arXiv subjects

A. M. Vinodkumar

Publications and source records attributed to A. M. Vinodkumar.

10 recordsLinked to original sources

arXiv2025

Spectral nature of Sco X-1 observed using the X-ray SPECtroscopy and Timing (XSPECT) payload on-board XPoSat

Scorpius X-1 is the brightest and first discovered X-ray source in the sky. Studying this source in the low-energy band has been challenging in the past due to its high brightness. However, with the X-ray SPECtroscopy and Timing (XSPECT) payload on-board Indias first X-ray Polarimetry Satellite (XPoSat), we have the capability to study the source despite its very high brightness, thanks to the fast (1 ms) readout of the instrument. We investigate the evolution of the spectral and timing properties of Sco X-1 across the horizontal, normal, and flaring branch, as observed with XSPECT. We examine changes in the spectral parameters as a function of position on the color-color diagram (CCD). Spectral studies indicate that the soft X-ray emission can be modeled using a multicolor disk component, with the inner disk temperature ranging from 0.6 to 0.8 keV. The hard component is described by a Comptonized continuum using either the nthComp or Comptb model with electron temperatures from 2.4 to 4.7 keV and optical depth between 5 and 14. Additionally, we observe the presence of an iron K-alpha line at 6.6 keV and an iron K-beta line at 7.6 keV. Both spectral models suggest a steep rise in Comptonization flux as well as disk flux in the flaring branch. An increase in neutron star blackbody temperature and inner disk temperature are also observed during flaring. The Z-track is driven by changes in the optical depth of the corona, the Comptonization flux and the disk flux and the inner disk temperature. No quasi-periodic oscillations are detected in any branch, suggesting their association with the high-energy spectrum.

astro-ph.HE↗

arXiv2025

First spectro-polarimetric study of the neutron star low-mass X-ray binary GX 9+1

We present the first spectro-polarimetric study of the bright atoll source GX 9+1, using the simultaneous Imaging X-ray Polarimetry Explorer (IXPE), and Neutron star Interior Composition Explorer (NICER) observations. The source was observed to remain in the soft state, with no changes in state throughout the observation period. The source does not show significant polarization in the 2-8 keV energy range. However, a significant polarization (3.3 sigma) was detected in the 2-3 keV range, with a polarization degree of 3.3 +/- 0.8% and a polarization angle of 11 +/- 7 deg. We used the simultaneous energy spectra from NICER (0.6 - 11 keV) and IXPE (2-8 keV) to study the spectral properties of the source during observations. The observed spectrum of the source can be well described by a combination of Comptonized blackbody emission from the neutron star surface (compbb model in XSPEC) and thermal Comptonized component with seed photons from the accretion disc. The spectral properties of GX 9+1 during the observation are consistent with those of other bright atoll-sources in the soft state. However, the high polarization degree observed in the low-energy band does not align with previous IXPE observations of other atoll-sources. This observed polarization in the source is attributed to the strong polarization of the Comptonized blackbody component. We discuss the results from the spectro-polarimetric studies in the context of various accretion disc and coronal geometries of the source.

astro-ph.HE↗

arXiv2023

Decay Pattern of Pygmy States Observed in Neutron-Rich 26 Ne

Coulomb excitation of the exotic neutron-rich nucleus Ne26 on a Pb208 target was measured at 58 MeV/u in order to search for low-lying E1 strength above the neutron emission threshold. This radioactive beam experiment was carried out at the RIKEN Accelerator Research Facility. Using the invariant mass method in the Ne25+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV excitation energy. By performing a multipole decomposition of the differential cross section, a reduced dipole transition probability of B(E1)=0.49+-0.16e2fm2 is deduced, corresponding to 4.9+-1.6% of the Thomas-Reiche-Kuhn sum rule. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured. The extracted decay pattern is not consistent with several mean-field theory descriptions of the pygmy states.

nucl-ex↗

arXiv2021

Running coupling constant in thermal $ϕ^4$ theory up to two loop order

Using the imaginary time formalism in thermal field theory, we derive running coupling constant and running mass in two loop order. In the process, we express the imaginary time formalism of Feynman diagrams as the summation of non-thermal quantum field theory (QFT) Feynman diagrams with coefficients that depend on temperature and mass. Renormalization constants for thermal $ϕ^4$ theory were derived using simple diagrammatic analysis. Our model links the non-thermal QFT and the imaginary time formalism by assuming both have the same mass scale $μ$ and coupling constant $g$. When these results are combined with the renormalization group equations (RGE) and applied simultaneously to thermal and non-thermal proper vertex functions, coupling constant and running mass with implicit temperature dependence are obtained. We evaluated pressure for scalar particles in two loop orders at zero external momentum limit by substituting the running mass result in the quasi-particle model.

hep-th↗

arXiv2017

Physics Potential of the ICAL detector at the India-based Neutrino Observatory (INO)

