| 1. |
- Dasgupta, Saswata, et al.
(författare)
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Interpreting the Hi 21-cm cosmology maps through Largest Cluster Statistics. Part I. Impact of the synthetic SKA1-Low observations
- 2023
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; :5
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Tidskriftsartikel (refereegranskat)abstract
- We analyse the evolution of the largest ionized region using the topological and morphological evolution of the redshifted 21-cm signal coming from the neutral hydrogen distribution during the different stages of reionization. For this analysis, we use the "Largest Cluster Statistics" - LCS. We mainly study the impact of the array synthesized beam on the LCS analysis of the 21-cm signal considering the upcoming low-frequency Square Kilometer Array (SKA1-Low) observations using a realistic simulation for such observation based on the 21cmE2E-pipeline using OSKAR. We find that bias in LCS estimation is introduced in synthetic observations due to the array beam. This in turn shifts the apparent percolation transition point towards the later stages of reionization. The biased estimates of LCS, occurring due to the effect of the lower resolution (lack of longer baselines) and the telescope synthesized beam will lead to a biased interpretation of the reionization history. This is important to note while interpreting any future 21-cm signal images from upcoming or future telescopes like the SKA, HERA, etc. We conclude that one may need denser uv-coverage at longer baselines for a better deconvolution of the array synthesized beam from the 21-cm images and a relatively unbiased estimate of LCS from such images.
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| 2. |
- Gehlot, B. K., et al.
(författare)
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Degree-scale galactic radio emission at 122 MHz around the North Celestial Pole with LOFAR-AARTFAAC
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 662
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Tidskriftsartikel (refereegranskat)abstract
- Aims. Contamination from bright diffuse Galactic thermal and non-thermal radio emission poses crucial challenges in experiments aiming to measure the 21-cm signal of neutral hydrogen from the cosmic dawn (CD) and Epoch of Reionisation (EoR). If not included in calibration, this diffuse emission can severely impact the analysis and signal extraction in 21-cm experiments. We examine large-scale diffuse Galactic emission at 122 MHz around the North Celestial Pole, using the Amsterdam-ASTRON Radio Transient Facility and Analysis Centre (AARTFAAC-) High Band Antenna (HBA) system.Methods. In this pilot project, we present the first-ever wide-field image produced with a single sub-band of the data recorded with the AARTFAAC-HBA system. We demonstrate two methods, multi-scale CLEAN and shapelet decomposition, to model the diffuse emission revealed in the image. We used angular power spectrum metrics to quantify different components of the emission and compared the performance of the two diffuse structure modelling approaches.Results. We observed that the point sources dominate the angular power spectrum (ℓ(ℓ + 1)Cℓ/2π≡Δ2(ℓ)) of the emission in the field on scales of ℓ ≳ 60 (≲3 degree). The angular power spectrum after subtraction of compact sources is flat within the 20 ≲ ℓ ≲ 200 range, suggesting that the residual power is dominated by the diffuse emission on scales of ℓ ≲ 200. The residual diffuse emission has a brightness temperature variance of Δℓ=1802 = (145.64 ± 13.61) K2 at 122 MHz on angular scales of 1 degree, and it is consistent with a power law following Cℓ ∝ ℓ−2.0 in the 20 ≲ ℓ ≲ 200 range. We also find that, in the current set-up, multi-scale CLEAN is suitable to model the compact and diffuse structures on a wide range of angular scales, whereas the shapelet decomposition method better models the large scales, which are of the order of a few degrees and wider.
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| 3. |
- Georgiev, Ivelin, 1995-, et al.
