Spin Temperature Research Articles

Mechanisms of Separation of the Elastic Step and the Thermal Step

AbstractThe elastic interaction causes a two‐step conversion from the low‐spin (LS) to the high‐spin state (HS) following a femtosecond laser pulse excitation. These steps occur on different timescales: a spin conversion (elastic step) in a short time due to pressure propagation and a late spin conversion (thermal step) resulting from heat diffusion. In this paper, we provide a brief review of models for spin crossover phenomena, emphasizing the role of the difference of degeneracy of the HS state and LS state and elastic interaction due to changes in local structures. We also review theoretical approaches to these effects. Additionally, to analyze the dynamical process of spin conversion after photo‐irradiation, it is necessary to consider mechanisms of heat transfer among spins. Recently, we proposed a two‐temperature model that incorporates both lattice and spin temperatures, which enables us to reproduce the two steps under an early uniformization of the lattice temperature, highlighting the importance of the timescales of these processes. Furthermore, we explore possible mechanisms to separate the two steps by examining the different timescales of processes of spin conversions by mechanical and entropic forces, and also by considering changes of local temperature due to spin conversion.

Emergent behaviors of two thermodynamically consistent inertial spin models

We propose two thermodynamically consistent inertial spin models generalizing the inertial spin model for the ensemble of mechanical particles with spin. For the collective dynamics modeling consistent with an entropy principle, we use two modeling ideas based on Hamiltonian formulation and flocking formalism for the coupling of the inertial spin model and temperatures introduced by Ha and Ruggeri [Arch. Ration. Mech. Anal. 223, 1397 (2017)]. The resulting models admit several constants of motions and are consistent with the entropy principle. We present several sufficient frameworks leading to the asymptotic vanishing of spins and velocity alignment. We also implement several numerical simulations and compare them with analytical results.

Measuring Bipartite Spin Correlations of Lattice-Trapped Dipolar Atoms.

We demonstrate a bipartition technique using a superlattice architecture to access correlations between alternating planes of a mesoscopic array of spin-3 chromium atoms trapped in a 3D optical lattice. Using this method, we observe that out-of-equilibrium dynamics driven by long-range dipolar interactions lead to spin anticorrelations between the two spatially separated subsystems. Our bipartite measurements reveal a subtle interplay between the anisotropy of the 3D dipolar interactions and that of the lattice structure, without requiring single-site addressing. We compare our results to theoretical predictions based on a truncated cumulant expansion and a new cluster semiclassical method that we use to investigate correlations at the microscopic scale. Comparison with a high-temperature analytical model reveals quantum thermalization at a high negative spin temperature.

Testing Bayesian inference of GRMHD model parameters from VLBI data

ABSTRACT Recent observations by the Event Horizon Telescope (EHT) of supermassive black holes M87* and Sgr A* offer valuable insights into their space–time properties and astrophysical conditions. Utilizing a library of model images ($\sim 2$ million for Sgr A*) generated from general-relativistic magnetohydrodynamic (GRMHD) simulations, limited and coarse insights on key parameters such as black hole spin, magnetic flux, inclination angle, and electron temperature were gained. The image orientation and black hole mass estimates were obtained via a scoring and an approximate rescaling procedure. Lifting such approximations, probing the space of parameters continuously, and extending the parameter space of theoretical models is both desirable and computationally prohibitive with existing methods. To address this, we introduce a new Bayesian scheme that adaptively explores the parameter space of ray-traced, GRMHD models. The general relativistic radiative transfer code IPOLE is integrated with the EHT parameter estimation tool THEMIS. The pipeline produces a ray-traced model image from GRMHD data, computes predictions for very long baseline interferometric (VLBI) observables from the image for a specific VLBI array configuration and compares to data, thereby sampling the likelihood surface via a Markov chain Monte Carlo scheme. At this stage we focus on four parameters: accretion rate, electron thermodynamics, inclination, and source position angle. Our scheme faithfully recovers parameters from simulated VLBI data and accommodates time-variability via an inflated error budget. We highlight the impact of intrinsic variability on model fitting approaches. This work facilitates more informed inferences from GRMHD simulations and enables expansion of the model parameter space in a statistically robust and computationally efficient manner.

