state and the second hard state respectively , the names come from the shape of the X-ray spectrum in binaries X ».
Were you surprised by this result?
«Traditionally the physics of accretion has been developed to explain the behavior of binaries X, which are well known because they change frequently and their evolution can be studied. In Agn, the accretion disk could have different physical properties, it should be much colder than that of X binaries, and this means that it may not be dominated by the pressure of the gas but by the pressure of radiation and the magnetic field. Despite the differences, this diagram tells theorists that there appear to be very similar states in black holes throughout the mass interval: surely there are specific features, but the general evolution seems very similar.
Is it the first time that the different growth states in Agn have been identified?
«It is not the very first time but in our opinion this is the clearest result because we have solved some systematics present in previous attempts. Our diagram is the one that most closely resembles that of X binaries, where the systems describe a trajectory in the shape of a ‘q’. In 2006, for example, Körding and co-workers had done similar work but without normalizing the system’s brightness for the black hole’s mass, because they didn’t have these mass estimates. So in their study there was a dispersion of a factor of 100-1000 in the vertical axis. With this noise, the shape a ‘q’ cannot be clearly seen, which instead is perceived in our diagram ».
What did you deduce about the physics of these systems from this analysis?
“The fact that the jet dominates the radio emission during the hard state is seen very well in the X binaries. Seeing the same phenomenon in the Agn – in the diagram it is the region with the black dots, where the emission of the jet is strong – reinforces much parallelism “.
There are differently colored regions in the diagram. What do they represent?
«The regions colored in red, green and blue indicate the different types of active galaxies according to the classical classification, based on the properties of their optical spectrum. The difference between Seyfert 1 (in red) and Seyfert 2 (in green) has traditionally been interpreted as a difference in the orientation of the system relative to us. Liners (in blue) are typically fainter active nuclei usually found in older galaxies. But there is a relatively recently discovered class of active galaxies that are the ‘ changing-look Agn‘: they are active galaxies that change from one type to another, and their brightness can also change a lot. They could be systems in transition, and it would be very interesting to understand how they move in the diagram when the properties of the spectrum change, because they are systems that could be in transition between one accretion state and another ».
What is special about these Agn changing looks ?
«They are very rare, you have to look at many to find one, while in binaries the variability in brightness is a common feature. They are galaxies where changes are seen that could indicate changes in the disk, in the central region. There is certainly a lot of physics to learn in these systems. Before, many of them were not known because they are rare events, but now there are surveys that map the sky every day and we are discovering that these Agn changing looks are a more frequent phenomenon than previously thought ».
How will your study continue?
“The data comes from Spitzer, who is no longer operational now. The next telescope to be able to observe the same lines in the infrared will be James Webb space telescope (Jwst). Part of this work began to prepare the science of the Spica mission , together with Prof. Luigi Spinoglio here at Iaps, which was then very drastically canceled by ESA. Even our recently accepted observation proposal for Jwst derives from the work done for Spica which therefore did not die at all, it gave rise to a lot of science ».
What exactly do you plan to do with Jwst?
“Jwst will see the lines we use to build this diagram with a much higher resolution. So we expect that when Jwst departs and begins measuring a statistically representative amount of these active galaxies, we can use these and other even fainter lines to better understand where the Agn are in this diagram. Another possibility is to use Euclid to do a similar thing, but from the perspective ».
Why Euclid precisely?
“The diagram we have made now is for the galaxies of the local universe. Euclid will be able to do this for the time when galaxies were in formation, 10 billion years ago. It is certainly not a difficult thing to do, you have to identify the lines that Euclid will see in those galaxies. If we can adapt our methodology to use the optician’s lines, we will have many more. And there is not only Euclid but also the Sloan Digital Sky Survey from the ground and then there will be the Roman space telescope : many of the future missions will map galaxies all over the sky, this amount of information is extremely useful for us ».
What can be learned new with a larger sample of galaxies?
“The current sample has 167 galaxies. If the statistic increases, the regions can be better defined. For now we have divided the Agn into 4 types, but if you have a statistic of hundreds of thousands of galaxies you can study the dependencies on other parameters such as the mass of the black hole, the type of galaxy, etc. In the family of active galaxies there is a gigantic taxonomy, and so it will be possible to highlight some classes with special properties to understand what happens differently ».
Featured image: Artist’s impression of the stellar-mass black hole Cygnus X-1. Credits: Nasa
To know more:
- Read on Monthly Notices of the Royal Astronomical Society the article “ X-ray binary accretion states in Active Galactic Nuclei? Sensing the accretion disc of supermassive black holes with mid-infrared nebular lines “by Juan A. Fernández-Ontiveros and Teo Muñoz-Darias
Provided by INAF