IndIGO-D: Probing Compact Binary Coalescences in the Decihertz GW Band

TL;DR

IndIGO-D proposes a decihertz space-based gravitational-wave mission using a three-spacecraft heliocentric L-shaped interferometer with 1000 km arms to bridge the gap between LISA and ground-based detectors. The study develops the detector geometry, full frequency-domain antenna response, and a fiducial sensitivity curve, quantifying horizon distances for BNS and IMBH binaries and demonstrating strong multi-band and early-warning capabilities. Simulated GW170817-like events show pre-merger sky localization improving from ~$21\ \mathrm{deg}^2$ months before merger to ~$0.3\ \mathrm{deg}^2$ hours before, enabling prompt electromagnetic follow-up and multi-messenger campaigns with facilities like the Rubin Observatory. IndIGO-D thus enables long, early inspiral tracking, precise chirp-mass measurements, and tests of GR across frequency bands by complementing both LISA and terrestrial detectors, while also probing environmental effects such as dark matter spikes around IMBHs.

Abstract

We study IndIGO-D, a decihertz gravitational-wave mission concept, focusing on a specific configuration in which three spacecraft fly in formation to form an L-shaped interferometer in a heliocentric orbit. The two orthogonal arms share a common vertex, providing a space-based analogue of terrestrial Michelson detectors, while operating in an optimised configuration that yields ppm-level arm-length stability. Assuming 1000 km arm length, we analyse the orbital motion and antenna response, and assess sensitivity across the [0.1 - 10] Hz band bridging LISA and next-generation ground-based interferometers. Using fiducial sensitivity curves provided by the IndIGO-D collaboration, we compute horizon distances for different source classes. Intermediate-mass black-hole binaries with masses $10^{2}$ - $10^{3} \, M_\odot$ are detectable to redshifts $z \sim 10^{3}$, complementing the reach of LISA and terrestrial detectors. Binary neutron star systems are observable to a horizon distance of $z \lesssim 0.3$, allowing continuous multi-band coverage with Voyager-class interferometers from the decihertz regime to merger. A Bayesian parameter-estimation study of a GW170817-like binary shows that the sky localization area improves from $\sim 21 \,\mathrm{deg}^2$ at one month to $0.3 \,\mathrm{deg}^2$ at six hours pre-merger! These sky areas are readily tiled by wide-field time-domain telescopes such as the Rubin Observatory, whose $9.6 \,\mathrm{deg}^2$ field of view and r-band depth enable high-cadence, repeated coverage of GW170817-like kilonovae at this distance and beyond. IndIGO-D exploits the rapid evolution of binaries in the decihertz band to bridge the gap between millihertz and terrestrial observations, enabling early warnings on timescales from months to hours and enhancing the prospects for multi-band and multi-messenger discoveries.

Figures (8)

• Figure 1: The orbits of the three spacecraft constituting IndIGO-D. The spacecraft 1 at the vertex orbits the Sun in ecliptic plane at a distance of 1 AU, while the orbits of spacecraft 2 and 3 are inclined by $60^\circ$ with respect to ecliptic which results in the most stable flight configuration such that the inter spacecraft separation remain nearly constant throughout the motion.
• Figure 2: Flexing of the arms: fractional variation of the inter-spacecraft separation (computed using the approximated orbits described by \ref{['Eq:anomaly_relation_approximated']}) along the two arms over one orbital period of the constellation. The separation varies by ${\sim 7.5\,\mathrm{m}}$ over a drift timescale ${t_{\rm drift} \sim \mathcal{O}(\mathrm{months})}$ within a single orbital period.
• Figure 3: Variation in the magnitude of the antenna pattern functions across the whole sky in nearly one orbital period of the spacecraft constellation, assuming long-wavelength limit. Grids in the sky-map correspond to the ecliptic longitude and ecliptic latitude that marks the position in the sky in the SSB frame. Note that at any instant there exists 4 blind and 2 bright spots where the magnitude of antenna patterns are 0 and 1, respectively. The 4 blind spots lie in the detector's plane along the lines making an angle of $45^\circ$ with the detector's arms. The 2 bright spots lie perpendicular to the detector's plane, above and below the detector.
• Figure 4: Comparison of detector sensitivities and horizon distances. Note that IndIGO-D is sensitive to binary neutron star systems out to a horizon distance of $\sim 1000$ Mpc, similar to LIGO-Voyager Adhikari2018Voyager. (a) Noise sensitivities: Detector noise amplitude spectral densities (ASD) for various detectors, along with the ASD of the GW170817- and GW150914-like GW signals, given by $2\sqrt{f}\tilde{h}(f)$. The red-filled circles represent the time before the merger at a specific GW frequency. (b) Horizon distance (right axis) and corresponding redshift (left axis) as functions of source-frame total mass for equal-mass compact binaries. Detection distances are evaluated for 48 uniformly distributed sky positions, with the inclination angle fixed to zero (face-on). The upper solid line indicates the horizon distance, while the dark and light shaded bands indicate the distances enclosing $50\%$ and $90\%$ of sky locations, respectively.
• Figure 5: Detection fractions (SNR $\geq 8$) for simulated merging binary neutron star systems with component masses $(1.4, 1.4)\, \rm{M}_\odot$, isotropic orientations, and a population distributed uniformly in comoving volume out to ${z = 0.33}$. IndIGO-D and Voyager detect 17.72% and 26.04% of the sources, respectively. All sources detected by IndIGO-D will also be detected by Voyager. The overlap illustrates the multi-band science potential, in which IndIGO-D provides long-duration tracking and pre-merger localisation in the dHz band for events later observed in the Voyager band.