arXiv subjects

S. K. Ghosh

Publications and source records attributed to S. K. Ghosh.

At least 37 records · Page 2Linked to original sources

arXiv2021

A new technique of linseed oil coating in bakelite RPC and the first test results

Single gap Resistive Plate Chamber (RPC) is one of the very popular gaseous detectors used in high-energy physics experiments nowadays. It is a very fast detector having low cost of fabrication. The RPCs are usually built using glass or bakelite plates having high resistivity $\sim~10^{10}-10^{11}$ $Ω$~cm. Bakelite RPCs are generally fabricated with a linseed oil coating inside to make the inner electrode surface smoother which helps to reduce the micro discharge probability. Linseed oil coating also reduces the surface UV sensitivity dramatically and effectively protect the bakelite surfaces from the Hydrofluoric Acid (HF), produced by the interaction of fluorine with the water vapour. There is a conventional way to do this linseed oil coating after making the gas gap as done in experiments $e.g.$ ALICE, CMS etc. A new technique is introduced here to do the linseed oil coating on the bakelite plate before making the gas gap. 100\% Tetrafluoroethane (C$_2$H$_2$F$_4$) gas is used to test the RPC module in the avalanche mode with cosmic rays. Conventional NIM electronics is used for this study. The efficiency and noise rate are measured. In this article, the detailed method of fabrication and the first test results are presented.

physics.ins-det↗

arXiv2021

Study of charging-up effect for a single mask triple GEM detector

With the advancement of the accelerator systems and the requirements of high luminosity particle beams to reach different physics goals, detectors with good position resolution and high rate handling capability have become essential for designing any High Energy Physics (HEP) experiments. The Gas Electron Multiplier (GEM) detectors are widely used in many HEP experiments as a tracking device because of their good spatial resolution and rate handling capability. The presence of the dielectric medium inside the active volume of the GEM detector changes its behaviour when exposed to external radiation. This mechanism is commonly referred as the charging-up effect. In this article, the effect of the charging-up phenomenon and the initial polarisation effect of the dielectric on the gain of the chamber are reported for a single mask triple GEM chamber with Ar/CO2 gas mixture.

physics.ins-det↗

arXiv2021

The Cold Dust Content of the Nearby Galaxies IC 5325, NGC 7496, NGC 7590, and NGC 7599

Star-forming galaxies are rich reservoirs of dust, both warm and cold. But the cold dust emission is faint alongside the relatively bright and ubiquitous warm dust emission. Recently, evidence for a very cold dust component has also been revealed via millimeter/submillimeter photometry of some galaxies. This component, despite being the most massive of the three dust components in star-forming galaxies, is by virtue of its very low temperature, faint and hard to detect together with the relatively bright emission from warmer dust. Here we analyze the dust content of a carefully selected sample of four galaxies detected by IRAS, WISE, and SPT, whose spectral energy distributions (SEDs) were modeled to constrain their potential cold dust content. Low-frequency radio observations using the GMRT were carried out to segregate cold dust emission from non-thermal emission in millimeter/submillimeter wavebands. We also carried out AstroSat/UVIT observations for some galaxies to constrain their SED at shorter wavelengths so as to enforce energy balance for the SED modeling. We constructed their SEDs across a vast wavelength range (extending from ultraviolet to radio frequencies) by assembling global photometry from GALEX FUV+NUV, UVIT, Johnson BRI, 2MASS, WISE, IRAC, IRAS, AKARI, ISOPHOT, Planck HFI, SPT, and GMRT. The SEDs were modeled with CIGALE to estimate their basic properties, in particular to constrain the masses of their total and very cold dust components. Although the galaxies' dust masses are dominated by warmer dust, there are hints of very cold dust in two of the targets, NGC 7496 and NGC 7590.

astro-ph.GA↗

arXiv2021

[CII] emission properties of the massive star-forming region RCW36 in a filamentary molecular cloud

