Papers by Shashikant Gupta
arXiv (Cornell University), Nov 21, 2022
Gamma Ray Bursts (GRB) are among the brightest objects in the Universe and hence can be observed ... more Gamma Ray Bursts (GRB) are among the brightest objects in the Universe and hence can be observed up to a very high redshift. Properly calibrated empirical correlations between intensity and spectral correlations of GRBs can be used to estimate the cosmological parameters. However, the possibility of the evolution of GRBs with the redshift is a long-standing puzzle. In this work, we used 162 long-duration GRBs to determine whether GRBs below and above a certain redshift have different properties. The GRBs are split into two groups, and we fit the Amati relation for each group separately. Our findings demonstrate that estimations of the Amati parameters for the two groups are substantially dissimilar. We perform simulations to investigate whether the selection effects could cause the difference. Our analysis shows that the differences may be intrinsic, and the selection effects are not their true origin.
arXiv (Cornell University), May 9, 2022
Understanding the explosion mechanism of type Ia supernova is among the most challenging issues i... more Understanding the explosion mechanism of type Ia supernova is among the most challenging issues in astrophysics. Accretion of matter on a carbon-oxygen white dwarf from a companion star is one of the most important keys in this regard. Our aim is to study the effects of WD composition on various parameters during the accretion of helium rich matter at a slow rate. We have used the computer simulation code "Modules for Experiments in Stellar Astrophysics" (MESA) to understand the variations in the properties such as specific heat (C P) and degeneracy parameter (η). The profile of specific heat shows a discontinuity and that of the degeneracy parameter shows a dip near the ignition region. As expected, the size of WD decreases and g increases during the accretion. However, a red-giant-like expansion is observed after the rapid ignition towards the end. Our study explains the reason behind the delay in onset of helium ignition due to the difference in carbon abundance in a CO-WD. We find that white dwarfs of the lower abundance of carbon accrete slightly longer before the onset of helium ignition.
Journal of Astrophysics and Astronomy, Apr 20, 2023
Understanding the explosion mechanism of type Ia supernova is among the most challenging issues i... more Understanding the explosion mechanism of type Ia supernova is among the most challenging issues in astrophysics. Accretion of matter on a carbon-oxygen white dwarf from a companion star is one of the most important keys in this regard. Our aim is to study the effects of WD composition on various parameters during the accretion of helium rich matter at a slow rate. We have used the computer simulation code "Modules for Experiments in Stellar Astrophysics" (MESA) to understand the variations in the properties such as specific heat (C P) and degeneracy parameter (η). The profile of specific heat shows a discontinuity and that of the degeneracy parameter shows a dip near the ignition region. As expected, the size of WD decreases and g increases during the accretion. However, a red-giant-like expansion is observed after the rapid ignition towards the end. Our study explains the reason behind the delay in onset of helium ignition due to the difference in carbon abundance in a CO-WD. We find that white dwarfs of the lower abundance of carbon accrete slightly longer before the onset of helium ignition.
Physics Letters B
Recent observations of Type Ia supernovae (SNe) by SH0ES collaboration (R11 and R16) diverge from... more Recent observations of Type Ia supernovae (SNe) by SH0ES collaboration (R11 and R16) diverge from the value reported by recent CMBR observations utilising the Planck satellite and application of the ΛCDM cosmological model by at least 3σ. It is among the most challenging problems in contemporary cosmology and is known as the Hubble tension. The SNe Ia in R11 and R16 were calibrated through cepheid variables in three distinct galaxies: Milky Way, LMC, and NGC4258. Carnegie Hubble Program (CHP) observations of type Ia SNe calibrated using the tip of the red giant approach yielded a somewhat different estimate for the Hubble constant. This decreased the Hubble tension from over 3σ to below 2σ. It is a legitimate question to answer whether there are any issues with SNe Ia calibration and to investigate whether the Hubble tension is real or not. We use statistical techniques namely, ANOVA, K-S test, and ttest to examine whether the cepheid calibration is host-dependent. Our analysis shows that (i) both R11 and R16 data suffer from non-Gaussian systematic effects, (ii) H0 values in the sub-samples (different anchorbased) in both R11 and R16 groups are significantly different at a 99% confidence level, and (iii) neglecting the metal-rich MW sample does not reduce the H0 value significantly, and thus Hubble tension persists. A small reduction in the Hubble constant could be linked to the differences in the host environment. Hence instead of using a single universal relation environment based slope and zero point should be preferred.
Research in Astronomy and Astrophysics
The Hubble constant (H 0), which represents the expansion rate of the universe, is one of the mos... more The Hubble constant (H 0), which represents the expansion rate of the universe, is one of the most important cosmological parameters. The recent measurements of H 0 using the distance ladder methods such as type Ia supernovae are significantly greater than the CMB measurements by Planck. The difference points to a crisis in the standard model of cosmology termed Hubble tension. In this work we compare different cosmological models, determine the Hubble constant and comment on the Hubble tension using the data from differential ages of galaxies. The data we use are free from systematic effects as the absolute age estimation of the galaxies is not needed. We apply the Bayesian approach along with the commonly used maximum likelihood method to estimate H 0 and calculate the AIC scores to compare the different cosmological models. The non-flat cosmological model provides a higher value for matter density as well as the Hubble constant compared to the flat ΛCDM model. The AIC score is sm...
