arXiv subjects

Ritesh Ghosh

Publications and source records attributed to Ritesh Ghosh.

At least 19 recordsLinked to original sources

arXiv2026

Beyond leading-logarithm photon production from two-loop diagrams in a hot QCD medium

We investigate high-energy photon production from a quark-gluon plasma by evaluating the imaginary part of the two-loop photon self-energies in thermal QCD. Working within the imaginary-time formalism, we derive analytical expressions for both the leading-logarithmic and the beyond-leading-logarithmic contributions to the photon production rate. The rates obtained within the thermal field theoretical framework agree with those derived from kinetic theory calculations for the relevant photon-production processes.

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arXiv2026

The emergence of X-ray emission lines during relativistic radio-jet formation in the changing-look active galactic nucleus 1ES 1927+654

We present results from a comprehensive multi-wavelength monitoring campaign of the changing-look active galactic nucleus 1ES 1927+654 during the onset and evolution of a relativistic radio jet $\sim$(May 2022 - August 2025), using observations from XMM-Newton, Swift, TNG, ZTF, VLA, and VLBA. The soft X-ray emission lines at $\sim 0.56$ keV and $\sim 1$ keV have appeared with variable strength and width during the formation of the nascent jet. We note that the $\sim 1$ keV feature has been persisting since the post-2017 flare phase. We also report the detection of a broad ($\sim 800$ eV) FeK emission feature at $(6-7)$ keV in the $\sim 70$ ks stacked EPIC-pn spectra, marking the first such detection, which historically was lacking in this source. The joint spectral fitting of XMM-Newton EPIC-pn and RGS data reveals the presence of ionized absorbers in 2022 ($\log{\xi\mathrm{/erg\ cm\ s^{-1}}}\sim 1.5\pm 0.3$, $\mathrm{N_H} \sim 2.5\pm 0.9\times 10^{20} \mathrm{cm^{-2}}$), but weaker than that detected during the high accretion state in 2018 (Eddington ratio, $\lambda_{\rm Edd}>1$). The absorption features further weakened in 2023-2025 and were marginally detectable ($\mathrm{N_H} \le 10^{20}\mathrm{cm^{-2}}$). The entire scenario is suggestive of a real-time transition of the accretion flow (from $\lambda_{\rm Edd}>1$ to $\lambda_{\rm Edd}\sim 0.3$) during which the winds become weaker and the jet starts to form and evolve. Furthermore, both the soft X-ray $(0.3-2)$ keV and 5 GHz radio fluxes, which increased by factors of $\sim 10$ and $\sim 60$, respectively, since 2022, have recently plateaued at elevated levels, indicating a stabilized accretion disk, corona, and jet configuration.

astro-ph.HE↗

arXiv2026

Magnetized bottom-up thermalization in heavy-ion collisions

We investigate how a strong magnetic field generated in noncentral heavy-ion collisions may modify the bottom-up equilibration scenario. In the conventional weak-coupling picture, the earliest stages of the evolution are dominated by overoccupied gluons, while quark production is parametrically delayed. In a background magnetic field, however, additional inelastic channels become kinematically allowed or enhanced, most notably gluon decay into quark-antiquark pairs, $g\to q+\bar q$. Using parametric estimates, we show that for sufficiently strong fields, with $|eB|$ approaching the saturation scale squared, $Q_s^2$, magnetic-field-induced quark production can become important during the earliest stages of bottom-up evolution. This mechanism can populate the hard quark sector, modify the chemical composition of the pre-equilibrium matter, and provide an additional pathway toward chemical equilibration. We also discuss possible back-reaction effects, including quark-antiquark annihilation, depletion of the hard-gluon sector, and the potential feedback of early quark production on the electromagnetic conductivity of the medium. This exploratory study of a magnetically assisted bottom-up scenario provides a natural extension of the standard framework, with qualitative predictions that depend sensitively on the lifetime and spacetime profile of the magnetic field.

hep-ph↗

arXiv2026

Neutrino production mechanisms in strongly magnetized quark matter: Current status and open questions

We review the main neutrino emission mechanisms operating in dense quark matter under strong magnetic fields, with particular emphasis on conditions expected in the interiors of compact stars. We discuss the direct Urca and neutrino synchrotron processes in unpaired quark matter, incorporating the effects of Landau-level quantization. For the direct Urca process, the quantization of the electron energy spectrum plays a critical role, whereas quark quantization can often be neglected at sufficiently high baryon densities. The resulting field-dependent neutrino emissivity is anisotropic and exhibits an oscillatory behavior as a function of magnetic-field strength. We explore the implications of these effects for magnetar cooling and for possible anisotropic neutrino emission that could contribute to pulsar kicks. In addition, we review the $\nu\bar{\nu}$ synchrotron emission process, which, although subdominant, provides valuable insights into the interplay between magnetic fields and weak interactions in dense quark matter. Overall, our analysis highlights the nontrivial influence of strong magnetic fields on neutrino production in magnetized quark cores, with potential consequences for the thermal and dynamical evolution of compact stars.

