arXiv subjects

Ritesh Ghosh

Publications and source records attributed to Ritesh Ghosh.

At least 37 records · Page 2Linked to original sources

arXiv2023

Impact of chiral asymmetry and magnetic field on passage of an energetic test parton in a QCD medium

We study the dependence of collisional energy loss of a test parton moving with a high velocity on the chiral imbalance and magnetic field in the QCD medium. A semi-classical approach is adopted to estimate the parton energy loss that takes into account the back-reaction on the parton due to the polarization effects of the QCD medium while traversing through the medium. We find that the motion of the parton is sensitive to the chiral asymmetry in the medium. Further, we investigate the effect of magnetic field-induced anisotropy on the energy transfer between the moving parton and the medium. Our results show that the energy loss of the parton is strongly influenced by the strength of the magnetic field as well as the relative orientation of the motion of the parton and the direction of the magnetic field in the medium.

hep-ph↗

arXiv2022

Collective modes of gluons in an anisotropic thermo-magnetic medium

We study the collective modes of gluons in an anisotropic thermal medium in the presence of a constant background magnetic field using the hard-thermal loop (HTL) perturbation theory. The momentum space anisotropy of the medium has been incorporated through the generalized $`$Romatschke-Strickland' form of the distribution function, whereas, the magnetic modification arising from the quark loop contribution has been taken into account in the lowest Landau level approximation. We consider two special cases: (i) a spheroidal anisotropy with the anisotropy vector orthogonal to the external magnetic field and (ii) an ellipsoidal anisotropy with two mutually orthogonal vectors describing aniostropies along and orthogonal to the field direction. The general structure of the polarization tensor in both cases are equivalent and consists of six independent basis tensors. We find that the introduction of momentum anisotropy ingrains azimuthal angular dependence in the thermo-magnetic collective modes. Our study suggests that the presence of a strong background magnetic field can significantly reduce the growth rate of the unstable modes which may have important implications in the equilibration of magnetized quark-gluon plasma.

hep-ph↗

arXiv2022

Magnetic field-dependent electric charge transport in hadronic medium at finite temperature

Electric charge transport of hadronic matter at finite temperature and magnetic field is studied within the linear sigma model. Anisotropic transport coefficients associated with the charge transport are estimated both in the weak and strong regimes of the magnetic field using the transport theory approach. In a weakly magnetized medium, the magnetic field effects are incorporated through the Lorentz force term in the Boltzmann equation. Strong magnetic field puts further constraints on the motion of charged particles through Landau quantization. Magnetic field-dependent thermal relaxation time is obtained from interaction rates of hadrons with the S-matrix approach by considering the Landau level kinematics of the charged hadrons. Mean-field effects are embedded in the analysis through the temperature-dependent hadron masses. Further, the hadronic medium response to a time-varying external electric field is studied in weak and strong magnetic field regimes. It is seen that electromagnetic responses of the hadronic matter have a strong dependence on the mean-field effects, sigma mass, the strength of the external fields, and its evolution in the medium.

hep-ph↗

arXiv2022

Anisotropic tomography of heavy quark dissociation by using general propagator structure at finite magnetic field

In this work we have explored the imaginary part of the Heavy Quark (HQ) potential and subsequently the dissociation of heavy quarkonia at finite temperature and magnetic field. With respect to earlier investigations on this topic, present work contain three new ingredients. First one is considering all Landau level summation, for which present work can be applicable in entire magnetic field domain - from weak to strong. Second one is the general structure of the gauge boson propagator in a hot magnetized medium, which is used here in heavy quark potential problem first time. Third one is a rich anisotropic structure of the complex heavy quark potential, which explicitly depends on the longitudinal and transverse distance. By comparing with earlier references, we have attempted to display our new contributions by plotting heavy quark potential tomography and dissociation probability at finite temperature and magnetic field.

hep-ph↗

arXiv2022

The origin of the vanishing soft X-ray excess in the changing-look Active Galactic Nucleus Mrk 590

