Connecting low-redshift LISA massive black hole mergers to the nHz stochastic gravitational wave background
arxiv(2024)
摘要
Pulsar Timing Array (PTA) experiments worldwide recently reported evidence of
a nHz stochastic gravitational wave background (sGWB) compatible with the
existence of slowly inspiralling massive black hole (MBH) binaries (MBHBs). The
shape of the signal contains valuable information about the evolution of z<1
MBHs above 10^8 M_⊙, suggesting a faster dynamical evolution of
MBHBs towards the gravitational-wave-driven inspiral or a larger MBH growth
than usually assumed. In this work, we investigate if the nHz sGWB could also
provide constraints on the population of merging lower-mass MBHBs (<
10^7 M_⊙) detectable by LISA. To this end, we use the
semi-analytical model applied to the
suite of simulations. We generate a population of MBHs
compatible simultaneously with current electromagnetic and nHz sGWB constraints
by including the possibility that, in favourable environments, MBHs can accrete
gas beyond the Eddington limit. The predictions of the model show that the
global (integrated up to high-z) LISA detection rate is not
significantly affected when compared to a fiducial model whose nHz sGWB signal
is ∼ 2 times smaller. In both cases, the global rate yields ∼ 12
yr^-1 and is dominated by systems of 10^5-6 M_⊙. The main
differences are limited to low-z (z<3), high-mass (> 10^6
M_⊙) LISA MBHBs. The model compatible with the latest PTA results
predicts up to ∼ 1.6 times more detections, with a rate of ∼1
yr^-1. We find that these LISA MBHB systems have 50% probability of shining
with bolometric luminosities >10^43 erg/s. Hence, in case PTA results
are confirmed and given the current MBH modelling, our findings suggest there
will be higher chances to perform multimessenger studies with LISA MBHB than
previously expected.
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