AI 中文总结
本研究利用NICER与NuSTAR首次综合研究超紧凑X射线双星4U 1812-12，探测其硬谱态，通过relxill模型分析光谱特征，发现吸积盘参数及时序特性，为中子星低质量X射线双星研究提供新数据。
AI 中文摘要
源4U 1812-12是一颗持续存在、弱变的低质量X射线双星，包含一颗中子星。该源于2019年至2021年被NICER观测，更近的2025年被NuSTAR观测。在NICER与NuSTAR观测期间，该源被探测到处于硬谱态，其热光度约为1.90×10³⁶ erg s⁻¹。NuSTAR的3-70 keV能谱具有吸积盘的软热辐射、冕的硬康普顿化辐射以及其在吸积盘上的反射特征；该能谱中存在明显的吸积盘反射特征，使用自洽相对论反射模型{\tt relxill}拟合。反射建模显示，吸积盘处于中等电离态（log ξ≈2.72），延伸至接近中子星表面（Rᵢₙ≲1.72 R_ISCO），观测倾角较小（i≈25度）。假设磁场（B）截断吸积盘，得到B≲2.54×10⁸ G，与中子星低质量X射线双星（NS LMXBs）的典型值相当。NICER的1.0-9.5 keV能谱同样具有软热成分和主导的硬康普顿化成分；在NICER观测期间，吸积盘温度在约0.69-0.84 keV内小幅度变化，而幂律光子指数Γ则有0.8-1.5的大幅变化，表明康普顿化辐射发生了显著改变。此外，NICER时序分析显示存在宽带非周期性变异性，在0.379±0.008 Hz和0.724±0.025 Hz处有显著的准周期振荡（QPO）类特征，其分数均方根振幅分别为2.9±0.6%和4.1±0.5%。
英文摘要
The source 4U 1812-12 is a persistent, weakly variable low-mass X-ray binary containing a neutron star. The source was observed by NICER between 2019 and 2021 and, more recently, by NuSTAR in 2025. During the NICER and NuSTAR observations, the source was detected in a hard spectral state with a bolometric luminosity of $\sim 1.90\times 10^{36}$ ergs s$^{-1}$. Its $3-70$ keV NuSTAR spectrum is characterized by a soft thermal emission from the disc, a hard Comptonized emission from the corona, and its reflection from the accretion disc. The NuSTAR energy spectrum exhibits the clear presence of disc reflection features, fitted using a self-consistent relativistic reflection model {\tt relxill}. Our reflection modeling indicates a moderately ionized accretion disc (log\:$ξ\sim2.72$) extending close to the neutron star surface ($R_{in}\lesssim 1.72\:R_{ISCO}$), and viewed through a small inclination angle ($i\sim 25$ degrees). Assuming that the magnetic field ($B$) truncates the disc, we found $B\lesssim 2.54\times 10^{8}$ G, comparable to the typical values observed for NS LMXBs. The $1.0-9.5$ keV NICER spectra are also characterized by a soft thermal component and a dominant hard Comptonized component. During NICER observations, the disc temperature exhibits a small variation within $\sim 0.69-0.84$ keV. In contrast, the power law photon index, $Γ$, exhibits a large variation of $\sim 0.8-1.5$, implying a substantial change in the Comptonized emission. Moreover, NICER timing analysis reveals broadband aperiodic variability with significant QPO-like features at $0.379\pm 0.008$ Hz and $0.724\pm 0.025$ Hz, having fractional rms amplitudes of $2.9\pm 0.6\%$ and $4.1\pm 0.5\%$, respectively.
Comments15 pages,13 figures,6 tables,Submitted to ApJ
