[Paper Review] Multi-frequency monitoring of gamma-ray loud blazars: I. Light curves and spectral energy distributions

U. Bach, C. M. Raiteri+24 more|ArXiv.org|Dec 6, 2006

Astrophysics and Cosmic Phenomena32 citations

TL;DR

This study presents multi-frequency radio-to-optical light curves and spectral energy distributions (SEDs) for 35 gamma-ray loud blazars, based on 18 months of coordinated monitoring. It reveals significant variability across all bands, with flares propagating from high to low frequencies in sources like 3C 454.3, providing critical constraints for jet emission models and supporting high-energy missions like GLAST and AGILE.

ABSTRACT

Context: Being dominated by non-thermal emission from aligned relativistic jets, blazars allow us to elucidate the physics of extragalactic jets, and, ltimately, how the energy is extracted from the central black hole in radio-loud active galactic nuclei. Aims: Crucial information is provided by broad-band spectral energy distributions (SEDs), their trends with luminosity and correlated multi-frequency variability. With this study we plan to obtain a database of contemporaneous radio-to-optical spectra of a sample of blazars, which are and will be observed by current and future high-energy satellites. Methods: Since December 2004 we are performing a monthly multi-frequency radio monitoring of a sample of 35 blazars at the antennas in Medicina and Noto. Contemporaneous near-IR and optical observations for all our observing epochs are organised. Results: Until June 2006 about 4000 radio measurements and 5500 near-IR and optical measurements were obtained. Most of the sources show significant variability in all observing bands. Here we present the multi-frequency data acquired during the first eighteen months of the project, and construct the SEDs for the best-sampled sources.

Motivation & Objective

• To establish a contemporaneous database of radio-to-optical SEDs for gamma-ray loud blazars to support high-energy satellite observations.
• To investigate the correlation between multi-frequency variability and the physical mechanisms driving flares in relativistic jets.
• To determine the time lag between flux variations at different frequencies to constrain jet dynamics and emission region size.
• To analyze the spectral shape evolution during flares to infer electron energy distribution and emission processes.
• To provide long-term, multi-wavelength data for testing models of blazar variability, including shock-in-jet and lighthouse effects.

Proposed method

• Monthly radio monitoring at 5, 8, and 22 GHz using the Medicina and Noto radio telescopes (INAF).
• Simultaneous near-IR and optical photometry coordinated for all observing epochs across the 35 blazar sample.
• Construction of contemporaneous SEDs using data from the same observation epochs to avoid time-delay biases.
• Analysis of flux variability trends across frequencies to identify time lags and correlated behavior.
• Use of archival data and cross-identification with X-ray and TeV observations from missions like AGILE and GLAST.
• Application of standard SED fitting techniques to infer synchrotron and inverse-Compton components and their evolution.

Experimental results

Research questions

• RQ1 How do flux variations in blazars correlate across radio, near-IR, and optical bands, and what time lags are observed?
• RQ2 What is the spectral evolution of SEDs during major flares, such as in 3C 454.3, and what does it imply about electron energy distribution?
• RQ3 To what extent do flares propagate from high to low frequencies, and what does this imply about the emission region size and jet dynamics?
• RQ4 How do achromatic and chromatic variability behaviors differ across the sample, and what do they suggest about emission mechanisms?
• RQ5 How do the observed multi-frequency light curves compare with predictions from shock-in-jet or lighthouse models?

Key findings

• Over 4,000 radio measurements and 5,500 near-IR/optical measurements were collected over 18 months, forming a high-cadence contemporaneous data set.
• The source 3C 454.3 (2251+158) underwent a record optical outburst with R = 12.0 (M_B ≈ -31.4), the most luminous quasar state observed to date.
• Flares in 3C 454.3 were observed to propagate from 22 GHz radio to lower frequencies, with the optical peak preceding the radio peak.
• In 1226+023 (3C 273), radio flares at 22 GHz preceded those at 8 and 5 GHz, indicating a time lag consistent with shock propagation models.
• BL Lac (2200+420) exhibited both achromatic and chromatic variability, with a second flare appearing simultaneously at all frequencies about 250 days after an initial 22 GHz flare.
• Most sources displayed significant variability on monthly timescales, with complex variability patterns indicating diverse emission mechanisms.