Top Ten for Chandra’s Fourth Anniversary
Chandra X-Ray Observatory — August 25, 2003 :: Among its array of revelations in the past 12 months, news about black holes pulled in most of the headlines with four of the top five stories from Chandra during its fourth year in operation.
#1 — The Crab Nebula
But it was the venerable Crab Nebula, with the spectacular Chandra/Hubble movie production of its amazing pulsar and dazzling tornado of high-energy particles and magnetic fields, that captured first place as our most popular image.
In this image, A wisp can be seen moving outward at half the speed of light from the upper right of the inner ring around the pulsar. The wisp appears to merge with a larger outer ring that is visible in both X-ray and optical images.
The inner X-ray ring consists of about two dozen knots that form, brighten and fade. As a high-speed wind of matter and antimatter particles from the pulsar plows into the surrounding nebula, it creates a shock wave and forms the inner ring. Energetic shocked particles move outward to brighten the outer ring and produce an extended X-ray glow.
Enormous electrical voltages generated by the rotating, highly magnetized neutron star accelerate particles outward along its equator to produce the pulsar wind. These pulsar voltages also produce the polar jets seen spewing X-ray emitting matter and antimatter particles perpendicular to the rings.
#2 — Supermassive Black Holes in Galaxy NGC 6240
Attracting the top spot in media coverage was the galaxy NGC 6240, which was discovered to have not one, but two supermassive black holes orbiting each other in the nucleus of the galaxy.
As the first definite identification of a binary supermassive black hole system, this discovery stimulated a great deal of scientific interest, and a flood of questions to our web site. People wanted to know when the two black holes would merge (a few hundred million years), whether the galaxy would survive (yes), whether we would survive (yes), and if the merger had happened already, given that NGC 6240 is 400 million light years away (probably).
Previous X-ray observatories had shown that the central region was an X-ray source, but astronomers did not know what was producing the X-rays. Radio, infrared, and optical observations had detected two bright nuclei, but their exact nature also remained a mystery.
Chandra was able to show that the X-rays were coming from the two nuclei, and determine their X-ray spectra. These cosmic fingerprints revealed features that are characteristic of supermassive black holes – an excess of high-energy photons from gas swirling around a black hole, and X-rays from fluorescing iron atoms in gas near black holes.
Over the course of the next few hundred million years, the two supermassive black holes, which are about 3000 light years apart, will drift toward one another and merge to form one larger supermassive black hole. This detection of a binary black hole supports the idea that black holes grow to enormous masses in the centers of galaxies by merging with other black holes.
NGC 6240 is a prime example of a “starburst” galaxy in which stars are forming, evolving, and exploding at an exceptionally rapid rate due to a relatively recent merger (30 million years ago). Heat generated by this activity created the extensive multimillion degree Celsius gas seen in this image.
#3 — Supermassive black hole Sagittarius A*
The supermassive black hole Sagittarius A* (Sgr A*) located at the center of our galaxy makes a repeat appearance near the top of our list in both scientific importance and popularity.
Since it is the nearest supermassive black hole, it seems a good bet to remain high on everyone’s list in years to come as astronomers seek a much better understanding of how supermassive black holes grow and how they influence the evolution of the galaxies in which they reside.
During the two-week observation period, Sgr A* flared up in X-ray intensity half a dozen or more times. The cause of these outbursts is not understood, but the rapidity with which they rise and fall indicates that they are occurring near the event horizon, or point of no return, around the black hole.
Even during the flares the intensity of the X-ray emission from the vicinity of the black hole is relatively weak. This suggests that Sgr A*, weighing in at 3 million times the mass of the Sun, is a starved black hole, possibly because explosive events in the past have cleared much of the gas from around it.
Evidence for such explosions was revealed in the image – huge lobes of 20 million-degree Centigrade gas (the red loops in the image at approximately the 2 o’clock and 7 o’clock positions) that extend over dozens of light years on either side of the black hole. They indicate that enormous explosions occurred several times over the last ten thousand years.
Further analysis of the Sgr A* image is expected to give astronomers a much better understanding of how the supermassive black hole in the center of our galaxy grows and how it interacts with its environment. This knowledge will also help to understand the origin and evolution of even larger supermassive black holes found in the centers of other galaxies.
#4 — Chandra Deep Field North
Another black hole story that could make big news in the future was first reported this year. By combing the data from the Chandra Deep Field North and Chandra Deep Field South images, astronomers made a high-energy “core sample” of the early universe.
In addition to hundreds of supermassive black holes, they found seven mysterious sources that were not detected by the Hubble Space Telescope. These X-ray sources are candidates for the most distant supermassive black holes ever observed.
More than 500 X-ray sources are present in this high-energy core sample of the early universe. Most of the sources are supermassive black holes located in the centers of galaxies. If the number of supermassive black holes seen in this patch of the sky is typical, the total number detectable over the whole sky at this level of sensitivity would be 300 million.
By combining the Chandra and Hubble data for this field, astronomers can take a census of the fraction of young galaxies that contain active supermassive black holes back to a time when the universe was only about one billion years old, less than 10% of its present age. The data show that these very distant supermassive black holes are rare, more so than expected.
The data indicate that it takes about 700 million years for a supermassive black hole to accumulate the millions of solar masses of gas needed to produce a powerful X-ray source. The relatively slow growth of the supermassive black holes may be due to a reduced gas supply created when early generations of massive stars exploded as supernovas and blew gas out of the galaxies.
The Rest of the Top 10
Rounding out the top five stories this past year was a series of Chandra images of oppositely directed jets from XTE J1550, a stellar black hole in our galaxy. These observations made at intervals over a four-year period allowed scientists to trace the evolution of large-scale X-ray jets for the first time. Such jets are thought to be one of the primary ways that black holes interact with their environment. Jets from XTE J1550
Jets figured in one of the most beautiful images of the year, which was also one of the most popular with the press and the public. The composite X-ray, radio and optical image of the galaxy Centaurus A presents a stunning tableau of a galaxy in turmoil. A broad band of dust and cold gas is bisected at an angle by opposing jets of high-energy particles blasting away from the supermassive black hole in its nucleus. Two large arcs of multimillion degree gas indicate that a titanic explosion rocked the galaxy about ten million years ago. Galaxy Centaurus A
Four other discoveries that made the top ten list illustrate the wide range of phenomena that can be studied by Chandra:
X-rays produced by fluorescent radiation from oxygen atoms gave astronomers a unique look at the sparse upper atmosphere of Mars, about 45 kilometers (45 miles) above its surface, and provided evidence for a faint halo of X-rays that extends out to 7,000 kilometers above the surface. Upper Atmosphere of Mars
The image of M83 revealed an ethereal beauty, with a graceful spiral of neutron stars and black holes around a blazing, starburst heart of multimillion degree gas. Spiral Galaxy M83
A fascination with the macabre no doubt accounted for some of the popularity of the “Black Widow” pulsar, a neutron star that is destroying its stellar companion. The science was also intriguing, as the image provided the first direct evidence of an elongated cocoon of high-energy particles around this old pulsar and that is speeding through interstellar space. Black Widow Pulsar
Finally, Chandra’s image of Tycho’s supernova remnant reveals in elaborate detail the turbulent debris created by the stellar catastrophe that was observed by the Danish astronomer Tycho Brahe in the year 1572. We have come a long way from that observation –made before the invention of the telescope– which provided the first evidence that stars are not eternal. Tycho’s supernova remnant
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