arXiv subjects

Umananda Dev Goswami

Publications and source records attributed to Umananda Dev Goswami.

At least 37 records · Page 2Linked to original sources

arXiv2025

Mapping Nearby Galaxies with Apache Point Observatory: Group and field galaxies' morphologies in the colour-magnitude plane

This study involves the use of integral field spectroscopy (IFS) data from Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) to investigate whether the morphology influences the environmental dependence of galaxies' colours and the colour-magnitude planes. The galaxies are classified into six morphologies (Elliptical, Lenticular, Early-type, Intermediate-type, Late-type spirals and Irregular) and further in field and group environments. The distributions of colours ($B-V, B-R, u-g, g-r, r-i$ and $i-z$) are compared between field and group environments and then the colour-magnitude planes are analysed. It is observed that Intermediate and Late-types spirals preferentially exist in field environments while Early-type spirals exist in groups. The colours and colour-magnitude planes of Elliptical, Lenticular, Early-type and Intermediate-type spirals depend on the environment while for the Late-type and Irregular galaxies, their dependence on the environment is very weak. The study concludes that the dependence of colours and colour-magnitude planes on the environment is influenced by morphology.

astro-ph.GA↗

arXiv2025

Strong Lensing Effect and Quasinormal Modes of Oscillations of Black Holes in $\boldsymbol{f(R,T)}$ Gravity Theory

In this work, we analyze the strong lensing phenomenon and quasinormal modes (QNMs) in the case of black holes (BHs) surrounded by fluids within the framework of $f(R,T)$ gravity, adopting a minimally coupled model of the theory. Our analysis is conducted for three surrounding fields corresponding to three different values of the parameter $ω$ of the equations of state, each representing a unique class of BH solutions. A universal method developed by V.~Bozza is employed for strong lensing analysis and the WKB approximation method to compute the QNMs of oscillation of the BHs. The influences of the model parameters $β$ and $c_2$ on the deflection angle and associated lensing coefficients are analyzed. Our findings on lensing reveal that smaller values of $β$ and $c_2$ cause photon divergence at larger impact parameters as well as the lensing results converge to the Schwarzschild limit. Extending the analysis to the supermassive BH Sgr A*, we examine the outermost Einstein rings, estimate three lensing observables: angular position $\vartheta_{\infty}$, angular separation $s$ and relative magnification $r_\text{mag}$ for the BHs. For a specific values of $β$ and $c_2$, BHs with different field configurations exhibit substantial variations in their observable properties. The variation of amplitude and damping of QNMs with respect to the model parameter $β$ and $c_2$ is analyzed for the BHs. We found that the $β$ parameter has a direct correlation with the amplitude and an inverse relation with the damping of the QNMs, while $c_2$ has direct correlation with amplitude as well as damping. Further, we use the time domain analysis to verify the results and found a good match between the two methods.

gr-qc↗

arXiv2025

Anisotropic cosmology in Bumblebee gravity theory

The Bumblebee vector model of spontaneous Lorentz symmetry breaking (LSB) in Bianchi type I (BI) Universe to observe its effect on cosmological evolution is an interesting aspect of study in anisotropic cosmology. In this study, we have considered a Bumblebee field under vacuum expectation value condition (VEV) with BI metric and studied the cosmological parameters along with observational data. Further, we have studied the effect of anisotropy and the Bumblebee field in cosmic evolution. We have also studied the effect of both anisotropy and Bumblebee field while considering the Universe as a dynamical system. We have found that there are some prominent roles of both anisotropy and the Bumblebee field in cosmic evolutions. We have also observed an elongated matter-dominated phase as compared to standard cosmology. Moreover, while studying the dynamical system analysis, we have also observed the shift of critical points from standard $Λ$CDM results showing the anisotropy and the Bumblebee field effect.

gr-qc↗

arXiv2025

Ultra-high energy cosmic rays with UFA-15 source model in Bumblebee gravity theory

