We study linear perturbations against static spherically symmetric background configurations of general relativity with a real scalar field (SF), which is minimally coupled with gravity; it is nonlinear due to the presence of the self-action potential. The background solutions have a naked singularity at the center of the configuration. The focus is on the stability of the background and fundamental frequencies of the quasinormal modes (QNM) of the axial perturbations in the Regge-Wheeler gauge. The problem is reduced to one hyperbolic master equation with an effective potential W-eff, which turns out to be positive for a general non-negative SF potential; this ensures the linear stability with respect to this kind of perturbations. For numerical simulations, the SF potential was chosen in the power-law form V(phi) similar to phi(2n) with 2 < n <= 40. We extracted the fundamental frequencies of QNM for different n and various sets of the background configuration parameters. The results show that even for a small background SF, there is a significant difference between the fundamental frequencies and ones in case of the Schwarzschild background. The results are also compared with the case of the Fisher-Janis-Newman-Winicour background dealing with a massless linear scalar field.

We study spherically symmetric configurations of the quadratic f(R) gravity [f(R)=R−R2/6μ2]. In the case of a purely gravitational system, we have fully investigated the global qualitative behavior of all static solutions satisfying the conditions of asymptotic flatness. These solutions are proved to be regular everywhere except for a naked singularity at the center; they are uniquely determined by the total mass M and the “scalar charge” Q characterizing the strength of the scalaron field at spatial infinity. The case Q=0 yields the Schwarzschild solution, but an arbitrarily small Q≠0 leads to the appearance of a central naked singularity having a significant effect on the neighboring region, even when the space-time metric in the outer region is practically insensitive to the scalaron field. Approximation procedures are developed to derive asymptotic relations near the naked singularity and at spatial infinity, and the leading terms of the solutions are presented. We have investigated the linear stability of the static solutions with respect to radial perturbations satisfying the null Dirichlet boundary condition at the center and numerically estimate the range of parameters corresponding to stable/unstable configurations. In particular, the configurations with sufficiently small Q turn out to be linearly unstable. Published by the American Physical Society 2024

Approximately one hundred sources of very-high-energy (VHE) gamma rays are known in the Milky Way. A survey of the entire Galactic Plane in the energy range from a few tens of GeV to a few hundred TeV has been proposed as a Key Science Project for the upcoming Cherenkov Telescope Array Observatory (CTAO). This article presents the status of the studies towards the Galactic Plane Survey (GPS). We build and make publicly available a sky model that combines data from observations of known gamma-ray emitters with state-of-the-art physically-driven models of synthetic populations of the main classes of established Galactic VHE sources, as well as of interstellar emission from cosmic-ray interactions in the Milky Way. We also perform an optimisation of the observation strategy. We use the improved sky model and observation strategy to simulate GPS data that are analysed using the methods and software tools under development for real data. We show that the GPS has the potential to increase the number of known Galactic VHE emitters by almost a factor of five. This corresponds to the detection of more than two hundred pulsar wind nebulae and a few tens of supernova remnants at average integral fluxes one order of magnitude lower than in the existing sample above 1 TeV, therefore opening the possibility to perform unprecedented population studies. The GPS also has the potential to provide new VHE detections of binary systems and pulsars, and to detect bright PeVatrons. Furthermore, the GPS will constitute a pathfinder for deeper follow-up observations of these source classes. Finally, we show that we can extract from GPS data an estimate of the contribution to diffuse emission from unresolved sources, and that there are good prospects of detecting interstellar emission and statistically distinguishing different scenarios. (Abridged)

Galaxy clusters are expected to be dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at gamma-ray energies and are predicted to be sources of large-scale gamma-ray emission due to hadronic interactions in the intracluster medium. We estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuse gamma-ray emission from the Perseus galaxy cluster. We perform a detailed spatial and spectral modelling of the expected signal for the DM and the CRp components. For each, we compute the expected CTA sensitivity. The observing strategy of Perseus is also discussed. In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratio within the radius $R_{500}$ down to about $X_{500}<3\times 10^{-3}$, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index $\alpha_{\rm CRp}=2.3$. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure $\alpha_{\rm CRp}$ down to about $\Delta\alpha_{\rm CRp}\simeq 0.1$ and the CRp spatial distribution with 10% precision. Regarding DM, CTA should improve the current ground-based gamma-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to $\sim 5$, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models with $\tau_{\chi}>10^{27}$s for DM masses above 1 TeV. These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario.

