Quantifying Suppression of the Cosmological 21-cm Signal due to Direction Dependent Gain Calibration in Radio Interferometers

The 21-cm signal of neutral hydrogen - emitted during the Epoch of Reionization - promises to be an important source of information for the study of the infant universe. However, its detection is impossible without sufficient mitigation of other strong signals in the data, which requires an accurate knowledge of the instrument. Using the result of instrument calibration, a large part of the contaminating signals are removed and the resulting residual data is further analyzed in order to detect the 21-cm signal. Direction dependent calibration (DDC) can strongly affect the 21-cm signal, however, its effect has not been precisely quantified. In the analysis presented here we show how to exactly calculate what part of the 21-cm signal is removed as a result of the DDC. We also show how a-priori information about the frequency behavior of the instrument can be used to reduce signal suppression. The theoretical results are tested using a realistic simulation based on the LOFAR setup. Our results show that low-order smooth gain functions (e.g. polynomials) over a bandwidth of ~10\,MHz - over which the signal is expected to be stationary - is sufficient to allow for calibration with limited, quantifiable, signal suppression in its power spectrum. We also show mathematically and in simulations that more incomplete sky models lead to larger 21-cm signal suppression, even if the gain models are enforced to be fully smooth. This result has immediate consequences for current and future radio telescopes with non-identical station beams, where DDC might be necessary (e.g. SKA-low).Comment: Submitted to MNRAS on 10-Aug-201

Radio Astronomical Image Formation using Constrained Least Squares and Krylov Subspaces

Image formation for radio astronomy can be defined as estimating the spatial power distribution of celestial sources over the sky, given an array of antennas. One of the challenges with image formation is that the problem becomes ill-posed as the number of pixels becomes large. The introduction of constraints that incorporate a-priori knowledge is crucial. In this paper we show that in addition to non-negativity, the magnitude of each pixel in an image is also bounded from above. Indeed, the classical "dirty image" is an upper bound, but a much tighter upper bound can be formed from the data using array processing techniques. This formulates image formation as a least squares optimization problem with inequality constraints. We propose to solve this constrained least squares problem using active set techniques, and the steps needed to implement it are described. It is shown that the least squares part of the problem can be efficiently implemented with Krylov subspace based techniques, where the structure of the problem allows massive parallelism and reduced storage needs. The performance of the algorithm is evaluated using simulations

Exergy in Photovoltaic/Thermal Nanofluid-Based Collector Systems

This chapter focuses on the exergy analysis of photovoltaic/thermal (PVT) systems using nanofluid. The PVT hybrid systems are designed to harness solar energy more efficiently. The thermodynamic theory of exergy in PVT systems is explained in details. The existing researches used various models to perform the exergy analysis for performance evaluation of the PVT systems. These models and formulations are compared with each other to achieve a widely used theory for a better comparison of the results. The exergy analysis is an effective tool to evaluate the performance of PVT systems. The exergy efficiency enhancement in PVT systems and the effect of nanofluid from the literature are presented. The literature survey suggests that the increase in the flow rate increases the exergy efficiencies in collector-based PVT. Using nanofluid as optical filters of solar radiation results in higher exergy efficiencies compared to collector-based PVT systems. According to the recent publications, the long-term thermophysical stability of nanofluid and cost-based exergy analysis still require further investigations

Effects of an Electric Field on Water Uptake in Single Roots of Intact Onion Plants

Water uptake by a single root of an onion plant (Allium cepa) was measured potentially before, during and following exposure of shoots to an external electric field (EEF). The field strength used was 9kV/m DC and AC (60-Hz) brought about a statistically significant increase (44-71%) in water uptake if the shoot chamber was at almost 100% humidity

Gene Expression Analysis of SOX2, NANOG, KLF4, OCT4, and REX1 Genes in Cord Blood Mononuclear Cells Treated with External Stresses

Background: Induced pluripotent stem cells (iPSCS) can be obtained from autologous cells for therapeutic purposes. So, far, many studies have been done to produce induced pluripotent cells by transferring specific pluripotency genes using different methods. In this study, pluripotency gene expression induced by external stresses was assessed in cord blood mononuclear cells.Methods: In this experimental study, mononuclear cells were isolated from umbilical cord blood. Isolated cells were divided into three groups. The first group had been exposed to HCL (pH 5.7) for 25 minutes and then transferred to the medium with normal pH. The second group was triturated with hamilton syringe for 15 min (external pressure), and the last group was considered the control group and did not receive treatment. Then, total RNA was extracted on Day 7. Gene expression of OCT4, SOX2, NANOG, REX1, and KLF4 was evaluated using qRT-PCR.Results: Gene expression of OCT4, NANOG, REX1, and KLF4 was increased after exposure to acidic pH and external pressures in comparison with control cells (P < 0.05). SOX2 gene expression was decreased in cells exposed to acidic pH but increased by external pressure.Conclusions: Exposure of umbilical cord blood mononuclear cells to acidic pH and external pressure lead to re-activation of pluripotency genes in mature cells. These findings indicate that mature cells may be reprogrammed with manipulation of environmental conditions