We explore the effects of Bumblebee gravity on the propagation of ultra-high energy cosmic rays (UHECRs) using astrophysical sources modelled in the Unger-Farrar-Anchordoqui (UFA) framework (2015), which includes star formation rate (SFR), gamma-ray bursts (GRBs), and active galactic nuclei (AGN). We compute the density enhancement factor for various source separations distances ($d_\text{s}$s) up to 100 Mpc within the Bumblebee gravity scenario. Additionally, we calculate the CRs flux and their suppression, goodness-of-fit values obtained from comparisons with observational data from the Pierre Auger Observatory (PAO) and the Telescope Array experiment data for the flux and the Levenberg-Marquardt algorithm for suppression. The anisotropy in the CRs arrival directions is examined, with corresponding goodness-of-fit values obtained from the PAO surface detector data (SD 750 and SD 1500). Finally, we present skymaps of flux and anisotropy under different model assumptions, providing insights into the observational signatures of UHECRs in Bumblebee gravity. We show that Bumblebee gravity stands as a viable cosmological model for explaining key observational features of UHECRs, including spectrum, composition and anisotropy. Our results show that increasing the Bumblebee gravity parameter $l$ enhances the density factor $ξ$, particularly at low energies, highlighting Lorentz violation's impact on CRs' propagation. Larger $d_\text{s}$ values amplify deviations from the $Λ$CDM model, with AGN sources dominating at high energies and GRB/SFR sources at lower energies. The skymaps indicate the structured flux patterns at large $d_\text{s}$ and structured anisotropies at higher energies.

astro-ph.HE↗

arXiv2025

Investigating the effects of gravitational lensing by Hu-Sawicki $\boldsymbol{f(R)}$ gravity black holes

In this work, gravitational lensing in the weak and strong field limits is investigated for black hole spacetime within the framework of Hu-Sawicki $f(R)$ gravity. We employ the Ishihara et al. approach for weak lensing and adopt Bozza's method for strong lensing to explore the impact of Hu-Sawicki model parameters on lensing phenomenon. The deflection angles are computed and analyzed in both the field limits. Our investigation in the weak as well as the strong lensing reveals that in the case of Hu-Sawicki black holes, photons exhibit divergence at smaller impact parameters for different values of the model parameters compared to the Schwarzschild scenario and the photon experiences negative deflection angle when impact parameter moves towards the larger impact parameter values. Additionally, by calculating strong lensing coefficients we study their behavior with model parameters. The strong lensing key observables associated with the lensing effect viz. the angular position $\vartheta_{\infty}$, angular separation $s$ and relative magnification $r_\text{mag}$ are estimated numerically by extending the analysis to supermassive black holes $\text{SgrA}^*$ and $\text{M87}^*$ and analyzed their behavior concerning the parameters for each black hole. The analysis shows that $\text{SgrA}^*$ demonstrates larger values of $\vartheta_{\infty}$ and $s$ relative to $\text{M87}^*$.

gr-qc↗

arXiv2025

Analysing Ultra High Energy Cosmic Rays' Anisotropy in $\boldsymbol{f(R, T)}$ Gravity Theory

In this study, we investigated the anisotropy of diffusive Ultra-High Energy Cosmic Rays (UHECRs) by employing three cosmological models: two models from the $f(R, T)$ gravity theory and the other is the standard $Λ$CDM model. The primary objective of this work was to ascertain the role of the $f(R, T)$ gravity theory in comprehending the anisotropy of UHECRs without implicitly endorsing the conventional cosmology. We parameterized the magnetic field and the source distance in anisotropy calculations to align well with the observational data from the Pierre Auger Observatory for all the cosmological models. An uncertainty band is presented along with the $χ^2$ test for all cosmological models to demonstrate the goodness of fitting. Our findings revealed that the amplitude of the anisotropy is highly sensitive to these cosmological models. Notably, the $f(R, T)$ models exhibited a lower amplitude of anisotropy (i.e., more isotropy), while the $Λ$CDM model predicted a comparatively higher amplitude at most of the energies considered.

astro-ph.HE↗

arXiv2025

Isolated and group environment dependence of stellar mass and different star formation rates

In this study, we explored the impact of isolated and group environments on stellar mass, star formation rate (SFR), and specific star formation rate (SSFR, i.e., the rate of star formation per unit stellar mass) using the galaxy dataset from the Sloan Digital Sky Survey Data Release 12 (SDSS DR12) for $z\lesssim0.2$. To mitigate the Malmquist bias, we partitioned the entire dataset into fifteen subsamples with a redshift bin size of $Δz = 0.01$ and examined the environmental dependencies of these properties within each redshift bin. A strong correlation between environment, stellar mass, SFR, and SSFR was observed across nearly all redshift bins. In the lower redshift bins $(z<0.1)$, the proportion of galaxies within the isolated environment exceeded that within the group environment. On the other hand, in the higher redshift bins $(z\geq 0.12)$, the isolated environment's galaxy fraction is found to be lower than that of the group environment. For the intermediate redshift bins $(0.1 \leq z < 0.12)$, an approximately equal proportion of galaxies is observed in both isolated and group environments.

