科学家推测黑洞在超新星爆炸中扮演了新角色
Newborn Black Holes Boost Explosive Power of SupernovaeAn international team of scientists, including two astronomers from NASA's Marshall Space Flight Center in Huntsville, Ala., have observed a supernova with peculiar radio emission. In the Jan. 28 issue of Nature, the team -- led by Zsolt Paragi of the Joint Institute for Very Long Baseline Interferometry in Europe, or JIVE -- reveals new details of these highly energetic explosions.
Supernova SN 2007gr was discovered less than five days after its explosion at a distance of just 35 million light years away -- one of the closest Type Ic supernovae ever seen in radio waves from Earth. Supernovae mark the violent deaths of massive stars via core collapse, followed by a gigantic explosion which expels their outer layers in an expanding fireball. As the explosion ejecta expand, they cool and slow down, and their emission progressively move to longer wavelengths, from X-rays to radio waves. SN 2007gr was close enough and found early enough to be a great candidate for extensive follow-up observations.
"The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent of the speed of light," said Chryssa Kouveliotou, an astrophysicist at the Marshall Center who co-authored the new study. "Yet in this new object we're tracking gas moving some 20 times faster than this."
Type Ic supernovae also have been associated with another very energetic phenomenon in high energy astrophysics: Gamma Ray Bursts. Though most of the energy in these bursts is released by gamma-ray jets traveling almost at the speed of light no such fast expansion evidence has yet been observed from other Type Ic supernovae.
Kouveliotou quickly put together an international team of radio astronomers. Led by Paragi and JIVE, the team included 14 members from 12 institutions spread over seven countries -- including the United States, the Netherlands, Hungary, the United Kingdom, Canada, Australia and South Africa.
The team proposed to use the highest-resolution imaging technique -- Very Long Baseline Interferometry, or VLBI -- to collect the extremely faint emission from SN 2007gr and reveal details of the explosion process. With the VLBI technique, multiple radio telescopes thousands of kilometers from one another carry out measurements simultaneously. The astronomers exploited the electronic VLBI capabilities of the European VLBI Network, by which the data are streamed in real time from the telescopes to the joint institute's central data processor in the Netherlands.
A rapid analysis of the SN 2007gr data, obtained 22 days after initial discovery, showed that the source was still visible in the radio. Based on this result, the team carried out further observations with the European VLBI Network and the Green Bank Telescope in Pocahontas County, W.V. For the first time ever, scientists measured mildly relativistic expansion in such a source.
One other instrument, the Westerbork Synthesis Array Telescope played an important role in obtaining this result due to its large collecting area, which significantly improved the sensitivity of the VLBI observations. Westerbork telescope observations, combined with radio data from the Very Large Array in Socorro, N.M., provided an independent measurement of the total flux density, or brightness, of the source.
In the second VLBI observation, the source was less bright when measured in very high resolution with the global VLBI network of radio telescopes than it was with the Westerbork telescope. The team of radio astronomers concluded that the latter had captured all the radio waves coming from the supernova, while the VLBI zoomed in so much that it only observed part of the source.
"It was the synergy between these radio observatories that led to our discovery," said Alexander van der Horst, a NASA postdoctoral program fellow in Huntsville and a co-author on the Nature article. "Zooming in and out on the supernova quickly led us to the conclusion that the ejecta had to be expanding very fast."
Although it showed peculiar radio properties, SN 2007gr was otherwise a normal type Ic supernova. It appears that only a small fraction of the matter ejected in the explosion reached a velocity of at least half the speed of light -- what scientists call mildly relativistic speed. According to the emerging picture, this mildly relativistic matter was beamed into a bipolar narrow cone, or jet. The team concluded that it is possible that most or all type Ic supernovae produce bipolar jets, but the energy content of these outflows varies dramatically, whereas the total energy of the explosions is much more standard.
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For the first time, astronomers have uncovered two supernovae explosions with properties similar to a gamma-ray burst but without the gamma rays, leading them to suspect that newborn black holes are providing the extra boost.
High-speed jets, traveling at nearly the speed of light, are usually only associated with gamma-ray bursts, resulting from the dramatic finale of a massive star's life. Once the star has run out fuel it collapses into a neutron star or black hole, blasting the rest of its material out into space in a supernova explosion. Around one in one hundred core-collapse supernovae produce gamma-ray bursts, but the most common type of supernovae blasts the star’s material outward in a roughly-spherical pattern at speeds of just three percent of the speed of light.
“In every respect, these objects look like gamma-ray bursts, except that they produced no gamma rays,” says Alicia Soderberg at the Harvard-Smithsonian Center for Astrophysics. Soderberg's team studied supernova explosion SN 2009bb, which was discovered in March last year in the spiral galaxy NGC 3278, located about 130 million light-years away. The group found that their radio observations showed material racing away from the heart of the explosion at around 85 percent the speed of light.
