Active galactic nuclei (AGNs) and black hole X-ray binaries (BHBs) are the key directions in high-energy astrophysics. It is found that the black hole mass, X-ray luminosity and radio luminosity follow a simple and linear relationship in logarithmic space (see Figure). This tight correlation, known as the “fundamental plane” of black hole activity, plays an important role in numerous research fields, such as the AGN feedback and the formation of relativistic jets.
Like human beings, black holes are the power sources in AGNs and BHBs, through eating (accreting) surrounding matter. Interestingly, most of the black holes in the galaxies of local universe are in the state of starvation, i.e. the nuclei of these galaxies are extremely inactive and they are classified as quiescent AGNs. In the fundamental plane research, one problem is whether or not the quiescent AGNs follow the same fundamental plane as that of normal ones.
Recently, researchers at Shanghai Astronomical observatory (SHAO) of Chinese Academy of Sciences solved this problem and made new achievement. They found that the quiescent AGNs follow a different radio/X-ray correlation (thus a different fundamental plane) from that of normal AGNs. They further proposed to investigate the fundamental plane under a revised parameter space. This study has been published in The Astrophysical Journal.
This study was conducted based on the largest quiescent AGN sample to date, which includes 75 sources. “Through a Bayesian Monte Carlo simulation approach, based on a much larger sample size, we confirm a previous result that quiescent AGNs are different from normal ones, i.e. they follow a much steeper correlation between radio and X-rays.” XIE Fuguo said.
Researchers then argued that the fundamental plane should be investigated under a revised parameter space, i.e. the impact of black hole mass on the luminosities should be properly taken into account in advance, in order to understand them more physically. This new parameter space will benefit in dividing/separating different accretion modes (which normally corresponds to different relative luminosities).
What is the physical reason for the quiescent AGNs to follow a steeper radio/X-ray correlation? "Whether or not there is a turnover in the radio/X-ray correlation, and where the turnover happens in relative luminosity, are of vital importance theoretically as they will help people to understand the origin of X-ray emission in these quiescent AGNs." YUAN Feng said. "For example, theoretically it is argued that the X-ray emission of quiescent AGNs is from relativistic jet, while that of normal AGNs comes from the hot accretion flow."
Figure: Fundamental plane of black hole activity in AGNs. (Image by XIE Fuguo)
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