X-Ray Spectral Variability and Rapid Variability of the Soft X-Ray Spectrum Seyfert 1 Galaxies Arakelian 564 and Ton S180

By R. Edelson, T. Turner, K. Pounds, S. Vaughan, A. Markowitz, H. Marshall, P. Dobbie, Robert Warwick Astronomy Department Ucla, X-ray Astronomy Group Leicester University, L. E. B. J. A. S. Center and 3 more. 2001

330 citations recorded by the source corpus.

The bright, soft X-ray spectrum Seyfert 1 galaxies Ark 564 and Ton S180 were monitored for 35 days and 12 days, respectively, with ASCA and RXTE (and EUVE for Ton S180). These represent the most intensive X-ray monitoring of any such soft-spectrum Seyfert 1 to date. Light curves were constructed for Ton S180 in six bands spanning 0.1-10 keV and for Ark 564 in five bands spanning 0.7-10 keV. The short-timescale (hours-days) variability patterns were very similar across energy bands, with no evidence of lags between any of the energy bands studied. The fractional variability amplitude was almost independent of energy band, unlike hard-spectrum Seyfert 1 galaxies, which show stronger variations in the softer bands. It is difficult to simultaneously explain soft Seyfert galaxies stronger variability, softer spectra, and weaker energy dependence of the variability relative to hard Seyfert galaxies. There was a trend for soft- and hard-band light curves of both objects to diverge on the longest timescales probed (~weeks), with the hardness ratio showing a secular change throughout the observations. This is consistent with the fluctuation power density spectra that showed relatively greater power on long timescales in the softest bands. The simplest explanation of all of these is that two continuum emission components are visible in the X-rays: a relatively hard, rapidly variable component that dominates the total spectrum and a slowly variable soft excess that only shows up in the lowest energy channels of ASCA. Although it would be natural to identify the latter component with an accretion disk and the former with a corona surrounding it, a standard thin disk could not get hot enough to radiate significantly in the ASCA band, and the observed variability timescales are much too short. It also appears that the hard component may have a more complex shape than a pure power law. The most rapid factor of 2 flares and dips occurred within ~1000 s, in Ark 564 and a bit more slowly in Ton S180. The speed of the luminosity changes rules out viscous or thermal processes and limits the size of the individual emission regions to ≲15 Schwarzschild radii (and probably much less), that is, to either the inner disk or small regions in a corona.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

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In Astronomy and Astrophysics

1. A. C. Fabian
2. H. Netzer
3. D. M. Alexander
4. C. Reynolds
5. Jenny E. Greene
6. S. Vaughan
7. P. Uttley
8. Bing Zhang

