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Stefano Bianchi. AGN without BLR New clues for a new scenario. A. Marinucci, G. Matt, F. Nicastro, F. Panessa, X. Barcons, F. Carrera, F. La Franca, N. Sacchi, A. Corral, L. Monaco, A. Ruiz, M. Brightman, K. Nandra, A. Wolter. “The X-ray Universe 2011” Berlin – June 28 th 2011.

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Stefano Bianchi

AGN without BLR

New clues for a new scenario

A. Marinucci, G. Matt, F. Nicastro, F. Panessa, X. Barcons, F. Carrera,

F. La Franca, N. Sacchi, A. Corral, L. Monaco, A. Ruiz,

M. Brightman, K. Nandra, A. Wolter

“The X-ray Universe 2011”

Berlin – June 28th 2011


The detection of broad optical lines in the polarized spectrum of the archetypical Seyfert 2, NGC 1068, led Antonucci (1993) to postulate the equivalence between Type 1 and Type 2 AGN, any observational difference due to obscuration along the line-of-sight to the source

However, about half of the brightest Seyfert 2 galaxies appear not to have hidden broad-line regions (HBLR) in their optical spectra, even when high-quality spectro-polarimetric data are analysed (e.g. Tran 2001, 2003)


Are NHBLR \'different\' from HBLR sources?

Many pieces of evidence have been presented to support the view that HBLR Sy2s

are intrinsically more powerful than non-HBLR Sy2s,

rather than \'less obscured\' or \'starburst-contaminated\' (see e.g. Tran 2001,2003)

It appears that HBLRs share

many similar large-scale characteristics with Sy1s, being their counterparts as predicted by the UM, while non-HBLRs could be a different class of sources

Tran (2003)

In a model proposed by Nicastro (2000), the creation of the BLR is connected with disk instabilities occurring in proximity of a transition radius at which the accretion disk changes from gas-pressure dominated to radiation-pressure dominated

Since this transition radius becomes smaller than the innermost stable orbit for very low accretion rates (and therefore luminosities), very weak AGN should lack the BLR

Nicastro (2000)


The existence of a critical threshold in luminosity and/or Eddington rate has been confirmed on observational grounds (e.g. Nicastro 2003, Bian & Gu 2007, Shu et al. 2007, Wu et al. 2011)

However, the separation between the two classes is still controversial, and results depend on the choice of the sample and the methods adopted to estimate the accretion rates

Nicastro et al. 2003

Indeed, most of these works suffer from highly uncertain estimates of the Eddington ratios, due to very indirect methods to estimate the bolometric luminosity and/or BH masses (e.g. from the [OIII] emission lines)


Our sample is based on the spectro-polarimetric surveys performed byTran (1992, 1995, 2001, 2003), Young et al. (1996), Moran et al (2000) and Lumsden et al. (2001)

Marinucci et al. in prep.

  • In order to have reliable estimates of the accretion rate for our objects, we require:
  • a measure of the stellar velocity dispersion σ* to estimate the BH mass
  • good SNR (>150 counts) X-ray observations from the most sensitive instruments to estimate the bolometric luminosity

Following these requirements, our final sample is composed by 21 sources with polarized hidden broad lines and 18 sources without


Marinucci et al. in prep.

When using only Compton-thin sources, a separation between HBLR and non-HBLR sources is clear, both in luminosity (KS test: 99.5%) and in accretion rate (KS test: 99.9%)

The largest separations between the two distributions are at logLbol=43.90 and log Lbol/LEdd=−1.88

Marinucci et al. in prep.

Including the Compton-thick sources (correction factor 70), the separation between HBLR and non-HBLR is less sharp (KS tests: 98.2% and 99.1%)

We note that no HBLR source falls below the Eddington ratio threshold, while there is a number of non-HBLR Compton-thick sources with Eddington ratios larger than the threshold


This mixing could be due to the larger uncertainties involved in determining the intrinsic luminosity of Compton-thick sources, because of the unknown geometry of the absorbers and reflectors

This is indeed what happens if the bolometric luminosities are estimated from [OIII]

Marinucci et al. in prep.

However, these uncertainties could explain the presence of few non-HBLR closely above the threshold, but it is unlikely that the much higher accretion rates are wrongly estimated

This is also supported by the fact that no HBLR is found below the threshold, which is what we would expect if the mixing were caused by random uncertainties


Are non-HBLR Compton-thick sources \'more obscured\'?