Many models of dark matter (DM) are now widely considered and probed intensively with accelerators, underground detectors, and astrophysical experiments. Among the various approaches, high-energy astrophysical observations are extremely useful to complement laboratory searches for some DM candidates. In the near future, the Cherenkov Telescope Array (CTA) should enable us to access much heavier weakly interacting massive particles, as well as a broad range of other DM candidates. In this talk, we describe DM searches with CTA.

Background. Ring-like structures may appear in some images of accretion disks around relativistic compact objects, which represent a possible alternative to conventional black holes. We present a qualitative treatment of photometric microlensing effects in case of such structures. Methods. Amplification curves are constructed for simple models of a thin accretion disk which is the source in the gravitational lens system. Two models of a source consisting of two unconnected regions are considered: (i) a circular area in the center and a ring, (ii) two rings with comparable integral brightness. We use the linear caustic approximation. The graphs show amplification curves giving the increase of the microlensed radiation flux when the caustic moves in the direction of its “tail” (the region, where there is an amplification). We first considered microlensing of a single ring. The simulation was carried out for a number of radii and thicknesses of the ring, observed in full face. Results. As expected, when the caustic is far from the source and its tail covers the source, the amplification is approximately the same as in the case of a point source. Then, when the caustic approaches the left side of the ring, the graph reaches a maximum, which is sharper, the smaller the thickness of the ring. Thereafter, the total amplification decreases relatively slowly. When the caustic approaches the right side of the ring, there is a sharp drop almost to zero over a small interval of the order of the ring thickness. In the case of two rings the amplification curve can be obtained by a superposition of different elements, yielding a graph with two maxima, the height of which depends on the contribution of the components, and two “knees”. The latter circumstance distinguishes the case of two rings from that with one ring and a region in the center. Conclusions. We summarize that in the presence of ring regions, characteristic features of the light curves appear, which can serve as an indication of the possible existence of these regions. However, different models can give quite similar light curves, and additional independent data are needed to unambiguously identify the source.

The local Cosmic Ray (CR) energy spectrum exhibits a spectral softening at energies around 3 PeV. Sources which are capable of accelerating hadrons to such energies are called hadronic PeVatrons. However, hadronic PeVatrons have not yet been firmly identified within the Galaxy. Several source classes, including Galactic Supernova Remnants (SNRs), have been proposed as PeVatron candidates. The potential to search for hadronic PeVatrons with the Cherenkov Telescope Array (CTA) is assessed. The focus is on the usage of very high energy γ-ray spectral signatures for the identification of PeVatrons. Assuming that SNRs can accelerate CRs up to knee energies, the number of Galactic SNRs which can be identified as PeVatrons with CTA is estimated within a model for the evolution of SNRs. Additionally, the potential of a follow-up observation strategy under moonlight conditions for PeVatron searches is investigated. Statistical methods for the identification of PeVatrons are introduced, and realistic Monte-Carlo simulations of the response of the CTA observatory to the emission spectra from hadronic PeVatrons are performed. Based on simulations of a simplified model for the evolution for SNRs, the detection of a γ-ray signal from in average 9 Galactic PeVatron SNRs is expected to result from the scan of the Galactic plane with CTA after 10 h of exposure. CTA is also shown to have excellent potential to confirm these sources as PeVatrons in deep observations with O(100) hours of exposure per source.

