[Paper Review] A Warning on the GeV-TeV Connection in Blazars

L. Costamante|arXiv (Cornell University)|Aug 3, 2012

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This paper warns that extrapolating Fermi-LAT GeV spectra to the TeV band is unreliable for blazars, as VHE spectra can be significantly harder than HE spectra due to multi-component emission. The study shows that Fermi-LAT data cannot be used as a robust upper limit for intrinsic VHE spectra, challenging common assumptions in EBL and redshift studies.

ABSTRACT

Fermi-LAT spectra at high energies (HE, 0.1-100 GeV) are often extrapolated to very high energies (VHE, >100 GeV) and considered either a good estimate or an upper limit for the blazars intrinsic VHE spectrum. This assumption seems not well justified, neither theoretically nor observationally. Besides being often softer, observations do indicate that spectra at VHE could be also harder than at HE, even when adopting the limit of Gamma=1.5. Results based on such straightforward GeV-TeV extrapolations are in general not reliable, and should be considered with caution.

Motivation & Objective

• To challenge the widespread assumption that Fermi-LAT spectra at GeV energies can serve as a reliable upper limit for intrinsic VHE spectra in blazars.
• To investigate whether VHE spectra can be harder than GeV spectra, especially in high-energy-peaked BL Lacs (HBLs).
• To assess the implications of such spectral upturns for extragalactic background light (EBL) modeling and redshift estimation in uncertain-source blazars.
• To demonstrate that current one-zone SSC models and HE-to-VHE extrapolations are insufficient due to multi-component emission physics.
• To advocate for improved understanding of blazar emission mechanisms before relying on spectral extrapolations for EBL or redshift constraints.

Proposed method

• Analyzing multi-wavelength SEDs of known TeV blazars (e.g., Mkn 501, PKS 2155-304, PKS 2005-489) across multiple epochs to detect spectral upturns or new components.
• Using EBL-corrected VHE data (e.g., from VERITAS, H.E.S.S.) to compare intrinsic VHE photon indices with Fermi-LAT HE indices.
• Evaluating the consistency of observed VHE spectra (e.g., Γ ≈ 1.5–1.6 in 1ES 0229+200) with theoretical expectations and EBL absorption models.
• Comparing Fermi-LAT spectral indices with VHE spectral indices across multiple sources to identify cases where VHE spectra are harder than HE.
• Assessing the limitations of one-zone SSC models in explaining multi-component SEDs observed in flaring and long-timescale states.
• Re-evaluating the use of Γ = 1.5 as a benchmark for intrinsic VHE spectra in light of observed hard spectra and theoretical possibilities.

Experimental results

Research questions

• RQ1 Can the intrinsic VHE spectrum of a blazar be significantly harder than its Fermi-LAT GeV spectrum, even when the latter is steep?
• RQ2 To what extent do multi-component emission processes in blazar jets invalidate the assumption that HE spectra extrapolate reliably to VHE?
• RQ3 Is the commonly used Γ = 1.5 limit for intrinsic VHE spectra a robust constraint, or can harder spectra (Γ < 1.5) be physically plausible?
• RQ4 How do spectral upturns in the 100 MeV–10 TeV band affect the reliability of EBL and redshift constraints derived from Fermi-LAT data?
• RQ5 Can one-zone SSC models reliably predict VHE emission from HE data when multiple emission components coexist in the jet?

Key findings

• VHE spectra in blazars can be significantly harder than their GeV spectra, with intrinsic photon indices as hard as Γ ≈ 1.5–1.6 observed in sources like 1ES 0229+200.
• The Fermi-LAT spectrum at GeV energies cannot be reliably used as an upper limit for the intrinsic VHE spectrum, especially in HBLs where the HE and VHE bands sample opposite sides of the SED hump.
• Spectral upturns in the 100 MeV–10 TeV band are observationally plausible and may already be present in Fermi-LAT data (e.g., Mkn 501), though not yet statistically confirmed.
• The superposition of multiple emission components—especially in flaring states or over long timescales—can produce concave SEDs, invalidating simple power-law extrapolations.
• Observations of hard VHE spectra in low-EBL environments imply that such spectra are physically possible, even if not yet directly detected in simultaneous HE-VHE data.
• Current EBL constraints based on Fermi-LAT extrapolations are systematically uncertain due to unmodeled multi-component physics, and should be revised using more robust EBL models.