arXiv subjects

Abdurakhmon Nosirov

Publications and source records attributed to Abdurakhmon Nosirov.

12 recordsLinked to original sources

arXiv2026

Assessing the Power-Law Emissivity Assumption in X-ray Reflection Spectroscopy: A Simulation-Based Evaluation of Different Coronal Geometries

The emissivity profile assumed in X-ray reflection spectroscopy significantly impacts black hole spin measurements. Using simulated NuSTAR spectra generated for lamppost and disk-like coronae with the relxill model suite, we evaluate systematic biases introduced when fitting with power-law or broken power-law emissivity profiles. We find that a simple power-law can accurately recover spins for low-height lamppost coronae with long exposures and low inclination angle, while broken power-laws introduce degeneracies when the simple power-law already performs adequately. However, for extended or high-height coronae, especially at high inclinations, both models produce large systematic biases unresolved by longer exposure times. Our results demonstrate that power-law approximations are reliable for compact coronae, highlighting the need for geometry-specific models in complex cases.

astro-ph.HE↗

arXiv2026

Testing Black Holes with Interstellar Missions: II. Flyby Probes

Recently, we demonstrated that while an interstellar mission to the nearest black hole remains highly speculative and extraordinarily challenging, it is not entirely implausible within the coming decades. Given that such a mission would likely take about a hundred years and require substantial financial and human investment, it is essential to assess whether it could investigate black holes and test General Relativity to a degree that cannot be achieved by Solar System observatories for the foreseeable future. In Paper I, we assumed the capability to decelerate the spacecraft and presented a preliminary study of how orbiting probes could test the nature of the compact object. In this second paper, we study how the black hole can be tested without decelerating the spacecraft, using flyby probes.

gr-qc↗

arXiv2026

Dense, multi-phase accretion disk atmosphere in the low-luminosity state of black hole transientV4641 Sgr

We present soft X-ray spectroscopy of the black-hole X-ray binary V4641~Sgr with the \textit{XMM-Newton} Reflection Grating Spectrometer (RGS). The RGS spectrum shows narrow emission features from N\,\textsc{vi--vii} and O\,\textsc{vii--viii} superimposed on a partially covered disk blackbody continuum. A blind Gaussian search confirms the presence of significant lines at the expected rest wavelengths. He-like triplet ratios (high $G$, low $R$) and full photoionization modelling both indicate a dense, photoionized plasma. Small redshifted velocities of $\sim 540$--$720\ \mathrm{km\ s^{-1}}$ are suggested, which are consistent with quasi-static or slowly flowing gas away from the observer after accounting for systematics. Photoionization modelling requires two \textsc{xstar} components with an intermediate ionization parameter ($\logξ\simeq 3.1$) and a low ionization parameter ($\logξ\simeq 0.36$), respectively. The simultaneous EPIC-pn spectrum suggests highly ionized Fe emission structures, hinting at an additional, more highly ionized component. These results imply the existence of a radially extended, multiphase, and dense disk atmosphere in the source. We compare the source with other X-ray binaries showing similar emission lines. V4641~Sgr shares a similarly high inclination with other sources; however, the presence of low ionization emission lines distinguishes it from the rest.

astro-ph.HE↗

arXiv2026

Cosmological Constraints on Minimal Cubic Galileon Models in Teleparallel Gravity

