Space and astronomy news
Imagine an object with the mass of the Sun, crushed down to the size of Manhattan. Now set that object spinning hundreds of times a second, blasting out powerful beams of radiation like a lighthouse. That’s a pulsar, one of the most exotic objects in the Universe.
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The Crab Nebula, or M1 (the first object in Messier’s famous catalog), is a supernova remnant and pulsar wind nebula. The name – Crab Nebula – is due to the Earl of Rosse, who thought it looked like a crab; it’s not in the constellation Cancer (the Crab), rather Taurus (the Bull).
The supernova which gave rise to the Crab Nebula was seen widely here on Earth in 1054 (and so it’s called SN 1054 by astronomers); it is perhaps the most famous of the historical supernovae. It is certainly one of the brightest (estimated to be –7 at peak), partly because it is so close (only 6,300 light-years away), and partly because it’s not hidden by dust clouds. The expansion of the nebula – as in seen-to-be-getting-bigger, rather than the-gas-is-moving-very-fast – was first confirmed in 1930.
As it was a core collapse supernova (a massive star which ran out of fuel), it left behind a neutron star; by chance, we are in line with its ‘lighthouse beam’, so we see it as a pulsar (all young neutron stars are pulsars, but not all of them have beams which point to us in one part of the cycle). It’s a pretty fast pulsar; the neutron star rotates once every 33 milliseconds. Because it’s so young and so close, the Crab Nebula pulsar was the first to be detected in the visual waveband, and also in x-rays and gamma rays. Being the source of the tremendous output of energy, from both the pulsar wind nebula and the pulsar itself, and as energy is conserved, the pulsar is slowing down, at a rate of 15 microseconds per year.
The inner part of the Crab Nebula, the pulsar wind nebula, contains lots of really hot (‘relativistic’) electrons spiraling around magnetic fields; this creates the eerie blue glow … synchrotron radiation. This makes the Crab Nebula one of the brightest objects in the x-ray and gamma ray region of the electromagnetic spectrum, and as it is a relatively steady source (unlike most high energy objects) it has given its name to a new astronomical unit, the Crab. For example, a new x-ray source may be 2 mCrab (milli-Crab), meaning 0.002 times as strong an x-ray source as the Crab Nebula.
This SEDS page has a lot more information on the Crab Nebula, both historical and contemporary.
Such an intensively studied object, no wonder there are lots of Universe Today stories on it; for example Nearly a Thousand Years After the Death of a Star, Giant Hubble Mosaic of the Crab Nebula, The Peculiar Pulsar in the Crab Nebula, Astronomers Locate High Energy Emissions from the Crab Nebula, and Evidence of Supernovae Found in Ice Core Sample.
Astronomy Cast’s Neutron Stars and Their Exotic Cousins has more on pulsars, and Nebulae more on nebulae.
Sources: Caltech Astronomy, SEDS, Stanford University SLAC
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A high-school student from West Virginia has discovered a new astronomical object, a strange type of neutron star called a rotating radio transient. Lucas Bolyard, a sophomore at South Harrison High School in Clarksburg, WV, made the discovery while participating in a project in which students are trained to search through data from the Robert C. Byrd Green Bank Telescope (GBT). Bolyard made the discovery in March, after he already had studied more than 2,000 data plots from the GBT and found nothing.
The project is the Pulsar Search Collaboratory (PSC), which allows students to do real scientific research by looking at data from the GBT, the largest radio telescope in the US.
“Lucas is one of the most enthusiastic students involved in the project,” said Duncan Lorimer, astronomer from West Virginia University. “He’s one of these youngsters that never gives up, he’s very persistent and he has all the attributes that a scientist should have.”
Rotating radio transients are thought to be similar to pulsars, superdense neutron stars that are the corpses of massive stars that exploded as supernovae. Pulsars are known for their lighthouse-like beams of radio waves that sweep through space as the neutron star rotates, creating a pulse as the beam sweeps by a radio telescope. While pulsars emit these radio waves continuously, rotating radio transients emit only sporadically, one burst at a time, with as much as several hours between bursts. Because of this, they are difficult to discover and observe, with the first one only discovered in 2006.
“This neutron star is rotating very rapidly, so you have something the size of city with the mass of the sun, spinning incredibly rapidly,” said Lorimer “which also has an incredibly large magnetic field which is how we detect it with radio telescopes.”
“These objects are very interesting, both by themselves and for what they tell us about neutron stars and supernovae,” said Maura McLaughlin, also from WVU. “We don’t know what makes them different from pulsars — why they turn on and off. If we answer that question, it’s likely to tell us something new about the environments of pulsars and how their radio waves are generated.”
