[Paper Review] The Role of Plasma Instabilities in the Propagation of Gamma-Rays from Distant Blazars

Andrey Saveliev, Carmelo Evoli+1 more|arXiv (Cornell University)|Nov 26, 2013

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper investigates plasma instabilities in the intergalactic medium (IGM) as an alternative mechanism to extragalactic magnetic fields (EGMF) for suppressing GeV gamma-ray flux from distant blazars. Using Monte Carlo simulations of electromagnetic cascades, it shows that relativistic electron-positron beams from TeV gamma-rays can be suppressed by two-stream instabilities in the IGM, with a critical IGM temperature of $ T \lesssim 5 \times 10^4\,\text{K} $ explaining the Fermi-HESS data for 1ES 0229+200, comparable to the suppression expected from $ B \gtrsim 10^{-16}\,\text{G} $ magnetic fields.

ABSTRACT

The observation in the GeV band of distant blazars has been recently used to put constraints on the Extragalactic Background Light (EBL) and Extragalactic Magnetic Fields (EGMF). To support such claims one has to assume that the leptonic component of the electromagnetic cascade initiated by blazar gamma-rays is deflected away by strong enough EGMF, suppressing the signal in the Fermi window. Apart from magnetic fields, the development of such a cascade might be affected by plasma instabilities due to interactions with the ionized component of the Intergalactic Medium (IGM). In this paper we model the electromagnetic cascade through a Monte Carlo simulation in which both effects are taken into account separately, and we derive constraints on these scenarios from the combined Fermi-HESS data set. In the specific case of 1ES 0229+200 observations, we show that both explanations of the GeV flux suppression are compatible with the available data, specifically by assuming a magnetic field of $B \gtrsim 10^{-16}\, m{G}$ or an IGM temperature of $T \lesssim 5 imes 10^{4}\, m{K}$ along the line of sight. Future observations of the spectra of high redshift ($z\lesssim 1$) TeV objects will help to distinguish magnetic field and plasma effects on electromagnetic cascades in the IGM.

Motivation & Objective

• To assess whether plasma instabilities in the ionized IGM can suppress the electromagnetic cascade of gamma-rays from distant blazars, offering an alternative to EGMF-induced deflection.
• To model the development of electromagnetic cascades in the IGM, including both plasma effects and magnetic field deflections, using a Monte Carlo simulation framework.
• To test whether plasma instability effects can reproduce the observed suppression of GeV flux in blazar spectra, particularly for 1ES 0229+200, as seen in combined Fermi and HESS data.
• To distinguish between plasma instability and magnetic field scenarios through future observations of high-redshift (z ≤ 1) TeV sources.
• To evaluate the role of IGM temperature and electron beam density in determining the strength of two-stream instabilities and their impact on cascade development.

Proposed method

• A Monte Carlo simulation models the full electromagnetic cascade initiated by TeV gamma-rays from blazars, including pair production on the EBL and Inverse Compton scattering on the CMB.
• The simulation separately includes the effects of extragalactic magnetic fields (EGMF) and plasma instabilities in the IGM, using established formalisms from Broderick et al. (2012) and Schlickeiser et al. (2012, 2013).
• The physical properties of the relativistic electron-positron beam are determined dynamically, with beam density derived from the gamma-ray luminosity and redshift.
• The growth rate of two-stream instabilities is calculated using the dispersion relation for unmagnetized plasma, with the transition between weak and strong regimes defined by the condition in Eq. (12).
• The relaxation time $ \tau_{\text{r}} $ for electrons due to plasma effects is computed and compared to cooling and interaction timescales.
• Spectra are generated for different IGM temperatures and magnetic field strengths, and compared to Fermi-HESS observations of 1ES 0229+200 to derive constraints.

Experimental results

Research questions

• RQ1 Can plasma instabilities in the IGM suppress the electromagnetic cascade of gamma-rays from distant blazars, offering a viable alternative to EGMF-induced deflection?
• RQ2 What IGM temperature is required for plasma instabilities to reproduce the observed GeV flux suppression in 1ES 0229+200, as seen in Fermi-HESS data?
• RQ3 How do the spectral signatures of plasma instability suppression differ from those of magnetic field deflection, particularly in terms of spectral features and redshift dependence?
• RQ4 What role does the transition between weak and strong blazar regimes play in determining the efficiency of plasma-driven cascade suppression?
• RQ5 Can future observations of high-redshift (z ≤ 1) TeV blazars distinguish between plasma instability and magnetic field scenarios?

Key findings

• Plasma instabilities in the IGM can significantly suppress the development of electromagnetic cascades from TeV gamma-rays, reducing the GeV flux observed by Fermi LAT.
• For the blazar 1ES 0229+200 at z = 0.14, a critical IGM temperature of $ T \lesssim 5 \times 10^4\,\text{K} $ is required to reproduce the observed GeV flux suppression, comparable to the suppression expected from $ B \gtrsim 10^{-16}\,\text{G} $ magnetic fields.
• The suppression mechanism due to plasma instabilities is stronger at higher electron energies (around TeV), where two-stream instabilities dominate, leading to a more efficient cascade suppression than magnetic deflection.
• A distinct second peak in the gamma-ray spectrum is predicted due to the transition between weak and strong instability regimes, which could serve as a unique observational signature.
• The spectral trends differ between the two scenarios: plasma instability suppression increases with redshift (due to stronger beam growth), while magnetic deflection suppression decreases with redshift.
• Future observations of high-redshift (z ≤ 1) TeV sources by HESS-II and CTA will be crucial to distinguish between plasma instability and magnetic field effects on electromagnetic cascades.