arXiv subjects

Ramanath Cowsik

Publications and source records attributed to Ramanath Cowsik.

13 recordsLinked to original sources

arXiv2025

The Correspondence Between Leaky-Box and Diffusion Models of Cosmic-Ray Propagation

The leaky-box model and the attendant concept of path-length distribution of cosmic rays were invented in the mid-1960's. Even though versatile computational packages such as GALPROP and DRAGON with the diffusion approach are now available for analyzing cosmic ray data, the concepts of the leaky-box and path-length distribution continue to be adopted extensively. We show here mathematically that there is a close correspondence between the two approaches: The path-length or resident-time of the leaky-box models are similar to 'impulse response functions' of complex dynamical systems and are intuitively transparent. The results provided by the leaky-box model are valid when used judiciously.

astro-ph.HE↗

arXiv2024

Mapping the AMS-02 and CALET Data onto the Source Spectra and Residence Times of Galactic Cosmic Rays

The recent measurements of the spectral intensities of cosmic-ray nuclei have suggested that the ratio of Boron to Carbon nuclei, $R(E)$, comprises two components, $R_1(E)$ which carries all of the energy dependence and the other $R_A$, a constant independent of energy per nucleon. This finding supports the earlier theoretical expectations and results of gamma-ray astronomy that one of these components is attributable to spallation in a cocoon like region surrounding the sources and the other in the general interstellar medium before cosmic rays leak away from the Galaxy. A new model-independent way of analyzing cosmic-ray spectra is presented here to shed light on the recent findings: Imposing the single constraint that the source function of B nuclei is minimal we use a cascade of rate equations to map point-by-point the observed cosmic ray spectra of p, B, C, O and Fe on to their source spectra and the two life-times of cosmic rays $τ_G$ in the Galaxy and $τ_S$ in the cocoons surrounding the sources. The model independent results show that the appropriate choice is $τ_S(E)$ is responsible for $R_1(E)$ and the source spectra are power laws with indices of $\sim-2.7$.

astro-ph.HE↗

arXiv2023

A Minimal Model for Understanding Secondary Cosmic Rays

We take a phenomenological approach in a minimal model to understand the spectral intensity of secondary cosmic-ray particles like positrons, antiprotons, Lithium, Beryllium and Boron. Our analysis shows that cosmic rays at $\sim$ GeV energies pass through a significant amount of matter in regions surrounding the sources. This grammage decreases with increasing cosmic-ray energy and becomes negligible beyond $\sim 100$ GeV. During the subsequent propagation in the interstellar medium cosmic rays of all energies up to $\sim 10^5$ GeV/$n$ pass through about 1-2 g cm$^{-2}$ of matter before leaking into the intergalactic medium. It is in the interstellar medium that the bulk of the positrons and antiprotons are generated. Also cosmic-ray nuclei like C, N, and O at all energies generate additional amounts of Li, Be and B nuclei with a spectrum similar to those of C, O etc. The implications of these findings of the minimal model to the observations of gamma rays and also the importance of spatial and temporal discreteness of cosmic-ray sources for modeling cosmic-ray propagation are briefly pointed out.

astro-ph.HE↗

arXiv2021

A New Torsion Balance for the Search of Long-range Interactions Coupling to Baryon and Lepton Numbers

We have developed a torsion balance with a sensitivity about ten times better than those of previously operating balances for the study of long range forces coupling to baryon and lepton number. We present here the details of the design and expected characteristics of this balance. Operation of this balance for a year will also result in improved bounds on long range interactions of dark matter violating Einstein's equivalence principle.

gr-qc↗

arXiv2019

Test of Einstein's Equivalence Principle by Galactic Dark Matter

We report here the results of operation of a torsion balance with a period of $\sim 1.27 \times 10^4$ s. The analysis of data collected over a period of $\sim$115 days shows that the difference in the accelerations towards the Galactic Center of test bodies made of aluminum and quartz was $(0.61 \pm 1.27) \times 10^{-15} \, \mathrm{ m \, s}^{-2}$. This sets a bound on the violation of the equivalence principle by forces exerted by Galactic dark matter which is expressed by the Eötvös parameter $η_{DM} = (1.32 \pm 2.68) \times 10^{-5}$, a significant improvement upon earlier bounds.

