Properties of relativistic jets in X-ray binaries

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Properties of relativistic jets in X-ray binaries

Dissertation

Erlangung des Doktorgrades (Dr. rer. nat.) zur Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn der

Richa Sharma von New Delhi, Indien aus

Bonn, 2021

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1. Gutachterin: Priv.-Doz. Dr. Maria Massi 2. Gutachter: Prof. Dr. Norbert Langer Tag der Promotion: 01.07.2021

Erscheinungsjahr: 2021

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To my parents and sister,

for their love and support.

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Abstract

X-ray binaries are stellar systems consisting of a compact object (either a neutron star or a black hole) accreting matter from a companion star. Some of the X-ray binaries form relativistic jets where plasma accelerates close to the speed of light. Relativistic jets have been extensively studied, and the most advanced research in this field investigates their structure, jet’s core position and short-term flaring events.

In this thesis, we initially work on understanding the structure of the jet wherein jet particles flow continuously. Radio observations have shown that the jet core for different frequencies is located at different positions along the jet axis, known as the core-shift effect. Additionally, jets show a flat spectrum in their emission over a range of frequencies. In the first project, we use a semi-analytical code of a synchrotron emitting, self-absorbed jet to find the relationship between the core-shift and flat spectrum. We study the jet characteristics by varying different parameters in the jet model. Our main results from this analysis are that the core moves upstream when the jet is viewed at a larger inclination angle and makes the spectra steeper. Our analytical results corroborate and explain the observed anisotropy of spectral index with the inclination angle. Additionally, when the relativistic electron density increases in the jet, the core moves downstream, making the spectra flatter. Thus, both core position and spectra are related and vary due to opacity change along the line of sight.

After investigating the jet properties where particles are continuously flowing, we study the jet’s case with transient particle injection episodes. In a few X-ray binaries, short-term variability is superimposed on the radio and X-ray emission. Therefore, we examine the radio and X-ray properties of an X-ray binary system: LS I +61^◦303. It is one of the most powerful radio-emitting system, which is highly periodic at all wavelengths over a time-scale of one month. We use radio data of the source obtained with the Westerbork synthesis radio telescope and X-ray data obtained with the Suzaku telescope. Timing analysis reveals periodic oscillations in the system of∼55 min (radio) stable over four days and∼2.5 h (X-ray) stable for 21 h, albeit at different epochs. We also compare our analysis with data from the literature and find that the periodic oscillations are always related to large radio outbursts but are independent of the outburst’s amplitude. These periodic features, which range from minute to hour time-scales, can be understood as magnetic reconnection events.

Inspecting deeper into the case where particle density in the jet increases abruptly for a short period, we present new simultaneous observations of LS I +61^◦303 using theXMM–Newtonand AMI-LA telescopes. The AMI-LA telescope observed the target source for∼11 h, andXMM–Newtontelescope observed it for∼6 h; thus, we obtain simultaneous radio/X-ray data for∼5 h. Using correlation analysis, we establish that the radio and X-ray emission in LS I +61^◦303 is correlated up to 81 per cent with zero time-lag. Moreover, after removing the long-term trend from the data, we find that even radio and X-ray emission variability is correlated up to 40 per cent. The results reveal that the radio and X-ray emission is due to the same population of electrons. If future observations can find a significant time-lag between emission at both wavelengths, it could imply either a synchrotron or inverse-Compton emission mechanism for the X-ray wavelengths.

In conclusion, we unify the core-shift and spectral properties in a steady jet. Furthermore, we show that the detection of periodic oscillations and correlation studies between radio and X-ray emission helps us understand X-ray binaries’ physical processes.

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C H A P T E R 1

Introduction

“The treasures hidden in the heavens is so rich, that the human mind shall never be lacking in fresh nourishment.”

– Johannes Kepler (In Terra inest virtus, quae Lunam del.)

Astrophysical jets are bipolar outflows of plasma that are present in many objects in the Universe, such as X-ray binaries (XRBs), active galactic nuclei (AGN), and young stellar objects (YSOs). For the first time, a jet-like feature was observed in the optical image of galaxy M87 in the early twentieth century (Curtis 1918). Clark et al. (1975) detected the first radio emission from an X-ray binary system SS 433, which was later proved to be coming from a jet (Spencer 1979). When the ionised plasma in the jet reaches speed close to the speed of light, then they are called relativistic jets. Such jets were first modelled for Active Galactic Nuclei (e.g. Blandford & Königl 1979). The models were later modified to explain X-ray binaries’ properties (e.g. Hjellming & Johnston 1988). The historical discoveries related to XRBs and AGNs opened a new field of study in astronomy. Even today, astronomers are working towards understanding the process of jet formation and its effects.

Jets are often observed in systems where matter accumulates around the central object in a disk known as the accretion disk. Jets liberate a significant amount of accretion power and transfer this energy and matter into their surrounding environment (Gallo et al. 2005; Tudose et al. 2006; Tetarenko et al. 2018). Subsequently, they affect star formation in the galaxy, inject heat and turbulence in the interstellar medium (ISM), and feed the ISM with magnetic fields, causing changes to the galaxy evolution (Heinz et al. 2008; Fabian 2012; Mirabel et al. 2014). X-ray binaries which consist of a stellar-mass black hole, mimic many phenomena observed in extragalactic objects called AGNs (objects that consist of supermassive black holes). Black holes of different masses follow the same scaling laws; for example, the length and time-scales of various processes in XRBs and AGNs are proportional to the black hole’s mass. Thus any phenomena observed in AGN can be observed on much shorter time-scales in XRBs.

Although jets are ubiquitous in many astrophysical objects, their formation, acceleration, and physical processes are still poorly understood. A comprehensive investigation of some critical questions regarding jets with the help of both analytical modelling and observations is the focus of this thesis.

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1.1 X-ray binaries

X-ray binaries are binary systems that consist of a compact object (either a neutron star or a stellar-mass black hole) orbiting and accreting matter from a companion star (also called the donor star, see Fig. 1.1). They are classified into two main groups depending on the donor star’s mass: low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs). If the mass of the donor star M_donor . 1M, then the binary system is called a low-mass X-ray binary. On the other hand, if the mass of the donorM_donor >10M, then the binary system is called a high-mass X-ray binary (e.g.

Tauris & van den Heuvel 2006).

The mass-transfer in X-ray binaries can occur via two different modes. The first scenario is that of Roche-lobe overflow. The Roche-lobe is the area surrounding a star where the material is gravitationally bound to it. The donor star fills its Roche-lobe, either due to the expansion of its envelope at a late evolutionary stage or due to decreasing orbital separation between the binary as a consequence of angular momentum losses. Roche-lobe overflow is the dominant accretion mechanism in LMXBs. The second scenario is predominant in HMXBs, where the compact object accretes matter via the strong stellar winds of the massive donor star, with a typical mass-loss rate ofMÛ_wind'10⁻⁶Myr⁻¹. Recent wind models have shown that an accretion disk can be formed even in HMXB due to inhomogeneous winds if the separation between the binaries is tight or stellar wind is slow (Karino et al. 2019).

