arXiv subjects

Mageshwaran Tamilan

Publications and source records attributed to Mageshwaran Tamilan.

6 recordsLinked to original sources

arXiv2026

Multifrequency evolution of the Integrated pulse profile of radio pulsars by implementing the inverse Compton mechanism

The Main Aim of this paper is to explain the emergence of new components of pulsars at higher radio bands by implementing the Inverse Compton Scattering Mechanism. From pulsar radio observation, it is seen that a couple of pulsars reveal new emission components at higher radio frequencies, although they show single-component emission at lower frequencies. We develop a brief outline, fostering inverse Compton scattering (ICS) of the low-frequency radio photons as a vulnerable source of scattering, susceptible to explaining the evolution of new components of some radio pulsars at higher bands. We couple the conventional curvature radiation (CR) mechanism and ICS, and suggest that the spectral convolution of the flux component individually from CR and the modulated template due to the ICS scattered component can be combined to reproduce such signatures associated with the diverse morphology of the integrated pulse profile. We reproduce the beam frequency diagram, the geometrical variation of different parameters of the emission geometry, as well as the multi-frequency evolution from theory. We have suitably tuned the input parameter space and given the combination of parameters that can tune to a particular scattered frequency in tabulated form. We conclude that ICS may be a responsible process for describing the emergence of new components in higher radio emission bands.

astro-ph.HE↗

arXiv2025

Time-Dependent Accretion Disks with Magnetically Driven Winds: Green's Function Solutions

We present Green's function solutions for a geometrically thin, one-dimensional Keplerian accretion disk that includes angular momentum extraction and mass loss due to magnetohydrodynamic (MHD) winds. The disk viscosity is assumed to vary radially as $ν\propto r^{n}$. We derive solutions for three types of boundary conditions applied at the inner radius $r_{\rm in}$: (i) zero torque, (ii) zero mass accretion rate, and (iii) finite torque and finite accretion rate, and investigate the time evolution of a disk with an initial surface density represented by a Dirac-delta function. The mass accretion rate at the inner radius decays with time as $t^{-3/2}$ for $n = 1$ at late times in the absence of winds under the zero-torque condition, consistent with Lynden-Bell \& Pringle (1974), while the presence of winds leads to a steeper decay. All boundary conditions yield identical asymptotic time evolution for the accretion and wind mass-loss rates, though their radial profiles differ near $r_{\rm in}$. Applying our solutions to protoplanetary disks, we find that the disk follows distinct evolutionary tracks in the accretion rate-disk mass plane depending on $ψ$, a dimensionless parameter that regulates the strength of the vertical stress driving the wind, with the disk lifetime decreasing as $ψ$ increases due to enhanced wind-driven mass loss. The inner boundary condition influences the evolution for $ψ< 1$ but becomes negligible at higher $ψ$, indicating that strong magnetically driven winds dominate and limit mass inflow near the boundary. Our Green's function solutions offer a general framework to study the long-term evolution of accretion disks with magnetically driven winds.

astro-ph.HE↗

arXiv2025

Self-Similar Solutions for Geometrically Thin Accretion Disks with Magnetically Driven Winds: Application to Tidal Disruption Events

We analytically derive self-similar solutions for a time-dependent, one-dimensional, magnetically driven accretion-disk-wind model based on the magnetohydrodynamic equations. The model assumes a geometrically thin, gas-pressure-dominated accretion disk and incorporates both magnetic braking and turbulent viscosity through an extended $α$-viscosity prescription in the vertical and radial directions, respectively. The $α$ parameter for the vertical stress is assumed to vary with the disk aspect ratio. We confirm that in the absence of a wind, our self-similar solutions agree with the classical solution of Cannizzo et al. (1990), in which the mass accretion rate follows a power-law decay with time as $t^{-19/16}$. This scaling has been widely used as a key indicator of the mass accretion rate in tidal disruption event (TDE) disks. In contrast, when a wind is present, both the mass accretion and mass loss rates decay more steeply than $t^{-19/16}$. Furthermore, we verify that the power-law indices of these rates are consistent with those obtained from the numerical simulations of Tamilan et al. (2024) at late times. In particular, our analytical solution demonstrates that magnetic braking leads to a more rapid decay of the mass accretion rate, mass loss rate, and bolometric luminosity. In the presence of a strong poloidal magnetic field, all three quantities asymptote to $t^{-5/2}$. This steep decay index can serve as a potential observational signature of magnetocentrifugally driven winds with strong poloidal magnetic fields in TDE disks.

