INTEGRAL observations suggest unified model for Active Galactic Nuclei requires a rethink

Aug 03, 2011
Artist's impression of an AGN according to the unified model. Credit: ESA/NASA, the AVO project and Paolo Padovani

Scrutinizing a large sample of Active Galactic Nuclei with INTEGRAL, astronomers have found that, unexpectedly, sources affected by stronger absorption at lower energies show an excess emission in the hard X-ray band when compared to their less obscured counterparts. The excess is likely due to reflection of X-rays off the same dense clouds responsible for absorption. Unaccounted for in the unified paradigm of Active Galactic Nuclei, the finding calls for a rethinking of some of this model's facets. The reflected radiation could also represent the long-sought missing part of the Cosmic X-ray Background.

Supermassive lie at the core of all , including the . Whereas the majority of them are inactive, a small fraction of these black holes are accreting the surrounding matter at extraordinary rates. This causes them to radiate profusely across the entire electromagnetic spectrum. Known as (AGN), these sources are often so bright that they outshine their and have been detected out to the far reaches of the observable Universe.

Astronomers explain the wide variety of features discerned in different classes of AGN in terms of the anisotropic geometry of the black hole's immediate surroundings. The accreting black hole is fed via a disc, and a thick, obscuring torus of gas and dust is believed to encompass the disc and to absorb part of the radiation it emits.

Depending on the orientation of the torus with respect to an observer's line of sight, the view to the may be obstructed to varying degrees. This 'unified' model, which provides a phenomenological interpretation of the observed AGN diversity, is now questioned, at least in part, by a new study performed with ESA's INTEGRAL observatory.

"The unified model predicts that all AGN exhibit the same behaviour when observed at hard X-ray wavelengths, regardless of the different emission they might show in other bands," explains Claudio Ricci, a PhD student from the Data Centre for Astrophysics (ISDC) at the University of Geneva, Switzerland. Ricci is the lead author on the paper reporting on the study and published online on 2 August 2011 in Astronomy & Astrophysics. "Absorption by the torus is weaker for photons with increasing energy, and it should have no effect at all, for the objects in our sample, on the hard X-ray photons probed by INTEGRAL," he adds.

To test this framework, Ricci and colleagues put together a sample of 165 AGN observed with INTEGRAL during its first 8 years of operation at energies between 20 and 250 keV. The sample comprises AGN subject to absorption at lower energies and also unabsorbed ones. "We expected to see no difference between classes, but instead we noticed two very distinct trends in the data," he notes.

Surprisingly, the sources affected by stronger absorption at lower energies (i.e. from the infrared to the soft X-ray bands) show an excess of emission, with respect to their less obscured counterparts, in the energy range 30–60 keV. No appreciable difference is observed at higher energies. The excess emission is a signature of X-rays being reflected off neutral hydrogen gas in the dense clouds that surround the black hole and disc, possibly the same clouds responsible for absorption at lower energies.

"A significantly stronger reflection in a certain class of AGN indicates that the environment of these objects is different," comments co-author Roland Walter, who is the Principal Investigator of the INTEGRAL team at the ISDC. "The discrepancy in the high-energy emission properties of different AGN types is unaccounted for in the leading theoretical framework and calls for a rethinking of some of its details," he adds.

In the light of the new data, the unified model's basic assumption that the same central engine powers all AGN is safe. However, the presumed existence, in all of them, of an anisotropic, toroidal structure needs revision. "We argue that various classes of AGN are characterised by a substantially different distribution of absorbing material around the black hole," notes Walter.

The team advocate a patchy but overall isotropic morphology for the absorber. In this scenario, the density of clouds around the central source would determine the amount of reflected X-rays, with denser environments resulting in stronger reflection. The astronomers speculate about whether such morphological variations could characterise objects at different evolutionary stages, although the issue is still unclear and the subject of further investigation.

The discovery of X-ray reflection by AGN has another, far-reaching implication in the debate about the Cosmic X-ray Background (CXB), a tenuous, diffuse background radiation that pervades the X-ray sky from all directions. The CXB consists of the cumulative X-ray radiation emitted by all unresolved active galaxies across cosmic history. As this diffuse background peaks in the hard X-ray regime probed by INTEGRAL, identifying its individual sources has been among the mission's main science goals ever since it began operations.

