Measuring the broad-band power spectra of active galactic nuclei with RXTE

By P. Uttley, I. McHardy, I. U. O. Southampton, U. Crete. 2002

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We develop a Monte Carlo technique to test models for the true power spectra of intermittently sampled light curves against the noisy, observed power spectra, and produce a reliable estimate of the goodness of fit of the given model. We apply this technique to constrain the broad-band power spectra of a sample of four Seyfert galaxies monitored by the Rossi X-ray Timing Explorer (RXTE ) over three years. We show that the power spectra of three of the AGN in our sample (MCG-6-30-15, NGC 5506 and NGC 3516) flatten significantly towards low frequencies, while the power spectrum of NGC 5548 shows no evidence of flattening. We fit two models for the flattening: a 'knee' model, analogous to the low-frequency break seen in the power spectra of BHXRBs in the low state (where the power-spectral slope flattens to α =0), and a 'high-frequency break' model (where the power-spectral slope flattens to α =1), analogous to the high-frequency break seen in the high- and low-state power spectra of the classic BHXRB Cyg X-1. Both models provide good fits to the power spectra of all four AGN. For both models, the characteristic frequency for flattening is significantly higher in MCG-6-30-15 than in NGC 3516 (by a factor of ~10), although both sources have similar X-ray luminosities, suggesting that MCG-6-30-15 has a lower black hole mass and is accreting at a higher rate than NGC 3516. Assuming linear scaling of characteristic frequencies with black hole mass, the high accretion rate implied for MCG-6-30-15 favours the high-frequency break model for this source, and further suggests that MCG-6-30-15, and possibly NGC 5506, may be analogues of Cyg X-1 in the high state. Comparison of our model fits with naive fits, where the model is fitted directly to the observed power spectra (with errors estimated from the data), shows that Monte Carlo fitting is essential for reliably constraining the broad-band power spectra of AGN light curves obtained to date.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/a35d0bad137572b21915a9c3901119c03f5cba9f

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Citations in the corpus

A long hard look at MCG–6-30-15 with XMM-Newton (2002, background): “As the variability is dominated by long timescale changes (Uttley et al. 2002; Vaughan et al. 2003a) this therefore reveals theenergy dependence of variations occurring on timescales comparable to the length of the observation (∼ 100 ks).”
A simple test for periodic signals in red noise (2004, background): “It is also now known that at low frequencies the power spectra of Seyferts break from a single power law (e.g. Uttley et al. 2002; Markowitz et al. 2003).”
X-ray reverberation around accreting black holes (2014, background): “…in the X-ray band, and these efforts have also stimulated the development of techniques to study irregularly-sampled AGN data in the Fourier domain (Uttley et al. 2002; Markowitz et al. 2003), or more recently, through a combination of Fourier and time-domain approaches (Miller et al. 2010a; Kelly…”
MCG–6-30-15: long time-scale X-ray variability, black hole mass and active galactic nuclei high states (2005, methodology): “It is now reasonably well established that the X-ray powerspectral densities (PSDs) of AGN are broadly similar to those of galactic black hole X-ray binary systems (GBHs) (McHardy 1988; Edelson & Nandra 1999; Uttley et al. 2002; Markowitz et al. 2003; McHardy et al. 2004).”
Active galactic nuclei: what’s in a name? (2017, background): “The Advanced Telescope for High ENergy Astrophysics (Athena37; Nandra et al. 2013) is an ESA-led mission planned for launch in 2028.”
Generating artificial light curves: revisited and updated (2013, background): “These are two very well understood spectral distortions induce by the sampling properties of the data set and they can be taken into account in the usual manner (e.g. Uttley et al. 2002).”
Soft X-Ray and Ultraviolet Emission Relations in Optically Selected AGN Samples (2005, background): “Longer timescale variability studies of Sy 1s reveal variability ofk100% of the mean count rate in some sources, with no obvious difference in the variability amplitude between higher and lower luminosity AGNs (Uttley et al. 2002; Uttley & McHardy 2004, and references therein).”
MODELING THE TIME VARIABILITY OF SDSS STRIPE 82 QUASARS AS A DAMPED RANDOM WALK (2010, methodology): “Each light curve was simulated over 100 years and then truncated to the 30–40 year segment in order to account for the additional variability and bias in the inferred SF∞, which may result from a red noise leak or an incorrect specification of the PSD model (see Uttley et al. 2002).”
Combined long and short timescale X-ray variability of NGC 4051 with RXTE and XMM-Newton (2003, methodology): “This work; U - Uttley et al. (2002); V - Vaughan, Fabian and Nandra (2003); VF - Vaughan and Fabian (2003); W - Wandell (2002) Mkn 766 are not well known (cf Wandel 2002).”
Correlated Multi-Wave Band Variability in the Blazar 3C 279 from 1996 to 2007 (2008, methodology): “We have applied an algorithm based on a method by Uttley et al. (2002) to obtain the broadband PSD of nonthermal radiation from the jet of 3C 279.”
On characterizing the variability properties of X-ray light curves from active galaxies (2003, background): “In most AGN timing studies however there are rarely enough data to construct averages in this way (but see Papadakis & Lawrence 1993 and Uttley et al. 2002 for more on PSD estimation for AGN).”
A method for the estimation of the significance of cross-correlations in unevenly sampled red-noise time series (2014, methodology): “…(2014) at U niversity of L ausanne on July 25, 2016 http://m nras.oxfordjournals.org/ D ow nloaded from estimates on the model of the light curves, and presented a method based on Uttley et al. (2002), which allows us to determine the best fit for a simple power-law PSD model for a light curve.”
A physical model for the continuum variability and QPO in accreting black holes (2011, methodology): “Nonetheless, we choose to useχ2 minimization for simplicity, but we then doa posteriorechecks on the goodness of fit using the rejection probability method of Uttley et al (2002) and Markewicz et al (2003).”
KEPLER OBSERVATIONS OF RAPID OPTICAL VARIABILITY IN ACTIVE GALACTIC NUCLEI (2011, background): “These are always much flatter, typically having high frequency slopes of −1 to −2 (Edelson & Nandra 1999, Uttley et al. 2002, Markowitz et al. 2003).”