Aims: To investigate properties of [CII]158 $μ$m emission of RCW36 in a dense filamentary cloud. Methods: [CII] observations of RCW36 covering an area of ~30 arcmin$\times$30 arcmin were carried out with a Fabry-Pérot spectrometer aboard a 100-cm balloon-borne far-infrared (IR) telescope with an angular resolution of 90 arcsec. By using AKARI and Herschel images, the spatial distribution of the [CII] intensity was compared with those of emission from the large grains and PAH. Results: The [CII] emission is spatially in good agreement with shell-like structures of a bipolar lobe observed in IR images, which extend along the direction perpendicular to the direction of a cold dense filament. We found that the [CII]--160 $μ$m relation for RCW36 shows higher brightness ratio of [CII]/160 $μ$m than that for RCW 38, while the [CII]--9 $μ$m relation for RCW36 is in good agreement with that for RCW38. Conclusions: The [CII] emission spatially well correlates with PAH and cold dust emissions. This means that the observed [CII] emission dominantly comes from PDRs. Moreover, the L_[CII]/L_FIR ratio shows large variation compared with the L_[CII]/L_PAH ratio. In view of the observed tight correlation between L_[CII]/L_FIR and the optical depth at $λ$=160 $μ$m, the large variation in L_[CII]/L_FIR can be simply explained by the geometrical effect, viz., L_FIR has contributions from the entire dust-cloud column along the line of sight, while L_[CII] has contributions from far-UV illuminated cloud surfaces. Based on the picture of the geometry effect, the enhanced brightness ratio of [CII]/160 $μ$m is attributed to the difference in gas structures where massive stars are formed: filamentary (RCW36) and clumpy (RCW38) molecular clouds and thus suggests that RCW36 is dominated by far-UV illuminated cloud surfaces compared with RCW38.

astro-ph.GA↗

arXiv2021

Chiral singlet superconductivity in the weakly correlated metal LaPt3P

Topological superconductors (SCs) are novel phases of matter with nontrivial bulk topology. They host at their boundaries and vortex cores zero-energy Majorana bound states, potentially useful in fault-tolerant quantum computation. Chiral SCs are particular examples of topological SCs with finite angular momentum Cooper pairs circulating around a unique chiral axis, thus spontaneously breaking time-reversal symmetry (TRS). They are rather scarce and usually feature triplet pairing: best studied examples in bulk materials are UPt3 and Sr2RuO4 proposed to be f-wave and p-wave SCs respectively, although many open questions still remain. Chiral triplet SCs are, however, topologically fragile with the gapless Majorana modes weakly protected against symmetry preserving perturbations in contrast to chiral singlet SCs. Using muon spin relaxation (muSR) measurements, here we report that the weakly correlated pnictide compound LaPt3P has the two key features of a chiral SC: spontaneous magnetic fields inside the superconducting state indicating broken TRS and low temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt3P is chiral d-wave singlet.

cond-mat.supr-con↗

arXiv2021

Curvit: An open-source Python package to generate light curves from UVIT data

Curvit is an open-source Python package that facilitates the creation of light curves from the data collected by the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat, India's first multi-wavelength astronomical satellite. The input to Curvit is the calibrated events list generated by the UVIT-Payload Operation Center (UVIT-POC) and made available to the principal investigators through the Indian Space Science Data Center. The features of Curvit include (i) automatically detecting sources and generating light curves for all the detected sources and (ii) custom generation of light curve for any particular source of interest. We present here the capabilities of Curvit and demonstrate its usability on the UVIT observations of the intermediate polar FO Aqr as an example. Curvit is publicly available on GitHub at https://github.com/prajwel/curvit.

