Lens Flare: Magnified X-Ray Binaries as Passive Beacons in SETI

Geometry of the lens system. At top, the source is at the focus of the lens, and the lens acts as a collimator; whereas, at bottom, the light from each point on the unfocused source (as in the light blue regions) diverges at infinity within a cone of opening angle θ?.

Low mass X-ray binaries (LMXBs) containing neutron stars are both extremely luminous and compact, emitting up to ~10^6 L_sun within a kilometer-scale boundary layer. This combination allows for easy modulation, motivating X-ray SETI.

When X-ray lenses smaller than planets (100 - 1,000 km) magnify the LMXB boundary layer, it brightens by a factor of several thousand for about a second. In addition, there should be occultation events where the neutron star is blocked out. Passive X-ray lenses could require little maintenance and the LMXB light source itself shines for millions of years, serving as an effective beacon for interstellar communication.

A very large number of lenses would be needed to ensure detection from all directions, however, and gathering material to construct them could be very difficult. Avoiding collisions between lenses and aiming them pose additional challenges. Both "lens flares" and eclipses of LMXBs are easily detectable in the Galaxy, although they would be rare events, occurring once per decade. Our X-ray instruments could detect the flares to several Mpc, but it is unlikely they would be observing the LMXB during a flare.

Brian C. Lacki
(Submitted on 1 Feb 2020)
Comments: Submitted, 15 pages, 4 figures, 1 table. An animated version of Figure 3 is available at this http URL
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Popular Physics (physics.pop-ph)
Cite as: arXiv:2002.00128 [astro-ph.HE] (or arXiv:2002.00128v1 [astro-ph.HE] for this version)
Submission history
From: Brian Lacki
[v1] Sat, 1 Feb 2020 02:20:37 UTC (1,716 KB)
https://arxiv.org/abs/2002.00128
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