gr-qc↗

arXiv2016

Project X: Physics Opportunities

Part 2 of "Project X: Accelerator Reference Design, Physics Opportunities, Broader Impacts". In this Part, we outline the particle-physics program that can be achieved with Project X, a staged superconducting linac for intensity-frontier particle physics. Topics include neutrino physics, kaon physics, muon physics, electric dipole moments, neutron-antineutron oscillations, new light particles, hadron structure, hadron spectroscopy, and lattice-QCD calculations. Part 1 is available as arXiv:1306.5022 [physics.acc-ph] and Part 3 is available as arXiv:1306.5024 [physics.acc-ph].

hep-ex↗

arXiv2015

X-Ray Polarimetry with the Polarization Spectroscopic Telescope Array (PolSTAR)

This paper describes the Polarization Spectroscopic Telescope Array (PolSTAR), a mission proposed to NASA's 2014 Small Explorer (SMEX) announcement of opportunity. PolSTAR measures the linear polarization of 3-50 keV (requirement; goal: 2.5-70 keV) X-rays probing the behavior of matter, radiation and the very fabric of spacetime under the extreme conditions close to the event horizons of black holes, as well as in and around magnetars and neutron stars. The PolSTAR design is based on the technology developed for the Nuclear Spectroscopic Telescope Array (NuSTAR) mission launched in June 2012. In particular, it uses the same X-ray optics, extendable telescope boom, optical bench, and CdZnTe detectors as NuSTAR. The mission has the sensitivity to measure ~1% linear polarization fractions for X-ray sources with fluxes down to ~5 mCrab. This paper describes the PolSTAR design as well as the science drivers and the potential science return.

astro-ph.IM↗

arXiv2015

Spectral Intensities of Antiprotons and the lifetime of Cosmic Rays in the Galaxy

In this paper we note that the spectral intensities of antiprotons observed in Galactic cosmic rays in the energy range ~ 1-100 GeV by BESS, PAMELA and AMS instruments display nearly the same spectral shape as that generated by primary cosmic rays through their interaction with matter in the interstellar medium, without any significant modifications. More importantly, a constant residence time of ~ 2.5 +/-0.7 million years in the Galactic volume, independent of the energy of cosmic rays, matches the observed intensities. A small additional component of secondary antiprotons in the energy below 10 GeV, generated in cocoon-like regions surrounding the cosmic-ray sources, seems to be present. We discuss this result in the context of observations of other secondary components like positrons and Boron, and conclude with general remarks about the origins and propagation of cosmic rays.

astro-ph.HE↗

arXiv2013

Properties of Galactic Dark Matter: Constraints from Astronomical Observations

The distributions of normal matter and of dark matter in the Galaxy are coupled to each other as they both move in the common gravitational potential. In order to fully exploit this interplay and to derive the various properties of dark matter relevant to their direct and indirect detection, we have comprehensively reviewed the astronomical observations of the spatial and velocity distributions of the components of normal matter. We then postulate that the phase-space distribution of dark matter follows a lowered-isothermal form and self-consistently solve Poisson's equation to construct several models for the spatial and velocity distributions of dark matter. In this paper, we compute the total gravitational potential of the normal and dark matter components and investigate their consistency with current observations of the rotation curve of the Galaxy and of the spatial and velocity distributions of blue horizontal-branch and blue straggler stars. Even with this demand of consistency, a large number of models with a range of parameters characterizing the dark matter distribution remain. We find the best choice of parameters, within the range of allowed values, for the surface density of the disk 55 M$_\odot$ pc$^{-2}$, are the following: the dark matter density at the Galactic center $ρ_{DM,c}\approx 100-250$ GeV cm$^{-3}$, the local dark matter density $ρ_{DM}(R_0) \approx 0.56-0.72$ GeV cm$^{-3}$, and the root-mean-speed of dark matter particles $< v_{DM}^{2}(R_0)> ^{1/2}\approx490-550$ km s$^{-1}$. We also discuss possible astronomical observations that may further limit the range of the allowed models.

astro-ph.GA↗

arXiv2013

Is there an unaccounted excess Extragalactic Cosmic Radio Background?

