arXiv subjects

Nitinkumar Bijewar

Publications and source records attributed to Nitinkumar Bijewar.

7 recordsLinked to original sources

arXiv2026

A NICER and AstroSat view of the neutron star low-mass X-ray binary 1A 1246-588

Neutron star (NS) low-mass X-ray binary (LMXB) systems depict a variety of X-ray spectral and timing features, which can be useful to probe the accretion-ejection mechanism in the strong gravity regime. Here, we study the relatively unexplored and faint NS LMXB 1A 1246-588, which is also an ultra-compact X-ray binary (UCXB) with a white dwarf donor. We investigate its temporal and spectral behavior using pointed NICER and AstroSat observations, supported by long-term MAXI/GSC monitoring. The MAXI light curve shows modest, recurrent outburst-like enhancements, providing the long-term flux context for interpreting the pointed observations. During the AstroSat observations in 2017, the source exhibits an absorbed 0.4-20 keV flux of $(1.18 \pm 0.02)$ x $10^{-10}$ $erg$ $cm^{-2}$ $s^{-1}$, while during the NICER observations in 2019, it spans an absorbed 0.5-10 keV flux range of $(0.7-3.7)$ x $10^{-10}$ $erg$ $cm^{-2}$ $s^{-1}$ and traces an atoll-like pattern in the hardness-intensity diagram. Broadband spectral modeling shows that the emission is well described by a soft blackbody and a hard Comptonized component, with no statistically required multicolor disk contribution. The blackbody temperature increases from 0.28 to 0.39 keV, with an emitting radius consistent within 6.9-13.8 km, while the Comptonization photon index varies from 1.8 to 2.3. We find that the observed spectral-state evolution is driven by a redistribution of accretion power between thermal emission from the NS boundary layer and Comptonized emission, consistent with atoll-type behavior. These results provide the first quantitative, multi-epoch view of accretion-state evolution in 1A 1246-588, revealing systematic changes in the thermal boundary-layer emission and the Comptonizing region in this UCXB system.

astro-ph.HE↗

arXiv2024

AstroSat timing and spectral analysis of the accretion-powered millisecond X-ray pulsar IGR J17591--2342

IGR J17591--2342, a transient accretion-powered millisecond X-ray pulsar, was discovered during its 2018 outburst. Here, we present a timing and spectral analysis of the source using {\it AstroSat} data of the same outburst. From the timing analysis, we obtain updated values of binary orbital parameters, which reveal an average pulsar spin frequency of 527.4256984(8) Hz. The pulse profiles can be fit well with four harmonically related sinusoidal components with fractional amplitudes of fundamental and second, third, and fourth harmonics as $\sim13$\%, $\sim$6\%, $\sim$0.9\%, $\sim$0.2\%, respectively. The energy-dependent study of pulse profiles in the range of $3-20$ keV shows that the fractional amplitude of both the fundamental and first overtone is consistent with being constant across the considered energy band. Besides, a decaying trend has been observed for both the fundamental and first overtone in the phase-delay versus energy relation resulting in soft X-ray (2.8-3.3 keV) phase lags of $\sim$0.05 and $\sim$0.13 with respect to $\leq 15$ keV photons, for the fundamental and first overtone, respectively. The combined spectra from the Large Area X-ray Proportional Counters and the Soft X-ray Telescope aboard {\it AstroSat} in the $1-18$ keV range can be fit well with an absorbed model consisting of a Comptonization, a blackbody and a Gaussian emission line component yielding as best-fit parameters a blackbody seed photon temperature $kT_{\rm bb}$ $\sim 0.95 \pm 0.03$ keV, and an electron temperature $kT_{\rm e}$ $\sim 1.54 \pm0.03$ keV. The spectral aspects suggest the scattering of photons from the accretion disc or the neutron star's surface.

astro-ph.HE↗

arXiv2019

Investigation of Thermoelectric properties of Magnetic Insulator FeRuTiSi Using First Principle Calculation

In this work, we have investigated the electronic structure and thermoelectric properties of quaternary heusler alloy, FeRuTiSi, using first principle DFT tools implemented in WIEN2k and BoltzTraP code. Electronic structure calculations using TB-mBJ potential shows appearance of flat band at the conduction band edge, thus electron in conduction band have the large effective mass (me*), and therefore mainly contribute for negatively large value of Seebeck coefficient (S). This alloy has indirect band gap of 0.59 eV, and shows the n-type transport behavior. Under the constant relaxation time approximation (tau = 10 -14 s), temperature dependent Seebeck coefficient, electrical conductivity (sigma), and electronic thermal conductivity (ke) were also estimated. The maximum figure-of-merit (ZT), for the FeRuTiSi compound is found to be ~0.86 at 840 K, with n-type doping, which suggests that this quaternary alloy can be a good candidate among the n-type material for thermoelectric applications in high-temperature reg

cond-mat.str-el↗

arXiv2019

Thermoelectric properties of Half-Metallic FeMnScGa Using First Principle Calculation

