BITS Faculty Publications
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Item Emission properties of non-rotating neutron stars with magnetic field using modified TOV equations(Springer, 2024-07) Mishra, Madhukar; Sarkar, Tapomoy GuhaEmission properties of the Astrophysical objects such as Neutron Stars are found using mass, pressure profile and thermal cooling rate. In this current work, we determine the cooling rate of spherically symmetric neutron star as a function of time and distance from the star’s centre using NSCool code. Here we first find the mass, pressure and baryon number density profile of the non-rotating neutron stars using modified Tolman-Oppenheimer-Volkoff (TOV) system of equations in the presence of intense magnetic field. We used here a constant value of magnetic field and a distance dependent magnetic field in TOV equations to obtain the profile. We employ three different equation of states to solve the TOV equations by assuming that the core of Neutron Stars is composed of a hadronic matter. By employing above profile, we obtain the cooling rate with and without magnetic field to examine the effect of magnetic field for three different equations of states. Observed temperature of a few Neutron Stars have also been plotted along with calculated values for comparison. Finally, emissivity of axions as a dark matter candidates has been calculated as a result of the nucleon Bremsstrahlung mechanism with and without magnetic field.Item Disentangling the growth rate of perturbations from the HI bias using only clustering data from galaxy surveys(2025-06) Sarkar, Tapomoy GuhaThis work serves two-fold purpose. Firstly, we provide an alternative to the traditional method of determining the growth rate of density perturbations . In usual practice, can not be directly measured from tracer clustering at some redshift without knowledge of the bias. While the bayron acoustic oscillation (BAO) imprint allows the determination of , redshift space anisotropy (RSD) allows the measurement of a quantity . To extract from , one usually requires some other data set. We show that precise BAO and RSD measurements in and around some key redshifts themselves can solely reconstruct without requiring any other data sets. Secondly, we extend this approach to another tracer, namely the post-reionization 21-cm brightness temperature intensity maps. We demonstrate that the measured from purely redshift space clustering allows us to measure the 21-cm bias, which is a largely unknown quantity. This may help interpret the observed intensity mapping signal in the future.Item Magnetic field and EOS effects on axion emission from the non-rotating neutron stars(Elsevier, 2025-12) Sarkar, Tapomoy GuhaWe present here a comparative study of different axion emission mechanism from the core of highly magnetized neutron stars. In this work, it is assumed that neutron stars is being cooled as a consequence of the emission of neutrinos and axionic dark matter particles from the core of the neutron stars. We employ three different equation of states APR, FPS,SLY to solve the modified TOV equations, determine the mass and the pressure profiles by assuming that core of Neutron Stars to be made up of a hadronic matter. By employing above profiles, we obtain the cooling rate of spherically symmetric NSs with and without magnetic field to see the impact of magnetic field employing the different equation of states. The same is also studied with and without considering axion emission within the star core. Luminosities of axions as a dark matter candidates, neutrinos and photons have been plotted as a function of time with and without magnetic field. A fraction of these axions may undergo photon conversion in the powerful magnetic field within NSs magnetosphere, producing broadband radio or X-ray waves. Finally, we show the effect of magnetic field within the possible axion mass range on the actual observables such as total X-ray flux and conversion probability as a result of different axion to photon conversion mechanisms.Item The dynamics of background cosmological evolution and structure formation in phase space: a semi-cosmographic reconstruction(Elsevier, 2025-11) Sarkar, Tapomoy GuhaThe Baryon Acoustic Oscillation (BAO) feature, imprinted in the transverse and radial clustering of dark matter tracers, enables the simultaneous measurement of the angular diameter distance DA(z) and the Hubble parameter H(z) at a given redshift. Further, measuring the redshift space anisotropy (RSD) allows us to measure the combination f8(z) ≡ fσ8(z). Motivated by this, we simultaneously study the dynamics of background evolution and structure formation in an abstract phase space of dynamical quantities , , and f8. We adopt a semi-cosmographic approach, whereby we do not pre-assume any specific dark energy model to integrate the dynamical system. The Luminosity distance is expanded as a Padé rational approximation in the variable . The dynamical system is solved by using a semi-cosmographic equation of state, which incorporates the dark matter density parameter along with the parameters of the Padé expansion. The semi-cosmographic DA(z), H(z) and fσ8(z), thus obtained, are fitted with BAO and RSD data from the SDSS IV. The reconstructed phase trajectories in the 3D (x, p, f8) space are used to reconstruct some diagnostics of background cosmology and structure formation. At low redshifts, a discernible departure from the ΛCDM model is observed. The geometry of the phase trajectories in the projected spaces allows us to identify three key redshifts where future observations may be directed for a better understanding of cosmic tensions and anomalies.Item Probing decaying dark matter using the post-eor hi 21-cm signal(2024-12) Sarkar, Tapomoy GuhaWe propose the HI 21-cm power spectrum from the post-reionization epoch as a probe of a cosmological model with decaying dark matter particles. The unstable particles are assumed to undergo a 2-body decay into a massless and massive daughter. We assume, that a fraction f of the total dark matter budget to be, unstable and quantify the decay using the life-time Γ−1 and the relative mass splitting ϵ between the parent and the massive daughter. The redshift space anisotropic power spectrum of the post-reionization 21-cm signal brightness temperature, as a tracer of the dark matter clustering, imprints the decaying dark matter model through its effect on background evolution and the suppression of power on small this http URL find that with an idealized futuristic intensity mapping experiment with a SKA-I Mid like radio-array, ϵ and Γ can be measured at 3.1% and 