Gravitational Lensing Signatures of Axion Dark Matter Minihalos in Highly Magnified Stars

Abstract

Axions are a viable candidate for Cold Dark Matter (CDM) which should generically form minihalos of sub-planetary masses from white-noise isocurvature density fluctuations if the Peccei-Quinn phase transition occurs after inflation. Despite being denser than the larger halos formed out of adiabatic fluctuations from inflation, axion minihalos have surface densities much smaller than the critical value required for gravitational lensing to produce multiple images or high magnification, and hence are practically undetectable as lenses in isolation. However, their lensing effect can be enhanced when superposed near critical curves of other lenses. We propose a method to detect them through photometric monitoring of recently discovered caustic transiting stars behind cluster lenses, under extreme magnification factors µ 10 3 -10 4 as the lensed stars cross microlensing caustics induced by intracluster stars. For masses of the first gravitationally collapsed minihalos in the range ∼ 10 −15 -10 −8 h −1 M , we show that axion minihalos in galaxy clusters should collectively produce subtle surface density fluctuations of amplitude ∼ 10 −4 -10 −3 on projected length scales of ∼ 10-10 4 AU, which imprint irregularities in the microlensing light curves of caustic transiting stars. We estimate that, inside a cluster halo and over the age of the Universe, most of these minihalos are likely to avoid dynamic disruption by encounters with stars or other minihalos.

Imprints of the early Universe on axion dark matter substructure

“…This tuning means that the brightness of the observed star is sensitive to surface density fluctuations in the cluster lens at that level. Thus, the presence of DM clumps within the cluster can then lead to O(1) brightness fluctuations if they produce surface density fluctuations of O(µ −1 ) on the relevant time and length scales [42]. This phenomenon has recently been observed for a small number of stars [89,90] using the Hubble Space Telescope.…”

“…[41][42][43]. We estimate the sensitivity of photometric microlensing [42] and pulsar timing [43] to axion minihalos produced from EMD below (the astrometry proposal of Ref. [41] is sensitive to heavier sub-halos that cannot arise from EMD).…”

“…However, new techniques to search for more diffuse objects have recently been proposed in Refs. [41][42][43]. We estimate the sensitivity of photometric microlensing [42] and pulsar timing [43] to axion minihalos produced from EMD below (the astrometry proposal of Ref.…”

“…Photometric monitoring of individual stars behind galaxy cluster lenses can be used to observe the imprints of substructure on microlensing light-curves [42]. The stars of interest are those which are very near to a cluster lens caustic; these stars are therefore highly magnified.…”

“…On the other hand, such compact structures can be searched for via lensing and pulsar timing. Future lensing [40][41][42] and pulsar-timing searches [43] will be able to probe compact DM substructure at an unprecedented level.…”