Recently, very high-energy (VHE) photons above 100 GeV were detected from several GRBs (including GRB 190114C, GRB 180720B and GRB 190829A) by IACTs during the afterglow phase. We discuss the origin of VHE emission of GRBs and its implications. We propose that the available broadband data of these GRBs can be modelled with the synchrotron plus synchrotron self-Compton (SSC) emission of the...
The Event Horizon Telescope (EHT) is a millimeter VLBI array observing supermassive black holes. Its 2017 observing run led to the first images of a black hole shadow in M87, in total intensity and later in polarization as well. These data and images have allowed us to conduct a black hole mass measurement and a null hypothesis test of general relativity, and to put significant constraints on...
We present results of Fermi-Large Area Telescope (LAT) light curve (LC) modelling of selected blazars. All objects have densely sampled and long-term LCs. For each blazar we generated three LCs with 7, 10, and 14 days binning, using the latest 4FGL catalogue and binned analysis provided within the fermipy package. The LCs were modelled with several tools: the Fourier transformation, the...
Blazars are a radio-loud subclass of AGN with relativistic jets closely aligned with our line of sight. The jet-emission in blazars is highly Doppler-boosted and non-thermal emission can be seen across the entire electromagnetic spectrum. These sources are highly variable across all timescales and display rapid flares across multiple wavelength bands. Blazar spectral energy distributions...
Blazars represent a subset of AGN with relativistic jets where the direction of a jet lies very close to our line of sight. The highly Doppler boosted emission from the blazar’s jet results in high apparent luminosities, and blazars display variability on periods from less than one day up to years. At optical wavelengths, the observed emission of the blazar is a superposition of the polarized...
This work provides the description of charged particle transport in magnetic fields via a correlated random walk of particles and derives a telegraph transport equation from first principles that describe the initial, ballistic, and later diffusive phases in a consistent manner. A novel high-performance numerical method based on the theoretical considerations is presented that propagates...
The spectral energy distribution of radio-loud AGN have a characteristic double bump structure, with emission at the lower wavelengths being dominated by synchrotron radiation from non-thermal electrons in relativistic jets. To investigate how the radio emission of these sources relate to the dynamical structure of the relativistic jets we model the synchrotron emission by using 3D hybrid...
We present results of an optical variability study of 44 newly identified blazar candidates behind the Magellanic Clouds. The sample contains candidates for 27 flat spectrum radio quasars (FSRQs) and 17 BL Lacertae objects (BL Lacs), with nine of them recognized as blazars, while the classification of the remaining objects is still uncertain. All objects possess high photometric accuracy and...
S5 1803+784 is a BL Lac object. Unlike most low synchrotron peaked (LSP) blazars, the spectrum of S5 1803+784 is poorly fitted with a single zone synchrotron self-Compton (SSC) leptonic jet model. This could be why recent multiwavelength studies show no clear correlation between the synchrotron emission and the gamma-ray emission in this blazar. We utilize a simple single-zone leptonic jet...
Blazars are a class of Active Galactic Nuclei (AGN) that are radio loud and have a small angle between the jet and the observer’s line of sight. In some cases, flaring events in one frequency band are not accompanied by flaring in other bands. Such events are termed orphan flares. The causes of this variabil- ity and conditions in and location of the high energy emission region are not...
Accreting systems in the centres of Active Galactic Nuclei (AGN) produce highly collimated relativistic jets.Magnetohydrodynamic simulations show that strong magnetic fields, twisted by the differential rotation of central black hole, play dynamically important role in these processes. Due to the dependence of the polarized radiation transfer coefficients on elementary composition and energy...
MIGHTEE is a galaxy evolution survey currently underway with the MeerKAT radio telescope. Once complete, the survey will cover 20 square degrees in four fields to a depth of 1 uJy rms/beam at 1.28 GHz, providing a unique combination of depthand breath. Crucially, the MIGHTEE fields have excellent multi-wavelength coverage, enabling a full census of galaxy properties. I will provide an overview...