(författare)
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The large-scale 21-cm power spectrum from reionization
- 2022
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 513:4, s. 5109-5124
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Tidskriftsartikel (refereegranskat)abstract
- Radio interferometers, such as the Low-Frequency Array and the future Square Kilometre Array, are attempting to measure the spherically averaged 21-cm power spectrum from the epoch of reionization. Understanding of the dominant physical processes which influence the power spectrum at each length-scale is therefore crucial for interpreting any future detection. We study a decomposition of the 21-cm power spectrum and quantify the evolution of its constituent terms for a set of numerical and semi-numerical simulations of a volume of (714 Mpc)3, focusing on large scales with k ≲ 0.3 Mpc−1. We find that after ∼10 per cent of the universe has been ionized, the 21-cm power spectrum follows the power spectrum of neutral hydrogen fluctuations, which itself beyond a certain scale follows the matter power spectrum. Hence the signal has a two-regime form where the large-scale signal is a biased version of the cosmological density field, and the small-scale power spectrum is determined by the astrophysics of reionization. We construct a bias parameter to investigate the relation between the large-scale 21-cm signal and the cosmological density field. We find that the transition scale between the scale-independent and scale-dependent bias regimes is directly related to the value of the mean free path of ionizing photons (λMFP), and is characterised by the empirical formula ktrans ≈ 2/λMFP. Furthermore, we show that the numerical implementation of the mean free path effect has a significant impact on the shape of this transition. Most notably, the transition is more gradual if the mean free path effect is implemented as an absorption process rather than as a barrier.
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| 4. |
- Greig, Bradley, et al.
(författare)
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Interpreting LOFAR 21-cm signal upper limits at z ≈ 9.1 in the context of high-z galaxy and reionization observations
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 501:1, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- Using the latest upper limits on the 21-cm power spectrum at z approximate to 9.1 from the Low Frequency Array (LOFAR), we explore the regions of parameter space which are inconsistent with the data. We use 21CMMC, a Monte Carlo Markov chain sampler of 21CMFAST which directly forward models the three dimensional (3D) cosmic 21-cm signal in a fully Bayesian framework. We use the astrophysical parametrization from 21CMFAST, which includes mass-dependent star formation rates and ionizing escape fractions as well as soft-band X-ray luminosities to place limits on the properties of the high-z galaxies. Further, we connect the disfavoured regions of parameter space with existing observational constraints on the Epoch of Reionization such as ultra-violet (UV) luminosity functions, background UV photoionization rate, intergalactic medium (IGM) neutral fraction, and the electron scattering optical depth. We find that all models exceeding the 21-cm signal limits set by LOFAR at z approximate to 9.1 are excluded at greater than or similar to 2 sigma by other probes. Finally, we place limits on the IGM spin temperature from LOFAR, disfavouring at 95 per cent confidence spin temperatures below similar to 2.6 K across an IGM neutral fraction range of 0.15 less than or similar to (x) over bar (HI) less than or similar to 0.6. Note, these limits are only obtained from 141 h of data in a single redshift bin. With tighter upper limits, across multiple redshift bins expected in the near future from LOFAR, more viable models will be ruled out. Our approach demonstrates the potential of forward modelling tools such as 21CMMC in combining 21-cm observations with other high-z probes to constrain the astrophysics of galaxies.
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| 5. |
- Kamran, Mohd, et al.
(författare)
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Redshifted 21-cm bispectrum - II. Impact of the spin temperature fluctuations and redshift space distortions on the signal from the Cosmic Dawn
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 502:3, s. 3800-3813
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of the 21-cm signal bispectrum (which quantifies the non-Gaussianity in the signal) from the Cosmic Dawn (CD). For our analysis, we have simulated the 21-cm signal using radiative transfer code GRIZZLY, while considering two types of sources (mini-QS05 and HMXBs) for Ly alpha coupling and the X-ray heating of the IGM. Using this simulated signal, we have, for the first time, estimated the CD 21-cm bispectra for all unique kappa-triangles and for a range of kappa modes. We observe that the redshift evolution of the bispectrum magnitude and sign follow a generic trend for both source models. However, the redshifts at which the bispectrum magnitude reaches their maximum and minimum values and show their sign reversal depends on the source model. When the Ly alpha coupling and the X-ray heating of the IGM occur simultaneously, we observe two consecutive sign reversals in the bispectra for small kappa-triangles (irrespective of the source models). One arising at the beginning of the IGM heating and the other at the end of Ly alpha-coupling saturation. This feature can be used in principle to constrain the CD history and/or to identify the specific CD scenarios. We also quantify the impact of the spin temperature (T-S) fluctuations on the bispectra. We find that T-S fluctuations have maximum impact on the bispectrum magnitude for small k-triangles and at the stage when Ly alpha coupling reaches saturation. Furthermore, we are also the first to quantify the impact of redshift space distortions (RSD), on the CD bispectra. We find that the impact of RSD on the CD 21-cm bispectra is significant (> 20 per cent) and the level depends on the stages of the CD and the k-triangles for which the bispectra are being estimated.