Direction-dependent Effects on Global 21 cm Detection

Cosmic dawn represents a critical juncture in cosmic history when the first population of stars emerged. The astrophysical processes that govern this transformation need to be better understood. The detection of redshifted 21 cm radiation emitted from neutral hydrogen during this era offers a direct window into the thermal and ionization state of the Universe. This emission manifests as differential brightness between spin temperature and the cosmic microwave background. The SARAS experiment aims to detect the sky-averaged signal in the frequency range 40–200 MHz. SARAS’s unique design and operational strategy to float the antenna over a water body minimizes spectral features that may arise due to stratified ground beneath the antenna. However, the antenna environment can be prone to configuration changes due to variations in critical design parameters such as conductivity and antenna tilt. In this paper, we connect the variations in antenna properties to signal detection prospects. By using realistic simulations of a direction- and frequency-dependent radiation pattern of the SARAS antenna and its transfer function, we establish critical parameters and estimate bias in the detectability of different models of the global 21 cm signal. We find a correlation between the nature of chromaticity in antenna properties and the bias in the recovered spectral profiles of 21 cm signals. We also find stringent requirements for transfer function corrections, which can otherwise make detection prospects prohibitive. We finally explore a range of critical parameters that allow robust signal detection.

The Local Group L-band Survey: The First Measurements of Localized Cold Neutral Medium Properties in the Low-metallicity Dwarf Galaxy NGC 6822

Abstract Measuring the properties of the cold neutral medium (CNM) in low-metallicity galaxies provides insights into heating and cooling mechanisms in early Universe-like environments. We report detections of two localized atomic neutral hydrogen (H i) absorption features in NGC 6822, a low-metallicity (0.2 Z ⊙) dwarf galaxy in the Local Group. These are the first unambiguous CNM detections in a low-metallicity dwarf galaxy outside the Magellanic Clouds. The Local Group L-band Survey (LGLBS) enabled these detections, due to its high spatial (15 pc for H i emission) and spectral (0.4 km s−1) resolution. We introduce LGLBS and describe a custom pipeline for searching for H i absorption at high angular resolution and extracting associated H i emission. A detailed Gaussian decomposition and radiative transfer analysis of the NGC 6822 detections reveals five CNM components, with key properties: a mean spin temperature of 32 ± 6 K, a mean CNM column density of 3.1 × 1020 cm−2, and CNM mass fractions of 0.33 and 0.12 for the two sightlines. Stacking nondetections does not reveal low-level signals below our median optical depth sensitivity of 0.05. One detection intercepts a star-forming region, with the H i absorption profile encompassing the CO (2−1) emission, indicating coincident molecular gas and a depression in high-resolution H i emission. We also analyze a nearby sightline with deep, narrow H i self-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and Hα emissions suggests a close link between the colder, denser H i phase and star formation in NGC 6822.

Soft photon heating: a semi-analytic framework and applications to 21-cm cosmology

ABSTRACT The presence of an abundant population of low-frequency photons at high redshifts (such as a radio background) can source leading order effects on the evolution of the matter and spin temperatures through rapid free–free absorptions. This effect, known as soft photon heating, can have a dramatic impact on the differential brightness temperature, $\Delta T_{\rm b}$, a central observable in 21-cm cosmology. Here, we introduce a semi-analytic framework to describe the dynamics of soft photon heating, providing a simplified set of evolution equations and a useful numerical scheme which can be used to study this generic effect. We also perform quasi-instantaneous and continuous soft photon injections to elucidate the different regimes in which soft photon heating is expected to impart a significant contribution to the global 21-cm signal and its fluctuations. We find that soft photon backgrounds produced after recombination with spectral index $\gamma \gt 3.0$ undergo significant free–free absorption, and therefore this heating effect cannot be neglected. The effect becomes stronger with steeper spectral index, and in some cases the injection of a synchrotron-like spectrum ($\gamma = 3.6$) can suppress the amplitude of $\Delta T_{\rm b}$ relative to the standard model prediction (where an additional radio background is absent), making the global 21-cm signal even more difficult to detect in these scenarios.