astro-ph.GA↗

arXiv2025

Magnetic Suppression of Cosmic Rays' Flux in $\boldsymbol{f(R)}$ and $\boldsymbol{f(Q)}$ Theories of Gravity

We investigate the effects of magnetic diffusion on the spectrum of ultra-high energy cosmic rays (UHECRs) from a cosmological perspective. To this end, we consider two modified theories of gravity (MTGs), namely, the $f(R)$ gravity and a symmetric teleparallel gravity, also known as $f(Q)$ gravity. Utilizing these two MTGs, we calculate the suppression in the flux of UHECRs for a collection of sources. Non-evolution (NE) and cosmic star formation rate (SFR) scenarios have been considered in our calculation of the suppression factor. This study also includes a mixed composition scenario involving the nuclei upto iron (Fe). Furthermore, we provide a parameterization of the suppression factor for the proton and also for the mixed compositions within the $f(R)$ and $f(Q)$ theories, considering both NE and SFR scenarios. The influence of the turbulent magnetic field on the suppression factor is also incorporated in our work. Comparative analysis of all our results with the standard $Λ$CDM model reveals significant effects of MTGs on the suppression factor that the $f(R)$ power-law model predicts the lowest suppression factor, while the $f(Q)$ model predicts the highest, and interestingly the results from the standard model fall within the range predicted by these two cosmological models.

astro-ph.HE↗

arXiv2025

Ageing and Quenching: Influence of Galaxy Environment and Nuclear Activity in Transition Stage

This study aims to investigate whether the environment and the nuclear activity of a particular galaxy influence the ageing and quenching at the transition stage of the galaxy evolution using the volume-limited sample constructed from the twelve release of the Sloan Digital Sky Survey. To this end, the galaxies were classified into isolated and non-isolated environments and then each subsample was further classified according to their nuclear activity using the WHAN diagnostic diagram, and ageing diagram to obtain ageing and quenching galaxies. The ageing and quenching galaxies at the transition stage were selected for the rest of the analysis. Using the star formation rate and the $u-r$ colour-stellar mass diagrams, the study revealed a significant change of $0.03$ dex in slope and $0.30$ dex in intercept for ageing galaxies and an insignificant change of $0.02$ dex in slope and $0.12$ dex in intercept of the star formation main sequence between isolated and non-isolated quenching galaxies. Further, a more significant change in the number of ageing galaxies above, within and below the main sequence and the green valley was observed. On the other hand, an insignificant change in the number of quenching galaxies above, within and below the main sequence and the green valley was observed. The study concludes that ageing depends on the environment and the dependence is influenced by the nuclear activity of a particular galaxy while quenching does not depend on the environment and this independence is not influenced by the nuclear activity.

astro-ph.GA↗

arXiv2025

Main sequence of star formation and colour bimodality considering galaxy environment

This study involves the use of friend-of-friend method on the volume limited samples constructed from the Sloan Digital Sky Survey Data Release 12 (SDSS DR12) to classify the galaxies into isolated and non-isolated environments hence to investigate the influence of the galaxy environment on the main sequence of star formation, and colour bimodality. We classified the galaxies into the luminous volume-limited sample with $ -22.5\leq M_r \leq -20.5$ (mag), and the faint volume-limited sample with $-20.5 \leq M_r \leq -18.5$ (mag). Using the WHAN diagnostic diagram we assignedthe samples into star-forming, strong AGN, weak AGN, and retired galaxies based on their environment (isolated and non-isolated). The friend-of-friend method was successful in producing consistent results regarding the stellar mass-SFR and stellar mass-colour known relations. Apart from that the decrease in the slope of the main sequence for star-forming galaxies by $0.04$ dex and intercept by $0.39$ dex for the luminous sample was observed while the faint sample a decrease of $0.08$ dex in slope and $0.74$ dex in intercept was observed between isolated and non-isolated galaxies. A significant difference on the number of galaxies between isolated and non-isolated galaxies within, above and below the main sequence by $7.47\%$, $28.51\%$, $14.59\%$ for the luminous sample while for the faint sample by $16.15\%$, $32.60\%$, $35.23\%$ on average, respectively are observed. A significant difference in the number of galaxies in the blue cloud, green valley, and red sequence by $10.30\%$, $20.61\%$, $5.74\%$ for luminous sample while for faint sample by $28.46\%$, $41.36\%$, $8.95\%$ on average, respectively was observed. The study concludes that the galaxy environment influences the shaping and positioning of galaxies along the star formation main sequence and colour bimodality.