We have investigated the Balmer line profile of the AGN linked to the X-ray source RXS J08182+0122. We present a low-resolution spectrum of this target that we obtained at SAAO and subsequently analysed. This AGN displays broad emission lines with a double bump profile that is characteristic of fast moving gas in a Keplerian orbit around a supermassive black hole. This source is particularly...
In this work we investigate the nature of multi-wavelength variability of blazars from a purely numerical approach. We use a time-dependent one-zone leptonic blazar emission model to simulate multi-wavelength variability by introducing stochastic parameter variations in the emission region. These stochastic parameter variations are generated by Monte Carlo methods and have a characteristic...
Relativistic jets and disc-winds are typically observed in BH-XRBs and AGNs. However, many physical details of jet launching and the driving of disc winds from the underlying accretion disc (AD) are still not fully understood. This study will investigate the role of different flow parameters on the launching of jets, driving disc-winds, and the dynamical properties of the disc. We will also...
The last decade has seen a significant gain in both space and ground-based monitoring capabilities, producing vastly better coverage of BH X-ray binaries during their (rare) transient outbursts. This interval also included two of the 3 brightest X-ray outbursts ever observed, namely V404 Cyg in 2015, and MAXI J1820+070 in 2018, as well as Swift J1357.2-0933, the first such system to show a...
X-ray binaries are among the brightest objects of our Galaxy in the high-energy domain (0.1-100 keV). Despite a relatively good knowledge of their basic emission mechanisms, we still lack a complete understanding of their time, energy, and luminosity dependence. We will present results obtained from a detailed study of a transient Be X-ray pulsar 4U 1901+03. This source was followed by NuSTAR...
Gamma-ray binaries are a rare subclass of high mass binary systems that display non-thermal emission peaking at energies greater than 1 MeV. All the identified systems contain an O/Oe or B/Be type star and a compact object in the mass range of a neutron star or black hole. Correctly interpreting how these sources can produce non-thermal emission, up to very high energy gamma-rays, depends on...
Gamma-ray binaries are a rare subclass of high mass binary systems, where the non-thermal emission peaks in the gamma-ray regime. Two scenarios have been proposed to explain the production of the emission; in the pulsar wind scenario the compact object is proposed to be a rapidly rotating pulsar, and the emission originates from particle acceleration that occurs at the shock that forms between...
Be X-ray binaries, which make up the largest subclass of the high mass X-ray binary systems, comprise a neutron star in an eccentric orbit around Be star companion with a geometrically thin Keplerian disc. The interaction of the neutron star with the Be disc results in the accretion of matter leading to X-ray outbursts. The X-ray outbursts occur in two flavours: type I (or normal, with...
Hydra A is a type I Fanaroff-Riley radio galaxy that hosts a pair of 300-kiloparsec diameter radio lobes that are being powered by one of the most of the powerful AGN outbursts known to date. Radio observations provide us with an excellent probe for the study of high-energy particles residing in the lobes. The MeerKAT radio telescope carried out observations of Hydra A, from which we obtained...
Extragalactic radio sources selected for geodetic and astrometric VLBI observations are expected to have a core-dominated point-like structure. However, it is a well-known fact that extragalactic radio sources often exhibit time- and frequency-dependent intrinsic source structure at milliarcsecond (mas) scale, making them subjects of routine monitoring. In this talk, I will present mas...
Previous work on time-dependent shock-acceleration and radiation transfer in relativistic jets has successfully reproduced many spectral variability features of blazars if flaring activity is mediated by increasingly efficient diffusive shock acceleration. However, flaring events exhibiting a significant increase of the Compton dominance, or even "orphan" gamma-ray flares, are very difficult...
Blazars – active galaxies with the jet pointing at Earth – emit across all electromagnetic wavelengths. The so-called one-zone model has described well both quiescent and flaring states, however it cannot explain the radio emission. In order to self-consistently describe the entire electromagnetic spectrum, extended jet models are necessary. Notably, kinetic descriptions of extended jets can...
