X-ray variability of SS 433: effects of the supercritical accretion disc

Abstract

We study a stochastic variability of SS 433 in the 10 −4 − 5 × 10 −2 Hz frequency range based on RXTE data, and on simultaneous observations with RXTE and optical telescopes. We find that the cross-correlation functions and power spectra depends drastically on the precession phase of the supercritical accretion disc. When the wind funnel of the disc is maximally open to the observer, a flat part emerges in the power spectrum; a break is observed at the frequency 1.7 × 10 −3 Hz, with a power-law index β ≈ 1.67 at higher frequencies. The soft emission forming mostly in the jets, lags behind the hard and optical emission. When the observer does not see the funnel and jets (the 'edge-on' disc), the power spectrum is described by a single power-law with β ≈ 1.34 and no correlations between X-ray ranges are detected. We investigated two mechanisms to explain the observed variability at the open disc phase, 1) reflection of radiation at the funnel wall (X-rays and optical) and 2) the gas cooling in the jets (X-rays only). The X-ray variability is determined by the contribution of both mechanisms, however the contribution of the jets is much higher. We found that the funnel size is (2 − 2.5) × 10 12 cm, and the opening angle is ϑ f ∼ 50 • . X-ray jets may consist of three fractions with different densities: 8 × 10 13 , 3 × 10 13 and 5 × 10 11 cm −3 , with most of the jet's mass falling within the latter fraction. We suppose that revealed flat part in the power spectrum may be related to an abrupt change in the disc structure and viscous time-scale at the spherization radius, because the accretion disc becomes thick at this radius, h/r ∼ 1. The extent of the flat spectrum depends on the variation of viscosity at the spherization radius.

Outbursts of the intermediate-mass black hole HLX-1: a wind-instability scenario

“…The inferred emission region for this extended component is ∼ 10 12 − 10 13 cm (Marshall et al 2012;Kubota et al 2010;Krivosheyev et al 2009;Cherepashchuk et al 2007;Kotani et al 1996), which is as large or larger than the size of the accretion disk (Rout ≈ 2 × 10 12 cm: Gies et al 2002). The observed X-ray luminosity of SS 433 (LX ≈ 10 36 erg s −1 ) is ∼10 −3 -10 −4 times the true luminosity (Khabibullin & Sazonov 2016;Fabrika et al 2015;Medvedev & Fabrika 2010), because only a small fraction of X-rays are scattered at all angles by the outflow while most of the radiation is emitted inside the polar-axis funnel (Atapin et al 2015;Medvedev & Fabrika 2010). If a similar fraction of the X-ray luminosity is scattered by a wind above the disk plane in HLX-1, the pressure exerted by the irradiation flux would satisfy the condition given in Phillips & Podsiadlowski (2002) and cause an intermittent mass transfer.…”

Ultraluminous X-ray sources with flat-topped noise and QPO

“…Examining the fast X-ray variability of SS 433 (Revnivtsev et al 2006), it has been found that its power density spectrum is well-fitted by a broken power-law with a flat component (P ∝ f 0 ) below 10 −3 Hz (Atapin et al 2015;Atapin & Fabrika 2016). This type of variability is similar to the flat-topped noise (hereafter FTN) observed in BHBs at the state of low accretion rate, i.e.…”

“…The linear rms-flux relation is observed in binary systems with black holes and neutron stars (Uttley & McHardy 2001;Gleissner et al 2004;Uttley 2004), active galactic nuclei (Vaughan et al 2003b gests that this mechanism must work in these systems as well. Moreover, the Lyubarskii's model was successfully applied to the variability of the supercritical accretion disc of SS 433 (Revnivtsev et al 2006;Atapin et al 2015;Atapin & Fabrika 2016).…”

“…In such a case the spherization radius becomes a trigger, which regulates the matter supply into the supercritical area of the disc. We suppose that if the viscosity at Rsp varies in a random and independent manner ("white noise") as it was assumed for all the radii (Lyubarskii 1997), this effect should produce a flat portion in the PDS (Atapin et al 2015;Middleton et al 2015b). In the suggested model the FTN arises from fluctuations occurring at the spherization radius, and the break frequency f b in the PDS is determined by ta at Rsp.…”

X-ray variability of SS 433: Evidence for supercritical accretion

“…Analyzing the RXTE observations of SS 433, we detected this break in the X-ray range (Atapin et al, 2015). In addition, we found the shape of the power spectrum and the presence of a break to depend on the precession phase: the break appears when the funnel is turned toward the observer.…”