astro-ph.IM↗

arXiv2020

In orbit performance of UVIT over the 5 years

Over the last 5 years, UVIT has completed observations of more than 500 proposals with ~ 800 unique pointings. In addition, regular planned monitoring observations have been made and from their analysis various key parameters related to in orbit performance of UVIT have been quantified. The sensitivities of the UV channels have remained steady indicating no effect of potential molecular contamination confirming the adequacy of all the protocols implemented for avoiding contamination. The quality of the PSF through the years confirms adequacy of thermal control measures. The early calibrations obtained during the Performance Verification (PV) phase have been further revised for more subtle effects. These include flat fields and detector distortions with greater precision. The operations of UVIT have also evolved through in orbit experience, e.g. tweaking of operational sequencing, protocol for recovery from bright object detection (BOD) shutdowns, parameters for BOD thresholds, etc. Finally, some effects of charged particle hits on electronics led to opimised strategy for regular resetting. The Near-UV channel was lost in one of such operations. All the above in-orbit experiences are presented here.

astro-ph.IM↗

arXiv2020

Performance of the UVIT Level-2 Pipeline

Performance of the Level-2 pipeline, which translates the UVIT data created by the ISRO's ground segment processing systems (Level-1) into astronomer ready scientific data products, is described. This pipeline has evolved significantly from experiences during the in orbit mission. With time, the detector modules of UVIT developed certain defects which led to occasional corruption of imaging and timing data. This article will describe the improvements and mitigation plans incorporated in the pipeline and report its efficacy and quantify the performance.

astro-ph.IM↗

arXiv2020

Study of Galactic Structure Using UVIT/AstroSat Star Counts

The structure of our Galaxy has been studied from ultraviolet (UV) star counts obtained with the Ultra-Violet Imaging Telescope (UVIT) on board the {\em AstroSat} satellite, in Far-UV (FUV) and Near-UV (NUV) bands. The F154W (BaF2) and N263M (NUVB4) filters were used in the FUV and NUV bands, respectively. The point sources are separated from the extra-galactic sources of UVIT observations using infrared (IR) color cut method. The observed UVIT star counts match well with the simulations obtained from the Besançon model of stellar population synthesis towards several Galactic directions. We also estimated the scale length and scale height of the thick disc and the scale height of the thin disc using the space density function and the exponential density law for the stars of intermediate Galactic latitudes. The scale length of the thick disc ranges from 3.11 to 5.40 kpc whereas the scale height ranges from 530$\pm$32 pc to 630$\pm$29 pc. The scale height of the thin disc comes out to be in the range of 230$\pm$20 pc to 330$\pm$11 pc.

astro-ph.GA↗

arXiv2020

Attenuation of electromagnetic radiation in Nuclear Track Detectors

A systematic study of the attenuation of electromagnetic radiation in Nuclear Track Detectors (NTDs) is carried out. The attenuation of gamma-ray, X-ray, UV, visible, and infrared radiation in NTDs are investigated using NaI(Tl) detector, Gas Electron Multiplier (GEM) detector, UV-Vis spectrophotometer, and FTIR spectrophotometer respectively. The values of some important parameters (e.g., optical depth, attenuation coefficient, etc.) of three commercially available NTDs (PET, Makrofol r and CR-39 r ), at the relevant region of the electromagnetic spectrum, is determined. The details of the experimental techniques and the results are also presented in this paper.

hep-ex↗

arXiv2020

Cosmic ray flux and lockdown due to COVID19 in Kolkata -- any correlation?

Cosmic ray muon flux is measured by the coincidence technique using plastic scintillation detectors in the High Energy Physics detector laboratory at Bose Institute, Kolkata. Due to the COVID19 outbreak and nationwide complete lockdown, the laboratory was closed from the end of March 2020 till the end of May 2020. After lockdown, although the city is not in its normal state, we still were able to take data on some days. The lockdown imposed a strict restriction on the transport service other than the emergency ones and also most of the industries were shut down in and around the city. This lockdown has significant effect on the atmospheric conditions in terms of change in the concentration of air pollutants. We have measured the cosmic ray flux before and after the lockdown to observe the apparent change if any due to change in the atmospheric conditions. In this article, we report the measured cosmic ray flux at Kolkata (22.58$^{\circ}$N 88.42$^{\circ}$E and 11~m Above Sea Level) along with the major air pollutants present in the atmosphere before and after the lockdown.