4.64% around their fiducial values of ϵ=0.01 and Γ=0.074Gyr−1 respectivelyItem Probing dark energy using anisotropies in the clustering of post-EoR H i distribution(IOP, 2025) Sarkar, Tapomoy GuhaWe propose an anisotropy quantifier of the H i 21-cm signal traditionally used to clock the astrophysics of the reionization era as a post-reionization dark energy diagnostic. We find that the anisotropy probe can be measured at SNR ∼ 10 in both auto-correlation and in cross-correlation with the Ly-α forest over a wide z and k-range. We propose to use the BAO signature on the anisotropy signal to measure ( H(z), DA(z)). Subsequently, we put constraints on a dark energy model involving a negative cosmological constant on top of a quintessence scalar field and find that such a model is consistent with futuristic observations.Item A semi-cosmographic approach to study cosmological evolution in phase space(2025-03) Sarkar, Tapomoy GuhaThe signature of Baryon Acoustic Oscillation in the clustering of dark-matter tracers allows us to measure (DA(z),H(z)) independently. Treating these as conjugate variables, we are motivated to study cosmological evolution in the phase space of dimensionless variables x=H0DA/c and p=dx/dz. The dynamical system (x(z),p(z)) can be integrated for a known set of equation of state parameters for different matter/energy components. However, to avoid any preference for specific dark energy models, we adopt a cosmographic approach. We consider two scenarios where the Luminosity distance is expanded as Padé rational approximants using expansion in terms of z and (1+z)1/2 respectively. However, instead of directly using the Padé ratios to fit kinematic quantities with data, we adopt an alternative approach where the evolution of the cold dark matter sector is incorporated in our analysis through a semi-cosmographic equation of state, which is then, used to solve the dynamical problem in the phase space. The semi-cosmographic (DA(z),H(z)), thus obtained, is fitted with BAO and SNIa data from DESI DR1 + eBOSS and Pantheon+ respectively. We also consider a futuristic 21-cm intensity mapping experiment for error projections. We further use the semi-cosmographic fitting to reconstruct some diagnostics of background cosmology and compare our results for the two scenarios of Padé expansions.Item The Imprint of the Baryon Acoustic Oscillations (BAO) in the Cross-correlation of the Redshifted HI 21-cm Signal and the Ly-alpha Forest(ARXIV, 2011-12) Sarkar, Tapomoy GuhaThe cross-correlation of the Ly-alpha forest and redshifted 21-cm emission has recently been proposed as an observational tool for mapping out the large-scale structures in the post-reionization era z < 6. This has a significant advantage as the problems of continuum subtraction and foreground removal are expected to be considerably less severe in comparison to the respective auto-correlation signals. Further, the effect of discrete quasar sampling is less severe for the cross-correlation in comparison to the Ly-alpha forest auto-correlation signal. In this paper we explore the possibility of using the cross-correlation signal to detect the baryon acoustic oscillation (BAO). To this end, we have developed a theoretical formalism to calculate the expected cross-correlation signal and its variance. We have used this to predict the expected signal, and estimate the range of observational parameters where a detection is possible. For the Ly-α forest, we have considered BOSS and BIGBOSS which are expected have a quasar density of 16 deg^{-2} and 64 deg^{-2} respectively. A radio interferometric array that covers the redshift range z=2 to 3 using antennas of size 2 m * 2 m, is well suited for the 21-cm observations. It is required to observe 25 independent fields of view, which corresponds to the entire angular extent of BOSS. We find that it is necessary to achieve a noise level of 1.1 * 10^{-5} K^2 and 6.25 * 10^{-6} mK^2 per field of view in the 21-cm observations to detect the angular and radial BAO respectively with BOSS. The corresponding figures are 3.3 * 10^{-5} mK^2 and 1.7 * 10^{-5} mK^2 for BIGBOSS. Four to five independent radio interferometric arrays, each containing 400 antennas uniformly sampling all the baselines within 50 m will be able to carry out these observations in the span of a few years.Item The post-reionization neutral hydrogen - a cosmological probe(COSPAR Scientific Assembly, 2012-07) Sarkar, Tapomoy GuhaIn the absence of complex astrophysical processes that characterize the reionization era, bulk of the post-reionization neutral gas is believed to be housed in dense self shielded DLA clouds. The neutral hydrogen from this epoch can be seen either through the 21-cm emission or Lyman-alpha absorption. We present the cross-correlation of the redshifted 21-cm emission from neutral hydrogen (HI) in the post-reionization era with the Ly-alpha forest as a new probe of the large scale matter distribution in the redshift range z=2 to 3 with a significantly lesser problem arising from foreground contamination. Though the 21-cm and the Lyman-alpha forest signals originate from different astrophysical systems, they are both expected to trace the underlying dark matter distribution on large scales. The post-reionization 21-cm signal is studied using numerical N-body simulation to justify its use as a biased tracer. We investigate the imprint of baryon acoustic oscillation (BAO) in the cross-correlation power spectrum and the feasibility of a detection of the BAO scale with future radio interferometric observations and quasar surveys. The multi-frequency angular power spectrum, shall allow the detection of the BAO feature along both radial and transverse directions, opening new avenues for precision cosmology and enhance our understanding of Dark energy.Item The cross-correlation of redshited 21-cm signal and Lyman-α forest: A cosmological probe(IOP, 2014) Sarkar, Tapomoy GuhaWe have investigated the cross-correlation of the Lyman-α forest and redshifted 21-cm emission as a new observational probe of the large scale structures in the post-reionization era, with a significant advantage that the problems of continuum subtraction and foreground removal, and other systematics are expected to be considerably less servere in comparison to the respective auto-correlation signals. In this paper, we have explored the possibility of detecting the baryon acoustic oscillation in the cross-correlation signal. We have developed a formalism to calculate the expected cross-correlation signal and its covariance. We have used this to predict the expected signal and noise for a range of observational parameters.