Posts Tagged ‘black holes’

A Theoretical Link Found Between Radio Jets and Gamma Rays

Wednesday, May 6th, 2009 by Bellatrix

Scientists from the Max Plank Institute for Radio Astronomy in Bonn Germany using NASA’s Fermi Gamma Ray Space Telescope and the world’s largest radio telescope array have solidified a theoretical link between radio jets coming from the center of active galaxies and gamma ray bursts. This is a fine demonstration and use of new technology combined with an innovative use of existing technology.

Active galaxies are extremely bright galaxies which emitted oppositely directed jets of charged particles from their centers traveling near the speed of light.  Some, called Blazars, are especially bright because their jets are orientated along our line of sight.  These jets glow brightly in the radio part of the spectrum and in the 90’s it was hinted with the Chandra X-ray Observatory that they might emit in the higher energy parts of the spectrum as well. Astronomers believe these jets arise from matter that is falling into the central massive black holes of the galaxies, but the exact processes behind them is not well understood; which makes them the object of much study.

Now the Fermi telescope uses it’s Large Area Telescope, LAT, to scan the entire sky every 3 hours getting snapshots of the gamma ray bursts throughout the sky and monitor flares. Gamma ray bursts are the highest energy form of light below cosmic rays, and the origins of these gamma ray bursts is still undetermined; the objective of Fermi is to help clarify these origins. 

The new study was part of the MOJAVE program, which is a long-term study of the jets form active galaxies using primarily the VLBA. The VLBA is the National Science Foundation’s Very Long Baseline Array, a set of 10 radio telescopes located from Hawaii to the Virgin Islands and operated by the National Radio Astronomy Observatory in New Mexico.  Signals from these 10 different telescopes are combined and the array acts like a single enormous radio dish more than 8,500 kilometers across. The VLBA can resolve details about a million times smaller than Fermi and50 times smaller than any optical telescope.

Astronomers combined data from the VLBA and LAT. Active galaxies detected in the LAT’s first few months of operations generally possess brighter and more compact radio jets than galaxies the LAT did not see. Moreover, an active galaxy’s radio jets tend to be brighter in the months following any gamma-ray flares observed by the LAT.  A correlation was also found between active galaxies with the brightest gamma ray emission and those with the fastest jets.

The scientists were also able to use this data to study a phenomenon known as Doppler boosting. Doppler boosting makes radio-emitting blobs look brighter and appear to move faster than the speed of light due to the angle at which it is viewed and the fact the speed of the particles is close to the speed of light.  The VLBA data shows that the bigger the Doppler boost seen in a radio jet, the more likely it is that Fermi recorded it as a gamma ray source. Also, many objects found by Fermi to be extreme in gamma rays are broadcasting strong bursts of radio emission at the same time. 

All of this data points to the conclusion that the portion of an active galaxy’s radio jet closest to the galaxy’s center is also the source of the gamma rays.  These findings show us a very interesting and before unknown link between two “sides” of one object and possibly one process. This may bring astronomers one-step closer to solving two very large mysteries: the processes behind the jets and the exact processes or origins of gamma ray bursts. It could turn out to be quite nice and convenient if both questions could have the same answer, or an answer that comes from the same place. This new finding is also a demonstration of the use of the new technology of Fermi the space telescope that was designed just to study gamma ray bursts, a first of its kind, and the technology behind the VLBA, using standard radio telescopes in a new way to improve their usefulness

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Microquasars: Black Holes and Normal Stars an Interesting Mix

Tuesday, December 2nd, 2008 by Bellatrix

Astronomers using two different telescopes and two different systems have started learning about microquasars. They’re learning new things that can then hopefully be applied to full size quasars as well.

A quasar is an extremely powerful, luminous and distant active galactic nucleus. While there was initially some controversy over the nature of these objects, there is now a scientific consensus that a quasar is a compact region surrounding the central supermassive black hole of a galaxy. Quasars show a very high redshift, meaning they are located a great distance from us. Quasars are active because the central black hole is accreting a lot of material. Near the black hole, intense magnetic fields in the disk accelerate material into tight jets that flow in opposite directions away from the hole.

Microquasars is a two-body system consisting of a stellar mass size black hole and a star, usually a red giant. The giant star is feeding material to the black hole. Which, needless to say creates some interesting dynamics. Astronomers have been looking at two systems, Swift1753.3-0127 and GX339-4, with the European Southern Observatory’s Very Large Telescope and NASA’s Rossi X-ray Timing Explorer to study microquasars. Microquasars are not only closer but change more rapidly, so a process that may take a normal quasar a year to undergo might only take a microquasar a few minutes.

Astronomers had thought that the visible light emission coming form microquasars was coming form far out in the accretion disk and thus did not give much information about the main actions going on. However, they were wrong. They now know that the optical and x-ray emission are intrinsically linked, probably by the same immense magnetic fields that hurl material into near light speed jets.

The data shows that light output typically drops just before x-ray output undergoes a large spike. The rapid variations in the x-ray and optical emission must have a common origin. The cool thing about discovering such patterns that stand out amidst chaotic fluctuations of light is that they give us a new handle on understanding the underlying physics. The best candidate is the strong magnetic fields as the dominant process behind it all.