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| 6. |
- Ma, Qing-Bo, et al.
(författare)
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POLAR-I : linking the 21-cm signal from the epoch of reionization to galaxy formation
- 2023
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Ingår i: Monthly notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 522:3, s. 3284-3297
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Tidskriftsartikel (refereegranskat)abstract
- To self-consistently model galactic properties, reionization of the intergalactic medium, and the associated 21-cm signal, we have developed the algorithm polar by integrating the one-dimensional radiative transfer code grizzly with the semi-analytical galaxy formation code L-Galaxies 2020. Our proof-of-concept results are consistent with observations of the star formation rate history, UV luminosity function, and the CMB Thomson scattering optical depth. We then investigate how different galaxy formation models affect UV luminosity functions and 21-cm power spectra, and find that while the former are most sensitive to the parameters describing the merger of haloes, the latter have a stronger dependence on the supernovae feedback parameters, and both are affected by the escape fraction model.
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| 7. |
- Majumdar, Suman, et al.
(författare)
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Quantifying the non-Gaussianity in the EoR 21-cm signal through bispectrum
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 476:3, s. 4007-4024
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Tidskriftsartikel (refereegranskat)abstract
- The epoch of reionization (EoR) 21-cm signal is expected to be highly non-Gaussian in nature and this non-Gaussianity is also expected to evolve with the progressing state of reionization. Therefore the signal will be correlated between different Fourier modes (k). The power spectrum will not be able capture this correlation in the signal. We use a higher order estimator - the bispectrum - to quantify this evolving non-Gaussianity. We study the bispectrum using an ensemble of simulated 21-cm signal and with a large variety of k triangles. We observe two competing sources driving the non-Gaussianity in the signal: fluctuations in the neutral fraction (X-HT) field and fluctuations in the matter density field. We find that the non-Gaussian contribution from these two sources varies, depending on the stage of reionization and on which k modes are being studied. We show that the sign of the bispectrum works as a unique marker to identify which among these two components is driving the non-Gaussianity. We propose that the sign change in the bispectrum, when plotted as a function of triangle configuration cos 0 and at a certain stage of the EoR can be used as a confirmative test for the detection of the 21-cm signal. We also propose a new consolidated way to visualize the signal evolution (with evolving (x) over bar (HT) or redshift), through the trajectories of the signal in a power spectrum and equilateral bispectrum i.e. P(k) - B(k, k, k) space.
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| 8. |
- Majumdar, Suman, et al.
(författare)
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Redshifted 21-cm bispectrum – I. Impact of the redshift space distortions on the signal from the Epoch of Reionization
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 499:4, s. 5090-5106
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Tidskriftsartikel (refereegranskat)abstract
- The bispectrum can quantify the non-Gussianity present in the redshifted 21-cm signal produced by the neutral hydrogen (HI) during the Epoch of Reionization (EoR). Motivated by this, we perform a comprehensive study of the EoR 21-cm bispectrum using simulated signals. Given a model of reionization, we demonstrate the behaviour of the bispectrum for all unique triangles in k space. For ease of identification of the unique triangles we parametrize the k-triangle space with two parameters, namely the ratio of the two arms of the triangle (n = k2/k1) and the cosine of the angle between them (cos θ). Furthermore, for the first time we quantify the impact of the redshift space distortions (RSD) on the spherically averaged EoR 21-cm bispectrum in the entire unique triangle space. We find that the real space signal bispectra for small and intermediate k1-triangles (k1≤0.6Mpc−1) is negative in most of the unique triangle space. It takes a positive sign for squeezed, stretched, and linear k1-triangles, specifically for large k1 values (k1≥0.6Mpc−1). The RSD affects both the sign and magnitude of the bispectra significantly. It changes (increases/decreases) the magnitude of the bispectra by 50−100 per cent without changing its sign (mostly) during the entire period of the EoR for small and intermediate k1-triangles. For larger k1-triangles, RSD affects the magnitude by 100−200 per cent and also flips the sign from negative to positive. We conclude that it is important to take into account the impact of RSD for a correct interpretation of the EoR 21-cm bispectra.
