[View Abstracts by Session]

Session: Cosmology and Relativistic Astrophysics

1. Name: Ms. Vera Agalianou (University of Ioannina)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: The rocket effect mechanism in neutron stars in supernova
   Abstract:
   Although it is commonly assumed that the magnetic field axis of a neutron star passes through its center, recent research has revealed that this may not always be the case. In particular, the millisecond pulsar J0030+0451 has shown evidence of a displaced magnetic field axis, which could activate the electromagnetic rocket effect. This effect occurs when the off-center dipole magnetic field generates asymmetric electromagnetic radiation, which exerts a net force, causing the star to accelerate. This mechanism could potentially account for the high spatial velocities of pulsars ($~10^3$ km/s). In this study, we investigate the impact of the electromagnetic rocket effect on young pulsars associated with supernova remnants. We compare observational data on characteristic quantities such as the braking index and proper motion with results obtained from the rocket effect. By utilizing a Markov Chain Monte Carlo analysis, we explore the necessary conditions, such as the initial spin period and distance between the magnetic axis and the star's center, to generate a velocity kick that approaches the present velocity. Our findings suggest that the electromagnetic rocket effect can account for typical pulsar transverse velocities, given an initial spin period of 3.8 ms and a dipole field located approximately 7 km from the star's center. We also examine the effect of the rocket effect on the braking index of a neutron star and find that, for the sample studied, its impact is negligible. Finally, we apply the rocket effect model to the pulsars J0030+0451 and J0538+2817, which are potential candidates for this mechanism.
2. Name: Mr. Yannis Anastasiadis (National & Kapodistrian Univ. of Athens)
   Coauthors: No coauthors were included.
   Type: Poster
   Title: Analytical GRMHD Jet Models
   Abstract:
   Jets manifest in a wide variety of astrophysical phenomena. We study them using a semi-analytical model of the General Relativistic Magnetohydrodynamic (GRMHD) equations in Kerr metric that describes the jet near the rotation axis, assuming steady state, and axisymmetry. The model is constructed by expanding the rotating black hole metric and the forces, with respect to the polar angle about the rotation axis. This results in all the physical quantities being expressed as functions of the radial distance. The goal is to solve for the bulk acceleration, the shape of the jet, and understand how these depend on the magnetic field. The addition to the previous semi-analytical models that expand the metric around the rotation axis, is that the flow is governed by a polytropic equation of state. The solutions in this presentation start from a stagnation surface very close to the event horizon and become highly relativistic at large distances.
3. Name: Mr. John Antoniadis (IA-FORTH)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Updates on the search for low-frequency gravitational waves with pulsar timing arrays
   Abstract:
   I will provide a brief update on the search for nanohertz gravitational waves using the second data release of the European Pulsar Timing Array
4. Name: Dr. Danai Antonopoulou (JBCA, Manchester)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Large-scale superfluidity in neutron stars
   Abstract:
   The majority of known isolated neutron stars is observed as pulsars, allowing the precise determination of their spin frequency over time. In the long-term, pulsars spin down due to rotational energy losses to their environment; however many of them — and especially the younger ones — occasionally display abrupt spin-up events known as glitches. The relaxation of pulsar rotation following the spin-up perturbation is very slow, a direct evidence that the neutrons in the stellar interior are in a superfluid state. I will review the phenomenology of glitches and discuss how observations combine with the latest theoretical models of neutron star internal dynamics to provide insights into dense matter physics.
5. Name: Ms. Eleni Antonopoulou (National & Kapodistrian Univ. of Athens)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Tackling the SgrA* orbital motion riddle with a new General-Relativistic Ray Tracing code
   Abstract:
   The Gravity instrument detected a hotspot orbiting SgrA*, the supermassive black hole in the Galactic Center, during the observed Near-Infrared flares of 2018. The Gravity Collaboration fitted the trajectory of the July 22 hotspot with a circular Keplerian orbit of a few gravitational radii, yet the short orbital period and broad angular extent of the observed trajectory disfavor this fit. Motivated by these results, we developed a new Python code for General-Relativistic Radiative Transfer calculations in the Kerr spacetime. We describe the tests used to evaluate our code’s results and move on to investigate the kinematics of the aforementioned orbit, including various synchrotron emission models in our research.
6. Name: Dr. Stella Boula (IFJ PAN)
   Coauthors: Niemiec Jacek (IFJ PAN)
   Amano Takanobu (University of Tokyo)
   Type: Oral
   Title: Generalized Hybrid Kinetic Simulations of Particle Acceleration at Merger Shocks in Galaxy Clusters
   Abstract:
   The question of how cosmic rays are accelerated to high energies in galaxy clusters remains unresolved. Radio relics, formed by relativistic electrons produced at merger shocks, emit synchrotron radiation and are believed to be a potential source of high-energy cosmic rays, gamma-rays, and neutrinos. Recent studies have focused on electron acceleration at low Mach number merger shocks propagating in hot intracluster mediums. Using multi-scale turbulence in the shock, including ion-scale shock surface rippling, our recent PIC simulation results demonstrate that stochastic shock-drift acceleration can provide electron acceleration in these conditions. A newly developed generalized fluid-particle hybrid numerical code can handle fluid electrons, ions, and an arbitrary number of kinetic species, including energetic particles. Our findings include discussions on the structure of quasi-perpendicular shocks obtained from standard hybrid simulation models, large-scale 2D and 3D simulations for varying parameters, such as plasma beta, magnetic field obliquity angle, and shock Mach number, under subcritical and supercritical conditions. Furthermore, we present our first preliminary results on electron acceleration in turbulent shocks by tracking the trajectories of an additional energetic electron population self-consistently implemented into our hybrid simulation model.