Here, we have investigated the thermoelectric properties of FeMnScGa alloy by combined use of full potential linearized augmented plane-wave (FP-LAPW) method and Boltzmann transport theory implemented in Wien2K and BoltzTraP code, respectively. Using the TB-mBJ potential, half-metallic nature is clearly observed with energy band gap of ~0.41 eV for down spin channel. Transport coefficients in 200-1000 K temperature range are investigated under the constant relaxation time approximations (tau=10-14 s). Total contributions to the Seebeck coefficients from up and dn-spin channels are estimated by using two-current model. The calculated values of Seebeck coefficients are found to be negative in the entire temperature region under present study, which suggests the n-type characteristic of this alloy. The magnitude of Seebeck coefficient is found to be increasing with temperature and exhibit the large value ~ -87 micro-V/K in 700-1000 K temperature range. The maximum in magnitude of total electrical conductivity value is found to be ~2.0 x 105 ohm-1m-1 at 1000 K. The present study shows, with half-metallic electronic structure, FeMnScGa have very good thermoelectric behavior, and can be suitable for thermoelectric application in high temperature region.

cond-mat.str-el↗

arXiv2014

Low energy secondary cosmic ray flux (gamma rays) monitoring and its constrains

Temporal variation of secondary cosmic rays (SCR) flux was measured during the several full and new moon and days close to them at Department of Physics, University of Mumbai, Mumbai (Geomagnetic latitude: 10.6 N), India. The measurements were done by using NaI (Tl) scintillation detector with energy threshold of 200 keV. The SCR flux shows sudden enhancement for approximately about 2 hour in counts during couple of events out of all experimental observations. The maximum Enhancement SCR flux is about 200% as compared to the diurnal trend of SCR temporal variations. Weather parameters (temperature and relative humidity) were continuously monitored during all observation. The influences of geomagnetic field, interplanetary parameters and tidal effect on SCR flux have been considered. Summed spectra corresponding to enhancement duration indicates appearance of atmospheric radioactivity which shows single gamma ray line. Detail investigation revealed the presence of radioactive Ar 41 . This measurements puts limitations on low energy SCR flux monitoring. This paper will help many researcher who want to measure SCR flux during eclipses and motivate to find unknown mechanism behind decrease/ increase during solar/lunar eclipse.

astro-ph.EP↗

arXiv2014

Lunar eclipse induces disturbance in the lunar exosphere

Given the renewed scientific interest in lunar exploration missions, complete understanding of lunar near surface environment and its exosphere under different conditions is of paramount importance. Lunar exosphere has been extensively studied by ground based observations [18,19,20,21,22,23] and hypothesized by different models[1,2,3,4,5,6,7,8,9,10,11,12,13]. In present work, we have discussed overlooked possible sources behind changes in the lunar exosphere when the Moon passes through the penumbra and umbra of the Earth during a lunar eclipse. The dusty turbulent environment due to planetary shadow is not only confined to lunar studies and exploration, but it can also be extended to all terrestrial airless bodies in the universe with a dusty surface e.g. some planets, planetary satellites, asteroids etc.

astro-ph.EP↗

arXiv2012

Unexpected enhancement in secondary cosmic ray flux during the total lunar eclipse of December 10, 2011

Temporal variation of secondary cosmic rays (SCR) flux was measured during the total lunar eclipse on December 10, 2011 and the subsequent full moon on January 8, 2012. The measurements were done at Department of Physics, University of Mumbai, Mumbai (Geomagnetic latitude: 10.6 N), India using NaI (Tl) scintillation detector by keeping energy threshold of 200 KeV. The SCR flux showed approximately 8.1% enhancement during the lunar eclipse as compared to the average of pre- and post-eclipse periods. Weather parameters (temperature and relative humidity) were continuously monitored and their correlations with temporal variation in SCR flux were examined. The influences of geomagnetic field, interplanetary parameters and tidal effect on SCR flux were considered. Qualitative analysis of SCR flux variation indicates that the known factors affecting SCR flux fail to explain observed enhancement during the eclipse. This enhancement during lunar eclipse and widely reported decrease during solar eclipses may unravel hitherto unnoticed factors modulating SCR flux.

physics.space-ph↗