A Search for Brief Optical Flashes Associated with the SETI Target KIC 8462852 PDF Print

 

 

Point-like events generated by an object moving across the field-of-view of VERITAS over the course of 28.9 seconds on MJD 57283. For details, see Figure 3 below.

Reference: U. Abeysekara et al. (The VERITAS Collaboration), Astrophysical Journal 818:  L33, 2016

Full text version

ArXiv: ArXiV:1602.00987

Contacts: Jamie Holder

 

 

KIC 8462852 is an ordinary F-type star, which occasionally dims by more than 20%. Stars like this are usually very stable, and so the most obvious explanation is that something is blocking some of the light from the star from reaching us. The dimming is too large to be caused by a planet orbiting the star, and so various ideas have been suggested to explain the effect: a family of comets near to the star is one possibility. An alternative proposal is that there is a huge artificial structure around the star - a so-called "alien megastructure". This is, of course, extremely unlikely, but it identifies the star as an interesting target for SETI (search for extra-terrestrial intelligence) studies.

SETI searches usually take place in the radio, but there is increasing interest in also searching at optical and infra-red wavelengths. An intense laser beam fired towards us from a star 1000 light years away could appear thousands of times brighter than the light from the star itself, but last for just a few billionths of a second. The idea is similar to "free space optical communication" - a method which has already been used by NASA to transmit high-definition TV images from the Moon. In order to detect the laser pulse at the Earth, you need large mirrors, and fast, sensitive light detectors - which is where VERITAS comes in.

VERITAS has four telescopes, each with a diameter of 12 m, equipped with extremely fast photomultiplier tube cameras. While they are primarily used for gamma-ray studies, they are also among the largest optical telescopes in the world. The image quality is poor, compared to an astronomical optical telescope, since they are designed to record the large images of gamma-ray air showers, which typically have a larger angular size than the Moon. However, it is good enough to search for distant optical pulses, which would appear as brief, point-like flashes in the same place in the field-of-view of each camera.

VERITAS has a full gamma-ray observing program, and using the limited observing time to search for very speculative SETI signals is difficult to justify. Fortunately, we have been running since 2007, and have made numerous observations of the region of sky around KIC 8462852 just by chance, in our searches for gamma-ray emitters. This paper reports on an analysis of this archival data, designed to identify brief optical pulses. No such signal is seen, but we demonstrate that such searches are extremely sensitive - a pulse producing just 1 photon per m2 at the telescopes is easily detectable. With an archive of around 10,000 hours of observations, the techniques developed can be used to perform SETI studies for a good fraction of the entire sky, and also to look for very short timescale astrophysical phenomena.

 

FITS files:

 

Figures from paper (click to get full size image):

 


Figure 1:  The VERITAS array in Arizona (left) and the PMT camera of a single telescope (right), that covers a total field-of-view of diameter 3o.5. The hexagonal reflective light cones reduce the dead space between the circular PMTs.
Figure 2: Cosmic ray events in the four VERITAS multi-pixel PMT cameras. The diameter of the fi eld-of-view is 3.5o, and the color scale indicates the intensity of Cherenkov light in each PMT in FADC digital counts (where 5.3 digital counts corresponds to one photo-electron). The left panel shows an event initiated by a high-energy cosmic ray, with large angular extent. The right panel shows a fainter, lower-energy cosmic ray event, which is still easily distinguished from a distant point source due to the large parallactic displacement between the image locations in each camera.

Figure 3: Point-like events generated by an object moving across the field-of-view of VERITAS over the course of 28.9 seconds on MJD 57283. Left: A single event viewed by all four telescopes. Right: A subset of the eight recorded events illustrating the motion of the image across the camera of a single telescope.

Figure 4: The wavelength-dependent efficiency for detection of an optical beacon by VERITAS, illustrating the eff ects of interstellar reddening, the Earth's atmosphere (generated using MODTRAN (Berk et al. 2006)), the mirror reflectivity (VERITAS measurement), and the PMT quantum effi ciency both before and after the PMT upgrade in 2012 (VERITAS measurement). The "Total" curve multiplies these eff ects together (assuming the post-2012 quantum efficiency), giving a peak efficiency for detection at 420 nm.

Last Updated on Friday, 27 May 2016 13:16
 

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