hep-ex↗

arXiv2020

The disintegrating old open cluster Czernik 3

We have performed a detailed analysis of the Czernik 3 (Cz3) open cluster by using deep near-infrared photometry taken with TIRCAM2 on 3.6m Devasthal optical telescope along with the recently available high quality proper motion data from the {\it Gaia} DR2 and deep photometric data from Pan-STARRS1. The cluster has a highly elongated morphology with fractal distribution of stars. The core and cluster radii of the cluster are estimated as 0.5 pc and 1.2 pc, respectively. We have identified 45 stars as cluster members using the {\it Gaia} proper motion data. The distance and age of the cluster are found to be $3.5\pm0.9$ kpc and $0.9^{+0.3}_{-0.1}$ Gyr, respectively. The slope of the mass function $`Γ'$ in the cluster region, in the mass range $\sim$0.95$<$M/M$_\odot$$<$2.2, is found to be $-1.01\pm0.43$. The cluster shows the signatures of mass-segregation and is dynamically relaxed (dynamical age=10 Myr). This along with its small size, big tidal radius, low density/large separation of stars, and elongated and distorted morphology, indicate that the Cz3 is a loosely bound disintegrating cluster under the influence of external tidal interactions.

astro-ph.GA↗

arXiv2020

Stability study and time resolution measurement of Straw Tube detectors

Straw tube detectors are single wire proportional counters that are widely used as a tracking device. We have carried out R$\&$D with a straw tube detector prototype. The motivation of this work is to study the stability of the performance in terms of gain and energy resolution of the straw tube detectors under high rate radiation. Two different methods are incorporated to perform this study. The gain and energy resolution of the detector are studied along with its variation with ambient temperature and pressure. X-ray from a radioactive source is used to irradiate the detector and the same source is also used to monitor the energy spectra simultaneously for calculation of gain. Variation of the gain and energy resolution of the straw tube detector under X-ray irradiation in Ar/CO$_2$ gas mixture is discussed in this article. We have also estimated the time resolution of the straw tube detectors that can be best achieved with cosmic rays as trigger for the same gas mixture. The details of the measurement process and the experimental results are presented in this article.

physics.ins-det↗

arXiv2020

Time-reversal symmetry breaking in the noncentrosymmetric Zr$_3$Ir superconductor

We report the discovery of Zr$_3$Ir as a new type of unconventional noncentrosymmetric superconductor (with $T_c = 2.3$ K), here investigated mostly via muon-spin rotation/relaxation ($μ$SR) techniques. Its superconductivity was characterized using magnetic susceptibility, electrical resistivity, and heat capacity measurements. The low-temperature superfluid density, determined via transverse-field $μ$SR and electronic specific heat, suggests a fully-gapped superconducting state. The spontaneous magnetic fields, revealed by zero-field $μ$SR below $T_c$, indicate the breaking of time-reversal symmetry in Zr$_3$Ir and, hence, the unconventional nature of its superconductivity. By using symmetry arguments and electronic-structure calculations we obtain a superconducting order parameter that is fully compatible with the experimental observations. Hence, our results clearly suggest that Zr$_3$Ir represents a new member of noncentrosymmetric superconductors with broken time-reversal symmetry.

cond-mat.supr-con↗

arXiv2020

A study of the secondary cosmic $γ$ -ray flux in India during the Great American solar eclipse on 21$^{st}$ August 2017

