Probing fundamental physics with Extreme Mass Ratio Inspirals: a full Bayesian inference for scalar charge

Abstract

Extreme Mass Ratio Inspirals (EMRIs) are key sources for the future space-based gravitational wave detector LISA, and are considered promising probes of fundamental physics. Here, we present the first complete Bayesian analysis of EMRI signals in theories with an additional massless scalar, which could arise in an extension of General Relativity or of the Standard Model of Particle Physics. We develop a waveform model accurate at adiabatic order for equatorial eccentric orbits around spinning black holes. Using full Bayesian inference, we forecast LISA's ability to probe the presence of new fundamental fields with EMRI observations.

Figures (9)

• Figure 1: Histograms of posterior samples of the scalar charge inferred by LISA observations of EMRIs with different orbital configurations of central black hole mass $M$ and dimensionless spin $a$, compact object mass $\mu$, initial eccentricity $e_0$ and time to plunge $T$. The colored vertical dashed lines show the one-sided $95\%$ credible interval of the distribution. All EMRI systems are characterized by an SNR of 50.
• Figure 2: Marginalized posterior distribution of an EMRI system with scalar charge $d=0.025$ and source parameters $M=10^5\,M_\odot$, $\mu=5\,M_\odot$, $a=0.95$, $e_0=0.4$, $T=2$ yrs and SNR=50. The estimated median and 95% credible interval are $0.0244^{+0.006} _{-0.007}$.
• Figure 3: Posterior distribution of the Gauss-Bonnet coupling mapped from the scalar charge constraints (see Fig. \ref{['fig:charge_bound']}). The black dotted line in the right panel shows the bound Gao:2024rel from the observation of the gravitational wave event GW230529 GW230529, while the black solid line shows the best forecasted bound for LVK Voyager configuration obtained in Perkins:2020tra. We use a different normalization with respect to Gao:2024relPerkins:2020tra (see Appendix \ref{['app:normalization_action']}).
• Figure 4: Trajectory evolution of semi-latus rectum and eccentricity for four EMRIs with different component masses and $a=0.95$. For reference, we show the location of the grid points used for interpolation in the $(p,e)$ plane for a constant dimensionless spin slice at $a=0.9562665680261776$, where the largest value of $p$ reached for this dimensionless spin is approximately $p\approx 30$.
• Figure 5: Spectrogram of the gravitational wave signal output of the AAK waveform obtained from an EMRI system with parameters $M=10^6 \, M_\odot, \mu=10\,M_\odot,e_0=0.4,d=0.0025,p_0=8.3 ,T=2\,{\rm yrs}$. The different color bands represent the different harmonics, and their color intensity represents their power.