astro-ph.GA↗

arXiv2024

Quasinormal modes, thermodynamics and shadow of black holes in Hu-Sawicki f(R) gravity theory

We derive novel black hole solutions in a modified gravity theory, namely the Hu-Sawicki model of $f(R)$ gravity. After obtaining the black hole solution, we study the horizon radius of the black hole from the metric and then analyse the dependence of the model parameters on the horizon. We then use the 6th-order WKB method to study the quasinormal modes of oscillations (QNMs) of the black hole perturbed by a scalar field. The dependence of the amplitude and damping part of the QNMs are analysed with respect to variations in model parameters and the errors associated with the QNMs are also computed. After that, we study some thermodynamic properties associated with the black hole such as its thermodynamic temperature as well as greybody factors. It is found that the black hole has the possibility of showcasing negative temperatures. We also analyse the geodesics and derive the photon sphere radius as well as the shadow radius of the black hole. The photon radius is independent of the model parameters while the shadow radius showed a fair amount of dependence on the model parameters. We tried to constrain the parameters with the help of Keck and VLTI observational data and obtained some bounds on $m$ and $c_{2}$ parameters.

gr-qc↗

arXiv2024

Galactic dynamics in the presence of scalaron: A perspective from $\boldsymbol{f(R)}$ gravity

We consider $f(R)$ modified gravity theory incorporating the chameleon mechanism to address galactic dynamics. By employing the metric formalism and utilizing a conformal transformation, we simplify the field equations and describe the extra degree of freedom $f_{R}$ via a scalar field (scalaron) with chameleonic behavior. A recently proposed $f(R)$ model is analyzed to illustrate this behavior effectively. Subsequently, the rotational velocity equation including the scalaron's contribution is derived for a test particle in a static, spherically symmetric spacetime. Then we generate rotation curves and fit them to observational data of thirty seven galaxies using two fitting parameters, $M_0$ and $r_c$, the total mass and core radius of a galaxy respectively.

gr-qc↗

arXiv2024

Anisotropies of Diffusive Ultra-high Energy Cosmic Rays in $f(R)$ Gravity Theory

Understanding the anisotropy of ultra high-energy cosmic rays (UHECRs) is crucial for unraveling the origins and propagation mechanisms of these enigmatic particles. In this work, we studied the dipolar anisotropy of UHECRs in the diffusive regime by considering three cosmological models: the standard $Λ$CDM model, $f(R)$ gravity power-law model and the Starobinsky model. This work aims to see the role of the $f(R)$ gravity theory in understanding the anisotropy of UHECRs without condoning the standard cosmology. We found that the amplitude of the dipolar anisotropy is sensitive to these cosmological models, with the $f(R)$ power-law model predicting the largest amplitude, while the $Λ$CDM model predicting the smallest amplitude at most of the energies in the range considered. The predicted amplitude of the Starobinsky model lies within the range of the $Λ$CDM one. This work not only provides a way for exploration of UHECRs anisotropy within different cosmological contexts but also may pave the way for new avenues of research at the intersection of high-energy astrophysics.

astro-ph.HE↗

arXiv2024

Dynamical system analysis of LRS-BI Universe with f(Q) gravity theory

Considering the Universe as a dynamic system, the understanding of its evolution is an interesting aspect of study in cosmology. Here, we investigate the anisotropic locally rotationally symmetric (LRS) Bianchi type-I (LRS-BI) spacetime under the $f(Q)$ gravity of symmetric teleparallel theory equivalent to the GR (STEGR) as a dynamical system and try to understand the role of anisotropy in the evolution of its various phases. For this work, we consider two models, viz., $f(Q) = -\,(Q+ 2Λ)$ and $f(Q) = -\, βQ^{n}$ to study the critical points and stability of the LRS-BI Universe. In both the models, it is found that the various phases like radiation-dominated, matter-dominated, and dark energy-dominated phases are heteroclinically connected, and there is some role of anisotropy in the evolution of these phases of the Universe. However, for both the models, there are some unphysical solutions too depending upon the values of model parameters $β$ and $n$ along with the anisotropic parameter $α$.