We have studied the nature and origin of the soft X-ray excess detected in the interesting changing-look AGN (CLAGN) Mrk~590 using two decades of multi-wavelength observations from \xmm{}, \suzaku{}, \swift{} and \nustar{}. In the light of vanishing soft excess in this CLAGN, we test two models, "the warm Comptonization" and "the ionized disk reflection" using extensive UV/X-ray spectral analysis. Our main findings are: (1) the soft X-ray excess emission, last observed in 2004, vanished in 2011, and never reappeared in any of the later observations, (2) we detected a significant variability ($\sim300\%$) in the observed optical-UV and power-law flux between observations with the lowest state ($L_{\rm bol} = 4.4\times 10^{43}\, erg\, s^{-1}$, in 2016) and the highest state ($L_{\rm bol} = 1.2\times 10^{44}\, erg\, s^{-1}$, in 2018), (3) the UV and power-law fluxes follow same temporal pattern, (4) the photon index showed a significant variation ($Γ=1.88^{+0.02}_{-0.08}$ and $Γ=1.58^{+0.02}_{-0.03}$ in 2002 and 2021 respectively) between observations, (5) no Compton hump was detected in the source spectra but a narrow Fe$K_α$ line is present in all observations, (6) we detected a high-energy cut-off in power-law continuum ($92^{+55}_{-25} \rm keV$ and $60^{+10}_{-08} \rm keV$) with the latest \nustar{} observations, (7) the warm Comptonization model needs an additional diskbb component to describe the source UV bump, (8) there is no correlation between the Eddington rate and the soft excess as found in other changing-look AGNs. We conclude that given the spectral variability in UV/X-rays, the ionized disk reflection or the warm Comptonization models may not be adequate to describe the vanishing soft excess feature observed in Mrk~590.

astro-ph.HE↗

arXiv2022

Shear Viscosity of hadronic matter at finite temperature and magnetic field

We calculate the transport coefficient of hadronic matter in the presence of temperature and magnetic field using the linear sigma model. In the relaxation time approximation, we estimate the shear viscosity over entropy density $η/s$. The point-like interaction rates of hadrons are evaluated through the $S$-matrix approach in the presence of a magnetic field to obtain the temperature and magnetic field-dependent relaxation time. We observe that the transport coefficients are anisotropic in the presence of the magnetic field. We calculate the temperature and magnetic field-dependent anisotropic shear viscosity coefficients by incorporating the estimated relaxation time. The value of viscosity over entropy density is lower in the presence of a magnetic field than the value of it in a thermal medium. The behavior of the perpendicular components of the shear-viscous coefficient is also discussed. We consider the temperature-dependent hadron masses from mean-field effects in this work.

hep-ph↗

arXiv2022

Heavy quark potential and LQCD based quark condensate at finite magnetic field

In the present work, we have studied heavy quarkonia potential in hot and magnetized quark gluon plasma. Inverse magnetic catalysis (IMC) effect is incorporated within the system through the magnetic field modified Debye mass by modifying the effective quark masses. We have obtained the real and imaginary part of the heavy quark potential in this new scenario. After the evaluation of the binding energy and the decay width we comment about the dissociation temperatures of the heavy quarkonia in presence of magnetic field.

hep-ph↗

arXiv2022

Limits on the hard X-ray emission from the periodic fast radio burst FRB 180916.J0158+65

FRB 180916.J0158+65 is one of the nearest, periodically repeating, and actively bursting fast radio burst (FRB) which has been localized to the outskirts of a spiral galaxy. In this work we study the FRB with the hard X-ray $14-195$ keV data from the Burst Alert Telescope (BAT) on board The Neil Gehrels Swift Observatory. BAT uses coded mask technology giving a localization of $\lesssim 3$ arc-minute in the hard X-ray band, along with an accurate background estimation. BAT has been observing the source location in survey mode since February 2020. The survey mode observations involves background subtracted spectra, integrated over a time span ranging $300-2000$ seconds, at the source location (from Feb 2020-Jan 2022). We analyzed all the $\sim 230$ survey mode observations from BAT and checked for any signal in any of the observations. We did not detect any signal at $>5σ$ confidence level in any of the observations. We could estimate a $5σ$ upper limit on the $14-195$ keV flux, which ranged between $4.5\times 10^{-10} - 7.6\times 10^{-9}\, \rm erg\, cm^{-2}\, s^{-1}$. At the source distance this relates to a $5σ$ upper limit on luminosity of $5.08\times 10^{44}- 8.5\times 10^{45} \rm erg\, s^{-1}$. With this estimate, we could rule out any persistent X-ray emission, at the source location for these snapshots of BAT observations.

