Saturday, September 1, 2007
By now, you’d think that astronomers would have found all the different classes of AGN — extraordinarily energetic cores of galaxies powered by accreting supermassive black holes. AGN such as quasars, blazars, and Seyfert galaxies are among the most luminous objects in our Universe, often pouring out the energy of billions of stars from a region no larger than our solar system.
But by using Swift and Suzaku, the team has discovered that a relatively common class of AGN has escaped detection. These objects are so heavily shrouded in gas and dust that virtually no light gets out.
Evidence for this new type of AGN began surfacing over the past two years. Using Swift’s Burst Alert Telescope (BAT), a team led by Tueller has found several hundred relatively nearby AGNs that were previously missed because their visible and ultraviolet light was smothered by gas and dust. The BAT was able to detect high-energy X-rays from these heavily blanketed AGNs because, unlike visible light, high-energy X-rays can punch through thick gas and dust.
AGNs are surrounded by a donut-shaped ring of material, which partially obscures our view of the black hole. Our viewing angle with respect to the donut determines what type of object we see. But a team member, thinks these newly discovered AGN are completely surrounded by a shell of obscuring material. We can see visible light from other types of AGN because there is scattered light. But in these two galaxies, all the light coming from the nucleus is totally blocked.
Another possibility is that these AGN have little gas in their vicinity. In other AGN, the gas scatters light at other wavelengths, which makes the AGN visible even if they are shrouded in obscuring material.
By missing this new class, previous AGN surveys were heavily biased, and thus gave an incomplete picture of how supermassive black holes and their host galaxies have evolved over cosmic history. But still future surveys may be wrong.
A brief flash of light had been observed two years before this rare cataclysm occurred. This precursor signal, which had never been seen before, raises hopes that astronomers will be able to "predict" explosions and observe stars as they enter the very last moments of their existence.
On 9 October 2006, two years after a Japanese amateur astronomer observed a flash of light in the constellation Lynx in UGC4904, the appearance of an object ten times brighter in the same spot attracted the attention of a European consortium, which mobilized a bank of telescopes.
The supernova, named SN2006jc, reached the maximum luminosity that is characteristic of the most powerful star explosions, more than a billion times brighter than the Sun. Astronomers divide these explosions into two broad categories – supernovae types I and II.
The type I category refers to the disintegration of a small, compact star, known as a white dwarf, which has been made unstable by the accumulation of matter coming from a companion.
The type II category, on the other hand, refers to the explosion of a massive star. In the first case, very little hydrogen and helium is seen in the explosion, whereas in the second type of explosion, these two elements are predominant. SN2006jc does not fit into either category and so has been catalogued in a special sub-category, Ic.
This rarity can be explained by the great mass of the star concerned. It is probably a star of 60 to 100 solar masses which has lost a great quantity of mass previously. Here it is only the central part, a core of carbon and oxygen of 15 to 25 solar masses that explodes. Thus, most of the elements in the explosion come from the core of the star, while the helium observed is only found around the edge and comes from the star's envelope, which was ejected earlier.
Supernova SN2006jc is the only known example of a star explosion for which a flash of light was observed two years earlier. For this reason, it opens up new horizons for predicting massive star explosions. Eta Carinae could be one example of a star similar to SN2006jc close to our galaxy. It also experienced an outburst of luminosity making it the second brightest star in the sky in 1843. A future outburst could be the sign of an imminent explosion.