Blazar variability and time lags at millimetre wavelengths: insights from Metsähovi and ALMA monitoring

Blazars, a subclass of active galactic nuclei (AGN) with relativistic jets directed close to our line of sight, dominate the extragalactic gamma-ray sky and are characterised by strong variability across the electromagnetic spectrum. Millimetre observations are particularly valuable because they probe emission regions near the jet base, where high-energy flares are likely produced. In this study, we investigate the variability of the blazar 3C 279 using long-term light curves from Metsähovi (37 GHz) and ALMA Bands 3, 6, and 7 (84–345 GHz). Multiple time-series methods including the Interpolated Cross Correlation Function (ICCF), Z-transformed Discrete Correlation Function (ZDCF), JAVELIN, and PyROA, were applied to search for inter-band time lags. In most cases, higher-frequency emission was found to lead lower-frequency variations, consistent with opacity-driven delays in the jet. However, certain deviations from this pattern suggest a more complex emission geometry or the influence of additional variability processes. We also quantified variability amplitudes using the fractional variability parameter (\(F_{\text{var}}\) ), which increases systematically from ≈ 0.29 at 37 GHz to ≈ 0.38 at 345 GHz, consistent with more compact and energetic emission regions at higher frequencies. These findings underscore the importance of coordinated, multi-frequency millimetre monitoring in constraining the connection between jet dynamics and high-energy emission, and they highlight the potential role of the Africa Millimetre Telescope (AMT) in future long-term blazar monitoring from the southern hemisphere.