The evolution of the disc variability along the hard state of the black hole transient GX 339-4

Abstract

We report on the analysis of hard-state power spectral density function (PSD) of GX 339-4 down to the soft X-ray band, where the disc significantly contributes to the total emission. At any luminosity probed, the disc in the hard state is intrinsically more variable than in the soft state. However, the fast decrease of disc variability as a function of luminosity, combined with the increase of disc intensity, causes a net drop of fractional variability at high luminosities and low energies, which reminds the well-known behaviour of disc-dominated energy bands in the soft state. The peak-frequency of the high-frequency Lorentzian (likely corresponding to the high-frequency break seen in active galactic nuclei, AGN) scales with luminosity, but we do not find evidence for a linear scaling. In addition, we observe that this characteristic frequency is energy-dependent. We find that the normalization of the PSD at the peak of the high-frequency Lorentzian decreases with luminosity at all energies, though in the soft band this trend is steeper. Together with the frequency shift, this yields quasi-constant high frequency (5-20 Hz) fractional rms at high energies, with less than 10 percent scatter. This reinforces previous claims suggesting that the high frequency PSD solely scales with BH mass. On the other hand, this constancy breaks down in the soft band (where the scatter increases to ∼ 30 percent). This is a consequence of the additional contribution from the disc component, and resembles the behaviour of optical variability in AGN.

Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries

“…Also the peaked component above 10 Hz becomes more pronounced at the highest energies. Although detailed studies of the energy dependence over the 0.5-10 keV energy of the island state PDS are sparse, the complex energy dependence we have found resembles that what has been found for the hard state PDS in several black-hole systems (e.g., Wilkinson & Uttley 2009;Kalamkar et al 2013;De Marco et al 2015). The hard state of black-hole systems resembles in many aspect the island state of atoll sources, so it is not surprising that also the energy dependence in the 0.5-10 keV energy range shows similarities.…”

Multiwavelength spectroscopy of the black hole candidate MAXI J1813-095 during its discovery outburst

“…Disc variability has been reported in several BHXRBs (e.g. Wilkinson & Uttley 2009;De Marco et al 2015) and seems to be a characteristic feature of low luminosity hard states. All in all, the X-ray spectral characteristics of MAXI J1813-095 clearly favour a BH accretor.…”

The very faint hard state of the persistent neutron star X-ray binary SLX 1737–282 near the Galactic Centre

“…As the effect of obscuration by lowly ionised material is more conspicuous in the soft energy band, spectral hardening should be observed in correspondence with dip events (e.g. White & Mason 1985;Frank et al 1987;Kuulkers et al 2013;Ponti et al 2016;De Marco et al 2015). To test this possibility, we plot in Fig.…”