Energy spectra of X-ray quasi- periodic oscillations in accreting black hole binaries Piotr Życki & Małgorzata Sobolewska § Nicolaus Copernicus Astronomical Center, Warsaw, Poland § present address: Durham University, Durham, U.K. Andrzej Niedźwiecki Łódź University, Łódź, Poland Prague, 21 August 2006

X-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 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 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? 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 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 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-f ) 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 σ 2 (E)] Energy spectrum of the variable component, if it is a separate component which varies

Energy 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 QPO When time averaged spectra are soft, the QPO spectra are harder than time averaged spectra Sobolewska & Życki 2006

Observed 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 process Described by two main physical parameters: heating rate of the plasma cooling rate by soft photons

Modulation 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 rate Spectral variability folded with QPO period r.m.s./mean variability Energy spectra QPO energy spectrum is softer than the time averaged spectrum

Consequences 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-f 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 high-f QPO Życki & Niedźwiecki, in prep.