[Paper Review] Fermi observations of TeV AGN

S. Fegan, D. A. Sanchez⋆|arXiv (Cornell University)|Dec 23, 2009

Particle Detector Development and Performance3 citations

TL;DR

This study presents Fermi-LAT observations of 28 TeV-selected AGN over 5.5 months, detecting 21 in the GeV band. It finds that most exhibit hard GeV spectra (median Γ ≈ 1.9) and show increasing spectral curvature with redshift, providing strong evidence for extragalactic background light (EBL) absorption in the TeV band, with spectral breaks consistent with EBL attenuation models.

ABSTRACT

We report on observations of TeV-selected AGN made during the first 5.5 months of observations with the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope (Fermi). In total, 28 TeV AGN were selected for study. The Fermi observations show clear detections of 21 of these TeV-selected objects. Most can be described with a power law of spectral index harder than 2, with a spectral break generally required to accommodate the TeV measurements. Evidence for systematic evolution of the gamma-ray spectrum with redshift is presented and discussed in the context of the EBL.

Motivation & Objective

• To study the high-energy gamma-ray emission of TeV-selected AGN using Fermi-LAT data to bridge GeV and TeV energy regimes.
• To investigate the connection between GeV and TeV emission in extragalactic sources, particularly blazars.
• To examine spectral evolution with redshift as a probe of extragalactic background light (EBL) absorption effects.
• To test whether observed spectral breaks between GeV and TeV bands are consistent with EBL attenuation or intrinsic source curvature.
• To provide a complete multi-wavelength picture of high-energy emission for TeV AGN, guiding future observations.

Proposed method

• Conducted 5.5 months of Fermi-LAT observations targeting 28 TeV-selected AGN, focusing on sources with known redshifts.
• Used the Large Area Telescope (LAT) to detect gamma rays in the 20 MeV to >300 GeV energy range.
• Applied a test statistic (TS > 25) to identify significant GeV detections, ensuring 5σ confidence level.
• Fitted GeV spectra with power laws and compared extrapolations to archival TeV data from H.E.S.S., MAGIC, VERITAS, and CANGAROO.
• Analyzed the difference between GeV and TeV spectral indices (ΔΓ) as a function of redshift to assess EBL absorption effects.
• Evaluated intrinsic spectral curvature by comparing extrapolated GeV spectra with actual TeV measurements, identifying deviations indicating curvature or absorption.

Experimental results

Research questions

• RQ1 Do TeV-selected AGN show detectable gamma-ray emission in the GeV band with Fermi-LAT?
• RQ2 How do the GeV and TeV spectral indices compare across the energy range, and does this difference evolve with redshift?
• RQ3 Is the observed spectral break between GeV and TeV bands consistent with absorption by the extragalactic background light (EBL)?
• RQ4 Can the observed spectral evolution with redshift be explained by EBL attenuation, or does it require intrinsic spectral curvature?
• RQ5 Are there systematic differences in spectral hardness between TeV AGN and other AGN populations (e.g., BL Lacs, FSRQs) in the GeV band?

Key findings

• Fermi-LAT detected GeV emission from 21 out of 28 TeV-selected AGN, with a median photon index Γ = 1.9 in the GeV band.
• The GeV spectra of these TeV AGN are significantly harder than those of typical BL Lacs (Γ = 2.0) and FSRQs (Γ = 2.4) from the LBAS sample.
• For 15 sources, the difference between GeV and TeV spectral indices (ΔΓ) increases with redshift, reaching ΔΓ ≥ 1.5 at z > 0.1.
• Extrapolation of GeV spectra to TeV energies under-predicts measured flux for flaring sources like 3C 279, indicating extreme flaring states not captured by simple power-law extrapolation.
• For PKS 2005-489, the GeV extrapolation over-predicts the TeV flux, suggesting intrinsic spectral curvature in the source's intrinsic spectrum.
• The observed redshift-dependent spectral evolution is best explained by absorption on the extragalactic background light (EBL), providing model-independent evidence for EBL attenuation.