arXiv subjects

Xiaoming Bi

Publications and source records attributed to Xiaoming Bi.

4 recordsLinked to original sources

arXiv2026

Bridging the Population Synthesis of Supermassive Binary Black Holes and the Gravitational Wave Background

Pulsar Timing Arrays (PTAs) constrain population properties of supermassive binary black holes (SMBHBs) through the observation of the gravitational wave background (GWB). Unlike other approaches that interpolate population-synthesis libraries or only consider the mean of the strain spectrum, here we capture its full strain probability density directly from semi-analytic population models. We apply our new method to the semi-analytic SMBHB population model, independently reproducing the parameter estimation for this model performed by the NANOGrav Collaboration with their 15-yr data. We also show the extent to which discrete SMBHB contributions to the GWB resolve degeneracies in the population parameter space. Finally, using the source-count intensity as the intermediate product in our calculation, we map PTA observations, as a proof of principle, to the SMBHB model based on the galaxy merger prescriptions from numerical hydrodynamical simulations "Illustris". We find the effect of delay times $\tau$ between kiloparsec and subparsec SMBHB separations following galaxy mergers, finding $h_{\rm c}$ spanning $(1$-$6)\times10^{-16}$ and $N_{\rm c}$ spanning $(0.3$-$7.1)\times10^{-3}$ for $\tau$ up to 8 Gyr.

astro-ph.HE↗

arXiv2026

A Joint Optimal Search for Gravitational Waves from Resolved and Unresolved Supermassive Binary Black Holes with Pulsar Timing Arrays

We introduce, from first principles, a joint model of the gravitational wave background (GWB) and brightest supermassive black hole binary (SMBHB) sources that may be individually resolvable in Pulsar Timing Array (PTA) searches for gravitational waves. We propose the characteristic number of SMBHB sources, $N_{\rm c}$, as a detection statistic for the astrophysical origin of the GWB. We then demonstrate how the brightest SMBHBs assist in resolving $N_{\rm c}$. Applying our method to the simulated NANOGrav 15-year data, which replicates all aspects of real data's known noise, observations, and the inferred GWB power spectrum, we demonstrate direct astrophysical limits on the strain amplitude of individually resolvable SMBHBs. We find that 21 of 114 SMBHB candidates from active galactic nuclei observations are in tension with the NANOGrav's observations. In contrast, only one candidate is in tension with the NANOGrav data based on the upper limits reported in the original analysis. Constraining the Poisson-specific characteristic number of SMBHBs, $N_{\rm c}$, at ${\rm yr}^{-1}$, we outline implications for the population properties of SMBHBs. Based on our new model applied to the simulated NANOGrav data, we calculate the probability of detecting GWs from isolated SMBHB in the 15-year data to be 2\% at the ${\rm SNR}=5$ level. Our projection towards the expected NANOGrav 20-year data suggests an increase to 5\%. With this, we estimate the probability of finding an outlier with an SNR of 2 in the NANOGrav 20-year data to be $40\%$.

astro-ph.HE↗

arXiv2024

The response of the Moon to gravitational waves

The response of the Moon to gravitational waves (GWs) is used by some of the proposed lunar GW detectors like the Lunar Gravitational-wave Antenna (LGWA) to turn the Moon into an antenna for GWs. The deep connection between the lunar internal structure, its geophysical environment and the study of the Universe is intriguing, but given our limited understanding of the Moon today, it also makes it very difficult to predict the science potential of lunar GW detectors accurately. Lunar response models have been developed since the Apollo program, but there is evidence coming from seismic measurements during the Apollo missions that the models are not good enough and possibly underestimating the lunar GW response especially in the decihertz frequency band. In this paper, we will provide an extension of Freeman Dyson's half-space model to include horizontally layered geologies, which allows us to carry out computationally efficient calculations of the lunar GW response above 0.1\,Hz compared to the normal-mode simulations used in the past. We analyze how the results depend on the values of geometric and elastic parameters of the layered geological model, and we find that modifications of the geological model as required to explain Apollo seismic observations can boost the lunar GW response.

gr-qc↗

arXiv2024

Doppler Tracking Data of Martian Mission Tianwen-I and Upper Limit of Stochastic Gravitational Wave Background

Two way ranging data for spacecraft tracking of China's first Martian mission Tianwen-I is analysed. Shortly before the spacecraft entered the Mars parking orbit, the two way coherent microwave link between the spacecraft and the Earth resembles a long arm gravitational wave interferometer, with both the spacecraft and the Earth regarded as in an approximate free falling state. By carefully selecting and analysing data segments of the time series of the two way ranging data during this time span, a parametric statistical model is built for the data segments and an upper limit for the stochastic gravitational waves background (SGWB) is then estimated within the frequency window 0.1Hz to 0.1 mHz. The upper bound improves considerably on those obtained before. In particular, around the deci-Hz band, there is a three orders improvement on the bound obtained previously by the two way ranging data of the Chang e 3 mission. Scientific applications of the upper bound is then considered and a weak upper bound is worked out for axions which is a promising candidate for ultra light dark matter.

gr-qc↗