2009/02/16 by Hua Feng, Philip Kaaret, P. Kaaret · 1 citation
Engineering · Physics and Astronomy · #Astrophysical Phenomena and Observations #Galaxies: Formation, Evolution, Phenomena #Mechanics and Biomechanics Studies #astro-ph.CO #astro-ph.HE
paper · pdf · doi:10.1088/0004-637x/696/2/1712
published as Astrophys.J.696:1712-1726,2009 · 17 pages, 9 tables, 10 figures, accepted by ApJ
arxiv created 2009/02/16 · openalex publication_date 2009/04/27 · arxiv updated 2010/05/25 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
We examined spectral evolution in ultraluminous X-ray sources (ULXs) with apparent luminosities of about 10 40 erg s −1 . Based on new results in this paper, and those reported in the literature, two common spectral behaviors were found. Some ULXs in starburst galaxies have varying luminosity ( L ) but remain in the hard state with power-law spectra and a constant, hard photon index (Γ). Other ULXs, such as NGC 5204 X-1, show a correlation between L and Γ. We interpret this L –Γ correlated phase as an intermediate state with hybrid properties from the thermal dominant and steep power-law states. When the spectra of NGC 5204 X-1 are fitted with a multicolor disk blackbody plus power-law model, the X-ray luminosity increases with the effective temperature of the accretion disk in a manner similar to that found in stellar-mass black hole X-ray binaries, suggesting that the emission arises from an accretion disk. The luminosity, disk size, and temperature suggest that NGC 5204 X-1 harbors a compact object more massive than stellar-mass black holes. In contrast, the disk model in IC 342 X-1 is ruled out because the luminosity decreases as the temperature increases; sources with such behavior may represent a class of objects with super-Eddington accretion. Also, we report a peculiar soft spectral feature from IC 342 X-2 and variability on a timescale of 20 ks from Holmberg II X-1. More observations are needed to test these results.