Characterizing the variability of the extragalactic sources used for calibration in the Atacama Large Millimeter/Sub-millimeter Array ( ALMA ) is key to assess the flux scale uncertainty of science observations .
To this end , we model the variability of 39 quasars which have been used by ALMA as secondary flux calibrators using continuous time stochastic processes .
This formalism is specially adapted to the multi-frequency , quasi-periodic sampling which characterizes the calibration monitoring of ALMA .
We find that simple mixtures of Ornstein-Uhlenbeck processes can describe well the flux and spectral index variability of these sources for Bands 3 and 7 ( 91.5 and 103.5 , and 343.5 GHz , respectively ) .
The spectral shape of the calibrators are characterized by negative spectral indices , mostly between -0.35 and -0.80 , and with additional concavity .
The model provides forecasts , interpolations , and uncertainty estimations for the observed fluxes that depend on the intrinsic variability of the source .
These can be of practical use for the ALMA data calibrator survey and data quality assurance .