We present the results from the measurement of secondary cosmic gamma-ray flux using a NaI(Tl) scintillator detector during a total solar eclipse. The unique feature of this experiment is that it was carried out at a place where the solar eclipse was not observable. The total solar eclipse of August 21, 2017, was visible in most of the regions of North America during the day, whereas India, falling on the other half of the globe missed this particular eclipse. Our aim was to measure and examine if there are any variations in the secondary cosmic ray (SCR) flux at Kolkata, India due to the occurrence of the eclipse in America. Detailed experimental techniques used for this experiment are mentioned in this article. Method of data analysis and results are presented. We observe unexpected dehancement and enhancement of SCR flux in certain energy regions.

astro-ph.HE↗

arXiv2020

Additional Calibration of the Ultra-Violet Imaging Telescope on board AstroSat

Results of the initial calibration of the Ultra-Violet Imaging Telescope (UVIT) were reported earlier by Tandon et al. (2017). The results reported earlier were based on the ground calibration as well as the first observations in orbit. Some additional data from the ground calibration and data from more in-orbit observations have been used to improve the results. In particular, extensive new data from in-orbit observations have been used to obtain (a) new photometric calibration which includes (i) zero-points (ii) flat fields (iii) saturation, (b) sensitivity variations (c) spectral calibration for the near Ultra Violet (NUV; 2000 - 3000 Angstroms) and far Ultra-Violet (FUV; 1300 - 1800 Angstroms) gratings, (d) point spread function and (e) astrometric calibration which included distortion. Data acquired over the last three years show continued good performance of UVIT with no reduction in sensitivity in both the UV channels.

astro-ph.IM↗

arXiv2019

A Classifier to Detect Elusive Astronomical Objects through Photometry

The application of machine learning principles in the photometric search of elusive astronomical objects has been a less-explored frontier of research. Here we have used three methods: the Neural Network and two variants of k-Nearest Neighbour, to identify brown dwarf candidates using the photometric colours of known brown dwarfs. We initially check the efficiencies of these three classification techniques, both individually and collectively, on known objects. This is followed by their application to three regions in the sky, namely Hercules (2 deg x 2 deg), Serpens (9 deg x 4 deg) and Lyra (2 deg x 2 deg). Testing these algorithms on sets of objects that include known brown dwarfs shows a high level of completeness. This includes the Hercules and Serpens regions where brown dwarfs have been detected. We use these methods to search and identify brown dwarf candidates towards the Lyra region. We infer that the collective method of classification, also known as ensemble classifier, is highly efficient in the identification of brown dwarf candidates.

astro-ph.SR↗

arXiv2019

Investigation of the Globular Cluster NGC 2808 with the Ultra-Violet Imaging Telescope

Globular clusters represent stellar laboratories where observations can be used to validate models of stellar evolution. In this study, we put forth new ultraviolet (UV) photometric results of stars in the Galactic globular cluster NGC 2808. NGC 2808 is known to host multiple stellar populations that include at least four distinct groups of horizontal Branch (HB) stars. We have observed this cluster with the AstroSat-UltraViolet Imaging Telescope in two far-UV (FUV) and five near-UV (NUV) filters, respectively. These UV filters enable the identification of HB populations of stars. The results from four NUV filters exhibit bimodal distributions in magnitude histograms. The nature of bimodality has been investigated on the basis of distinct stellar types contributing to those bands. The color-magnitude diagrams constructed using FUV and NUV filters enable the location of hot stellar populations, viz. stars belonging to Red HB (RHB), Blue HB, Extreme HB, Blue Hook branch and post-Asymptotic Giant Branch. Prominent gaps are observed in the UV color-magnitude diagrams. We report for the first time, a photometric gap in a NUV color-magnitude diagram, that segregates the RHB population of this cluster into two groups, that are likely to be associated with distinct generations of stars. We have constructed and examined the GAIA color-magnitude diagram of the optical counterparts of the hot UV stars. We also investigate the spatial density distributions of various groups of stars in the cluster and comment on the proposed formation models of multiple populations.

astro-ph.SR↗