A joint Suzaku and Chandra spectroscopy study of hard X-ray emission from the Arches cluster

We present the results of a joint Suzaku and Chandra X-ray spectroscopic study of the Arches cluster in the Galactic center region. The superb spectroscopic performance of XIS on-board Suzaku brought a tight constraint of the spectrum of the Arches X-rays. We found that the spectrum consists of a thermal plasma, a hard power-law tail, and two Gaussian components. The plasma component with a temperature of ∼2.2 keV is established from highly ionized Ca XIX and Fe XXV Kα emission lines as well as the lack of Fe XXVI Kα line. The two Gaussian lines represent the Kα and Kβ lines from iron at lower ionization stages at ∼6.4 keV and ∼7.1 keV. Both the line centers and the intensity ratio of these two lines are consistent with the neutral iron. The hard power-law tail with a photon index of ∼0.7 was found to have no pronounced iron K edge feature. In comparison with ACIS spectra constructed separately for point-like and diffuse emission by using the unprecedented spatial capability of Chandra, we conclude that the thermal component is from the ensemble of point-like sources plus thermal diffuse emission concentrated at the cluster center, while the Gaussian and the hard tail components are from the non-thermal diffuse emission extended in a larger scale. In the band-limited images of the XIS field, the distribution of the 7.5-10.0 keV emission resembles that of the 6.4 keV emission, including the local excess at the Arches cluster. This strongly suggests that the power-law emission is related to the 6.4 and 7.1 keV lines in the underlying physics. The full discussion can be found in Tsujimoto et. al. [M. Tsujimoto, Y. Hyodo and K. Koyama, Publ. Astron. Soc. Jpn. 59 (2007), S229]. For point-like X-ray emission, three Arches sources are exceptionally bright among cluster members. The brightest spectrum of them requires two temperature plasma with an extinction of 2x1023 cm-2. The estimated hard-band luminosity of approx. = 1035 erg s-1, which is brighter than any known stellar sources in our Galaxy, might be a consequence of colliding wind shocks with an extreme mass loss rate (∼10-4 (solar mass) yr-1) indicated from near-infrared and radio continuum observations. (author)