7. Name: Dr. Ioannis Contopoulos (RCAAM, Academy of Athens)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Novel Features of the Pulsar Magnetosphere
   Abstract:
   In the past 5 years new high resolution global numerical simulations of the pulsar magnetosphere have appeared in the literature. We will present their main results regarding the origin of the pulsar wind and high-energy radiation, and we will comment on their significance and limitations. We will also discuss new theoretical approaches for the future.
8. Name: Mr. Ioannis Dimitropoulos (University of Patras)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Sgr A* flares, an electromagnetic signal and more
   Abstract:
   Sagittarius A* is a super massive black hole in the center of our galaxy observed across the electromagnetic spectrum for decades. Moreover, it is the primary target of the Event Horizon Telescope (EHT) and the GRAVITY collaboration, which observe in the 230 GHz (EHT) and the NIR (GRAVITY). The compact radio emission is expected to be thermal radiation from the vicinity of the black hole. The NIR emission exhibits strong variability and flares which are observed from the Galactic center every 6-8 hours. Flaring activity has been attributed to reconnection events close to the Black hole. We perform general-relativistic magneto- hydrodynamic to model the accretion flow of SgrA*, and employ thermal emission models to reproduce the emission at 230 GHz, analyzing the regions close to the black hole that this emission is generated. Furthermore, we monitor the magnetic re-connection events in the simulation and model the non-thermal emission at the NIR. Our investigation focuses on the production of flares from Sgr A* observed in the NIR from GRAVITY, our aim is to reproduce a model that calculates lightcurves at these frequencies.
9. Name: Prof. Dimitrios Giannios (Purdue Univ.)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Particle Acceleration in Blazar jets
   Abstract:
   Relativistic jets are a common manifestation of accreting black holes. Blazars are jets from supermassive black holes moving close to our line of sight. A common hypothesis for jet formation is that they are launched by powerful magnetic fields that thread the black hole. Here, I discuss the trip of the jet from the black hole to the much larger scales where it radiates. I argue that the jet emission is result of MHD instabilities that result in dissipation in the jet through the process of magnetic reconnection. I will review our latest understanding of the physics of magnetic reconnection and show that it could naturally produce the emitting plasmoids commonly invoked when modeling the blazar flares. Our 3D first-principle simulations of magnetic reconnection show that the dominant mechanism for (fast) particle acceleration involves particles that escape the current sheet and are accelerated by the large-scale electric field in the reconnection upstream. If the reconnection layers extend to a large fraction of the jet cross section, this mechanism can accelerate hadrons to ultra-high energies.
10. Name: Prof. Konstantinos Gourgouliatos (University of Patras)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Pulsar - Supernova Remnant Associations: Implications for magnetic field evolution and pulsar initial periods
    Abstract:
    Neutron stars form through core-collapse supernova explosions which leave behind, along with the compact object, a supernova remnant (SNR). We have studied the SNR cat (Ferrand & Safi-Harb 2012), and identified 56 pulsars for which there exists both an estimate of the characteristic age of the pulsar and that of the hosting SNR. The discrepancy between these age estimates provides a means to quantify possible deviations from the pulsar spin-down dipole model and estimate their initial periods. We find that distribution of magnetic fields and periods for radio pulsars are both well described using the lognormal distribution. The mean magnetic field is log_{10}[B/G] = 12.44 and standard deviation is σ_B = 0.44. Magnetars and central compact objects do not follow the same distribution. The mean initial period is log_{10}P0[P/s]=−1.04(+0.15−0.2) and standard deviation is σ_p=0.53(+0.12−0.08). We show that the normal distribution does not describe the initial periods of NSs sufficiently well.
11. Name: Dr. Dimitrios Kantzas (LAPTh/CNRS)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: The contribution of X-ray binaries in the cosmic ray and neutrino spectra
    Abstract:
    Since their discovery, cosmic rays (CRs) remain among the most mysterious phenomena of modern Physics. The dominant sources, as well as the exact acceleration mechanisms, remain unknown. The CRs up to the ``knee’’ have traditionally been considered to originate entirely in the shock waves of supernova remnants (SNRs), however, due to the lack of a “smoking-gun” TeV counterpart in many cases, as well as the new population of non-SNR Galactic PeVatrons, this scenario has been recently questioned. In this talk, I will motivate how the small-scale analogues of active galactic nuclei, namely black-hole X-ray binaries (BHXBs), can potentially contribute to the Galactic CR spectrum. Based on a new multi-zone, lepto-hadronic jet model to take advantage of the entire broadband multiwavelength spectra observed by BHXBs, I will discuss how to properly estimate the neutrino and γ-ray emissions and how these two compare to current observations. Finally, I will discuss the contribution of these sources to the diffuse γ-ray and neutrino spectra detected by Fermi and HESS, and IceCube, respectively.
