2004年4月28日通讯！

vmzy

Newsletter #22
April 28, 2004  
SETI@home Reobservation Report
Eric Korpela, Jeff Cobb, Dan Werthimer, Matt Lebofsky

A bit more than a year ago the SETI@home team was granted 24 hours at the Arecibo observatory to reobserve the best candidate locations detected by volunteers running the SETI@home screen saver. (For details, check out the Planetary Society's reports on the reobservations here.)

At Arecibo, we were able to observe 226 points on the sky, containing many of the best SETI@home candidates, several candidates found by the SERENDIP project, and some interesting astronomical objects, including a few known planetary systems and external galaxies.

We sent out the data resulting from these observations to our SETI@home volunteers (that means you). To people who got this data, there was no visible difference between it, and ordinary SETI@home data. For each of these work-units, we scanned the verified results for signals similar to the candidates.

One of the more difficult aspects of the analysis was determining whether a match to the candidate was good enough to be considered significant. We expected about a 10% chance that due to random noise a candidate would get a better score than it had originally.

In the process we found some bugs in our scoring algorithms. Now that we've sorted those out we can talk about our results. As we expected, for most candidates, no matching signals were found and therefore, their scores got worse. The exception was Gaussian candidates with a wide frequency window (non-barycentric Gaussians). Most of these got better scores. It turns out that because of the way SETI@home detects Gaussians, their candidate score is missing a mathematical term that all the other signal types have. We're looking for ways to derive the missing term so we can fix the problems with scores of these Gaussians.

For all of the other signal types, only one candidate was found whose score improved. (Remember, we expected about a 10% chance that we'd find one of these.) It is a Gaussian-type signal with a narrow frequency window. Normally, that would get us excited, but unfortunately, the properties of the signal don't seem consistent with a real signal. In a narrow frequency window, we would expect to find Gaussians with low Doppler drift rates (ones whose frequency is not changing rapidly with time). Unfortunately this Gaussian candidate consists of signals whose Doppler Drift rates are between 10 and 50 Hz per second. These would drift out of our 125 Hz matching window in a few seconds, so if we had looked at that part of the sky even a few seconds later (in any of our observations of this part of the sky), we wouldn't have found a match. Even so, we'll keep an eye on this spot on the sky.

The candidates we reobserved came from the first 2 years of SETI@home observations, so we've still got some work to do. We're working to identify a new set of candidates, and we will ask to be allowed to look at them with the Arecibo telescope. SETI@home will continue with a new software architecture (BOINC) and new hardware at Arecibo (ALFA, SERENDIP V) that will perform a comprehensive survey of the sky.

译文如下：

22号通讯
2004年4月28日




SETI@home 复查报告
Eric Korpela, Jeff Cobb, Dan Werthimer, Matt Lebofsky




大约一年前SETI@home被获准允许在Arecibo天文台对由志愿者通过SETI@home屏保程序找到的有效位置进行24小时的复查。(欲知详情, 请到这里查看行星社团的复查报告。)

在Arecibo,我们对天空的226个点进行了观测，他们含有大部分SETI@home有效信号，一部分由SERENDIP项目发现；另外，还有一些我们感兴趣的天文目标，包括一些已知的行星系统和河外星系。

我们把这些复查数据包发给了志愿者(也就是你). 对得到这些数据包的志愿者而言，它们和普通的SETI@home数据包是没有任何区别的。对每个数据包，我们都把信号和以前的结果进行了对比。

我们经过深思熟虑才确定一个数据包到底是否有效。我们认为一个数据包因为随机噪音的干扰大约有10%的可能性会出现得分高于实际值的情况。

在复查过程中我们发现我们的评分算法有些许的不足。现在我们已经把那些有待讨论的结果挑拣出来了。我们希望，对于大多数有效结果都没有问题，如若不然，它们的分值会变低。但宽频高斯 (无重心高斯) 数据除外，它们的分值会变高。SETI@home计算高斯的方法已经证明了以上结论， 它们缺少了一个数据项，然而其它数据类型都有这个数据项。我们正在努力找到这个数据项以纠正这些高斯数据的评分问题。

对于其它数据类型而言，我们发现有一个数据包的分值提高了。 (记住, 我们认为这种数据包出现的几率只有10%左右) 它是一个窄频高斯型数据。一般来说，我们很高兴发现这种数据，但不幸的是，这个信号和一个真正的信号有些不同。在一个窄频谱中，我们认为只会发现低速的多普勒漂移高斯曲线(单位时间里频率变化速度较低)。不幸的是这些该死数据含有10～50Hz/秒的多普勒漂移。它们会在几秒内就漂出我们的125Hz的频谱， 所以如果我们几秒后对天空的这个区域再进行观测 (对这个区域的任何观测)，我们都不会找到和以前完全相同的信号。虽然如此，我们仍会留意天空里的这个区域。

复查的数据是由SETI@home 2年里的观测数据中提取的。 我们仍在努力寻找新的一组有效数据，然后用Arecibo天文望远镜对它们进行复查。SETI@home在新的软件平台(BOINC)和新的Arecibo 硬件(ALFA, SERENDIP V)继续运行，会对天空进行更加全面的观测。

翻译：VMZY

Lightalt

感谢vmzy翻译