In Nuclear and High Energy Physics

1. A. C. Fabian
2. F. Tavecchio
3. C. Dermer
4. A. Marscher
5. D. Maoz
6. E. Orlando
7. Y. Kovalev
8. W. N. Brandt

Citations in the corpus

• GX 339—4: the distance, state transitions, hysteresis and spectral correlations (2004, methodology): “Then, we have calculated it for both weighted (see Appendix) and unweighted (Edelson et al. 2002) averages.”
• A long hard look at MCG–6-30-15 with XMM-Newton (2002, background): “2 of Edelson et al. (2002), but as discussed in their appendix should be considered only as approximations since they strictly assume the lightcurves are drawn from independent Gaussian processes.”
• Non‐linear X‐ray variability in X‐ray binaries and active galaxies (2005, background): “Recent studies of X-ray light curves of the NLS1 Ark 564 (Edelson et al., 2002), NGC 4051 (McHardy et al., 2004) and the Seyfert 1 AGNs MCG-6-3015 and Mrk 766 (Vaughan, Fabian & Nandra, 2003; Vaughan et al., 2003), also show clear linear rms-flux relations in these AGN¶.”
• X-ray continuum variability of MCG-6-30-15 (2002, methodology): “Uttley & McHardy (2001) showed that both X-ray binary and AGN light curves show a tight, linear correlation between flux and rms variability amplitude (see also Edelson et al. 2002).”
• INSIGHTS INTO THE HIGH-ENERGY γ-RAY EMISSION OF MARKARIAN 501 FROM EXTENSIVE MULTIFREQUENCY OBSERVATIONS IN THE FERMI ERA (2011, methodology): “In order to account for the individual flux measurement errors (σerr, i), we used the “excess variance” as an estimator of the intrinsic source variance (Nandra et al. 1997; Edelson et al. 2002).”
• GAMMA-RAY LIGHT CURVES AND VARIABILITY OF BRIGHT FERMI-DETECTED BLAZARS (2010, methodology): “We also quantify the variability amplitude of all the LBAS sources, using the “normalized excess variance” (Nandra et al. 1997; Edelson et al. 2002).”
• Relativistic iron K X-ray reverberation in NGC 4151 (2011, background): “The RMS spectrum (Edelson et al. 2002) for the low and high intervals is also different (Fig.”
• THE LICK AGN MONITORING PROJECT 2011: SPECTROSCOPIC CAMPAIGN AND EMISSION-LINE LIGHT CURVES (2015, background): “…is similar to the normalized excess scatter and is defined as Fvar = √ σ2f − δ2rms 〈 f 〉 , (2) whereσ2f is the variance of the flux values in the light curve, δrms is the rms measurement uncertainty on the fluxes, and〈 f 〉 is the mean flux (Rodríguez-Pascual et al. 1997; Edelson et al. 2002).”
• Variable Very High Energy γ-Ray Emission from Markarian 501 (2007, methodology): “In order to account for the individual flux measurement errors (σerr,i), we used the ’excess variance’ (Nandra et al 1997; Edelson et al 2002) as an estimator of the intrinsic source variance.”
• SMARTS OPTICAL AND INFRARED MONITORING OF 12 GAMMA-RAY BRIGHT BLAZARS (2012, background): “…“excess variance,” which is the fractional root-meansquared variability amplitude normalized by the mean flux: Fvar = √ S2 − σ2 err x2 , (1) where S is the variance, σerr is the observational uncertainty, and x is the mean of the data (Nandra et al. 1997; Edelson et al. 2002; Vaughan et al. 2003).”
• On characterizing the variability properties of X-ray light curves from active galaxies (2003, methodology): “The problem of how to assess the uncertainty on the excess variance (orFvar) is a long-standing one (e.g. Nandra et al. 1997; Turner et al. 1999; Edelson et al. 2002).”
• Combined long and short timescale X-ray variability of NGC 4051 with RXTE and XMM-Newton (2003, background): “Subsequently, better quality lightcurves have provided stronger evidence of a linear rms-flux relation in AGN (in Akn 564, Edelson et al. 2002; and MCG– 6-30-15, Vaughan et al. 2003).”
• Reverberation Mapping of High-Luminosity Quasars: First Results (2006, background): “…light curve passed the test, we list for it in column (8) the intrinsic normalized variability measure, N ¼ 100( 2 2)1=2/f̄ (used by Kaspi et al. 2000), and in column (9) we list the fractional variability amplitude, Fvar, and its uncertainty (Rodriguez-Pascual et al. 1997; Edelson et al. 2002).”
• PS16dtm: A Tidal Disruption Event in a Narrow-line Seyfert 1 Galaxy (2017, background): “However, given that NLS1 galaxies are known to be highly X-ray variable on timescales as short as days (Boller et al. 1993, 1996; Edelson et al. 2002) and that this variability is typically correlated with UV/optical variability (Uttley 2006; Grupe et al. 2010), it is significant that a long-term…”
• BLAZARS IN THE FERMI ERA: THE OVRO 40 m TELESCOPE MONITORING PROGRAM (2010, background): “…tests, such as the variability index (e.g., Aller et al. 1992); the fluctuation index (e.g., Aller et al. 2003); the modulation index (e.g., Kraus et al. 2003); the fractional variability amplitude (e.g., Edelson et al. 2002; Soldi et al. 2008); and χ2 tests of a null hypothesis of non-variability.”