TY - JOUR
T1 - Detailed spectral study of an ultra-luminous compact X-ray source, M 81 X-9, in the disk-dominated state
AU - Tsunoda, Naoko
AU - Kubota, Aya
AU - Namiki, Masaaki
AU - Sugiho, Masahiko
AU - Kawabata, Kiyoshi
AU - Makishima, Kazuo
PY - 2006
Y1 - 2006
N2 - We report on the results of detailed spectral studies of an ultra-luminous X-ray source (ULX), M 81 X-9 (Holmberg IX X-1), conducted with XMM-Newton on 2001 April 22 and with ASCA on 1999 April 6. On both occasions, the source showed an unabsorbed 0.5-10keV luminosity of ∼2 × 1040 erg s-1 (assuming a distance of 3.4 Mpc) and a soft spectrum, apparently represented by a multi-color disk model with an innermost disk temperature of 1.3-1.5 keV. Adding a power-law model further improved the fit. However, as previously reported, the high luminosity cannot be reconciled with the high disk temperature within the framework of the standard accretion disk radiating at a sub-Eddington luminosity. Therefore, we modified the multi-color disk model, and allowed the local disk temperature to scale as ∝ r-p on the distance r from the black hole, with p being a free parameter. We then found that the XMM-Newton and the ASCA spectra can be both successfully reproduced with p ∼ 0.6 and the innermost disk temperature of 1.4-1.8 keV. These flatter temperature profiles suggest a deviation from the standard Shakura-Sunyaev disk, and are consistent with predictions of a slim-disk model.
AB - We report on the results of detailed spectral studies of an ultra-luminous X-ray source (ULX), M 81 X-9 (Holmberg IX X-1), conducted with XMM-Newton on 2001 April 22 and with ASCA on 1999 April 6. On both occasions, the source showed an unabsorbed 0.5-10keV luminosity of ∼2 × 1040 erg s-1 (assuming a distance of 3.4 Mpc) and a soft spectrum, apparently represented by a multi-color disk model with an innermost disk temperature of 1.3-1.5 keV. Adding a power-law model further improved the fit. However, as previously reported, the high luminosity cannot be reconciled with the high disk temperature within the framework of the standard accretion disk radiating at a sub-Eddington luminosity. Therefore, we modified the multi-color disk model, and allowed the local disk temperature to scale as ∝ r-p on the distance r from the black hole, with p being a free parameter. We then found that the XMM-Newton and the ASCA spectra can be both successfully reproduced with p ∼ 0.6 and the innermost disk temperature of 1.4-1.8 keV. These flatter temperature profiles suggest a deviation from the standard Shakura-Sunyaev disk, and are consistent with predictions of a slim-disk model.
KW - Accretion, accretion disks
KW - Black hole physics
KW - X-rays: individual (M81 X-9, Holmberg IX X-1)
KW - X-rays: stars
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U2 - 10.1093/pasj/58.6.1081
DO - 10.1093/pasj/58.6.1081
M3 - Article
AN - SCOPUS:33846862658
VL - 58
SP - 1081
EP - 1088
JO - Publication of the Astronomical Society of Japan
JF - Publication of the Astronomical Society of Japan
SN - 0004-6264
IS - 6
ER -