A 600 minute near-infrared lightcurve of Sagittarius A*

TL;DR

This study presents the longest near-infrared lightcurve of Sagittarius A*, addressing quasi-periodicity and time lag discrepancies, and finds no significant evidence for transient QPOs or periodic signals.

Contribution

It provides a combined, long-duration near-infrared lightcurve of Sgr A* from Keck and VLT data, clarifying previous conflicting reports on periodicity and time lags.

Findings

No confirmed ~20 min quasi-periodicity.

Consistent ~100 min time lag between IR and mm data.

No significant transient QPO detected.

Abstract

We present the longest, by a factor of two, near-infrared lightcurve from Sgr A* - the supermassive black hole in the Galactic center. Achieved by combining Keck and VLT data from one common night, which fortuitously had simultaneous Chandra and SMA data, this lightcurve is used to address two outstanding problems. First, a putative quasi-periodicity of ~20 min reported by groups using ESO's VLT is not confirmed by Keck observations. Second, while the infrared and mm-regimes are thought to be related based on reported time lags between lightcurves from the two wavelength domains, the reported time lag of 20 min inferred using the Keck data of this common VLT/Keck night only is at odds with the lag of ~100 min reported earlier. With our long lightcurve, we find that (i) the simultaneous 1.3 millimeter observations are in fact consistent with a ~100 min time lag, (ii) the different methods of NIR photometry used by the VLT and Keck groups lead to consistent results, (iii) the Lomb-Scargle periodogram of the whole NIR lightcurve is featureless and follows a power-law with slope -1.6, and (iv) scanning the lightcurve with a sliding window to look for a transient QPO phenomenon reveals for a certain part of the lightcurve a 25 min peak in the periodogram. Using Monte Carlo simulations and taking the number of trials into account, we find it to be insignificant.