arXiv subjects

Tarek M. Kamel

Publications and source records attributed to Tarek M. Kamel.

6 recordsLinked to original sources

arXiv2026

Multiband Color Monitoring of 3I/ATLAS through Ground-Based Relay Observations

We present multiband, long-baseline photometric observations of interstellar comet 3I throughout its 2025--2026 apparition using coordinated ground-based global relay observations. Our dataset combines measurements from professional observatories and citizen-operated Unistellar eVscopes distributed worldwide, providing dense temporal coverage from 2025 July 2 through 2026 April 1 and spanning the comet's pre- and post-perihelion trajectory. Broadband photometry was obtained in bandpasses equivalent to the Johnson--Cousins $B$ (436 nm), $V$ (545 nm), and $R$ (641 nm) filters and the Sloan $g$ (477 nm), $r$ (623 nm), and $i$ (763 nm) filters. The photometry was measured using projected aperture radii of approximately 10{,}000~km to provide a consistent probe of the inner coma across the heterogeneous dataset. We measure representative mean colors of $B-V=0.86\pm0.06$, $V-R=0.50\pm0.03$, $B-R=1.36\pm0.08$, and $g-r=0.58\pm0.08$, demonstrating a persistently red optical coma. Constant-color models provide an adequate description of the data, with little evidence for long-term color evolution with time or heliocentric distance despite substantial changes in the coma's brightness, gas production, and volatile composition. This suggests that the ensemble-averaged optical scattering properties of the coma remained relatively stable over the period sampled by our observations, even as other properties of the coma evolved. These observations provide the first densely sampled, apparition-long characterization of the broadband optical colors of an interstellar comet and establish a benchmark for comparison with future interstellar objects.

astro-ph.EP↗

arXiv2024

Investigating the hyperbolic and hybrid scalar field cosmologies with varying cosmological constant in $f(R,T)$ gravity

This paper investigated two scalar field cosmological models in $f(R,T)$ gravity with cosmic transit and varying cosmological constant $Λ(t)$.The cosmological constant tends to have a tiny positive value in the current epoch.The scalar field pressure $p_ϕ$ shows a sign reversal for a normal scalar field. For the phantom field, the scalar potential $V(ϕ)$ is negative and the energy density $ρ_ϕ=E_k+V$ takes negative values when the equation of state parameter $ω_ϕ$ is less than $-1$. While the weak energy condition WEC implies that the total energy density $ρ=\sum_iρ_i\geq 0$, we still can have a negative $ρ$ term as long as the total $ρ$ is positive. In the current work we argue that the WEC, $ρ=\sum_i ρ_i \geq 0$ and $p_i+ρ_i \geq 0$, is not violated but with an instability for the second model at late-times. For a scalar field $ϕ$, The condition $ρ_ϕ+p_ϕ=ρ_ϕ (1+ω_ϕ)=2E_k\geq 0$ allows for $ρ_ϕ<0$ if $ω_ϕ<-1$. The causality and energy conditions have been discussed for both models. The cosmology in both models was studied using a given function $a(t)$ derived from the desired cosmic behavior, which is the opposite of the traditional view.

gr-qc↗

arXiv2024

Stability Analysis Of Fractional Relativistic Polytropes

In astrophysics, the gravitational stability of a self-gravitating polytropic fluid sphere is an intriguing subject, especially when trying to comprehend the genesis and development of celestial bodies like planets and stars. This stability is the sphere's capacity to stay in balance in the face of disruptions. We utilize fractional calculus to explore self-gravitating, hydrostatic spheres governed by a polytropic equation of state ¶=Kρ^{1+1/n}. We focus on structures with polytropic indices ranging from 1 to 3 and consider relativistic and fractional parameters, denoted by σand α, respectively. The stability of these relativistic polytropes is evaluated using the critical point method, which is associated with the energetic principles developed in 1964 by Tooper. This approach enables us to pinpoint the critical mass and radius at which where polytropic spheres shift from stable to unstable states. The results highlight the critical relativistic parameter where the polytrope's mass peaks, signaling the onset of radial instability. For polytropic indices of 1, 1.5, 2, and 3 with a fractional parameter α, we observe stable relativistic polytropes for σvalues below the critical thresholds of σ= 0.42, 0.20, 0.10, and 0.0, respectively. Conversely, instability emerges as σsurpasses these critical values. Our comprehensive calculations reveal that the critical relativistic value (σ_{CR}) for the onset of instability tends to increase as the fractional parameter α decreases.

gr-qc↗

arXiv2023

Big rip in Swiss-cheese Brane-worlds with cosmic transit

We study the big rip scienario in Swiss-cheese Brane-worlds. The results obtained have been found to be independent of the value of the cosmological constant $Λ$ whether its positive, negative or zero. Negative tension branes are not allowed in the current model. There is a sign flipping in cosmic pressure corresponding to the sign flipping in the deceleration parameter from positive to negative. The evolution of the EOS parameter shows the presence of three phases: the matter dominant decelerating era,The accelerated-Quitessence phase, and the phantom phase. The evolution of the potential and Kinetic term also shows a change of sign. The energy conditions and cosmographic parameters have also been investigated.

gr-qc↗

arXiv2022

A non-singular closed bouncing universe without violation of null energy condition

A matter bouncing entropy-corrected cosmological model has been suggested. The model allows only positive curvature with negative pressure and no violation of the null energy condition. The result obtained in this paper is supported by some recent theoretical works where the combination of positive spatial curvature and vacuum energy leads to non-singular bounces with no violation of the null energy condition. An important feature of the current model is that evolutions of the cosmic pressure, energy density and equation of state parameter are independent of the values of the prefactors $α$ and $β$ in the corrected entropy-area relation. The validity of the classical and the new nonlinear energy conditions has been discussed. The cosmographic parameters have been analyzed

gr-qc↗

arXiv2021

Note on Dark Energy and Cosmic Transit in a scale-invariance cosmology

In general, the laws of physics are not invariant under a change of scale. To find out whether the 'scale-invariance hypothesis' corresponds to nature or not, a careful examination to its implications is required. As a consequence, the scale-invariance cosmological models need to be carefully checked with many tests in order to confirm or disconfirm them. In this paper, three different toy models have been introduced in the framework of a scale-invariance cosmology to examine dark energy and cosmic transit. Although cosmic transit exists in the three models, the pressure stays always negative during cosmic evolution. In addition, there is always a singularity in the evolution of the equation of state parameter which is not suitable for a complete investigation of dark energy evolution. The undesirable features of the parameters have been discussed, and a comparison with other cosmological contexts has been done.

gr-qc↗