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arXiv2025

Magnetoviscosity of relativistic plasma

Using first-principles quantum field-theoretical methods, we investigate the shear and bulk viscosities of strongly magnetized relativistic plasmas. The analysis is performed within the weak-coupling approximation and utilizes known results for the fermion damping rates in the Landau-level representation, $\Gamma_{n}(p_{z})$, which are dominated by one-to-two and two-to-one processes in the presence of a strong magnetic field. The transverse and longitudinal components of the viscosities are derived using Kubo's linear response theory. Our results reveal a pronounced anisotropy in both shear and bulk viscosities induced by the magnetic field. In the case of an electron-positron plasma, where the weak-coupling approximation is well justified, the dimensionless longitudinal shear viscosity $\eta_{\parallel}/T^3$ increases rapidly with the magnetic field strength, while the transverse component $\eta_{\perp}/T^3$ decreases and can even drop below the KSS bound at sufficiently large fields. In contrast, both the dimensionless longitudinal and transverse bulk viscosities, $\zeta_{\perp}/T^3$ and $\zeta_{\parallel}/T^3$, initially rise from small values, reach a maximum, and then gradually decrease toward zero. We find that the bulk viscosity is highly sensitive to the longitudinal and transverse components of the sound velocity, particularly at high magnetic fields, indicating that its quantitative values should be interpreted with caution. We also calculate an additional cross viscosity, which is negative and whose magnitude increases with the magnetic field strength. Finally, we discuss the physical implications of these magnetoviscosity results in the contexts of magnetar physics and the strongly magnetized quark-gluon plasma produced in heavy-ion collisions.

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arXiv2025

Quarkonium in a QCD medium with momentum-dependent relaxation time

In this study, we explore the properties of quarkonia in a hot QCD medium using a newly proposed collision kernel that consistently incorporates the particle's momentum dependence into the relaxation time scale of the medium. The longitudinal component of the gluon self-energy, along with the Debye screening mass, is computed within the one-loop hard thermal loop framework by incorporating non-equilibrium corrections. A modified kinetic theory with an extended relaxation time approximation is employed to model the non-equilibrium dynamics of the QCD medium. The sensitivity of the heavy quarkonia potential to the momentum dependence of the relaxation time is studied. Further, we studied the binding energy and thermal width of quarkonia states within this new kinetic theory. Sizable variations in the temperature behavior of these quantities are observed in comparison with the standard relaxation time approximation method due to the particle momentum dependence on the relaxation timescale of the QCD medium. Our findings highlight that accounting for the microscopic nature of the collision timescale is crucial for understanding the quarkonium behavior in a QCD medium.

hep-ph↗

arXiv2025

Review of heat and charge transport in strongly magnetized relativistic plasmas

We review field-theoretic studies of charge transport in hot relativistic plasmas under strong magnetic fields and extend the analysis to thermal conductivity. The calculations rely on accurately determining the fermion damping rate. Using the Landau-level representation, these damping rates are computed exactly at leading order and incorporated into the Kubo formula to obtain the thermal and electrical conductivity tensors. Our analysis reveals that the mechanisms underlying longitudinal and transverse transport differ significantly. Strong magnetic fields markedly suppress transverse charge transport by confining particles within localized Landau orbits, allowing transport only through quantum transitions between these discrete states. In contrast, longitudinal charge transport is enhanced, as it primarily depends on the reduced scattering probability of particles moving along the direction of the magnetic field. The anisotropy of thermal conductivity is also nontrivial but less pronounced since its underlying transport mechanism is different. We also examine the modification of the Wiedemann--Franz law in strongly magnetized plasmas.

