arXiv subjects

Oindrila Ghosh

Publications and source records attributed to Oindrila Ghosh.

8 recordsLinked to original sources

arXiv2026

Magnetic field spreading from stellar and galactic dynamos into the exterior

The exteriors of stellar and galactic dynamos are usually modeled as current-free potential fields. A more realistic description might instead be that of a force-free magnetic field. Here, we suggest that, in the absence of outflows, neither of these reflect the actual behavior when the magnetic field spreads diffusively into a more poorly conducting turbulent exterior outside dynamo. In particular, we explain why the usual ordering, in which the dipole magnetic field is the most slowly decaying one, is altered, and why the quadrupole can develop a toroidal component that decays even more slowly with radial distance. This is a robust feature that persists even for spatially nonuniform magnetic diffusivities. It is most clearly seen for spherical dynamo volumes and becomes more complicated for oblate ones. In either case, however, these fields are confined within a magnetosphere, beyond which the field strength drops exponentially. We demonstrate that the Faraday displacement current, which plays a role in a vacuum, can safely be neglected in all cases. The superposition of magnetic fields from galaxies in the outskirts of voids between galaxy clusters therefore cannot explain the magnetization of the intergalactic medium in voids, reinforcing the conventional expectation that these fields are of primordial origin. For quadrupolar configurations, the synchrotron emission from the magnetosphere is found to be constant along concentric rings. The dipolar and quadrupolar configurations display large-scale radial trends that are potentially distinguishable with existing radio telescopes.

astro-ph.HE↗

arXiv2025

Can galactic magnetic fields diffuse into the voids?

Cosmic voids are magnetized at the level of at least $10^{-17}$ G on Mpc scales, as implied by blazar observations. We show that an electrically conducting plasma is present in the voids, and that, because of the plasma, \emph{diffusion} into the voids of galactic fields generated by a mean-field dynamo is far too slow to explain the present-day void magnetization. Indeed, we show that even in the presence of turbulence in the voids, dynamo-generated galactic fields diffuse out to a galactocentric radius of only 200-400 kpc. Therefore, it is challenging to meet the required volume filling-factor of the void magnetic field. We conclude that a primordial origin remains the most natural explanation to the space-filling weak fields in voids.

astro-ph.CO↗

arXiv2025

Heavy Axions Can Disrupt $γ$-ray Bursts

Axion-like particles (ALPs) can be produced in the hot dense plasma of fireballs that develop in the initial stage of $γ$-ray burst (GRB) outflows. They can transport an enormous amount of energy away from the jet by propagating out of the fireball. The photons produced by the eventual decay of such ALPs do not reach a sufficient density to re-thermalize through pair production, preventing fireball re-emergence. Thus, the production of heavy ALPs disrupts the fireball and dims GRBs, allowing bright GRB observations to strongly constrain the existence of heavy ALPs. By adding ALP interactions to existing models of GRB fireballs, we set competitive bounds on the ALP-photon coupling down to $g_{a γγ} \sim 4 \times 10^{-12}~{\mathrm{GeV}^{-1}}$ for ALPs in the mass range of 200 MeV - 5 GeV.

astro-ph.HE↗

arXiv2023

Light Dark Matter in a Blazar-heated Universe

Prompt emissions from TeV blazars pair produce off the extragalactic background light and the highly energetic resulting pair beams then cascade through inverse Compton scattering to give rise to secondary gamma-rays. Such reprocessed cascade emission that can be associated with individual blazar sources has not been detected thus far. The absence of pair halos around these sources, along with the non-observation of isotropic gamma-ray background excess, seems to suggest that collective plasma effects, such as beam-plasma instabilities, can play a crucial role in alleviating this GeV-TeV tension by transferring the energy from the pair beams into the background plasma of the intergalactic medium (IGM). This has profound implications not only for TeV astrophysics, but also the strength of the intergalactic magnetic field and properties of dark matter (DM). A direct consequence of the instability losses and IGM heating is the modification of thermal history at late times, which suppresses structure formation particularly in baryonically underdense regions, potentially holding a clue towards resolving the small-scale crisis in cosmology. In a blazar-heated universe, the observation of dwarf galaxies and Lyman-$α$ measurements present a favoured mass range for DM candidates such as light axion-like particles.

astro-ph.CO↗

arXiv2023

Evolution of Relativistic Pair Beams: Implications for Laboratory and TeV Astrophysics

