<p>AGN's are divided into roughly 3 types, Quasars, Seyfert galaxies and radio galaxies. All of these produce light, each in their own electromagnetic spectrum and intensity like quasar emits across all, seyfert galaxies have less intensity, and radio galaxies emit in the radio spectrum.</p>
<p>However they oscillate, at an intensity, frequency known as Quasi-periodic oscillation which happens in the case of a compact object like black holes, neutron stars and white dwarfs along the edge of the accretion disk near the congestion zone. This quasi-periodic oscillations are noticed  on the lightcurves. Quasiperiodic oscillations occur mainly because of the dynamics of the accretion disk, and the relativistic effects.</p>
<p>So to sum up, the periodic oscillations observed on the light curve are quasi-periodic oscillations which occur mainly because of the dynamics of the accretion disk, and the relativistic effects such as relativistic beaming.</p>
<p>Note: Most of the times, Quasars are not completely periodic and it has signs of slight variability though it is mostly periodic. And it may not be related to QPOs at all, it may be just some background noise and cause stochastic variability, because analysing lightcurves is an extremely difficult and tedious task and may require machine power</p>
<p>Thanks, Hope it helps you!</p>

CC BY-SA 4.0
2023-08-07T15:15:51.643Z
2023-08-09T10:36:34.487Z
2023-08-09T10:36:34.487Z
Arjun
<p>I don't think what you're seeing is periodicity; my bet is that it's just stochastic red noise.</p>
<p>Searches for periodicity or quasi-periodic oscillations (QPOs) in AGN light curves is a difficult and at times contentious task. Plenty of phenomena have been invoked to explain candidates. I kinda like the short summary in the introduction to <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.508.3975K/abstract" rel="nofollow noreferrer">Krishnan et al. 2021</a>; any list should include interactions with a binary companion, jet precession, and accretion disk hot spots, depending on the wavelength in question.</p>
<p>However, one enormous problem with searching for and trying to verify periodicity claims is that AGN light curves are almost always dominated by stochastic variations. One common model is a damped random walk (see e.g. <a href="https://ui.adsabs.harvard.edu/abs/2009ApJ...698..895K/abstract" rel="nofollow noreferrer">Kelly et al. 2009</a>), explained as arising from thermal fluctuations within the accretion disk, although there's plenty of debate as to what's really going on. The resulting light curves can, in some cases, look periodic, but there's really nothing there.</p>
<p>Quantitatively, too, this red noise can trip up standard periodicity search techniques. In an appendix, <a href="https://ui.adsabs.harvard.edu/abs/2010ApJ...721.1014M/abstract" rel="nofollow noreferrer">MacLeod et al. 2010</a> give examples of how naively computing a Lomb-Scargle periodogram can produce spurious candidates because the method compares a periodic signal to white noise, whereas the comparison should really be between a periodic signal and red noise. You need to use far more robust methods to truly find periodicity or QPOs.</p>
<p>That's not to say that all periodicity candidates are spurious! There are increasingly better methods that take this red noise into account, and some AGN, like the famous <a href="https://en.wikipedia.org/wiki/OJ_287" rel="nofollow noreferrer">OJ 287</a>, show strong evidence for periodicity due to a binary companion. But most of the time, variations are just due to red noise. My bet is that that's what you're seeing.</p>

CC BY-SA 4.0
2023-08-08T17:52:11.343Z
Another classic paper on this is https://articles.adsabs.harvard.edu/pdf/1978ComAp...7..103P
John Doty
CC BY-SA 4.0
2023-08-09T14:29:34.907Z
@JohnDoty That's an interesting one. Quite the writing style. . .
HDE 226868
CC BY-SA 4.0
2023-08-07T15:19:34.800Z
2023-08-08T13:09:02.967Z
2023-08-08T13:09:02.967Z
HDE 226868
<p>So apparently they are all lightcurves of (likely candidates) Active Galactic Nuclei and in some way or the other they all appear to be periodic:
<a href="https://i.sstatic.net/tCEqq.png%5Bs%5D" rel="nofollow noreferrer"><img src="https://i.sstatic.net/tCEqq.png%5Bs%5D" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/MbNLn.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/MbNLn.png" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/77Qnw.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/77Qnw.png" alt="enter image description here" /></a>
I'd like to know the reason for this and the underlying physical mechanism.
<br>
Thanks!</p>

