Pointlike Gamma Ray Sources as Signatures of Distant Accelerators of Ultrahigh Energy Cosmic Rays

We discuss the possibility of observing distant accelerators of ultrahigh energy cosmic rays in synchrotron gamma rays. Protons propagating away from their acceleration sites produce extremely energetic electrons during photopion interactions with cosmic microwave background photons. If the accelerator is embedded in a magnetized region, these electrons will emit high energy synchrotron radiation. The resulting synchrotron source is expected to be pointlike, steady, and detectable in the GeV-TeV energy range if the magnetic field is at the nanoGauss level.

Figure 1

Effective loss length for electrons. Solid line refers to Compton and pair-production processes in the CMB and radio background (assumed to have a low frequency cutoff at $2\sim \mathrm{MHz}$ [22]). Dashed lines refer to synchrotron losses in a magnetic field of 10, 1, and 0.1 nG (curves 1, 2, and 3).

Figure 2

Photon spectra for a source at a distance of 1 Gpc in a 20 Mpc magnetized region. The luminosity in UHECRs is ${10}^{46}\mathrm{erg}/\mathrm{s}$, the proton spectral index is $\delta =2$. Top: $E_{\mathrm{cut}}={10}^{21}\mathrm{eV}$, magnetic field 0.5 (curve 1), 5 (2), 50 nG (3). Bottom: $E_{\mathrm{cut}}=5\times {10}^{20}$, ${10}^{21}$, $5\times {10}^{21}\mathrm{eV}$, magnetic field is 1 nG. Dotted lines represent the intrinsic spectra, solid lines show the effect of absorption in the infrared background. The sensitivities of GLAST and of a generic IACT are shown.