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Energy spectra of X-ray quasi-Energy spectra of X-ray quasi-periodic oscillations in periodic oscillations in
accreting black hole binariesaccreting black hole binaries
Piotr Życki & Małgorzata SobolewskaPiotr Życki & Małgorzata Sobolewska§§
Nicolaus Copernicus Astronomical Center, Warsaw, PolandNicolaus Copernicus Astronomical Center, Warsaw, Poland§ § present address: Durham University, Durham, U.K.present address: Durham University, Durham, U.K.
Andrzej NiedźwieckiAndrzej NiedźwieckiŁódź University, Łódź, PolandŁódź University, Łódź, Poland
Prague, 21 August 2006
X-rays from accreting black hole binariesX-rays from accreting black hole binaries
Random, non-periodic variability
Broad-band power spectra
X-rays emitted in flares (bursts) of radiation
X-rays from accreting black holes binariesX-rays from accreting black holes binaries
Energy spectra with two components:
Thermal, 1 keV, emission from an accretion disk
Hard power law spectrum (with cutoff at 100 keV) – Comptonization of disk photons in a hot plasma
Quasi-periodic oscillationsQuasi-periodic oscillations
Visible in PDS as a narrow (but not a δ–like) feature.
Usually, QPO would not be visible in the light curve.
A quasi-periodic component of variability in the lightcurve
Why are QPO interesting and important?Why are QPO interesting and important?
1. A well-defined characteristic frequency
2. Appear in all classes of accreting compact objects (black holes, neutron stars, white dwarfs)
3. Broad range of frequencies (from 10-3 Hz to 1 kHz)
4. Low–f (1-10 Hz) QPO appear when the state of the source changes
5. kHz-QPO: motion very close to central object
6. Pairs of QPO often present (beat-frequency phenomenon?, resonances?)
Studies of QPOStudies of QPO
Most studies concentrate on finding the “clock”…
… ignoring the fact that it is hard X-rays that are being modulated
Energy spectra of (low-Energy spectra of (low-f f ) QPO) QPO
Construct power density spectrum (PDS) for each energy channel.
Describe the shape of PDS, e.g. broken power law + Lorentzian QPOs.
Integrate the Lorentzians over frequency.
This gives the strength of the QPO as a function of energy, QPO(E)
[it’s really σ2(E)]
Energy spectrum of the variable component, if it is a separate component which varies
Energy spectra of QPOEnergy spectra of QPO
Energy [keV]
Sobolewska & Życki 2006
Disk emission not observed in the QPO spectra
QPO spectrum harder than the time averaged spectrum
Observed energy spectra of QPOObserved energy spectra of QPO
When time averaged spectra are soft, the QPO spectra are harder than time averaged spectra
Sobole
wsk
a &
Życk
i 2006
Observed energy spectra of QPOObserved energy spectra of QPO
Hard spectral state Intermediate state
When time averaged spectra are hard, the QPO spectra are softer than time averaged spectra
Sobolewska & Życki 2006
Energy spectra from inverse-Compton processEnergy spectra from inverse-Compton process
Described by two main physical parameters:
heating rate of the plasma
cooling rate by soft photons
Modulation of heating rateModulation of heating rate
Spectral variability folded with QPO period
r.m.s./mean variability
Energy spectra
QPO energy spectrum is harder than the time averaged spectrum
Życki & Sobolewska 2005
Modulation of cooling rateModulation of cooling rate
Spectral variability folded with QPO period
r.m.s./mean variability
Energy spectra
QPO energy spectrum is softer than the time averaged spectrum
ConsequencesConsequences
When the QPO spectra are harder than time averaged spectra, they are driven by oscillations of the hot plasma, rather than oscillations of the cold disk.
Disk component NOT seen in the QPO spectrum
but …
All physical QPO models focus on oscillations of the cold disk.
High-High-ff QPO in accreting black holes QPO in accreting black holes
Often (always?) appear in pairs, with frequency ratio 3:2
A model: resonance between two epicyclic motions
X-ray modulation from relativistic effects (emission intrinsically constant)
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