Rossi X‐Ray Timing ExplorerSpectrum of A2319

The cluster of galaxies A2319 was observed in 1999 for ~160 ks by the PCA and HEXTE instruments aboard the Rossi X-Ray Timing Explorer ( RXTE ) satellite. No noticeable variability is seen in the emission measured by either instrument over the ~8 week observation. The quality of the data allows, for the first time, a meaningful search for emission whose spectral properties are distinct from those of the primary thermal emission with (previously measured) mean temperature in the range 8-10 keV. Fitting the RXTE data by a single thermal component, we obtain kT = 8.6 ± 0.1 (all errors are 90% confidence limits), a low iron abundance Z Fe ~ 0.16 ± 0.02, and large positive residuals below 6 keV and between 15 to 30 keV. The quality of the fit is drastically improved if a second component is added. A two-temperature model yields kT 1 ≃ 10.1 ± 0.6, kT 2 ≃ 2.8 ± 0.6, and of Z Fe ~ 0.23 ± 0.03, which is consistent with previously measured values. An equally good fit is obtained by a combination of a primary thermal and a secondary nonthermal component with kT ≃ 8.9 ± 0.6 and a power-law (photon) index α ≃ 2.4 ± 0.3. We have repeated the analysis by performing joint fits to both these RXTE measurements and archival ASCA data. At most, 25% of the RXTE secondary component could be present in the ASCA data. Allowing for this difference, very similar results were obtained with only somewhat different values for the temperature ( kT = 9.2 ± 0.5) and power-law index (α ≃ 2.5 ± 0.3) in the latter model. The deduced value of α is consistent with the measured spectrum of extended radio emission in A2319. Identifying the power-law emission as Compton scattering of the radio-emitting electrons by the cosmic microwave background (CMB), we obtain B ~ 0.1-0.3 μG for the volume-averaged magnetic field, and ~ 4 × 10 -14 ( R /2 Mpc) -3 ergs cm -3 for the mean energy density of the emitting electrons in the central region (radius R ) of A2319.