Synchrotron self-Comptonized emission of low energy cosmic ray electrons in the Universe

Abstract

Abstract. Most of the Universe's populations of low energy cosmic ray electrons in the energy range of 1-100 MeV still manage to elude detection by our instruments, since their synchrotron emission is at too low frequencies. We investigate a mechanism which can lead to observable emission of such electron populations: synchrotronself Comptonization (SSC). The inverse Compton (IC) scattering can shift otherwise unobservable low-frequency 10 kHz-10 MHz photons into observable radio, infrared (IR) or even more energetic wave-bands. The resulting emission should be polarized. We also consider IC scattering of the cosmic microwave background (CMB) and the cosmic radio background (CRB). Electron spectral aging due to synchrotron, IC and adiabatic losses or gains influences the resulting spectrum. The predicted radiation spectrum is a strong function of the history of the source, of the low energy spectrum of relativistic electrons, and of redshift. It has typically two maxima, and a decrement in between at CMB frequencies. Detection will provide a sensitive probe of the environment of radio galaxies. We demonstrate that the fossil remnants of powerful radio galaxies are promising detection candidates, especially when they are embedded in a dense intra-cluster medium (ICM). GHz peaked sources (GPS) have very low SSC luminosities, which may, however, extend into the X-ray or even the gamma ray regime. Clusters of galaxies with relativistic electron populations may be another detectable sources. Fossil radio plasma released by our own Galaxy could be revealed by its large angular scale SSC flux. We discuss the expected detectability of these sources with new and upcoming instruments such as LOFAR, GMRT, EVLA, ATA, ALMA, PLANCK, and HERSCHEL.

Radio and X-ray detectability of buoyant radio plasma bubbles in clusters of galaxies

“…Similarly, the radio luminosity of the bubble at a given frequency declines rapidly with increasing cluster radius and suddenly vanishes when cooling has removed the emitting electrons. After that point, only a weak flux of synchrotron-self Comptonized emission remains (Enßlin & Sunyaev 2002). Figure 3 illustrates these dependencies for an example with parameters similar to Perseus A.…”

Giant radio relics in galaxy clusters: reacceleration of fossil relativistic electrons?

“…3), indicating that weak shocks (M ∼ 3) dom-9 Here we use the monochromatic approximation of synchrotron emission (see App. B of Enßlin & Sunyaev 2002) where the synchrotron kernel is replaced by a delta distribution, δ(ν − νs) with νs = 3eB sin θ γ 2 /(2πmec), where θ is the CRes' pitch angle and which gives the exact synchrotron formula for a power-law electron population with spectral index α = 3 and attains only order unity corrections (< 20 per cent) for small spectral changes ∆α < 0.5.…”

On the escape of cosmic rays from radio galaxy cocoons

“…In such a case they would be long-lived and would therefore be able to produce a nonthermal Comptonisation signature in the cosmic microwave background (Enßlin & Kaiser 2000;Enßlin & Sunyaev 2002). Further, if old, invisible radio plasma with confined relativistic electrons is dragged into a shock wave of the large-scale structure formation, its radio emission is possibly revived and forms the observed cluster radio relics (Enßlin & Brüggen 2002, and references therein).…”

“…On the other hand, if the relativistic electrons are efficiently confined for cosmological time-scales in radio cocoons, they are shielded from severe energy losses by Coulomb interaction with the environmental gas. In such a case they would be long-lived and would therefore be able to produce a nonthermal Comptonisation signature in the cosmic microwave background (Enßlin and Kaiser, 2000;Enßlin and Sunyaev, 2002). Further, if old, invisible radio plasma with confined relativistic electrons is dragged into a shock wave of the large-scale structure formation, its radio emission is possibly revived and forms the observed cluster radio relics (Enßlin and Brüggen, 2002, and references therein).…”