A deep survey of the Large Magellanic Cloud at ~0.1-100TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N157B, N132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3-2.4 pending a flux increase by a factor >3-4 over ~2015-2035. Large-scale interstellar emission remains mostly out of reach of the survey if its >10GeV spectrum has a soft photon index ~2.7, but degree-scale 0.1-10TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above >100GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1-10% of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within <100pc. Finally, the survey could probe the canonical velocity-averaged cross section for self-annihilation of weakly interacting massive particles for cuspy Navarro-Frenk-White profiles.

We study linear perturbations against static spherically symmetric background configurations of General Relativity with a real scalar field (SF), which is minimally coupled with gravity; it is non-linear due to the presence of the self-action potential. The background solutions have a naked singularity at the center of the configuration. The focus is on the stability of the background and fundamental frequencies of the quasi-normal modes (QNM) of the axial perturbations in the Regge-Wheeler gauge. The problem is reduced to one hyperbolic master equation with an effective potential W_ eff, which turns out to be positive for a general non-negative SF potential; this ensures the linear stability with respect to this kind of perturbations. For numerical simulations, the SF potential was chosen in the power-law form V(ϕ)∼ϕ^2n with 2<n≤ 40. We extracted the fundamental frequencies of QNM for different n and various sets of the background configuration parameters. The results show that even for a small background SF, there is a significant difference between the fundamental frequencies and ones in case of the Schwarzschild background. The results are also compared with the case of the Fisher-Janis-Newman-Winicour background dealing with a massless linear scalar field.

We study self-gravitating multi-pion systems (pion stars) in a state of the Bose condensate. To ensure stability of such stars, it is assumed that they are immersed in the lepton background. Two different phenomenological equations of state (EoS) for the pion matter are used, some of them having the first order phase transition. The model parameters are chosen to reproduce the recent lattice QCD data at zero temperature and large isospin chemical potential. It is shown that the mass-radius diagrams of pion stars obtained with phenomenological EoS are close to ones calculated in the ideal gas model. We analyze properties of neutrino clouds which are necessary for stabilizing the pion stars.

We study effects of the particles coupling with scalar field (SF) on the distribution of stable circular orbits (SCO) around the naked singularity described by the well-known Fisher-Janis-Newman-Winicour solution. The power-law and exponential models of the particle--SF interaction are analyzed. The focus is on the non-connected SCO distributions. We show that coupling between particles and SF can essentially complicate the topology of the SCO distributions. In particular, it can lead to new non-overlapping SCO regions, which are separated by unstable orbits and/or by regions where the circular orbits do not exist.

Gravitational microlensing of elliptical sources in the linear caustic approximation is considered, based on the formula for the power-law distribution of brightness over the source plane; , where . This expression is convolved with the brightness amplification of the point source (linear caustic approximation). We have constructed the total amplification coefficients (amplification curves) that allow us to estimate the increase in the integral brightness of the source in microlensing processes for various orientations with respect to the linear caustic. The averaged amplification curves are built within the framework of a simplified statistical model when the source is crossed by linear caustics with an isotropic distribution of velocities (with fixed caustic strength and velocity modulus) , . The value of appears to be independent of the model parameter . The resulting estimates are qualitatively consistent with the conclusions obtained for the Gaussian source model, although there are differences related to the shape of the source brightness distribution. The results show that in view of the accuracy of the modern photometry, the effects of ellipticity are significant; however, they are obscured by other effects due to various conditions of a specific microlensing event. Moreover, registration of the ellipticity signal in real observations requires long-term monitoring with a large number of microlensing events.