12. Name: Ms. Despina Karavola (National & Kapodistrian University of Athens)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Gamma-rays from Synchrotron radiation of Bethe-Heitler pairs in Active Galactic Nuclei
    Abstract:
    Jets from Active Galactic Nuclei are considered to be efficient astrophysical particle accelerators, producing electromagnetic spectra that in most cases extend to the γ-ray regime. These spectra are usually modeled with the use of solely leptonic processes, such as electron synchrotron radiation and inverse Compton scattering. Even though the results are in agreement with the observational data, the nature of an acceleration mechanism able to create such energetic electrons remains an open question. We present an alternative to the latter by exploiting the energy reservoir provided to the proton population present in the jet, by the magnetic field. Highly relativistic protons can interact with low energy photons from the jet, so as to produce very energetic electron-positron pairs through a procedure known as Bethe-Heitler pair production. In this contribution we examine under which conditions the produced Bethe-Heitler pairs can emit γ-rays, thus relaxing the requirement of accelerating electrons to ultra-relativistic energies. Alongside, we were able to approximate the energy spectrum of the produced pairs by an analytical function, a quantity that otherwise should be calculated by the integration of the Bethe-Heitler cross section, a task which is computational expensive.
13. Name: Ms. Elpida Koutsantoniou (National & Kapodistrian Univ. of Athens)
    Coauthors: No coauthors were included.
    Type: Poster
    Title: Algorithms & radiation dynamics near black holes
    Abstract:
    We study systems of hot accretion disks orbiting astrophysical black holes. These disks have temperatures of a few keVs and emit robustly in the band of X-rays. The accretion disk material then reabsorbs this radiation and develops unilateral braking due to the Poynting-Robertson effect. This alters the material dynamics, modifying the plasma kinematics and the equilibrium conditions. We discuss the results of our computational algorithms that calculate the generated radiation field of accretion disks in curved spacetime. We examine various configurations of disks, estimating the magnitude and the geometrical distribution of these fields. We consider what consequences these recorded radiation forces can have on the arrangements and investigate their possible evolution. Our results include clear indications that the primary factor affecting the radiation field magnitude is the material density gradient rather than the accretion disk volume. We also record evidence of early jet collimating tendencies due to the radiation. Another noteworthy implication of such procedures is the consequent generation of astrophysically significant magnetic fields in small timescales via the Cosmic Battery mechanism. These magnetic fields later twirl and expand leading to the formation of system outflows. Subsequently, the fields are injected into the interstellar or intergalactic space by the jets. Finally, we investigate the assorted information we can retrieve from observations of such structures, such as estimations of the central black hole spin and the system dynamics.
14. Name: Prof. Michael Kramer (MPIfR)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Radio pulsars as probes of fundamental physics - and more
    Abstract:
    Radio pulsars are not fascinating but also very useful tools to probe a wide range of physics. Finding them, especially when they are in binary systems, is challenging but unlocks a wide range of fundmanetal physics that can be explored and tested. A prime example is the study of relativistic gravity, including their usage as detectors of gravitational waves, or the study of the equation-of-state of extreme matter. As radio sources, pulsars are only poorly understood, though remarkable process has been made recently. As an end product of a massive stars, one can use pulsars to probe the physics of core collapse supernovae, while the high degree of polarisation makes them a superb probes to study the Galactic magnetic field. This talk tries to give an overview of pulsar physics and their applications, remembering Prof Seiradakis' contributions to the field.
15. Name: Prof. Nikolaos Kylafis (Univ. of Crete & FORTH)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: A quantitative explanation of the radio -- X-ray correlation in black-hole X-ray binaries
    Abstract:
    Accretion of matter onto compact objects is tightly connected with ejection of matter from them. After all, it is the accretion flow that feeds the outflow. Despite many efforts, this connection has not become clear. In particular, the observed correlation between the radio and X-ray fluxes in the hard state of black-hole X-ray binaries (BHXRBs) has been around for more than two decades now. It is currently accepted that the hard X-rays in BHXRBs come from Comptonization in the corona and the radio emission from the jet. The jet and the corona, however, are separate entities with hardly any communication between them, apart from the fact that the jet is fed from the corona. It is also widely accepted that the accretion flow around black holes in BHXRBs consists of an outer thin disk and an inner hot flow. From this hot inner flow, an outflow must emanate in the hard and hard-intermediate states of the source. By considering Compton up-scattering of soft disk photons in the outflow (i.e., in an outflowing "corona") as the mechanism that produces the hard X-ray spectrum, we have been able to explain quantitatively a number of observed correlations. Here, we investigate whether this outflowing "corona" can explain the observed radio - X-ray correlation also. We consider a parabolic outflow and compute the radio emission at 8.6 GHz coming from it, as well as the power-law photon-number spectral index Γ of the Comptonized hard X-rays produced in it. Thus, we have a correlation between the computed radio flux F_R at 8.6 GHz and the computed spectral index Γ of the hard X-ray spectrum. This correlation is actually a theoretical prediction, since both F_R and Γ are computed from the model and, to our knowledge, no such correlation has been constructed from observations for the hard and the hard-intermediate states. This prediction can be confirmed or rejected in future outbursts of GX 339-4. From observations of GX 339-4, we also produce a correlation between the observed X-ray flux F_X and the observed index Γ. Thus, for each value of Γ, observed/computed, we have the corresponding values of the observed F_X and the computed F_R, which we plot one against the other. We find that, in the hard state of GX 339-4, our idea reproduces the observed correlation of F_R ~ (F_X)^0.6 . In addition, in the hard-intermediate state of GX 339-4, we predict that, in future outbursts of the source, the F_R will exhibit first a sudden increase and then a sharp drop within a very narrow range of values of F_X . Also, since in a parabolic outflow the density is largest at its bottom, the transverse optical depth at the bottom of the outflow is very large (typically 10 to 100) and so the soft input photons, that are Comptonized there, see something like a "slab" above their emission. This may explain naturally the observed X-ray polarization from BHXRBs.