Gene Expression Analysis of SOX2, NANOG, KLF4, OCT4, and REX1 Genes in Cord Blood Mononuclear Cells Treated with External Stresses

Background: Induced pluripotent stem cells (iPSCS) can be obtained from autologous cells for therapeutic purposes. So, far, many studies have been done to produce induced pluripotent cells by transferring specific pluripotency genes using different methods. In this study, pluripotency gene expression induced by external stresses was assessed in cord blood mononuclear cells.Methods: In this experimental study, mononuclear cells were isolated from umbilical cord blood. Isolated cells were divided into three groups. The first group had been exposed to HCL (pH 5.7) for 25 minutes and then transferred to the medium with normal pH. The second group was triturated with hamilton syringe for 15 min (external pressure), and the last group was considered the control group and did not receive treatment. Then, total RNA was extracted on Day 7. Gene expression of OCT4, SOX2, NANOG, REX1, and KLF4 was evaluated using qRT-PCR.Results: Gene expression of OCT4, NANOG, REX1, and KLF4 was increased after exposure to acidic pH and external pressures in comparison with control cells (P < 0.05). SOX2 gene expression was decreased in cells exposed to acidic pH but increased by external pressure.Conclusions: Exposure of umbilical cord blood mononuclear cells to acidic pH and external pressure lead to re-activation of pluripotency genes in mature cells. These findings indicate that mature cells may be reprogrammed with manipulation of environmental conditions

The AARTFAAC Cosmic Explorer:Observations of the 21-cm power spectrum in the EDGES absorption trough

The 21-cm absorption feature reported by the EDGES collaboration is several times stronger than that predicted by traditional astrophysical models. If genuine, a deeper absorption may lead to stronger fluctuations on the 21-cm signal on degree scales (up to 1~Kelvin in rms), allowing these fluctuations to be detectable in nearly 50~times shorter integration times compared to previous predictions. We commenced the "AARTFAAC Cosmic Explorer" (ACE) program, that employs the AARTFAAC wide-field imager, to measure or set limits on the power spectrum of the 21-cm fluctuations in the redshift range $z = 17.9-18.6$ ($\Delta\nu = 72.36-75.09$~MHz) corresponding to the deep part of the EDGES absorption feature. Here, we present first results from two LST bins: 23.5-23.75h and 23.5-23.75h, each with 2~h of data, recorded in `semi drift-scan' mode. We demonstrate the application of the new ACE data-processing pipeline (adapted from the LOFAR-EoR pipeline) on the AARTFAAC data. We observe that noise estimates from the channel and time-differenced Stokes~$V$ visibilities agree with each other. After 2~h of integration and subtraction of bright foregrounds, we obtain $2\sigma$ upper limits on the 21-cm power spectrum of $\Delta_{21}^2 < (8139~\textrm{mK})^2$ and $\Delta_{21}^2 < (8549~\textrm{mK})^2$ at $k = 0.144~h\,\textrm{cMpc}^{-1}$ for the two LST bins. Incoherently averaging the noise bias-corrected power spectra for the two LST bins yields an upper limit of $\Delta_{21}^2 < (7388~\textrm{mK})^2$ at $k = 0.144~h\,\textrm{cMpc}^{-1}$. These are the deepest upper limits thus far at these redshifts.Comment: 16 pages, 10 figures, accepted for publication in MNRA

The first power spectrum limit on the 21-cm signal of neutral hydrogen during the Cosmic Dawn at z=20-25 from LOFAR

Observations of the redshifted 21-cm hyperfine line of neutral hydrogen from early phases of the Universe such as Cosmic Dawn and the Epoch of Reionization promise to open a new window onto the early formation of stars and galaxies. We present the first upper limits on the power spectrum of redshifted 21-cm brightness temperature fluctuations in the redshift range z = 19.8-25.2 (54-68 MHz frequency range) using 14 h of data obtained with the LOFAR-Low Band Antenna (LBA) array. We also demonstrate the application of a multiple pointing calibration technique to calibrate the LOFAR-LBA dual-pointing observations centred on the North Celestial Pole and the radio galaxy 3C220.3, We observe an unexplained excess of similar to 30-50 per cent in Stokes / noise compared to Stokes V for the two observed fields, which decorrelates on greater than or similar to 12 s and might have a physical origin. We show that enforcing smoothness of gain errors along frequency direction during calibration reduces the additional variance in Stokes I compared Stokes V introduced by the calibration on sub-band level. After subtraction of smooth foregrounds, we achieve a 2 sigma upper limit on the 21-cm power spectrum of Delta(2)(21) &lt;(14561 mK)(2) at k similar to 0.038 h cMpc(-1) and Delta(2)(21) &lt;(14886 mK)(2) at k 0.038 h cMpc(-1) for the 3C220 and NCP fields respectively and both upper limits are consistent with each other. The upper limits for the two fields are still dominated by systematics on most k modes.</p