Previous analysis showed that with X-ray and gamma ray energies above 30 keV, the Crab is the strongest persistent source in the sky. Its flux (energy emission) was known to be above 10 eV.
However, recent work has shown the energy levels are much higher than previously thought. Scientists found emissions at more than 100 GeV (gigaelectronvolts) – 100 billion times more energetic than visible light.
The creation of the Crab Nebula corresponds to the bright SN 1054 supernova, which was recorded by Chinese astronomers in 1054 AD. The Crab Nebula itself was first observed in 1731 by John Bevis. The nebula was independently rediscovered in 1758 by Charles Messier as he was observing a bright comet. Messier catalogued it as the first entry in his catalogue of comet-like objects. The Earl of Rosse observed the nebula at Birr Castle in 1848, and referred to the object as the Crab Nebula because a drawing he made of it looked like a crab.
In the early 20th century, the analysis of early photographs of the nebula taken several years apart revealed that it was expanding. Tracing the expansion back revealed that the nebula must have become visible on Earth about 900 years ago. Historical records revealed that a new star bright enough to be seen in the daytime had been recorded in the same part of the sky by Chinese astronomers in 1054. Given its great distance, the daytime 'guest star' observed by the Chinese could only have been a supernova—a massive, exploding star, having exhausted its supply of energy from nuclear fusion and collapsed in on itself.
Recent analysis of historical records have found that the supernova that created the Crab Nebula probably appeared in April or early May, rising to its maximum brightness of between apparent magnitude −7 and −4.5 (brighter than everything in the night sky except the Moon) by July. The supernova was visible to the naked eye for about two years after its first observation. Thanks to the recorded observations of Far Eastern and Middle Eastern astronomers of 1054, Crab Nebula became the first astronomical object recognized as being connected to a supernova explosion.
Trackball
In computing, a trackball is a pointing input device. It consists of a ball held by a socket containing sensors to detect a rotation of the ball about two axes. It is like an upside-down mouse with a ball that sticks out. The user rolls the ball with the thumb, fingers or the palm of the hand to move a cursor and can click one of two buttons (identical to mouse buttons) near the trackball to select desktop objects or position the cursor for text entry. Currently only three major companies: Logitech, A4Tech and Kensington make many of them. Microsoft was once a major producer.
Large trackballs are sometimes seen on computerized special-purpose workstations, such as the radar consoles in an air traffic control room, or sonar equipment on a ship or submarine, or computer-aided design. Potentiometers capture the trackball orientation.
Many notebook and laptop computers in the late 20th century used a small trackball. It was usually located in front of the keyboard toward the user. IBM's ThinkPad series of notebook computers used a "pointing stick", called a TrackPoint, that was integrated into the middle of the keyboard keys.
Binary pulsar
A binary pulsar is a pulsar with a binary companion, often a white dwarf or neutron star. In at least one case, the double pulsar PSR J0737-3039, the companion star is another pulsar as well.
Binary pulsars are one of the few objects which allow physicists to test general relativity in the case of a strong gravitational field. Although the binary companion to the pulsar is usually difficult or impossible to observe, the timing of the pulses from the pulsar can be measured with extraordinary accuracy by radio telescopes. Binary pulsar timing has indirectly confirmed the existence of gravitational radiation and verified Einstein's general theory of relativity.
Two objects orbiting do not do so in absolutely circular paths. the paths are virtually always elliptical. So twice a circuit they are closest, and twice a circuit they are furthest away. This is obvious for the Earth and Sun, but the idea applies much more widely.
When the two bodies are close, the gravitational field is stronger, and the passage of time is slowed. With pulsars, the time between pulses (or ticks) is lengthened. As the pulsar clock travels more slowly through the weakest part of the field it regains time. This is a relativistic time delay. It is the difference between what one would expect to see if the pulsar were moving at a constant distance and speed around its companion, and what is actually observed.
Binary pulsars are one of the few tools scientists have to detect evidence of gravitational waves. Einstein’s theory of general relativity predicts that two neutron stars would emit gravitational waves as they orbit a common center of mass, which would carry away orbital energy, and cause the two stars to draw closer together. As the two stellar bodies draw closer to one another, often one pulsar will absorb matter from the other, causing a violent accretion process. This interaction can heat the gas being exchanged between the bodies and produce X-ray light which can appear to pulsate, causing binary pulsars to occasionally be referred to as X-ray binaries. This flow of matter from one stellar body to another is known as an accretion disk. Millisecond pulsars (or MSP's) create a sort of "wind", which in the case of binary pulsars can blow away the magnetosphere of the neutron stars and have a dramatic effect on the pulse emission.
The first binary pulsar, PSR B1913+16 or the "Hulse-Taylor binary pulsar" was discovered in 1974 at Arecibo by Joseph Taylor and Russell Hulse, for which they won the 1993 Nobel Prize in Physics. Pulses from this system have been tracked, without glitches, to within 15 μs since its discovery.
The 1993 Nobel Prize was awarded to Joseph Taylor and Russell Hulse after they discovered two such stars. While Hulse was observing a new pulsar, named PSR B1913+16, he noticed that the frequency with which it pulsed fluctuated. It was concluded that the simplest explanation was that the pulsar was orbiting another star very closely at a high velocity. Hulse and Taylor determined that the stars were equally heavy by observing these pulse fluctuations, which led them to believe the other spacial object was also a neutron star.
The observations made of the orbital decay of this star system was a near perfect match to Einstein’s equations. Relativity predicts that over time a binary system’s orbital energy will be converted to gravitational radiation. Data collected by Taylor and his colleagues of the orbital period of PRS B1913+16 supported this relativistic prediction. They reported in 1983 that there was a difference in the observed minimum separation of the two pulsars compared to that expected if the orbital separation had remained constant. In the decade following its discovery the system’s orbital period had decreased by about 76 millionths of a second per year. This means the pulsar was approaching its maximum separation more than a second earlier than it would have if the orbit had remained the same. Subsequent observations continue to show this decrease.
Zacarias Moussaoui
Zacarias Moussaoui (, Zakariyyā Mūsawī; born May 30, 1968) is a French citizen who plead guilty to conspiring to kill citizens of the United States as part of the September 11 attacks. As a result of his conviction, he is serving a life sentence without parole at the Federal ADX Supermax prison in Florence, Colorado.
Massari