16. Name: Dr. Sam Lander (University of East Anglia, Norwich)
    Coauthors: Gourgouliatos Konstantinos (University of Patras)
    Type: Oral
    Title: Breaking a neutron star’s crust
    Abstract:
    Like any elastic medium, the solid crust of a neutron star has a maximum stress it can withstand before it fails. What happens once this stress is exceeded is not fully understood, but it is believed to lead to the observed activity of magnetars - neutron stars with the strongest known magnetic fields in the Universe. We will discuss efforts to simulate this failure under high magnetic stress through numerical simulations - both with coupled differential equations, and through a cellular automaton model. The applications of the work stretch beyond magnetar activity, however, and may also have implications for the prospects of observing continuous gravitational waves from neutron stars.
17. Name: Dr. Ioannis Liodakis (FINCA)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Relativistic jets in the discovery era of X-ray polarimetry
    Abstract:
    X-ray polarization is a crucial probe of the magnetic field structure and emission processes in astrophysical systems. This is particularly true for relativistic jets from supermassive black holes, where polarimetry allows us to investigate their composition and particle acceleration mechanisms. Until now, polarization observations have been limited to the radio-to-optical range, thereby leaving a gap in our understanding of the processes and physical conditions in the most energetic objects in the Universe. The recently launched Imaging X-ray Polarimetry Explorer (IXPE), NASA's very first X-ray polarization mission, offers a radical new way of studying high-energy processes in these jets. I will discuss the multiwavelength polarization observations and results from the first year of IXPE observations of radio galaxies and blazars.
18. Name: Mr. Argyrios Loules (National & Kapodistrian Univ. of Athens )
    Coauthors: Vlahakis Nektarios (National and Kapodistrian University of Athens)
    Type: Oral
    Title: Unveiling the Effects of Electromagnetic Dissipation on Relativistic Jets
    Abstract:
    Relativistic astrophysical jets are highly energetic, collimated plasma outflows ubiquitous in extreme astrophysical environments. Owing to their intriguing and complex nature, the dynamics of these outflows have been extensively studied over the past few decades, with a large number of works dedicated to the investigation of the mechanisms which accelerate and collimate them. In the present work relativistic astrophysical jets are studied under the assumption of non-zero electrical resistivity with the aim of uncovering the impact of electromagnetic dissipation upon their dynamics and morphology. We expand the equations of resistive relativistic MHD in spherical geometry in the vicinity of the polar axis and retrieve polynomials in the polar angle θ. These polynomials provide the differential equations which govern the radial profiles of the jet parameters and its electromagnetic field. The analytical solutions retrieved from this system of equations unveil the impact of electromagnetic dissipation on the jet’s acceleration and collimation mechanisms and allow for a comparison between the dynamics of resistive jets and their ideal MHD counterparts.
19. Name: Ms. Cloe Mahe (Universite de la Cote d'Azur)
    Coauthors: No coauthors were included.
    Type: Poster
    Title: A synchrotron cooling model for the NIR/X-ray flares of Sgr A*
    Abstract:
    Sgr A*, the supermassive black hole at the center of our Galaxy, is accreting mass from its surroundings at an extremely low rate of about 3e-13 M_sun/yr. Sgr A* is therefore a unique laboratory to study the physical processes at work in quiescent accreting systems. The observation of flares with hour-long duration in the near-infrared and X-ray wavelengths from Sgr A* suggests the formation of active flaring regions in the accretion flow of Sgr A* filled with relativistic electrons that produce non-thermal radiation. The aim of this project is to fit the 2014 NIR/X-ray flare from Sgr A* with a synchrotron cooling model using Markov-Chain Monte Carlo simulations. By obtaining the posterior distributions for the parameters describing the flaring region, such as size and magnetic field strength, we are able to study the degeneracies between model parameters and their evolution with time. Our results support a scenario where a hot spot in the accretion of Sgr A* with kG magnetic fields and size of ~1/1000 of the Schwarzschild radius is the production site of the non-thermal flares. Our method can be easily applied to a larger sample of NIR/X-ray flares in order to extract a statistical sample for the properties of the flaring regions.
20. Name: Mr. Nikolaos Mandarakas (IA-FORTH & Univ. of Crete)
    Coauthors: No coauthors were included.
    Type: Poster
    Title: Optical afterglow polarization of GRB210619B
    Abstract:
    We have measured the optical polarization of the afterglow of the extremely bright gamma-ray burst (GRB) GRB210619B in the time window ∼5967-8245 seconds post-burst, using the RoboPol polarimeter in Skinakas, Crete. We report a 5σ significant measurement of polarization P = 1.5 ± 0.3 at polarization angle EVPA = 8 ± 6°. The polarization properties remain likely constant throughout our observations. We have demonstrated that the measured polarization is intrinsic to the GRB, as the host-induced polarization and the polarization induced by the interstellar are relatively negligible.
21. Name: Prof. Apostolos Mastichiadis (National & Kapodistrian Univ. of Athens)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: A study of natural frequencies in a dynamic corona – disk system
    Abstract:
    Black-hole X-ray binaries (BHXRBs) in the hard and hard-intermediate spectral and temporal states exhibit in their power spectra characteristic frequencies called type-C quasi-periodic oscillations (QPOs). Noting that the hot Comptonizing corona interacting with the cold accretion disk, both of which are central in understanding BHXRBs, is essentially a dynamical system, we write and solve the time-dependent equations that describe energy conservation. For a constant mass accretion rate, the dynamic corona – disk system exhibits dumped oscillations with characteristic frequencies similar to the ones observed in the power spectra of BHXRBs. Even small perturbations in the accretion rate are able to sustain the oscillations. We argue, therefore, that type-C QPOs in BHXRBs could, in principle, arise from the interaction of the hot Comptonizing corona with the much colder accretion disk.