Since the inflowing matter has orbital angular momentum, the donor star’s matter cannot directly fall to the compact object’s surface. Instead, it spirals around the accretor (compact object) and initially forms a ring around it. The matter rotates differentially, and because of viscous shear, the faster inner material of the ring loose angular momentum to the outer ring, thus spreading radially to form an accretion disk. During the accretion disk formation, half of the gravitational potential energy is converted to kinetic energy and the other half is radiated away (Frank et al. 1992). The ionised material in the inner disk reaches temperatures more than 10⁷K, and the emission is observed at X-ray wavelengths.

1.2 Relativistic jets

Relativistic jets are fundamental to accretion onto black holes of all scales - both stellar-mass black holes and supermassive black holes in AGNs - and accreting neutron star X-ray binaries (Fender 2006).

They are collimated and bipolar outflows of plasma expelled from the accreting material (Meier et al.

2001). Jets are observed from radio to infrared frequencies, and in some cases, they are observed at optical (Kanbach et al. 2001; Corbel et al. 2001; Curran et al. 2011) and X-ray frequencies (Pe’er &

Markoff 2012; Russell et al. 2013). The high-energy electrons in the jet gyrate around the magnetic field and emit radio synchrotron radiation. Model-fits to the spectra of X-ray binaries have shown that X-rays could be emitted from the jet’s base, with soft X-rays being dominated by synchrotron emission and the hard X-rays being dominated by the inverse-Compton process (Markoff et al. 2005;

Poutanen & Veledina 2014; Veledina et al. 2017).

The mechanism of jet launching, acceleration and collimation are still poorly understood in astrophysics. The model proposed by Blandford & Znajek (1977) considers that the jet is powered by the black hole’s rotation energy as it spins. Another model for jet launching was proposed by Blandford & Payne (1982), where they consider that the jet is powered by the angular momentum removed by the strong magnetic field lines leaving the accretion disk and extending to large distances.

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1.2 Relativistic jets

Figure 1.1: A schematic representation of an X-ray binary. The compact object accretes matter from the companion star in the form of an accretion disk and emits at X-ray wavelengths. The relativistic jets are an outflow of plasma from the system, which can be observed from radio wavelengths to very energetic gamma-rays.

A microquasar whose jet creates a small angle with respect to the line of sight is called a microblazar. The figure is taken from Mirabel (2006).

Observationally, the latter scenario is expected to be seen when we observe correlation in the jets’

X-ray and radio emission (e.g. Mirabel et al. 1998; Massi et al. 2020). On the other hand, McClintock et al. (2014) has also shown a correlation between the jets’ power and black holes’ spin in X-ray binaries.

X-ray binaries with radio-emitting jets are called microquasars. The name was coined by Mirabel et al. (1992) in analogy to quasars, a subclass of AGN. The microquasars whose radio jets make a small angle with respect to the line of sight are known as microblazars (see Fig. 1.1), analogous to blazars (another subclass of AGN).

1.2.1 Superluminal motion and Doppler boosting

Some jets appear to be travelling faster than the speed of light. This is known as the apparent superluminal motion and was first proposed by Rees (1966) even before the Very Long Baseline Interferometry (VLBI) observations started. To understand the phenomena, let us consider that plasma

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D 𝛃 𝛉

d

d’

Source, t=0

Source

,

t =Δt

D is ta nc e

Observer

0 25 50 75

Viewing angle, 0

5 10 15 20

Do pp ler fa cto r,

jet

= 2

jet

= 5

jet

= 10

Figure 1.2: Left: Schematic for calculating the effect of superluminal motion in the jet. Right: Doppler factor (δ) as a function of the viewing angle (θ) for different bulk Lorentz factor (Γ_jet).

in the jet is travelling at a relativistic speed,β=v/c(cis the speed of light), and at an angleθ with respect to the line of sight as shown in the left panel of Fig. 1.2. The plasma emits the first radio signal at timet =0 and a second signal at∆t. In the observer’s reference frame, the first signal arrives at t₁= D/cand the second signal arrives at

t₂=∆t+ D−d⁰

c =∆t+D/c−∆tβcosθ. (1.1)

The apparent speed then becomes (Pearson & Zensus 1987), β_app = βsinθ

1− βcosθ. (1.2)

The apparent velocity is maximum for cosθ_max = βand sinθ_max =q

1−β² =1/Γ_jet, whereΓ_jetis known as the Lorentz factor. Therefore,

β_app^max= βΓ_jet. (1.3)

IfΓ_jet >>1 (i.e. v∼c), therefore despite the jet velocityβ <1, the apparent velocityβ^max_app >1, i.e.

superluminal motion is observed.

Due to the relativistic effects in the jet, the frequency detected by the observer is

ν_obs=δν⁰, (1.4)

whereδis the relativistic Doppler factor. The Doppler factor is different for the two oppositely-directed

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1.2 Relativistic jets

Figure 1.3: Image of microquasar SS 433 observed by the Very Large Array (VLA) radio telescope. The source consists of either a neutron star or a black hole. The image traces the radio jet ejected from the source with particles travelling at 24–28% of the light speed. The jet in SS 433 precesses around its axis, tracing out a corkscrew pattern. The figure is taken from Blundell & Bowler (2004).

relativistic jets. For an approaching and receding jet, it is given by

δ_a= 1

Γ_jet(1−βcosθ), (1.5)

δ_r= 1

Γ_jet(1+βcosθ), (1.6)

respectively. The Doppler factor is a function of the viewing angle of the jet, also shown in the right panel of Fig 1.2. Due to this, the flux of the source is amplified when it is viewed at an angle close to the line of sight, and attenuated for the receding jet. The flux received from the approaching and receding jets are

S_a =S₀δ_a^k−α, (1.7)

S_r=S₀δ^k−α_r , (1.8)

respectively, whereS₀is the intrinsic flux density,αis the radio spectral index, parameterk = 2 for a continuous jet andk= 3 for discrete condensations. For a steady jet as will be described in Sect. 1.6.1, the total flux density observed is

S_total=S_aδ_a²^−α+S_rδ_r²^−α. (1.9)

1.2.2 Jet precession

The angle the jet axis makes with respect to the line-of-sight is known as the jet inclination/viewing angle. In many X-ray binaries as well as AGNs, it has been observed that the jet inclination angle varies with time, producing an S-shaped radio morphology. For example, the jet in SS 433 precesses

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around its axis and traces a corkscrew pattern in radio images as shown in Fig. 1.3 (Margon et al. 1984;

Blundell & Bowler 2004; Monceau-Baroux et al. 2014). More recently, Miller-Jones et al. (2019) found evidence of a precessing jet in V404 Cygni in its VLBI images. The jet in Circinus X-1 (Coriat et al. 2019) and 1E 1740.7-2942 (Luque-Escamilla et al. 2015) show hints of precession. Evidence for the presence of precessing jets in AGN is shown by Brown (1982), Rozgonyi & Frey (2016), and Britzen et al. (2019).

1.3 Emission mechanisms

X-ray binaries emit radiations that range from radio frequencies up to TeVγ-rays and is assumed to originate from different regions in the system. The emission can be either thermal or non-thermal.

The latter emission is divided into synchrotron and inverse-Compton processes.