Localized dopant motion across the 2D Ising phase transition

I investigate the motion of a single hole in 2D spin lattices with square and triangular geometries. While the spins have nearest neighbor Ising spin couplings JJ, the hole is allowed to move only in 1D along a single line in the 2D lattice with nearest neighbor hopping amplitude tt. The non-equilibrium hole dynamics is initialized by suddenly removing a single spin from the thermal Ising spin lattice. I find that for any nonzero spin coupling and temperature, the hole is localized. This is an extension of the thermally induced localization phenomenon [Phys. Rev. Res. 6, 023325 (2024)] to the case, where there is a phase transition to a long-range ordered ferromagnetic phase. The dynamics depends only on the ratio of the temperature to the spin coupling, k_BT / |J|kBT/|J|, and on the ratio of the spin coupling to the hopping J/tJ/t. I characterize these dependencies in great detail. In particular, I find universal behavior at high temperatures, common features for the square and triangular lattices across the Curie temperatures for ferromagnetic interactions, and highly distinct behaviors for the two geometries in the presence of antiferromagnetic interactions due geometric frustration in the triangular lattice.

Toward a Direct Measurement of the Cosmic Acceleration: The Pilot Observation of HI 21 cm Absorption Line at FAST

This study presents results on detecting neutral atomic hydrogen (H i) 21 cm absorption in the spectrum of PKS PKS1413+13 at redshift z = 0.24670041. The observation was conducted by FAST, with a spectral resolution of 10 Hz, using 10 minutes of observing time. The global spectral profile is examined by modeling the absorption line using a single Gaussian function with a resolution of 10 kHz within a 2 MHz bandwidth. The goal is to determine the rate of the latest cosmic acceleration by directly measuring the redshift evolution of the H i 21 cm absorption line with Hubble flow toward a common background quasar over a decade or longer time span. This will serve as a detectable signal generated by the accelerated expansion of the Universe at redshift z < 1, referred to as redshift drift ż or the SL effect. The measured H i gas column density in this DLA system is approximately equivalent to the initial observation value, considering uncertainties of the spin temperature of a spiral host galaxy. The high signal-to-noise ratio of 57, obtained at a 10 kHz resolution, strongly supports the feasibility of using the H i 21 cm absorption line in DLA systems to accurately measure the redshift drift rate at a precision level of around 10−10 per decade.

Effects of feedback-free starburst galaxies on the 21-cm signal and reionization history

ABSTRACT Different star formation models at Cosmic Dawn produce detectable signatures in the observables of upcoming 21-cm experiments. In this work, we consider the physical scenario of feedback-free starbursts (FFB), according to which the star formation efficiency (SFE) is enhanced in sufficiently massive haloes at early enough times, thus explaining the indication from the JWST for an excess of bright galaxies at $z \ge 10$. We model the contribution of FFBs to popII SFE and compute the impact these have on the 21-cm global signal and power spectrum. We show that FFBs affect the evolution of the brightness temperature and the 21-cm power spectrum, but they only have a limited effect on the neutral hydrogen fraction. We investigate how the observables are affected by changes in the underlying star formation model and by contribution from popIII stars. Finally, we forecast the capability of next-generation Hydrogen Epoch of Reionization Array (HERA) to detect the existence of FFB galaxies via power spectrum measurements. Our results show the possibility of a significant detection, provided that popII stars are the main drivers of lowering the spin temperature. Efficient popIII star formation will make the detection more challenging.

Mapper of the IGM spin temperature: instrument overview

ABSTRACT The observation of the global 21 cm signal produced by neutral hydrogen gas in the intergalactic medium (IGM) during the Dark Ages, Cosmic Dawn, and Epoch of Reionization requires measurements with extremely well-calibrated wideband radiometers. We describe the design and characterization of the Mapper of the IGM Spin Temperature (MIST), which is a new ground-based, single-antenna, global 21 cm experiment. The design of MIST was guided by the objectives of avoiding systematics from an antenna ground plane and cables around the antenna, as well as maximizing the instrument’s on-sky efficiency and portability for operations at remote sites. We have built two MIST instruments, which observe in the range 25–105 MHz. For the 21 cm signal, this frequency range approximately corresponds to redshifts 55.5 &amp;gt; z &amp;gt; 12.5, encompassing the Dark Ages and Cosmic Dawn. The MIST antenna is a horizontal blade dipole of 2.42 m in length, 60 cm in width, and 52 cm in height above the ground. This antenna operates without a metal ground plane. The instruments run on 12 V batteries and have a maximum power consumption of 17 W. The batteries and electronics are contained in a single receiver box located under the antenna. We present the characterization of the instruments using electromagnetic simulations and lab measurements. We also show sample sky measurements from recent observations at remote sites in California, Nevada, and the Canadian High Arctic. These measurements indicate that the instruments perform as expected. Detailed analyses of the sky measurements are left for future work.