22. Name: Dr. Konstantinos Migkas (Leiden University)
    Coauthors: No coauthors were included.
    Type: Poster
    Title: Probing cosmic isotropy with new X-ray clusters and the first eROSITA cluster catalog
    Abstract:
    The isotropy of the Universe at >200 Mpc scales is a crucial assumption of LCDM. Any significant, observational deviation from this consensus can strongly challenge the standard cosmological model. Multiwavelength scaling relations of galaxy cluster properties that do or do not depend on cosmological assumptions are an excellent and powerful tool for scrutinizing cosmic isotropy. The apparent angular variation of these relations provides us with numerous independent tests of cosmic isotropy. In Migkas et al. (2021) we detected a 9% anisotropy of the local Hubble constant at a $5.4sigma$ level. This anomaly can be attributed to a 900 km/s cluster bulk flow motion extending up to 500 Mpc. These results are in strong tension with LCDM. In this work, we present for the first time the cosmic isotropy results of 1) multiple new, more precise cluster scaling relations between the core-excised X-ray luminosity, gas mass, isophotal radius, and temperature of ~300 clusters, and 2) the 1st eROSITA cluster catalog. Amazingly, we show that both the much-improved scaling relations and the independent eROSITA cluster catalog confirm the detection of a cosmic anisotropy at the <1 Gpc Universe.
23. Name: Mr. Vasileios Mpisketzis (National & Kapodistrian Univ. of Athens)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Impact of inhomogeneous ejecta in jet dynamics and afterglow emission in binary neutron star mergers
    Abstract:
    Binary neutron star (BNS) systems are prime candidates as progenitors of gamma-ray bursts (GRB). After the remnant of the BNS merger collapses to a black hole, a jet is powered from the central engine that may break out of the merger ejecta, a massive dense envelope that forms upon collision. This ejecta could be anisotropic and inhomogeneous, which may affect the dynamical evolution of the outflow from the merger, the emerging jet, and the resulting electromagnetic counterparts. Here, we examine how an inhomogeneous ejecta affects the properties of the jet-ejecta cocoon, the propagation of the jet, and finally the afterglow radiated by the jet upon deceleration. We carry out hydro-dynamical simulations of relativistic jets launched within ejectas with prescribed inhomogeneous distributions of matter. Effects are mostly present in the high-latitude regions of the jet, where we find that a significant reservoir of energy can be present, depending on the ejecta structure. On the off-axis lines of sight expected for afterglow counterparts of GW-triggered BNS mergers, the early afterglow emission is particularly affected by the high-latitude structure of the jet.
24. Name: Mr. Dimitris Ntotsikas (University of Patras)
    Coauthors: Gourgouliatos Kostas (Dr, University of Patras)
    Lander Sam (Dr,University of East Anglia)
    Kontopoulos Ioannis ((Dr, Academy of Athens))
    Type: Poster
    Title: Twisted Pulsar Magnetospheres and their Current Sheets
    Abstract:
    Motivated by sudden energetic events in magnetars, we study force-free, twisted, axisymmetric, and relativistic pulsar magnetospheres. We obtain equilibrium solutions utilizing the method of simultaneous relaxation inside and outside the light cylinder. We introduce toroidal magnetic fields in the region of closed field lines to account for the twist. Our findings indicate that increasing the twist leads to a larger fraction of magnetic field lines of opening, which enlarges the polar caps and enhances the spin-down rate. In magnetospheres with significant twisting, the inner edge of the current sheet must lie within the light cylinder, as, otherwise the solution has magnetic field lines that are disconnected from the star. We apply these solutions to variations in the spin-down rates of magnetars and nulling and moding of pulsars.
25. Name: Prof. Foteini Oikonomou (NTNU)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: The Curious Maximum Rigidity Distribution of Ultra-high Energy Cosmic Ray Accelerators
    Abstract:
    A standard assumption among models of candidate source populations of ultra-high energy cosmic rays (UHECRs) is that all sources in a source population accelerate particles to the same maximum energy. Motivated by the fact that candidate astrophysical accelerators exhibit a vast diversity in terms of their relevant properties, such as luminosity, Lorentz factor, and magnetic field strength, we study the compatibility of a population of sources with non-identical maximum cosmic-ray energies with the observed energy spectrum and composition of UHECRs at Earth. For this purpose, we compute the UHECR spectrum emerging from a population of sources with a power-law, or broken-power-law, distribution of maximum energies applicable to a broad range of astrophysical scenarios. We find that for a wide range of studied models, the maximum energies of the UHECR accelerators must be nearly identical in order to be compatible with the UHECR data, in stark contrast to the variance expected for the astrophysical source models considered. A substantial variance of the maximum energy is only consistent with the UHECR data if the maximum energies of the UHECR sources follow a broken power-law distribution with a very steep spectrum above the break. However, in this scenario, the individual source energy spectra must be unusually hard with increasing energy output as a function of energy.
