X-ray Sources and Star Formation Activity in the Sgr B2 Cloud Observed   with Chandra
  We report the X-ray population study in the giant molecular cloud Sagittarius B2 (SgrB2). More than a dozen of X-ray cloud members (and candidates) are discovered with Chandra. Two bright X-ray sources are located near Sgr B2 Main, the most copious complex of the ultra compact HII sources. The X-ray spectra are fitted with a thin thermal plasma model of 5-10 keV temperature. The intrinsic luminosity after correcting the absorption of 5 x 10^23 H/cm^2 is about 10^33 erg/s. Although these two X-ray sources are attributable to young stellar objects (YSOs) in the same HII complex, they are in sharp contrast; one at the center of the HII complex exhibits strong K-shell transition lines of iron, while the other near the east has only weak lines. The other HII complexes, SgrB2 North and South, also show hard and highly absorbed X-ray emissions due possibly to the star formation activity. The composite X-ray spectrum of the other cloud member X-ray sources is fitted with a thin thermal plasma of about 10-keV temperature with the hydrogen column density (N_H) of 1.3 x 10^23 H/cm^2, and the individual X-ray luminosity of a few times of 10^31-32$ erg/s. These are likely to be a single or cluster of YSO(s), but neither radio nor infrared counterpart is found. An alternative scenario of isolated white dwarfs powered by the Bondi-Hoyle accretion from the dense cloud gas is also discussed. The X-ray spectra exhibit an additional 6.4-keV line of neutral or low-ionization irons, which indicates that the environment gas is concentrated near at the sources. 
S. Takagi, H. Murakami and K. Koyama