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تسجيل مستخدم جديدSwift Spectroscopy of the Accretion Disk Wind in the Black Hole GRO J1655-40
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M. Balakrishnan
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Chandra obtained two High Energy Transmission Grating (HETG) spectra of the stellar-mass black hole GRO J1655-40 during its 2005 outburst, revealing a rich and complex disk wind. Soon after its launch, the Neil Gehrels Swift Observatory began monitoring the same outburst. Some X-ray Telescope (XRT) observations were obtained in a mode that makes it impossible to remove strong Mn calibration lines, so the Fe K-alpha line region in the spectra was previously neglected. However, these lines enable a precise calibration of the energy scale, facilitating studies of the absorption-dominated disk wind and its velocity shifts. Here, we present fits to 15 Swift/XRT spectra, revealing variability and evolution in the outflow. The data strongly point to a magnetically driven disk wind: both the higher velocity (e.g., v ~ 10^4 km/s) and lower velocity (e.g., v ~ 10^3 km/s) wind components are typically much faster than is possible for thermally driven outflows (v < 200 km/s), and photoionization modeling yields absorption radii that are two orders of magnitude below the Compton radius that defines the typical inner extent of thermal winds. Moreover, correlations between key wind parameters yield an average absorption measure distribution (AMD) that is consistent with magnetohydrodynamic wind models. We discuss our results in terms of recent observational and theoretical studies of black hole accretion disks and outflows, and future prospects.
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We report on simultaneous Chandra/HETGS and RXTE observations of the transient stellar-mass black hole GRO J1655-40, made during its 2005 outburst. Chandra reveals a line-rich X-ray absorption spectrum consistent with a disk wind. Prior modeling of t
he spectrum suggested that the wind may be magnetically driven, potentially providing insights into the nature of disk accretion onto black holes. In this paper, we present results obtained with new models for this spectrum, generated using three independent photoionization codes: XSTAR, Cloudy, and our own code. Fits to the spectrum in particular narrow wavelength ranges, in evenly spaced wavelength slices, and across a broad wavelength band all strongly prefer a combination of high density, high ionization, and small inner radius. Indeed, the results obtained from all three codes require a wind that originates more than 10 times closer to the black hole and carrying a mass flux that is on the order of 1000 times higher than predicted by thermal driving models. If seminal work on thermally-driven disk winds is robust, magnetic forces may play a role in driving the disk wind in GRO J1655-40. However, even these modeling efforts must be regarded as crude given the complexity of the spectra. We discuss these results in the context of accretion flows in black holes and other compact objects.
During its 2005 outburst, GRO J1655-40 was observed twice with the Chandra High Energy Transmission Grating Spectrometer; the second observation revealed a spectrum rich with ionized absorption lines from elements ranging from O to Ni (Miller et al.
2006a, 2008; Kallman et al. 2009), indicative of an outflow too dense and too ionized to be driven by radiation or thermal pressure. To date, this spectrum is the only definitive evidence of an ionized wind driven off the accretion disk by magnetic processes in a black hole X-ray binary. Here we present our detailed spectral analysis of the first Chandra observation, nearly three weeks earlier, in which the only signature of the wind is the Fe XXVI absorption line. Comparing the broadband X-ray spectra via photoionization models, we argue that the differences in the Chandra spectra cannot possibly be explained by the changes in the ionizing spectrum, which implies that the properties of the wind cannot be constant throughout the outburst. We explore physical scenarios for the changes in the wind, which we suggest may begin as a hybrid MHD/thermal wind, but evolves over the course of weeks into two distinct outflows with different properties. We discuss the implications of our results for the links between the state of the accretion flow and the presence of transient disk winds.
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