The Cosmological Impact of Luminous TeV Blazars I: Implications of Plasma Instabilities for the Intergalactic Magnetic Field and Extragalactic Gamma-Ray Background (1106.5494v2)

Abstract: Inverse-Compton cascades initiated by energetic gamma rays (E>100 GeV) enhance the GeV emission from bright, extragalactic TeV sources. The absence of this emission from bright TeV blazars has been used to constrain the intergalactic magnetic field (IGMF), and the stringent limits placed upon the unresolved extragalactic gamma-ray background (EGRB) by Fermi has been used to argue against a large number of such objects at high redshifts. However, these are predicated upon the assumption that inverse-Compton scattering is the primary energy-loss mechanism for the ultra-relativistic pairs produced by the annihilation of the energetic gamma rays on extragalactic background light photons. Here we show that for sufficiently bright TeV sources (isotropic-equivalent luminosities >10^{42} erg/s) plasma beam instabilities, specifically the "oblique" instability, present a plausible mechanism by which the energy of these pairs can be dissipated locally, heating the intergalactic medium. Since these instabilities typically grow on timescales short in comparison to the inverse-Compton cooling rate, they necessarily suppress the inverse-Compton cascades. As a consequence, this places a severe constraint upon efforts to limit the IGMF from the lack of a discernible GeV bump in TeV sources. Similarly, it considerably weakens the Fermi limits upon the evolution of blazar populations. Specifically, we construct a TeV-blazar luminosity function from those objects presently observed and find that it is very well described by the quasar luminosity function at z~0.1, shifted to lower luminosities and number densities, suggesting that both classes of sources are regulated by similar processes. Extending this relationship to higher redshifts, we show that the magnitude and shape of the EGRB above ~10 GeV is naturally reproduced with this particular example of a rapidly evolving TeV-blazar luminosity function.

• The paper introduces plasma instabilities as a mechanism that rapidly dissipates energy from electron-positron pairs, suppressing the inverse-Compton cascade.
• It challenges traditional IGMF constraints by indicating that the absence of a GeV bump stems from oblique instabilities rather than a strong intergalactic magnetic field.
• The study aligns the TeV-blazar luminosity function with quasar trends, reconciling the model with the observed extragalactic gamma-ray background.

The Cosmological Impact of Luminous TeV Blazars

The paper "The Cosmological Impact of Luminous TeV Blazars I: Implications of Plasma Instabilities for the Intergalactic Magnetic Field and Extragalactic Gamma-Ray Background" by Broderick et al. explores the role of luminous TeV (tera-electronvolt) blazars in the cosmological context, particularly focusing on their influence through plasma instabilities on the intergalactic magnetic field (IGMF) and the unresolved extragalactic gamma-ray background (EGRB).

Overview and Main Findings

Blazars, a subclass of active galactic nuclei (AGNs) with their relativistic jets pointed towards the Earth, are prolific emitters of very high-energy gamma rays. The paper discusses the phenomenon of inverse-Compton (IC) cascades that result from the interaction of TeV gamma rays with extragalactic background light (EBL), leading to enhanced GeV emissions. Contrary to expectations, the observed absence of this GeV component in bright TeV blazars has been previously used to infer strong constraints on the IGMF.

However, this paper introduces the role of plasma instabilities, particularly the oblique instability, which can mediate the local dissipation of energy from ultra-relativistic electron-positron pairs produced in gamma-ray interactions. These instabilities, growing on timescales much shorter than the IC cooling rates, suppress the IC cascade, challenging the constraints on the IGMF derived from the lack of GeV emission.

Implications for IGMF and EGRB

1. IGMF Constraints: The findings suggest that the absence of a GeV bump does not necessarily indicate a strong IGMF but rather could be due to plasma beam instabilities dissipating energy locally. Therefore, previous constraints on the IGMF might need reassessment.
2. Blazar Population and EGRB: The paper suggests that the evolution of the TeV-blazar luminosity function, consistent with that of quasars, aligns well with the observed EGRB when plasma instabilities suppress IC cascades. This challenges prior conclusions that distant blazars do not significantly contribute to the EGRB due to ICC quenching.

Methodological Focus

The paper utilizes a framework where plasma instabilities are hypothesized to play a key role in energy dissipation, beyond the traditionally considered IC processes. The analytical and numerical treatment of these instabilities, and their growth timescales, provides insight into how an isotropic and dissipative environment affects the fate of gamma-ray-generated pairs.

Future Directions

Further investigation into the non-linear evolution of these instabilities within the blazar beams will be crucial to fully understanding their role in energy dissipation. Observations with higher sensitivity, potentially with upcoming Cherenkov Telescope Array (CTA) surveys, may provide empirical data to test these theoretical predictions.

Conclusion

The paper presents a significant shift in our understanding of how luminous TeV blazars impact the cosmic environment, especially regarding their contribution to the EGRB and the inferred properties of the IGMF. By highlighting the influence of plasma instabilities, it opens new avenues for exploring the interactions between blazar emissions and the intergalactic medium, challenging conventional interpretations of gamma-ray observational data.