The upcoming 50 kt magnetized iron calorimeter (ICAL) detector at the India-based Neutrino Observatory (INO) is designed to study the atmospheric neutrinos and antineutrinos separately over a wide range of energies and path lengths. The primary focus of this experiment is to explore the Earth matter effects by observing the energy and zenith angle dependence of the atmospheric neutrinos in the multi-GeV range. This study will be crucial to address some of the outstanding issues in neutrino oscillation physics, including the fundamental issue of neutrino mass hierarchy. In this document, we present the physics potential of the detector as obtained from realistic detector simulations. We describe the simulation framework, the neutrino interactions in the detector, and the expected response of the detector to particles traversing it. The ICAL detector can determine the energy and direction of the muons to a high precision, and in addition, its sensitivity to multi-GeV hadrons increases its physics reach substantially. Its charge identification capability, and hence its ability to distinguish neutrinos from antineutrinos, makes it an efficient detector for determining the neutrino mass hierarchy. In this report, we outline the analyses carried out for the determination of neutrino mass hierarchy and precision measurements of atmospheric neutrino mixing parameters at ICAL, and give the expected physics reach of the detector with 10 years of runtime. We also explore the potential of ICAL for probing new physics scenarios like CPT violation and the presence of magnetic monopoles.

physics.ins-det↗

arXiv2013

The interaction of 11Li with 208Pb

Background: 11Li is one of the most studied halo nuclei. The fusion of 11Li with 208Pb has been the subject of a number of theoretical studies with widely differing predictions, ranging over four orders of magnitude, for the fusion excitation function. Purpose: To measure the excitation function for the 11Li + 208Pb reaction. Methods: A stacked foil/degrader assembly of 208Pb targets was irradiated with a 11Li beam producing center of target beam energies from above barrier to near barrier energies (40 to 29 MeV). The intensity of the 11Li beam (chopped) was 1250 p/s and the beam on-target time was 34 hours. The alpha-decay of the stopped evaporation residues was detected in a alpha-detector array at each beam energy in the beam-off period (the beam was on for <= 5 ns and then off for 170 ns). Results: The 215At evaporation residues were associated with the fusion of 11Li with 208Pb. The 213,214At evaporation residues were formed by the breakup of 11Li into 9Li + 2n, with the 9Li fusing with 208Pb. The 214At evaporation residue appears to result from a "quasi-breakup" process. Conclusions: Most of 11Li + 208Pb interactions lead to breakup with a small fraction (<= 11%) leading to complete fusion.

nucl-ex↗

arXiv2007

Search for low lying dipole strength in the neutron rich nucleus $^{26}$Ne

Coulomb excitation of the exotic neutron-rich nucleus $^{26}$Ne on a $^{nat}$Pb target was measured at 58 A.MeV in order to search for low-lying E1 strength above the neutron emission threshold. Data were also taken on an $^{nat}$Al target to estimate the nuclear contribution. The radioactive beam was produced by fragmentation of a 95 A.MeV $^{40}$Ar beam delivered by the RIKEN Research Facility. The set-up included a NaI gamma-ray array, a charged fragment hodoscope and a neutron wall. Using the invariant mass method in the $^{25}$Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV. The reconstructed $^{26}$Ne angular distribution confirms its E1 nature. A reduced dipole transition probability of B(E1)=0.49$\pm$0.16 $e^2fm^2$ is deduced. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is obtained. The results are discussed in terms of a pygmy resonance centered around 9 MeV.

nucl-ex↗

arXiv2005

A search for deeply bound kaonic nuclear states

We have measured proton and neutron energy spectra by stopping negative kaons on liquid helium4. Two distinct peak structures were found on both spectra, which were assigned to the formation of new kinds of strange stribaryons. In this paper, we summarize both results.

nucl-ex↗

arXiv2004

Indication of a strange tribaryon S^+ from the ^4He(stopped K^-,n) reaction

We measured the neutron time-of-flight spectrum in the ^4He(stopped K^-, n) reaction by stopping negative kaons in a superfluid helium target. A clear enhancement was observed in the neutron spectrum, which indicates the formation of a strange tribaryon of charge +1 with a mass and width of M = 3141 +/- 3 (stat.) +4/-1 (sys.) MeV/c^2 and G < 23 MeV/c^2. This state, denoted as S^+(3140), is about 25 MeV/c^2 higher than the previously observed S^0(3115) T=1.

nucl-ex↗

arXiv2003

Search for pairing-vibration states of even Ca isotopes in $^{40}$Ca+$^{208}$Pb transfer reactions

Multi-nucleon transfer reactions in $^{40}$Ca+$^{208}$Pb have been studied at several bombarding energies close to the Coulomb barrier. Light reaction products have been identified in mass and charge with a time-of-flight spectrometer. The energy spectra of the inclusive two neutron pick-up channel show a population in a narrow region of excitation energies which corresponds to the predicted energy of pairing vibration states in $^{42}$Ca.

nucl-ex↗