分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The discrepancy between the value of the Hubble constant $H_0$ measured from the local distance ladder and from the cosmic microwave background is the most serious challenge to the standard $\Lambda$CDM model. Various models have been proposed to solve or relieve it, but no satisfactory solution has been given until now. Here, we report a late-time transition of $H_{0}$, i.e., $H_0$ changes from a low value to a high one from early to late cosmic time, by investigating the Hubble parameter $H(z)$ data based on the Gaussian process (GP) method. This finding effectively reduces the Hubble crisis by 70\%. Our results are also consistent with the descending trend of $H_0$ measured using time-delay cosmography of lensed quasars at 1$\sigma$ confidence level, and support the idea that the Hubble crisis arises from new physics beyond the standard $\Lambda$CDM model. In addition, in the $\Lambda$CDM model and $w$CDM model, there is no transition behavior of $H_{0}$.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Fast radio bursts (FRBs) are cosmic sources emitting millisecond-duration radio bursts. Although several hundreds FRBs have been discovered, their physical nature and central engine remain unclear. The variations of Faraday rotation measure and dispersion measure, due to local environment, are crucial clues to understanding their physical nature. The recent observations on the rotation measure of FRB 20201124A show a significant variation on a day time scale. Intriguingly, the oscillation of rotation measure supports that the local contribution can change sign, which indicates the magnetic field reversal along the line of sight. Here we present a physical model that explains observed characteristics of FRB 20201124A and proposes that repeating signal comes from a binary system containing a magnetar and a Be star with a decretion disk. When the magnetar approaches the periastron, the propagation of radio waves through the disk of the Be star naturally leads to the observed varying rotation measure, depolarization, large scattering timescale, and Faraday conversion. This study will prompt to search for FRB signals from Be/X-ray binaries.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Recent observations discovered that some repeating fast radio bursts (FRBs) show complicated variations and reversals of Faraday rotation measures (RMs), indicating that the sources of these FRBs are embedded in a dynamically magnetized environment. One possible scenario is that repeating FRBs are generated by pulsars in binary systems, especially containing a high-mass companion with strong stellar outflows. Here, we study the RM variations caused by stellar winds, and a possible stellar disc. If the magnetic field is radial in the stellar wind, RM will not reverse except if the magnetic axis inclination angle is close to $90 ^\circ$. For the toroidal magnetic field in the wind, RM will reverse at the super conjunction. For the case of the toroidal field in the disc, the RM variations may have a multimodal and multiple reversal profile because the radio signals travel through different components of the disc during periastron passage. We also apply this model to FRB 20180916B. By assuming its 16.35-day period is from a slowly rotating or freely precessing magnetar, we find that the secular RM variation can be explained by the periastron passage of a magnetar in a massive binary system. In addition, the clumps in the stellar wind and disc can cause short time-scale ($< 1$ day) variations or reversals of RM. Therefore, long-term monitoring of RM variations can reveal the environments of repeating FRBs.
点击量
待评议
点击量
下载量
评论