Cubic Galileon cosmological models provide a well-motivated framework for investigating late-time cosmic acceleration beyond the standard $Λ$CDM paradigm. In this work, we study observational constraints on cubic Galileon models within the teleparallel gravity framework, where deviations from the standard teleparallel equivalent of general relativity are encoded through the model parameter $b_1$. We consider two scalar-field potentials, namely quadratic and exponential potentials, and analyze four representative scenarios: quadratic and exponential potentials with $b_1$ treated as a free parameter, together with the corresponding cases in which $b_1=2$ is fixed. Using the $\text{Pantheon}^+$ Type Ia supernova sample, cosmic chronometer measurements, SH0ES information, and baryon acoustic oscillation data, we constrain the cosmological and model parameters and compare the observational viability of the different scenarios. We find that the considered teleparallel cubic Galileon models can accommodate the late-time expansion history, although the statistical preference depends on the choice of potential and on whether $b_1$ is fixed or varied. In particular, the fixed-$b_1$ model with a quadratic potential provides the most competitive fit among the Galileon scenarios when BAO data are included, showing a lower $χ^2_{\min}$ than $Λ$CDM and comparable support according to the AIC criterion. However, the BIC criterion continues to favor the minimal $Λ$CDM model because of the larger parameter space of the extended models. These results suggest that teleparallel cubic Galileon cosmologies remain phenomenologically viable, while a stronger claim regarding the Hubble tension requires further consistency tests.

gr-qc↗

arXiv2026

Assessing systematic uncertainties from spectral re-analysis of Cyg X-1 with different coronal geometries

In this work, we carry out a new spectral reanalysis of NuSTAR and Suzaku observations of the disk reflection spectra in the stellar-mass black hole X-ray binary Cyg~X-1. We compare three types of models: a broken power-law disk emissivity profile with no assumption about the coronal shape used in the previous work of the same observations, a compact lamppost corona, and an extended disk-like corona motivated by recent X-ray polarization results. Our goal is to measure the systematic uncertainties caused by the assumed geometry, with a focus on key parameters such as the black hole spin and the inclination of the inner accretion disk. We find that the disk-like corona gives a fit that is statistically similar to the broken power-law and lamppost models, but it leads to more physically reasonable results, such as a lower inclination angle of about $30^{\circ}$. By using a variable disk density model, we measure the disk density to be $n_{\rm e}\approx10^{20}$\,cm$^{-3}$, which is similar to earlier results. While the extended corona model infers a wider allowed parameter space for black hole spin and the inner radius of the disk-shaped coronal region, this reflects the additional physical freedom of the model. Even so, the disk-like corona remains a strong and physically well-motivated candidate for explaining the X-ray emission from Cyg~X-1.

astro-ph.HE↗

arXiv2026

Testing Black Holes with Interstellar Missions: I. Orbiting Probes

Recently, we showed that the possibility of an interstellar mission to the closest black hole, while highly speculative and extremely challenging, is not completely unrealistic within the next few decades. Since such a mission might last around a century and require significant financial and human resources, it is crucial to assess whether it can truly study black holes and test General Relativity at levels unattainable by observational facilities in the Solar System for many years. In this manuscript, we assume the capability to decelerate the spacecraft and present a preliminary study of how probes orbiting a black hole could test the nature of the compact object.

gr-qc↗

arXiv2026

Searching for Isolated Black Hole Candidates within 15 pc of the Solar System in Gaia DR3

Theoretical models predict that the Galaxy hosts $10^8$-$10^9$ black holes formed from the complete gravitational collapse of heavy stars and that most of these black holes are isolated, without any companion. Within 15 pc of the Solar System ($\sim 50$ ly), there may be a few black holes. If located inside one of the Local Interstellar Clouds - which occupy 5-20% of this local volume - an isolated black hole could produce detectable electromagnetic emission via accretion from the interstellar medium, given the capabilities of current or near-future observatories. However, precise predictions remain challenging due to large uncertainties in the expected accretion spectra. Outside these clouds, the accretion rate would be too low; according to our models, the resulting electromagnetic flux is well below the detection thresholds of current and near-future observational facilities. While astrometric detection via gravitational perturbation of nearby stars is conceivable, the local stellar density is too low for this method to be realistically successful. We have searched the Gaia DR3 catalog for candidate isolated black holes accreting from the interstellar medium and identified five sources. All candidates lie close to the Galactic plane, making them likely spurious astrometric solutions, for instance caused by unmodelled background sources (crowding) and/or unmodelled binarity. Our search for infrared and radio emission from these sources further suggests that they are unlikely to be black holes accreting from the interstellar medium.

