Cosmic-ray electrons and the diffuse gamma-ray spectrum

Abstract

Short Title: Cosmic-ray electrons and diffuse gamma-ray spectrum PACS Numbers: 98.70.Sa Cosmic-rays (including sources, origin, acceleration, and interactions) 95.85.Pw Gamma-ray 98.70.Vc Background radiations AbstractThe bulk of the diffuse galactic gamma-ray emission above a few tens of GeV has been conventionally ascribed to the decay of neutral pions produced in cosmic-ray interactions with interstellar matter. Cosmic-ray electrons may, however, make a significant contribution to the gamma-ray spectrum at high energies, and even dominate at TeV-PeV energies depending on their injection spectral index and acceleration cut-off energy. If the injection spectrum is flat, the highest energy electrons will also contribute a diffuse hard X-ray/soft gamma-ray flux via synchrotron emission, and this may offer an explanation for the OSSE observation of a steep spectrum below a few MeV from the inner Galaxy. We perform a propagation calculation for cosmic-ray electrons, and use the resulting interstellar electron spectrum to obtain the gamma-ray spectrum due to inverse Compton, synchrotron and bremsstrahlung interactions consistently from MeV to PeV energies. We compare our results with available observations from satellite-borne telescopes, opticaľ Cerenkov telescopes and air shower arrays and place constraints on the injection spectrum of cosmic-ray electrons. With future observations at TeV-PeV energies it should be possible to determine the average interstellar spectrum of cosmic-ray electrons, and hence estimate their spectrum on acceleration.

Observation of Multi‐TeV Diffuse Gamma Rays from the Galactic Plane with the Tibet Air Shower Array

“…The density distribution of ISPF in their calculations is based on the compilations by Bloemen (1985) and Mathis, Mezger, & Panagia (1983). The curves from Tateyama & Nishimura (2001) are consistent with the estimations by Porter & Protheroe (1997) considering a different region of jbj. The present Tibet data, especially at 10 TeV, together with the HEGRA-IACT data, give the most stringent upper limit for the IC model, although these data cannot clearly rule out the IC model with a source electron spectral index of 2.0.…”

“…2 component (BGHS) in the region of 140 l 225 and jbj 2 . The IC gamma rays calculated by Porter & Protheroe (1997) for the region 50 l 200 and jbj 10 for source spectral indices of 2.0 (PP2.0) and 2.4 (PP2.4) are shown, as well as the ones by Tateyama & Nishimura (2001) in the region of l ¼ 180 and jbj 2 for source spectral indices of 2.0 (TN2.0) and 2.4 (TN2.4). The present data are also not sufficient to rule out the IC model with a spectral index of 2.0.…”

“…2 due to the collision of cosmic-ray hadrons with the ISM is drawn by a solid curve (BGHS) that is for the region of 20 l 55 and jbj 2 , deduced from their original paper, which is based on the matter density distribution compiled by Fichtel & Kniffen (1984) and Bloemen, Blits, & Hermsen (1984). For inverse Compton gamma rays induced by energetic electrons, the calculation by Porter & Protheroe (1997) is shown by dashed curves for source electron spectral indices of 2.0 (PP2.0) and 2.4 (PP2.4) in the direction l ¼ 0 and b ¼ 0 , the Galactic center. Similar theoretical curves calculated by Tateyama & Nishimura (2001) are shown by dot-dashed lines with source spectral indices of 2.0 (TN2.0) and 2.4 (TN2.4) in the direction l ¼ 0 and jbj 2 .…”

“…Detailed calculations of IC gamma rays above 50 MeV were made by Chi et al (1989), which were more than 50% of the total diffuse intensity at a medium Galactic latitude of jbj ¼ 10 20 . Porter & Protheroe (1997) indicated that in such a high-energy region, cosmic-ray electrons may create a significant part of the diffuse gamma rays, depending on their injection spectral index and acceleration cutoff energy. Pohl & Esposito (1998) remarked that most of radio synchrotron spectra of supernova remnants (SNRs) are well represented by power-law indices around 0.5, corresponding to an electron injection index of about 2.0 (Green 1995).…”

A search for gamma-ray emission from the Galactic plane in the longitude range between $\mathsf{37}^\circ$ and $\mathsf{43}^\circ$

“…Above 1 GeV, data show an excess in gamma-ray flux over model predictions (Hunter et al 1997; see however Aharonian & Atoyan 2000). At these energies, contributions from inverse Compton scattering of electrons start to become relevant (see, e.g., Porter & Protheroe 1997). In response, revised models speculate that the local measurements of the electron flux may not be representative for the entire Galaxy (Porter & Protheroe 1997;Pohl & Esposito 1998).…”