Quantifying the Scientific Potential of Intermediate and Extreme Mass Ratio Inspirals with the Laser Interferometer Space Antenna

Abstract

The Laser Interferometer Space Antenna (LISA) will enable precision studies of Extreme and Intermediate Mass Ratio Inspirals (EMRIs/IMRIs), providing unique probes of astrophysical environments of galactic nuclei and strong-field gravity. Using a fully relativistic pipeline across primary masses $m_1 \in [5\times10^4, 10^7]\,M_\odot$ and secondary masses $m_2 \in [1, 10^4]\,M_\odot$, we map instrumental performance directly to detection horizons and parameter measurement precision. EMRIs with $m_1 = 10^7\,M_\odot$ and $m_2 \sim 1\,M_\odot$ are the most sensitive to instrument degradation, with redshift horizons at $z \sim 0.01$, while IMRIs are the least sensitive to degradation and reach redshifts $z \sim 1-3$. All prograde systems considered achieve sub-percent spin precision within three months of observation. The full 4.5-year mission increases the horizon of systems with $m_1 = 10^7\,M_\odot$ and $m_2 \sim 1\,M_\odot$ by a factor of $\sim 4$ and improves sky localization by one to two orders of magnitude reaching $ < 10\,\mathrm{deg}^2$. IMRI detection is robust against degradation, but their parameter estimation is more vulnerable due to fewer cycles in band. With the full baseline, EMRI observations constrain scalar dipole emission and Kerr quadrupole deviations below ground-based bounds by one to two orders of magnitude. We release the accompanying software and an interactive website to enable the community to rapidly quantify the scientific potential of EMRIs and IMRIs.

Figures (11)

• Figure 1: Top: Electromagnetic observations of massive black holes with primary mass $m_1$ and redshift $z$ that could potentially host companions and therefore be the target of LISA observations. We present estimates of Quasars (blue dots), Active Galactic Nuclei (AGN) (black cross), Quasi Periodic Eruptions and Oscillations (QPE and QPO) (violet diamond), and Tidal Disruption Events (TDEs) (green plus). These redshift and mass measurements are characterized by large uncertainties and are only representative of the capabilities of electromagnetic observations. Bottom: EMRIs and IMRIs component masses considered in this work, source-frame primary $m_1$ and secondary mass $m_2$. The horizontal shaded area (yellow) shows the ground-based masses from gravitational wave observations of the LIGO-Virgo-KAGRA collaboration. The diagonal lines indicate constant mass ratios $m_2/m_1$.
• Figure 2: GW frequency evolution as a function of time (left) and eccentricity (right) for an inspiral of three months for different primary mass $m_1$ and prograde (solid line) and retrograde dimensionless spin $a$ (dashed line). The GW frequency is given by twice the azimuthal fundamental frequency of the orbit. The mass ratio is fixed to $m_{\text{2}}/m_{\text{1}} = 10^{-3}$ and the horizontal lines indicate the Nyquist frequency corresponding to different sampling rates $\Delta t$.
• Figure 3: LISA Time Delay Interferometry sensitivity curves for 2nd-generation A and E channels. The blue solid line represents the instrumental noise only, while the dashed and dashed-dotted lines include the galactic white dwarf confusion foreground estimated for durations of 1.5 and 4.5 years, respectively. At frequencies above $10^{-2}$ Hz, we apply a smoothing of the sensitivity curve, shown as a dashed red line, to avoid numerical artifacts due to the zeros of the response. At frequencies below $10^{-4}$ Hz, we impose a smooth increase to account for the lower-frequency limit guaranteed by the requirements.
• Figure 4: EMRI/IMRI Redshift horizon for different source-frame primary masses $m_{\text{1}}$ and dimensionless spin $a=+0.99$ with source-frame secondary mass $m_{\text{2}}$ for a detection threshold of $\rm SNR = 30$. The observational window and the time to plunge are set to $T = 0.25$ years. We present estimates of Quasars (blue dots), Active Galactic Nuclei (AGN) (black cross), Quasi Periodic Eruptions and Oscillations (QPE and QPO) (violet diamond), and Tidal Disruption Events (TDEs) (green plus).
• Figure 5: Relative measurement precision for source-frame primary mass $m_1$ (top panel) and dimensionless spin $a$ (bottom panel) as a function of component masses $m_{\text{1}}, m_{\text{2}}$ for prograde orbits ($a = 0.99$) at fixed median SNR of 30 and three-month observations.