Indian Journal of Science and Technology, 2021
In binary systems, the helium accretion onto carbon-oxygen (CO) white dwarfs (WDs) plays a vital ... more In binary systems, the helium accretion onto carbon-oxygen (CO) white dwarfs (WDs) plays a vital role in many astrophysical scenarios, especially in supernovae type Ia. Moreover, ignition density for accretion ratė M 10 −9 M yr −1 in helium accreting CO white dwarfs decides the triggering mechanism of a supernova explosion which could be either off-centre helium flash or central carbon flash. We aim to study the accretion of helium with a slow accretion rate 5 × 10 −10 M yr −1 onto relatively cool and hot white dwarfs of different abundances of carbon and oxygen. The simulation code "Modules for Experiments in Stellar Astrophysics" (MESA) has been used for our study. We analyze the variation in several properties like surface gravity (g), helium luminosity (L He), and effective temperature (T e f f) during the accretion phase of the white dwarfs. We also calculate the ignition density (ρ He) and ignition temperature (T He) of helium burning. As expected, the size of WD decreases and g increases during the accretion. However, a red-giant-like expansion is observed after the rapid ignition towards the end. The dependence of helium accreting WD evolution on its composition has also been explored in this study. We find that white dwarfs of the lower abundance of carbon accrete slightly longer before the onset of helium ignition.
Communications in Computer and Information Science, 2021
Type Ia Supernovae (SNe Ia) represents a distinct class of stellar objects which is crucial for c... more Type Ia Supernovae (SNe Ia) represents a distinct class of stellar objects which is crucial for cosmological applications such as estimating expansion rate of the universe and measuring the matter density at large scales. Spectroscopic and photometric observations (Light Curves) are used to identify these objects. Recent studies indicate heterogeneity among SNe Ia which is a vital issue for cosmological studies. We investigate the existence of subgroups within SNe Ia class using the photometric properties such as decline rate of light curve and the position of SN in the host galaxy. For this study, a sample of 40 SNe Ia from Asiago Supernova Catalogue and Calan/Tololo survey is taken. Density Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm is implemented on the data sample. Our results show the presence of subgroups in the SNe Ia sample. Strong correlation among various parameters of SNe Ia is found in each subgroup .
arXiv (Cornell University), Nov 1, 2022
Recent observations of Type Ia supernovae (SNe) by SH0ES collaboration (R11 and R16) diverge from... more Recent observations of Type Ia supernovae (SNe) by SH0ES collaboration (R11 and R16) diverge from the value reported by recent CMBR observations utilising the Planck satellite and application of the ΛCDM cosmological model by at least 3σ. It is among the most challenging problems in contemporary cosmology and is known as the Hubble tension. The SNe Ia in R11 and R16 were calibrated through cepheid variables in three distinct galaxies: Milky Way, LMC, and NGC4258. Carnegie Hubble Program (CHP) observations of type Ia SNe calibrated using the tip of the red giant approach yielded a somewhat different estimate for the Hubble constant. This decreased the Hubble tension from over 3σ to below 2σ. It is a legitimate question to answer whether there are any issues with SNe Ia calibration and to investigate whether the Hubble tension is real or not. We use statistical techniques namely, ANOVA, K-S test, and ttest to examine whether the cepheid calibration is host-dependent. Our analysis shows that (i) both R11 and R16 data suffer from non-Gaussian systematic effects, (ii) H0 values in the sub-samples (different anchorbased) in both R11 and R16 groups are significantly different at a 99% confidence level, and (iii) neglecting the metal-rich MW sample does not reduce the H0 value significantly, and thus Hubble tension persists. A small reduction in the Hubble constant could be linked to the differences in the host environment. Hence instead of using a single universal relation environment based slope and zero point should be preferred.
Research in Astronomy and Astrophysics, 2021
Different measurements of the Hubble constant (H 0) are not consistent, and a tension between the... more Different measurements of the Hubble constant (H 0) are not consistent, and a tension between the CMB based methods and cosmic distance ladder based methods has been observed. Measurements from various distance based methods are also inconsistent. To aggravate the problem, the same cosmological probe (Type Ia SNe for instance) calibrated through different methods also provides different values of H 0. We compare various distance ladder based methods through the already available unique data obtained from the Hubble Space Telescope (HST). Our analysis is based on parametric (t-test) as well as non-parametric statistical methods such as the Mann-Whitney U test and Kolmogorov-Smirnov test. Our results show that different methods provide different values of H 0 and the differences are statistically significant. The biases in the calibration would not account for these differences as the data have been taken from a single telescope with a common calibration scheme. The unknown physical e...