Another supernova, SN 2007gr, was discovered in August 2007 in the 35 million light year distant galaxy NGC 1058, and studied by a team of astronomers including Chryssa Kouveliotou, Alexander van der Horst and Zsolt Paragi. The fastest outflows from this supernova reached around 60 percent that of the speed of light, but yet searching through archival data for possible gamma-ray signals revealed none.
It is very unusual that such low-energy radiation – radio waves – can signal a very high-energy event, and the only way to explain the observations, says Soderberg, is by a central engine powering the high speed jets. In this scenario, material falling toward the core enters a swirling disc surrounding the new neutron star or black hole, which produces jets of material accelerated from the poles of the disc.
Until now, no such engine-driven supernova had been found any way other than by detecting gamma rays emitted by it. “Discovering such a supernova by observing its radio emission, rather than through gamma rays, is a breakthrough,” says Soderberg. With the new capabilities of the Expanded VLA coming soon, the astronomers believe they will be able to find more examples in the future through radio observations than with gamma-ray satellites.
The absence of gamma-rays in these explosions is still a mystery. “We know that the gamma-ray emission is beamed in such blasts, and this one may have been pointed away from Earth and thus not seen,” suggests Soderberg. Another possibility is that the gamma rays were ‘smothered’ as they tried to escape the star. “This is perhaps the more exciting possibility since it implies that we can find and identify engine-driven supernovae that lack detectable gamma rays and thus go unseen by gamma-ray satellites.
The scientists hope that future observations will find what causes the difference between the “ordinary” and the “engine-driven” core-collapse supernovae. “There must be some rare physical property that separates the stars that produce the ‘engine-driven’ blasts from their more-normal cousins,” says Soderberg. “We’d like to find out what that property is.”
One popular idea is that such stars have an unusually low concentration of elements heavier than hydrogen, but this does not appear to be the case in this study. “We’ve now found evidence for the unsung crowd of supernovae – those with relatively dim and mildly relativistic jets that only can be detected nearby,” comments Kouveliotou. “These likely represent most of the population.”
The observations were made using the National Science Foundation’s Very Large Array (VLA) radio telescope, the Robert C. Byrd Green Bank Telescope in West Virginia, the European Very Long Baseline Interferometry Network and NASA's space-based Swift observatory, and the results published in the current issue of the journal Nature. 在内容上我没有直译 编辑了一下 整合了两篇新闻稿