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| 9. |
- Maurya, Naresh Chandra, et al.
(författare)
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Exciton many-body interactions and charge transfer in CsPbBr3/graphene derivatives
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:15
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Tidskriftsartikel (refereegranskat)abstract
- Charge separation and many-body interactions at the interface of the light-absorbing semiconductor and contact layer are of crucial importance to the photophysical properties and optoelectronic device performance. Here, we report the exciton many-body interactions and charge transfer dynamics at the interface of metal halide perovskite nanocrystals and graphene derivatives [graphene oxide (GO) and reduced GO (RGO)] using ultrafast transient absorption (TA) and time-resolved photoluminescence (PL) measurements. At the early timescales, the TA spectra of CsPbBr3/GO and CsPbBr3/RGO show an asymmetric derivative feature originating from the exciton many-body interactions. The band gap renormalization and binding energies of exciton and biexciton of CsPbBr3 nanocrystals are significantly reduced in CsPbBr3/GO(RGO) due to the charge transfer and change in the dielectric environment, respectively. More specifically, the exciton (biexciton) binding energy of CsPbBr3 nanocrystals, originally 38 +/- 2 (34 +/- 1) meV, decreases to 27 +/- 1 (22 +/- 1) meV in CsPbBr3/RGO and 17 +/- 1 (15 +/- 1) meV in CsPbBr3/GO. Furthermore, we observe a reduction in the Auger recombination rate and exciton PL quenching in CsPbBr3/GO and CsPbBr3/RGO, corroborating the charge transfer mechanism. Our systematic studies successfully describe photoexcited charge transfer from CsPbBr3 nanocrystals to GO (RGO) in 7.0 +/- 0.4 (4.2 +/- 0.1) ps, which is one order of magnitude faster than the charge transfer for other acceptor materials such as metal oxide, fullerene, anthraquinone, 1-aminopyrene, and phenothiazine. Our results offer insights and guidance for perovskite-based high-performance optoelectronic devices.
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| 10. |
- Mondal, Rajesh, et al.
(författare)
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Statistics of the epoch of reionization 21-cm signal - I. Power spectrum error-covariance
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 456:2, s. 1936-1947
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Tidskriftsartikel (refereegranskat)abstract
- The non-Gaussian nature of the epoch of reionization (EoR) 21-cm signal has a significant impact on the error variance of its power spectrum P(k). We have used a large ensemble of seminumerical simulations and an analytical model to estimate the effect of this non-Gaussianity on the entire error-covariance matrix C-ij. Our analytical model shows that C-ij has contributions from two sources. One is the usual variance for a Gaussian random field which scales inversely of the number of modes that goes into the estimation of P(k). The other is the trispectrum of the signal. Using the simulated 21-cm Signal Ensemble, an ensemble of the Randomized Signal and Ensembles of Gaussian Random Ensembles we have quantified the effect of the trispectrum on the error variance C-ii. We find that its relative contribution is comparable to or larger than that of the Gaussian term for the k range 0.3 = k <= 1.0Mpc-1, and can be even similar to 200 times larger at k similar to 5Mpc(-1). We also establish that the off-diagonal terms of C-ij have statistically significant non-zero values which arise purely from the trispectrum. This further signifies that the error in different k modes are not independent. We find a strong correlation between the errors at large k values (>= 0.5Mpc(-1)), and a weak correlation between the smallest and largest k values. There is also a small anticorrelation between the errors in the smallest and intermediate k values. These results are relevant for the k range that will be probed by the current and upcoming EoR 21-cm experiments.
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