astro-ph.HE↗

arXiv2022

Simultaneous view of the FRB~180301 with FAST and NICER during a bursting phase

FRB180301 is one of the most actively repeating fast radio bursts (FRBs) which has shown polarization angle changes in its radio burst emission, an indication for their likely origin in the magnetosphere of a highly-magnetized neutron star. We carried out a multi-wavelength campaign with the FAST radio telescope and NICER X-ray observatory to investigate any possible X-ray emission temporally coincident with the bright radio bursts. The observations took place on 2021 March 4, 9 and 19. We detected five bright radio bursts with FAST, four of which were strictly simultaneous with the NICER observations. The peak flux-density of the radio bursts ranged between $28-105$ mJy, the burst fluence between $27-170$ mJy-ms, and the burst durations between $1.7-12.3$ ms. The radio bursts from FRB~180301 exhibited complex time domain structure, and sub-pulses were detected in individual bursts, with no significant circular polarisation. The linear degree of polarisation in L-band reduced significantly compared to the 2019 observations. We do not detect any X-ray emission in excess of the background during the 5ms, 10ms, 100ms, 1sec and 100sec time intervals at/around the radio-burst barycenter-corrected arrival times, at a $>5σ$ confidence level. The $5σ$ upper limits on the X-ray a) persistent flux is $<7.64\times 10^{-12}\, \rm erg\, cm^{-2}\, s^{-1}$ , equivalent to $L_{\rm X}<2.50 \times 10^{45} \rm erg\, s^{-1}$ and b) 5 ms fluence is $<2\times 10^{-11} \rm erg\, cm^{-2}$, at the radio burst regions. Using the $5$ ms X-ray fluence upper limit, we can estimate the radio efficiency $η_{R/X} \equiv L_{\rm Radio}/L_{\rm X-ray} \gtrsim 10^{-8}$. The derived upper limit on $η_{R/X}$ is consistent with both magnetospheric models and synchrotron maser models involving relativistic shocks.

astro-ph.HE↗

arXiv2022

A radio, optical, UV and X-ray view of the enigmatic changing look Active Galactic Nucleus 1ES~1927+654 from its pre- to post-flare states

The nearby type-II AGN 1ES1927+654 went through a violent changing-look (CL) event beginning December 2017 during which the optical and UV fluxes increased by four magnitudes over a few months, and broad emission lines newly appeared in the optical/UV. By July 2018 the X-ray coronal emission had completely vanished, only to reappear a few months later. In this work we report the evolution of the radio, optical, UV and X-rays from the pre-flare state through mid-2021 with new and archival data from the Very Long Baseline Array (VLBA), the European VLBI Network, the Very Large Array (VLA), the Telescopio Nazionale Galileo (TNG), Gran Telescopio Canarias (GTC), The Neil Gehrels Swift observatory and XMM-Newton. The main results from our work are: (1) The source has returned to its pre-CL state in optical, UV, and X-ray; the disk-corona relation has been re-established as has been in the pre-CL state, with an $α_{\rm OX}\sim 1.02$. The optical spectra are dominated by narrow emission lines. (2) The UV light curve follows a shallower slope of $\propto t^{-0.91\pm 0.04}$ compared to that predicted by a tidal disruption event. We conjecture that a magnetic flux inversion event is the possible cause for this enigmatic event. (3) The compact radio emission which we tracked in the pre-CL (2014), during CL (2018) and post-CL(2021) at spatial scales $<1$ pc was at its lowest level during the changing look event in 2018, nearly contemporaneous with a low $2-10$ keV emission. The radio to X-ray ratio of the compact source $L_{\rm Radio}/L_{\rm X-ray}\sim 10^{-5.5}$, follows the Gudel-Benz relation, typically found in coronally active stars, and several AGN. (4) We do not detect any presence of nascent jets at the spatial scales of $\sim 5-10$ pc.

astro-ph.HE↗

arXiv2021

A study of shear viscosity coefficient of quark matter in Polyakov quark meson model for three flavors

