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X-Ray Binaries

Bachelor and master thesis

Tutor: Sara Motta
Representative image for thesis
Representation of an X-ray binary. (Credit: NASA/R. Hynes)
Background - X-ray binaries:
Throughout the Universe the combination of a deep potential well and an accretion disc, (which forms when matter is gravitationally captured by a celestial body) leads to the generation of fast, collimated outflows called jets.
This process, still poorly understood, occurs in proto-planetary discs and at the centre of galaxies alike, but around black hole (BHs) and neutron stars (NSs) it is taken to the extreme.
In a process known as feedback these so-called compact objects contrive to feed back to the surrounding space a large fraction of the energy and matter they could have swallowed, thereby acting to heat their environment rather than behaving only as sinks.
Feedback is important across a range of scales: from stellar-mass BHs and NSs in X-ray binaries (XRBs), to super-massive BHs powering the AGN, which via this process regulated the growth of massive galaxies.
The AGN and XRBs hosting BHs and NSs provide us with the best tests of General Relativity, but while the former evolve over decades to millenia, XRBs evolve rapidly, offering us the opportunity to probe on humanly accessible time-scales the energy and matter input/output around accreting objects.
The knowledge gained from studying XRBs can then be directly applied to AGN, where the inflow/outflow processes follow the same basic principles as around stellar-mass BHs.

Using observations from across the entire electromagnetic spectrum, and employing various techniques best-suited to extract the information stored in the data we investigate the physics of the accretion and outflow generation processes in X-ray binaries, with the aim of understanding the nature of such processes and the link between them both on stellar-mass scales, and on super-massive scales.
  • A study of the accretion process in active black hole and neutron stars across a state transition
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Essentially all accreting black hole and neutron star X-ray binaries show a number of accretion states, characterized by specific features in terms of their luminosity, spectral properties, fast time-variability, and outflows production.
The canonical accretion states include a hard and a soft state, and two intermediate states.
These are always crossed according to the same cyclical pattern: hard state > hard intermediate state > soft intermediate state > soft state > soft-intermediate state > hard intermediate state > hard state.
Sources spend a large amount of time (several months) in the hard and soft states, during which the spectral and timing properties of a source remain relatively constant, but the luminosity varies significantly.
Instead, the two intermediate states are crossed relatively rapidly (days or faster), and the spectral and timing properties vary dramatically, while the luminosity remains relatively stable.
There appears to be a correlation between the amount of time it takes for a source to go transition from the hard state to the soft state and the luminosity at which such transition occurs.

The aim of this thesis is to verify the existence of such correlation by means of spectral and timing analysis, and interpret it in the light of the most recent theoretical models describing the physics of accreting compact objects.
We will use X-ray data from the Rossi X-ray Timing Explorer satellite on a sample of Galactic black hole and neutron star X-ray binaries.
The results of this project will reveal important information of the accretion processes around compact objects, and will be relevant not only in the context of stellar mass compact objects, but will be also contrasted with and extended to the case of super-massive black holes powering the Active Galactic Nuclei.
  • A study of the long-term properties of Sco X-1
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Scorpius X-1 the first X-ray source ever discovered outside the Solar System, and also the first example of how accretion onto a compact object can lead to a release of gravitational energy so large and so efficient to produce radiation peaked in the X-rays.
Sco X-1 is a bright X-ray binary hosting a neutron star, which accretes at luminosities close or even exceeding the Eddington limit.
As such, this source is extremely bright in the X-rays, and hence can be easily observed even with instrumentation that are not particularly sensitive, but are able to observe a given source with very high cadence (often several times a day), such as the All Sky Monitors.
A very large dataset obtained with the MAXI mission on board the International Space Station is available for Sco X-1, almost completely unexplored, but which could hide crucial information to understand the behavior and properties of this important system.

