A Short-Timescale Optical Quasi-Periodic Oscillation in PKS\,0805$-$07 from High-Cadence TESS Observations

TL;DR

This paper reports the detection of a transient, short-timescale optical quasi-periodic oscillation in the high-redshift quasar PKS 0805-07 using high-cadence TESS data, with potential origins in accretion disk hotspots or jet instabilities.

Contribution

The study presents the first detection of a short-lived optical QPO in PKS 0805-07, utilizing advanced time-series analysis to characterize its transient nature and explore physical origins.

Findings

Detected a ~1.7-day QPO exceeding 99.99% confidence

QPO is transient, lasting about 5 cycles

Possible origins include accretion disk hotspots or jet instabilities

Abstract

We present a timing analysis of the high-cadence optical light curve of the high-redshift flat-spectrum radio quasar PKS\,0805$-$07 obtained during \textit{TESS} Sector~34 (MJD $59230.90$--$59239.90$). We search for short-timescale quasi-periodic oscillations (QPOs) using complementary time-series techniques, including the Lomb--Scargle periodogram (LSP) and the weighted wavelet $Z$-transform (WWZ), and evaluate their significance against red-noise variability using Monte Carlo simulations. The LSP reveals a dominant modulation at $f \approx 0.597\,\mathrm{d^{-1}}$ ($P \approx 1.7\,\mathrm{d}$) exceeding the $99.99\%$ confidence level, while the WWZ independently recovers a consistent timescale at the $\sim 99.9\%$ level and shows that the signal is temporally localized rather than persistent across the full light curve. The modulation spans $\sim 5$ coherent cycles, indicating a transient quasi-periodic feature. We discuss possible physical interpretations of the detected modulation. In a disk-based scenario, orbital motion of a hotspot near the innermost stable circular orbit implies a black hole mass of $M_{\rm BH}\sim7.2\times10^{8}\,M_\odot$, consistent with typical FSRQ values. Alternatively, magnetohydrodynamic kink instabilities in the relativistic jet can naturally produce day-scale variability for standard blazar parameters and account for the transient character of the signal. We conclude that the observed modulation is consistent with a compact, short-lived structure embedded within stochastic jet variability.