Long-term X-ray spectral variability in AGN from the Palomar sample observed bySwift

Abstract

We present X-ray spectral variability of 24 local active galactic nuclei (AGN) from the Palomar sample of nearby galaxies, as observed mainly by Swift. From hardness ratio measurements, we find that 18 AGN with low accretion rates show hardening with increasing count rate, converse to the softer-when-brighter behaviour normally observed in AGN with higher accretion rates. Two AGN show softening with increasing count rate, two show more complex behaviour, and two do not show any simple relationship.Sufficient data were available for the spectra of 13 AGN to be summed in fluxbins. In 9 of these sources, correlated luminosity-dependent changes in the photon index (Γ) of a power-law component are found to be the main cause of hardness variability. For 6 objects, with a low accretion rate as a fraction of the Eddington rate (ṁ Edd ), Γ is anticorrelated withṁ Edd , i.e. 'harder-when-brighter' behaviour is observed. The 3 higher-ṁ Edd -rate objects show a positive correlation between Γ anḋ m Edd . This transition from harder-when-brighter at lowṁ Edd to softer-when-brighter at highṁ Edd can be explained by a change in the dominant source of seed-photons for X-ray emission from cyclo-synchrotron emission from the Comptonising corona itself to thermal seed-photons from the accretion disc. This transition is also seen in the 'hard state' of black hole X-ray binaries (BHXRBs). The results support the idea that LINERs are analogues of BHXRBs in the hard state and that Seyferts are analogues of BHXRBs in either the high-accretion-rate end of the hard state or in the hard-intermediate state.

Quasar spectral variability from theXMM-Newtonserendipitous source catalogue

“…β does not depend on λ E either for −2 λ E 0, while it shows a weak trend toward smaller absolute values for λ E −2, suggesting a possible agreement with Connolly et al (2016). In fact, according to their work, there might be a transition to harder when brighter trend (β > 0) for very low Eddington ratio sources.…”

“…8, There is no clear trend for λ E −2. Even though the number of points for the first bin is scarce, the absolute value of β for very low Eddington ratio λ E −2 is smaller, suggesting that for even lower Eddington ratios it might become positive, transitioning to harder when brighter trend as found by Connolly et al (2016). Bottom: histogram of Eddington ratio is also shown.…”

“…The trend these studies have found is quite the opposite of the optical one, since a softer when brighter behaviour was found in both individual and ensemble studies, even at higher energies (Soldi et al 2014). One notable exception is the work by Connolly et al (2016), in which the authors analysed a sample of local low-luminosity AGNs and found harder when brighter behaviour for low-Eddington-ratio sources and softer when brighter behaviour for high-Eddingtonratio sources. These observations explain the observed difference in terms of dominant seed photons source, which in the first case is cyclo-synchroton emission from the Comptonising corona itself, while in the second case is thermal emission from the accretion disk.…”

Long-Term X-Ray Variability of Typical Active Galactic Nuclei in the Distant Universe

“…The counts of our sources in each epoch (median ∼350) generally cannot constrain Γ effectively; e.g., Brightman et al (2013) found that even a simple powerlaw model would require 1000 counts to obtain an uncertainty (90% confidence level) of Γ within 0.2. Studies of quasars and local luminous AGNs show that spectral variability generally follows a "softer when brighter" behavior (e.g., Sobolewska & Papadakis 2009;Gibson & Brandt 2012;Sarma et al 2015;Connolly et al 2016). 22 However, based on the empirical X-ray flux-Γ relations given by those studies, the Γ variability amplitudes for our sources are expected to be small and not detectable given the counts we have in each epoch.…”

The MEXSAS2 Sample and the Ensemble X-ray Variability of Quasars

“…The spectra of most Seyfert galaxies with Eddington ratios above 0.01 typically become steeper in their brighter phases (e.g., Markowitz et al, 2003;Sobolewska and Papadakis, 2009;Connolly et al, 2016), as well as for many galactic black hole binary systems (e.g., Remillard and McClintock, 2006;Done et al, 2007;Dong et al, 2014). This is known as "softer when brighter" behavior and translates to a negative β value according to Equation 2.…”