TY - JOUR
T1 - NGC 7793 P9
T2 - An Ultraluminous X-Ray Source Evolved from a Canonical Black Hole X-Ray Binary
AU - Hu, Chin Ping
AU - Kong, Albert K.H.
AU - Ng, C. Y.
AU - Li, K. L.
N1 - Publisher Copyright:
© 2018. The American Astronomical Society. All rights reserved..
PY - 2018/9/1
Y1 - 2018/9/1
N2 - Transient ultraluminous X-ray sources (ULXs) provide an important link bridging transient low-mass X-ray binaries and ULXs. Here we report the first discovery of both a canonical sub-Eddington outburst and an ultraluminous super-Eddington outburst from an unusual transient ULX, NGC 7793 P9, with a variability factor higher than 103. Its X-ray spectrum switches between the typical high/soft state and the steep power-law state during the canonical outburst. The inner radius of the accretion disk and the disk temperature-luminosity correlation suggest that P9 harbors a stellar-mass black hole (BH). At the beginning of the ultraluminous outburst, we observe a cool outer disk with a hard Comptonized spectrum, implying a transition to the ULX regime. When the luminosity increases to L 3 × 1039 erg s-1, P9 shows a significantly curved spectrum that can be described by either a slim disk or a strongly curved Comptonized corona. The phenomenological model suggests that the hot disk observed near the peak of the ultraluminous outburst is coincidentally consistent with the extension of the thermal track. Utilizing more physical Comptonized disk models, we suggest that the corona cools down and the apparent disk-like spectrum is dominated by soft Comptonization. The significant variability above 1 keV supports this two-component scenario. The spectral evolution can also be interpreted with the supercritical accretion model. All these indicate that a canonical BH X-ray binary can show properties of a ULX.
AB - Transient ultraluminous X-ray sources (ULXs) provide an important link bridging transient low-mass X-ray binaries and ULXs. Here we report the first discovery of both a canonical sub-Eddington outburst and an ultraluminous super-Eddington outburst from an unusual transient ULX, NGC 7793 P9, with a variability factor higher than 103. Its X-ray spectrum switches between the typical high/soft state and the steep power-law state during the canonical outburst. The inner radius of the accretion disk and the disk temperature-luminosity correlation suggest that P9 harbors a stellar-mass black hole (BH). At the beginning of the ultraluminous outburst, we observe a cool outer disk with a hard Comptonized spectrum, implying a transition to the ULX regime. When the luminosity increases to L 3 × 1039 erg s-1, P9 shows a significantly curved spectrum that can be described by either a slim disk or a strongly curved Comptonized corona. The phenomenological model suggests that the hot disk observed near the peak of the ultraluminous outburst is coincidentally consistent with the extension of the thermal track. Utilizing more physical Comptonized disk models, we suggest that the corona cools down and the apparent disk-like spectrum is dominated by soft Comptonization. The significant variability above 1 keV supports this two-component scenario. The spectral evolution can also be interpreted with the supercritical accretion model. All these indicate that a canonical BH X-ray binary can show properties of a ULX.
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U2 - 10.3847/1538-4357/aad5e2
DO - 10.3847/1538-4357/aad5e2
M3 - Article
AN - SCOPUS:85053126514
SN - 0004-637X
VL - 864
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
M1 - 64
ER -