26. Name: Dr. Theodoros Papanikolaou (National Observatory of Athens)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Primordial magnetic fields from primordial black hole disks
    Abstract:
    Large scale primordial magnetic fields (PMFs) threading the intergalactic medium are observed ubiquitously in the Universe playing a key role in the cosmic evolution while their origin constitutes one of the long-standing issues in cosmology. In the present talk, we propose a novel natural ab initio mechanism for the origin of such PMFs through the portal of supermassive primordial black holes (PBHs) forming between the Big Bang Nucleosynthesis and the recombination era. In particular, by considering PBHs furnished with a locally isothermal disk we study the generation of a Biermann battery induced seed magnetic field (MF) due to the vortexlike motion of the primordial plasma around the black hole. By considering monochromatic PBH mass distributions and deriving the relevant MF power spectrum we make a conservative estimate for the seed PMF in intergalactic scales and at redshift $z=30$, when typical galaxies are considered to form, which reads as $Bsimeq 10^{-30}mathrm{G}left(frac{ell_mathrm{R}}{10^6}right)^2left(frac{M_mathrm{PBH}}{10^{14}M_odot}right)^{5/2}$, where $M_mathrm{PBH}$ is the PBH mass and $ell_mathrm{R}equiv R_mathrm{d}/R_mathrm{ISCO}$, is the ratio of the radius of the disk, $R_mathrm{d}$ over the radius of the innermost stable circular orbit, $R_mathrm{ISCO}$. Interestingly enough, by requiring to seed a PMF of the order of $10^{-30}mathrm{G}$ necessary to give rise to a present day $10^{-18}mathrm{G}$ in intergalactic scales, we find a lower bound on the PBH mass within the range $[10^{10}- 10^{16}]M_odot$ depending on the radius of the PBH disk.
27. Name: Prof. Maria Petropoulou (National & Kapodistrian Univ. of Athens)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Multi-messenger signatures of magnetically dominated baryon-loaded blazar jets
    Abstract:
    Blazars are a rare class of active galactic nuclei (AGN) with relativistic jets pointing towards the observer. In this contribution, we present a simple, yet physically motivated, model for blazar emission. According to this, electrons and protons are energized via magnetic reconnection in parts of the jet where the plasma magnetization is still high (σ>1). The magnetization σ and bulk Lorentz factor Γ are related to the available jet energy per baryon, μ. We also adopt an observationally motivated relation between Γ and the mass accretion rate, which controls the luminosity of external radiation fields. I will present our results about the photon and neutrino jet emission as a function of the fundamental parameters μ, σ, and Γ, and discuss their implications about the contribution of the blazar population to the astrophysical neutrino flux measured by IceCube.
28. Name: Prof. Maria Petropoulou (National & Kapodistrian Univ. of Athens)
    Coauthors: Marcotulli Lea (Yale University)
    Ajello Marco (Clemson University)
    Boettcher Markus (North-West University)
    Coppi Paolo (Yale University)
    Costamante Luigi (Perugia University)
    Errando Manel (Washington University in St Louis)
    Garcia Javier (Caltech)
    Gokus Andrea (Washington University in St. Louis)
    Liodakis Yannis (University of Turku)
    Madejski Greg (Stanford University)
    Mc Bride Fe (Bowdoin College)
    Petropoulou Maria (University of Athens)
    Rani Bindu (NASA, Goddard Space Flight Center)
    Sbarrato Tullia (INAF)
    Stern Daniel (JPL)
    Tavecchio Fabrizio (INAF Osservatorio Astronomico di Brera)
    Zacharias Michael (LUTH, Meudon and Potchefstroom U.)
    Zhang Haocheng (NASA Goddard Space Flight Center)
    Wolton Dom (University of Hertfordshire)
    Type: Poster
    Title: The High Energy X-ray Probe (HEX-P): the most powerful jets through the lens of a superb X-ray eye
    Abstract:
    HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging (<10 arcsec FWHM) and broad spectral coverage (0.1-150 keV) with an effective area far superior to current facilities (including XMM-Newton and NuSTAR), to enable revolutionary new insights into a variety of important astrophysical problems. The unprecedented observational capability of HEX-P will enable us to study the most extreme jets in the Universe powered by supermassive black holes. The instrument's superior angular resolution will enable us to image jet structures, and its excellent sensitivity will uncover the bulk of their population in the early universe. Acceleration and radiative processes responsible for the majority of their X-ray emission will be pinned down by HEX-P excellent timing capabilities. For the first time, truly simultaneous soft- to hard X-ray coverage will enable us to study in detail the particle population responsible for these jets high-energy radiation. All in all, HEX-P is the ideal mission to unravel the science behind the most powerful jets in the universe, and the simulations presented here demonstrate so. More information on HEX-P, including the full team list, is available at https://hexp.org
29. Name: Mr. Pantelis Pnigouras (University of Alicante)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Dynamical tides in neutron star binaries: glimpsing the dense nuclear matter equation of state
    Abstract:
    During the late stages of a neutron star binary inspiral finite-size effects come into play, with the tidal deformability of the supranuclear density matter leaving an imprint on the gravitational-wave signal. As demonstrated in the case of GW170817---the first direct detection of gravitational waves from a neutron star binary---this can lead to constraints on the neutron star equation of state. As detectors become more sensitive, the hydrostatic response of the neutron star to the tidal field of its companion (equilibrium tide) needs to be supplemented by dynamical effects, such as oscillation mode resonances triggered by the orbital motion (dynamical tide). We calculate the contribution of the various stellar oscillation modes to the tidal deformability and demonstrate the (anticipated) dominance of the fundamental mode. We show what the impact of the matter composition is on the tidal deformability, as well as the changes induced by more realistic additions to the problem, e.g. the presence of an elastic crust, superfluidity, and rotation. Finally, based on this formulation, we develop a simple phenomenological model describing the effective tidal deformability of neutron stars and show that it provides a surprisingly accurate representation of the dynamical tide close to merger.