arXiv: Cosmology and Nongalactic Astrophysics, 2020
Different measurements of the Hubble constant ($H_{0}$) are not consistent and a tension between ... more Different measurements of the Hubble constant ($H_{0}$) are not consistent and a tension between the CMB based methods and cosmic distance ladder based methods has been observed. Measurements from various distance based methods are also inconsistent. To aggravate the problem, same cosmological probe (Type Ia SNe for instance) calibrated through different methods also provide different value of $H_{0}$. We compare various distance ladder based methods through the already available unique data obtained from Hubble Space Telescope (HST). Our analysis is based on parametric (T-test) as well as non-parametric statistical methods such as the Mann-Whitney U test and Kolmogorov-Smirnov test. Our results show that different methods provide different values of $H_0$ and the differences are statistically significant. The biases in the calibration would not account for these differences as the data has been taken from a single telescope with common calibration scheme. The unknown physical effec...
We use the \(\varDelta _{\chi }\) statistic introduced in Gupta and Saini (Mon Not R Astron Soc, ... more We use the \(\varDelta _{\chi }\) statistic introduced in Gupta and Saini (Mon Not R Astron Soc, 407:651, 2010, [5]) to study non-Gaussianity and directional dependence, in the high-z supernovae type Ia data. This dependence could arise due to departures from the cosmological principle or from direction dependent systematics in the data. We apply our statistic to largest and latest catalogue Union2 from Amanullah et al. (Astrophys J, 716:712, 2010, [11]). Our results indicate weak signature of non-Gaussianity and direction dependence in the data.
Monthly Notices of the Royal Astronomical Society, 2014
We use the ∆ χ 2 statistic introduced in Gupta, Saini & Laskar (2008); Gupta, Saini (2010) to stu... more We use the ∆ χ 2 statistic introduced in Gupta, Saini & Laskar (2008); Gupta, Saini (2010) to study directional dependence, in the high-z supernovae data. This dependence could arise due to departures from the cosmological principle or from direction dependent statistical systematics in the data. We apply our statistic to the gold data set from Riess et al. (2004) and Riess et al. (2007), and Union2 catalogue from Amanullah et al. (2010). Our results show that all the three data sets show a weak but consistent direction dependence. In 2007 data errors are Gaussian, however other two data sets show non-Gaussian features.
Journal of Cosmology and Astroparticle Physics, 2016
Assuming the Central Limit Theorem, experimental uncertainties in any data set are expected to fo... more Assuming the Central Limit Theorem, experimental uncertainties in any data set are expected to follow the Gaussian distribution with zero mean. We propose an elegant method based on Kolmogorov-Smirnov statistic to test the above; and apply it on the measurement of Hubble constant which determines the expansion rate of the Universe. The measurements were made using Hubble Space Telescope. Our analysis shows that the uncertainties in the above measurement are non-Gaussian.
Physics of the Dark Universe
Research in Astronomy and Astrophysics
We investigate direction dependence and non-Gaussian features in high-z cosmological data using ∆... more We investigate direction dependence and non-Gaussian features in high-z cosmological data using ∆ χ 2 and ∆ χ statistics and the Kolmogorov-Smirnov test. These techniques are applied on a set of calibrated long gamma-ray bursts (GRBs) and its combination with recent Type Ia supernovae data (Union2). Our statistical analysis shows a weak but consistent direction dependence in both the data sets. The analysis also indicates a non-Gaussian nature of errors in both data sets.
Indian Journal of Science and Technology
Research in Astronomy and Astrophysics, 2016
The nature of random errors in any data set is Gaussian is a well established fact according to t... more The nature of random errors in any data set is Gaussian is a well established fact according to the Central Limit Theorem. Supernovae type Ia data have played a crucial role in major discoveries in cosmology. Unlike in laboratory experiments, astronomical measurements can not be performed in controlled situations. Thus, errors in astronomical data can be more severe in terms of systematics and non-Gaussianity compared to those of laboratory experiments. In this paper, we use the Kolmogorov-Smirnov statistic to test non-Gaussianity in high-z supernovae data. We apply this statistic to four data sets, i.e., Gold data(2004), Gold data(2007), Union2 catalogue and the Union2.1 data set for our analysis. Our results shows that in all four data sets the errors are consistent with the Gaussian distribution.
The most detailed constraints on the accelerating expansion of the universe and details of nature... more The most detailed constraints on the accelerating expansion of the universe and details of nature of dark energy are derived from the high redshift supernova data, assuming that the errors in the measurements are Gaussian in nature. There is a possibility that there are direction dependent systematics in the data, either due to uncorrected, known physical processes or because there are tiny departures from the cosmological principle, making the universe slightly anisotropic. To investigate this possibility we introduce a statistic based on the extreme value theory and apply it to the gold data set from Riess et al. (2004). Our analysis indicates a systematic, direction dependent non-gaussianity at about one sigma level.
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Papers by Shashikant Gupta