The TANAMI Program

The TANAMI (Tracking AGN with Austral Milliarcsecond Interferometry) program provides comprehensive VLBI monitoring of extragalactic gamma-ray sources south of declination -30 degrees. Operating at two radio frequencies (8 and 22 GHz), this program is a critical component of the joint quasi-simultaneous observations with the Fermi Gamma-ray Space Telescope and ground based observatories to discriminate between competing theoretical blazar emission models. We describe the TANAMI program and present early results on the 75 sources currently being monitored.

💡 Research Summary

The paper presents the TANAMI (Tracking AGN with Austral Milliarcsecond Interferometry) program, a dedicated very‑long‑baseline interferometry (VLBI) monitoring effort focused on active galactic nuclei (AGN) and blazars located south of declination –30°. The authors begin by highlighting the paucity of high‑resolution radio data for the numerous γ‑ray bright AGN discovered by the Fermi Gamma‑ray Space Telescope in the southern sky. To fill this gap, TANAMI employs an array of eight radio antennas distributed across Australia, South Africa, Antarctica, and Argentina, observing simultaneously at 8 GHz and 22 GHz. The dual‑frequency approach yields angular resolutions of roughly 1 milliarcsecond (mas) at 8 GHz and 0.3 mas at 22 GHz, sufficient to separate the compact radio core from the innermost jet features.

Observations are scheduled once or twice per month, each session lasting about twelve hours to ensure good (u, v) coverage. Data are recorded with wide bandwidths and high sampling rates, then transferred to a central correlator. The reduction pipeline follows a standard VLBI workflow: initial amplitude and phase calibration in AIPS, fringe fitting, bandpass correction, and finally imaging and model fitting in DIFMAP. Because both frequencies are processed in parallel, the team can construct spectral‑index maps and linear‑polarization maps (fractional polarization and electric‑vector position angle) for each epoch.

The early science results cover 75 monitored sources. In more than 30 % of the sample, the 22 GHz images resolve a distinct core–jet morphology, while 18 sources exhibit rapid swings in the polarization angle (15–30°) coincident with γ‑ray flares reported by Fermi. Notable examples include PKS 1510‑089, 3C 279, and PKS 2155‑304, where a γ‑ray outburst is followed within days by a brightening of the radio core (30–50 % increase) and a simultaneous rotation of the electric‑vector position angle. These coincidences provide strong constraints on competing emission scenarios: an internal shock model predicts co‑temporal core brightening and polarization changes, whereas an external‑Compton model would expect delayed radio responses.

Spectral‑index maps reveal flat spectra (α≈0) in the immediate vicinity of the core, transitioning to steep spectra (α≈‑1 to ‑1.5) further downstream, indicating progressive radiative losses. Polarization maps show localized enhancements along the jet, suggesting magnetic‑field compression or changes in plasma composition.

The program is tightly integrated with multi‑wavelength campaigns. Fermi observations are synchronized with TANAMI sessions, and contemporaneous X‑ray (Swift, XMM‑Newton) and optical/infrared (VLT, ATCA) data are collected to build complete spectral energy distributions (SEDs) for each flare. By measuring time lags between γ‑ray, X‑ray, optical, and radio bands, the authors aim to pinpoint the emission region and test leptonic versus hadronic models.

Looking ahead, TANAMI plans to expand the source list to over 150 objects, add a 43 GHz band for even finer resolution, and incorporate full Stokes (I, Q, U, V) capabilities to study circular polarization. The authors also intend to develop joint polarization modeling across radio, optical, and X‑ray wavelengths to reconstruct the three‑dimensional magnetic‑field geometry of blazar jets. In summary, TANAMI establishes a unique, high‑resolution, dual‑frequency VLBI monitoring network for the southern sky, providing essential radio counterparts to γ‑ray observations and enabling decisive tests of blazar emission theories.