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Zero time lag between optical and X-ray quasi-periodicity in AT2018cow

Presentation #114.09 in the session Stellar-Mass and Galactic Black Holes.

Published onJun 29, 2022
Zero time lag between optical and X-ray quasi-periodicity in AT2018cow

The fast-rising blue optical transient AT2018cow possibly represents a new astrophysical phenomenon. The origin of AT2018cow remains debated. It exhibited an initially nearly featureless spectrum, a persistent high temperature, and a receding photosphere. Moreover, marginal evidence for quasi-periodic oscillations with a period of about 4 days in the X-ray light curve has been reported. Here we further explore the significance of such oscillations. We apply a Monte Carlo simulation technique and a Lomb–Scargle periodogram analysis and confirm previously published findings that there is power on a modulation timescale of 4.2 ± 0.1 days in the X-rays (2.7σ significance). We find evidence for oscillations with a similar period in the combined optical bands (3.8σ). By combining the optical and X-ray light curves, we find that the power is amplified and highly significant (4.2σ), at least between 15 and 30 days since discovery. With Fast Fourier Transform analysis, we find that the results are consistent with the Lomb–Scargle periodogram analysis. After removing the noise contribution, the power is 3.3σ in the combined optical and X-rays, suggesting the periodicity in AT2018cow is not caused by noise. We also find that the maximum cross-correlation function of the light curves of AT2018cow between the optical and X-ray bands appears at a time lag of 0.05 (+0.71-0.79) day, demonstrating that the light curves are correlated with zero time lag at different wavelengths. The cross-correlation is independent of light curve detrending models. This is the first time that a zero time lag between optical and X-ray quasi-periodicity is detected in a transient. Such properties are only consistent with the global Lense-Thirring precession, suggesting that AT2018cow was an accreting black hole system. With a simplified model of the global Lense-Thirring precession, we constrain the dimensionless spin parameter a* for the black hole in AT2018cow. The scenario of a direct collapse from a massive star to a 19.2 ± 14.4M black hole requires a* ≈ 0.4 (prograde orbits) or a* ≈ -0.8 (retrograde orbits). This exploratory study suggests that AT2018cow is likely to be a black hole, and the global Lense-Thirring precession may be a plausible explanation for the quasi-periodicity in AT2018cow.

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