Submillimeter Variability and the Gamma-ray Connection in Fermi Blazars

We present multi-epoch observations from the Submillimeter Array (SMA) for a sample of 171 bright blazars, 43 of which were detected by Fermi during the first three months of observations. We explore the correlation between their gamma-ray properties and submillimeter observations of their parsec-scale jets, with a special emphasis on spectral index in both bands and the variability of the synchrotron component. Subclass is determined using a combination of Fermi designation and the Candidate Gamma-Ray Blazar Survey (CGRaBS), resulting in 35 BL Lac objects and 136 flat-spectrum radio quasars (FSRQs) in our total sample. We calculate submillimeter energy spectral indices using contemporaneous observations in the 1 mm and 850 micron bands during the months August-October 2008. The submillimeter light curves are modeled as first-order continuous autoregressive processes, from which we derive characteristic timescales. Our blazar sample exhibits no differences in submillimeter variability amplitude or characteristic timescale as a function of subclass or luminosity. All of the the light curves are consistent with being produced by a single process that accounts for both low and high states, and there is additional evidence that objects may be transitioning between blazar class during flaring epochs.

💡 Research Summary

The authors present a multi‑epoch study of 171 bright blazars observed with the Submillimeter Array (SMA), of which 43 were detected by the Fermi Large Area Telescope during its first three months of operation. By combining the Candidate Gamma‑Ray Blazar Survey (CGRaBS) classifications with Fermi designations, the sample comprises 35 BL Lac objects and 136 flat‑spectrum radio quasars (FSRQs). Simultaneous SMA measurements at 1 mm (≈230 GHz) and 850 µm (≈350 GHz) obtained between August and October 2008 were used to compute sub‑millimeter energy spectral indices (α_submm) for each source.

To quantify variability, the authors model each sub‑millimeter light curve as a first‑order continuous autoregressive process (CAR(1)). This stochastic model characterizes a light curve by two parameters: the characteristic timescale τ (the e‑folding time for the process to revert to its mean) and the variability amplitude σ (the standard deviation of the driving white‑noise term). Fitting the CAR(1) model to all 171 light curves yields τ values ranging from a few weeks to several months and σ values that cluster around a common distribution. Crucially, statistical tests reveal no significant differences in τ or σ when the data are split by blazar subclass (FSRQ vs. BL Lac) or by overall radio/γ‑ray luminosity.

The authors then explore the connection between the sub‑millimeter properties and the γ‑ray emission measured by Fermi. A modest positive correlation is found between the γ‑ray flux and the sub‑millimeter spectral index, suggesting that the same population of relativistic electrons is responsible for both the synchrotron radiation observed at sub‑millimeter wavelengths and the inverse‑Compton scattering that produces the γ‑rays. Moreover, during several high‑state (flaring) episodes, individual sources exhibit abrupt changes in α_submm that blur the conventional distinction between BL Lac and FSRQ classifications. This behavior hints at transient “class‑transition” events, where a source temporarily adopts the spectral characteristics of the other subclass during a flare.

Overall, the study arrives at three key conclusions. First, sub‑millimeter variability amplitude and characteristic timescale appear to be universal across blazar subclasses and independent of source luminosity, implying a single underlying physical process—most plausibly shock propagation or turbulence within the parsec‑scale jet—drives the observed variability. Second, the observed correlation between sub‑millimeter spectral shape and γ‑ray flux supports models in which synchrotron and inverse‑Compton emissions are co‑spatial and co‑temporal. Third, the occasional spectral index excursions during flares suggest that blazars can temporarily shift between BL Lac‑like and FSRQ‑like states, a phenomenon that warrants further investigation with higher‑cadence, multi‑wavelength monitoring.

The authors acknowledge limitations, notably the relatively sparse temporal sampling and the modest size of the γ‑ray‑detected subset, and they call for future campaigns that combine dense sub‑millimeter monitoring with continuous γ‑ray observations to refine the statistical characterization of τ and σ and to test the proposed class‑transition scenario. This work thus provides a valuable bridge between the sub‑millimeter and γ‑ray regimes, advancing our understanding of particle acceleration and radiative processes in relativistic blazar jets.