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Abstract: We examined spectral evolution in ultraluminous X-ray sources ULXs withapparent luminosities of about 10^40 ergs-s. Based on new results in this paperand those reported in the literature, two common spectral behaviors were found.Some ULXs in starburst galaxies have varying luminosity L but remain in thehard state with power-law spectra and a constant, hard photon index Gamma.Other ULXs, such as NGC 5204 X-1, show a correlation between L and Gamma. Weinterpret this L-Gamma correlated phase as an intermediate state with hybridproperties from the thermal dominant and steep power-law states. When thespectra of NGC 5204 X-1 are fitted with a multicolor disk blackbody pluspower-law model, the X-ray luminosity increases with the effective temperatureof the accretion disk in a manner similar to that found in stellar-mass blackhole X-ray binaries, suggesting that the emission arises from an accretiondisk. The luminosity, disk size, and temperature suggest that NGC 5204 X-1harbors a compact object more massive than stellar-mass black holes. Incontrast, the disk model in IC 342 X-1 is ruled out because the luminositydecreases as the temperature increases; sources with such behaviors mayrepresent a class of objects with super-Eddington accretion. Also, we report apeculiar soft spectral feature from IC 342 X-2 and variability on a time scaleof 20 ks from Holmberg II X-1. More observations are needed to test theseresults.



Author: Hua Feng 1,2, Philip Kaaret 2 1 Tsinghua U., China, 2 U. of Iowa

Source: https://arxiv.org/







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