hep-ph↗

arXiv2025

Neutrino-antineutrino synchrotron emission from magnetized dense quark matter

Using the Kadanoff-Baym formalism, we perform a detailed study of neutrino-antineutrino synchrotron emission from strongly magnetized, dense quark matter under conditions relevant to compact stars. Starting from an exact expression for the emission rate that fully accounts for Landau-level quantization of quarks, we derive an approximate formula applicable in the regime where quark chemical potentials are much larger than all other relevant energy scales. We demonstrate that the emission rate is largely controlled by a single dimensionless ratio between two low-energy scales: the Landau-level spacing at the Fermi surface, $|e_f B|/\mu_{f}$, and the temperature of the quark matter, $T$. When the ratio $|e_f B|/(\mu_{f} T)$ approaches zero, many closely spaced Landau levels contribute to the emission, but the total rate vanishes as $B\to 0$. In the opposite limit, where the ratio is large, the rate is dominated by transitions between adjacent levels and is exponentially suppressed due to Landau-level quantization, which limits the thermal activation of quarks near the Fermi surface. Our results show that, even in the presence of the strongest magnetic fields expected in compact stars, the synchrotron emission remains suppressed by more than 3 orders of magnitude compared to the direct Urca process. This implies that such emission is unlikely to play any substantial role in the cooling of magnetized quark stars, at least those made of unpaired quark matter phases.

hep-ph↗

arXiv2025

Shear and bulk viscosity for a pure glue theory using an effective matrix model

At nonzero temperatures, the deconfining phase transition can be analyzed using an effective matrix model to characterize the change in holonomy. The model includes gluons and two-dimensional ghost fields in the adjoint representation, or ``teens''. As ghosts, the teen fields are responsible for the decrease of the pressure as $T \rightarrow T_d$, with $T_d$ the transition temperature for deconfinement. Using the solution of this matrix model for a large number of colors, the parameters of the teen fields are adjusted so that the expectation value of the Polyakov loop is close to the values from the lattice. The shear, $\eta$, and bulk, $\zeta$, viscosities are computed in weak coupling but nonzero holonomy. In the pure glue theory, the value of the Polyakov loop is relatively large in the deconfined phase, $\approx 1/2$ at $T_d$. Consequently, if $s$ is the entropy density, while $\eta/s$ decreases as $T\rightarrow T_d$, it is still well above the conformal bound. In contrast, $\zeta/s$ is largest at $T_d$, comparable to $\eta/s$, then falls off rapidly with increasing temperature and is negligible by $\sim 2 T_d$.

hep-ph↗

arXiv2025

Neutrino energy and momentum emission from magnetized dense quark matter

Using first-principles field-theoretic methods, we investigate neutrino emission from strongly magnetized dense quark matter under conditions relevant to compact stars. We develop a customized approximation that fully accounts for the Landau-level quantization of electron states while neglecting such quantization for quarks. This approach is well-justified in dense quark matter, where the chemical potentials of up and down quarks significantly exceed those of electrons. Our analysis provides a detailed exploration of the influence of strong magnetic fields on neutrino emission, including both the modification of the total emission rate and the emergence of emission asymmetry relative to the magnetic field direction. We further examine the role of temperature in smoothing the oscillatory behavior of neutrino emission as a function of magnetic field strength. Additionally, we study the interplay between the Landau-level quantization of electrons and the Fermi-liquid effects of quarks in modifying the phase space of relevant weak processes. Finally, we briefly discuss the broader implications of magnetic fields on stellar cooling processes and the potential contribution of asymmetric neutrino emission to pulsar kicks.

hep-ph↗

arXiv2025

Multi-wavelength observations of a jet launch in real time from the post-changing-look Active Galaxy 1ES 1927+654

We present results from a high cadence multi-wavelength observational campaign of the enigmatic changing look AGN 1ES 1927+654 from May 2022- April 2024, coincident with an unprecedented radio flare (an increase in flux by a factor of $\sim 60$ over a few months) and the emergence of a spatially resolved jet at $0.1-0.3$ pc scales (Meyer et al. 2024). Companion work has also detected a recurrent quasi-periodic oscillation (QPO) in the $2-10$ keV band with an increasing frequency ($1-2$ mHz) over the same period (Masterson et al., 2025). During this time, the soft X-rays ($0.3-2$ keV) monotonically increased by a factor of $\sim 8$, while the UV emission remained near-steady with $<30\%$ variation and the $2-10$ keV flux showed variation by a factor $\lesssim 2$. The weak variation of the $2-10$ keV X-ray emission and the stability of the UV emission suggest that the magnetic energy density and accretion rate are relatively unchanged, and that the jet could be launched due to a reconfiguration of the magnetic field (toroidal to poloidal) close to the black hole. Advecting poloidal flux onto the event horizon would trigger the Blandford-Znajek (BZ) mechanism, leading to the onset of the jet. The concurrent softening of the coronal slope (from $\Gamma= 2.70\pm 0.04$ to $\Gamma=3.27\pm 0.04$), the appearance of a QPO, and low coronal temperature ($kT_{e}=8_{-3}^{+8}$ keV) during the radio outburst suggest that the poloidal field reconfiguration can significantly impact coronal properties and thus influence jet dynamics. These extraordinary findings in real time are crucial for coronal and jet plasma studies, particularly as our results are independent of coronal geometry.

