Chandra measurements of non-thermal-like X-ray emission from massive, merging, radio halo clusters
Abstract
We report the discovery of spatially extended, non-thermal-like emission components in Chandra X-ray spectra for five of a sample of seven massive, merging galaxy clusters with powerful radio haloes. The emission components can be fitted by power-law models with mean photon indices in the range 1.5 < Γ < 2.0. A control sample of regular, dynamically relaxed clusters, without radio haloes but with comparable mean thermal temperatures and luminosities, shows no compelling evidence for similar components. Detailed X-ray spectral mapping reveals the complex thermodynamic states of the radio halo clusters. Our deepest observations, of the Bullet Cluster 1E0657-56, demonstrate a spatial correlation between the strongest power-law X-ray emission, highest thermal pressure and brightest 1.34 GHz radio halo emission in this cluster. We confirm the presence of a shock front in the 1E0657-56 and report the discovery of a new, large-scale shock front in Abell 2219. We explore possible origins for the power-law X-ray components. These include inverse-Compton scattering of cosmic microwave background photons by relativistic electrons in the clusters; bremsstrahlung from suprathermal electrons energized by Coulomb collisions with an energetic, non-thermal proton population; and synchrotron emission associated with ultrarelativistic electrons. Interestingly, we show that the power-law signatures may also be due to complex temperature and/or metallicity structure in clusters particularly in the presence of metallicity gradients. In this case, an important distinguishing characteristic between the radio halo clusters and control sample of predominantly cool-core clusters is the relatively low central X-ray surface brightness of the former. Our results have implications for previous discussions of soft excess X-ray emission from clusters and highlight the importance of further deep X-ray and radio mapping, coupled with new hard X-ray, γ-ray and TeV observations, for improving our understanding of the non-thermal particle populations in these systems.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 2009
- DOI:
- 10.1111/j.1365-2966.2009.15359.x
- arXiv:
- arXiv:0811.0834
- Bibcode:
- 2009MNRAS.399.1307M
- Keywords:
-
- magnetic fields;
- radiation mechanisms: non-thermal;
- galaxies: clusters: individual: 1E0657-56;
- A665;
- A2163;
- A2255;
- A2319;
- A2744;
- A2219;
- A576;
- A1795;
- A2204;
- A478;
- A2029;
- intergalactic medium;
- X-rays: galaxies: clusters;
- Astrophysics
- E-Print:
- Accepted for publication in MNRAS (24 pages, 13 figures). Improved discussion includes a new, possible explanation for `soft excess' X-ray emission from clusters as an artifact of metallicity/temperature structure and projection effects. Other physical explanations for the observed non-thermal-like X-ray emission also remain