Figure 1: The QPOs in the active galaxies RE J1034+396 and MS2254.9-3712. They are observed as narrow peaks of variability power imprinted on top of the broadband noise. The broadband noise is fit with a power-law model (red) and a bending power-law model with αlow = 1 (blue) and αlow = 0 (yellow). Using Bayesian statistics we showed that the simple power law model is preferred and the QPOs are significant at > 3σ. See Alston et al (2014) and Alston et al (2015) for more details.
The X-ray emission we observe from accreting black holes is highly variable. This variability is often described in terms of its power spectral density or PSD, which gives the variability amplitude as a function of temporal (Fourier) frequency. The shape of the power spectrum of accreting black holes is that of broadband noise. In black hole X-ray binaries (BHXRBs), narrow coherent peaks of variability power are imprinted on top of the broadband noise, known as quasi-periodic oscillations (QPOs). These are coherent signals arising in the innermost regions of the accretion flow in accreting BHXRBs (see Remillard & McClintock 2006 for a nice review). In principle they provide important information on the BH mass and spin as well as the structure of the strongly-curved spacetime close to the event horizon. If the accretion process is scale invariant then QPOs should also be present in active galactic nuclei (AGN), however they have been notoriously difficult to detect.
Two types of QPOs are observed in BHXRBs: low frequency (LFQPO) and high frequency (HFQPO). The HFQPOs are typically only observed when the source is accreting at or above the Eddington rate. Despite being observed more often in BHXRBs, AGN offer better insight into the phenomenon. We typically receive a factor of 103 more ct/s from BHXRBs than AGN. However, the timescale of the QPO scales inversely with the BH mass, so they are ~105 times lower in AGN. This means we actually receive ~100 times more photons per characteristic QPO timescale, making AGN better probes of this phenomenon.
RE J1034+396 was the first active galaxy with a robust detection of a quasi-periodic oscillation (QPO) in its X-ray lightcurve (Geirlinski et al 2008). The periodicity of the QPO was ~1 hr, or 2.6 x 10-4 Hz. This was previously detected in the 0.3-10 keV band of one 90 ks XMM-Newton observation, but was believed to be a transient feature. In a recent paper (Alston et al 2014), we showed that the QPO is indeed still present at the same frequency in the hard (1-4 keV) band of 4 further low-flux/spectrally harder XMM-Newton observations (see figure above, left). This increases the duration where a QPO is detected to 250 ks, allowing us study this phenomena in unprecedented detail (work in prep). RE J1034+396 is thought to be accreting above the Eddington rate, so with a Mbh ~ 2 x 106 Msun this QPO is most likely a HFQPO analogue.
Another QPO was recently detected in the active galaxy, MS2254.9-3712 (Alston et al 2015, see above figure, right). The period of the QPO is approximately 2 hrs, or 1.5 x 10-4 Hz. Given the BH mass of ~ 6 x 106 Msun and high mass accretion rate, this QPO is also most likely to be analogous to the HFQPOs observed in BHXRBs. This object shows similar spectral timing properties to RE J1034+396, including hard spectral variability, principle components analysis (PCA) and X-ray reverberation lags at the QPO frequency.
Origin of QPOs
The origin of QPOs observed in accreting BHs still remains a mystery. The frequency at which they are observed tells us the oscillations originate very close to the black hole: if we equate them with the frequency of Keplerian orbits and use our best black hole mass estimates, these features must occur within 6 Rg. This is consistent with where the inner edge of the accretion disc is expected to extend to. QPOs are therefore an important probe of the direct vicinity of black holes.