[Paper Review] Recent multi-wavelength campaigns in the Fermi-GST era

Lars Fuhrmann|arXiv (Cornell University)|Jul 2, 2010

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This paper reviews multi-wavelength (MW) campaigns conducted in the Fermi-GST era, focusing on three blazars—3C 279, PMN J0948+0022, and 3C 454.3—using coordinated observations from Fermi/LAT and ground/space-based facilities across the electromagnetic spectrum. Key results include the first detection of a spectral break above 100 MeV in a high-luminosity blazar, strong correlations across γ-ray, optical, and mm bands in 3C 454.3, and evidence for quasi-periodic variability and polarization swings linked to flaring states.

ABSTRACT

Since 2008 the Fermi/LAT instrument has delivered highly time-resolved gamma-ray spectra and detailed variability curves for a steadily increasing number of AGN. For detailed AGN studies the Fermi/LAT data have to be combined with, and accompanied by, dedicated ground- and space-based multi-frequency observations. In this framework, the Fermi AGN team has realized a detailed plan for multi-wavelength campaigns including a large suite of cm/mm/sub-mm band instruments. Many of those campaigns have been triggered, often for sources detected in flaring states. We review here a few interesting results recently obtained during three such campaigns, namely for the flat-spectrum radio quasar 3C 279, the Narrow Line Seyfert 1 PMN J0948+0022 and quasar 3C 454.3.

Motivation & Objective

• To investigate the physical mechanisms behind γ-ray emission and variability in active galactic nuclei (AGN) using coordinated multi-wavelength observations.
• To determine the location and properties of the γ-ray emission region by combining Fermi/LAT data with radio, optical, X-ray, and TeV observations.
• To study spectral evolution and variability patterns across the electromagnetic spectrum, especially in flaring states.
• To test models of jet emission such as synchrotron self-Compton (SSC) and external Compton (EC) scenarios through broad-band spectral energy distributions.
• To explore the role of shocks and magnetic fields in shaping variability and polarization behavior in relativistic jets.

Proposed method

• Conducting ad-hoc and long-term multi-wavelength campaigns triggered by Fermi/LAT flaring detections across γ-ray, radio, IR, optical, UV, X-ray, and TeV bands.
• Utilizing a large network of ground-based and space-based instruments, including Swift, RXTE, Spitzer, VLBA, EVN, MOJAVE, TANAMI, and Cherenkov telescopes (HESS, VERITAS).
• Performing time-series and cross-band correlation analysis between γ-ray, optical, and mm/sub-mm light curves to identify variability coherence.
• Measuring Doppler factors from radio variability and spectral evolution, with consistency checks using synchrotron self-absorption and γ-pair production models.
• Analyzing optical polarization angle swings to probe magnetic field reorganization during flares, as seen in 3C 279.
• Modeling spectral energy distributions with SSC and EC models, and testing shock-in-jet scenarios via evolution of synchrotron turnover frequency.

Experimental results

Research questions

• RQ1 What causes the observed spectral break above 100 MeV in the γ-ray spectrum of 3C 454.3, and is it intrinsic or due to absorption?
• RQ2 How are the rapid γ-ray flares in 3C 279 and 3C 454.3 related to variability in other bands, and what does this imply about the emission region's location?
• RQ3 What is the origin of the quasi-periodic variability (20-day and 60-day components) observed in 3C 454.3 across γ-ray, optical, and mm bands?
• RQ4 How do optical polarization angle swings during flares in 3C 279 relate to magnetic field reorganization and jet dynamics?
• RQ5 To what extent do the observed mm/sub-mm light curves and spectral evolution support the shock-in-jet model for blazar variability?

Key findings

• The Fermi/LAT data revealed a spectral break above 100 MeV in 3C 454.3, which may indicate an intrinsic break in the electron energy distribution or γ-ray absorption via photon-photon pair production.
• A strong correlation was found between γ-ray, optical (R band), and mm/sub-mm (SMA, IRAM 30-m) light curves in 3C 454.3, indicating coherent variability across the broad-band spectrum.
• Quasi-periodic variability with fast (≈20 days) and slow (≈60 days) components was detected in 3C 454.3 across γ-ray, optical, and mm bands, with similar start and stop times.
• The optical polarization angle in 3C 279 exhibited a smooth swing during the second γ-ray flare, suggesting magnetic field reorganization linked to the flaring activity.
• Doppler factors derived from radio variability (3–9) were consistent with those from spectral modeling, supporting the presence of relativistic outflows.
• The evolution of the synchrotron turnover frequency in 3C 454.3 aligns with the shock-in-jet model, though departures in the Compton phase suggest additional processes beyond simple shock models.