Pulsar timing arrays have found evidence for a low-frequency gravitational-wave background (GWB). Assuming that the GWB is produced by supermassive black hole binaries (SMBHBs), the next gravitational-wave (GW) signals astronomers anticipate are continuous waves (CWs) from single SMBHBs and their associated GWB anisotropy. The prospects for detecting CWs and anisotropy are highly dependent on the astrophysics of SMBHB populations. Thus, information from single sources can break degeneracies in astrophysical models and place much more stringent constraints than the GWB alone. We simulate and evolve SMBHB populations, model their GWs, and calculate their anisotropy and detectability. We investigate how varying components of our semianalytic model, including the galaxy stellar mass function, the SMBH–host galaxy relation (M _BH–M _bulge), and the binary evolution prescription, impact the expected detections. The CW occurrence rate is greatest for few total binaries, high SMBHB masses, large scatter in M _BH–M _bulge, and long hardening times. The occurrence rate depends most on the binary evolution parameters, implying that CWs offer a novel avenue to probe binary evolution. The most detectable CW sources are in the lowest frequency bin for a 16.03 yr PTA, have masses from ∼10⁹ to 10¹⁰ M _⊙, and are ∼1 Gpc away. The level of anisotropy increases with frequency, with the angular power spectrum over multipole modes ℓ varying in low-frequency C _ℓ>0/C ₀ from ∼5 × 10⁻³ to ∼2 × 10⁻¹, depending on the model; typical values are near current upper limits. Observing this anisotropy would support SMBHB models for the GWB over cosmological models, which tend to be isotropic.

中文翻译：

---

超越背景：来自超大质量黑洞双星的引力波各向异性和连续波

脉冲星计时阵列发现了低频引力波背景（GWB）的证据。假设 GWB 由超大质量黑洞双星 (SMBHB) 产生，天文学家预计的下一个引力波 (GW) 信号是来自单个 SMBHB 的连续波 (CW) 及其相关的 GWB 各向异性。检测连续波和各向异性的前景高度依赖于 SMBHB 群体的天体物理学。因此，来自单一来源的信息可以打破天体物理模型的简并性，并比单独的 GWB 施加更严格的约束。我们模拟和演化 SMBHB 群体，对其 GW 进行建模，并计算其各向异性和可检测性。我们研究半解析模型的不同组成部分，包括星系恒星质量函数、SMBH-宿主星系关系（中号 _BH –中号 _bulge）和二元进化处方会影响预期的检测。对于少数总双星、高 SMBHB 质量、大分散度，CW 出现率最高中号 _BH –中号 _膨胀，硬化时间长。发生率主要取决于双星演化参数，这意味着连续波为探测双星演化提供了一种新途径。最可检测的 CW 源位于 16.03 年 PTA 的最低频率箱中，质量从 ∼10 ⁹到 10 ¹⁰ 中号 _⊙，距离 ∼1 Gpc。各向异性水平随着频率的增加而增加，多极模式的角功率谱也随之增加ℓ低频变化C _ℓ>0 /C ₀从 ∼5 × 10 ^-3到 ∼2 × 10 ^-1，具体取决于模型；典型值接近当前上限。观察到这种各向异性将支持 GWB 的 SMBHB 模型，而不是倾向于各向同性的宇宙学模型。