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)

Energy spectra of hEnergy spectra of high-igh-ff QPO QPO

Życki & Niedźwiecki, in prep.