Revised bounds on local cosmic strings from NANOGrav observations
arxiv(2024)
Abstract
In a recent paper, the NANOGrav collaboration studied new physics
explanations of the observed pulsar timing residuals consistent with a
stochastic gravitational wave background (SGWB), including cosmic strings in
the Nambu-Goto (NG) approximation. Analysing one of current models for the loop
distribution, it was found that the cosmic string model is disfavored compared
to other sources, for example, super massive black hole binaries (SMBHBs). When
both SMBHB and cosmic string models are included in the analysis, an upper
bound on a string tension Gμ≲ 10^-10 was derived. However, the
analysis did not accommodate results from cosmic string simulations in an
underlying field theory, which indicate that at most a small fraction of string
loops survive long enough to emit GW. Following and extending our previous
study, we suppose that a fraction f_ NG of string loops follow NG
dynamics and emit only GWs, and study the three different models of the loop
distribution discussed in the LIGO-Virgo-KAGRA (LVK) collaboration analyses. We
re-analyze the NANOGrav 15yrs data with our signal models by using the NANOGrav
analysis code via the wrapper . We
find that loop distributions similar to LVK Model B and C yield higher Bayes
factor than Model A analyzed in the NANOGrav paper, as they can more easily
accommodate a blue-tilted spectrum of the observed amplitude. Furthermore,
because of the degeneracy of Gμ and f_ NG in determining the signal
amplitude, our posterior distribution extends to higher values of Gμ, and
in some cases the uppermost value of credible intervals is close to the Cosmic
Microwave Background limit Gμ≲ 10^-7. Hence, in addition to the
pulsar timing array data, further information about the fraction of long-lived
loops in a cosmic string network is required to constrain the string tension.
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