Twenty years of blazar monitoring with the INAF radio telescopes

TL;DR

This work documents the ROBIN program, a twenty-year INAF effort to monitor blazar variability in the radio with the Medicina and Noto telescopes across 5–43 GHz, building a ~21000-point database for 47 sources. It describes the observational setup, data reduction via the CAP pipeline, and initial variability analyses using the intrinsic modulation index $\overline{m}$ and the structure-function $SF_{1.5}'$, along with spectral indices between 8 and 24 GHz. The study shows a robust link between variability amplitude and spectral inversion, finds no statistically significant BL Lac–FSRQ difference in variability, and highlights the benefits and biases of the two variability estimators, advocating a combined approach. The ROBIN archive represents a valuable, long-term resource for probing jet physics in blazars and will be expanded in the coming years, with data available to the community on request and via CDS for published tables.

Abstract

The extreme variability of blazars, in both timescale and amplitude, is generally explained as the effect of a relativistic jet closely aligned to the observer's line-of-sight. Due to causality arguments, variability characteristics translate into spatial information about the emitting region of blazars. Since radiation at different wavelengths is emitted in different parts of the jet, multi-frequency observations provide us with a virtual view of the structure of the jet on different scales. Radio--gamma-ray correlations, moreover, are essential to reveal where and how the high-energy radiation is produced. We present here the observations collected within the blazar radio monitoring program that we are running at the Medicina and Noto telescopes. It aims at investigating how the variability characteristics and spectral energy distribution of blazars evolve in time. Since 2004, observation have been performed at 5, 8, 24, and 43 GHz on 47 targets, with monthly cadence; the monitoring program is still active at frequencies of 8 and 24 GHz. The database we built in more than twenty years of activity comprises to date about 21000 flux density measurements. Some basic analysis tools have been applied to the data to characterise the detected variability and offer a first glance at the wealth of information that such a program can provide about blazars.

Figures (13)

• Figure 1: The light curves of 2251+158 (i.e. 3C454.3) from the ROBIN program. The 5, 8, 24, and 43 GHz data are plotted as blue, orange, green, and magenta dots, respectively.
• Figure 2: Upper panel: the ROBIN light curves of BL Lacertae at 24 and 8 GHz (light green and orange dots, respectively) are shown together with the re-scaled (see text for the details) 20 GHz ones from the F-GAMMA program (dark green dots). Lower panel: spectral indices calculated between the combined ROBIN and F-GAMMA data at 20-24 GHz and the ROBIN ones at 8 GHz. The clear increasing trend of the spectral index (magenta line) reflects the increasing discrepancy between the light curves in the last 2000 days of observations.
• Figure 3: Comparison between the variability amplitude parameters, at 5 (blue dots), 8 (orange squares), 24 (green triangles), and 43 (magenta diamonds) GHz. The linear regression results are plotted as lines with the same colours.
• Figure 4: Minimum value of the spectral index calculated over all the available epochs is plotted versus redshift. Both a linear regression of the data (green line) and the average calculated over increasingly larger bins (in order to keep the number of contributing points comparable; orange diamonds) show a moderate anti-correlation between the two parameters.
• Figure 5: The mean variability parameters for BL Lacs and FSRQs are plotted separately as a function of frequency. On the upper panel are shown the $\overline{m}$ and $SF_{1.5}^\prime$ values, while on the lower one the fastness of the variability calculated through $SF_{1.5}^\prime/SF_{3.0}^\prime$.