# A Comparative Study of Multiwavelength Blazar Variability on Decades to Minutes Timescales

## Abstract

**:**

## 1. Introduction

## 2. Methodology: Light Curve Generation and PSD Analysis

## 3. PSD Construction: DFT

## 4. Result and Discussion

- The PSD slopes obtained from our analysis for the long-term $\gamma $-ray and optical light curves of the BL Lac objects and on timescales ranging from many years to weeks indicate that the statistical character of $\gamma $-ray flux changes ($\beta \sim $ 1; flickering/pink noise) is different than that for optical flux changes ($\beta \sim $ 1.5–2.9; red noise) (Figure 1 and Figure 2).
- An explicit comparison of the PSDs derived at $\gamma $-ray shows a clear indication of more variability power on timescales $\le $1000 days for OJ 287 as compared to PKS 0735+178 (Figure 3). On the other hand, the optical PSDs, show stronger variability on timescales longer than ∼100 days (Figure 4). Again, on the intra-night timescales, we see larger amplitude variability for OJ 287. However, inclusion of the Kepler spectrum shows a steepening on timescales below ∼10 days, indicating a possible intermittency of the process generating intra-night variability. We note that such intermittency is observed for PKS 0735+178 and in other blazars as well [9].
- OJ 287 is a well-known binary black-hole system with roughly ∼12 year quasi-periodic oscillation (QPO) seen in optical and radio light curves [18,19]. Our analysis using the DFT method does not reveal a clear peak in the PSD corresponding to historical optical light curve (Figure 4). This could be due to extremely sparse sampling of the light curve before the year 1970 or due to the transitory nature of the QPO behaviour [20].

## 5. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## Abbreviations

DFT | Discrete Fourier transform |

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1 |

**Figure 1.**Power spectral densities (PSDs) corresponding to multiwavelength light curves for the blazar PKS 0735+178. The dashed gray line and solid circles denote the raw and binned periodogram estimates, respectively, while the dashed horizontal line indicates the noise floor level due to the measurement errors. (

**a**) High energy $\gamma $-ray light curve using the Fermi-LAT data at photon energies 0.1–200 GeV (2008–2015) [9]. (

**b**) Daily averaged R-band light curve (1993–2015) [9]. (

**c**) Intra-night R-band light curve obtained on 15 December 2007 [12].

**Figure 2.**PSDs corresponding to multiwavelength light curves for the blazar OJ 287. The dashed gray line and solid circles denote the raw and binned periodogram estimates, respectively, while the dashed horizontal line indicates the noise floor level due to the measurement errors. (

**a**) High energy $\gamma $-ray light curve using the Fermi-LAT data at photon energies 0.1–300 GeV (2008–2016) [10]. (

**b**) Daily averaged historical R-band light curve (1990–2017) [10]. (

**c**) Kepler satellite light curve [10]. (

**d**) Intra-night R-band light curve obtained on 12 April 2005 [12].

**Figure 3.**Composite Fermi-LAT PSDs using the ‘binned’ periodogram estimates for the studied blazars covering ∼3 dex in temporal frequency.

**Figure 4.**Composite PSDs in the optical band using the ‘binned’ periodogram estimates for the studied blazars covering ∼6 dex in temporal frequency.

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**MDPI and ACS Style**

Goyal, A.
A Comparative Study of Multiwavelength Blazar Variability on Decades to Minutes Timescales. *Galaxies* **2018**, *6*, 34.
https://doi.org/10.3390/galaxies6010034

**AMA Style**

Goyal A.
A Comparative Study of Multiwavelength Blazar Variability on Decades to Minutes Timescales. *Galaxies*. 2018; 6(1):34.
https://doi.org/10.3390/galaxies6010034

**Chicago/Turabian Style**

Goyal, Arti.
2018. "A Comparative Study of Multiwavelength Blazar Variability on Decades to Minutes Timescales" *Galaxies* 6, no. 1: 34.
https://doi.org/10.3390/galaxies6010034