1.3.1 Thermal radiation

In X-ray binaries, the thermal radiation from the accretion disk can be regarded as blackbody radiation.

The intensity of black body radiation,I_ν^BBis a function of the temperatureT of the source and can be given by the Planck’s law (Rybicki & Lightman 1979)

I_ν^BB= 2hν³ c²

exp

hν k_BT

−1 −1

, (1.10)

wherek_Bis the Boltzmann constant andhis the Planck constant.

1.3.2 Synchrotron Radiation

Synchrotron radiation is produced when relativistic particles are accelerated due to magnetic field lines

Electromagnetic wave Magnetic field lines

electron

Figure 1.4: Electron gyrating around magnetic field lines producing synchrotron emission.

(see Fig. 1.4). In X-ray binaries, synchrotron radiation is observed from jets as a multi-wavelength continuum at radio and infrared wavelengths, though there is evidence that Hard X-rays can also be produced by the same mechanism (e.g.

Markoff et al. 2001). The total power emitted by a single electron of energyγ_em_ec², integrated over all frequencies is P∝ γ²_eB², whereBis the magnetic field in Gauss. Due to the synchrotron process, the relativistic particles lose energy, and this is called radiative cooling. The average rate of loss of energy is (Eq. 8.80 in Longair 1994)

− dE dt = 4

3σ_Tcγ_e²U_B, (1.11) whereσ_Tis the cross-section of Thompson scattering and U_Bis the energy density of the magnetic field.

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1.3 Emission mechanisms

Synchrotron self-absorption

In a physical scenario, a bunch of electrons contribute to the emission process. For a homogeneous

Optically

thick Optically thin

ν_"#$%

𝜈5/2 𝜈^{-(p -1)/2}

log 𝑣 log 𝐼,

𝜏 ~ 1

Cooling break

𝜈^-p/2

Figure 1.5: Spectra of synchrotron emission.

source with a constant magnetic field, an optically thin spectrum is generated due to the synchrotron mechanism, with a power-law slopeα=−(p−1)/2 as shown in the bottom panel of Fig. 1.5. The power-law spectrum is observed if there is no absorption. For synchrotron sources with brightness temperatureT_b >m_ec²/k(electron temperature), the electrons start to reabsorb the photons at lower frequencies giving rise to optically thick, self-absorbed emission with a power-law slope of 5/2, independent of the energy distribution of the electrons. Therefore, at the turn-over frequency from optically thin to thick emission, the synchrotron spectrum reaches a peak flux at ν_peak where optical depth, τ equals unity. The synchrotron emission for such electron peaks at a frequency (see Eq. 8.77 in Longair 1994)

ν_peak=2.8×10⁶Bγ²_e. (1.12)

The highest-energy particles lose their energy fastest, and it is shown by the cooling break in the synchrotron spectra.

Power-law distribution

If the electrons have a power-law distribution in energy

N(E)dE ∝E⁻^p, (1.13)

then consequently the total power emitted by them will also follow a power law given by

P(ν) ∝ ν^α, (1.14)

where p is the power-law index of the energy distribution of the electrons, and α is known as the spectral index. For this scenario, the location of the turn-over frequency depends on the density of relativistic electrons in the source and the magnetic field strength (Dulk 1985).

Low-energy

Photon High-energy Photon

High-energy

electron Low-energy

electron 1.3.3 Inverse-Compton Scattering

When an electron scatters-off a photon to gain energy, it is called Compton scattering. On the contrary, when a low frequency photon scatters-off a high-energy electron and gains energy, it is known as inverse-Compton (IC) scattering (see Fig. 1.6). It is another process apart from the synchrotron emission where relativistic electrons cool radiatively. The rate of loss of energy for IC scattering is given by (Eq. 9.41

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in Longair 1994)

dE_IC

dt ∝γ²_eU_photon, (1.15)

whereU_photonis the photon field energy density of the seed photon.

In X-ray binaries with radio jets, IC scattering can produce X-rays andγ-rays (e.g. see Romero et al. 2017). If the seed photons are synchrotron photons and interact with the same population of synchrotron emitting electrons, then the process is called synchrotron self-Compton (SSC) (Atoyan

& Aharonian 1999). On the other hand, if the seed photons originate outside the jet and interact with jet electrons, the process is called external Compton (EC) (Georganopoulos et al. 2002). In X-ray binaries, EC has two contributions, i.e. seed photons come either from the accretion disk or the companion star. In high-mass X-ray binaries, EC of seed photons from the companion star is more dominant, whereas, in low-mass X-ray binaries, EC is mainly due to the scattering of seed photons of the accretion disk, as the companion star is not bright enough (e.g. Romero et al. 2017).

1.4 Jet composition

Understanding the jet composition is required to model the emission from jets. Two models have been proposed in the literature: the leptonic and the hadronic model. Even though the leptonic model has successfully described the emission processes in the jet, the hadronic model is equivalently intriguing.

Leptonic Models

In the leptonic model, radiative processes in the jet are associated with electrons and positrons. The presence of protons is considered to be inefficient in producing energy for radiation mechanisms.

Therefore, synchrotron emission and inverse-Compton scattering become the primary source of emission processes, with IC scattering as the primary source ofγ-ray production (e.g. Bloom &

Marscher 1996; Błażejowski et al. 2000).

Hadronic Models

In the hadronic model, radiative processes in the jet are associated with electrons, protons and nuclei (Romero et al. 2003; Aharonian et al. 2006). Both electrons and protons reach relativistic energies in the jet. Due to the interaction of relativistic protons from the jet with cold protons from the star, pions are produced (π⁰+π^±) (Böttcher et al. 2013). Due to its very short lifetime, pion decays, producing γ-rays,

π⁰→γ+γ. (1.16)

The charged particles undergo further decay producing neutrino,

π^±→µ^±+ν_µ/ν_µ, (1.17)

µ⁺→e⁺+ν_e+ν_µ, (1.18)

µ⁻→e⁻+ν_e+ν_µ. (1.19)

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1.5 Geometry of the jets To search for evidence for the Hadronic model, the neutrino flux can be detected on the Earth by the IceCube experiment (IceCube Collaboration et al. 2013, 2018).

1.5 Geometry of the jets

Jets are formed very close to the central object in both X-ray binaries and AGNs. In blazars, the jet has been observed at 10⁴−10⁶R_sch(R_schis the Schwarzschild radii) away from the central object (Marscher et al. 2008, 2010). High-resolution Very Long Baseline Interferometry (VLBI) study done by Hada et al. (2011) has shown that the jet core in the famous AGN M87 is even closer, i.e. only a few tens of R_schfrom the central object. From∼10²−10⁴R_sch, the jet accelerates to relativistic energies. Studying the jets’ geometry and confinement is vital to determine their formation, collimation, and acceleration regions. Most magnetohydrodynamical (MHD) simulations and analytical models consider the jet shape as conical or parabolic.

Typically, the jet shape is considered conical (e.g. Blandford & Znajek 1977; Blandford & Payne 1982; Kaiser 2006; Potter & Cotter 2012), where the sideways jet expansion velocity remains constant.

