Small-scale shear: peeling off diffuse subhalos with gravitational waves. (arXiv:2103.08618v2 [astro-ph.CO] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Choi_H/0/1/0/all/0/1">Han Gil Choi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Park_C/0/1/0/all/0/1">Chanung Park</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jung_S/0/1/0/all/0/1">Sunghoon Jung</a>
Subhalos at subgalactic scales ($Mlesssim 10^7 M_odot$ or $kgtrsim 10^3
,{rm Mpc}^{-1}$) are pristine testbeds of dark matter (DM). However, they are
too small, diffuse and dark to be visible, in any existing observations. In
this paper, we develop a complete formalism for weak and strong diffractive
lensing, which can be used to probe such subhalos with chirping gravitational
waves (GWs). Also, we show that NFW subhalos in this mass range can indeed be
detected individually, albeit at a rate of ${cal O}(10)$ or less per year at
BBO and others limited by small merger rates and large required SNR $gtrsim
1/gamma(r_0) sim 10^3$. It becomes possible as NFW scale radii $r_0$ are of
the right size comparable to the GW Fresnel length $r_F$, and unlike all
existing probes, their lensing is more sensitive to lighter subhalos. Further
remarkably, our formalism reveals that the frequency dependence of weak
lensing, what is actually the detectable effect, is due to emph{shear $gamma$
at $r_F$}. Not only is it consistent with an approximate scaling invariance,
but it also offers a new way to measure the mass profile at a successively
smaller scale of chirping $r_F propto f^{-1/2}$. Meanwhile, strong diffraction
that produces a blurred Einstein ring rather has a universal frequency
dependence, allowing only detections. These are further demonstrated through
semi-analytic discussions of power-law profiles. Our developments for a single
lens can be generalized and will promote diffractive lensing to a more concrete
and promising physics in probing DM and small-scale structures.
Subhalos at subgalactic scales ($Mlesssim 10^7 M_odot$ or $kgtrsim 10^3
,{rm Mpc}^{-1}$) are pristine testbeds of dark matter (DM). However, they are
too small, diffuse and dark to be visible, in any existing observations. In
this paper, we develop a complete formalism for weak and strong diffractive
lensing, which can be used to probe such subhalos with chirping gravitational
waves (GWs). Also, we show that NFW subhalos in this mass range can indeed be
detected individually, albeit at a rate of ${cal O}(10)$ or less per year at
BBO and others limited by small merger rates and large required SNR $gtrsim
1/gamma(r_0) sim 10^3$. It becomes possible as NFW scale radii $r_0$ are of
the right size comparable to the GW Fresnel length $r_F$, and unlike all
existing probes, their lensing is more sensitive to lighter subhalos. Further
remarkably, our formalism reveals that the frequency dependence of weak
lensing, what is actually the detectable effect, is due to emph{shear $gamma$
at $r_F$}. Not only is it consistent with an approximate scaling invariance,
but it also offers a new way to measure the mass profile at a successively
smaller scale of chirping $r_F propto f^{-1/2}$. Meanwhile, strong diffraction
that produces a blurred Einstein ring rather has a universal frequency
dependence, allowing only detections. These are further demonstrated through
semi-analytic discussions of power-law profiles. Our developments for a single
lens can be generalized and will promote diffractive lensing to a more concrete
and promising physics in probing DM and small-scale structures.
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