The anticorrelation between the hard X-ray photon index and the Eddington ratio in low-luminosity active galactic nuclei

Gu, Minfeng; Cao, Xinwu

doi:10.1111/j.1365-2966.2009.15277.x

Cited by 142 publications

(208 citation statements)

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“…This saturation is once again seen in observations of both AGN and BHXRBs, using a small number of observations from large samples of sources (Yang et al 2015;Gu & Cao 2009;Hernández-García et al 2013). …”

Section: Introductionmentioning

confidence: 99%

“…This softer-when-brighter behaviour is seen in both black hole X-ray binary systems (BHXRBs) (e.g. Gu & Cao 2009) and in AGN (e.g. M c Hardy et al 1998;Sobolewska & Papadakis 2009;Shemmer et al 2006).…”

Section: Introductionmentioning

confidence: 99%

“…In a sample of single-epoch observations of low-luminosity AGN (LLAGN), Gu & Cao (2009) found Γ to be anticorrelated with the accretion rate (i.e. 'harder-when-brighter').…”

Section: Introductionmentioning

confidence: 99%

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

Connolly

McHardy

Skipper

et al. 2016

Mon. Not. R. Astron. Soc.

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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.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Connolly

McHardy

Skipper

et al. 2016

Mon. Not. R. Astron. Soc.

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“…Although LINERs and more powerful AGN are located in the same plane, different authors have pointed out that the accretion mechanism in LINERs could be different from that in more powerful AGN (e.g., Gu & Cao 2009;Younes et al 2011). When a source accretes at a very low Eddington rate (R Edd < 10 −3 ), the accretion is dominated by radiatively inefficient accretion flows (RIAF, Narayan & Yi 1994;Quataert 2004).…”

Section: Variability Timescalesmentioning

confidence: 99%

“…Some efforts have been made to observationally investigate that relationship for LLAGN. Gu & Cao (2009) used a sample of 55 LLAGN (including 27 LINERs and 28 Seyferts) and found a regular anticorrelation between Γ and R Edd . However, when LINERs were considered alone, they did not find a strong correlation.…”

Section: Variability Timescalesmentioning

confidence: 99%

X-ray spectral variability of LINERs selected from the Palomar sample

Hernández-García

González‐Martín

Masegosa

et al. 2014

A&A

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Context. Variability is a general property of active galactic nuclei (AGN). The way in which these changes occur at X-rays is not yet clearly understood. In the particular case of low-ionization nuclear emission line region (LINER) nuclei, variations on the timescales from months to years have been found for some objects, but the main driver of these changes is still debated. Aims. The main purpose of this work is to investigate the X-ray variability in LINERs, including the main driver of these variations, and to search for possible differences between type 1 and 2 objects. Methods. We examined the 18 LINERs in the Palomar sample with data retrieved from the Chandra and/or XMM-Newton archives that correspond to observations gathered at different epochs. All the spectra for the same object were fitted simultaneously to study long-term variations. The nature of the variability patterns were studied by allowing different parameters to vary during the spectral fit. Whenever possible, short-term variations from the analysis of the light curves and long-term UV variability were studied. Results. Short-term variations are not reported in X-rays. Three LINERs are classified as non-AGN candidates in X-rays, all of them are Compton-thick candidates; none of them show variations at these frequencies, and two of them vary in the UV. Long-term X-ray variations were analyzed in 12 out of 15 AGN candidates; about half of them showed variability (7 out of the 12). At UV frequencies, most of the AGN candidates with available data are variable (five out of six). Thus, 13 AGN candidates are analyzed at UV and/or X-rays, ten of which are variable at least in one energy band. None of the three objects that do not vary in X-rays have available UV data. This means that variability on long-timescales is very common in LINERs. These X-ray variations are mainly driven by changes in the nuclear power, while changes in absorptions are found only for NGC 1052. We do not find any difference between type 1 and 2 LINERs, neither in the number of variable cases (three out of five type 1 and four out of seven type 2 LINERs), nor in the nature of the variability pattern. We find indications of an anticorrelation between the slope of the power law, Γ, and the Eddington ratio. Conclusions. LINERs are definitely variable sources irrespective of whether they are classified as optical type 1 or 2. Their BH masses, accretion rates, and variability timescales place them in the same plane as more powerful AGN at X-rays. However, our results suggest that the accretion mechanism in LINERs may be different. UV variations of some type 2 LINERs were found, this could support the hypothesis of a torus that disappears at low luminosities.

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The fundamental plane of black hole activity for quiescent sources

Dong

Liu

et al. 2021

Astron Nachr

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In this work, we construct a sample of black hole X-ray binaries (BH XRBs), low-luminosity active galactic nuclei (LLAGNs), and FR Is with wider distribution of Eddington ratios and re-explore their fundamental plane of BH activities, that is, log L R = X log L X + M log L M BH + c 0. We find that the quiescent BH sources follow a similar fundamental plane (ξ X ∼ 0.6) with sub-Eddington BH sources very well, while the strong radio sources (e.g., FR Is) follow a steeper fundamental plane (ξ X ∼ 1.30). We also find that the radio-X-ray correlation of quiescent BH sources are still on the extension of sub-Eddington BH sources and the strong radio sources follow a steeper correlation (ξ X ∼ 1.30). The results are consistent with recent observations in BH XRBs. It is predicted that the X-ray emissions of both sub-Eddington BH sources and quiescent BH sources originate from radiatively inefficient accretion mode, while the X-ray emissions of strong radio sources dominantly originate from the jet.

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The anticorrelation between the hard X-ray photon index and the Eddington ratio in low-luminosity active galactic nuclei

Cited by 142 publications

References 60 publications

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

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

X-ray spectral variability of LINERs selected from the Palomar sample

The fundamental plane of black hole activity for quiescent sources

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