TianQin: a space-borne gravitational wave detector

TL;DR

The paper presents TianQin, a space-borne gravitational-wave detector designed to operate in the millihertz band using a triangle of geocentric, drag-free spacecraft and LISA-like interferometry. By concentrating on a single reference source (notably J0806 HM Cancri) with known properties, the authors lay out a feasible, cost-conscious mission concept, including sensitivity goals, an error budget for the laser interferometer and disturbance reduction system, and a realistic technology status roadmap. They quantify the required performance (≈1 pm/√Hz displacement, ≈10^-15 m s^-2/√Hz acceleration noise) and outline observational strategies, annual science windows, and potential contingencies to mature critical technologies. If realized, TianQin would enable direct detection of millihertz GW signals and serve as a stepping stone toward a fleet of future space-based GW observatories.

Abstract

TianQin is a proposal for a space-borne detector of gravitational waves in the millihertz frequencies. The experiment relies on a constellation of three drag-free spacecraft orbiting the Earth. Inter-spacecraft laser interferometry is used to monitor the distances between the test masses. The experiment is designed to be capable of detecting a signal with high confidence from a single source of gravitational waves within a few months of observing time. We describe the preliminary mission concept for TianQin, including the candidate source and experimental designs. We present estimates for the major constituents of the experiment's error budget and discuss the project's overall feasibility. Given the current level of technology readiness, we expect TianQin to be flown in the second half of the next decade.

Figures (4)

• Figure 1: An illustration of the preliminary concept of TianQin, with J0806 being the reference source. The three TianQin spacecraft are denoted as SC1, SC2 and SC3. The plane of the celestial equator is also shown, together with the direction to J0806 in the sky.
• Figure 2: Time evolution of the preliminary TianQin orbits. Shown are the range rates (panels 1,2,3 and 7) and the subtended angles (panels 4,5,6 and 8) between each pair of the spacecraft (denoted as SC1, SC2, and SC3) assuming that the three TianQin spacecraft are on nearly identical orbits with a semi-major axis of $10^5$ km. The panels 1--6 show five-year spans, while the panels 7 and 8 show more detail in the first few months for the pair SC3-SC1 (the behavior of the pairs SC1-SC2 and SC2-SC3 is very similar). The effects of the Sun, the Moon and major planets in the solar system, multiple moments of the Earth's gravity up to the fifth order, and random noise at the level $10^{-12} ~{\rm m\, s}^{-2}\,$ (representing the residual effect of non-gravitational forces, which are largely canceled by the drag-free control) have been included in the simulation of the orbits.
• Figure 3: The expected sensitivity curve of TianQin. The curve for LISA and another short period binary source, SDSS J065133+2844 Brown2011apjl23, are also plotted for comparison. The magnitudes of sources include 90 days of integration time. The solid curves are obtained by using the all-sky and polarization-averaged transfer function (\ref{['val.Rw']}), while the dashed curve is for sources not only having the same inclination but also lying in the same sky direction as J0806.
• Figure 4: A schematic of the optical system on each spacecraft. The system includes a heterodyne interferometer optical bench, a frequency stabilization bench, a phase locking optical bench, an FPGA-based phase-meter and two telescopes. On the interferometer optical bench there are four laser interferometers and a signal acquisition pointing and tracking control optical system. On the frequency stabilization optical bench, there is the Fabry-Perot cavity used to stabilize the laser frequency through the Pound-Drever-Hall scheme. On the phase locking module, the slave laser head is heterodyne optical phase-locked with the frequency stabilized laser head, and two pairs of acousto-optic modulators (AOMs) are used to generate the heterodyne frequencies for heterodyne laser interferometer. All the interfering signals are detected with photo detectors, and we use an FPGA-based ultra high precision phase meter to read the phase of each signal.