Abstract
The recent discovery of fast transient events near critical curves of massive galaxy clusters, which are interpreted as highly magnified individual stars in giant arcs due to caustic crossing, opens up the possibility of using such microlensing events to constrain a range of dark matter models such as primordial black holes and scalar field dark matter. Based on a simple analytic model, we study lensing properties of a point mass lens embedded in a high magnification region, and derive the dependence of the peak brightness, microlensing time scales, and event rates on the mass of the point mass lens as well as the radius of a source star that is magnified. We find that the lens mass and source radius of the first event MACS J1149 Lensed Star 1 (LS1) are constrained, with the lens mass range of $0.1~M_\odot \lesssim M \lesssim 4\times 10^3M_\odot$ and the source radius range of $40~R_\odot \lesssim R \lesssim 260~R_\odot$. In the most plausible case with $M\approx 0.3~M_\odot$ and $R\approx 180~R_\odot$, the source star should have been magnified by a factor of $\approx 4300$ at the peak. The derived lens properties are fully consistent with the interpretation that MACS J1149 LS1 is a microlensing event produced by a star that contributes to the intra-cluster light. We argue that compact dark matter models with high fractional mass densities for the mass range $10^{-5}M_\odot \lesssim M\lesssim 10^2M_\odot$ are inconsistent with the observation of MACS J1149 LS1 because such models predict too low magnifications. Our work demonstrates a potential use of caustic crossing events in giant arcs to constrain compact dark matter.
Highlights
Kelly et al [1] reported the discovery of MACS J1149 Lensed Star 1 (LS1, known as “Icarus”), a faint transient near the critical curve of the massive cluster MACS J1149.6+2223
We find that the lens mass and source radius of the first event MACS J1149 Lensed Star 1 (LS1) are constrained, with the lens mass range of 0.1 M⊙ ≲ M ≲ 4 × 103 M⊙ and the source radius range of 40R⊙ ≲ R ≲ 260R⊙
We argue that compact dark matter models with high fractional mass densities for the mass range 10−5 M⊙ ≲ M ≲ 102 M⊙ are inconsistent with the observation of MACS J1149 LS1 because such models predict too low magnifications
Summary
Et al [1] reported the discovery of MACS J1149 Lensed Star 1 (LS1, known as “Icarus”), a faint transient near the critical curve of the massive cluster MACS J1149.6+2223. While typical magnifications of the brightest images of lensed quasars are ≈10–20, MACS J1149 LS1 is observed very close to the critical curve of the macro lens model where the magnification due to the macro lens model is estimated to be ≳300 Such high magnification of the macro mass model is possible because the giant arc directly crosses the critical curve and many stars in the giant arcs are located very near the critical curve. Caustic crossing events in giant arcs are new phenomena that may probe a different parameter space from previously known microlensing events This possibility of observing highly magnified images of individual stars in giant arcs was first discussed in [25], only the smooth cluster potential was considered in that work. Throughout the paper we adopt a cosmological model with the matter density Ωm 1⁄4 0.3, cosmological constant ΩΛ 1⁄4 0.7, and the Hubble constant H0 1⁄4 70 km s−1 Mpc−1
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