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

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.

Figures (4)

• Figure 1: Systematics-corrected TESS Sector 34 optical light curve of PKS 0805$-$07 obtained using the Quaver pipeline. The red points correspond to the hybrid reduction, while the blue points show the Simple PCA (SPO) light curve adopted for the timing analysis. The time axis is given in BTJD ($\mathrm{BTJD} = \mathrm{BJD} - 2457000$), and the flux is in instrumental $\mathrm{e^{-}\,s^{-1}}$.
• Figure 2: Lomb--Scargle periodogram of the TESS light curve of PKS 0805$-$07. A dominant peak is detected at $f=0.5968~{\rm d^{-1}}$ ($P\approx1.7$ d), exceeding the 99.9% confidence level derived from $2\times10^{4}$ Monte Carlo simulations following the method of emmanoulopoulos2013generating.
• Figure 3: Left: WWZ time--frequency map of the TESS light curve showing the evolution of power as a function of time (days) and frequency. Right: Average WWZ power spectrum with confidence levels derived from $10^{4}$ Monte Carlo simulations. The dashed cyan and red curves denote the 99.9% and 99.99% confidence levels, respectively. The dashed black line marks the dominant peak at $f=0.5977~{\rm d^{-1}}$ ($P\approx1.67$ d), which exceeds the 99.9% confidence threshold.
• Figure 4: Segment of the TESS optical light curve of PKS 0805$-$07 (1.5 hr binned) over the interval $\mathrm{BTJD}=2231.4047$--$2240.4047$, showing the quasi-periodic modulation. The black points with error bars represent the observed flux, while the red curve shows the best-fitting sinusoidal model with a period of $P\approx1.7$ d. The model reproduces the repeating flux enhancements over $\sim5$ cycles, consistent with the timescale identified in the LSP and WWZ analyses.