arXiv subjects

Sruthiranjani Ravikularaman

Publications and source records attributed to Sruthiranjani Ravikularaman.

4 recordsLinked to original sources

arXiv2025

Can cosmic rays explain the high ionisation rates in the Galactic centre?

The Central Molecular Zone (CMZ), located in the centre of the Milky Way, is a roughly cylindrical structure of molecular gas extending up to parsecs around the supermassive black hole Sagittarius A*. The average H2 ionisation rate in the CMZ is estimated to be 2e-14 s-1, which is 2-3 orders of magnitude higher than anywhere else in the Galaxy. Due to the high gas density in this region, electromagnetic radiation is rapidly absorbed, leaving low-energy cosmic rays (CRs) as the only effective ionising agents. Hence, a high CR density has been invoked to explain such high ionisation rates. However, a corresponding excess in gamma rays, which would result from interactions of high-energy CRs, has not been observed. This suggests that the supposed excess exists only in the low-energy CR spectrum. To constrain this unknown low-energy component, we first derive the high-energy CR injection spectra using gamma-ray and radio data, to which we add various low-energy components. We then propagate these injection spectra by numerically solving the CR transport equation using a Crank-Nicolson scheme. Testing multiple CR injection scenarios, we find that the energy required to sustain the observed ionisation rates is excessively high in every case. We conclude that CRs cannot be the exclusive ionising agents in the CMZ.

astro-ph.HE↗

arXiv2025

Testing Gamma/Hadron Separation for Ultra-High-Energy Cherenkov Astronomy

Dark100 is a planned array of six telescopes, using the Panoramic Search for Extraterrestrial Intelligence (PANOSETI) telescope system. It will operate as an imaging atmospheric Cherenkov telescope array, with a telescope design and array layout optimized for accessing gamma rays with tens of TeV to PeV energies. The science goals of Dark100 include the search for ultra-heavy dark matter, observations of Galactic Pevatrons, and the search for ultra-fast optical transients. Rejection of background cosmic rays is key to the sensitivity of the array. We present a first study of gamma/hadron separation based on simulated gamma rays and protons, focusing on the impact of the hadronic background models used in CORSIKA.

astro-ph.HE↗

arXiv2025

Cosmic rays cannot explain the high ionisation rates in the Galactic centre

The H$_2$ ionisation rate in the central molecular zone, located in the Galactic centre, is estimated to be $ζ\sim2\times10^{-14}~\mathrm{s}^{-1}$, based on observations of H$_3^+$ lines. This value is two to three orders of magnitude larger than that measured anywhere else in the Galaxy. Due to the high density of the gas in the central molecular zone, UV and X-ray photons do not penetrate this region. Hence, cosmic rays are expected to be the exclusive agents of ionisation. A high cosmic-ray density has been invoked to explain the unusually high ionisation rate. However, this excess is not seen in the $γ$-ray emission from this region, which is produced by high-energy cosmic rays. Therefore, an excess is expected only in the low-energy cosmic-ray spectrum. Here, we derive constraints on this hypothetical low-energy component in the cosmic-ray spectra, and we question its plausibility. To do so, we numerically solved the cosmic-ray transport equation in the central molecular zone. We derived stationary solutions under the assumption that cosmic rays are continuously injected by a source located in the Galactic centre. The high-energy component in the cosmic-ray spectrum was then fitted to available $γ$-ray and radio data, and a steep low-energy component was added to the cosmic-ray spectrum to explain the large ionisation rates. We find that injection spectra of $p^{-7}$ for protons below $p_\mathrm{enh,p}c\simeq780~\mathrm{MeV}$ and $p^{-5.2}$ for electrons below $p_\mathrm{enh,e}c=1.5~\mathrm{GeV}$ are needed to reach the observed ionisation rates. This corresponds to a cosmic-ray power of the order of $\sim10^{40-41}~\mathrm{erg}\,\mathrm{s}^{-1}$ injected at the Galactic centre. We conclude that cosmic rays alone cannot explain the high ionisation rates in the Galactic centre.

astro-ph.HE↗

arXiv2022

Cosmic ray production in superbubbles

We compute the production of cosmic rays in the dynamical superbubble produced by a cluster of massive stars. Stellar winds, supernova remnants and turbulence are found to accelerate particles so efficiently that the nonlinear feedback of the particles must be taken into account in order to ensure that the energy balance is not violated. High energy particles do not scatter efficiently on the turbulence and escape quickly after each supernova explosion, which makes both their intensity inside the bubble and injection in the interstellar medium intermittent. On the other hand, the stochastic acceleration of low energy particles hardens the spectra at GeV energies. Because cosmic rays damp the turbulence cascade, this hardening is less pronounced when nonlinearities are taken into account. Nevertheless, spectra with hard components extending up to 1 to 10 GeV and normalised to an energy density of 1 to 100 eV cm$^{-3}$ are found to be typical signatures of cosmic rays produced in superbubbles. Efficient shock reacceleration within compact clusters is further shown to produce hard, slightly concave spectra, while the presence of a magnetised shell is shown to enhance the confinement of cosmic rays in the bubble and therefore the collective plasma effects acting on them. We eventually estimate the overall contribution of superbubbles to the galactic cosmic ray content and show typical gamma-ray spectra expected from hadronic interactions in superbuble shells. In both cases, a qualitative agreement with observations is obtained.

astro-ph.HE↗