A Hard Look at Accretion Around Neutron Stars

[摘要] Neutron stars (NSs) are the most compact objects with a surface in the Universe. The only way to understand how matter behaves under the conditions found in NSs is to determine the equation of state (EoS) of ultradense, cold matter. The EoS sets the radius for a NS of a given mass, therefore, measurements of masses and radii can be used to rule out or confirm theoretical EoSs. One method to determine radii of NSs utilizes atomic lines that arise from the inner region of the accretion disk. These lines are broadened due to Doppler and relativistic effects from the motion of the disk and extreme gravity near the NS. The resolution and sensitivity of NuSTAR in the 3-79 keV bandpass have provided an unprecedented look at the innermost accretion flow onto NSs. Numerous observations have revealed clear disk reflection spectra, unbiased by detector effects or modeling degeneracies. In this dissertation I demonstrate the importance of these features in determining properties of the accretion disk and NS in low-mass X-ray binaries (LMXBs).The discovery of multiple emission lines that originate from different ionization states - and presumably radii - within the disk are presented in Chapter 2. Modeling of these lines does not return distinct radii, but this is promising nonetheless for future endeavors that can capture these features with higher signal-to-noise. In Chapter 3, a sample of persistently accreting NSs reveals tight constraints on the position on the inner disk close to the NS. This allows for regions on the mass-radius plane to be traced out, which are already comparable to constraints obtained from other methods to determine NS mass and radius. Moreover, in Chapter 4 I perform the first reflection study of the NS transient XTE J1709-267 as the source transitions to a higher accretion rate. Hence, I analyze these states separately to track changes in the inner accretion disk, but disk properties remain consistent. In Chapter 5, I demonstrate that Fe lines can be used to estimate the magnetic field strength in these systems to first-order by comparing to estimates from pulsations seen in accreting millisecond X-ray pulsars.With the growing number of NuSTAR observations of reflection spectra in NS LMXBs, I am able to look at the sample as a whole to explore how the inner disk radius changes as a function of mass accretion rate (Chapter 6). There is no clear correlation between the inner disk position and mass accretion rate; confirming previous studies. The recent launch of NICER now affords the opportunity to search for low-energy relativistic lines down to 0.25 keV using detectors that are also free of distortions at high flux levels. In Chapter 7, I perform the first NICER spectral study using observations of Serpens X-1. This confirmed the reflection nature of the Fe L blend for the first time in a NS system.Reflection studies of NS LMXBs provides information on NS radii, magnetic field strengths, potential boundary layers between the inner accretion flow and NS surface, as well as properties of the accreting material. The combined bandpass and sensitivity of NuSTAR and NICER opens a new opportunity to reveal the entire reflection spectrum, and to measure different observables within these systems (Chapter 8). Future studies will enhance our understanding of accretion and how matter behaves under ultradense, cold conditions.

[发布机构] University of Michigan

[效力级别] Accretion

[关键词] Neutron star;Accretion;X-rays;Astronomy;Science;Astronomy and Astrophysics