Analyses of measurements of the distribution of absolute brightness temperature over the radio sky have led recently to suggestions that there exists a substantial unexplained extragalactic radio background. Consequently, there have been numerous attempts to place constraints on plausible origins for the `excess'. We suggest here this expectation of a large extragalactic background, over and above that contributed by the sources observed in the surveys, is based on an extremely simple geometry adopted to model the Galactic emission and the procedure adopted in the estimation of the extragalactic contribution. In this paper, we derive the extragalactic radio background from wide-field radio images by using a more realistic modeling of the Galactic emission and decompose the sky maps at 150, 408, and 1420 MHz into anisotropic Galactic and isotropic extragalactic components. The anisotropic Galactic component is assumed to arise from a highly flattened spheroid representing the thick disc, embedded in a spherical halo, both centered at the Galactic center, along with Galactic sources, filamentary structures, Galactic loops and spurs. All components are constrained to be positive and the optimization scheme minimizes the sky area occupied by the complex filaments. We show that in contrast to the simple modeling of the Galactic emission as a plane parallel slab, the more realistic modeling yields estimates for the uniform extragalactic brightness that is consistent with expectations from known extragalactic radio source populations.

astro-ph.CO↗

arXiv2010

Development of a Thomson X-ray Polarimeter

We describe the current status of the design and development of a Thomson X-ray polarimeter suitable for a small satellite mission. Currently we are considering two detector geometries, one using rectangular detectors placed on four sides of a scattering element and the other using a single cylindrical detector with the scattering element at the center. The rectangular detector configuration has been fabricated and tested. The cylindrical detector is currently under fabrication. In order to compensate any pointing offset of the satellite, a collimator with a flat topped response has been developed that provides a constant effective area over an angular range. We have also developed a double crystal monochromator/polariser for the purpose of test and calibration of the polarimeter. Preliminary test results from the developmental activities are presented here.

astro-ph.IM↗

arXiv2010

Direct detection of WIMPs : Implications of a self-consistent truncated isothermal model of the Milky Way's dark matter halo

Direct detection of Weakly Interacting Massive Particle (WIMP) candidates of Dark Matter (DM) is studied within the context of a self-consistent truncated isothermal model of the finite-size dark halo of the Galaxy based on the "King model" of the phase space distribution function of collisionless DM particles. Our halo model takes into account the modifications of the phase-space structure of the halo due to the gravitational influence of the observed visible matter in a self-consistent manner. The parameters of the halo model are determined by a fit to a recently determined circular rotation curve of the Galaxy that extends up to $\sim$ 60 kpc. Unlike in the Standard Halo Model (SHM) customarily used in the analysis of the results of WIMP direct detection experiments, the velocity distribution of the WIMPs in our model is non-Maxwellian with a cut-off at a maximum velocity that is self-consistently determined by the model itself. For our halo model that provides the best fit to the rotation curve data, the 90% C.L. upper limit on the WIMP-nucleon spin-independent cross section from the recent results of the CDMS-II experiment, for example, is $\sim 5.3\times10^{-8}\pb$ at a WIMP mass of $\sim$ 71 GeV. We also find, using the original 2-bin annual modulation amplitude data of the DAMA experiment, that there exists a range of small WIMP masses, typically $\sim$ 2 -- 16 GeV, within which DAMA collaboration's claimed annual modulation signal purportedly due to WIMPs is compatible with the null results of other experiments. These results strengthen the possibility of low-mass ($\lsim 10\gev$) WIMPs as a candidate for dark matter as indicated by several earlier studies performed within the context of the SHM. A more rigorous analysis using DAMA bins over smaller intervals should be able to better constrain the "DAMA regions" in the WIMP parameter space within the context of our model.

astro-ph.CO↗

arXiv2009

Internal dynamics and dynamical friction effects in the dwarf spheroidal galaxy in Fornax

In the Fornax dwarf spheroidal galaxy the globular clusters are distributed widely, without any significant central concentration. Oh et al. pointed out that such a distribution is paradoxical: dynamical friction effects estimated using single-component King models would have forced the globular clusters to spiral down to the center of the galaxy well within a Hubble time. This paper is devoted to a discussion of this paradox. We describe a model in which the stars of the dwarf spheroidal galaxy are embedded in a cloud of dark matter, and each of these components is specified by its own phase space distribution function. This model allows us to fit self-consistently the observed luminosity profile and the spatial variation of the velocity dispersion of the stars. This fitting yields two basic parameters, related to the central density and velocity dispersion, that characterize the phase space distribution of dark matter. The dynamical friction effects calculated on the basis of this self-consistent model are small enough that the observed spatial distribution of the globular clusters poses no difficulty, and the apparent paradox is resolved. Thus we have at hand a model for Fornax that reproduces the main observed features of this dwarf spheroidal galaxy.

astro-ph.CO↗