Multiband Gravitational Wave Detection Prospects for M31 UCXB-1 System in Low and Middle Frequency Band

Abstract

The recent discovery of M31 UCXB-1, the first extragalactic ultracompact X-ray binary (UCXB) with an orbital period of $T_{\rm orb} \sim 465$ s, presents a unique laboratory for studying close binary evolution and an unprecedented target for continuous gravitational wave (GW) searches. Its identification as a strong candidate black hole-white dwarf (BH-WD) system, combined with its exceptionally short period and high X-ray luminosity, suggests it may be one of the most vital low-frequency GW sources in M31. In this \textit{Letter}, we investigate the detectability of its GW signal for future space-borne detectors in multiband GW detection. We find that while its signal-to-noise ratio (S/N) for low-frequency detectors remains marginal for high-confidence detection, middle-frequency detectors such as DECIGO and BBO are far more promising, potentially achieving S/N $\varrho>8$ within reasonable observational duration. With a primary mass of only $m_1 > 5.4M_\odot$ (or $6.6M_\odot$), the network of all low and middle frequency detector (or BBO alone) is capable of detecting GW from this system with a $\varrho > 8$, during 10-year observation. Furthermore, orbital eccentricity can enhance the GW strain at higher harmonics, further improving its detectability, especially for middle-frequency detectors. This study establishes M31 UCXB-1 as a key prototype of short-period UCXBs, cementing its role as a cornerstone for multiband, multi-messenger astrophysics and a vital bridge between X-ray astronomy and the future GW era.

Figures (11)

• Figure 1: Possible characteristic strain $h_{\rm c}(f)$ of GW emitted from this system [red ($m_1=3M_\odot$) and green ($m_1=20M_\odot$) points($T_{\rm obs}=4$ yr)/diamond($T_{\rm obs}=10$ yr)] and noise curve $h_{\rm n}(f)$ for different GW detectors in low and middle frequency band with different colors represent different GW detectors as shown in the legend. These dotted lines for Taiji and LISA represent the noise curves of GW detectors without Galactic binaries foreground confusion 2023PhRvD.107f4021L. And this black line with triangle (red line with $\times$) symbols represents characteristic strain $h_{{\rm c},i}$ for eccentric orbit with $e=0.2$(0.4), $m_1=3M_\odot$, and $T_{\rm obs}=4$ yr.
• Figure 2: S/N as the function of primary mass $m_1$ for $T_{\rm obs}=4$ yr, where the vertical black dotted line indicates the lower mass limit for black holes, $3M_\odot$; the horizontal black dashed line indicates threshold S/N $\varrho=8$. We utilize colorful dotted lines to represent S/N for GW detector networks.
• Figure 3: S/N as the function of primary mass $m_1$ for $T_{\rm obs}=10$ yr. It is similar to Figure \ref{['fig:SNR_m4']} except $T_{\rm obs}=10$ yr.
• Figure 4: The relative uncertainty $\Delta m_1/m_1$ versus BH mass $m_1$ for $T_{\rm obs}=4$ yr. We utilize different colors to represent different values of the S/N as indicated in color bar. Since a larger $m_1$ leads to a higher S/N $\varrho$, the constraining power on $m_1$ correspondingly increases, and the relative uncertainty $\Delta m_1/m_1$ becomes smaller.
• Figure 5: The relative uncertainty $\Delta m_1/m_1$ versus BH mass $m_1$ for $T_{\rm obs}=10$ yr. It is similar to Figure \ref{['fig:Dm_m10']}, except $T_{\rm obs}=10$ yr.