The supermassive black holes in the centers of some active galaxies (AGN) eject powerful relativistic jets which propagate over kpc scales, showing no significant momentum loss. Both observational evidence as well as theoretical considerations from MHD simulations of jets suggests that they are radially stratified, with a fast inner spine surrounded by a slower-moving outer sheath. The...
Her X-1 is an accreting X-ray binary with well-determined properties based on measurements of light- curves and spectra since its discovery in 1972. Since discovery, it has been observed with most major X-ray astronomy instruments. Observations of Her X-1 over the past decade include those from AstroSat, Swift, and MAXI. These have added to the range of phenomena detected from this system. The...
X-ray binaries (XRBs) comprise of compact objects, black hole or a neutron star, and a companion (donor) star. Depending on the mass of the companion, the XRB is classified as low-mass (Mdonor < 1 M⊙) or high-mass (Mdonor > ∼ 8 M⊙). SAX J1808.4–3658 is a low-mass neutron star XRB, the first accreting millisecond pulsar (AMXP) known. The pulsar was confirmed with the detection of 401 Hz...
The Imaging X-ray Polarimetry Explorer (IXPE), scheduled for launch in Fall 2021, will open up the window of X-ray polarimetry. IXPE will be joined by XL-Calibur in 2022, adding 15-75 keV polarimetric capabilities to IXPE’s 2-8 keV coverage.The exciting science topics that can be addressed with IXPE include observations of stellar mass black holes and observations of magnetars. I will report...
The spectral energy distributions (SEDs) of some blazars exhibit an ultraviolet (UV) and/or soft Xray excess, which can be modelled with different radiation mechanisms. Polarization measurements of the UV/X-ray emission from blazars may provide new and unique information about the astrophysical environment of blazar jets and could thus help to distinguish between different emission scenarios....
Active galactic nuclei (AGN) with relativistic jets, powered by mass accretion onto the central supermassive black hole of their host galaxies, are the most powerful persistent sources of broadband electromagnetic radiation in the Universe. Despite decades of multi-wavelength observations, there are still several key questions about AGN jet physics that remain open, such as "What is the plasma...
Blazars are potential candidates of cosmic-ray (CR) acceleration up to ultrahigh energies (UHE, E > 1017 eV). Association of a number of blazars with IceCube neutrino events supports this hypothesis. If the intergalactic magnetic field strength is reasonably low, the UHECRs escaping from the blazar jet will produce neutrinos and gamma rays along the line-of-sight (LoS) by interacting with the...
In 2015 the advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) detected the first ever gravitational event, gravitational wave event GW150914, with multiple new gravitational wave events, originating from both binary neutron stars and binary black hole (BBH) mergers, detected in subsequent years. In light of these detections, we simulate the dynamics of ambient test particles...
Ultraluminous X-ray sources (ULXs) were first discovered in the 1980s by the Einstein Observatory. The most interesting property of ULXs is that they exceed the Eddington luminosity of a neutron star. Data from the 2XMM-Newton and Chandra contain 470 ULX candidates in 238 galaxies (Walton et al. 2011) and 629 ULX candidates in 309 galaxies (Kovlakas et al. 2020), respectively. In this study,...
Emerging anomalies in both di and multi-lepton data from the LHC have been used to motivate for an extension to the Standard Model in the form of a second Higgs doublet and a singlet scalar (2HDM+S). Here we explore a dark matter candidate drawn from this model: a scalar particle that couples to the Standard Model through the 2HDM+S degrees of freedom. Using the best-fit 2HDM+S model from LHC...
At present, only a single Globular Cluster (GC) has plausibly been detected at very high energies (VHEs) by H.E.S.S. The future CTA is expected to detect more GCs in this band. We present results from an emission code that assumes millisecond pulsars (MSPs) to be sources of relativistic particles diffusing through GCs that will give broad-band radiation due to their interaction with the...