astro-ph.HE↗

arXiv2025

Measuring black hole spins with x-ray reflection spectroscopy: A GRMHD outlook

X-ray reflection spectroscopy has evolved as one of the leading methods to measure black hole spins. However, the question is whether its measurements are subjected to systematic biases, especially considering the possible discrepancy between the spin measurements inferred with this technique and those from gravitational wave observations. In this work, we use general relativistic magnetohydrodynamic (GRMHD) simulations of thin accretion disks around spinning black holes for modeling the accretion process, and then we simulate NuSTAR observations to test the capability of modern reflection models in recovering the input spins. For the first time, we model the electron density and ionization profiles from GRMHD-simulated disks. Our study reveals that current reflection models work well only for fast-rotating black holes. We model the corona as the base of the jet and we find that reflection models with lamppost emissivity profiles fail to recover the correct black hole spins. Reflection models with broken power-law emissivity profiles perform better. As we increase the complexity of the simulated models, it is more difficult to recover the correct input spins, pointing toward the need to update our current reflection models with more advanced accretion disks and coronal geometries.

astro-ph.HE↗

arXiv2025

Probing the Strong Gravity Region of Black Holes with eXTP

We present the novel capabilities of the enhanced X-ray Timing and Polarimetry (eXTP) mission to study the strong gravity region around stellar-mass black holes in X-ray binary systems and supermassive black holes in active galactic nuclei. eXTP can combine X-ray spectral, timing, and polarimetric techniques to study the accretion process near black holes, measure black hole masses and spins, and test Einstein's theory of General Relativity in the strong field regime. We show how eXTP can improve the current measurements of black holes of existing X-ray missions and we discuss the scientific questions that can be addressed.

astro-ph.HE↗

arXiv2025

Gauge invariant perturbations in teleparallel Horndeski gravity

We present in the form of a catalogue of the cosmological perturbations within the Bahamonde- Dialektopoulos-Levi Said (BDLS) theory, which serves as the teleparallel counterpart of Horndeski gravity. To understand structure formation in cosmological models, it is essential to study both the background and perturbative aspects of their cosmology. While extensive analysis of both Horndeski gravity and its teleparallel analog exists in the literature, a quantitative understanding requires a detailed examination of their cosmological perturbations. We review here all the different gauges for the scalar, vector and tensor perturbations of a cosmological background up to second order and we hope this will help people who work with observations, to incorporate it in existing codes.

gr-qc↗

arXiv2023

Stable bouncing solutions in Teleparallel Horndeski gravity: violations of the no-go theorem

In order to have singularity-free solutions at the beginning of the Universe, we need to violate the null energy condition. In the general class of Horndeski gravity, there are healthy NEC-violating solutions, which however are plagued with instabilities or some kind of pathologies in the history of the Universe; this is known as the no-go theorem. In this paper, we study the possibility of stable bouncing solutions in the Teleparallel analog of Horndeski gravity and we show explicitly that there exist healthy violations of the no-go theorem.

gr-qc↗

arXiv2023

Cosmological Perturbations in the Teleparallel analog of Horndeski gravity

In this work we study the cosmological perturbations in Bahamonde-Dialektopoulos-Levi Said (BDLS) theory, i.e. the teleparallel analog of Horndeski gravity. In order to understand the evolution of structure in a cosmological model, it is necessary to study its cosmology not only in the background but also perturbatively. Both Horndeski and its teleparallel analog have been analyzed a lot in the literature, but in order to study them quantitatively, we need to know their cosmological perturbations. That is why, we study here the scalar-vector-tensor decomposition of the theory and we also express the so-called alpha parameters in terms of the arbitrary functions of the theory, that designate the deviation from the ΛCDM model. We have explored tensor, vector and scalar perturbation of the action up to second order, which drastically opens up new possibilities on searches in the parameter space of scalar-tensor theories in the context of observations.

gr-qc↗