A group of researchers led by Paula Sánchez-Saez, a doctoral student in the Astronomy Department of the University of Chile, was able to determine that the rate of variability of light emitted by material absorbed by supermassive black holes in the cores of active galaxies is determined by the accretion rate, that is, the amount of material absorbed.
'The light emitted by a material whose brightness changes greatly over time, so we can talk about some changes. We know that it is changing, but we still do not know why when observing other objects, such as stars or galaxies without active nuclei, we see that their brightness is constant over time, but if we look at galaxies with active nuclei , their brightness both rises and falls, that is, is completely unpredictable. We studied how the amplitude of variability relates to the average brightness emitted by a supermassive black hole (AGN) and the accretion rate of AGN (that is, the amount of material absorbed by the black hole per year). The results of our analysis show that, contrary to what was previously thought, the only important physical property to explain the amplitude of variability is the AGN accretion rate, 'says Paula Sanchez in her publication.
The study found that there is only one physical property that can predict the variability of these objects: the accretion rate. 'The only thing that matters is how much material gets into this supermassive black hole. So if she is on a 'diet' or if she 'swallows', then it takes a lot of energy for her to 'eat' … that is, it determines whether a lot or a little. Our discovery is that the less they 'swallow', the more they change, 'explained Polina Lyra, a researcher in the Astronomy Department at the University of Chile and a researcher at the CATA Astrophysicist Center for Excellence.
For Paula Sanchez-Saez, the first author of the study, the importance of this discovery is to try to figure out what is the physical mechanism of this variability – one of the most inherent characteristics of active galactic nuclei.
'The results obtained in this study challenge the old paradigm that the amplitude of AGN variability depends mainly on the AGN luminosity. It was believed that the measurement of the mass of black holes is not always possible, therefore, the measurement of the accretion rate can be performed exactly for several objects at once. According to SDSS data, physical properties can be measured for about 2000 objects, which was also observed in the QUEST-La Silla AGN variability survey. In addition, from our observation, we were able to obtain very good quality light curves for a large sample of objects, so we could independently investigate the variability of each object, which was not possible before for most AGNs. With accurate measurements of the physical properties of AGNs, coupled with good characterization of the variability of individual AGNs, we were able to determine that the main factor determining the amplitude of variation is the accretion rate, or more, or in technical terms, the Eddington ratio. '
The data used in this work were taken from two sources at once. To analyze the variability, the scientists used data from the QUEST-La Silla AGN variability survey (led by Paulina Lira), which was carried out from 2010 to 2015, observing five extragalactic locations at once. To study the physical properties of AGNs, public spectral data from the Sloan Digital Sky Survey (SDSS) were used.
In the future, researchers plan to study the timeline of variability of these active galactic nuclei:
'Another very important property is the time scale of variability of these objects. To accurately measure this property, we need to have light curves that span over 10 years. Therefore, we must wait for future research, such as observations from the Large Synoptic Observation Telescope (LSST), to provide more photometric data that we can combine with current data, so we can combine this data with our data from the QUEST-La Silla AGN variability survey that will expand our light curves to about 20 years, 'concluded Paula.
