Ultra-Short-Period Massive Black Hole Binary Candidates in LSST as LISA “Verification Binaries”. (arXiv:2105.00005v1 [astro-ph.HE])

<a href="http://arxiv.org/find/astro-ph/1/au:+Xin_C/0/1/0/all/0/1">Chengcheng Xin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Haiman_Z/0/1/0/all/0/1">Zoltan Haiman</a>

The Legacy Survey of Space and Time (LSST) by the Vera C. Rubin Observatory

is expected to discover tens of millions of quasars. A significant fraction of

these could be powered by coalescing massive black hole (MBH) binaries, since

many quasars are believed to be triggered by mergers. We show that under

plausible assumptions about the luminosity functions, lifetimes, and binary

fractions of quasars, we expect the full LSST quasar catalogue to contain

between 20-100 million compact MBH binaries with masses $M=10^{5-9}M_{odot}$,

redshifts $z=0-6$, and orbital periods $P=1-70$ days. Their light-curves are

expected to be distinctly periodic, which can be confidently distinguished from

stochastic red-noise variability, because LSST will cover dozens, or even

hundreds of cycles. A very small subset of 10-150 ultra-compact ($Plesssim1$

day) binary quasars among these will, over $sim$5-15 years, evolve into the

mHz gravitational-wave (GW) frequency band and can be detected by

$textit{LISA}$. They can therefore be regarded as “$textit{LISA}$

verification binaries”, analogous to short-period Galactic compact-object

binaries. The practical question is how to find these handful of “needles in

the haystack” among the large number of quasars: this will likely require a

tailored co-adding analysis optimised for this purpose.

The Legacy Survey of Space and Time (LSST) by the Vera C. Rubin Observatory

is expected to discover tens of millions of quasars. A significant fraction of

these could be powered by coalescing massive black hole (MBH) binaries, since

many quasars are believed to be triggered by mergers. We show that under

plausible assumptions about the luminosity functions, lifetimes, and binary

fractions of quasars, we expect the full LSST quasar catalogue to contain

between 20-100 million compact MBH binaries with masses $M=10^{5-9}M_{odot}$,

redshifts $z=0-6$, and orbital periods $P=1-70$ days. Their light-curves are

expected to be distinctly periodic, which can be confidently distinguished from

stochastic red-noise variability, because LSST will cover dozens, or even

hundreds of cycles. A very small subset of 10-150 ultra-compact ($Plesssim1$

day) binary quasars among these will, over $sim$5-15 years, evolve into the

mHz gravitational-wave (GW) frequency band and can be detected by

$textit{LISA}$. They can therefore be regarded as “$textit{LISA}$

verification binaries”, analogous to short-period Galactic compact-object

binaries. The practical question is how to find these handful of “needles in

the haystack” among the large number of quasars: this will likely require a

tailored co-adding analysis optimised for this purpose.

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