"One of the observatory's milestones was the first accurate measurement of the CXB intensity in the energy range where its emission is stronger," notes Chris Winkler, INTEGRAL Project Scientist at ESA. Achieved in 2006, this result did not solve, but in fact deepened, a long-standing debate about the origin of the CXB. The measured intensity appeared to be far greater than that which astronomers could predict by modelling observations of individual AGN and summing their contributions. To match the measured value, a much larger number of heavily obscured AGN than has been observed was required.

Now, the study by Ricci and his colleagues proposes a fresh solution to the puzzle: when the reflected X-rays are added to the total budget of radiation sources in the CXB, invoking sources that have never been observed may no longer be necessary.

"The result relies on several years of data acquired with the most sensitive hard X-ray telescope currently in operation," comments Winkler. "By showing us where the missing part of the CXB lies, INTEGRAL might have solved a 30-year long mystery," he concludes.

Explore further: We're not alone—but the universe may be less crowded than we think

More information: C. Ricci, et al., "Reflection in Seyfert Galaxies and the Unified Model of AGN", Astronomy & Astrophysics, 2011, 532, A102

Related Stories

Mysterious X-rays from a Nearby Galaxy

Nov 13, 2009

( -- The nucleus of an active galaxy, an AGN, contains a massive black hole that is vigorously accreting material. In the process it typically ejects jets of particles and radiates brightly at ...

NASA Performs Headcount of Local Black Holes

Oct 06, 2006

NASA scientists using the Swift satellite have conducted the first complete census of galaxies with active, central black holes, a project that scanned the entire sky several times over a nine-month period.

Recommended for you

Observing the birth of a planet

13 hours ago

Astronomers at ETH Zurich have confirmed the existence of a young giant gas planet still embedded in the midst of the disk of gas and dust surrounding its parent star. For the first time, scientists are able ...

First stars in the universe left a unique signature

18 hours ago

Determining the chemical abundance pattern left by the earliest stars in the universe is no easy feat. A Lawrence Livermore National Laboratory (LLNL) scientist is helping to do just that.

NGC 2367: Buried in the heart of a giant

19 hours ago

This rich view of an array of colorful stars and gas was captured by the Wide Field Imager camera, on the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile. It shows a young open cluster ...

NASA missions monitor a waking black hole

Jun 30, 2015

NASA's Swift satellite detected a rising tide of high-energy X-rays from the constellation Cygnus on June 15, just before 2:32 p.m. EDT. About 10 minutes later, the Japanese experiment on the International ...

What is the habitable zone?

Jun 30, 2015

The weather in your hometown is downright uninhabitable. There's scorching heatwaves, annual tyhpoonic deluges, and snow deep enough to bury a corn silo.

User comments : 16

Adjust slider to filter visible comments by rank

Display comments: newest first

1.7 / 5 (6) Aug 03, 2011
Yes, the rethink could include these ideas. Rip Theory predicts that a black hole and the galaxy it is associated with evolve together. Therefore the associated active galactic nuclei would also go through changes (it would also evolve). The actual emission and reflection of radiation may be due to the active galactic nuclei (the dense matter that is being accelerated towards the event horizon).
© Copyright 2011 Thomas A. Sullivan
1 / 5 (3) Aug 03, 2011
Radiation and reflection may not be coming from or occurring at the black hole itself.
2.3 / 5 (3) Aug 03, 2011
It could have it's implications, but at the same time. You can realize that we are further than that and there are reasons we're not sharing.
5 / 5 (6) Aug 03, 2011
Neu....n......n.....neeeeeuuuuu.....nnnneuuuuutt........can't say it.
5 / 5 (3) Aug 03, 2011
Someone so self absorbed as to self copyright their own ideas in a comments section like this is not likely to intelligently add to a conversation or consider any contrary facts.

Also, that is not a copywritable idea. it is a theory with little supporting evidence, does not present yourself opportunity for financial gain in that format, and fails the original concept test.

real astronomers write papers and get credit for their work. They copyright the books and papers they write, not their ideas, because astronomical theory generally is not a process that is bankable in itself.
1 / 5 (7) Aug 04, 2011
Rethinking has been going on for some time, outside of control by the "politically correct" consensus opinions imposed on the Western scientific community by the united efforts of the UN's IPCC, the US National Academy of Sciences and the UK's Royal Society.