High-resolution VLBI images show an axisymmetrical jet with continuous plasma flow. Such a jet has been approximated by conical geometry (e.g. Krichbaum et al. 2006; Kovalev et al. 2007; Sokolovsky et al. 2011). MHD simulations have predicted that the jet has a parabolic shape near the central engine (McKinney et al. 2012). Using VLBI observations, Asada & Nakamura (2012) showed for the very first time that the jet in M87 transitions from a parabolic to conical shape at∼10⁵R_sch. Subsequent studies of AGNs have observed that the parabolic jet is very close to the central object (e.g. Hada et al. 2013; Mertens et al. 2016; Walker et al. 2018). In MHD simulations of Nakamura & Asada (2013), it was suggested that the region closest to the central object and dominated by black hole potential is initially confined due to the balance between the internal magnetic field and external gas pressure. After that, the jet expands adiabatically in a conical geometry. Different samples of AGNs were recently studied by Kovalev et al. (2020) and Pushkarev et al. (2017), where they found that for most AGNs, jets are conical from 100–1000 pc scales, but on smaller scales, the geometry is quasi-parabolic.

1.6 Characteristics of jets in black hole X-ray binaries

X-ray binaries with compact objects as black holes are known as black hole X-ray binaries (BHXRBs).

Based on the spectral and morphological properties, jets in accreting black hole X-ray binaries are mainly of two types (Dhawan et al. 2000; Fender 2001; Fender et al. 2004) and are described in the following sections.

1.6.1 Steady jets

Steady jets are compact, synchrotron self-absorbed jets that result in the classical flat/inverted spectrum, i.e. spectral indexα≥0 (Blandford & Königl 1979; Corbel et al. 2000; Stirling et al. 2001; Kaiser 2006). They have relatively low bulk velocity (bulk Lorentz factor,Γ_jet < 2, Fender et al. 2004).

In some of the synchrotron emitting jets, instead of the peak turn-over frequency described in Section 1.3.2, the spectrum is flat over several orders of magnitude in frequency (Fender 2001). The flat spectrum, as shown in Fig. 1.7, could be ascribed to the inhomogeneity present in the jet. Let us

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𝜈

^5/2

𝜈

^{-(p -1)/2}

log 𝑣

𝜈

^{𝛼 ≳ 0}

𝜈

^-p/2

lo g 𝐹

*

𝜈

𝑜𝑏𝑠

Figure 1.7: A schematic representation of a flat spectrum in a stratified, self-absorbed jet. Red circles represent the flux contribution from different segments of the jet. At observation frequency,ν_obs, the total spectrum is the result of each of these segments’ contribution. Figure adapted from Markoff (2010).

dive deeper into the concept. In a conical jet, the magnetic field and the number density of relativistic electrons decrease along the jet axis, creating different plasma conditions. Suppose we assume that the jet is divided into different segments, and each segment acts as an individual homogeneous synchrotron emitting source. In that case, this gives rise to several homogeneous self-absorbed synchrotron components at different turn-over frequencies (Blandford & Königl 1979), as shown in Fig. 1.7. The turn-over frequency location depends on the density of relativistic electrons in the jet segment and the magnetic field strength (Dulk 1985). If the jet is observed with a single-dish telescope, individual regions of the jet cannot be resolved. In such a case, the total flux observed at each frequency is an integrated flux having a contribution from different segments of the jet. As shown in Fig. 1.7, the individual spectrum that has a peak at observation frequencyν_obs, contributes the maximum to the total flux. The total flux also has smaller contributions from the individual spectrums of neighbouring segments as shown by the red circles. When the peak of self-absorbed emission at each frequency is approximately the same, a flat spectrum is observed (Markoff 2010).

When the same jet is mapped at high resolution, one observes a resolved jet. Since the total flux is due to different segments’ contribution, the jet is observed as an elongated Gaussian ellipse-like photosphere (orange ellipse in Fig. 1.7). The flat-spectrum compact region in the resolved jet is identified as a “core.” At any frequency, the core of the jet is the area where the optical depthτ_ν ≈1.

Its position changes as a function of frequency and this is known as the core-shift effect, i.e. the core for higher frequency is observed closer to the central object and vice-versa, as shown in Fig. 1.8. This characteristic feature of the jet is supported by Very Long Baseline Interferometer (VLBI) observations in AGNs (e.g. Marcaide & Shapiro 1983; Hada et al. 2011; Sokolovsky et al. 2011).

From observations, it is known that the flat spectrum extends in the entire radio band till the

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1.6 Characteristics of jets in black hole X-ray binaries

BH

Accretion disk

Higher frequency Lower frequency

𝜏(𝜈

_!

)~1

𝜏(𝜈

_"

)~1 𝜏(𝜈

_#

)~1 𝜏(𝜈

_$

)~1

𝜈

_!

> 𝜈

_"

> 𝜈

_#

> 𝜈

_$

Figure 1.8: Schematic diagram representing the core-shift effect in a jet. The ellipses depict the core (τ_ν≈1) positions at different frequencies, with the core of higher frequency closer to the central black hole than that of lower frequency.

millimetre range (Fender 2001; Markoff, Falcke & Fender 2001). Above this frequency range, the jet ceases to be self-absorbed, the spectrum breaks and a steep optically thin spectrum is observed with α=−(p−1)/2 (see Fig. 1.7). For X-ray binaries, this break is at infrared (IR) frequencies (Russell et al. 2013). According to Markoff (2010) and Polko et al. (2010), the location in the jet corresponding to the break frequency is the region where non-thermal particles are first accelerated, probably due to internal shocks (Malzac 2014, also see Romero et al. 2017). There are only eight black hole X-ray binaries whose jet breaks have been detected so far (Russell et al. 2013). The break is difficult to detect because, in some sources, the accretion disk and companion stars also emit at the IR frequencies (Gallo et al. 2007; Rahoui et al. 2011). A detailed model of steady jets will be discussed in Chapter 2 of this thesis.

1.6.2 Transient jets

Transient jets are associated with optically thin emission having spectral index−1≤α≤ −0.2 (Fender 2001). They are observed as the relativistic ejection of plasmoids detached from the centre and moving in opposite directions with bulk Lorentz factor,Γ_jet >2 (Fender et al. 2004, as shown in Fig. 1.9). This jet is unlike the steady jet, where plasma continuously flows in the jet from the centre (see Fig. 1.3).

In microquasars, the first indication of a transient jet’s presence was observed using high-resolution mapping of the system GRS 1915+105 (Mirabel & Rodríguez 1994). Except for GRS 1915+105, discrete ejecta has been directly observed only in a few X-ray binaries (e.g. Fender et al. 1999; Yang et al. 2010; Miller-Jones et al. 2012a, 2019).

The steady and transient jets are related to each other. During the jet formation, initially, a steady jet

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Figure 1.9: High-resolution radio images of multiple relativistic ejections of plasmoids from X-ray binary GRS 1915+105. The figure is taken from Fender et al. (1999).

Figure 1.10: Observation of X-ray binary GRS 1915+105 showing short-term radio variability simultaneous with X-ray dips and an increase in inner accretion radius. The figure is taken from Klein-Wolt et al. (2002).

