文章来源:未知 文章作者:enread 发布时间:2011-01-14 06:39 字体: [ ]  进入论坛

The Crab1 Nebula2(蟹状星云) , once considered to be a source of energy so stable that astronomers3 used it to calibrate4 their instruments, is dimming. LSU physicists5 Mike Cherry, Gary Case and graduate student James Rodi, together with an international team of colleagues using the Gamma-ray Burst Monitor, or GBM, on NASA's Fermi gamma-ray space telescope, discovered the anomaly(异常,不规则) . This revelation has proven astonishing for astronomers. The Crab Nebula, one of the most studied objects in the sky, is the wreckage6 of a star that exploded in 1054. Considered a cornerstone of astronomical7 research, it even inspired its own unit of measurement, the "millicrab," which is used as a standard for measuring the intensity8 from other high-energy sources.

The GBM instrument was launched into orbit in summer 2008. This summer, the scientists were working on a catalog of the high energy X-ray and gamma ray signals detected mainly from sources in the galaxy9 powered by black holes and neutron10 stars. As they were preparing the catalog, which has been accepted for publication in the Astrophysical Journal, they realized that the intensity coming from the Crab Nebula was dimming.

"We were using the Crab as our calibration source and comparing the other high energy sources to it," said Case. "But as we collected more and more data, we noticed that the intensity we were measuring for the Crab was going down. This was a rather startling discovery, and it took awhile for us to believe it."

The initial suspicion was that the instrument was losing sensitivity. The team then gathered data from three other sensitive X-ray and gamma ray observatories11 currently in orbit – NASA's Swift and Rossi X-Ray Timing12 Explorer, or RXTE, and the European Space Agency's International Gamma-Ray Astrophysics Laboratory, or INTEGRAL. The result was that all four instruments were seeing the same decrease in intensity of about 7 percent since the summer of 2008.

"Nearly every other source of high energy radiation in the sky shows evidence of explosive, time-variable, transient(短暂的) activity. The Crab was the exception," said Cherry. "It was the only object that was bright enough and steady enough to serve as a 'standard candle.'"

Colleen Wilson-Hodge, an astrophysicist at NASA's Marshall Space Flight Center in Hunstville, Ala., recently presented the findings at the American Astronomical Society meeting in Seattle.

"Now, for the first time, we're clearly seeing how much our candle flickers13(闪烁,颤动) ," she said. The findings will be published in the Astrophysical Journal Letters.

Additional analysis showed that the Crab Nebula has brightened and dimmed several times since 1999 on approximately a three-year time scale. The current decrease is the largest so far observed, and the international team will continue to monitor the Crab Nebula to observe how much the decline continues.

The cause of the changes is not understood, but apparently14 involves changes in the magnetic fields close to the nebula's central neutron star. Because of this news, the scientists said that astronomers will now need to find new ways to calibrate their instruments in flight and to explore the possible effects of the inconstant Crab Nebula emissions15 on past findings.


1 crab xoozE     
  • I can't remember when I last had crab.我不记得上次吃蟹是什么时候了。
  • The skin on my face felt as hard as a crab's back.我脸上的皮仿佛僵硬了,就象螃蟹的壳似的。
2 nebula E55zw     
  • A powerful telescope can resolve a nebula into stars.一架高性能的望远镜能从星云中分辨出星球来。
  • A nebula is really a discrete mass of innumerous stars.一团星云实际上是无数星体不连续的集合体。
3 astronomers 569155f16962e086bd7de77deceefcbd     
n.天文学者,天文学家( astronomer的名词复数 )
  • Astronomers can accurately foretell the date,time,and length of future eclipses. 天文学家能精确地预告未来日食月食的日期、时刻和时长。 来自《简明英汉词典》
  • Astronomers used to ask why only Saturn has rings. 天文学家们过去一直感到奇怪,为什么只有土星有光环。 来自《简明英汉词典》
4 calibrate vTvyu     
  • Pesticide levels in food are simply too difficult to calibrate.食品中杀虫剂的含量很难精确测定。
5 physicists 18316b43c980524885c1a898ed1528b1     
物理学家( physicist的名词复数 )
  • For many particle physicists, however, it was a year of frustration. 对于许多粒子物理学家来说,这是受挫折的一年。 来自英汉非文学 - 科技
  • Physicists seek rules or patterns to provide a framework. 物理学家寻求用法则或图式来构成一个框架。
6 wreckage nMhzF     
  • They hauled him clear of the wreckage.他们把他从形骸中拖出来。
  • New states were born out of the wreckage of old colonial empires.新生国家从老殖民帝国的废墟中诞生。
7 astronomical keTyO     
  • He was an expert on ancient Chinese astronomical literature.他是研究中国古代天文学文献的专家。
  • Houses in the village are selling for astronomical prices.乡村的房价正在飙升。
8 intensity 45Ixd     
  • I didn't realize the intensity of people's feelings on this issue.我没有意识到这一问题能引起群情激奋。
  • The strike is growing in intensity.罢工日益加剧。
9 galaxy OhoxB     
  • The earth is one of the planets in the Galaxy.地球是银河系中的星球之一。
  • The company has a galaxy of talent.该公司拥有一批优秀的人才。
10 neutron neutron     
  • Neutron is neutral and slightly heavier than the proton.中子是中性的,比质子略重。
  • Based on the neutron energy,the value of weighting factor was given.根据中子能量给出了相应的辐射权重因子的数值。
11 observatories d730b278442c711432218e89314e2a09     
n.天文台,气象台( observatory的名词复数 )
  • John Heilbron, The Sun in the Church: Cathedrals as Solar Observatories, 3-23. 约翰.海耳布隆,《教会里的太阳:教堂即太阳观测台》,第3-23页。 来自互联网
  • Meteorologists use satellites, land observatories and historical data to provide information about the weather. 气象学家使用卫星、上天文台和历史资料来提供有关天气的信息。 来自互联网
12 timing rgUzGC     
  • The timing of the meeting is not convenient.会议的时间安排不合适。
  • The timing of our statement is very opportune.我们发表声明选择的时机很恰当。
13 flickers b24574e519d9d4ee773189529fadd6d6     
电影制片业; (通常指灯光)闪烁,摇曳( flicker的名词复数 )
  • The fire flickers low. 炉火颤动欲灭。
  • A strange idea flickers in my mind. 一种奇怪的思想又在我脑中燃烧了。
14 apparently tMmyQ     
  • An apparently blind alley leads suddenly into an open space.山穷水尽,豁然开朗。
  • He was apparently much surprised at the news.他对那个消息显然感到十分惊异。
15 emissions 1a87f8769eb755734e056efecb5e2da9     
排放物( emission的名词复数 ); 散发物(尤指气体)
  • Most scientists accept that climate change is linked to carbon emissions. 大多数科学家都相信气候变化与排放的含碳气体有关。
  • Dangerous emissions radiate from plutonium. 危险的辐射物从钚放散出来。
TAG标签: energy space research