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Broadband radio emission from Kardashev Type II civilisations.

Submitter: Michael Garrett
Description: KIC8462852 (Boyajian et al. 2015 - http://arxiv.org/abs/1509.03622 ) has created quite some excitement as perhaps the first example of an alien megastructure eclipsing what appears to be a fairly normal, main sequence star. In particular, Wright et al. (see: http://arxiv.org/pdf/1510.04606.pdf) have suggested that the light curve is consistent with a Dyson swarm (see image above left for my poor attempt at how that might look "op afstand" (at a distance) and also https://en.wikipedia.org/wiki/Dyson_sphere). Other more conventional but also somewhat contrived explanations have their roots in disrupted cometary/planetary systems.

For the Dyson swarm scenario, the remarkable depth of the obscuration in the light curves of KIC8462852 (dips in stellar luminosity exceeding 20% are observed) suggests a scale of astro-engineering that is best associated with something approaching a Kardashev Type II civilisation (see https://en.wikipedia.org/wiki/Kardashev_scale ) - in other words, a civilisation with energy requirements of order 400E24 Watts or 400 YW (yottawatt) - i.e. around 14 orders of magnitude greater than the current total energy consumption of our own species on planet Earth).

If one assumes that advanced Type II civilisations use radio waves to communicate across their planetary system (and why wouldn't they...) one can ask the question about what level of broadband radio emission we might expect from such a civilisation. My best estimate of the global (waste/leakage) radio emission from our own Kardashev Type 0.7 civilisation (note it's a logarithmic scale...) is of order 50 MW (EIRP) at frequencies < 2000 MHz. This figure is likely an under-estimate, and is mostly based on the collective output of GSM base stations located around the globe.

In this back-of-an-envelope analysis, I make a key assumption that the waste radio emission generated by a communicating civilisation scales linearly with its total energy budget, and this then predicts a total isotropic waste radio emission output from a Type II civilisation associated with KIC8462852 of ~ 50E20 Watts or 2.5E12 Watts/Hz (10-2000 MHz). Since KIC8462852 is located at a distance of ~450 pc or ~1.4E19 metres, this output translates to a flux density of 0.1 Jansky, a level of output that is readily detectable by most radio telescopes today with integration times of only a few seconds. Note that as a main-sequence star, any natural emission from KIC8462852 is essentially undetectable at these sensitivity levels.

Somewhat disappointingly, radio surveys (e.g. NVSS and WENSS) detect no radio emission at the position of KIC8462852 although admittedly these represent very shallow observations (see images above - the WSRT WENNS 326 MHz image [centre], and the VLA NVSS 1400 MHz image [right] - the green cross hairs indicate the position of the KIC8462852. NVSS reaches 1-sigma r.m.s. noise levels better than 0.0025 Jy i.e. about 40 times better than the 0.1 Jy emission that we might predict from KIC8462852. With a resolution of 45 arcseconds, NVSS is unlikely to resolve any braodband radio emission associated with KIC8462852, even if this is distributed on scales associated with a highly capable space-faring civilisation active within an extended, colonised planetary system. Naturally, any conclusions based on this analysis depend crucially on the validity of these somewhat arbitrary assumptions - much deeper integrations by the JVLA and LOFAR are highly desirable in order to place even better limits (x1000) on any broadband radio emission associated with this system. A relatively shallow but very broadband observation by the Allen Telescope Array (see http://arxiv.org/abs/1511.01606, appearing after this AJDI was submitted) arrives at a similar conclusion suggested by the existing shallow radio surveys described here.

Finally, it should be noted that the full SKA and other next generation telescopes such as the NG VLA will go 50-100x deeper than current facilities, permitting interesting limits to be placed on nearby stellar systems within a few hundred light years from the Sun.
Copyright: ASTRON
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