DISCOVERY OF A RECOMBINING PLASMA IN Sagittarius A EAST: A RELIC OF photo-ionization BY THE PAST ACTIVITY OF Sagittarius A*?

Sagittarius (Sgr) A ^{*} is a supermassive black hole located at the Galactic Center (GC). Recent observations suggest that Sgr A ^{*} exhibited bright X-ray flares in the past (e.g., the X-ray reflection nebula located at the GC region, Sunyaev et al. 1993, ApJ, 407, 606; Koyama et al. 1996, PASJ, 48, 249; the recombining plasma in the south of the GC, Nakashima et al. 2013, ApJ, 773, 20). Sgr A East is a supernova remnant (SNR) with a high temperature plasma (Maeda et al. 2002, ApJ, 570, 671; Koyama et al. 2007, PASJ, 59, 237). Sgr A ^{*} is located inside of the SNR. Using the Suzaku archive data, we have obtained a high quality X-ray spectrum upto 12 keV. As a result, we have discovered a recombination continuum feature of Fe at ∼9 keV, indicating the recombining plasma (RP). The electron temperature, the initial ionization temperature and recombination parameters (plasma density multiplied by the recombination time) are ∼1.5 keV, ∼13 keV and ∼7_10 ^{11} s cm ^{_3}, respectively. In the RP spectrum, we discovered Heα lines of Cr and Mn for the first time. We found a hint that the ionization temperature in the near side from Sgr A ^{*} is larger than that of far side. Then one possible origin of the RP is photo-ionization by past X-ray flares of Sgr A ^{*}. Using the mean distance of Sgr A East from Sgr A ^{*}, best-fit initial ionization temperature and recombination parameter, we estimate the flare luminosity of Sgr A ^{*} in the past to be ∼10 ^{42} erg s ^{-1}.