The main aim of this thesis is to explore the dataset, with the goal of evidencing properties that only a long-term large database can reveal.
The results obtain via this projects will be contrasted with the known properties of the source (including its multi-wavelength behaviour), and interpreted accordingly.
  • Simultaneous radio and X-ray observations of X-ray binaries with MeerKAT and Swift
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The MeerKAT radio telescope is a large interferometer located in South-Africa, dedicated to the study of the radio sky between 1 and 4 GHz.
MeerKAT is one of the pathfinder for SKA, which will sign a new era of radio astronomy, characterized by high cadence, high sensitivity radio observations of the entire sky at moderate angular resolutions.
As part of the ThunderKAT project, which is dedicated to the study of both Galactic and extra-Galactic radio transients, we observe weekly interesting X-ray binaries located in the Southern Sky when they become active.
Simultaneous observations are taken with the Swift Observatory via the SwiftKAT large program (PI: S. E. Motta).

The aim of this thesis is to study both in radio and in the X-rays simultaneously a new outburst of a X-ray binary (either hosting a neutron star or a black hole) using brand new proprietary data collected with the MeerKAT radio telescope, and simultaneous X-ray data obtained with Swift.
We will extract radio images from the MeerKAT data, which may reveal the presence of transient relativistic jets which evolve over time.
The variations in the radio properties of the X-ray binary will be interpreted together with the results form the Swift light curves and energy spectra, with the aim of investigating the accretion-ejection coupling in the target.
The results will be then put in context of what is known of accreting compact objects, both stellar mass and super-massive.
  • Testing general Relativity on accreting black hole binaries: the case of Cyg X-1
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Cyg X-1 was discovered by a sounding rockets carrying X-ray sensitive instruments in 1964, and it was the first black hole X-ray binary ever discovered.
While the majority of black hole binaries are transient systems (i.e. they are X-ray bright only for relatively brief periods of time, and remain dim otherwise) Cyg X-1 is one of the few known persistent black holes systems in the Galaxy, which are relatively rare in the local Universe as it was established decades after its discovery.
Cyg X-1 has been studied thoroughly since its discovery, and an enormous dataset - still to be fully exploited - is available for studying this topical system.

The theory of General Relativity predicts the existence of characteristic frequencie associated to the motion of matter around a black hole.
These frequencies, if identified in the emission arising from an accreting black hole, would lead to key information on its fundamental properties: mass and intrinsic angular momentum (spin).
The Power Density Spectra of the X-ray lightcurves of accreting black holes are formed by features the characteristic frequencies of which can be associated to the frequencies predicted by General Relativity.

