Wide-Field Science – Regular
Tzu-Ching Chang / Jet Propulsion Laboratory, PI
Gravitational waves (GWs) are a new avenue of observing our Universe. So far, we have seen them in the ~10-100 Hz range, and there are hints that we might soon detect them in the nanohertz regime. Multiple efforts are underway to access GWs across the frequency spectrum; however, parts of the frequency space are currently not covered by any planned or future observatories. Our recent work has shown that photometric surveys can bridge the microhertz gap in the spectrum between LISA and Pulsar Timing Arrays (PTAs) through relative astrometric measurements. Similar to PTA measurements, these astrometric measurements rely on the correlated spacetime distortions produced by gravitational waves at Earth, which induce coherent, apparent stellar position changes on the sky. To detect the microhertz GWs with an imaging survey, a combination of high relative astrometric precision, a large number of observed stars, and a high cadence of exposures are needed. The Roman Galactic Bulge Time Domain Survey (RGBTDS) would have all of these components. Our program seeks to simulate relevant data and explore survey designs for the Roman mission to better estimate the sensitivity of Roman to gravitational waves from supermassive black holes in the microhertz regime.
Using analytic estimates, we calculated that the RGBTDS is sensitive to GWs with frequencies ranging from 7.7 × 10-8 Hz to 5.6 × 10-4 Hz, which opens up a unique GW observing window for supermassive black hole binaries and their waveform evolution. While the detection threshold assuming the currently expected performance proves too high for detecting individual GWs given the expected supermassive black hole binary population distribution, we showed that Roman would still be sensitive to the stochastic GWB with an estimated signal-to-noise (SNR) ~ 1 and set interesting limits to SMBH population and evolution. If the mean astrometric deflection, which is normally lost due to the use of guiding stars as reference for spacecraft pointing solution, could be recovered, a factor of 100 in sensitivity improvement could be expected. We will investigate this exciting prospect, which would allow confident detection of SMBH with a chirp mass Mc >107 M☉ out to 50 Mpc, and detection of stochastic GWB with an estimated SNR ~70.
In this proposal, working with mission and RGBTDS experts, we propose to study and simulate several key aspects of the RGBTDS, including the recovery of the mean astrometric deflection, as well as studying how modifications to the RGBTDS design could improve the sensitivity to GWs. Relying on these estimates, we will also develop an optimal statistic tailored to Roman to detect spatially and temporally coherent modulations and stochastic gravitational wave background modulations (e.g., from the superposition of many SMBHBs) in the RGBTDS data. Our program will guide the future development of the full simulation and analysis pipeline that will be required to make Roman a space based GW detector.