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Detection of Quasiperiodic Oscillations in the Blazar PKS 0735+178 with TESS

Using TESS observations from sectors 71 and 72, this study reports the detection of a modest flare and a transient quasiperiodic oscillation with an ~11.2-hour period in the optical light curve of the blazar PKS 0735+178, confirmed through multiple statistical analysis techniques.

Original authors: Shubham Kishore, Alok C. Gupta, Paul J. Wiita, Sandeep K. Mondal, M. Vivek

Published 2026-07-09

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CC0 1.0

Original authors: Shubham Kishore, Alok C. Gupta, Paul J. Wiita, Sandeep K. Mondal, M. Vivek

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Catching a Cosmic Rhythm

Imagine the universe is a giant, chaotic concert hall. Most of the time, the music (light) from the stars and galaxies is just a messy, random noise—like a crowd shouting without a rhythm. But sometimes, if you listen closely enough, you might hear a specific drumbeat repeating itself.

This paper is about astronomers catching one of those rare, rhythmic drumbeats coming from a very distant, very energetic galaxy called a blazar named PKS 0735+178.

The Cast of Characters

The Blazar (PKS 0735+178): Think of this as a cosmic lighthouse. It's a supermassive black hole at the center of a galaxy, shooting out a massive jet of particles (like a high-speed water hose) directly at Earth. Because it's pointing right at us, it looks incredibly bright and changes its brightness very quickly.
The Telescope (TESS): Usually, this satellite is looking for planets orbiting other stars. But in this case, the scientists used it like a high-speed camera to take a "movie" of this blazar's brightness over about 49 days.
The Gamma Rays (Fermi): This is a different telescope looking at the highest-energy light (gamma rays) from the same object. It's like checking if the lighthouse is also flashing a strobe light in a different color.

What They Found

The scientists looked at the "movie" of the blazar's light and found two main things:

1. A Short, Two-Part Flash (The Flare)
In the first part of their observation, the blazar suddenly got brighter. It wasn't just one big flash; it was like a double-blink.

The Analogy: Imagine a light bulb that flickers once, dims slightly, and then flickers again before settling down. This "double-flicker" lasted about 4.3 days. It was a modest event, but it showed the black hole was having a bit of a tantrum.

2. The Rhythmic Beat (The QPO)
In the second part of the observation, something even more interesting happened. The light didn't just flicker randomly; it started pulsing with a steady rhythm.

The Beat: The blazar brightened and dimmed over and over again, exactly every 11.2 hours.
The Significance: Finding a rhythm in a blazar is like finding a perfect metronome ticking inside a hurricane. The scientists used three different mathematical "stethoscopes" (statistical tools) to make sure this wasn't just a glitch or random noise. They confirmed the rhythm was real with a high level of confidence (about 99.9% sure).

What About the Gamma Rays?

The scientists also checked the gamma-ray data (the "strobing" light).

The Result: The gamma rays were mostly quiet. They didn't show the same rhythmic beating or the big flash that the optical light did.
The Takeaway: This suggests that the "flash" and the "rhythm" are happening in a specific part of the jet (likely closer to the black hole or in a specific zone) that doesn't necessarily affect the highest-energy gamma rays. It's like the engine of a car revving up (the optical light) while the exhaust pipe (the gamma rays) stays relatively calm.

Why Does This Rhythm Happen?

The paper suggests a few theories for why this 11.2-hour beat exists, using the black hole as the stage:

The Hot Spot Theory: Imagine a single, super-hot blob of gas orbiting the black hole like a planet. As it circles, it gets brighter when it faces us and dimmer when it turns away. If it takes 11.2 hours to do one lap, that's our rhythm.
The Jet Theory: The jet shooting out of the black hole might be twisting or wobbling (like a garden hose that's kinked). As the shockwave travels down this twisting hose, it hits "bumps" in the magnetic field, creating a rhythmic pulse of light.

The Black Hole's Weight

By knowing the rhythm (11.2 hours) and the distance to the galaxy, the scientists could estimate how heavy the central black hole is.

The Estimate: They calculated the black hole weighs between 60 million and 400 million times the mass of our Sun.
The Catch: Because we don't know exactly how fast the black hole is spinning, the weight estimate has a wide range. If it's spinning slowly, it's lighter; if it's spinning fast, it's heavier. But it's definitely a giant.

Summary

In simple terms, this paper is a report card saying: "We watched a distant, chaotic cosmic lighthouse for 49 days. We saw it do a quick double-flare, but more importantly, we caught it tapping its foot to a steady 11.2-hour beat. This rhythm helps us understand how the black hole is spinning and how heavy it is, even though the highest-energy light from the same object stayed quiet."

Technical Summary: Detection of Quasiperiodic Oscillations in the Blazar PKS 0735+178 with TESS

Problem and Context
Blazars, a subclass of radio-loud active galactic nuclei (AGN), exhibit non-thermal emission across the entire electromagnetic spectrum, dominated by relativistic jets aligned with the observer's line of sight. While their light curves (LCs) are generally non-periodic, the detection of quasiperiodic oscillations (QPOs) in various AGN subclasses has become a significant area of research, potentially offering insights into jet physics, accretion disk instabilities, or supermassive black hole (SMBH) properties. The blazar PKS 0735+178 ( $z=0.45$ ) has a history of variability studies, including a reported 13.8-year QPO and associations with neutrino events, yet short-timescale intraday variability (IDV) and transient QPOs remain challenging to characterize due to the stochastic nature of blazar emission and observational gaps. This study addresses the need to investigate short-timescale variability in PKS 0735+178 using high-cadence optical data to search for transient QPO signatures and associated flaring behavior.

