High Sensitivity Methodologies to Detect Radio Band Gravitational Waves

Abstract

Gravitational waves (GWs) can resonate with magnetic fields through the Gertsenshtein-Zeldovich effect, producing electromagnetic signals at the same frequency. In pulsar magnetospheres, this conversion may yield a faint radio-band signal that could be detected. In this work, we focus on two specific pulsars, PSR J1856-3754 and PSR J0720-3125, and use numerical simulations to evaluate how well the FAST and SKA2-MID telescopes could detect such signals. We consider transient events, including primordial-black-hole-like mergers, as well as stochastic backgrounds, including primordial GWs. To improve detection sensitivity, we propose four observational methods to lower the detectable energy-density limit of very high-frequency (VHF) GWs; the "Multiple Pulsars with Multiple Telescopes" (MPMT) method performs best because it allows cross-validation and rejection of false candidates. Under the assumption of nearly 6000 hours of observation at 3 GHz and a $5σ$ detection threshold, the minimum detectable characteristic strain is projected to be $h_c \approx 10^{-23}$ for transient events and $h_c \approx 10^{-33}$ for stochastic backgrounds. Under optimistic assumptions on integration time and conversion efficiency, these projections suggest that radio-band searches may approach the sensitivity needed to begin testing representative VHF GW scenarios. More broadly, this conversion in pulsar magnetospheres could be relevant to the origin of some repeating fast radio bursts in the our galaxy.

Figures (24)

• Figure 1: PIC simulation results of pulsar magnetospheres of PSR J1856-3754. The picture is divided into two parts. And there are several parts for each pulsar: the top panel shows a slice of the entire simulation in the plane at z = 0 for the three magnetic field directions, and the middle panel shows the electromagnetic field around the radius of the neutron star zoomed in on the top panel. Both panels show the magnetic fields in the order $Bx$, $By$, and $Bz$ in that order. The left side of the bottom panel shows the data used for the examination of the pulsar integral profile and the results of the comparison between the pulsar profile generated by our PIC simulation. In contrast, the right side shows the pulsar parameters and a schematic of the gravitational waves crossing the magnetic field in the direction of the observational LOS.
• Figure 2: PIC simulation results of pulsar magnetospheres of PSR J0720-3125. This figure shows the results in the same order as Figure 1.
• Figure 3: Reference intensity‑response profiles, frequency‑dependent broadening, and simulated time series for GW-EM conversion in pulsar magnetospheres. Top-left: normalised intensity-response profiles for the two pulsars at selected frequencies, shown for the short-transient case at a fixed rotational phase and produced with the filtering pipeline’s bandpass, sampling, and normalisation; these are intensity/flux-density response kernels, not phase-resolved GW-strain templates. Top‑right: frequency‑dependent temporal broadening adopted to model propagation and instrumental effects such as interstellar scintillation and scattering, shown for FAST and SKA2‑MID. Middle and bottom: simulated intensity time series for PSR J1856-3754 observed with FAST (middle) and SKA2‑MID (bottom). Left column displays transient injections as short, band‑limited excesses; right column shows persistent or stochastic‑background cases in which long integrations reveal the rotation‑modulated intensity envelope from the phase‑dependent magnetospheric response.
• Figure 4: Reference intensity‑response profiles, frequency‑dependent broadening, and simulated time series for GW-EM conversion in pulsar magnetospheres upon reception by FAST and SKA2-MID of PSR J0720-3125. This figure shows the results in the same order as Figure 3.
• Figure 5: The linear polarization of a GW radio signal varies with frequency. The two diagrams at the top panel show the radio signals from PSR J1856-3754 and PSR J0720-3125, with FAST's signal represented by a coloured solid line and SKA2-MID's signal represented by a coloured dashed line. The bottom panel shows the fit of the depolarisation theory of our signals to several examples of intragalactic FRB signals, where the coloured solid lines indicate the results of our fits to the repeated FRBs, and the coloured dashed lines indicate the results of our fits to the non-repeated FRBs. Different FRBs are represented by different shapes whose colours indicate the magnitude of the RM, with the line-polarised error bars of repeated FRBs shown as black solid lines, and those of non-repeated FRBs shown as red solid lines.