两组分别由Zsolt Paragi牵头的NASA马歇尔空间飞行中心的天文学家和由哈佛大学史密森天文中心的天文学家Soderberg，对2007年爆发的超新星SN2007gr和2009年爆发的超新星SN2009bb进行了对比分析，这两个超新星爆发和伽马射线暴很类似，但是却没有发现伽马射线！所以他们推测是新诞生的黑洞产生了额外的影响。
一颗大质量的恒星在生命最后一刻以伽马射线暴的形式结束，还伴随着接近光速的高速喷流。恒星的燃料用完后，则会坍缩成中子星或者黑洞，在一次超新星爆炸中将所有剩余物质抛向太空。超新星爆炸通过一种极其“暴力”的方式由核心开始崩溃，紧接着就是最猛烈的殉爆，绝对光度超过太阳光度100亿倍，中心温度可达100亿摄氏度，向外急剧膨胀的外层高温气体层最高可达数倍太阳系直径。随着爆炸喷射物的范围的扩大，逐渐冷却放慢，波长也逐渐向长波方向移动，由X射线波长移动向无线电波，大约只有一小部分的超新星爆发产生伽马射线暴，大多数的超新星爆发只是把物质以光速百分之三向外喷射。
2007年，距地球3500万光年的超新星SN2007gr爆炸五天后的情形被观测到，其也是离地球最近的超新星之一，而且在发现时间上具有优势，可以为后续观测取得不错的效果。在起初发现之后的22天内，快速分析SN 2007gr的数据，显示该射电源仍然可以在无线电波长探测到，Paragi牵头的欧洲甚长基线干涉测量联合研究小组，使用了欧洲甚长基测量网和西佛吉尼亚的绿岸射电望远镜对该射电源进行了轻度相对论扩张观测，韦斯特博克合成阵列望远镜在数据收集方面扮演了重要角色，由于其巨大的收集区域，显著提高了甚长基干涉测量的灵敏度。联合来自新墨西哥州Socorro甚大阵列的无线电数据，提供了一个独立测量射电源的总通量密度或者亮度的渠道。发现SN2007gr物质喷流达到了0.6倍光速，却从未在数据中发现伽马射线。哈佛史密森天文中心的Soderberg研究了2009年发现的距地球1亿3000万光年的超新星SN 2009bb的爆发，发现了射电源，并且显示喷射物质以0.85倍的光速远离爆炸中心。
图注：具有“中央引擎驱动”的超新星爆发
“如此低能量的射电波却可以映射成高能射电波是相当不寻常的，所以只有一种方法解释：其核心有个“中央引擎”驱动高速喷射流。这个中央引擎是一个新生的中子星或者是一个黑洞，它产生物质喷流来自吸积盘的极点。” Soderberg说，“直到目前还没有探测到伽马射线辐射来自这类具有中央引擎驱动的超新星，发现这类超新星是通过观测射电源，而不是通过观测伽马射线，是一个突破。随着新一代的射电阵列的建成，天文学家相信他们在将来使用射电观测要比通过伽马射线观测发现更多的超新星。”
至于这类超新星爆发为什么没有伽马射线仍然是一个迷，Soderberg解释：“我们知道伽马射线在这样的爆炸中是被汇集成类似定向波束了，而并没有指向地球，所以不能被观测到。另一种可能是当伽马射线逃逸时被“窒息”了，也就是说逃逸不了了，这种解释提出后更令人兴奋，我们可以发现并确认具有“中央引擎驱动”的没有伽马射线特征的超新星，也因此他们不可能被伽马射线探测卫星观测到。
科学家希望在将来的观测中找到，“普通类型”和“中央引擎驱动”超新星的区别。这里必然存在某种特别的物理现象导致了这种“中央引擎驱动”爆炸。还有一种流行的观点认为，一种不寻常的低浓度元素比氢元素重，但是这个观点不在这次研究范围之内。Kouveliotou说：“我们现在发现的证据来自那些只有相对论喷流和轻度相对论喷流的超新星，现在看起来，这些超新星代表了今后研究的方向。
还没来得及跑出来 就被事件视界出口转内销了... 至少能减少地球被喷的机会 翻译得很不错啊。极点建议翻译为两极。
我更愿意相信是黑洞喷流没朝向地球，而不是所谓被黑洞吸收了。要知道，就算是强大的活动星系核心的黑洞，也不会把黑洞喷流给拉回来的，更不用说恒星级黑洞了。 本帖最后由 gohomeman1 于 2010-2-2 10:28 编辑
我比较孤陋，好像超新星爆发时的能量转移机制还不清晰吧。我只知道，对Ia超新星的模拟已经比较充分，能够基本解释其爆发过程了。而普通模式的，即II型超新星，最大的问题是怎么形成爆发的，能量转移机制貌似没说清。以前一般是认为Fe/Ni核心收缩为中子星后，大量引力势能通过激波的方式转移给核心区，并继续在恒星内传播，所过之处触发了外部多个壳层（比如O、Ne、C、He）瞬间的极快速核反应，巨大的能量释放导致整个恒星立刻爆发。但后来认为这种激波无法传递给外面的壳层，但是爆发却是实实在在的。
所谓中央引擎，就是说中心已经收缩为黑洞，并在恒星内核区形成了吸积盘的过程。这类恒星初始质量至少在太阳的15倍以上，一般应该在20倍以上吧。现在有好多观测表明，此类超新星的前身是蓝超巨星而不是红超巨星。爆发时，恒星外部还是球形的气壳，中心黑洞形成的相对论性喷流冲破了气壳并形成强烈的辐射（就像图2那样），当然这种辐射比中心喷流本身朝向我们的辐射弱多了 ，这也是说为何观测到的强度相对低得多，而且主要是射电波的原因。 翻译得很不错啊。极点建议翻译为两极。
我更愿意相信是黑洞喷流没朝向地球，而不是所谓被黑洞吸收了。要知道，就算是强大的活动星系核心的黑洞，也不会把黑洞喷流给拉回来的，更不用说恒星级黑洞了。 ...
gohomeman1 发表于 2010-2-2 09:59 http://www.astronomy.com.cn/bbs/images/common/back.gif
谢谢指点····嘿嘿 恒星级的黑洞，靠恒星气体的吸积数量，能够达到产生喷流吗？对黑洞我了解太少，原本以为只有星系核那种规模的才能吸积出喷流 8# kajiryouji
恒星级黑洞都是通过吸积盘的喷流来确认的，另外一个条件是在双星系统中，这样才能算出其质量是否超过了3倍太阳。
黑洞喷流具有是相对论性喷流，速度非常高，这与白矮星、中子星的吸积盘喷流也是一个大区别。
不要以为吸积的物质很多，其实每年消耗的很少。黑洞引擎的质量——能量转换效率非常高，比热核反应能转换的质量大得多了。 不要以为吸积的物质很多，其实每年消耗的很少。黑洞引擎的质量——能量转换效率非常高，比热核反应能转换的质量大得多了（原来如此，怪不得我说类星体辐射量这么巨大。） 翻译的不错 支持一下 楼主真乃神人也！赞一个！ ::070821_01.jpg::来这学习下~::070821_14.jpg::
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