We have computed shear viscosity coefficient for hot and dense QCD matter within Polyakov loop extended quark meson(PQM) model for three flavors. Viscosity coefficient is estimated using Boltzmann equation within the relaxation time approximation. Relaxation times for quarks are calculated from elastic scattering of quarks and antiquarks via gluon exchange. Also the mean free paths of strange and light quarks are estimated for this cases. The calculations are performed at finite chemical potential. Thermodynamic properties like energy, pressure, entropy densities are also computed and compared with lattice QCD data.

nucl-th↗

arXiv2021

A weakening Compton hump and soft X-ray excess detected in the Seyfert-1 galaxy MCG-02-58-22

We have carried out an extensive X-ray spectral study of the bare Seyfert-1 galaxy MCG--02--58--22 to ascertain the nature of the X-ray reprocessing media, using observations from Suzaku (2009) and simultaneous observations from XMM-Newton and NuSTAR (2016) . The most significant results of our investigation are: 1. The primary X-ray emission from the corona is constant in these observations, both in terms of the power law slope ($Γ=1.80$) and luminosity ($L_{2-10 \rm keV}= 2.55\times 10^{44} $ erg/s). 2. The soft excess flux decreased by a factor of two in 2016, the Compton hump weakened/vanished in 2016, and the narrow FeK$α$ emission line became marginally broad ($σ=0.35\pm0.08$ keV) and its flux doubled in 2016. 3. From physical model fits we find that the normalization of the narrow component of the FeK$α$ line does not change in the two epochs, although the Compton hump vanishes in the same time span. Since the primary X-ray continuum does not change, we presume that any changes in the reprocessed emission must arise due to changes in the reprocessing media. Our primary conclusions are: A. The vanishing of the Compton hump in 2016 can probably be explained by a dynamic clumpy torus which is infalling/outflowing, or by a polar torus wind. B. The torus in this AGN possibly has two structures: an equatorial toroidal disk (producing the narrow FeK$α$ emission) and a polar component (producing the variable Compton hump), C. The reduction of the soft-excess flux by half and increase in the FeK$α$ flux by a factor of two in the same period cannot be adequately explained by ionized disk reflection model alone.

astro-ph.HE↗

arXiv2021

Anisotropic pressure and quark number susceptibility of strongly magnetized QCD medium

In this work, we compute the hard thermal loop pressure of quark-gluon plasma within strong magnetic field approximation at one-loop order. Magnetic field breaks the rotational symmetry of the system. As a result, the pressure of QGP becomes anisotropic and one finds two different pressures along the longitudinal (along the magnetic field direction) and transverse direction. Similarly, the second-order quark number susceptibility, which represents the fluctuation of the net quark number density, also becomes anisotropic. We compute the second order QNS of deconfined QCD matter in strong field approximation considering same chemical potential for two quark flavors.

hep-ph↗

arXiv2021

Chiral susceptibility in dense thermo-magnetic QCD medium within HTL approximation

We have computed the chiral susceptibility in quark-gluon plasma in presence of finite chemical potential and weak magnetic field within hard thermal loop approximation. First we construct the massive effective quark propagator in a thermomagnetic medium. Then we obtain completely analytic expression for the chiral susceptibility in weak magnetic field approximation. In the absence of magnetic field the thermal chiral susceptibility increases in presence of finite chemical potential. The effect of thermomagnetic correction is found to be very marginal as temperature is the dominant scale in weak field approximation.

hep-ph↗

arXiv2021

Dynamics of QCD Matter -- current status

In this article, there are 18 sections discussing various current topics in the field of relativistic heavy-ion collisions and related phenomena, which will serve as a snapshot of the current state of the art. Section 1 reviews experimental results of some recent light-flavored particle production data from ALICE collaboration. Other sections are mostly theoretical in nature. Very strong but transient magnetic field created in relativistic heavy-ion collisions could have important observational consequences. This has generated a lot of theoretical activity in the last decade. Sections 2, 7, 9, 10 and 11 deal with the effects of the magnetic field on the properties of the QCD matter. There are several unanswered questions about the QCD phase diagram. Sections 3, 11 and 18 discuss various aspects of the QCD phase diagram and phase transitions. Recent years have witnessed interesting developments in foundational aspects of hydrodynamics and their application to heavy-ion collisions. Sections 12, 15, 16 and 17 of this article probe some aspects of this exciting field. Transport coefficients together with their temperature- and density-dependence, are essential inputs in hydrodynamical calculations. Sections 5, 8 and 14 deal with calculation/estimation of various transport coefficients (shear and bulk viscosity, thermal conductivity, relaxation times, etc.) of quark matter and hadronic matter. Sections 4, 6 and 13 deals with interesting new developments in the field. Section 4 discusses color dipole gluon distribution function at small transverse momentum in the form of a series of Bells polynomials. Section 6 discusses the properties of Higgs boson in the quark gluon plasma using Higgs-quark interaction. Section 13 discusses modification of coalescence model to incorporate viscous corrections and application of this model.