By comparing the average Lx/L[OIII] in the Compton-thick sources of our sample to that derived by Lamastra et al. (2010) for Compton-thin sources, we estimate a correction factor of ~70 for the intrinsic luminosity of these objects

Non-HBLR Compton-thick sources preferentially have lower values of the Lx/L[OIII] ratio. However, a K-S test on the two distributions is not significant

We also verified if only the 9 non-HBLR Compton-thick sources that fall in the region with Lbol/LEdd>-1.88 could be different than all the other Compton-thick sources in our sample with respect to this ratio, but a K-S test is again not significant


It appears that there are two classes of non-HBLR:

  • those with low accretion rates, really lacking the BLR,
  • those with high accretion rate, likely hosting the BLR, but something prevents us from observing it

The second class is composed only by Compton-thick sources, so the nucleus is severely obscured by intervening absorbers. This could be explained within the framework of standard UM: more inclined sources should intercept a larger column density of the torus, and may obscure the medium responsible for the scattering of the BLR photons (e.g. Shu 2007). This scenario would also explain the (albeit not significantly) lower Lx/L[OIII] ratios found in non-HBLR Compton-thick sources, as a geometrical effect due to the shrinking of the reflecting area with respect to the line-of-sight

Heisler et al. 1997


If the BLR cannot exist in weakly accreting AGN, we would expect genuine Seyfert 2 galaxies, without any evidence of obscuration of their nuclei

\'Unabsorbed Seyfert 2s\' do exist, and the best examples (where the lack of the optical broad lines and of the X-ray obscuration are unambiguosly found in simultaneous observations with high SNR) have Eddington rates lower than the accretion rate threshold estimated by Marinucci et al. (in prep.)



Bianchi et al. (2008)

Simultaneous observations: it\'s not an artefact of variability!


Adapted from Panessa & Bassani (2002)‏

Matt et al. (in prep.)

NGC3147 is NOT Compton-thick:

  • Neutral iron Kα EW: ~130 eV
  • X-ray/IR/[OIII] flux diagrams
  • No excess above 10 keV

Strong iron line: outer disk, torus (without BLR!)?




No iron lines

NGC3660: EW<100 eV

Q2130: unconstrained

Panessa et al. 2010



(see also Brightman & Nandra 2008)

Bianchi et al. in prep.


How many?

Among the \'good\' candidates

  • Panessa & Bassani (2002) sample: 17 Sy2s galaxies with NH<1022 cm−2 and very \'unlikely\' to be Compton-thick

Excluding the sources where a column density (even if much lower than the one expected from the optical properties) is actually measured and misclassified sources

5 sources: only 1 is a true unabsorbed (NGC3147)

The others are either Compton-thick or misclassified (starburst)

\'Naked\' AGN from Hawkins (2004): Sy2s with rapid optical variability

6 sources: 1 unabsorbed Sy2 (Q2130-427), 3 still to be properly verified

Brightman & Nandra (2008) sample: Sy2s from the IRAS 12 μm sample of Rush, Malkan & Spinoglio (1993) with good quality X-ray data, and NH < 1022 cm−2), 6 sources: 2 unabsorbed Sy2s (NGC3660 and again NGC3147)


Apparently, not so many...

How many more?

  • Among all the X-ray unobscured AGN observed in targeted observations by XMM-Newton (CAIXA: Bianchi et al. 2009), only 6 have an Eddington rate lower than the accretion rate threshold found by Marinucci et al. (in prep.)

NGC3147 & NGC3660: unabsorbed Sy2s!

NGC4579 & NGC5033: unabsorbed Sy2 candidates in the Panessa & Bassani (2002) sample. However, weak broad optical lines are reported for both objects (see e.g. Shi 2010): very interesting!

PG1011-040: barely below the threshold, it is strongly X-ray weak with respect to its multi-wavelength emission, so that the accretion rate derived from its X-ray luminosity is likely to be orders of magnitude lower than the real one (Gallagher et al. 2001, Vasudevan & Fabian 2007)

NGC7213: the only known object where the iron Kα line is entirely produced in the BLR, which is visible in the optical spectrum (Bianchi 2008)! However, this is also the only bright Sy1 known to lack reprocessing features from a Compton-thick torus (Bianchi et al. 2003,2004 – Lobban et al 2010)



It appears that the presence of the BLR depends on the accretion rate:

lowly accreting sources cannot form the BLR

  • However, the detectability of the BLR in polarized light does also depends on geometry/obscuration
  • Two classes of non-HBLR:
  • those with low accretion rates, really lacking the BLR,
  • those with high accretion rate, likely hosting the BLR, but something prevents us from observing it

Unabsorbed Sy2s exist and are the counterparts of \'real\' non-HBLR

They do have low accretion rate

They are a small fraction of the Seyfert population (?)