A Hard Look at Accretion Around Neutron Stars

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.PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/150033/1/rmludlam_1.pd

Spectral and timing evolution of the black hole transient MAXI J1727-203 with NICER

MAXI J1727-203 is a new X-ray transient discovered on 5 June 2018. A hard-to-soft state transition at the beginning of the outburst led to the identification as a black hole candidate. MAXI J1727-203 was monitored with the Neutron Star Interior Composition Explorer (NICER) on an almost daily basis from the beginning of the outburst. We present a spectral and timing analysis of the full outburst of the source, which lasted approximately four months. A preliminary spectral analysis suggest that the accretion disk component can was detected throughout the entire outburst, with temperatures ranging from ~0.4 keV (in the soft state), down to ~0.2 keV near the end of the outburst when the source was in the hard state. The power spectrum in the hard state shows broadband noise up to 10 Hz, with no detection of any quasi-periodic oscillations. We argue that the system's characteristics are not consistent with those expected for a neutron star and that they are particularly reminiscent of the black hole X-ray binaries XTE J1118+480 and Cyg X-1

NICER Detects a Soft X-Ray Kilohertz Quasi-periodic Oscillation in 4U 0614+09

We report on the detection of a kilohertz quasi-periodic oscillation (QPO) with the Neutron Star Interior Composition Explorer (NICER). Analyzing approximately 165 ks of NICER exposure on the X-ray burster 4U 0614+09, we detect multiple instances of a single-peak upper kHz QPO, with centroid frequencies that range from 400 to 750 Hz. We resolve the kHz QPO as a function of energy, and measure, for the first time, the QPO amplitude below 2 keV. We find the fractional amplitude at 1 keV is on the order of 2% rms, and discuss the implications for the QPO emission process in the context of Comptonization models. Key words: accretion, accretion disks – stars: neutron – X-rays: binaries – X-rays: individual (4U 0614+0

A NICER look at the Aql X-1 hard state

We report on a spectral-timing analysis of the neutron star low-mass X-ray binary (LMXB) Aql X-1 with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station (ISS). Aql X-1 was observed with NICER during a dim outburst in 2017 July, collecting approximately 50 ks of good exposure. The spectral and timing properties of the source correspond to that of a (hard) extreme island state in the atoll classification. We find that the fractional amplitude of the low-frequency (<0.3 Hz) band-limited noise shows a dramatic turnover as a function of energy: it peaks at 0.5 keV with nearly 25% rms, drops to 12% rms at 2 keV, and rises to 15% rms at 10 keV. Through the analysis of covariance spectra, we demonstrate that band-limited noise exists in both the soft thermal emission and the power-law emission. Additionally, we measure hard time lags, indicating the thermal emission at 0.5 keV leads the power-law emission at 10 keV on a timescale of ∼100 ms at 0.3 Hz to ∼10 ms at 3 Hz. Our results demonstrate that the thermal emission in the hard state is intrinsically variable, and is driving the modulation of the higher energy power-law. Interpreting the thermal spectrum as disk emission, we find that our results are consistent with the disk propagation model proposed for accretion onto black holes.United States. National Aeronautics and Space Administration. Neutron Star Interior Composition ExplorerUnited States. National Aeronautics and Space Administration. Astrophysics Research and Analysis Progra