“The X-ray Universe 2011” Berlin – June 28th 2011

Stefano BianchiStefano Bianchi

AGN without BLRNew 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 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)

Tran (2003)

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 sharemany similar large-scale characteristics with Sy1s, being their counterparts as predicted by the UM,

while non-HBLRs could be a different class of sources

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

Are NHBLR 'different' from HBLR sources?

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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)

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

Mari

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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)

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

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

Marinucci et al. in prep.

Marinucci et al. in prep.

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]

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

Marinucci et al. in prep.

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

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

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

Heisler et al. 1997

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

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.)

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NGC3147NGC3147log(Lbol/LEdd)~-4

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

Matt et al. (in prep.)NGC3147 is NOT Compton-thick:NGC3147 is NOT Compton-thick:

Neutral iron Kα EW: ~130 eVX-ray/IR/[OIII] flux diagrams

No excess above 10 keV

Adapted from Panessa & Bassani (2002)Adapted from Panessa & Bassani (2002)

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

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Q2131−427Q2131−427log(Lbol/LEdd)~-2.6

NGC3660NGC3660log(Lbol/Ledd)~--2

(see also Brightman & Nandra 2008)

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No iron linesNGC3660: EW<100 eVQ2130: unconstrained

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 sources5 sources: only 1 is a true unabsorbed (NGC3147)

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

Among the 'good' candidates

'Naked' AGN from Hawkins (2004): Sy2s with rapid optical variability6 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)

How many?How many?

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.)

Apparently, not so many...

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)

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!

NGC3147 & NGC3660: unabsorbed Sy2s!

How many more?How many more?

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-HBLRThey do have low accretion rate

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

SummarySummary