The results - many published in "underground papers" since the 21-28 Feb 1972 decision to control government science in order to avoid mutual nuclear annihilation - are summarized in these three papers:

1. "Earth's Heat Source - The Sun", Energy and Environment 20, 131-144 (2009):

2. Neutron repulsion, The APEIRON Journal, preprint, in press, 19 pages (2011) :

3. "Is the Universe Expanding?", The Journal of Cosmology 13, 4187-4190 (2011):


With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo
1 / 5 (1) Aug 04, 2011
"The astronomers speculate about whether such morphological variations could characterise objects at different evolutionary stages, although the issue is still unclear and the subject of further investigation." - article's authorship

Finding objects at exactly the same evolutionary stage has the potential to confine the variables.

An "identical twin" (including identical cloud density) comparison has potential to underscore the researchers' new solution proposals.

A difficult undertaking. Assigning any process 'bankablity'.
'For the good of humankind' are words of chitin from insects discarding their shells of metamorphosis.

Thought control is bankable. With or without copyright.
No one possessing the process of thought control applies for copyright. That is self defeating and humanity wins.
5 / 5 (1) Aug 04, 2011
Neu....n......n.....neeeeeuuuuu.....nnnneuuuuutt........can't say it.

Too late!
5 / 5 (1) Aug 04, 2011
Oliver K. Manuel:
- many published in "underground papers" since the 21-28 Feb 1972 decision to control government science in order to avoid mutual nuclear annihilation -

So why would the government control science to avoid M.A.D.? I am curious.
1 / 5 (5) Aug 04, 2011
Thanks for your comments. I updated the historical review.

Climategate, and the collapse of Western economies and the USA space program apparently result from agreements that Henry Kissinger negotiated with leaders of China in 1971.

The addendum suggests 1971 was the turning point, that was later made official when Richard Nixon went to China with Henry Kissinger in 1972.



Again, thanks for your comments.

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo
1 / 5 (1) Aug 05, 2011
Oliver, you question other scientists credibility and integrity yet forgot the most important title off the list of noteworthy things youve been involved in- "convicted sex offender". But seriously oliver, give it a break. not even you can really believe that garbage coming from your mouth all the time can you? You should visit your local psych ward asap if you do. You've obviously got paranoid delusions, obsessive compulsive disorder, and dementia. Please for the good of society and yourself do so. Honestly I'm scared you might start molesting young children again.
1 / 5 (1) Aug 05, 2011
by garbage i mean olivers conspiracy theory, i know he believes in neutron repulsion heart and soul and no one can change that
2.3 / 5 (3) Aug 06, 2011
Science placates:
'Question everything'*
'Assume nothing'*

Questionable Clichés. Highly contagious. Highly virulent.

The word 'control' finds it's source of origin in such placation.

Eventually leading to pathological states of mind (or in this case, government states) harboring mindsets such as:

*Ergo, never trust science.
To avoid M.A.D. never trust.
Trust is good. Control is better.

Most people trust. A perquisite for health and/or progress.

So why would the government control science to avoid M.A.D.?
So why would the government trust science to avoid M.A.D.?

We have:
Mutual Distrust.

And no reason not to believe this.

1 / 5 (4) Aug 07, 2011
And no reason not to believe this.

The Bilderberg Sun, Climategate & Economic Crisis



If you can't open the links, the document will on request be sent to you as a pdf file or Word doc
5 / 5 (1) Aug 07, 2011
Science placates

@hush1: Could you tell me what language you're writing in? I'd like to run it through a translation utility and see what it says.
1.8 / 5 (5) Aug 17, 2011
"We argue that various classes of AGN are characterised by a substantially different distribution of absorbing material around the black hole," notes Walter.

My reaction to that observation is simply: If the AGN is instead periodically expelling material, in explosive ejections of varying magnitude, one would expect a wide variety of condensing structures in the vicinity of various AGN's. One would not expect only one type of structure, as one might if one assumed that the ejected material was solely a result of material accreting onto the core.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.