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Spectra and variability of accreting black holes
In this thesis, we present various results concerning the structure of matter close to black holes when they are actively accreting. The two components (apart from the black hole itself) most relevant to this work are the accretion disc and X-ray emitting corona. We use various methods to help determine their location, properties and relation to each other.
In chapters 2 and 3, we consider how thermal emission from the accretion disc is affected by the coronal emission. Chapter 2 presents an analysis of a sample of supermassive black hole (SMBH) sources, showing that in many of these sources, variability in the accretion disc emission is driven by heating from the central source. Chapter 3 presents results from a dedicated campaign on a particularly X-ray variable object, IRAS 13324-3809, which however does not show disc variability in response to X-ray variations. We discuss potential reasons for this.
Chapters 4 and 5 consider what we can learn about accreting black holes from their X-ray spectra. Chapter 4 describes the use of X-rays which reflect from the disc to study properties of the disc such as its innermost extent (governed in part by the black hole spin) and ionisation state. Chapter 5 analyses hard X-ray spectra of two sources, including the electron temperature of the corona, and considers the physical process which could produce the observed temperature.
Finally, in chapter 6, we use a number of the techniques utilised previously to perform an in-depth study of the recent outburst from the stellar-remnant black hole in the MAXI 1820+070 X-ray binary system in our own Galaxy. As they are much smaller, such systems can show far more drastic state changes on observable timescales than SMBH can; the reasons for these state changes are not yet fully explained. The reflected emission shows that the disc extends close to the black hole, which has not been universally accepted in the state observed here. That the reflecting disc extends so close to the black hole also constrains the possible causes of characteristic timescales (QPOs) in the variability.Funded by a STFC studentship
The Link Between Neutron Star X-ray Binaries and Spider Pulsars
The discovery of transitional millisecond pulsars (tMSPs) provided conclusive
proof that neutron star (NS) low-mass X-ray binaries (LMXBs) comprise part of
the evolutionary pathway towards binary millisecond pulsars (MSPs). Redback and
black widow `spider' pulsars are a sub-category of binary MSPs that `devour'
their companions through ablation - the process through which material is
lifted from the stellar surface by a pulsar wind. In addition to reducing the
companion star's mass, ablation introduces observable characteristics like
extended, energy-dependent and asymmetric eclipse profiles in systems observed
at a sufficiently high inclination. Here, we present a detailed study and
comparison of the X-ray eclipses of two NS LMXBs;
J1858.60814 and EXO 0748676. Some of the X-ray eclipse characteristics
observed in these two LMXBs are similar to the radio eclipse characteristics of
eclipsing redback and black widow pulsars, suggesting that they may also host
ablated companion stars. X-ray irradiation or a pulsar wind could drive the
ablation. We conduct orbital phase-resolved spectroscopy for both LMXBs to map
the column density, ionization and covering fraction of the material outflow.
From this, we infer the presence of highly ionized and clumpy ablated material
around the companion star in both systems. We term LMXBs undergoing ablation,
, and speculate that they may be the progenitors of
redback pulsars under the assumption that ablation begins in the LMXB stage.
Therefore, the false widows could provide a link between LMXBs and spider
pulsars. The detection of radio pulsations during non-accreting states can
support this hypothesis.Comment: 21 pages, 15 figures. Accepted for publication in MNRAS. The full
version of Table 1 is available as online supplementary material from the
MNRAS websit
Outflow Legacy Accretion Survey: unveiling the wind driving mechanism in BHXRBs
Transient black-hole X-ray binaries viewed at high inclinations display blue-shifted absorption lines in their X-ray spectra. These features are the signatures of powerful, hot and equatorial accretion disk winds being driven from these systems in their luminous soft states. Remarkably, blue-shifted absorption lines have recently also been discovered in optical and NIR recombination lines and ultraviolet resonance features. These features must also be produced in an outflow, but the physical conditions traced by these outflows are different. Despite this, the characteristic Doppler velocities of all three types of signatures are comparable, yet they have never been observed simultaneously. It is therefore completely unclear if they are associated with distinct outflows (e.g. driven by different mechanisms) or simply with different regions/phases within the same outflow. Here, we propose to answer this question by carrying out simultaneous time-resolved spectroscopy of a high-inclination system in the X-ray, ultraviolet and optical bands, in its two distinct physical configurations (hard- and soft-states). This will allow us to test if the three types of wind features are present simultaneously, and, if so, whether they display correlated variability and/or velocity structure