30. Name: Dr. Felix Pötzl (IA-FORTH)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Results from the SMILE pilot study to distinguish milli-lens systems
    Abstract:
    The existence of very low mass (<~ 10^9 M_sun) dark matter (DM) halos is a critical prediction of the Lambda CDM paradigm, and yet still has to be proven observationally. Low mass DM halos, nearly void of stars, may possibly be detected only through the gravitational effect they exert on ordinary matter. The SMILE (Search for MIlli LEnses) project aims at probing the number density of low mass (~ 10^6 - 10^9 M_sun) DM halos searching for gravitational lens systems at milliarcsecond scales (milli-lenses), where the lens is expected to have a mass in the range of interest. This is achieved by studying radio images of active galactic nuclei made with Very-Long-Baseline Interferometry (VLBI). In a recent pilot project, we have searched for milli-lens candidates in a sample consisting of 13,828 compact radio sources from the Astrogeo VLBI FITS image database. Forty candidates with compact double structures have been found using a citizen-science approach, for which I will present the ongoing analysis of follow-up observations with the European VLBI Network at 5 and 22 GHz in phase-referencing mode. These observations with increased sensitivity and frequency coverage allow us to better constrain the nature of the lens candidates. We find some interesting sources that are still viable lens candidates, given constrains such as surface brightness ratio, stability of flux density ratio of components over time, and their spectrum. Other sources are potential compact symmetric objects or even binary black hole candidates, and thus also deserve closer attention.
31. Name: Dr. Brian Reville (MPIK)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Non-thermal particle acceleration at relativistic shocks - astrophysical implications
    Abstract:
    Astrophysical shocks are commonly associated to sites of significant non-thermal emission such as Gamma Ray Bursts, AGN hotspots, or pulsar wind nebulae. The particles that generate the non-thermal emission are thought to be accelerated via Fermi shock acceleration, a process that has been well-studied in non-relativistic scenarios such as supernova remnants, but less well explored in the context of the above-mentioned relativistic systems. I will summarise the current theoretical understanding of particle acceleration at relativistic shocks, highlighting the role of the global geometry, and the possible implications for sources of ultra-high energy gamma-rays and cosmic rays. Powerful jets carrying helical magnetic fields are found to provide favourable conditions for the acceleration of UHECRs.
32. Name: Mr. Dimitrios Skiathas (University of Patras)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Simulations of magnetic field evolution in crusts and cores of neutron stars. The impact of ambipolar diffusion.
    Abstract:
    Magnetic fields in neutron stars are responsible for a variety of spectacular phenomena ranging from flares to gamma-ray and fast radio bursts. The evolution of the magnetic field in neutron stars is driven by three mechanics, Ohmic dissipation, Hall effect and ambipolar diffusion. The strength of each mechanism, depends on the electron number density, the electric conductivity and the properties of the interactions between the different particles, thus it is not the same in the whole region of the neutron star. Ohmic dissipation and Hall effect drive the evolution of magnetic field in the crust, while ambipolar diffusion dominates the evolution of the magnetic field in the core of strongly magnetized neutron stars. The evolution of the magnetic field in a neutron star under the effect of ambipolar diffusion is the focus of our study. We have developed a code to integrate the induction equation, by discretizing it using a central finite difference scheme, on an axisymmetric domain. We simulate the evolution, both in the crust and the core of the neutron star, for four different configurations of the initial magnetic field: a pure poloidal field, a pure toroidal field, a mixed field and the case where the toroidal field is confined in the region of closed poloidal field lines. We investigate the influence of ambipolar diffusion in the evolution as well as the final state of the magnetic field. The consequences of the evolution is that the toroidal field is driven near the crust – core interface, inside the region of closed poloidal field lines.
33. Name: Mr. Stamatios Ilias Stathopoulos (National & Kapodistrian Univ. of Athens )
    Coauthors: No coauthors were included.
    Type: Oral
    Title: LeHaMoC: a novel radiation code for high-energy astrophysics
    Abstract:
    Recent associations of high-energy neutrinos with active galactic nuclei (AGN) have revived the interest in leptohadronic models of radiation from astrophysical sources. The rapid increase in multi-messenger observations requires fast numerical models that may be applied to large source samples. In this contribution, we introduce LeHaMoC, a newly developed code for solving (using an implicit difference scheme) the Fokker-Planck equations of photons and relativistic particles (e.g., electrons, positrons, protons, neutrinos) produced in a homogeneous magnetized source that may also be expanding. Our code offers several notable benefits compared to other existing codes, such as flexibility, speed, and precision. We demonstrate the capabilities of LeHaMoC by presenting two astrophysical applications. We first fit the spectral energy distribution of a jetted AGN using emcee, a Python implementation of the affine invariant Markov chain Monte Carlo ensemble sampler. We then model the high-energy neutrino signal associated with the Seyfert galaxy NGC 1068, and the spectra of the induced electromagnetic cascade. Both applications showcase the versatility of our code and its ability to accurately predict the observed high-energy photon and neutrino emission from these sources. Additionally, it can be easily customized to model a variety of high-energy astrophysical sources and has the potential to become a widely utilized tool in multi-messenger astrophysics.
