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Register a new userStrong disc winds traced throughout outbursts in black-hole X-ray binaries
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Recurring outbursts associated with matter flowing onto compact stellar remnants (black-holes, neutron stars, white dwarfs) in close binary systems, provide strong test beds for constraining the poorly understood accretion process. The efficiency of angular momentum (and thus mass) transport in accretion discs, which has traditionally been encoded in the $alpha$-viscosity parameter, shapes the light-curves of these outbursts. Numerical simulations of the magneto-rotational instability that is believed to be the physical mechanism behind this transport find values of $alpha sim 0.1-0.2$ as required from observations of accreting white dwarfs. Equivalent $alpha$-viscosity parameters have never been estimated in discs around neutron stars or black holes. Here we report the results of an analysis of archival X-ray light-curves of twenty-one black hole X-ray binary outbursts. Applying a Bayesian approach for a model of accretion allows us to determine corresponding $alpha$-viscosity parameters, directly from the light curves, to be $alpha sim$0.2--1. This result may be interpreted either as a strong intrinsic rate of angular momentum transport in the disc, which can only be sustained by the magneto-rotational instability if a large-scale magnetic field threads the disc, or as a direct indication that mass is being lost from the disc through substantial mass outflows strongly shaping the X-ray binary outburst. Furthermore, the lack of correlation between our estimates of $alpha$-viscosity and accretion state implies that such outflows can remove a significant fraction of disc mass in all black hole X-ray binary accretion states, favouring magnetically-driven winds over thermally-driven winds that require specific radiative conditions.
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Black hole low-mass X-ray binaries (BH LMXBs) evolve in a similar way during outburst. Based on the X-ray spectrum and variability, this evolution can be divided into three canonical states: low/hard, intermediate and high/soft state. BH LMXBs evolve from the low/hard to the high/soft state through the intermediate state in some outbursts (here called full outbursts). However, in other cases, BH LMXBs undergo outbursts in which the source never reaches the high/soft state, here called Failed-Transition outburst (FT outbursts). From a sample of 56 BH LMXBs undergoing 128 outbursts, we find that $sim$36% of these BH LMXBs experienced at least one FT outburst, and that FT outbursts represent $sim$33% of the outbursts of the sample, showing that these are common events. We compare all the available X-ray data of full and FT outbursts of BH LMXBs from RXTE/PCA, Swift/BAT and MAXI and find that FT and full outbursts cannot be distinguished from their X-ray light curves, HIDs or X-ray variability during the initial 10-60 days after the outburst onset. This suggests that both types of outbursts are driven by the same physical process. We also compare the optical and infrared (O/IR) data of FT and full outbursts of GX 339-4. We found that this system is generally brighter in O/IR bands before an FT outburst, suggesting that the O/IR flux points to the physical process that later leads to a full or an FT outburst. We discuss our results in the context of models that describe the onset and evolution of outbursts in accreting X-ray binaries.
We review the current status of studies of disc atmospheres and winds in low mass X-ray binaries. We discuss the possible wind launching mechanisms and compare the predictions of the models with the existent observations. We conclude that a combination of thermal and radiative pressure (the latter being relevant at high luminosities) can explain the current observations of atmospheres and winds in both neutron star and black hole binaries. Moreover, these winds and atmospheres could contribute significantly to the broad iron emission line observed in these systems.
In the last decade, high-resolution X-ray spectroscopy has revolutionized our understanding of the role of accretion disk winds in black hole X-ray binaries. Here I present a brief review of the state of wind studies in black hole X-ray binaries, focusing on recent arguments that disk winds are not only extremely massive, but also highly variable. I show how new and archival observations at high timing and spectral resolution continue to highlight the intricate links between the inner accretion flow, relativistic jets, and accretion disk winds. Finally, I discuss methods to infer the driving mechanisms of observed disk winds and their implications for connections between mass accretion and ejection processes.
X-ray flux from the inner hot region around central compact object in a binary system illuminates the upper surface of an accretion disc and it behaves like a corona. This region can be photoionised by the illuminating radiation, thus can emit different emission lines. We study those line spectra in black hole X-ray binaries for different accretion flow parameters including its geometry. The varying range of model parameters captures maximum possible observational features. We also put light on the routinely observed Fe line emission properties based on different model parameters, ionization rate, and Fe abundances. We find that the Fe line equivalent width $W_{rm E}$ decreases with increasing disc accretion rate and increases with the column density of the illuminated gas. Our estimated line properties are in agreement with observational signatures.
In the last decade, X-ray spectroscopy has enabled a wealth of discoveries of photoionised absorbers in X-ray binaries. Studies of such accretion disc atmospheres and winds are of fundamental importance to understand accretion processes and possible feedback mechanisms to the environment. In this work, we review the current observational state and theoretical understanding of accretion disc atmospheres and winds in low-mass X-ray binaries, focusing on the wind launching mechanisms and on the dependence on accretion state. We conclude with issues that deserve particular attention.
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