24
P. A. Caraveo
Currently, three Cherenkov observatories are collecting data, namely:
H.E.S.S. (High-Energy Stereoscopic System), in Namibia (1800 m asl), encompassing 4 telescopes of 12 m diameter arranged to form a square with a 28 m
diameter telescope at the center;
MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov), in La Palma
(2225 m asl), characterized by two 17 m diameter telescopes;
VERITAS (Very Energetic Radiation Imaging Telescope Array System), in
Arizona (1400 m asl), encompassing 4 telescopes of 12 m diameter.
Although the optical quality of the Cherenkov telescopes is not comparable to
that of the optical ones, their big collecting area coupled with large field of view and
ultrafast camera have fostered the idea to use them to perform searches for fast optical
signals from nearby stars. Such optical flashes from a point-like source should light a
single pixel which should appear in the same position in the images collected simultaneously by all telescopes, as opposed to the characteristically elongated pattern
due to a gamma-ray induced shower. The possibility to perform the search for fast
optical signals without interfering with the normal data collection is certainly a bonus
making it possible to monitor targets which happen to be in the telescope field of
view.
Thus, after half a century from their common start, gamma-ray astronomy and
the search for interstellar communications crossed their paths.
The technique to use Cherenkov Telescopes to search for extraterrestrial signals,
described e.g. in [10], was applied by VERITAS to study the peculiar planetary
system KIC 8,462,852 [11] which had received much attention since its puzzling
behavior prompted explanations based on improbable alien scenarios. The VERITAS
team analyzed archival data collected over several observing runs from 2009 to 2015
when the star happened to be serendipitously in the telescopes’ field of view. Atmospheric (possibly meteoric) flashes as well as flashes moving through the field of
view, probably due to satellite reflections, were seen, but nothing was detected in the
direction of KIC 8,462,852. However, the resulting upper limit of 1 photon/m
2 is by
far the lowest achieved so far in the search for optical flashes showing the potentialities of Cherenkov telescopes to detect very short optical variabilities during normal
operations. Thus, independently from the optical SETI searches, this capability could
contribute to the growing field of time domain astronomy.
4.3 A Promising Future
The sensitivity to rapid optical flashes will significantly improve in the future owing
to the construction of the most ambitious Cherenkov Observatory known as CTA
(Cherenkov Telescope Array) whose baseline design encompasses more than one
hundred telescopes divided into two observatories, one in the Northern Hemisphere
and one in the Southern Hemisphere, so as to cover the whole celestial sphere [12].
P. A. Caraveo
Currently, three Cherenkov observatories are collecting data, namely:
H.E.S.S. (High-Energy Stereoscopic System), in Namibia (1800 m asl), encompassing 4 telescopes of 12 m diameter arranged to form a square with a 28 m
diameter telescope at the center;
MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov), in La Palma
(2225 m asl), characterized by two 17 m diameter telescopes;
VERITAS (Very Energetic Radiation Imaging Telescope Array System), in
Arizona (1400 m asl), encompassing 4 telescopes of 12 m diameter.
Although the optical quality of the Cherenkov telescopes is not comparable to
that of the optical ones, their big collecting area coupled with large field of view and
ultrafast camera have fostered the idea to use them to perform searches for fast optical
signals from nearby stars. Such optical flashes from a point-like source should light a
single pixel which should appear in the same position in the images collected simultaneously by all telescopes, as opposed to the characteristically elongated pattern
due to a gamma-ray induced shower. The possibility to perform the search for fast
optical signals without interfering with the normal data collection is certainly a bonus
making it possible to monitor targets which happen to be in the telescope field of
view.
Thus, after half a century from their common start, gamma-ray astronomy and
the search for interstellar communications crossed their paths.
The technique to use Cherenkov Telescopes to search for extraterrestrial signals,
described e.g. in [10], was applied by VERITAS to study the peculiar planetary
system KIC 8,462,852 [11] which had received much attention since its puzzling
behavior prompted explanations based on improbable alien scenarios. The VERITAS
team analyzed archival data collected over several observing runs from 2009 to 2015
when the star happened to be serendipitously in the telescopes’ field of view. Atmospheric (possibly meteoric) flashes as well as flashes moving through the field of
view, probably due to satellite reflections, were seen, but nothing was detected in the
direction of KIC 8,462,852. However, the resulting upper limit of 1 photon/m
2 is by
far the lowest achieved so far in the search for optical flashes showing the potentialities of Cherenkov telescopes to detect very short optical variabilities during normal
operations. Thus, independently from the optical SETI searches, this capability could
contribute to the growing field of time domain astronomy.
4.3 A Promising Future
The sensitivity to rapid optical flashes will significantly improve in the future owing
to the construction of the most ambitious Cherenkov Observatory known as CTA
(Cherenkov Telescope Array) whose baseline design encompasses more than one
hundred telescopes divided into two observatories, one in the Northern Hemisphere
and one in the Southern Hemisphere, so as to cover the whole celestial sphere [12].