4. X-ray emission from inverse-Compton scattering
The same electrons that emit the synchrotron radiation of the relic should inverse-Compton scatter microwave background photons to the X-ray range. Goldschmidt & Rephaeli (1994) calculated the flux from 1253+275 to be:
Since only a very low thermal X-ray background is produced at such a large cluster radius, this inverse-Compton flux should be best detectable at keV energies. The predicted flux in the ROSAT 0.5-2.4 keV band is . The observed X-ray energy flux from the Coma cluster is plotted in Fig. 2 of White et al. (1993), and is at the position of the relic (assuming a emission temperature of 8 keV). This can be translated into an observed flux of coming from the field covered by the relic. Comparing this number with the predicted flux implies a field strength of G, since most of the observed X-ray emission should belong to the X-ray blob centered on NGC 4839. This field strength is in good agreement with the field strength derived by minimum energy arguments and pressure equilibrium, given in Sect. 3.3. It has to be noted though that if a break in the synchrotron spectrum below the observed range (150 MHz - 4.75 GHz) indicates a break in the relativistic electron population, the resulting inverse-Compton flux in the ROSAT band would be largely reduced, since radio frequency translates into inverse Compton energy via in the monochromatic approximation of the synchrotron and inverse Compton formulae (Blumenthal & Gould 1970). Assuming a break in the radio spectral index of 0.5 at 10 MHz or 100 MHz still gives lower limits of G or G, respectively. The latter value can be regarded as a hard lower limit to the relic field strength.
1253+275 should be an interesting object for future X-ray telescopes, such as XMM and AXAF, because its large-scale inverse Compton flux might be detected, or stronger limits to the magnetic fields and/or the low-energy electron spectrum can be derived. The radio halo of the Coma cluster should have a larger inverse-Compton flux, but there the keV range is polluted by thermal emission and has to be avoided by using other energy bands (Rephaeli et al. 1994, Enßlin & Biermann 1997).
© European Southern Observatory (ESO) 1998
Online publication: March 23, 1998