Methodology
The authors analyzed optical data from the Transiting Exoplanet Survey Satellite (TESS) covering sectors 71 and 72 (October 19 – December 7, 2023), spanning approximately 49 days.

Data Acquisition and Reduction: Full Frame Images (FFI) were processed using the lightkurve Python package. Differential photometry was performed using a comparison star ('D') selected for low crowding. The analysis involved aperture photometry, background subtraction, and normalization against the comparison star's pixel response function to correct for instrumental artifacts. The data were segmented into four intervals (1/71, 2/71, 1/72, 2/72) to mitigate the effects of TESS's inherent 2–4 day data gaps, which complicate periodogram analysis over full sectors.
Gamma-ray Data: Fermi-LAT Pass 8 data (0.1–300 GeV) for the same epoch were analyzed using Fermipy to construct a 1-day binned light curve and Test Statistic (TS) map, providing a multi-wavelength context.
Statistical Analysis:
- Fractional Variability ( $F_{var}$ ): Calculated to quantify intrinsic variability amplitudes for each segment, accounting for Poisson noise.
- Weighted Wavelet Z-transform (WWZ): Applied to the segment 2/72 data to identify transient periodicities in the time-frequency domain, utilizing a Morlet mother wavelet.
- Lomb-Scargle Periodogram (LSP): Used to estimate the power spectral density (PSD) and identify significant frequency peaks. The PSD was modeled as a red noise power law ( $P(\nu) = A\nu^{\alpha} + c$ ) plus white noise. Significance was assessed using Bayesian Information Criterion (BIC) comparisons and analytical methods following Vaughan (2005).
- Monte Carlo Simulations: To validate significance, $10^5$ synthetic light curves with similar statistical properties (PSD shape and PDF) were generated to determine False Alarm Probabilities (FAP).
- Phase Dispersion Minimization (PDM): Employed as a non-Fourier method to detect non-sinusoidal periodicities, minimizing the variance of phase-folded data.

Key Results

Optical Flaring: In sector 71 (segment 1/71), a modest flare lasting $\sim4.3$ days was detected, composed of two sub-flares (C1 and C2). C1 appeared symmetric, while C2 was asymmetric with a slower decay. The variability amplitude ( $F_{var}$ ) in this segment was $\sim3\%$ , rising to $>5\%$ during the flare components.
Transient QPO Detection: In sector 72 (segment 2/72), a transient QPO was identified with a period of $\sim11.2$ $\sim 11.2$ hours ( $\sim2.15$ $\sim 2.15$ day $^{-1}$ $^{- 1}$ ).
- Significance: The peak was detected at a local significance of $4.11\sigma$ and a global significance of $3.06\sigma$ using the LSP method with red-noise modeling.
- Validation: Monte Carlo simulations yielded a False Alarm Probability (FAP) of 0.00035 ( $\sim3.58\sigma$ ). PDM analysis confirmed the signal with a dip in the $\Theta$ -statistic corresponding to an FAP of $\sim0.0003$ ( $\sim3.62\sigma$ ).
- Duration: The QPO feature was transient, appearing primarily between BTJD 3281.3 and 3284.1.
Gamma-ray Correlation: The Fermi-LAT light curve showed the source in a quiescent state throughout the observation window. While modest flux increases were noted during the optical flare and near the QPO epoch, the uncertainties were large, and no significant correlation or periodicity could be established in the $\gamma$ -ray band.
PSD Characteristics: All four segments exhibited Power Spectral Density slopes ( $\alpha$ ) $\gtrsim 2$ , consistent with typical optical emission from blazars.

Significance and Claims
The paper reports the detection of a short-duration ( $\sim11.2$ hours) QPO in the optical light curve of PKS 0735+178, a timescale characteristic of intraday variability. The authors suggest that such QPOs may arise from:

Jet Mechanisms: Turbulence behind shocks, precession of a helical jet, or magnetohydrodynamic instabilities causing enhanced emission at specific viewing angles due to Doppler boosting.
Disk Mechanisms: Instabilities in the accretion disk, such as a dominant hot spot near the innermost stable circular orbit (ISCO) or pulsational modes.

Using the QPO period and assuming an origin in the orbital motion of a blob near the ISCO, the authors estimate the central SMBH mass. For a non-spinning (Schwarzschild) black hole, the mass is estimated at $(6.10 \pm 1.05) \times 10^7 M_{\odot}$ ; for a maximally rotating (Kerr) black hole, it is $(3.88 \pm 0.67) \times 10^8 M_{\odot}$ . These values are consistent with previous estimates derived from intraday variability timescales.

The lack of a strong $\gamma$ -ray counterpart suggests that the optical and $\gamma$ -ray emission regions may not be co-spatial, or that the mechanisms driving the optical QPO do not significantly impact the high-energy emission. The study underscores the utility of TESS for detecting transient, short-timescale QPOs in blazars, which are often obscured by longer-term stochastic variability or observational gaps in ground-based data.