A Fourier transform analysis of 2.5 million spectra in the Sloan Digital Sky Survey was carried out to detect periodic spectral modulations. Signals having the same period were found in only 234 stars overwhelmingly in the F2 to K1 spectral range. The signals cannot be caused by instrumental or data analysis effects because they are present in only a very small fraction of stars within a narrow spectral range and because signal to noise ratio considerations predict that the signal should mostly be detected in the brightest objects, while this is not the case. We consider several possibilities, such as rotational transitions in molecules, rapid pulsations, Fourier transform of spectral lines and signals generated by Extraterrestrial Intelligence (ETI). They cannot be generated by molecules or rapid pulsations. It is highly unlikely that they come from the Fourier transform of spectral lines because too many strong lines located at nearly periodic frequencies are needed. Finally we consider the possibility, predicted in a previous published paper, that the signals are caused by light pulses generated by Extraterrestrial Intelligence to makes us aware of their existence. We find that the detected signals have exactly the shape of an ETI signal predicted in the previous publication and are therefore in agreement with this hypothesis. The fact that they are only found in a very small fraction of stars within a narrow spectral range centered near the spectral type of the sun is also in agreement with the ETI hypothesis. However, at this stage, this hypothesis needs to be confirmed with further work. Although unlikely, there is also a possibility that the signals are due to highly peculiar chemical compositions in a small fraction of galactic halo stars.
The objects listed in Tables 1 and 2 should also be observed with large telescopes to obtain spectra with high resolutions and high signal to noise ratios that would allow studying the signals in greater details to definitely confirm that they are not data reduction or instrumental effects.
They considered the possibility that the signals are caused by intensity pulses generated by Extraterrestrial Intelligence (ETI), as suggested by Borra (2012), to make us aware of their existence. The shape of the detected signals has exactly the shape predicted by Borra (2012). The ETI hypothesis is strengthened by the fact that the signals are found in stars having spectral types within a narrow spectral range centered near the G2 spectral type of the sun. Intuitively, we would expect stars having a spectral type similar to the sun to be more likely to have planets capable of having ETI. This is a complex and highly speculative issue (see Lammer et al. 2009) and we shall not delve on it. Let us however note that all of the active optical SETI observational projects listed in Tarter (2001) search for signals in Solar-type stars. Reines and Marcy (2002) and Howard et al. (2004) searched for signals generated by lasers in nearby solar stars. In particular, let us note that Howard et al. (2004) searched for nanosecond optical pulses from nearby solar stars
The ETI hypothesis requires that all different ETI transmitters choose to broadcast with the same time separation of pulses and one may wonder why they do so. This is a highly speculative issue that may have several explanations. A possible explanation that makes sense is that all ETI use the same time separation to make it clear that the pulses all come from ETI.
Arxiv - Searching for extraterrestrial intelligence signals in astronomical spectra, including existing data [Borra 2012]
At first sight, one may question the validity of the ETI hypothesis because of the energy required to send the pulses to distant stars. The energy issue is discussed in Borra (2012) that shows that technology presently available on Earth could be used to send signals having the energy needed to be detected 1000 light years away. Obviously, more advanced civilizations would have technologies capable to generate much stronger signals; Borra (2012) elaborates on this. As an illustration of this, just imagine how the suggestion made by Borra (2012) would have been considered if submitted in 1950, before the invention of the laser, when it would have suggested the use of a light bulb to send the signal.