“They also tell us there are more neutron stars than we knew about before, and that means there are more supernova explosions. In fact, we now almost have more neutron stars than can be accounted for by the supernovae we can detect,” she added.
“I was home on a weekend and had nothing to do, so I decided to look at some more plots from the GBT,” Bolyard said. “I saw a plot with a pulse, but there was a lot of radio interference, too. The pulse almost got dismissed as interference,” he added.
Nonetheless, he reported it, and it went on a list of candidates for McLaughlin and Lorimer to re-examine, scheduling new observations of the region of sky from which the pulse came. Disappointingly, the follow-up observations showed nothing, indicating that the object was not a normal pulsar. However, the astronomers explained to Bolyard that his pulse still might have come from a rotating radio transient.
Confirmation didn’t come until July. Bolyard was at the NRAO’s Green Bank Observatory with fellow PSC students. The night before, the group had been observing with the GBT in the wee hours, and all were very tired. Then Lorimer showed Bolyard a new plot of his pulse, reprocessed from raw data, indicating that it is real, not interference, and that Bolyard is likely the discoverer of one of only about 30 rotating radio transients known.
Suddenly, Bolyard said, he wasn’t tired anymore. “That news made me full of energy,” he exclaimed. “My friends were really excited because they think I’m going to be famous!”
As of a year ago, Bolyard said he wouldn’t have thought of becoming astronomer, but this has given him second thoughts. “Making this discovery has made me very excited to get into a scientific field,” he said. “It’s a lot of hard work, but it’s worth it.”
The PSC, led by NRAO Education Officer Sue Ann Heatherly and Project Director Rachel Rosen, includes training for teachers and student leaders, and provides parcels of data from the GBT to student teams. The project involves teachers and students in helping astronomers analyze data from 1500 hours of observing with the GBT. The 120 terabytes of data were produced by 70,000 individual pointings of the giant, 17-million-pound telescope. Some 300 hours of the observing data were reserved for analysis by student teams.
Learn more about the PSC and Bolyard’s discovery on the Sept. 18 edition of 365 Days of Astronomy.
NRAO has a video about the discovery.
The student teams use analysis software to reveal evidence of pulsars. Each portion of the data is analyzed by multiple teams. In addition to learning to use the analysis software, the student teams also must learn to recognize man-made radio interference that contaminates the data. The project will continue through 2011.
“The students get to actually look through data that has never been looked through before,” Rosen said. From the training, she added, “the students get a wonderful grasp of what they’re looking at, and they understand the science behind the plots that they’re looking at.”
Source: NRAO
A group of astronomers have discovered something they never expected to find. The scientists were studying white dwarf stars, hoping to learn if white dwarfs could be responsible for the cosmic rays that zip through our galaxy and occasionally strike earth. But instead, what they found was that a certain white dwarf star known as AE Aquarii acts like a Pulsar, challenging our understanding of white dwarfs.
Astronomers had believed white dwarfs were inert stellar corpses that slowly cool and fade away, but this recently observed white dwarf star emits pulses of high-energy X-rays as it whirls around on its axis.
A group of astronomers from the US and Japan used the Suzaku X-Ray Observatory, a JAXA and NASA telescope in Earth orbit to make the new observations.
“AE Aquarii seems to be a white dwarf equivalent of a pulsar,” says Yukikatsu Terada, from the Institute of Physical and Chemical Research in Wako, Japan. “Since pulsars are known to be sources of cosmic rays, this means that white dwarfs should be quiet but numerous particle accelerators, contributing many of the low-energy cosmic rays in our galaxy.”
Some white dwarfs, including AE Aquarii, spin very rapidly and have magnetic fields millions of times stronger than Earth’s. These characteristics give them the energy to generate cosmic rays. But the Suzaku observatory also detected sharp pulses of hard X-rays. After analyzing the data, the astronomy team realized that the hard X-ray pulses match the white dwarf’s spin period of once every 33 seconds.
The hard X-ray pulsations are very similar to those of the pulsar in the center of the Crab Nebula. In both objects, the pulses appear like a lighthouse beam, and a rotating magnetic field is thought to be controlling the beam. Astronomers think that the extremely powerful magnetic fields are trapping charged particles and then flinging them outward at near-light speed. When the particles interact with the magnetic field, they radiate X-rays.
“We’re seeing behavior like the pulsar in the Crab Nebula, but we’re seeing it in a white dwarf,” says Koji Mukai of NASA Goddard Space Flight Center in Greenbelt, Md. The Crab Nebula is the shattered remnant of a massive star that ended its life in a supernova explosion. “This is the first time such pulsar-like behavior has ever been observed in a white dwarf.”
Original News Source: NASA/Goddard Spaceflight Center Press Release