Authors: John D. Timlin III, W. N. Brandt, S. Zhu, H. Liu, B. Luo, and Q. Ni
First Author’s Institution: Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA; Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
Status: Published in MNRAS. Open access on arXiv.
Fueled by the accretion of gas and dust onto compact supermassive black holes, active galactic nuclei (AGN) shine like distant lighthouses on the Universe’s dark shores. At a closer glance, a “corona” of hot gas (analogous to that of our own Sun) encircles the inner accretion disk of these energetic objects and produces X-rays. Astronomers believe this emission be intricately connected to the violent swirl of matter at the galactic center. However, the true nature of these coronal regions remains uncertain.
To better understand the dynamics of AGN coronal regions and their dependency on accretion, today’s authors analyze quasar observations using the Chandra X-Ray Observatory. To ensure they are probing coronal activity that is dependent on accretion flow, the researchers exclude radio-loud AGN, which often experience X-ray variations from jets, and AGN with broad absorption lines, which tend to feature X-ray variability due to changing column densities (the X-rays are absorbed). They net a final sample of 462 AGN, which were collectively observed ~ 1,600 times for nearly ~ 12 years.
A Deeper Understanding
The authors determine that the X-ray fluctuations in their sample are not consistent with a Gaussian distribution, which suggests that these measurements are not due to random variations in coronal emission. Rather, the authors suspect that the variations more likely arise from a supplementary physical mechanism. This strengthens the notion that a physical process, such as accretion, is regulating the coronal X-ray variability.
They also measure the frequency of extreme X-ray variations in three epochs: ∆t < 5.3 days, 5.3 ≤ ∆t ≤ 294 days, and ∆t ≥ 294 days (Figure 1). They find that fluctuations tend to increase with longer timescales. This indicates that the accretion processes near the black hole require sufficient time to influence the corona.
Figure 1. Distributions of count ﬂux ratio for observations that are separated by ∆t < 5.3 days (ﬁlled light blue, 284 points), 5.3 ≤ ∆t ≤ 294 days (dashed blue; 297 points), and ∆t ≥ 294 days (open red; 318 points) in the full sample. The vertical dashed line depicts a ﬂux ratio of one, and the black arrows show the X-ray limits and directions. A visual inspection of the three distributions suggests that the intermediate and long timescale bins display larger count ﬂux ratios than the short timescale distribution which implies that extreme X-ray variations occur more frequently at larger ∆t. (Figure 5 in the paper)
To The Extreme
The authors report two AGN in their sample with dramatic X-ray variability (≥ 9.85x) that is likely due to accretion.
This AGN was observed five times – three times in August/September 2002 and two times in September/ October 2014 (Figure 2). The X-ray measurements increased between these two epochs (3,472 days) by a factor of ~ 10!
Figure 1: The X-ray variability as a function of the Chandra start time for five observations of SDSS J1417+5223. The values above the data points report the decimal year of the Chandra observations. (Figure 10 in the paper)
This AGN was observed 22 times between 2005 and 2008 (Figure 3). The X-ray count flux in this AGN increased by 21 on a timescale of 439 days. However, unlike in the broader sample, they find extreme variability on short time-scales and measure a 12x increase in 2.73 days. The author’s reason that additional spectral observations of this object are required to better understand it’s X-ray variability.
Figure 2: The X-ray count ﬂux as a function of the Chandra start time for 22 observations of SDSS SDSS J1420+5254. The values above the data points report the approximate decimal year of the Chandra observations. (Figure 11 in the paper)
Time Tells All
Today’s paper showcases the truly active nature of AGN. Using only ~ 12 years of Chandra data, the authors analyzed ~ 1,600 observations for 462 quasars that likely feature X-ray variability that stems from their inner accretion disks. Doing so, they confirm that a physical process (likely accretion) is responsible for the observed fluctuations and that X-ray variability tends to increase over longer timescales.
These observations exemplify the strength of time-domain astronomy in understanding the fundamental nature of AGN. As more AGN are analyzed over longer periods of time, and in additional wavelengths, astronomers will gain a deeper understanding of these fascinating phenomena.