astro-ph.HE↗

arXiv2024

Anisotropic charge transport in strongly magnetized relativistic matter

We investigate electrical charge transport in hot magnetized plasma using first-principles quantum field theoretical methods. By employing Kubo's linear response theory, we express the electrical conductivity tensor in terms of the fermion damping rate in the Landau-level representation. Utilizing leading-order results for the damping rates from a recent study within a gauge theory, we derive the transverse and longitudinal conductivities for a strongly magnetized plasma. The analytical expressions reveal drastically different mechanisms that explain the high anisotropy of charge transport in a magnetized plasma. Specifically, the transverse conductivity is suppressed, while the longitudinal conductivity is enhanced by a strong magnetic field. As in the case of zero magnetic field, longitudinal conduction is determined by the probability of charge carriers to remain in their quantum states without damping. In contrast, transverse conduction critically relies on quantum transitions between Landau levels, effectively lifting charge trapping in localized Landau orbits. We examine the temperature and magnetic field dependence of the transverse and longitudinal electrical conductivities over a wide range of parameters and investigate the effects of a nonzero chemical potential. Additionally, we extend our analysis to strongly coupled quark-gluon plasma and study the impact of the coupling constant on the anisotropy of electrical charge transport.

hep-ph↗

arXiv2024

Late-time radio brightening and emergence of a radio jet in the changing-look AGN 1ES 1927+654

We present multi-frequency (5-345 GHz) and multi-resolution radio observations of 1ES 1927+654, widely considered one of the most unusual and extreme changing-look active galactic nuclei (CL-AGN). The source was first designated a CL-AGN after an optical outburst in late 2017 and has since displayed considerable changes in X-ray emission, including the destruction and rebuilding of the X-ray corona in 2019-2020. Radio observations prior to 2023 show a faint and compact radio source typical of radio-quiet AGN. Starting in February 2023, 1ES 1927+654 began exhibiting a radio flare with a steep exponential rise, reaching a peak 60 times previous flux levels, and has maintained this higher level of radio emission for over a year to date. The 5-23 GHz spectrum is broadly similar to gigahertz-peaked radio sources, which are understood to be young radio jets less than ~1000 years old. Recent high-resolution VLBA observations at 23.5 GHz now show resolved extensions on either side of the core, with a separation of ~0.15 pc, consistent with a new and mildly relativistic bipolar outflow. A steady increase in the soft X-ray band (0.3-2 keV) concurrent with the radio may be consistent with jet-driven shocked gas, though further observations are needed to test alternate scenarios. This source joins a growing number of CL-AGN and tidal disruption events which show late-time radio activity, years after the initial outburst.

astro-ph.HE↗

arXiv2024

Electrical conductivity of hot relativistic plasma in a strong magnetic field

We employ first-principles quantum field theoretical methods to investigate the longitudinal and transverse electrical conductivities of a strongly magnetized hot quantum electrodynamics (QED) plasma at the leading order in coupling. The analysis employs the fermion damping rate in the Landau-level representation, calculated with full kinematics and exact amplitudes of one-to-two and two-to-one QED processes. In the relativistic regime, both conductivities exhibit an approximate scaling behavior described by $\sigma_{\parallel,\perp} = T \tilde{\sigma}_{\parallel,\perp}$, where $\tilde{\sigma}_{\parallel,\perp}$ are functions of the dimensionless ratio $|eB|/T^2$ (with $T$ denoting temperature and $B$ magnetic field strength). We argue that the mechanisms for the transverse and longitudinal conductivities differ significantly, leading to a strong suppression of the former in comparison to the latter.

