The remarkable timing properties of a ‘hypersoft’ state in GRO J1655-40

Abstract

We report the identification and study of an unusual soft state of the black hole lowmass X-ray binary GRO J1655-40, observed during its 2005 outburst by the Rossi Xray Timing Explorer. Chandra X-ray grating observations have revealed a high massoutflow accretion disc wind in this state, and we show that the broadband X-ray spectrum is remarkably similar to that observed in the so-called 'hypersoft' state of the high mass X-ray binary Cyg X-3, which possesses a strong stellar wind from a Wolf-Rayet secondary. The power-spectral density (PSD) of GRO J1655-40 shows a bending power-law shape, similar to that of canonical soft states albeit with larger fractional rms. However, the characteristic bend-frequency of the PSD is strongly correlated with the X-ray flux, such that the bend-frequency increases by two decades for less than a factor 2 increase in flux. The strong evolution of PSD bend-frequency for very little change in flux or X-ray spectral shape seems to rule out the suppression of high-frequency variability by scattering in the wind as the origin of the PSD bend. Instead, we suggest that the PSD shape is intrinsic to the variability process and may be linked to the evolution of the scale-height in a slim disc. An alternative possibility is that variability is introduced by variable absorption and scattering in the wind. We further argue that the hypersoft state in GRO J1655-40 and Cyg X-3 is associated with accretion close to or above the Eddington limit.

An Optically Thick Disk Wind in Gro J1655–40?

“…What is unusual in the hypersoft state is not only the characteristics of the SED but also fast X-ray variability. As reported by Uttley & Klein-Wolt (2015), the high-frequency part of the flat noise component was remarkably reduced in the hypersoft state, compared with that in the normal high/soft state. The break frequency in the high/soft state was located at »5 Hz, whereas during the hypersoft state it moved to lower frequencies with decreasing X-ray flux to reach »0.1 Hz at the Chandra observation.…”

“…The hard tail in the normal high/soft state is suppressed in the spectrum on MJD 53461, which is dominated by the soft X-ray fluxes below 10 keV. As reported by Uttley & Klein-Wolt (2015), this X-ray spectrum can be described with an MCD plus an unusually steep power-law component with a photon index of >5. This peculiar hypersoft state was steadily seen from a few days before the Chandra observation, after making rapid and frequent transitions between the normal high/soft and hypersoft states in the preceding »10 days.…”

“…4. The absorption feature at »8 keV is always seen, and it becomes deeper as the observed X-ray flux decreases (Uttley & Klein-Wolt 2015). 5.…”

“…We note that, in Figure 2, a significant depression is present at ≈8 keV in the SED on MJD 53461, which is likely an ionized iron-K edge along with absorption lines produced by the disk winds. This feature was always visible during the hypersoft state and deepened without significant energy shifts as the observed X-ray flux decreased (see Uttley & Klein-Wolt 2015).…”

“…Recently, Uttley & Klein-Wolt (2015) have found that unusual properties in X-ray short-term variability and continuum spectra were seen around the Chandra observation, from a comprehensive study of Rossi X-ray Timing Explorer (RXTE) data, covering almost the entire outburst episode in 2005. When a deep absorption structure emerged at ∼8 keV, the hard X-ray flux was greatly reduced and a very soft X-ray spectrum appeared.…”