Supernova remnants (SNRs), star formation regions (SFRs), and pulsar wind nebulae (PWNe) are prime candidates for Galactic PeVatrons. The nonthermal high-energy (HE, $\varepsilon>100 \textrm{ MeV}$) and very high-energy (VHE, $\varepsilon>100 \textrm{ GeV}$) $\gamma$-ray emission from these sources should be a promising manifestation of acceleration processes. We investigate the possibility to explain the HE and VHE $\gamma$-ray emission from the sky region of the magnetar SGR 1900+14 as a signature of cosmic rays accelerated in above mentioned sources. To this end, we simulate the $\gamma$-ray emission from the extended Fermi-LAT HE source 4FGL J1908.6+0915e, the extended VHE H.E.S.S. source candidate HOTS J1907+091, and the point-like HAWC TeV source 3HWC J1907+085, which are spatially coincident with the SNR G42.8+0.6, the magnetar SGR 1900+14 and the star forming region W49A. The simulations are performed within the hadronic and leptonic models. We show that the observed $\gamma$-ray emission from the region of the magnetar SGR 1900+14 can, in principle, include contributions of different intensities from all three types of (potentially confused) sources. The considered in detail cases of a magnetar-connected but still undetected SNR and a PWN are the most promising ones, but with a serious requirement on the energy reserve of radiated CR particles - of order of $10^{51}d_{\textrm{10kpc}}^{2}$ erg for sources at a distance of $d\sim 10$ kpc. Such energy reserve can be provided by the magnetar-related Hypernova and/or magnetar wind nebula remnant created by the newborn millisecond magnetar with the large supply of rotational energy $E_{\textrm{rot}}\sim 10^{52}\textrm{ erg}$.

Magnetar wind nebulae (MWNe), created by new-born millisecond magnetars, and magnetar giant flares are PeVatron candidates and even potential sources of ultra high energy ($E>10^{18} \textrm{ eV}$) cosmic rays (UHECRs). Nonthermal high-energy (HE, $E>100 \textrm{ MeV}$) and very high-energy (VHE, $E>100 \textrm{ GeV}$) $\gamma$-ray emission from magnetars' neighbourhoods should be a promising signature of acceleration processes. We investigate a possibility of explaining HE and VHE $\gamma$-ray emission from the vicinity of the magnetar SGR 1900+14 by cosmic rays accelerated in a Supernova remnant of a magnetar-related Supernova and/or in a MWN. Simulation of the observed HE (the extended Fermi-LAT source 4FGL J1908.6+0915e) and VHE (the extended H.E.S.S. source candidate HOTS J1907+091 and the point-like HAWC TeV source 3HWC J1907+085) $\gamma$-ray emission, spatially coincident with the magnetar SGR 1900+14, was carried out in the framework of hadronic (pp collisions with a subsequent pion decay) and leptonic (inverse Compton scattering of low energy background photons by ultrarelativistic electrons) models. We show that under reasonable assumptions about parameters of the circumstellar medium the observed $\gamma$-ray emission of Fermi-LAT 4FGL J1908.6+0915e, H.E.S.S. HOTSJ1907+091 and 3HWC J1907+085 sources may be explained or at least considerably contributed by a (still undetected) magnetar-connected Hypernova remnant and/or a MWN created by new-born millisecond magnetar with a large reserve of rotational energy $E_{rot}\sim 10^{52}\textrm{ erg}$.

Astrophysical observations provide strong evidence that more than 80 matter in the Universe is in the form of dark matter (DM). Two leading candidates of particles beyond the Standard Model that could constitute all or a fraction of the DM content are the so-called Weakly Interacting Massive Particles (WIMPs) and Axion-Like Particles (ALPs). The upcoming Cherenkov Telescope Array, which will observe gamma rays between 20 GeV and 300 TeV with unprecedented sensitivity, will have unique capabilities to search for these DM candidates. A particularly promising target for WIMP searches is the Galactic Center. WIMPs with annihilation cross sections correctly producing the DM relic density will be detectable with CTA, assuming an Einasto-like density profile and WIMP masses between 200 GeV and 10 TeV. Regarding new physics beyond DM, CTA observations will also enable tests of fundamental symmetries of nature such as Lorentz invariance.