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1.6 Characteristics of jets in black hole X-ray binaries is observed with flat/inverted radio emission. This jet changes to a transient jet with an optically thin emission. It has been deduced that the radio flaring during the transient jet is due to internal shocks where the relativistic plasma runs into the pre-existing slower-moving plasma of the steady jet (Fender et al. 2004, 2009). More details on this topic will be discussed in Section 1.7.

When a transient jet is observed with a single-dish telescope, the radio emission shows short-term variability superimposed on the long-term emission. These variabilities could range from minute–hour time-scales. Figure 1.10 shows an example of one such observation of microquasar GRS 1915+105, where Klein-Wolt et al. (2002) observed variations in the radio flux density of∼40 min. The rise in the radio variability pattern (top pane) is simultaneous with dips in the X-ray emission (middle panel). These short time-scale and small amplitude variations in the flux are known as quasi-periodic oscillations (QPOs). Furthermore, the authors showed computations that during the decreased X-ray emission, the accretion discs’ inner radius increases (Fig. 1.10 bottom panel), either due to the material ejected from the disk into the jet or being advected into the black hole (Belloni et al. 1997a,b).

Short-term radio variability has been studied over the years in a few sources. In black hole X-ray binary V404 Cyg, Han & Hjellming (1992) observed 20–120 min sinusoidal variations during the decay of its radio outburst. In GRS 1915+105, Rodríguez & Mirabel (1997) observed sinusoidal oscillations of∼30 min. Simultaneous radio and X-ray observations have shown 20–40 min oscillations at both frequencies (Pooley & Fender 1997), and Fender & Pooley (1998) presented the first infrared oscillation event of∼26 min. Fender et al. (2002) showed that the oscillations are not limited to the flux itself but are also mirrored in the source’s spectral index. In another black hole X-ray binary Cygnus X-1, short-term radio variability of 1 h was observed. Cyg X-3 has also shown variability in its radio flux density and spectral index (Zimmermann et al. 2015).

Understanding the origin of short-term radio variability and QPOs is a subject undergoing intense research. According to one hypothesis, the radio variability in the optically thin transient jets is due to multiple shocks in the jet. This shock-in-jet model was initially proposed by Marscher &

Gear (1985). The short radio outbursts or variability are due to shocks that move along the jet and accelerate particles in situ (Kaiser et al. 2000; Klein-Wolt et al. 2002). Another hypothesis states that radio variability can occur due to multiple ejections of blobs of plasma in the jet and is supported by observations (Fender et al. 1999), as shown in Fig. 1.9. The ejected blobs of plasma could either originate in the jet itself (Sironi et al. 2016; Petropoulou et al. 2016) or the accretion disk. In the case of disk origin, the emission is observed initially at the X-ray wavelengths and later at infrared to radio wavelengths as adiabatic expansion (e.g., see Mirabel et al. 1998). More about variability studies will be discussed in Chapter 3 and 4 of this thesis.

Magnetic reconnection

The formation and ejection of plasma blobs either in the jet or in the accretion disk could be due to magnetic reconnection. It is a phenomenon where the magnetic field lines of opposite polarity rearrange topologically (Fig. 1.11). Consequently, magnetic energy is released in the form of kinetic energy, which accelerates the particles. Giannios (2013) showed that during this process, the reconnection layer fragments into several plasmoids. Magnetic reconnection was initially observed in the Sun, where magnetic energy is accumulated over a long time and then gets released (e.g. see Shibata &

Tanuma 2001). However, in recent years, it has been invoked to explain the rapid variability in the TeV and X-ray emission of blazars (Giannios et al. 2009; Narayan & Piran 2012). Multiple periodic ejections could result from blobs forming in multiple reconnection regions, as shown in Fig. 1.11 or

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Magnetic field, B

Plasma flow

BH

Accretion disk

Emitting blobs

JET

Figure 1.11: A schematic showing magnetic reconnection in the jet. The inset shows when the magnetic field of opposite polarities come together, plasma gets heated, and blobs are emitted. Figure adapted from Giannios et al. (2009).

due to intrinsic instabilities in a single reconnection region, such as the tearing instability. The process of removing angular momentum from the accretion disk to the steady jet can stimulate more twists in the magnetic field, making magnetic reconnection a more likely phenomenon.

Geometrical effects

The radio flux density seen by the observer can get affected if the jet is non-axisymmetric and/or has a helical magnetic field configuration. In a non-axisymmetric jet, the magnetic field and number of electrons can have a higher density in individual sections of the jet. If a relativistic shock encounters a higher density (either magnetic field or particle density or both) along its path, then the flux density increases. A precessing jet can further amplify this effect due to Doppler boosting, giving rise to a successive increase in flux and causing apparent QPOs (Rani et al. 2010).

1.7 The disk–jet connection

X-ray binaries are unique sources to understand accretion and ejection processes. Their multi- wavelength observations show that the properties of accretion flow and jet formation are related to each other. In fact, most systems undergo a cyclic change in jet and accretion pattern. There are episodes of increased accretion during which the jet undergo significant changes (e.g. Fender & Gallo 2014). This section presents the research done so far in understanding the radio outbursts and X-ray spectral states in black hole X-ray binaries (see, e.g. Fender et al. 2004; Remillard & McClintock

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1.7 The disk–jet connection

(I)

(IV)

0.1 1

X-ray spectral hardness 10

100 1000

X-raycountrate

Low/Hard state Hard intermediate Soft intermediate

High/Soft

Hard intermediate

X-ray Spectral hardness 0.1

X-ray count rate 100010100

(III)

Γ

_!"#

< 2

(II)

Γ

_!"#

> 2

Figure 1.12: Hardness-intensity diagram (HID) representing the X-ray states in black hole X-ray binaries. The data points to generate the plot are of X-ray binary GX 339–4 during its 2002/2003 outburst (Belloni et al.

2005). The grey dashed line represents the end point of each X-ray state. The blue colour represents the corona, the orange colour represents the accretion disk and the yellow colour shows the jet. The figure is adapted from Fender et al. (2004).

2006; Fender et al. 2009; Belloni et al. 2011).

1.7.1 Accretion states

The black hole X-ray binaries undergo various accretion modes classified into canonical states depending on their spectral hardness and are represented on a hardness-intensity diagram (HID), as shown in Fig. 1.12. The spectral hardness is defined as the ratio between the observed X-ray counts in hard to soft X-rays. It indicates which process dominates the X-ray emission: thermal (soft spectrum) or non-thermal emission (hard power-law). The length of the cycle in HID and each spectral state is different for different sources. Sometimes there could be failed state transitions (Pottschmidt et al.

2001).

The X-ray spectra of black hole X-ray binaries can be described by a power law of the form N(E) ∝E^−Γ, whereN(E)is the photon number density,Eis the photon energy, andΓis the photon index. In the initial rise phase of an X-ray outburst, the source traces an upward path along the right hand vertical axis of Fig. 1.12. In this state, an X-ray binary has low X-ray luminosity and a hard power-law component in its X-ray spectrum with an X-ray photon indexΓ∼1.4–2.4 (McClintock &

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Remillard 2006) and an exponential cut-off around 100 keV (∼60 keV for neutron stars). This state is called the “low/hard state.” The hard power-law component in the spectra is explained due to the presence of corona: an optically thick region of hot electrons near the black hole and accretion disk (see, e.g. Romero et al. 2017). In a few cases, this state also shows a weak blackbody component in its spectra due to a truncated accretion disk. The state is characterised by the presence of a steady jet, as was discussed in Section 1.6.1. As the outburst progresses, the X-ray luminosity and radio emission increase due to the increased accretion rate, and the system evolves along the vertical line, as in Fig. 1.12.