Marsh et al. detected radio and optical pulsations from the binary system AR Scorpii (AR Sco). This system, with an orbital period of 3.55 h, is composed of a cool, low-mass star and a white dwarf (WD) with a spin period of 1.95 min. Takata et al. also detected X-ray pulsations from this source. These observations indicate no presence of an accretion disk or column. Buckley et al. found that...
Previous studies indicate that AR Sco’s surrounding region complicates the search for Gamma-ray emission from this source. The fact that AR Sco lies close to the Galactic plane and strong nearby VHE Fermi sources, make it difficult to constrain and quantify an upper-limit of the emission from AR Sco’s location in the sky. In this study, a search for high energy gamma-ray emission was...
Here we present the first report of pulsed emission at the spin period of the white dwarf in MeerKAT data, which is also the first report of pulsed emission at the spin period of the white dwarf in radio frequencies (L-band). Further support for the pulsar-like behaviour of AE Aqr is the detection of pulsed emission at the spin period of the white dwarf in AE Aqr in Fermi-LAT data. By...
Ground-based Air-Cherenkov telescopes have detected pulsations at energies above 50 GeV from a growing number of Fermi pulsars. These include the Crab, Vela, PSR B1706-44 and Geminga, with the first two having pulsed detections above 1 TeV. There appears to be VHE emission that is an extension of the Fermi spectra to high energies as well as additional higher-energy components that require a...
Astronomy for development is making great strides in Namibia. Forged by a collaboration between the Universities of Oxford and Namibia, together we are using astronomy as a means for capacity-building and to benefit Namibia socio-economically. Namibia is already recognised as a world leader in sustainable tourism; astronomy offers great potential to expand and diversify the market with minimal...
The very-high-energy (VHE; E>100 GeV) gamma-ray emission observed from a number of Supernova rem- nants (SNRs) suggests that particles are accelerated to high energies at the shock of the remnants. However, it is extremely difficult to determine which particles are responsible for this emission as both protons (through hadronic interactions and subsequent pion decay) and electrons (through...
Being less than a thousand years old, Kes 75 (G29.7-0.3) represents one of the youngest composite supernova remnants in the Milky Way. It contains the nebula of PSR J1846-0258, a glitching young pulsar with a particularly high spin-down luminosity of 8e36 erg/s. This pulsar has furthermore manifested magnetar-like bursts in 2006. The H.E.S.S. Collaboration detected gamma-ray emission from HESS...
Gamma-ray bursts (GRB’s) are highly energetic impulses of γ−rays that are categorised into two major categories, namely long and short GRBs. Their distinction lies in their duration, the former lasts for more than 2 s whilst the latter lasts for less than 2 s. Their prompt emission has an energy range of keV to GeV energy band. On the 10th of April 2021, the \emph{Fermi} Large Area Telescope...
I will briefly introduce fast radio bursts and highlight recent developments of the field from a theorist’s perspective, as well as exciting observational developments presented at FRB 2021. I will also discuss the low-twist magnetar model for FRBs.
The number of detected gamma-ray millisecond pulsars (MSPs) has been increasing rapidly since the launch of the Fermi Large Area Telescope (LAT) in 2008. The Third Fermi LAT Pulsar Catalogue (3PC) will present high-quality data for about 250 gamma-ray pulsars based on 11 years of observation. Out of that number, more than 100 are MSPs. The stability of MSPs light curves makes them interesting...
Despite the growing amount of observational data, the physical conditions in the vicinity of supermassive black holes (SMBH) are still poorly understood. Thanks to its proximity, Sgr A*, being the SMBH located in the center of our Galaxy, represents an ideal target to probe physical processes in the surroundings of massive compact objects, including details of accretion/advection flows,...
The Neutron Star Interior Composition Explorer (NICER) began science operations from the International Space Station in 2017 July, conducting observations in X-rays (0.4-12 keV) with sub-microsecond time resolution. Accomplishments to date are briefly reviewed for several types of sources. Pulse profiles for rotation-powered millisecond pulsars are modeled to constrain the equation of state of...