The Thermonuclear X-Ray Bursts of 4U 1730-22
We present observations of the historic transient 4U 1730â22 as observed with the Neutron Star Interior Composition Explorer (NICER). After remaining in quiescence since its 1972 discovery, this X-ray binary showed renewed outburst activity in 2021 and 2022. We observed 4U 173â22 extensively with NICER, detecting a total of 17 thermonuclear X-ray bursts. From a spectroscopic analysis, we find that these X-ray bursts can be divided into a group of bright and weak bursts. All bright bursts showed 1â2 s rise times and a photospheric radius expansion phase, while the weak bursts showed a slower âŒ5 s rise with a tendency for concave shapes. From the photospheric radius expansion flux, we estimate the source distance at 6.9 ± 0.2 kpc. We consider various interpretations for our observations and suggest that they may be explained if accreted material is burning stably at the stellar equator and unstable ignition occurs at a range of higher latitudes
Disk, Corona, Jet Connection in the Intermediate State of MAXI J1820+070 Revealed by NICER Spectral-timing Analysis
We analyze five epochs of Neutron star Interior Composition Explorer (NICER) data of the black hole X-ray binary MAXI J1820+070 during the bright hard-to-soft state transition in its 2018 outburst with both reflection spectroscopy and Fourier-resolved timing analysis. We confirm the previous discovery of reverberation lags in the hard state, and find that the frequency range where the (soft) reverberation lag dominates decreases with the reverberation lag amplitude increasing during the transition, suggesting an increasing X-ray emitting region, possibly due to an expanding corona. By jointly fitting the lag-energy spectra in a number of broad frequency ranges with the reverberation model reltrans, we find the increase in reverberation lag is best described by an increase in the X-ray coronal height. This result, along with the finding that the corona contracts in the hard state, suggests a close relationship between spatial extent of the X-ray corona and the radio jet. We find the corona expansion (as probed by reverberation) precedes a radio flare by ~5 days, which may suggest that the hard-to-soft transition is marked by the corona expanding vertically and launching a jet knot that propagates along the jet stream at relativistic velocities
Guiding structures with multiply connected cross-sections: evolution of propagation in external fields at complex Robin parameters
Properties of the two-dimensional ring and three-dimensional infinitely long
straight hollow waveguide with unit width and inner radius in the
superposition of the longitudinal uniform magnetic field and
Aharonov-Bohm flux are analyzed within the framework of the scalar Helmholtz
equation under the assumption that the Robin boundary conditions at the inner
and outer confining walls contain extrapolation lengths and
, respectively, with nonzero imaginary parts. It is shown that,
compared to the disk geometry, the annulus opens up additional possibilities of
varying magnetization and currents by tuning imaginary components of the Robin
parameters on each confining circumference; in particular, the possibility of
restoring a lossless longitudinal flux by zeroing imaginary part of the
total transverse energy is discussed. The energy turns real under
special correlation between the imaginary parts of and
with the opposite signs what physically corresponds to the
equal transverse fluxes through the inner and outer interfaces of the annulus.
In the asymptotic case of the very large radius, simple expressions are derived
and applied to the analysis of the dependence of the real energy on
and . New features also emerge in the magnetic
field influence; for example, if, for the quantum disk, the imaginary energy
is quenched by the strong intensities , then for the annulus this
takes place only when the inner Robin distance is real;
otherwise, it almost quadratically depends on with the corresponding
enhancement of the reactive scattering. Closely related problem of the hole in
the otherwise uniform medium is also addressed for real and complex
extrapolation lengths with the emphasis on the comparative analysis with its
dot counterpart.Comment: 37 pages, 9 figure
Extending the Baseline for SMC X-1's Spin and Orbital Behavior with NuSTAR Stray Light
StrayCats, the catalog of NuSTAR stray light observations, contains data from bright X-ray sources that fall within crowded source regions. These observations offer unique additional data with which to monitor sources such as X-ray binaries that show variable timing behavior. In this work, we present a timing analysis of stray light data of the high-mass X-ray binary SMC X-1, the first scientific analysis of a single source from the StrayCats project. We describe the process of screening stray light data for scientific analysis, verify the orbital ephemeris, and create both time- and energy-resolved pulse profiles. We find that the orbital ephemeris of SMC X-1 is unchanged and confirm a long-term spin-up rate of ÎœË=(2.52±0.03)Ă10â11 Hz s-1. We also note that the shape of SMC X-1's pulse profile, while remaining double peaked, varies significantly with time and only slightly with energy
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