[Paper Review] Overview of KAGRA : KAGRA science

KAGRA Collaboration, T. Akutsu+245 more|arXiv (Cornell University)|Aug 6, 2020

Pulsars and Gravitational Waves ResearchPhysics and Astronomy541 references61 citations

TL;DR

The paper surveys KAGRA science prospects, including baseline KAGRA and near-term upgrades (KAGRA+) across stellar-mass BBHs, IMBHs, neutron star binaries, accreting binaries, isolated NSs, supernovae, early Universe GWs, gravity tests, late-time cosmology, and multimessenger opportunities.

ABSTRACT

KAGRA is a newly build gravitational-wave observatory, a laser interferometer with 3 km arm length, located in Kamioka, Gifu, Japan. In this paper in the series of KAGRA-featured articles, we discuss the science targets of KAGRA projects, considering not only the baseline KAGRA (current design) but also its future upgrade candidates (KAGRA+) for the near to middle term (~5 years).

Motivation & Objective

• Motivate the science cases accessible with current and near-future ground-based GW detectors including bKAGRA and upgrades (KAGRA+).
• Assess formation channels and GW signatures for stellar-mass BBHs, IMBHs, BNSs, accreting binaries, and isolated NSs.
• Explore the impact of KAGRA+ upgrades on parameter estimation, horizon distances, and network capabilities in the LVK (LIGO-Virgo-KAGRA) framework.
• Discuss prospects for GWs from the early Universe, tests of gravity, and multimessenger observations with KAGRA integrated into a global network.

Proposed method

• Review and synthesize science targets enabled by current bKAGRA and proposed KAGRA+ upgrades (LF, HF, 40 kg, FDSQZ, Combined).
• Compare detector sensitivity curves and forecast SNR and parameter estimation errors using Fisher information matrix analyses for representative BBH systems.
• Analyze how low-, middle-, and high-frequency sensitivity changes affect detection horizons, mass-ratio sensitivity, and multimessenger localization.
• Evaluate the potential to detect GWs from IMRIs/IMBHs, NS binaries, and early-Universe backgrounds across upgrade scenarios.

Experimental results

Research questions

• RQ1 What are the primary gravitational-wave sources KAGRA can observe today and under near-term upgrades?
• RQ2 How do different KAGRA+ upgrade configurations (LF, HF, 40 kg, FDSQZ, Combined) impact SNR, parameter estimation, and detection horizons for BBHs and IMBHs?
• RQ3 Can KAGRA + LVK networks discern formation channels (isolated binaries vs dynamical formation) and measure neutron star EOS or tidal effects?
• RQ4 What is KAGRA's role in multimessenger observations and cosmology with GW and EM/neutrino counterparts?

Key findings

• KAGRA+ upgrades substantially improve SNR and parameter precision for stellar-mass BBHs compared with 2G networks, with the Combined configuration delivering the largest gains.
• Low-frequency optimization (LF) greatly enhances detectability of intermediate-mass-ratio inspirals (IMRIs) and increases the accessible volume for IMBH systems.
• Improvements in sky localization and distance estimation with an expanded detector network (including KAGRA+) aid host-galaxy identification and astrophysical inference.
• Observations across BBH mass ranges, NS binaries, and GWB signatures can help distinguish formation channels, probe neutron-star EOS via tidal effects, and test gravity in strong fields.
• KAGRA’s role in a LVK network improves cosmological measurements (e.g., Hubble constant via standard sirens) and enables richer multimessenger science.