Missing cascades from TeV blazar beams indicate that collective plasma effects may play a significant role in their energy loss. It is possible to mimic the evolution of such highly energetic pair beams in laboratory experiments using modern accelerators. The fate of the beam is governed by two different processes, energy loss through the unstable mode and energetic broadening of the pair beam through diffusion in momentum space. We chalk out this evolution using a Fokker-Planck approach in which the drift and the diffusion terms respectively describe these phenomena in a compact form. We present particle-in-cell simulations to trace the complete evolution of the unstable beam-plasma system for a generic narrow Gaussian pair beam for which the growth rate is reactive. We show that the instability leads to an energetic broadening of the pair beam, slowing down the instability growth in the linear phase, in line with the analytical and numerical solutions of the Fokker-Planck equation. Whereas in a laboratory experiment the change in the momentum distribution is an easily measured observable as a feedback of the instability, the consequence of diffusive broadening in an astrophysical scenario can be translated to an increase in the opening angle of the pair beam.

astro-ph.HE↗

arXiv2023

Where are the Cascades from Blazar Jets? An Emerging Tension in the $γ$-ray sky

Blazars are among the most powerful accelerators and are expected to produce a bright TeV $γ$-ray flux. However, TeV $γ$-rays are attenuated by interactions with intergalactic radiation before reaching Earth. These interactions produce cascades that transfer TeV power into the GeV band, powering both extended halos around bright sources and a large contribution to the isotropic $γ$-ray background (IGRB). Using conservative blazar models and recent IGRB measurements, we rule out scenarios where blazars effectively transfer their multi-TeV power into GeV $γ$-rays. Three possible solutions include: (1) strong spectral cuts on bright blazars, which are increasingly in tension with local blazar data, (2) collective plasma effects that can prevent the development of blazar cascades, the effectiveness of which is debated, (3) an increase in the $γ$-ray opacity from axion-like particles.

astro-ph.HE↗

arXiv2022

WISPLC: Search for Dark Matter with LC Circuit

The focus on dark matter search has expanded to include low-mass particles such as axions or axion-like particles, and novel theoretical schemes extending the phenomenological landscape, within QCD and beyond, also garnered additional interest in recent decades. Assuming dark matter is composed of axions, in presence of a solenoidal magnetic field, they induce a displacement current that gives rise to a toroidal magnetic field. The Weakly Interacting Slender Particle detection with LC circuit (WISPLC) is a precision direct detection experiment that will search for light dark matter candidates such as axion-like particles in parts of the parameter space previously unexplored. We present two detection schemes of the signal in a pickup loop capturing the flux of this toroidal magnetic field. WISPLC operates in a broadband and a resonant scheme where a LC circuit is used to enhance the signal with an expected Q factor $\sim 10^4$. Taking into account the irreducible flux noise of the detector, we estimate the sensitivity of the experiment in the axion mass range between $10^{-11}$ eV and $10^{-6}$ eV to reach a detectable axion-photon coupling of $g_{aγγ}\approx 10^{-15}~\mathrm{GeV}^{-1}$, making it possible to probe mass ranges corresponding to ultralight axions motivated by string theory. The WISPLC experiment is fully funded and currently in the construction phase.

hep-ex↗

arXiv2020

Axion Gegenschein: Probing Back-scattering of Astrophysical Radio Sources Induced by Dark Matter

We investigate a novel technique for the astrophysical detection of axions or axion-like particles in the dark matter halo of the Milky Way based on stimulated decay of axions, which we call axion gegenschein emission. Photons from the brightest known radio sources with a frequency equal to half the axion mass stimulate axion decay while propagating through the dark matter halo, causing radio emission in a direction precisely opposite to the incoming photon in the axion rest-frame and creating a countersource for every radio source, with an image smoothed by the dark matter velocity dispersion. We calculate the flux of the axion gegenschein countersource of Cygnus A, the brightest extragalactic radio source, and the limits that can be set with SKA to the axion-photon coupling constant $g_{aγ}$. We find this method to be more powerful than previous proposals based on searching for radio emission from axion decay in nearby dwarf galaxies or the Milky Way. The forecasted limits remain considerably higher than predictions from QCD axion models, and limits that can be set with laboratory searches of radio waves generated in resonant cavities with strong magnetic fields similar to the ADMX experiment, although this observation would directly measure a column density of dark matter through the Galactic halo and is therefore not affected by possible substructure in the dark matter distribution.

astro-ph.CO↗