arXiv subjects

Juhan Raidal

Publications and source records attributed to Juhan Raidal.

6 recordsLinked to original sources

arXiv2026

The Heavy Tailed Non-Gaussianity of the Supermassive Black Hole Gravitational Wave Background

We study the non-Gaussian features of the gravitational wave (GW) background generated by a population of inspiraling supermassive black hole (SMBH) binaries. We show that the SMBH GW amplitude distribution (GWAD) features a universal heavy power-law tail $\propto A^{-4}$, while the low-amplitude tail depends on the SMBH merger rate and the energy-loss mechanisms of the binaries. The distribution of the induced timing residuals inherits this heavy tail. As a result, the ensemble averaged statistical moments of order three and higher diverge, limiting their usefulness as measures of non-Gaussianity, and the GW background from SMBH binaries exhibits the single loud source principle, according to which the strongest signals are more likely to be caused by a small number of loud sources. We confirm that the variance-averaged Gaussian approximation accurately describes the timing residual statistics. This approximation justifies a factored likelihood structure that combines standard Gaussian-process PTA posteriors with the non-Gaussian population prior, enabling consistent incorporation of non-Gaussian effects into SMBH model inference. We provide a fast and flexible Python implementation to compute the distribution of timing residuals from a given SMBH merger rate or GWAD.

astro-ph.CO↗

arXiv2026

CMB anisotropies from cosmic (super)strings in light of ACT DR6

We present updated constraints on cosmic string and superstring parameters derived from Cosmic Microwave Background (CMB) anisotropies. The constraints are obtained via Markov Chain Monte Carlo (MCMC) analyses of the full \textit{Planck} temperature and polarization data combined with the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6). For ordinary cosmic strings, we constrain the string tension $G\mu$, the string wiggliness parameter $\alpha$, and the self-chopping efficiency $\tilde{c}$. For cosmic superstrings, we constrain the fundamental string tension $G\mu_F$, the string coupling $g_s$, and a parameter $w$ describing the volume of the compact extra dimensions. In both cases, we find significantly tighter bounds on the string tension compared to previous analyses, obtaining $2\sigma$ upper limits of $G\mu < 3.66\times10^{-8}$ and $G\mu_F < 1.38\times10^{-8}$. We also discuss the significant prior-dependence of these results. The computational pipeline used in this work, including a modified version of \texttt{CAMB} capable of computing CMB anisotropies sourced by any active network described via unequal-time correlators, is released publicly as \texttt{CAMBactive} \cite{Raidal_CAMBactive_CAMB_extension_2026}.

astro-ph.CO↗

arXiv2025

The stochastic gravitational wave background from cosmic superstrings

We study the stochastic gravitational wave background sourced by a network of cosmic superstrings and demonstrate that incorporating higher-mass string species, beyond the fundamental string, is crucial for accurately modelling the resulting gravitational wave spectrum across frequencies ranging from nanohertz to kilohertz. Using the multi-tension velocity-dependent one-scale model to evolve the cosmic superstring network, we perform several fits to the NANOGrav 15-year dataset and obtain expectation values for the fundamental string tension, string coupling and effective size of compact extra dimensions. We find that the cosmic superstring best-fits are comparable in likelihood to Supermassive Black Hole models, thought by many to be the leading candidate explanation of the signal. The implications of the best-fit spectra are discussed within the context of future gravitational wave experiments. We obtain expectation values for the fundamental string tension of $\log_{10}(G\mu_1)=-11.4^{+0.3}_{-0.2}$($-11.5^{+0.3}_{-0.2}$) for gravitational waves originating from large cuspy (kinky) cosmic superstring loops and $\log_{10}(G\mu_1)=-9.7^{+0.7}_{-0.7}$($-9.9^{+1.0}_{-0.5}$) for small cuspy (kinky) loops. We also place $2\sigma$ upper bounds on the string coupling, finding $g_s<0.7$ in all cases, and comment on the implication of our results for the effective size of the compact extra dimensions.

astro-ph.CO↗

arXiv2024

Statistics of the supermassive black hole gravitational wave background anisotropy

We study the statistical properties of the anisotropy in the gravitational wave (GW) background originating from supermassive black hole (SMBH) binaries. We derive the distribution of the GW anisotropy power spectrum coefficients, $C_{l\geq1}/C_0$, in scenarios including environmental effects and eccentricities of the SMBH binaries. Although the mean of $C_{l\geq1}/C_0$ is the same for all multipoles, we show that their distributions vary, with the low $l$ distributions being the widest. We also find a strong correlation between spectral fluctuations and the anisotropy in the GW signal. We show that the GW anisotropy can break the degeneracy between the scenarios including environmental effects or eccentricity. In particular, we find that existing NANOGrav constraints on GW anisotropy begin to constrain SMBH scenarios with strong environmental effects.

astro-ph.CO↗

arXiv2024

Eccentricity effects on the supermassive black hole gravitational wave background

We studied how eccentricity affects the gravitational wave (GW) spectrum from supermassive black hole (SMBH) binaries. We developed a fast and accurate semi-analytic method for computing the GW spectra, the distribution for the spectral fluctuations and the correlations between different frequencies. As GW emission circularizes binaries, the suppression of the signal strength due to eccentricity is relevant for signals from wider binaries emitting at lower frequencies. Such a feature is present in the signal observed at pulsar timing arrays. We found that when orbital decay of the SMBH binaries is driven by GWs only, the shape of the observed signal preferred highly eccentric binaries $\langle e \rangle_{2\,{\rm nHz}} = 0.83^{+0.04}_{-0.05}$. However, when environmental effects were included, the initial eccentricity could be significantly lowered, yet the scenario with purely circular binaries was still mildly disfavored.

astro-ph.CO↗

arXiv2024

Gravitational Waves from SMBH Binaries in Light of the NANOGrav 15-Year Data

The NANOGrav Collaboration has recently announced evidence for nHz gravitational waves (GWs), in the form of a Hellings-Downs angular correlation in the common-spectrum process that had been observed previously by them and other Pulsar Timing Arrays (PTAs). We analyze the possibility that these GWs originate from binary supermassive black holes (SMBHs) with total masses $\gtrsim 10^9\, M_{\odot}$. The spectral index of the GW signal differs at 95 % CL from that predicted for binary evolution by GW emission alone, and we find $> 3 σ$ evidence that environmental effects such as dynamical friction with gas, stars, and dark matter may be affecting the binary evolution. We estimate the required magnitude and spectrum of such environmental effects and comment on their possible implications for measurements of GWs at higher frequencies.

astro-ph.CO↗