At some point during the passage, the spectra of the X-ray binary begins to soften, and the source moves counter-clockwise in the HID, where the steady jet switches off and a transient jet is observed (see Section 1.6.2). This is known as the “intermediate state.” When the steady jet is turned off, jet quenching is observed where the radio flux density decreases by about 2.5 orders of magnitude (Russell et al. 2011, 2019). The X-ray spectrum has a steep power-law component with an X-ray photon index Γ ∼ 2.5. The X-ray emission originates from a geometrically thin, optically thick accretion disk extending to the innermost stable circular orbit (ISCO). The source in this phase transitions from a

“hard intermediate state” to a “soft intermediate state.”

Following this state, the source transits to the “high/soft” X-ray state, where no radio jet is observed.

However, some radio emission is traced when the transient jet’s material moves downstream and interacts with the interstellar medium (Corbel et al. 2002; Rushton et al. 2017). The X-ray emission is dominated by soft X-rays fitted with a blackbody component and originating from the inner accretion disk.

As the accretion rate starts to decrease, the source starts to harden, moving from an “intermediate state" and gradually reaching the “quiescent state.” During the transition, the jet relaunches and is observed at radio, mm- and IR frequencies (Miller-Jones et al. 2012b; Kalemci et al. 2013). It is observed that the majority of X-ray binaries spend most of their lifetime in the quiescent state.

1.8 AGN unification model and jet orientation

The central region in many galaxies consists of a bright nucleus called the active galactic nucleus (AGN). It consists of a supermassive black hole in the centre with a mass in the range of 10⁶⁻¹⁰M compared to microquasars, consisting of a black hole of a few solar masses. Studies have shown that accretion and jet physics is similar in black holes of all masses and is governed by simple scaling laws.

Therefore in this section, we discuss some critical aspects of AGN.

AGN emit radiation over the entire electromagnetic band, from radio toγ-rays, just like X-ray binaries. Their structure constitutes a supermassive black hole in the centre, accretion disk, hot corona, broad-line region (BLR) of hot gas, narrow-line region (NLR), obscuring torus and relativistic jets. AGN are divided into different categories depending on their detection criteria (e.g. narrow and broad emission lines), flux density, polarisation, and spectral properties. The two most broad categories include radio-loud and radio-quiet AGN (e.g. Kellermann et al. 2016), as shown in Fig. 1.13.

Radio-loud AGNs are dominated by relativistic radio jets, whereas jets are absent in radio-quiet AGN.

Depending on the strength of emission lines in their optical spectra, both radio-loud and radio-quiet AGN are sub-divided into type-1 and type-2 objects. Type-1 objects show narrow and broad emission lines in their spectra, and type-2 objects show only narrow emission lines due to BLR being obscured from the dusty gas of torus when the source is observed edge-on.

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1.8 AGN unification model and jet orientation

supermassive black holeaccretion diskobscuring torus broad line region

narrow line region jet

Seyfert 1 Seyfert 2

NLRG

BLRG, Type 1 QSO BL Lac FSRQ

NLRG, Type II QSO

Low power High power

Blazar

FR-I FR-II

Radio-quiet (RQ) AGNRadio-loud (RL) AGN

`

Figure 1.13: Schematic representation of the current classification of AGN according to the unification model.

Figure adapted from Britto et al. (2016).

Figure 1.14: Plot representing variation in the spectral index (α) with the inclination angle (θ) to the line of sight. The spectral index of FR-I and FR-II radio galaxies are flatter for small angles than for larger angles. The figure is taken from Fine et al. (2011).

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Classifying AGNs depending on the jet’s orientation is the most favoured AGN classification to date, where sources are accompanied by intrinsic variation in their properties. Scheuer & Readhead (1979) were the first to introduce the unification scheme for the AGN. Since then, many attempts have been made to unify that different AGN are the same objects viewed from different angles by the observer (e.g. Antonucci 1993; Urry & Padovani 1995). A schematic representation of AGN division based on the source’s orientation is shown in Fig. 1.13 and further detailed in Meier (2012).

One of the interesting classes is Fanaroff–Riley radio galaxies (Fanaroff & Riley 1974): FR-I and FR-II because of their observed spectral properties. FR-I consists of compact radio sources with bright cores and low-power jet lobes. On the other hand, FR-II consists of extended radio structures with high-power radio lobes and faint cores. When FR-II radio galaxies are viewed edge-on, they show a steep spectrum with narrow emission lines. As the orientation decreases, the object is observed as a steep-spectrum quasar, and with a pole-on view, the object shows enhanced radio core emission (BL Lac and flat-spectrum quasars). When FR-I radio galaxies are viewed edge-on, they show weak/no narrow emission lines but steep spectra and when viewed pole-on, the object can show BL-Lac properties with weak optical emission (Jackson & Wall 1999). The flat spectra for an edge-on view can be attributed to the superposition of multiple Doppler-boosted self-absorbed radio peaks. Despite indications that flat spectra are observed in sources that are viewed close to the line of sight, there can still be a scatter between theoretical and observational data due to intrinsic property changes. Figure 1.14 shows the spectral index of FR-I and FR-II objects obtained from a semi-empirical simulation performed by Wilman et al. (2008). At small angles to the line of sight, the objects exhibit a flatter spectrum than those objects at larger angles. Fine et al. (2011) has shown that this relation can be used as an orientation indicator for AGN sources whose viewing angle is unknown. Such studies have also been performed by Wills & Browne (1986) and Brotherton (1996).

In fact, in Hovatta et al. (2014), the distribution of spectral index for BL Lac objects (sources with small inclination angles) are concentrated towards flatter spectra. More details about the importance of studies related to change in inclination angle are discussed in Chapter 2.

1.9 High-mass X-ray binary LS I +61

^◦

303

LS I +61^◦303 is a high-mass X-ray binary in our galaxy at a distance of 2.45⁺⁰−0^..²¹26kpc (Arnason et al.

2021), which was discovered by Gregory & Taylor (1978) in a galactic plane survey. It consists of a compact object (neutron star or black hole) orbiting a massive star which shows optical spectra of a B0 Ve star (Casares et al. 2005). The massive star rotates rapidly and loses mass in the form of a decretion disk. The source is highly eccentric with an eccentricity of 0.72±0.15 based on emission lines from the Be star (Casares et al. 2005), whereas a recent study that assumed emission only from the Be disk results in lower eccentricity (Kravtsov et al. 2020). A schematic representation of the system is shown in Fig. 1.15.

The compact object in LS I +61^◦303 is suggested to be a black hole (Punsly 1999; Massi et al. 2017, 2020) or a neutron star (Maraschi & Treves 1981; Dubus 2006). Following the definition by Fender (2001), microquasars consist of a double-sided radio jet along with displaying a flat radio spectrum.