At this stage, the ETI generation of the spectral modulation is a hypothesis that needs to be confirmed with further work. This can be done by repeatedly observing the stars in Tables 1 and 2 with photoelectric detectors capable of detecting very rapid intensity signals. However ETI may not necessarily send us pulses at all times so that a lack of detections in some stars may not necessarily signify that ETI does not exist. The reason why ETI may not send pulses
at all times may simply come from the fact that the signals must be sent to a very large number of stars so that too much energy would be required to send pulses to all stars at all times. The kind of signal that we have detected can be generated by pairs of pulses that have the same time separation t but with the pairs sent with time separations significantly larger than (Borra 2012). One could therefore look for the ETI pulses using techniques similar to the one described in Leeb et al. (2013) because pairs of pulses separated by a constant value of t = 1.6465 10^-13 seconds could be sent with a periodicity having a period much larger than t (e.g. 10^-6 seconds). Leeb et al. (2013) estimate that a telescope having a 1.7-m diameter could detect signals from a G2V star 500 ly distant so that this type of signal could be detected in stars to distances as large as 2000 ly with existing telescopes. However the detected stars in Tables 1 and 2 are at distances greater than 8000 ly, so that a 30-m telescope would be needed to observe the stars listed in tables 1 and 2
The new fourth phase of the SDSS will include observations from the Southern Hemisphere for the first time. The southern observations will be taken from the Irénée du Pont Telescope at Las Campanas Observatory in northern Chile (Latitude 29° 0′ 52.56″ S, Longitude 70° 41′ 33.36″ W, Elevation 2380m). The du Pont telescope is a Ritchey-Chrétien 2.5-meter f/7.5 telescope with a Gascoigne corrector lens.
Abstract - Searching for extraterrestrial intelligence signals in astronomical spectra, including existing data
The main purpose of this article is to make Astronomers aware that Searches for Extraterrestrial Intelligence can be carried out by analyzing standard astronomical spectra, including those they already have taken. Simplicity is the outstanding advantage of a search in spectra. The spectra can be analyzed by simple eye inspection or a few lines of code that uses Fourier transform software. Theory, confirmed by published experiments, shows that periodic signals in spectra can be easily generated by sending light pulses separated by constant time intervals. While part of this article, like all articles on searches for ETI, is highly speculative the basic physics is sound. In particular, technology now available on Earth could be used to send signals having the required energy to be detected at a target located 1000 light years away. Extraterrestrial Intelligence (ETI) could use these signals to make us aware of their existence. For an ETI, the technique would also have the advantage that the signals could be detected both in spectra and searches for intensity pulses like those currently carried out on Earth.
A major issue comes from the fact that the energy needed should not be absurdly large. We shall therefore compare the energy requirements with the technology presently available on Earth. For this, we shall use the analysis in Howard et al, (2004) who considered the energy requirements for an ETI trying to communicate with nanosecond optical pulses. They considered the feasibility of interstellar communications with technology available at the time the paper was written. They assumed communications within a 1000 light years diameter region surrounding the Earth that would contain about 1 million sun-like stars. They assumed that a diode pumped laser similar to the Helios laser designed at Lawrence Livermore National Laboratory for inertial confinement fusion would be used. They assumed 10-meter diameter telescopes to send and receive the signals. Considering a beam that would exit the transmitting telescope with a 20 mas angle, giving a 6 AU wide beam at 1000 ly, they compute that a 3 ns pulse generated by the Helios laser, would give at the receiving 10-meter telescope 1500 photons for each 3 ns pulse. Considering that the
Helios laser generates pulses with a 10 Hz frequency, we see that the receiver would observe 15,000 photons/second.
ExtraTerrestrial Intelligence (ETI) could signal its existence to others by sending light pulses with time separations of the order of 10^-9 to 10^-15 seconds that could be detected in spectra. Signals with time separations considerably larger than nanoseconds would however be difficult to detect because the resolution of the spectroscopic equipment would be insufficient to resolve the spectroscopic signature. One also could detect spectroscopic signals from ETIs that send bursts with periodic time signals (e.g. pairs of pulses) separated by longer time scales (e.g milliseconds). The other advantage of this procedure is that the signals could also be detected in SETIs that look for intensity pulses within short time scales. For example searches for nanosecond pulses in the optical region (Howard et al (2004).
The physical requirements (e.g. energy) needed to communicate within a 1000 ly radius are reasonable. They could be met with lasers and telescopes presently available on Earth