Insights into the 21 cm field from the vanishing cross-power spectrum at the epoch of reionization

ABSTRACT The early stages of the epoch of reionization, probed by the 21 cm line, are sensitive to the detailed properties and formation histories of the first galaxies. We use 21cmfast and a simple, self-consistent galaxy model to examine the redshift evolution of the large-scale cross-power spectrum between the 21 cm field and line-emitting galaxies. A key transition in redshift occurs when the 21 cm field shifts from being positively correlated with the galaxy distribution to being negatively correlated. Importantly, this transition redshift is insensitive to the properties of the galaxy tracers but depends sensitively on the thermal and ionization histories traced through the 21 cm field. Specifically, we show that the transition occurs when both ionization fluctuations dominate over 21 cm spin temperature fluctuations and when the average spin temperature exceeds the temperature of the cosmic microwave background. We illustrate this with three different 21 cm models which have largely the same neutral fraction evolution but different heating histories. We find that the transition redshift has a scale dependence, and that this can help disentangle the relative importance of heating and ionization fluctuations. The best prospects for constraining the transition redshift occur in scenarios with late X-ray heating, where the transition occurs at redshifts as low as z ∼ 6–8. In our models, this requires high-redshift galaxy surveys with sensitivities of $\sim 10^{-18}~\rm erg\,s^{-1}\,cm^{-2}$ for optical lines and $\sim 10^{-19}~\rm erg\,s^{-1}\,cm^{-2}$ for far-infrared lines. Future measurements of the transition redshift can help discriminate between 21 cm models and will benefit from reduced systematics.

Unified understanding of the breakdown of thermal mixing dynamic nuclear polarization: The role of temperature and radical concentration

We reveal an interplay between temperature and radical concentration necessary to establish thermal mixing (TM) as an efficient dynamic nuclear polarization (DNP) mechanism. We conducted DNP experiments by hyperpolarizing widely used DNP samples, i.e., sodium pyruvate-1-13C in water/glycerol mixtures at varying nitroxide radical (TEMPOL) concentrations and microwave irradiation frequencies, measuring proton and carbon-13 spin temperatures. Using a cryogen consumption-free prototype-DNP apparatus, we could probe cryogenic temperatures between 1.5 and 6.5 K, i.e., below and above the boiling point of liquid helium. We identify two mechanisms for the breakdown of TM: (i) Anderson type of quantum localization for low radical concentration, or (ii) quantum Zeno localization occurring at high temperature. This observation allowed us to reconcile the recent diverging observations regarding the relevance of TM as a DNP mechanism by proposing a unifying picture and, consequently, to find a trade-off between radical concentration and electron relaxation times, which offers a pathway to improve experimental DNP performance based on TM.

Abstract 5018: Evaluation of whole blood fractionation conditions for optimal liquid biopsy NGS applications