astro-ph.HE↗

arXiv2025

Revealing Physical Properties of a Tidal Disruption Event: iPTF16fnl

Tidal disruption event (TDE) iPTF16fnl shows a relatively low optical flare with observationally very weak X-ray emission and the spectroscopic property that the helium emission line from the source dominates over the hydrogen emission line at early times. We explore these observed signatures by calculating spectral emission lines with the publicly available code, CLOUDY. We estimate five physical parameters by fitting the observed optical UV spectra on multiple days to a theoretical model of a steady-state, slim disk with a spherical outflow. The resultant key parameters among them are black hole mass $M_{\bullet} \sim 4.43 \times 10^5 M_{\odot}$, stellar mass $M_{\star} \sim 0.46 M_{\odot}$, and wind velocity $v_{\rm w} \sim 5382.7~{\rm km~s^{-1}}$. The disk-wind model also estimates the radiative efficiency to be $0.01\lesssimη\lesssim0.04$ over the observational time, resulting in the disk being radiatively inefficient. In our CLOUDY model, the filling factor of the wind is also estimated to be 0.8, suggesting that the wind is moderately clumpy. We reveal that the helium-to-hydrogen number density ratio of the wind lies between 0.1 and 0.15, which is nearly the same as the solar case, suggesting the tidally disrupted star is originally a main sequence star. Because the optical depth of the helium line is lower than the hydrogen line by two orders of magnitude, the helium line is significantly optically thinner than the hydrogen line. Consequently, our results indicate that the helium line luminosity dominates the hydrogen line luminosity due to the optical depth effect despite a small helium-to-hydrogen number density ratio value.

astro-ph.HE↗

arXiv2025

Steady-State Solutions for a Geometrically Thin Accretion Disk with Magnetically-Driven Winds

We present steady-state solutions for a one-dimensional, magnetically-driven accretion disk wind model based on magnetohydrodynamic equations. We assume a geometrically thin, gas-pressure-dominated accretion disk, incorporating both magnetic braking and turbulent viscosity introduced by an extended alpha-viscosity prescription. Additionally, the vertical stress parameter is assumed to scale with the disk aspect ratio. We confirm that the derived solutions result in standard disk solutions when the wind is absent. We find that the mass accretion rate decreases as the disk mass falls inward, while the mass loss rate increases with radius. The disk spectrum emitted from the magnetically-driven disk wind can be observed without interference from the wind medium because the wind is significantly optically thin. The spectral luminosity is proportional to $ν^{1/3}$ in the intermediate, multicolor-blackbody wavebands, in the absence of wind, as predicted by standard disk theory. However, in the presence of wind, it follows a different power-law dependence on frequency over the same range. A deviation from the spectral slope of $1/3$, particularly a negative spectral slope, is a clear indicator of the presence of a magnetically driven wind. We also discuss an observational strategy to test our model with multi-wavelength observations.

astro-ph.HE↗

arXiv2024

Evolution of Tidal Disruption Event Disks with Magnetically Driven Winds

We present a time-dependent, one-dimensional, magnetically-driven disk wind model based on magnetohydrodynamic (MHD) equations, in the context of tidal disruption events (TDEs). We assume that the disk is geometrically thin and gas-pressure dominated, and explicitly accounts for magnetic braking and turbulent viscosity through an extended alpha-viscosity prescription. We find a particular wind solution for a set of basic equations that satisfies the necessary and sufficient conditions for vertically unbound MHD flows. The solution shows that the disk evolves with mass loss due to wind and accretion from the initial Gaussian density distribution. We confirm that the mass accretion rate follows the power law of time $t^{-19/16}$ at late times in the absence of wind, which matches the classical solution of Cannizzo et al. (1990). We find that the mass accretion rate is steeper than the $t^{-19/16}$ curve when the wind is present. Mass accretion is also induced by magnetic braking, known as the wind-driven accretion mechanism, which results in a faster decay with time of both the mass accretion and loss rates. In the disk emission, the ultraviolet (UV) luminosity is the highest among the optical, UV, and X-ray luminosities. While the optical and X-ray emission is observationally insignificant without magnetic braking, the X-ray emission is brighter at late times, especially in the presence of magnetic braking. This provides a possible explanation for observed delayed X-ray flares. Our model predicts that late-time bolometric light curves steeper than $t^{-19/16}$ in UV-bright TDEs are potentially compelling indicators of magnetically driven winds.

astro-ph.HE↗