Abstract
ABSTRACT The geometry of the inner accretion flow in the hard and hard-intermediate states of X-ray binaries remains controversial. Using Neutron star Interior Composition Explorer observations of the black hole X-ray binary MAXI J1820+070 during the rising phase of its 2018 outburst, we study the evolution of the timing properties, in particular the characteristic variability frequencies of the prominent iron K α line. Using frequency-resolved spectroscopy, which is robust against uncertainties in the line profile modelling, we find that reflection occurs at large distances from the Comptonizing region in the bright hard state. During the hard-to-soft transition, the variability properties suggest that the reflector moves closer to the X-ray source. In parallel, the peak of the iron line shifts from 6.5 to ∼7 keV, becoming consistent with that expected of from a highly inclined disc extending close to the black hole. We additionally find significant changes in the dependence of the root-mean-square (rms) variability on both energy and Fourier frequency as the source softens. The evolution of the rms-energy dependence, the line profile, and the timing properties of the iron line as traced by the frequency-resolved spectroscopy all support the picture of a truncated disc/inner flow geometry.
Highlights
Black hole X-ray binaries (BH XRBs) display a large range of spectral states, linked to a striking variety in variability properties
Using Neutron star Inner Composition Explorer (NICER) observations of the black hole X-ray binary MAXI J1820+070 during the rising phase of its 2018 outburst, we study the evolution of the timing properties, in particular the characteristic variability frequencies of the prominent iron Kα line
Using frequency-resolved spectroscopy, we find that reflection occurs at large distances from the Comptonizing region in the bright hard state
Summary
Black hole X-ray binaries (BH XRBs) display a large range of spectral states, linked to a striking variety in variability properties. The states can be broadly classified as “hard” or “soft” based on the spectral hardness, with more detailed state definitions being made based on parameters such as spectral shape, broad-band variability, and the presence or absence of quasi-periodic oscillations and jets (e.g., Remillard & McClintock 2006; Belloni & Motta 2016). Variations of the physical parameters and geometry of these components can explain the large differences between the spectral states. Spectral studies clearly indicate the presence of a cool, geometrically thin and optically thick accretion disc (Shakura & Sunyaev 1973) in the soft state; in the low/hard state, a region of hot plasma where soft photons are Comptonized into a hard spectrum has been identified (see, e.g., Zdziarski & Gierliński 2004; Remillard & McClintock 2006; Done et al 2007, for reviews). There is currently no consensus on the location of this region
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