3 research outputs found
Predictions for Fourier-resolved X-ray spectroscopy from the model of magnetic flare avalanches above an accretion disc with hot ionized skin
The magnetic flare avalanches model of Poutanen & Fabian for X-ray
variability of accreting black holes is combined with computations of vertical
structure of illuminated accretion discs in hydrostatic equilibrium. The latter
predict the existence of a hot ionized skin, due to the thermal instability of
X-ray illuminated plasma. The presence of such ionized skin, with properties
dependent on disc radius, introduces a dependence of the emitted X-ray spectrum
on the position on the disc. If the position is related to the time scale of
the flares, the X-ray energy spectra (both the primary continuum and the
reprocessed component) gain an additional dependence on Fourier frequency,
beside that resulting from spectral evolution during a flare. We compute the
Fourier frequency resolved spectra in this model and demonstrate that the
presence of the hot skin introduces trends opposite to those observed in black
hole binaries. Furthermore, the flare profile is strongly constrained, if the
Fourier frequency dependence due to spectral evolution is to agree with
observations.Comment: 10 pages, MNRAS, in pres
Repetitive patterns in rapid optical variations in the nearby black-hole binary V404 Cygni
ブラックホール近傍から出る規則的なパターンを持つ光の変動を可視光で初めて捉えることに成功 -ブラックホールの「またたき」を直接目で観測できる機会に期待-. 京都大学プレスリリース. 2016-01-07.How black holes accrete surrounding matter is a fundamental yet unsolved question in astrophysics. It is generally believed that matter is absorbed into black holes via accretion disks, the state of which depends primarily on the mass-accretion rate. When this rate approaches the critical rate (the Eddington limit), thermal instability is supposed to occur in the inner disk, causing repetitive patterns of large-amplitude X-ray variability (oscillations) on timescales of minutes to hours. In fact, such oscillations have been observed only in sources with a high mass-accretion rate, such as GRS 1915+105 (refs 2, 3). These large-amplitude, relatively slow timescale, phenomena are thought to have physical origins distinct from those of X-ray or optical variations with small amplitudes and fast timescales (less than about 10 seconds) often observed in other black-hole binaries--for example, XTE J1118+480 (ref. 4) and GX 339−4 (ref. 5). Here we report an extensive multi-colour optical photometric data set of V404 Cygni, an X-ray transient source containing a black hole of nine solar masses (and a companion star) at a distance of 2.4 kiloparsecs (ref. 8). Our data show that optical oscillations on timescales of 100 seconds to 2.5 hours can occur at mass-accretion rates more than ten times lower than previously thought. This suggests that the accretion rate is not the critical parameter for inducing inner-disk instabilities. Instead, we propose that a long orbital period is a key condition for these large-amplitude oscillations, because the outer part of the large disk in binaries with long orbital periods will have surface densities too low to maintain sustained mass accretion to the inner part of the disk. The lack of sustained accretion--not the actual rate--would then be the critical factor causing large-amplitude oscillations in long-period systems