Optical SETI with Imaging Cherenkov Telescopes J. Holder a, P. Ashworth a, S. LeBohec b, H.J. Rose a, T.C. Weekes c a) School of Physics and Astronomy, University of Leeds, UK b) Department of Physics, University of Utah, Salt Lake City, USA c) Harvard-Smithsonian Center for Astrophysics, USA 1)Schwartz, R. & Townes, 1961, Nature, 190, 205 2)Howard, A.W. et al., 2004, Ap.J., 613, )Lampton, M,, 2000, in ASP Conf. Ser. 213:Bioastronomy 99, 565 4)Wright, S. A. et al., 2001, in Proc. SPIE 4273, SETI in the Optical Spectrum III, 173 Abstract The idea of searching for optical signals from extraterrestrial civilizations has become increasingly popular over the last five years, with dedicated projects at a number of observatories. The method relies on the detection of a brief (~few ns), intense light pulse with fast photon detectors. Ground-based gamma-ray telescopes such as the Whipple 10 m, providing a large mirror area and equipped with an array of photomultiplier tubes (PMTs) are ideal instruments for this kind of observation if the background of cosmic-ray events can be rejected. We report here on a method for searching for optical SETI pulses, using background discrimination techniques based on the image shape. Introduction Until now, SETI projects have chosen to focus on radio searches for signals from extraterrestrial civilizations. Schwartz and Townes [1] were the first to suggest the idea of searching at optical wavelengths, and this idea has become more attractive as the potential of powerful pulsed laser signals has been realised. Howard et al. [2] recently noted that, using current technology (a 10m reflector as the transmitting and receiving aperture, and a 3.7 MJ pulsed laser source), a 3 ns optical laser pulse could be produced which would be easily detectable at a distance of 1000 ly, outshining starlight from the host system by a factor of 10,000. A number of dedicated projects to detect such a signal are now operating [2, 3, 4]. They typically consist of a ~1 m diameter reflector of astronomical quality, instrumented with 2-3 PMTs with which to discriminate the pulses from the steady background light, and sensitive to pulses with intensities greater than ~100 photons m -2. The optical qualities of the receiver are not critical; it acts essentially as a ‘light bucket’ with which to collect as many photons as possible. Imaging Cherenkov gamma-ray telescopes, with mirror areas >100 m-2 and typically instrumented with hundreds of fast PMTs could potentially perform the same task with greatly increased sensitivity, if the background of cosmic-ray air shower events can be removed. A number of authors have discussed the possibility of using Cherenkov telescopes for optical SETI [5, 6, 7], in this paper we present an analysis method for searching for optical pulses in archival data taken with the Whipple 10 m telescope. The Whipple 10 m telescope (left) has a total mirror area of 90m 2 and is equipped with an imaging camera of 379 PMTs with 0.12 spacing, giving a total field-of-view of diameter 2.5. An optical laser pulse from the direction of a candidate star will appear as a point source in the camera. If the telescope optics were perfect, such a signal would never trigger the telescope readout, since the photons would all fall within one PMT and the 3-PMT multiplicity trigger condition would not be met; however, the point spread function (PSF) for the Whipple telescope is comparable to the PMT spacing. The figure to the right shows the trigger efficiency as a function of pulse intensity at the ground for two values of the PSF and for two locations in the field-of-view; one at the centre of the central PMT (corresponding to the worst case triggering scenario), the other offset to a position equidistant from three PMTs (best case). The system has a trigger efficiency >80% for all cases for pulses of ~10 ph m -2, implying a sensitivity a factor of 10 greater than dedicated OSETI instruments. The Whipple telescope is clearly able to trigger on point-source optical pulses; however, we must also be able to discriminate these pulses from the overwhelming background of cosmic-ray images. The figure to the left shows the photon distribution in the focal plane for a cosmic ray event (the circle shows the full field-of-view). The photon distribution for an OSETI flash will be bright, compact, symmetrical and co-located with the position of a candidate star, as illustrated in the figure to the right. The data are analysed by parameterizing each event with an ellipse. The symmetrical nature of OSETI flashes can be exploited by selecting ellipses with an ellipticity (=length/width) < 1.5. This reduces the background by 80%, while retaining 97.5% of OSETI images. Close to the edge of the camera images become highly elliptical due to truncation effects and optical aberrations; we therefore select only images with an angular distance < 1 from the camera centre. The OSETI flashes are also compact. We define the ellipse radius R=(width 2 + length 2 ). The figure left shows R plotted against the sum over all the charge in the image for cosmic rays (blue) and for OSETI flashes simulated with a range of intensities (black), with the ellipticity and distance cuts applied. Bright OSETI flashes are clearly discriminated; selecting events below and right of the grey line removes all but one of the original 31,000 cosmic-ray events. The final tool for discrimination is the image location. If a candidate star is chosen, the OSETI flash should originate from the star position in the field-of-view. The figure right shows the results of a 28 minute targeted observation of one such candidate identified by Howard et al. [2]. Only events within 0.05 of the star position are selected, and the background in the OSETI pulse region of the plot is reduced to negligible levels. A search of the Whipple 10 m archive for OSETI pulses is currently underway using these techniques. HIP ) Covault, C. et al., 2001, in Proc. SPIE 4273, SETI in the Optical Spectrum III, 173 6) Eichler, D. & Beskin, G., 2001, Astrobiology, 1, 489 7) Armada, A., Cortina, C., Martinez, M., 2004 Proc. 14 th Int. School of Cos. Ray Astrophys.