The aim of this thesis is to test the predictions of General Relativity using the data from Sco X-1, by means of Fourier Analysis applied to the X-ray data collected over the past decade on Cyg X-1 with the Rossi X-ray Timing Explorer.
Based on the association between the relativistic frequencies and specific power spectral density components we will obtain an independent estimate of the black hole spin, a measurement that is usually extremely challenging, but that is crucial to understand the fundamental properties of a black hole.
  • Timing-based spin and mass measurement of neutron star X-ray binaries
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In the theory of General Relativity (GR), the frequencies that characterize the motion of matter close to a rotating compact object (a neutron star or a black hole) can differ substantially from the classical frequencie in Newtonian gravity. GR predicts the existence of three different orbital frequencies: the relativistic Keplerian orbital frequency (in the azimuthal direction) and the related epicyclic frequencies: the vertical and radial epicyclic frequency (the so-called epicyclic frequencie).
GR predicts that the motion of matter in the immediate surroundings of black holes (BHs) carries the signature of yet untested strong-field gravity effects which are amongst the fundamental consequences of Einstein’s theory.
Quasi-periodic oscillations (QPOs) are commonly observed in the X-ray light curves of stellar-mass accreting compact objects. QPOs are the result of almost periodic fluctuations of the amount of light arising from an astronomical source and are detected and studied mainly through the analysis of a power density spectrum (PDS), produced via the Fourier analysis of the signal. QPOs are not coherent signals and in a PDS take the form of relatively narrow peaks with a characteristic centroid frequency measurable with high precision and a finite width. Different QPO types have been observed so far over a wide range of frequencies (from a tenth to several hundreds Hz) and in some cases multiple QPOs are seen simultaneously in the PDS of accreting sources.
The relativistic-precession mode (RPM) associates three types of QPOs observable in the PDS of accreting black hole binaries with a combination of the Keplerian frequency and the epicyclic frequencies. The nodal precession frequency or Lense-Thirring frequen (= Keplerian frequency - vertical epicyclic frequency) is associated with oscillations usually observed at low (0.1-20 Hz) frequencies, while the periastron precession (= Keplerian frequency - radial epicyclic frequency) and the Keplerian frequency are associated to two different types of high-frequency (30-450 Hz) QPOs, known as "lower" and "upper" high-frequency QPO (HFQPO) respectively. The analytical form of the keplerian, periastron precession, and nodal frequency only depend on three parameters: the mass and the spin of the black hole, and the radius at which the QPOs are generated. In 2014 the RPM was used to establish a successful method to self-consistently estimate the mass and the spin of a BH based only on the detection of QPOs.
Since high frequency QPOs are rare in black holes, the RPM has been so far used only to estimate the mass and spin of the black hole in three systems. Neutron stars, instead, commonly show the three types of QPOs that are relevant for the RPM, and hence they offer the unique opportunity to test this model on a large sample of systems. So far only two neuron star XRBs have been tested.

The aim of this thesis is to perform a detailed test of the RPM on a famous NS system, Sco X-1, which is an important source for a number of reasons. besides being relatively nearby, this is the primary target for the search of continuous gravitational waves. Thus, measuring the spin of the neutron star will have a high impact in field of the gravitational wave astronomy. We will analyze the QPOs observed in Sco X-1 and we will attempt the first timing-based estimate of the spin of this source using the RPM. We will make an assessment of the validity of the RPM for the study of neutron stars, and we will test a version of the model specifically developed to describe the complex space time near the surface of a neutron star. Our results will be compared with the current understanding of neutron star X-ray binaries and their variability. This work will deliver important insights into the properties of neutron stars, which will be relevant in a broad context.
  • Unveiling the nature of the obscured phase of the black hole X-ray binary GRS 1915+105
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GRS 1915+105 is one of the best-studied Galactic BH X-ray binaries. Located at a radio parallax distance of ~8.6 kpc, GRS 1915+105 hosts a stellar mass black hole of approximately 12 solar masses, believed to accrete erratically close to its Eddington limit. This system was the first Galactic BH observed to display relativistic super-luminal radio ejections, and it is still considered the archetypal Galactic source of relativistic jets.
GRS 1915+105 appeared as a bright transient in August 1992 and over the past 30 years, it has alternated between extraordinary activity phases with strong and variable radio and X-ray emission, and long periods of time of reduced activity, with relatively low radio and X-ray fluxes.
In July 2018 GRS 1915+105 entered an unusually extended low-flux X-ray phase, interrupted by a few months during which the source was extremely active in radio, but remained dim in the X-rays, most likely as a consequence of presence of absorbing material local to the source that was shielding the X-ray emission. More recently, at the end of 2021, GRS 1915+105 entered an even lower phase, which is characterized by the absence of any variability at all wavelength, and an X-ray and radio luminosity consistent with an unusually high quiescence.

The aim of this thesis is to characterize the emission of GRS 1915+105 during the years from 2018 onwards, to understand the properties of the accretion flow and how they are connected with those of the jet observed in radio. We will study the X-ray emission using unpublished data collected by the Swift satellite and the NICER mission to reveal the physical processes at the base of the behavior of the source, and we will compare them with the properties of the radio emission observed with the MeerKAT radio telescope and the AMI-Large Array.
The results of this analysis will be put in context of what is known of GRS 1915+105 and of X-ray binaries in general.