34. Name: Mr. Petros Stefanou (Universidad de Valencia, Universidad de Alicante)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Modelling Force-Free Magnetospheres of Isolated Neutron Stars
    Abstract:
    We present stationary force-free magnetospheric models for non-rotating (magnetars) and rotating (pulsars) isolated neutron stars. For 2D models, we use Physics-Informed Neural Networks, a modern, flexible approach in solving PDEs. We introduce networks that are trained for general boundary conditions and source terms that can produce new solutions with very small computational cost, after training. We show that past, known results are recovered reliably and demonstrate the potential of this approach in certain applications, such as the coupling of the magnetosphere with the interior evolution. We employ a finite differences Grad-Rubin solver for 3D models. We present novel results for twisted, current-filled magnetospheres in the non-axisymmetric and time-independent regime. Our results are in-line with MHD simulations as well as with observations.
35. Name: Dr. Georgios Vasilopoulos (NKUA/IASA)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: From BeXRBs to ULXPs: connecting the “dots”
    Abstract:
    Spin-evolution in accreting objects is dictated by well known principles and can provide indirect estimates of the magnetic field of the NS. Moreover, the observed flux from a system is a good proxy of the mass accretion rate onto the compact object. However, in ULXPs things become complicated. The accretion disk is no longer Keplerian and the flux can show variations on super-orbital scales that are related to disk precession and can result in beaming effects. Thus to study the spin-up of NS above the Eddington limit we need to take into account these extra effects. In my talk I will introduce evidence from observational campaigns of ULXPs that help us understand the flux-mass accretion coupling in ULXPs that provide input to torque modeling in super-Eddington accretion. An ideal case study are very bright BeXRBs – like SMC X-3, RX J0209.6-7427 and Swift J0243.6+6124 – that can exceed the Eddington limit and can help us explore the complicated parameter space of torque modeling.
36. Name: Dr. Georgios Vasilopoulos (NKUA/IASA)
    Coauthors: Tzavelas Anastasios (NKUA)
    Petropoulou Maria (NKUA)
    Type: Poster
    Title: Application of neural networks to spectral fitting of extragalactic jet emission
    Abstract:
    Jets from supermassive black holes in the centers of active galaxies are the most powerful persistent sources of electromagnetic radiation in the Universe. Jets emit non-thermal radiation, with luminosity as high as $10^{48}$ erg/s, and a spectral energy distribution (SED) that spans many decades in photon energy, i.e. from radio waves to gamma rays. To infer the physical conditions in the otherwise out-of-reach regions of extragalactic jets we usually rely on SED fitting, i.e. we compare observed SEDs with radiative models in order to measure the goodness-of-fit of the model to the data, and determine the best-fit values. Radiative models for jet emission are primarily numerical, as they require the solution of a system of stiff coupled partial differential equations describing the evolution of the distribution functions of radiating particles (relativistic leptons and/or hadrons) and photons. Due to their time dependence such numerical codes tend to have a high computational complexity, especially in the case of hadronic models, so each run can last from a few minutes up to a couple of hours. If such a model were to be used in a Markov Chain Monte Carlo (MCMC) algorithm, the overall execution time would become prohibitively long. In this work, machine learning is used to tackle the problem of high computational complexity in order to reduce the model evaluation time. An existing numerical code is executed in order to compose a leptonic dataset, which in turn is used to train a neural network. The trained neural network can make predictions in a few milliseconds and thus can be used instead of the actual numerical code. We use the trained network to fit observational data from an extra-galactic jet using Bayesian methods based on random walks and nested sampling algorithms. Our results demonstrate that our approach offers a viable alternative that could potentially replace expensive algorithms, especially when searching multi-parameter spaces.
37. Name: Mr. Andreas Vitsos (University of Patras)
    Coauthors: No coauthors were included.
    Type: Poster
    Title: Dynamical friction due to fuzzy dark matter on non-spherically symmetric satellites
    Abstract:
    Fuzzy dark matter is a plausible dark matter candidate in the form of an ultralight bosonic particle, whose mass-energy is ∼ 10^(-22) eV and its corresponding de Broglie wavelength is of kiloparsec scale. Fuzzy dark matter exhibits wave behaviour in scales comparable to a galactic core, which could not appear in conventional cold dark matter models. The presence of fuzzy dark matter in galactic clusters will impact the motion of their members through dynamical friction. In this work, we present simulations of the dynamical friction on satellites traversing an initially uniform fuzzy dark matter halo. We focus on the satellites whose shapes are beyond spherical symmetry described by ellipsoidal and logarithmic potentials. The wakes created on the fuzzy dark matter halo due to the passage of such satellites are qualitatively different from those generated by spherically symmetric ones. Furthermore, we find that the same satellite may experience a drag differing by a factor of 5 depending on its ellipticity and the direction of motion. Finally, we find that the dynamical friction time-scale is close to Hubble time, assuming a satellite of 10^11 M⊙ traversing at 10^3 km/s a FDM halo whose mean density is ∼ 10^6 M⊙ kpc^(-3).
38. Name: Prof. Nektarios Vlahakis (National & Kapodistrian Univ. of Athens)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: The Kelvin-Helmholtz instability in Relativistic Jets
    Abstract:
    The Kelvin-Helmholtz instability is one of the most common instabilities in fluid dynamics that occurs whenever two fluids in contact are in relative motion. Its application to Astrophysical Jets is discussed, based on results from linear stability analysis. The presentation focuses on how the growth rate is modified by the presence of magnetic fields, the relativistic bulk motion, and the curvature of the interface between the jet and its environment.