Nuclear Millimeter Continuum Emission in Radio-Quiet Active Galactic Nuclei: an ALMA Spectral and Variability Study
Unresolved millimeter (mm) emission is observed in radio-quiet (RQ) Active Galactic Nuclei (AGN), but its physical origin remains debated. Self-absorbed synchrotron emission from the compact X-ray corona is one of the leading candidates. Previous support for this coronal synchrotron origin has mainly been based on variability and spectral indices in a few RQ AGN, with a lack of (quasi-)simultaneous broadband data for larger samples. We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of nine nearby ($z \sim 0.008-0.064$) RQ AGN, obtained quasi-simultaneously at 100, 145, and 233 GHz. Based on power-law fits to the spectral energy distributions, we measure millimeter spectral indices ranging from $-0.05^{+0.09}_{-0.09}$ to $0.88^{+0.12}_{-0.11}$. Furthermore, across two 100 GHz observing epochs per source, separated by $4-11$ days, we detect flux density variability of $30-56$% (>$3σ$ significance level) in three out of nine sources. Spectral slopes alone are insufficient to constrain the origin of the millimeter emission; however, the observed short-timescale variability and inferred brightness temperature values are consistent with a non-thermal emission mechanism arising from a compact emitting region with a size of $R_{\rm mm}< 10^3 - 10^4 R_{\rm g}$, such as the corona or a compact jet. Furthermore, we find a clear correlation between the 100/233 GHz and X-ray flux in our sample, and establish a new linear correlation between 145 GHz and X-ray emission.