Simultaneous Monitoring of X-ray and Radio Variability in Sagittarius A*

TL;DR

This study presents nine simultaneous X-ray and radio observations of Sagittarius A*, revealing potential time lags between flares but no definitive correlation, highlighting the need for more extensive monitoring to understand their relationship.

Contribution

First simultaneous X-ray and radio observations of Sgr A* showing potential time lags and emphasizing the necessity for more comprehensive data to clarify correlations.

Findings

Radio peaks can lag X-ray flares by up to 176 minutes.

No statistically significant correlation found between X-ray and radio variability.

Long-duration, simultaneous monitoring is essential for understanding Sgr A*'s variability.

Abstract

Monitoring of Sagittarius A* from X-ray to radio wavelengths has revealed structured variability --- including X-ray flares --- but it is challenging to establish correlations between them. Most studies have focused on variability in the X-ray and infrared, where variations are often simultaneous, and because long time series at sub-millimeter and radio wavelengths are limited. Previous work on sub-mm and radio variability hints at a lag between X-ray flares and their candidate sub-millimeter or radio counterparts, with the long wavelength data lagging the X-ray. However, there is only one published time lag between an X-ray flare and a possible radio counterpart. Here we report 9 contemporaneous X-ray and radio observations of Sgr A*. We detect significant radio variability peaking $\gtrsim$176 minutes after the brightest X-ray flare ever detected from Sgr A*. We also report other potentially associated X-ray and radio variability, with the radio peaks appearing $\lesssim$80 minutes after these weaker X-ray flares. Taken at face value, these results suggest that stronger X-ray flares lead to longer time lags in the radio. However, we also test the possibility that the variability at X-ray and radio wavelengths is not temporally correlated. We cross-correlate data from mismatched X-ray and radio epochs and obtain comparable correlations to the matched data. Hence, we find no overall statistical evidence that X-ray flares and radio variability are correlated, underscoring a need for more simultaneous, long duration X-ray--radio monitoring of Sgr A*.