Studies of accretion disc luminosities and quasiperiodic oscillations around black holes may help us understand the gravitational properties of black hole spacetime. This work is devoted to studying the radiation properties of the accretion disk around the black holes in Kalb–Ramond gravity. We investigate the event horizon of the black hole spacetime and calculate the effective gravitational mass of the spacetime. Also, we analyze the circular motion of test particles in the black hole spacetime. The effects of the black hole charge and KR parameters on the particles’ effective mass, energy, and angular momentum at circular orbits and innermost stable circular orbits are studied. The frequency of Keplerian orbits and the radial and vertical oscillations of the particles along stable orbits are calculated and applied to analyze the existence of QPO in relativistic precession, warped disc, and epicyclic resonance models. QPO orbits’ locations with ratios of upper and lower frequencies of twin-peaked QPOs 3:2, 4:3, and 5:4 are analyzed compared to ISCO. We also obtain constrain values for the black hole mass, charge, KR field parameter, and QPO orbits found using Markovian chain Monte Carlo (MCMC) simulations for stellar mass (XTE J1550, GRS 1915+105), intermediate mass (M82-X1), and supermassive black holes (Sgr A*). Finally, we explore the radiative properties of the accretion disk around the charged black hole in KR gravity, such as the total radiation flux, accretion disc temperature, and differential luminosity.

Testing gravity theories is an important and interesting issue in relativistic astrophysics using astrophysical observations. In recent years, Event horizon telescope collaboration provided valuable data from shadow of supermassive black holes located at the center of the galaxies Milky Way and Meissner 87, which may help to get information about the spacetime around the black holes. In this work, first, we consider rotating black hole solution for charged black hole with the NUT charge in the presence of clouds of strings, and study the event horizon properties of spacetime around rotating twisting and charged black holes in the cloud of strings and quintessential fields. The null geodesics of the spacetime is derived and the shadow case, radius of the shadow of the black and its distortion, due to the effects of the black hole spin, also analyzed. It is obtained that gravitational effects of black hole charge and quintessential field compensate each other and the rotating black hole’s shadow can be circular. We obtain constraints on charge and spin parameters of supermassive black holes Sgr $$\hbox {A}^*$$A ∗ and $$\hbox {M87}^*$$M87 ∗ using observational data from their shadow (size and observational angle) for various values of the NUT charge, cloud of strings and quintessential field parameters. Moreover, we also investigate gravitational capture cross-section for electromagnetic waves and emission rate of the black hole radiation energy through Hawking evaporation.

In this paper, we aim to investigate the dynamics of magnetized particles around magnetically and electrically charged static black holes (BHs) in Einstein–Maxwell-scalar theory. First, we explore the possible values of EMS parameters for which the spacetime geometry represents a BH spacetime. Since there is no interaction between the electric field of the electrically charged BH and the proper magnetic field of the magnetized test particle. Therefore, we consider BH immersed in an external asymptotically uniform magnetic field. We explore the properties of an external magnetic field around a charged EMS BH. Moreover, we also explored the effects of BH charge and EMS theory parameters on particle’s energy and angular momentum in the circular stable orbits, together with the radius of innermost stable circular orbits. All the obtained results are compared with the acquired results of the Reissner–Nordström BH. Finally, we investigate the behaviour of the center of mass energy of colliding magnetized and electrically charged particles around the EMS BH.

General relativity (GR) is a well-tested theory of gravity in strong and weak field regimes. Many modifications to this theory were obtained, including different scalar, vector, and tensor fields to the GR with non-minimal coupling to gravity. Kalb–Ramond (KR) gravity is also a modified theory formulated in the presence of a bosonic field. One astrophysical way to test gravity is by studying the motion of test particles in the spacetime of black holes (BH). In this work, we study the circular motion of charged particles and explore energetic processes around charged BHs in KR theory. First, we investigated the event horizon radius and analyzed horizon-no horizon regions in the BH charge and KR parameter space. Considering the Coulomb interaction, we derive and analyze the effective potential for charged particles around a charged KR BH. We investigate charged particles’ angular momentum and energy corresponding to circular orbits. We also investigate how the KR non-minimal coupling parameter affects the radius of the innermost stable circular orbits, the corresponding energy, and the angular momentum. We also investigated the electric Penrose process and charged-particle collisions near the KR BH. The presence of the nonzero KR parameter results in a decrease in the energy efficiency of the Penrose process. Also obtained is that the KR parameter’s positive (negative) values cause a decrease (increase) in the center of mass energy of colliding particles near the BH horizon.

We study the dynamics of charged test particles around an electrically charged black hole in Einstein–Maxwell-scalar (EMS) gravity. The event horizon properties of the spacetime around the black hole are explored and the upper limit for the EMS theory parameters corresponding to extreme charge and minimal value of the event horizon are found. The effective potential for the radial motion of the charged particles at the equatorial plane is investigated. Specific energy and angular momentum of the particles corresponding to circular stable orbits are also studied. We also investigate the effects of the EMS parameter and the black hole charge on innermost stable circular orbits (ISCOs). We also investigate synchrotron radiation of charged particles in the spacetime of the charged black hole in EMS gravity. We also explore electric Penrose and Bañados–Silk–West processes near the black hole horizon, where we analyse in detail the effects of EMS parameters on energy efficiency in the Penrose process and critical angular momentum that allows colliding particles near the horizon, together with the center of mass energy in charged particles collisions.