gr-qc↗

arXiv2024

Propagation and Fluxes of Ultra High Energy Cosmic Rays in f(R) Gravity Theory

We study the effect of diffusion of ultra-high energy cosmic ray (UHECR) protons in the presence of turbulent magnetic fields in the light of the $f(R)$ theory of gravity. The $f(R)$ theory of gravity is a successful modified theory of gravity in explaining the various aspects of the observable Universe including its current state of expansion. Here, we consider the two most studied $f(R)$ gravity models, viz., the power-law model and the Starobinsky model. For these two models, the diffusive character of the propagation of UHECR protons is explored in terms of their density enhancement (a measure of how the density of UHECRs changes due to their diffusion and interactions in the intergalactic medium). Ankle, instep, and Greisen-Zatsepin-Kuzmin cutoff are spectral characteristics that extragalactic UHECRs acquire when they propagate through the cosmic microwave background. All these characteristics are analysed through the diffusive flux as well as its modification factor. We compare the UHECR protons spectra calculated for the considered $f(R)$ gravity models with the available data of the Telescope Array and Pierre Auger Observatory. Both models of $f(R)$ gravity predict energy spectra of UHECRs with all experimentally observed features, which lay well within the range of combined data of both experiments. However, this work is only to investigate the possible effects of $f(R)$ gravity theory on the UHECRs propagation, using pure proton composition as a simplified case study. At this stage, our results cannot be used to favour or disfavour the $f(R)$ cosmology over the $Λ$CDM cosmology.

hep-th↗

arXiv2024

Quasinormal modes, temperatures and greybody factors of black holes in a generalized Rastall gravity theory

We introduce a modification in the energy-momentum conservation violating Rastall's theory of gravity and obtain a Reissner-Nordström-type black hole solution in spacetime surrounded by a cloud of strings and charge fields. We examine the horizons of the black hole along with the influence of the parameters of the model on it. The scalar quasinormal modes (QNMs) of oscillations of the black hole are also computed using the 6th order WKB approximation method. It is seen that the Rastall parameter $β$ and the newly introduced energy-momentum tensor trace parameter $α$ as well as the charge parameter $q$ and strings field parameter $a$ influence the amplitude and damping of the QNMs. From the metric function, we obtain the temperature of the black hole and study the effects of the four model parameters $β$, $α$, $q$ and $a$ on the temperature. We then examine the greybody factors associated with the black hole and the corresponding total absorption cross-section for it. It is seen that the modification we introduced in the Rastall theory has a drastic effect on various properties of the black hole and may lead to interesting outcomes in future when the better detection techniques will be available with the LISA and the Einstein Telescope.

gr-qc↗

arXiv2024

AstroSat Observation of Recent Outburst in the Be/X-ray Binary LS V +4417/RX J0440.9+4431

A Be/X-ray binary system known as RX J0440.9+4431 (or LS V +44 17) is a potential member of the uncommon gamma-ray binary class. With an orbital period of 150 days, this system consists of a neutron star and a Be star companion. The MAXI observatory discovered an X-ray outburst from the source in December of 2022. Early in January, the outburst reached its peak, which was then followed by a decrease and a subsequent rebrightening. The X-ray flux exceeded 1 Crab in the 15-50 keV range at this second peak. AstroSat observations were conducted close to the peak of the second outburst, from January 11 to January 12, 2023. We report here the results of our search for 3-80 keV X-ray emission in the data of the AstroSat's LAXPC detector. It is found that the pulse period of the source is around 208 seconds. The source is found to be emitting more in the softer part of the X-ray energy range. The spectral characteristics can be described by employing a power-law model with an exponential cutoff, along with a blackbody component, interstellar absorption and an additional 6.4 keV iron fluorescence line.

astro-ph.HE↗

arXiv2023

Galactic rotation curves of spiral galaxies and dark matter in f(R,T) gravity theory

Galactic rotation curve is a powerful indicator of the state of the gravitational field within a galaxy. The flatness of these curves indicates the presence of dark matter in galaxies and their clusters. In this paper, we focus on the possibility of explaining the rotation curves of spiral galaxies without postulating the existence of dark matter in the framework of $f(\mathcal{R},T)$ gravity, where the gravitational Lagrangian is written by an arbitrary function of $\mathcal{R}$, the Ricci scalar and of $T$, the trace of energy-momentum tensor $T_{μν}$. We derive the gravitational field equations in this gravity theory for the static spherically symmetric spacetime and solve the equations for metric coefficients using a specific model that has minimal coupling between matter and geometry. The orbital motion of a massive test particle moving in a stable circular orbit is considered and the behavior of its tangential velocity with the help of the considered model is studied. We compare the theoretical result predicted by the model with observations of a sample of nineteen galaxies by generating and fitting rotation curves for the test particle to check the viability of the model. It is observed that the model could almost successfully explain the galactic dynamics of these galaxies without the need of dark matter at large distances from the galactic center.

gr-qc↗