An overview of the problems associated with the possible going beyond the basic cosmological model is given. The focus is mainly on issues related to astronomical observations and the modifications of general relativity, which use a cosmological scalar field or nonlinear in curvature terms in gravitational Lagrangian. The main points are as follows: the problems of flatness and horison, discussion of the “Hubble Tension” and the simplest known models with one scalar field and the modified gravity. The general opinion is that that there is a number of ways to solve problems with the Hubble constant, but none of them explains all the completeness of the observational data. They are either phenomenological in nature or contain arbitrary functions that must be determined from experiment or from some fundamental principles. Therefore, it is natural to first turn to those models that either do not introduce “new entities”, or include a minimum number of new physical elements and a minimum number of additional parameters.

The discovery of gravitational waves, high-energy neutrinos or the very-high-energy counterpart of gamma-ray bursts has revolutionized the high-energy and transient astrophysics community. The development of new instruments and analysis techniques will allow the discovery and/or follow-up of new transient sources. We describe the prospects for the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory, for multi-messenger and transient astrophysics in the decade ahead. CTA will explore the most extreme environments via very-high-energy observations of compact objects, stellar collapse events, mergers and cosmic-ray accelerators.

Теорія гравітаційного лінзування вивчає загально-релятивістські ефекти при поширенні електромагнітного випромінювання. У даній роботі розглянуто ефекти, залежні від довжини хвилі, при (мікро)лінзуванні протяжного гаусівського джерела на точковій масі за стандартних припущень щодо некогерентності різних елементів джерела. Отримано аналітичні вирази для спектра потужності мікролінзованого випромінювання, що єефективними за великого джерела. Коли центр джерела, маса та спостерігач розташовані на одній прямій, знайдено спектр потужності в замкненій формі через гіпергеометричну функцію. У випадку загального розташування цю величину знайдено у формі ряду. Отримано асимптотичні вирази для спектра потужності за великого розміру джерела та за високих частот.

We study stable circular orbits (SCO) around static spherically symmetric configuration of General Relativity with a non-linear scalar field (SF). The configurations are described by solutions of the Einstein-SF equations with monomial SF potential V ( φ ) = | φ | 2 n , n > 2 , under the conditions of the asymptotic flatness and behavior of SF φ ∼ 1 /r at spatial infinity. We proved that under these conditions the solution exists and is uniquely defined by the configuration mass M > 0 and scalar "charge" Q . The solutions and the space-time geodesics have been investigated numerically in the range n ≤ 40 , | Q | ≤ 60 , M ≤ 60 . We focus on how nonlinearity of the field affects properties of SCO distributions (SCOD), which in turn affect topological form of the thin accretion disk around the configuration. Maps are presented showing the location of possible SCOD types for different M, Q, n . We found many differences from the Fisher-Janis-Newman-Winicour metric (FJNW) dealing with the linear SF, though basic qualitative properties of the configurations have much in common with the FJNW case. For some values of n , a topologically new SCOD type was discovered that is not available for the FJNW metric. All images of accretion disks have a dark spot in the center (mimicking an ordinary black hole), either because there is no SCO near the center or because of the strong deflection of photon trajectories near the singularity.

In view of the famous problem with the “Hubble constant tension” there is a number of approaches to modify the cosmological equations and correspondingly modify Hubble parameter H(z) in order to to relieve the tension between the “early” and “late” Hubble constants. f(R)– gravity is one of such possible modifications. We discuss how to choose the Lagrangian in the f(R)– gravity on account of observational data within the homogeneous isotropic cosmology. The equation is obtained that enable us to derive f(R) for given Hubble parameter H(z). This yields a second order differential equation with corresponding degrees of freedom. If H(z) corresponds to that obtained from usual Friedmann equations, this equation yields a condition for f(R) to mimic the observable quantities of the standard ΛCDM with the above-mentioned freedom. To reduce this freedom on needs additional considerations, which involve the other observable quantities, such as those which appear in considerations of cosmological perturbations on the isotropic and homogeneous background. Also, we consider the reverse problem to find for given f(R). This is fulfilled within an approximation in case of small deviation of f(R) from the General Relativity value.