Using high-resolution radio images, LS I +61^◦303 has been observed to have both double-sided radio morphology (top panel of Fig. 1.16) (Massi et al. 2012) and flat radio spectrum extending over the entire cm radio band (Zimmermann et al. 2015). The jet in LS I +61^◦303 has the same S-shaped structure as the precessing jet in microquasar SS433 (see Fig. 1.3). Furthermore, LS I +61^◦303

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1.9 High-mass X-ray binary LS I +61^◦303

Be star

Compact object

Orbit direction

Figure 1.15: Schematic representation of high-mass X-ray binary system LS I +61^◦303, with a compact object orbiting a Be star.

follows the radio characteristics of microquasars with optically thick radio outburst followed by an optically thin radio emission, as shown in the bottom panel of Fig. 1.16 (Massi & Kaufman Bernadó 2009; Massi et al. 2020). Massi et al. (2020) showed evidence of periodic accretion and ejection processes twice along the orbit of LS I +61^◦303 by studying its radio, X-ray and gamma-ray light curves. Additionally, Massi, Migliari & Chernyakova (2017) investigated the correlation between the X-ray luminosity and photon index of the system and found that it follows the relationship of black holes V404 Cygni and Swift J1357.2-0933.

The alternate theory suggests that LS I +61^◦303 is a pulsar (highly magnetised neutron star). Dhawan, Mioduszewski & Rupen (2006) performed high-resolution radio imaging of LS I +61^◦303 using VLBA and interpreted that the varying one-sided radio morphology is present due to a cometary tail pointed away from the high-mass star. According to the numerical calculations in Dubus (2006), radio images of LS I +61^◦303 could be modelled using a young pulsar model. In this model, the shocked wind material between the pulsar’s relativistic wind and the stellar wind produce a comet-shaped tail in the images. Even though no pulses have been detected from the source, the source can host either a pulsar or rotating neutron star. However, the pulses could be suppressed because of the wind’s optical depth due to free-free absorption (Zdziarski et al. 2010).

1.9.1 Periodic nature of LS I +61^◦303

LS I +61^◦303 exhibits periodicities over the entire electromagnetic spectrum from radio to very high energies (radio, Taylor & Gregory 1982, optical, Mendelson & Mazeh 1989, H-α, Paredes et al. 1994;

Zamanov et al. 1999, X-ray, Harrison et al. 2000; Zhang et al. 2010; Chernyakova et al. 2012, GeV, Abdo et al. 2009; Jaron et al. 2018, TeV, Albert et al. 2006). It has an orbital periodP_orb=26.4960±

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Figure 1.16: Top: VLBI observation of LS I +61^◦303 showing double-sided radio morphology indicating the presence of jet in the system (Massi, Ros & Zimmermann 2012). Bottom: Radio characteristic of LS I +61^◦303 along the system’s orbit, showing that the spectral index reachesα≥0 twice along the orbit (Massi & Kaufman Bernadó 2009).

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1.10 Radio and X-ray telescopes 0.0028 days (Gregory 2002) with orbital phase defined as,

Φ= t−t₀ P₁ −int

t−t₀ P₁

, (1.20)

where,t₀ =MJD 43366.275. For historical reasons, the periastron phase for the system is atΦ= 0.23

±0.02 (Casares et al. 2005).

As mentioned earlier, high-resolution interferometric images of LS I +61^◦303 shows changes in the jet’s position angle. In the context of the microquasar scenario, Massi et al. (2004) explained the changes as being present due to variations in the angle between the jet and the line-of-sight, i.e. a precessing jet in LS I +61^◦303. Because of the precession, the Doppler factor continuously varies, giving rise to changes in morphology. For a small angle with the line-of-sight, the approaching jet gets boosted, and the receding jet gets attenuated. Timing analysis of the radio data reveals a precession period in its spectra along with the orbital period (Massi & Jaron 2013; Massi, Jaron & Hovatta 2015;

Massi & Torricelli-Ciamponi 2016). Using VLBA astrometry, Wu et al. (2018) found that the jet core traces a closed elliptical trajectory on the sky plane with a precession period,P₂ =26.926±0.005 days.

The two similar periods observed in LS I +61^◦303, i.e. orbital (P₁) and precession period (P₂), interfere with each other, giving rise to the long-term periodP_beat=P_long= _ν ¹

1−ν₂ =1626±48days.

The beat period modulates the average of P₁ andP₂, giving rise to the periodic radio outburst at P_average=26.704±0.05 (Ray et al. 1997; Massi & Jaron 2013; Jaron & Massi 2013). These results solved the long-standing problem of the orbital shift of the radio outburst (Gregory et al. 1999).

1.10 Radio and X-ray telescopes

Radio astronomy deals with studying the electromagnetic spectrum at the lowest frequencies. Radio radiations in the Universe can be observed from ground-based telescopes. The ‘radio window’ ranges from a few MHz to THz. The low-frequency limit to the observed radio band is set because of the ability of the Earth’s ionosphere to reflect longer radio waves, i.e. ν < 10 MHz. Also, human-made interference, such as mobile radars, can hinder radio observations, i.e. ν <300 MHz, bypassed only by radio frequency interference (RFI) mitigation. On the other hand, the high-frequency radio emission is absorbed by the tropospheric water vapours, but it can be surpassed by building telescopes at high altitudes.

X-ray astronomy deals with studying the high-energy universe and uses wavelengths from∼0.008 nm to 8 nm. The X-ray radiation in this entire band is opaque to us because it is absorbed by the Earth’s atmosphere. Therefore in order to detect X-ray emission from the Universe, X-ray satellites are built.

This thesis work has used data from various telescopes, both radio and X-ray, as shown in Fig. 1.17 and discussed below.

Arcminute microkelvin imager

The Arcminute Microkelvin Imager (AMI) is situated in Mullard Radio Astronomy Observatory near Cambridge. It constitutes two interferometric radio telescope arrays: small array (SA) and large array (LA). The telescope receiver covers a frequency range from 13.1–17.9 GHz, with 4096 channels and each channel bandwidth of 1.22 MHz (Zwart et al. 2008; Hickish et al. 2018). For our observations,

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Figure 1.17: Radio and X-ray telescopes. Top to bottom: XMM–Newton(Credit: ESA), AMI-LA (Credit:

University of Cambridge, Institute of Astronomy), Westerbork synthesis radio telescope (Credit: Wijnholds et al. 2010), Effelsberg 100-m radio telescope (Credit: MPIfR), and Suzaku telescope (Credit: NASA).

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1.10 Radio and X-ray telescopes we used the AMI-LA, which consists of eight telescopes separated by a distance of 18–110 m, with each telescope having a diameter of 12.8 m (Fig. 1.17).

Westerbork synthesis radio telescope

The Westerbork synthesis radio telescope (WSRT) is situated in Westerbork, Netherlands and is operational since 1970. It operates on the technique of interferometry and aperture synthesis (Baars

& Hooghoudt 1974). It consists of fourteen steerable telescopes in equatorial mount, each having a diameter of 25 m. They are built in a linear fashion going for 2.7 km. Two of the fourteen telescopes can also move on rails. The receiver in the telescope covers a frequency range from 120 MHz to 8.3 GHz.