hep-ph↗

arXiv2024

The complex heavy-quark potential with the Gribov-Zwanziger action

Gribov-Zwanziger prescription in Yang-Mills theory improves the infrared dynamics. In this work, we study the static potential of a heavy quark-antiquark pair with the HTL resummed perturbation method within the Gribov-Zwanziger approach at finite temperature. The real and imaginary parts of the heavy quark complex potential are obtained from the one-loop effective static gluon propagator. The one-loop effective gluon propagator is obtained by calculating the one-loop gluon self-energies containing the quark, gluon, and ghost loop. The gluon and ghost loops are modified in the presence of the Gribov parameter. We also calculate the decay width from the imaginary part of the potential. We also discuss the medium effect of heavy quark potential with the localized action via auxiliary fields.

hep-ph↗

arXiv2024

The fermion self-energy and damping rate in a hot magnetized plasma

We derive a general expression for the fermion self-energy in a hot magnetized plasma by using the Landau-level representation. In the one-loop approximation, the Dirac structure of the self-energy is characterized by five different functions that depend on the Landau-level index $n$ and the longitudinal momentum $p_z$. We derive general expressions for all five functions and obtain closed-form expressions for their imaginary parts. The latter receive contributions from three types of on-shell processes, which are interpreted in terms of Landau-level transitions, accompanied by a single photon (gluon) emission or absorption. By making use of the imaginary parts of the self-energy functions, we also derive the Landau-level dependent fermion damping rates $\Gamma_{n}(p_z)$ and study them numerically in a wide range of model parameters. We also demonstrate that the two-spin degeneracy of the Landau levels is lifted by the one-loop self-energy corrections. While the spin splitting of the damping rates is small, it may be important for some spin and chiral effects. We argue that the general method and the numerical results for the rates can have interesting applications in heavy-ion physics, astrophysics, and cosmology, where strongly magnetized QED or QCD plasmas are ubiquitous.

hep-ph↗

arXiv2024

Energy loss of a fast moving parton in Gribov-Zwanziger plasma

The Gribov-Zwanziger prescription applied within Yang-Mills theory is demonstrated to be an efficient method for refining the theory's infrared dynamics. We study the collisional energy loss experienced by a high-energetic test parton as it traverses through the Grivov plasma at finite temperature. To achieve this, we employ a semi-classical approach that considers the parton's energy loss while accounting for the back-reaction induced by the polarization effects due to its motion in the medium. The polarization tensor of the medium is estimated within a non-perturbative resummation considering the Gribov-Zwanziger approach. The modification of the gluon and ghost loops due to the presence of the Gribov parameter plays a vital role in our estimation. We observe that the non-perturbative interactions have a sizable effect on the parton energy loss. Further, we discuss the implications of our findings in the context of relativistic heavy-ion collisions.

hep-ph↗

arXiv2023

A re-emerging bright soft-X-ray state of the changing-look Active Galactic Nucleus 1ES~1927+654: a multi-wavelength view

1ES1927+654 is a nearby active galactic nucleus that has shown an enigmatic outburst in optical/UV followed by X-rays, exhibiting strange variability patterns at timescales of months-years. Here we report the unusual X-ray, UV, and radio variability of the source in its post-flare state (Jan 2022- May 2023). Firstly, we detect an increase in the soft X-ray (0.3-2 keV) flux from May 2022- May 2023 by almost a factor of five, which we call the bright-soft-state. The hard X-ray 2-10 keV flux increased by a factor of two, while the UV flux density did not show any significant changes ($\le 30\%$) in the same period. The integrated energy pumped into the soft and hard X-ray during this period of eleven months is $\sim 3.57\times 10^{50}$ erg and $5.9\times 10^{49}$ erg, respectively. From the energetics, it is evident that whatever is producing the soft excess (SE) is pumping out more energy than either the UV or hard X-ray source. Since the energy source presumably is ultimately the accretion of matter onto the SMBH, the SE emitting region must be receiving the majority of this energy. In addition, the source does not follow the typical disc-corona relation found in AGNs, neither in the initial flare (in 2017-2019) nor in the current bright soft state (2022-2023). We found that the core (<1 pc) radio emission at 5 GHz gradually increased till March 2022 but showed a dip in August 2022. The Güdel Benz relation ($L_{\rm radio}/L_{\rm X-ray}\sim 10^{-5}$), however, is still within the expected range for radio-quiet AGN and further follow-up radio observations are currently being undertaken.

astro-ph.HE↗