FALCON catalogue links millimeter and gamma-ray emission in blazars

Turning calibration observations into a catalogue

Blazars are active galactic nuclei whose jets point nearly toward Earth. Material moving at relativistic speeds makes their radiation appear brighter through Doppler boosting. Their emission stretches from radio waves to gamma rays, but the physical relationship between these different components remains an open question.

In an arXiv manuscript submitted on November 5, 2026, and accepted for publication in The Astrophysical Journal, Kanako Narita, Yoshiyuki Inoue, Bunyo Hatsukade and Yasushi Fukazawa introduce FALCON, short for FERMI–ALMA Calibrator ObservatioN. They compare sources used to calibrate the Atacama Large Millimeter/submillimeter Array with the third release of the Fourth LAT AGN Catalog, or 4LAC-DR3.

Millimeter observations are useful because they probe compact regions near the jet base, with relatively little contamination from the host galaxy or extended radio structures. ALMA repeatedly observes bright, compact calibration sources, providing an archive that can support population studies as well as telescope calibration.

What the researchers matched

The team restricted its ALMA observations to January 2013 onward because earlier measurements could have underestimated flux uncertainties. It then examined the angular separations between ALMA calibrators and Fermi-LAT sources, estimating chance alignments by shuffling the ALMA sources’ right ascensions while preserving their declinations.

The adopted matching radius was 0.10 degrees, or six arcminutes. At that radius, the authors estimate approximately 1% contamination from chance coincidences and an overlooked fraction of about 17% of genuine associations. These are statistical estimates of the matching procedure, not guarantees for individual objects.

After resolving ambiguous matches and removing duplicates across ALMA bands, FALCON contains 949 unique gamma-ray-associated calibrators. Sources without reliable redshifts remain in the catalogue, but cannot enter calculations requiring luminosities. Redshift measures the stretching of light associated with cosmic expansion and is used here to estimate distance.

A correlation that survives a distance check

The luminosity analysis uses smaller, overlapping samples: 642 sources in ALMA Band 3, 469 in Band 6 and 593 in Band 7. Together, these include 649 distinct objects. They require valid positive flux measurements and redshifts, and exclude sources with millimeter luminosities below 10^40 ergs per second.

Across all three bands, higher millimeter luminosity is associated with higher gamma-ray luminosity. The full-sample Spearman rank correlations range from 0.83 to 0.85. This statistic measures how consistently the rankings of two quantities rise together.

Distance is a major complication: luminosities in both wavebands depend on distance, potentially producing an apparent relationship. After statistically controlling for redshift, the correlation coefficients fall to 0.48, 0.55 and 0.48 for Bands 3, 6 and 7, respectively, but remain statistically significant. The fitted relationships are close to proportional, although individual sources show substantial scatter.

The measurements were not simultaneous. ALMA values are medians of observations across multiple epochs, whereas Fermi-LAT fluxes are integrated over its catalogue observing interval. The authors interpret the surviving relationship as evidence for a long-term connection between the emissions, rather than a measurement of coordinated individual flares.

Spectral differences and tentative subclass trends

Gamma-ray-detected calibrators also have flatter millimeter spectra than calibrators without a Fermi counterpart. Between ALMA Bands 3 and 6, their median spectral indices are −0.60 and −0.80, respectively. Under the paper’s convention, the less negative value means flux density declines less steeply as frequency increases. The distributions differ statistically, but overlap.

Within the detected population, the redshift-controlled luminosity correlation appears stronger for high-synchrotron-peaked BL Lac objects than for flat-spectrum radio quasars. These categories differ in their spectral properties. However, the authors emphasize that their confidence intervals overlap substantially, and some subclass samples are small. The apparent progression is suggestive, not conclusive.

What the findings do not establish

The authors discuss a shared dependence on long-term jet power, together with Doppler boosting, as a possible explanation. Different mechanisms for producing gamma rays could contribute to the subclass patterns. The correlations do not uniquely establish those mechanisms or show that both emissions originate in exactly the same location.

Selection remains another limitation. ALMA calibrators are chosen for observational purposes, not through a uniform flux-limited survey. The luminosity analysis includes only gamma-ray-detected sources with usable redshifts, and residual selection effects may remain after the redshift correction. FALCON therefore provides evidence for a population-level connection within the selected sample, rather than a complete account of all blazar jets.