Kavli Affiliate: Qingjuan Yu
| First 5 Authors: Xiao Guo, Qingjuan Yu, Youjun Lu, ,
| Summary:
Massive black holes (MBHs) exist in the Galactic center (GC) and other nearby
galactic nuclei. As natural outcome of galaxy mergers, some MBHs may have a
black hole (BH) companion. In this paper, assuming that the MBHs in the GC and
some nearby galaxies are in binaries with orbital periods ranging from months
to years (gravitational-wave frequency $sim1-100$,nHz), we investigate the
detectability of gravitational-waves from these binary MBHs (BBHs) and
constraints on the parameter space for the existence of BBHs in the GC, LMC,
M31, M32, and M87, that may be obtained by current/future pulsar timing array
(PTA) observations. We find that a BBH in the GC, if any, can be revealed by
the Square Kilometer Array PTA (SKA-PTA) if its mass ratio
$qgtrsim10^{-4}-10^{-3}$ and semimajor axis $asim20-10^3$,AU. The existence
of a BH companion of the MBH can be revealed by SKA-PTA with $sim20$-year
observations in M31 if $qgtrsim10^{-4}$ and $asim10^2-10^4$,AU or in M87 if
$qgtrsim10^{-5}$ and $asim10^3-2times10^4$,AU, but not in LMC and M32 if
$qll1$. If a number of milli-second stable pulsars with distances
$lesssim0.1-1$,pc away from the central MBH in the GC, LMC, M32, or M31, can
be detected in future and applied to PTAs, the BH companion with mass even down
to $sim100M_odot$, close to stellar masses, can be revealed by such PTAs.
Future PTAs are expected to provide an independent way to reveal BBHs and
low-mass MBH companions in the GC and nearby galaxies, improving our
understandings of the formation and evolution of MBHs and galaxies.
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