So again what we once thought was wrong, we learned something new, but realize how much we don’t know yet. This data is a new clue about very mysterious and not yet understood systems. We still don’t know exactly what’s going on in these dynamic systems, but we have one more piece of the puzzle.

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Medium Sized Black Holes Might be a Reality

Monday, October 6th, 2008 by Bellatrix

For many years astronomers have debated and speculated about the existence of intermediate mass black holes. Well an answer to the question may now be available. It looks as though one of these mysterious types of black holes has been discovered in one of the Milky Way’s globular clusters. However, this discovery may not have all the answers to the mystery.

For many years the only kinds of black holes that have been found are stellar sized ones and super massive ones. Stellar sized black holes form as a result of the death of a massive star and can range up to maybe 50 solar masses. Super massive black holes reside at the center of almost all galaxies and weigh millions if not billions of solar masses. But observations were lacking for any black holes in between these sizes.

Globular Clusters are dense formulations of stars that orbit in the outskirts of a galaxy. They reside in what is known as the halo of the galaxy. They tend to be very old and are generally no longer creating new stars. They are fairly common, we know of about 200 hundred that belong to the Milky Way.

So it was in the globular cluster known as Omega Centauri that this intermediate black hole is thought to reside. Omega Centauri is one of the largest and most massive clusters belonging to the Milky Way, and is about 17,000 light years from earth. Using NASA’s Hubble space telescope and the Gemini observatory in Chile astronomers were able to note that the stars at the center of the cluster were orbiting something with very fast speeds. These fast speeds and the absence of anything we can see suggests it is a black hole that the stars are orbiting. The astronomers then used theoretical models combined with this data to calculate the mass of this black hole at about 40,000 solar masses.

This is actually the second discovery of a medium sized black hole suggesting the first was not just a fluke and that they may be common. However, no theory currently exists to explain why they exist, that is, to explain how they could have formed. However, there existence might be important for the theory of how super massive black holes form. One of the current theories states that in an early galaxy a “seed” black hole would be needed of about this size. The black hole would feed to grow to super massive size.

However, one issue might be that Omega Centauri is not a normal globular cluster, and many theorize that it is actually the remnant of a dwarf galaxy that was gobbled up by the Milky Way. Models that compare data of galaxy mass with super massive black hole mass show that a dwarf galaxy like the one predicted for Omega Centauri would have had a black hole of this size. So this would suggest not that these intermediate black holes are the seeds for super massive ones but actually play the same role as the super massive ones on a smaller scale.

More of these intermediate sized black holes will need to be discovered before any questions about formation or purpose can be answered.

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Little Stars with Jets

Monday, September 29th, 2008 by Bellatrix

There are many objects in the universe that have jets of material exploding from them. A few examples are neutron stars, black holes, quasars, and protostars. Well now we can add brown dwarfs to that list. One wonders what causes these jets and if brown dwarfs can have them what’s next?

A brown dwarf is like a failed star. It’s cool and small, with a mass range of between 10 to 90 Jupiter masses. These objects are not massive enough to start nuclear burning like normal stars. They can be hard to observe since they are so small and don’t give off nearly as much light as a normal star. There is some debate about how to distinguish a brown dwarf from a giant planet like Jupiter. There are some differences; they all have about the same radii so if the mass is higher than about 10 Jupiter masses, they have a higher density and are usually not considered a planet. Also with brown dwarfs water is always found in a gaseous state where in giant planets it condenses to ice; also planets usually have ammonia in their atmospheres while brown dwarfs do not.

Now the brown dwarf called 2MASS1207-3932 has a mass of about 24 Jupiter masses with a companion planet of about 5 Jupiter masses. This brown dwarf also has a disk around it like that seen in young stars. This is the smallest object ever observed to have a jet. The jet is moving at a speed of a few kilometers per second and stretches about 1 billion kilometers; it is also much smaller and less bright than jets seen in regular stars. Astronomers had observed jets from one other brown dwarf, so with this new discovery a pattern is emerging. Its discovery suggests that these brown dwarfs form in a similar manner to normal stars but also that outflows are driven out by objects as massive as hundreds of millions of solar masses down to Jupiter-sized objects.

Astronomers were not able to observe the jets directly. Astronomers had to use the powerful Very Large Telescope (VLT) , and only an instrument called UVES could provide the sensitivity and resolution required to “see” the jet. The results highlight the incredible level of quality of instruments available today. With ever more powerful and sensitive instruments we are observing more of these faint objects and are able to learn much more about brown dwarfs, their properties, and how they develop.

This discovery tells us more about the development of brown dwarfs, but also raises some new questions. Does this mean that giant planets also have jets that we haven’t detected yet? If not, why not? What is the cut off threshold between the two? Also, what role exactly do these jets play in the life of the brown dwarf? If not all brown dwarfs have jets, what are the resulting differences between ones that do and ones that don’t? Hopefully, as we are able to observe more of these objects with our better instruments we will learn the answers to these questions.

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