TianQin: a space-borne gravitational wave detector

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.

• The paper introduces the TianQin mission with a novel design of three drag-free spacecraft to detect gravitational waves in the millihertz range.
• It details advanced laser interferometry and disturbance reduction techniques achieving exceptional sensitivity for faint gravitational signals.
• The mission targets the binary system RX J0806.3+1527, promising to extend gravitational wave astronomy and test aspects of general relativity.

Overview of TianQin: A Space-Borne Gravitational Wave Detector

The proposed TianQin mission represents a significant endeavor in the ongoing efforts to directly detect gravitational waves (GWs) and enhance our understanding of the Universe through gravitational wave astronomy. This paper introduces the conceptual framework and anticipated capabilities of TianQin, a space-borne detector aimed primarily at identifying gravitational waves within the millihertz frequency range. It outlines the mission design, technological requirements, and scientific goals associated with launching this detector, which relies on a constellation of three drag-free spacecraft arranged in a precise triangular formation.

Scientific Motivation

The detection of gravitational waves offers transformative potential for testing aspects of general relativity and advancing our understanding of various astrophysical phenomena. Although ground-based detectors like LIGO and Virgo have successfully identified gravitational waves, their sensitivity is limited to specific frequency bands. Space-borne detectors like TianQin aim to cover different frequency ranges that ground-based detectors cannot. TianQin is particularly focused on signals in the millihertz range, which could be generated by sources such as ultra-compact galactic binaries and coalescing massive black holes.

Mission and Technological Design

The mission concept for TianQin involves deploying three spacecraft in an equilateral triangle configuration, each equipped with advanced laser interferometry systems to measure the minuscule distance variations between the spacecraft caused by passing gravitational waves. Drag-free technology is employed to minimize non-gravitational forces, ensuring the precision necessary to detect faint gravitational signals. TianQin's development emphasizes utilizing currently advanced or near-ready technology, aiming for deployment in the latter half of the next decade.

Technical Aspects

• Laser Interferometry: TianQin's design leverages laser interferometry to detect GW-induced motions. The system strives for a positional sensitivity of about 1 pm/Hz$^1/2$, harnessed through advanced frequency stabilization techniques and noise-canceling algorithms such as time-delay interferometry.
• Disturbance Reduction: The spacecraft are required to maintain an inertial frame of reference against non-gravitational disturbances. This involves careful engineering of the inertial sensors and disturbance reduction systems to achieve accuracy in residual acceleration noise to the order of $10^{-15}$ m/s$^2$/Hz$^1/2$.
• Trajectory and Formation Flying: Geocentric orbits were chosen for practical accessibility, leveraging the gravitational stability and economic efficiency of placements close to Earth. This choice aids in deploying advanced technology from existing space infrastructure, significantly impacting cost-effectiveness.

Sensitivity and Source Targeting

TianQin’s sensitivity is designed to be optimized for detecting waves from specific reference sources, notably the binary system RX J0806.3+1527 (J0806), identified due to its distinctive emission in the millihertz range. The mission aims for a high signal-to-noise ratio, facilitated by taking advantage of J0806's predictable frequency and phase characteristics, streamlining the identification process.

The team's approach to focusing on a known strong source minimizes complications such as managing diverse signal sources, allowing for targeted design and operational refinement to ensure the robustness and success of the mission.

Practical and Theoretical Implications

The implications of TianQin's research extend to practical advancements in launching cost-effective and technologically ready gravitational astronomy missions. By refining technologies that support very precise measurements in space, TianQin sets a precedent for future missions that could comprehensively survey gravitational phenomena across a broader range of sources. Theoretically, successful detection of GWs with TianQin could provide data fundamental to validating and extending theoretical models in astrophysics and cosmology.

Future Prospects

Upon achieving its primary detection goals, TianQin could potentially evolve into a more generalized observatory capable of contributing to a wide array of gravitational wave research objectives. Enhancing international collaboration on such missions also offers pathways to consolidate efforts for a more cohesive, global understanding of gravitational wave physics.

In sum, TianQin represents a viable, scientifically rich, and technologically grounded approach to extending gravitational wave astronomy into frequency ranges and astrophysical inquiries currently inaccessible from terrestrial efforts. As the paper accurately outlines, focused preparatory strategies and leveraging of cutting-edge technology are pivotal components steering the mission towards its proposed launch.