Abstract “For research use only. Not for diagnostic procedures.” Introduction: Liquid biopsy is a non-invasive alternative to tissue biopsies for cancer profiling using cell-free total nucleic acids (cfTNA; including cell free DNA (cfDNA) and cell free RNA (cfRNA)), the biological analytes derived from tumor cells floating in blood. However, it is crucial to optimize the whole blood fractionation workflows, since limited access to a suitable centrifuge at blood collection sites poses a challenge to this critical fractionation step, causing delays in processing the blood, leading to decreased sensitivity of variant detection and increased occurrence of false positives. Current study shows a systematic comparison of the effects of 8 different blood fractionation conditions for spin speeds, temperatures and duration on the levels of cfTNA derived from K2-EDTA-derived blood from multiple healthy and non-small cell lung cancer (NSCLC) donors, using next generation sequencing (NGS). Methods: CfTNA from K2-EDTA derived plasma was isolated using the MagMAXTM Cell-Free Total Nucleic Acid kit and the Genexus™ Cell free total nucleic acid kit, utilizing the KingFisher TM Flex and GenexusTMpurification systems. Yield and integrity of cfTNA isolated was evaluated by Qubit™, Agilent Tapestation and quantitative real time PCR (qRT-PCR). Furthermore, the impact of these test conditions on quality control specifications of DNA and RNA was assessed by performing sequencing on an Ion TorrentTM GenexusTM sequencing instrument using the AmpliSeqTM HD target amplification assay, that reports sequence variations in DNA for 42 cancer related genes and RNA in 7 cancer related genes. Results: Results showed comparable performance for all the 8 blood fractionation conditions tested, for yield and cfDNA profiles compared to control condition, indicating that there is no significant impact of these conditions on either yield or integrity of the cfDNA isolated. In addition, NGS data also showed that on target reads and median molecular coverage for DNA are comparable for all the conditions tested. However, sequencing data demonstrated that one test condition showed significantly better performance for RNA quality control specifications in terms of RNA mapped reads and molecular coverage for process controls used in the assay, compared to all other conditions tested. Conclusion: Data suggests that one of the whole blood fractionation conditions tested impact the cfRNA recovery from plasma without negatively affecting the yield, and integrity of isolated cfDNA and shows increased sensitivity for RNA process controls in NGS. Citation Format: MADHU JASTI, Sergio Hernandez, Nick Siepert, Lokesh Kodandaramaswamy, Thilanka Jayaweera, Jeoffrey Schageman, Angie Cheng, Susanne Lopez, Miao Der Chen. Evaluation of whole blood fractionation conditions for optimal liquid biopsy NGS applications [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5018.

The temperature of the neutral interstellar medium in the Galaxy

ABSTRACT Atomic Hydrogen-21 cm transition (H i) is an excellent tracer to study and understand the properties of the atomic gas in the Galaxy. Using the Westerbork Synthesis Radio Telescope, we observed 12 quasar sightlines to detect galactic H i in absorption. We achieve an optical depth rms of ∼1−2 × 10−3, essential to detect the warm neutral medium (WNM). We detect H i absorption in all our sightlines except along 1006+349, for which we set a strict upper limit on the spin temperature as 〈Ts〉 &amp;gt; 570 K. We find around 50 per cent of our sightlines have 〈Ts〉 &amp;gt; 500 K, indicating a WNM dominance. Further, we calculate an upper limit of the cold neutral medium (CNM) fraction along our sightlines and find a median CNM fraction of ∼0.12. With our observations, we reconfirm the existence of a threshold column density of ∼2 × 1020$\rm atoms \, cm^{-2}$ to form CNM in the interstellar medium (ISM). Using a two-temperature model of the H i disc, we explore the distribution of spin temperature in the Galactic ISM. We find that a simple fixed axisymmetric two-temperature model could not produce either the observed column density or the integral optical depth. This indicates the existence of a more complex distribution of spin temperatures in the Galaxy.

Optical spectroscopy of comets using Hanle Echelle Spectrograph (HESP)

ABSTRACT Observing the vibrational/rotational lines in a comet’s optical spectrum requires high-resolution spectroscopy, as they are otherwise seen as a blended feature. To achieve this, we have obtained medium and high-resolution (R (λ/Δλ) = 30 000 and 60 000) spectra of several comets, including C/2015 V2 (Johnson), 46P/Wirtanen, 41P/Tuttle–Giacobini–Kresák, and 38P/Stephan–Oterma, using the Hanle Echelle Spectrograph (HESP) mounted on the 2-m Himalayan Chandra Telescope (HCT) in India. The spectra effectively cover the wavelength range 3700–10 000 Å, allowing us to probe the various vibrational bands and band sequences to identify the rotational lines in the cometary molecular emission. We were also able to separate the cometary Oxygen lines from the telluric lines and analyse the green-to-red (G/R) forbidden oxygen [O i] ratios in a few comets. For comets C/2015 V2, 46P, and 41P, the computed G/R ratios, 0.04 ± 0.01, 0.04 ± 0.01, and 0.08 ± 0.02, respectively, point to H2O being a major source of Oxygen emissions. Notably, in the second fibre pointing at a location 1000 km away from the photocentre of comet 46P, the G/R ratio reduced by more than half the value observed in the first fibre, indicating the effects of quenching within the inner coma. We also measured the NH2 ortho-to-para ratio of comet 46P to be about 3.41 ± 0.05 and derived an ammonia ratio of 1.21 ± 0.03 corresponding to a spin temperature of ∼26 K. With these, we present the results of the study of four comets from different cometary reservoirs using medium and high-resolution optical spectroscopy, emphasizing the capabilities of the instrument for future cometary studies.