CC BY-SA 4.0
2023-08-07T14:49:56.343Z
I'm not sure I see any evidence of periodicity -- can you expand on what you see? Some of the pattern is almost certainly due to gaps between observations. Also, can you add a source for those plots?
HDE 226868
CC BY-SA 4.0
2023-08-07T20:47:22.813Z
Who says they are periodic?
ProfRob
<quasars><light-curve><active-galaxy>
On the "periodic" lightcurves of Active Galactic Nuclei
Ambica Govind
# On the "periodic" lightcurves of Active Galactic Nuclei

(Asked by: username_0 on 2023-08-07)

So apparently they are all lightcurves of (likely candidates) Active Galactic Nuclei and in some way or the other they all appear to be periodic:
[![enter image description here](https://i.sstatic.net/tCEqq.png%5Bs%5D)](https://i.sstatic.net/tCEqq.png%5Bs%5D)
[![enter image description here](https://i.sstatic.net/MbNLn.png)](https://i.sstatic.net/MbNLn.png)
[![enter image description here](https://i.sstatic.net/77Qnw.png)](https://i.sstatic.net/77Qnw.png)
I'd like to know the reason for this and the underlying physical mechanism.
  
Thanks!

**Tags:** quasars, light-curve, active-galaxy
> **Comments:**
> **username_1**: I'm not sure I see any evidence of periodicity -- can you expand on what you see? Some of the pattern is almost certainly due to gaps between observations. Also, can you add a source for those plots?  
> **username_2**: Who says they are periodic?  

---

## 2 answers

### Answer by username_1 on 2023-08-07. Score: 8 | Accepted answer

I don't think what you're seeing is periodicity; my bet is that it's just stochastic red noise.

Searches for periodicity or quasi-periodic oscillations (QPOs) in AGN light curves is a difficult and at times contentious task. Plenty of phenomena have been invoked to explain candidates. I kinda like the short summary in the introduction to [Krishnan et al. 2021](https://ui.adsabs.harvard.edu/abs/2021MNRAS.508.3975K/abstract); any list should include interactions with a binary companion, jet precession, and accretion disk hot spots, depending on the wavelength in question.

However, one enormous problem with searching for and trying to verify periodicity claims is that AGN light curves are almost always dominated by stochastic variations. One common model is a damped random walk (see e.g. [Kelly et al. 2009](https://ui.adsabs.harvard.edu/abs/2009ApJ...698..895K/abstract)), explained as arising from thermal fluctuations within the accretion disk, although there's plenty of debate as to what's really going on. The resulting light curves can, in some cases, look periodic, but there's really nothing there.

Quantitatively, too, this red noise can trip up standard periodicity search techniques. In an appendix, [MacLeod et al. 2010](https://ui.adsabs.harvard.edu/abs/2010ApJ...721.1014M/abstract) give examples of how naively computing a Lomb-Scargle periodogram can produce spurious candidates because the method compares a periodic signal to white noise, whereas the comparison should really be between a periodic signal and red noise. You need to use far more robust methods to truly find periodicity or QPOs.

That's not to say that all periodicity candidates are spurious! There are increasingly better methods that take this red noise into account, and some AGN, like the famous [OJ 287](https://en.wikipedia.org/wiki/OJ_287), show strong evidence for periodicity due to a binary companion. But most of the time, variations are just due to red noise. My bet is that that's what you're seeing.

> **Comments:**
> **username_4**: Another classic paper on this is https://articles.adsabs.harvard.edu/pdf/1978ComAp...7..103P  
> **username_1**: @JohnDoty That's an interesting one. Quite the writing style. . .  

---

### Answer by username_3 on 2023-08-07. Score: 3

AGN's are divided into roughly 3 types, Quasars, Seyfert galaxies and radio galaxies. All of these produce light, each in their own electromagnetic spectrum and intensity like quasar emits across all, seyfert galaxies have less intensity, and radio galaxies emit in the radio spectrum.

However they oscillate, at an intensity, frequency known as Quasi-periodic oscillation which happens in the case of a compact object like black holes, neutron stars and white dwarfs along the edge of the accretion disk near the congestion zone. This quasi-periodic oscillations are noticed on the lightcurves. Quasiperiodic oscillations occur mainly because of the dynamics of the accretion disk, and the relativistic effects.

So to sum up, the periodic oscillations observed on the light curve are quasi-periodic oscillations which occur mainly because of the dynamics of the accretion disk, and the relativistic effects such as relativistic beaming.

Note: Most of the times, Quasars are not completely periodic and it has signs of slight variability though it is mostly periodic. And it may not be related to QPOs at all, it may be just some background noise and cause stochastic variability, because analysing lightcurves is an extremely difficult and tedious task and may require machine power

Thanks, Hope it helps you!


---

