After having received so many results from the hundreds of thousands of enthusiastic users
of SETI@home, we have begun analyzing these results for signs of an extraterrestrial
signal. Many of these analyses include simple calibration tests to ensure that the data
are valid and makes sense scientifically. One of these calibration analyses, however,
yielded a rather interesting result. We took a sample of 50 million spikes received from
users, and plotted these, displaying the number of spikes at each power. The graphs
displaying the results of this plot are below, plotted both on a standard scale and a log
scale (in which the y-axis values grow increasing larger).
Both graphs display an odd hornlike spike at around power 240,000, implying an excess
of radio frequency interference around that power. This goes against what we expect, since
in theory there would be fewer spikes at greater powers. The horn could not be explained
very well by radio frequency interference, however, since it seems to have a great deal of
spikes at slightly lower powers, but virtually none past a cutoff power of roughly
265,000. In an attempt to determine the nature of these spikes, we investigated which
frequencies contributed most to the spikes within the "horn".
Again, there seemed to be no explanation for the strange spike "horn" we had
found. A standard emitter of radio frequency interference, such as a radar or satellite
passing overhead, would tend to emit the high powered transmissions over a very small
frequency range. As shown by the graph above, there was no significant correlation between
the spike horn and any particular frequency, which ruled out much of the standard radio
frequency interference. Although it is possible that a spread-spectrum satellite emitting
over a wide frequency range could cause interference like this, a single satellite would
not be over Arecibo long enough to result in as many spikes as were found.
Finally, a display of the days the spikes originated from yielded the answer. The vast
majority of the spikes were coming from January 17th-21st, 2000. It turns out that on
these days, the Arecibo telescope receiver was temporarily broken.