Effelsberg 100-m radio telescope

The Effelsberg radio telescope with a diameter of 100 m (Hachenberg et al. 1973) is situated in Bad Münstereifel, Germany. It is one of the largest fully steerable radio telescopes on Earth. The telescope’s construction is based on homologous distortion, i.e. in any position, the reflector will maintain a parabolic shape, only the focal point will shift. This ensures high-sensitivity at high frequencies. The telescope is operational at both primary and secondary (Gregorian) focus with a 6.5 m secondary mirror. The receiver in the telescope covers a frequency range from 300 MHz to 96 GHz.

XMM–Newtontelescope

X-ray Multi-Mirror (XMM) - Newton is a space telescope (Mason et al. 1995) launched in 1999. It can observe at X-ray (0.1–15 keV) and optical wavelengths simultaneously. With a 10.8 m length and 16.16 m width, it has a large collecting area. XMM–Newtonis orbiting the Earth in a highly elliptical orbit (HEO) with an orbital period of 47.86 h. The advantage of HEO is that it can observe targets without any interruption for extended time intervals. The disadvantage is that, during periastron passage, the telescope temporarily observes higher particle background due to the Earth’s magnetosphere.

Suzaku telescope

Suzaku is a space-based telescope (Mitsuda et al. 2007) that can observe broadband emission simultaneously from∼0.2–500 keV. The detectors included onboard the Suzaku telescope includes an X-ray spectrometer (XRS), four X-ray imaging spectrometers (XIS) (Koyama et al. 2007), and a hard X-ray detector (HXD) (Takahashi et al. 2007). The mirrors of XIS and XRS are made of thin foil, which helps achieve large effective areas at low cost. It is placed in the low-earth orbit (LEO), therefore it is protected from the Earth’s magnetosphere, and the particle background is low. Therefore, such telescopes can be used to observe low surface brightness objects. The satellite has an orbital period of 1.6 h, hence decreasing the time interval for which a target source can be observed simultaneously.

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1.11 Overview of the thesis

Accretion of matter onto a compact object (neutron star or black hole) is often accompanied by collimated relativistic jet outflows that remove the accretion disk’s angular momentum. As we have explained in this chapter, jets play a vital role in the Universe, but their formation process is complicated. This thesis aims to explore the answers to the following questions:

• How and why does the jet structure, i.e. spectral index and core position, changes in a steady jet for different jet parameters?

• When do we observe periodic variable emission in X-ray binaries? And why are they present only at some epochs?

• How are the radio and X-ray emission related to each other? What could be the physical process behind the emission?

We answer these questions by studying the jets in X-ray binaries, both analytically and observationally.

Below is a description of how we achieve our aims in this thesis.

Exploring opacity effect on core-shift and spectral properties of self-absorbed jets The steady jet has two main attributes: core-shift and the flat spectrum. Core-shift has been observed in VLBI observations of the jet (e.g. Plavin et al. 2019), and the conditions which form a flat spectrum was initially studied by Blandford & Königl (1979) for AGNs and later developed for X-ray binaries (e.g. Kaiser (2006)). In Chapter 2, we study these two attributes of the jet by using a semi-analytical code of Massi & Torricelli-Ciamponi (2014) and try to understand how they are related to each other.

Core-shift and flat spectra are observed in synchrotron emitting jets. Therefore, in our analysis, we consider a self-absorbed, synchrotron emitting conical jet filled with continuously flowing particles.

We study the jet structure by varying different parameters in the jet: 1) frequency from 2–8 GHz, 2) jet inclination angle from 5^◦–75^◦, 3) magnetic field configuration (both parallel and perpendicular), and 4) relativistic electron density.

Hour time-scale QPOs in the X-ray and radio emission of LS I +61^◦303

Transient particle injection episodes are characterised by short-term variability superimposed on the main radio and X-ray emission of X-ray binaries. In Chapter 3, we obtain new radio data of the X-ray binary system LS I +61^◦303 using the WSRT telescope and the X-ray data by the Suzaku telescope from literature (Chernyakova et al. 2017). The source is chosen due to its periodic radio outbursts.

We study the radio data spanning over four days and X-ray data over 21 h, both observed at different epochs. Using time series analysis, we investigate variability in the source. We also discuss the main reasons for the presence or absence of variability along the orbit of LS I +61^◦303.

Radio/X-ray correlations and variability in the X-ray binary LS I +61^◦303

Equipped with the results from Chapter 3, in Chapter 4, we study the variable emission in the X-ray binary system LS I +61^◦303 using new radio and X-ray observations simultaneously observed by AMI-LA,XMM–Newton, and Effelsberg 100-m telescopes. The source is observed at radio frequency

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1.11 Overview of the thesis bands 13–15.5 and 15.5–18 GHz for almost 11 h and at X-ray energy 0.3–10 keV for almost 6 h. In total, we get 5 h of simultaneous radio and X-ray observations. Using statistical methods, we analyse the data to see if the two emissions are related to each other or not. This chapter again uses time-series analysis to find any simultaneous periodic features in the source’s variability. Understanding the relationship between different wavelengths is an essential tool to shed light on the physical processes responsible for the emission at different wavelengths.

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Exploring opacity effect on core-shift and spectral properties of self-absorbed jets

This article has been submitted to theAstrophysical Journal. R. Sharma, M. Massi and G. Torricelli-Ciamponi.

Abstract

Two fundamental characteristics of self-absorbed radio jets are their core-shift and the flat spectrum.

Aimed to study the conditions for spectral flattening, in this work, we explore the core position as a function of frequency, magnetic field alignment, relativistic electron density and jet inclination angle, monitoring the corresponding evolution of the related spectrum. We use a physical model of a synchrotron emitting jet with emission at 2 GHz, 5 GHz, 7 GHz, 8 GHz and 15 GHz and study it by changing the orientation of the jet and the relativistic electron density. Two cases of the jet are analysed, magnetic field parallel and perpendicular to the jet axis. We confirm the core-shift effect at all frequencies and find that the core position varies with the jet’s orientation. The greater the inclination angle, the closer the core is to the base of the jet. The core position also varies with the relativistic electron density in the jet. Changes in the spectral index and positions of the cores along the jet are related to each other, with spectral flattening towards smaller inclination angles and/or larger electron density, i.e. when the cores at different frequencies are at their largest distance from the jet base.

2.1 Introduction

Relativistic jets are produced when the inflowing, accreting plasma around the central black hole gets expelled and collimated due to magnetic forces (e.g. Meier et al. 2001). Single dish radio observations of the jets show a flat or inverted spectrum, i.e. spectral indexα≥ 0, with flux densityS∝ν^α(e.g.

Fender 2001). Broadband jet-model fits to the spectra of X-ray binaries (Markoff et al. 2001, 2003) show that the flat spectrum extends all over and beyond the radio band. The conditions leading to the development of a flat spectrum were initially studied by Blandford & Königl (1979). It was suggested that a compact, self-absorbed jet could explain the jet flat-spectrum properties (e.g. Falcke

& Biermann 1996). Kaiser (2006) created an analytical model to investigate the flat spectra of the jets.

Properties of relativistic jets in X-ray binaries