hep-ph↗

arXiv2021

Short-Timescale Variability of the Blazar Mrk 421 from AstroSat and Simultaneous Multi-Wavelength Observations

We study the multi-wavelength variability of the blazar Mrk 421 at minutes to days timescales using simultaneous data at $γ$-rays from Fermi, 0.7-20 keV energies from AstroSat, and optical and near-infrared (NIR) wavelengths from ground-based observatories. We compute the shortest variability timescales at all of the above wavebands and find its value to be ~1.1 ks at the hard X-ray energies and increasingly longer at soft X-rays, optical and NIR wavelengths as well as at the GeV energies. We estimate the value of the magnetic field to be 0.5 Gauss and the maximum Lorentz factor of the emitting electrons ~1.6 x $10^5$ assuming that synchrotron radiation cooling drives the shortest variability timescale. Blazars vary at a large range of timescales often from minutes to years. These results, as obtained here from the very short end of the range of variability timescales of blazars, are a confirmation of the leptonic scenario and in particular the synchrotron origin of the X-ray emission from Mrk 421 by relativistic electrons of Lorentz factor as high as $10^5$. This particular mode of confirmation has been possible using minutes to days timescale variability data obtained from AstroSat and simultaneous multi-wavelength observations.

astro-ph.HE↗

arXiv2020

An X-ray spectral study of the origin of reflection features in bare Seyfert 1 galaxy ESO 511--G030

The reprocessed X-ray emission from Active Galactic Nuclei (AGN) is an important diagnostic tool to study the dynamics and geometry of the matter surrounding the supermassive black holes (SMBHs). We present a broadband (optical-UV to hard X-ray) spectral study of the bare Seyfert 1 galaxy, ESO~511--G030, using multi-epoch Suzaku and XMM-Newton data from 2012 and 2007 respectively. The broadband spectra of ESO~511--G030 exhibit a UV bump, a prominent soft-excess below 2 keV, a relatively broad ($σ=0.08 - 0.14$ keV) Fe emission line at 6.4 keV and a weak Compton hump at E > 10 keV. The soft X-ray excess in ESO~511--G030 can be described either as the thermal Comptonization of disk seed photons by a warm ($0.40^{+0.02}_{-0.02}$ keV), optically thick ($τ= 12.7^{+0.5}_{-0.4}$) and compact ($< 15 \rm r_{g}$) corona or as the blurred reflection from an untruncated and moderate to highly ionized accretion disk. However, for the blurred reflection, the model requires some extreme configuration of the disk and corona. Both these models prefer a rapidly spinning black hole ($a>0.78$) and a compact corona, indicating a relativistic origin of the broad Fe emission line. We found an inner disk temperature of $\sim 2-3$ eV that characterises the UV bump and the SMBH accretes at a sub-Eddington rate ($λ_{Edd} = 0.004-0.008$).

astro-ph.HE↗

arXiv2020

Covariant formulation of gluon self-energy in presence of ellipsoidal anisotropy

In this work, a covariant formulation of the gluon self-energy in presence of ellipsoidal anisotropy is considered. It is shown that the general structure of the gluon self-energy can be written in terms of six linearly independent projection tensors. Similar to the spheroidal anisotropy, mass scales can be introduced for each of the collective modes considering the static limits. With a simplified ellipsoidal generalization of the Romatschke-Strickland form, the angular dependencies of the mass scales are studied. It is observed that, compared to the spheroidal case, additional unstable mode may appear in presence of ellipsoidal anisotropy depending upon the choice of the parameters.

hep-ph↗