Frequency-dependent time delays in Sgr A* from a broadband VLA observation
Year: 2026
Type: article
Abstract: Frequency-dependent radio time delays have been measured in Sgr A* light curves, with higher-frequency emission generally leading lower-frequency emission. These frequency-dependent lags have been interpreted in terms of an organized jet-like outflow, expanding plasma components, or magnetic-flux eruptions, although their origin and recurrence remain uncertain. We aim to search for and quantify frequency-dependent radio time delays in a new high-cadence Karl G. Jansky Very Large Array (VLA) observation of Sgr A*. Using the improved broad 12--37 GHz frequency coverage on the VLA, we measured the timing of the dominant variability feature and compared the resulting lag--wavelength slope with previous measurements. Sgr A* was observed with the VLA on 28 May 2022 in the Ku, K, and Ka bands over an sim6 hour track. We extracted both band-averaged and 2 GHz subband light curves and measured the delays using a peak-time method and a locally normalized cross-correlation function. The light curves show one broad variability feature across the observed frequency range, with the maximum occurring progressively later at lower observing frequencies. Both timing methods recover a positive lag--wavelength relation, with a slope of order 40 . This value agrees with time lag results reported over more than 15 years from the first VLA observations at 22--43 GHz to later multifrequency VLA and ALMA campaigns. -1 The sign and magnitude of the estimated delay are consistent with outward propagation through an optically thick synchrotron-emitting region. The recurrence of comparable delays over almost two decades suggests that frequency-dependent radio lags are a persistent feature of Sgr A* variability. Broadband, multi-epoch VLA monitoring of Sgr A* provides a useful probe of its radio variability, but robust lag measurements require high cadence and improved $(u,v)$ coverage of both Sgr A* and the surrounding extended emission such as by combining different VLA configurations.
Source: Astronomy and Astrophysics
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7223208707