Hard X-ray emission from a giant molecular cloud/massive star-forming region NGC 6334

Abstract

Hard X-rays from a Giant Molecular Cloud (GMC)/H(sub II) region/massive (OB-type) star-forming region, NGC 6334, have been detected with the X-ray satellite ASCA (Advanced Satellite for Cosmology and Astrophysics). The emission is described with a thin thermal plasma of 11.1+7.5keV/-3.6 keV which is the highest among known stellar objects. The hard component (2-10 keV) is emitted from the star-forming regions observed as the five far-infrared (FIR) cores (I-V). The X-ray luminosity of NGC 6334 and the FIR cores (I-V) are 5.4 x 10(exp 33) and 0.9-2.1 x 10(exp 33) erg s(exp -1), respectively, in the 0.5-10 keV range. The ratios of the X-ray luminosity to the bolometric luminosity of the five FIR cores are L(sub X)/L(sub bol) approximately equal to 10(exp -6), which is an order of magnitude larger than that of evolved OB-type stars (L(sub X)/L(sub bol) approximately equal to 10(exp -7)), and is two orders of magnitude smaller that that of low-mass Pre-Main Sequence (PMS) stars (L(sub X)/L(sub bol) approximately equal to 10(exp -4)). The X-ray ionization rate calculated from the X-ray luminosity is greater than 10 (exp -17) s(exp -1) within a radius of to 0.5 pc from the X-ray source. If L(sub X)/M (approximately equal to 10(exp -5) solar luminosity/solar mass) obtained in NGC 6334 represents a typical one among GMCs, the X-ray luminosity expected from all GMCs in the Galaxy amounts to 4.1 x 10(exp 37) erg s(exp -1), which can account for 20 percent of the hard component of the galactic ridge X-ray emission.

X線衛星ASCA(「あすか」)により、GMC(巨大分子雲)のH(sub �)領域/大質量(OB型)星形成領域のNGC6334から硬X線を検出した。既知の天体中では最高の値である11.1+7.7keV/-3.6keVの薄い熱的プラズマでその放射を記述した。2〜10keVの硬成分が、遠赤外(FIR)コア領域(1〜5)として観測される星形成領域から放射される。NGC6334とFIRコアのX線輝度は、0.5〜10keVの領域ではそれぞれ5.4×10(exp 33)と0.9〜2.1×10(exp 33)erg/sであった。X線輝度と5つのFIRコアの放射輝度の比はL(sub X)/L(sub bol)〜1×10(exp -6)であり、進化しているOB型の星のそれ(L(sub X)/L(sub bol)〜1×10(exp -7))よりも1桁高く、低質量主系列前(PMS)の星のそれ(L(sub X)/L(sub bol)〜1×10(exp -4))よりは2桁低い。X線輝度から計算したX線の電離度は、X線源から半径0.5pc内では10(exp -17)/sより大きい。もしNGC6334で得られた値、L(sub X)/M(〜10(exp -5)(太陽輝度)/(太陽質量))がGMCの代表的な値であるならば、銀河内の全てのGMCから期待されるX線の輝度は4.1×10(exp 37)erg/sとなり、銀河リッジX線放射の硬成分の20%を説明できることになる。