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Subject [Research] THE VARIABILITY TIME SCALE - ACCRETION RATE RELATION OF ACTIVE GALACTIC NUCLEI
Writer admin Date 2017-03-15 14:35:04

 

 

THE VARIABILITY TIME SCALE - ACCRETION RATE RELATION OF ACTIVE GALACTIC NUCLEI

 

 

 

 

PhD stedent Jongho Park

(first author) 

Prof. Sascha Trippe

(Advisor/Corresponding author)

 

 

Active galactic nuclei (AGN) are the strongest sources of energy in the universe, being up to 10,000 times brighter than our home galaxy, the Milky Way. Their radiation is powered by accretion of interstellar gas into supermassive black holes located in the centers of galaxies; each of those black holes has a mass of a few million to a few billion times the mass of the sun.


The brightness of AGN is highly variable and can change by more than one order of magnitude within few years. The amount of variability is stronger over longer time scales and shows no periodicities.
In statistics terms, AGN are sources of "red noise": the power of variability is proportional to the sampling frequency (or inverse variability time scale) taken to a power beta, with beta being smaller than zero.

 

SNU PhD student Jongho Park and Prof. Sascha Trippe analyzed the radio light curves of 43 bright AGN. The light curves, which were provided by the University of Michigan Radio Astronomy Observatory (UMRAO), span about 30 years in time. Park and Trippe calculated the power law indices beta for each light curve and searched for correlations with the physical properties of the target AGN (like, e.g., black hole mass). Surprisingly, their analysis found a correlation between beta and the accretion rate, which is the amount of mass a black hole accretes per time: beta is proportional to the accretion rate taken to the power of 0.25.
In other words: if a black hole accretes more mass per time, variations of its brightness take more time.

This new relation is not yet understood, and investigations of its cause continue.


REFERENCE: Park, J. & Trippe, S. 2017, ApJ, 834, 157
[http://iopscience.iop.org/article/10.3847/1538-4357/834/2/157/meta]


 

 

 

FIGURE CAPTION:
 The variability time scale - accretion rate relation in logarithmic representation, showing the time scale parameter beta (corrected for Doppler boosting) as function of accretion disk luminosity (accretion rate times squared light speed). The parameter alpha, shown in the left panel, is the slope of the best-fit line.