Caustics and velocity caustics in the diffuse interstellar medium at high Galactic latitudes

Context. The far-infrared (FIR) distribution at high Galactic latitudes, observed with Planck, is filamentary with coherent structures in polarization. These structures are also closely related to H I filaments with coherent velocity structures. There is a long-standing debate about the physical nature of these structures. They are considered either as velocity caustics, fluctuations engraved by the turbulent velocity field or as cold three-dimensional density structures in the interstellar medium (ISM). Aims. We discuss different approaches to data analysis and interpretation in order to work out the differences. Methods. We considered mathematical preliminaries for the derivation of caustics that characterize filamentary structures in the ISM. Using the Hessian operator, we traced individual FIR filamentary structures in H I from channel maps as observed and alternatively from data that are provided by the velocity decomposition algorithm (VDA). VDA is claimed to separate velocity caustics from density effects. Results. Based on the strict mathematical definition, the so-called velocity caustics are not actually caustics. These VDA data products may contain caustics in the same way as the original H I observations. Caustics derived by a Hessian analysis of both databases are nearly identical with a correlation coefficient of 98%. However, the VDA algorithm leads to a 30% increase in the alignment uncertainties when fitting FIR/H I orientation angles. Thus, the VDA velocity crowding concept fails to explain the alignment of FIR/H I filaments at |b| &gt; 20°. We used H I absorption data to constrain the physical nature of FIR/H I filaments and determine spin temperatures and volume densities of FIR/H I filaments. H I filaments exist as cold neutral medium (CNM) structures; outside the filaments no CNM absorption is detectable. Conclusions. The CNM in the diffuse ISM is exclusively located in filaments with FIR counterparts. These filaments at high Galactic latitudes exist as cold density structures; velocity crowding effects are negligible.

The depletion of star-forming gas by AGN activity in radio sources

Abstract Cold, neutral interstellar gas, the reservoir for star formation, is traced through the absorption of the 21-cm continuum radiation by neutral hydrogen (H i). Although detected in one hundred cases in the host galaxies of distant radio sources, only recently have column densities approaching the maximum value observed in Lyman- $\alpha$ absorption systems ( $N_{{\textrm{H}\,\scriptsize{\textrm{I}}}}\sim 10^{22}$ $\textrm{cm}^{-2}$ ) been found. Here, we explore the implications these have for the hypothesis that the detection rate of H i absorption is dominated by photo-ionisation from the active galactic nucleus (AGN). We find, with the addition all of the current searches for H i absorption at $z\geq0.1$ , a strong correlation between the H i absorption strength and the ionising photon rate, with the maximum value at which H i is detected remaining close to the theoretical value in which all of the neutral gas would be ionised in a large spiral galaxy ( $Q_{{\textrm{H}\,\scriptsize{\textrm{I}}}} = 2.9\times10^{56}$ ionising photons s $^{-1}$ ). We also rule out other effects (excitation by the radio continuum and changing gas properties) as the dominant cause for the decrease in the detection rate with redshift. Furthermore, from the maximum theoretical column density, we find that the five high column density systems have spin temperatures close to those of the Milky Way ( $T_{\textrm{spin}} \lesssim 300$ K), whereas, from our model of a gaseous galactic disc, the H i detection at $Q_{{\textrm{H}\,\scriptsize{\textrm{I}}}} =2.9\times10^{56}$ s $^{-1}$ yields $T_{\textrm{spin}}\sim10\,000$ K, consistent with the gas being highly ionised.

Simulating the Detection of the Global 21 cm Signal with MIST for Different Models of the Soil and Beam Directivity

The Mapper of the IGM Spin Temperature (MIST) is a new ground-based, single-antenna, radio experiment attempting to detect the global 21 cm signal from the Dark Ages and Cosmic Dawn. A significant challenge in this measurement is the frequency dependence, or chromaticity, of the antenna beam directivity. MIST observes with the antenna above the soil and without a metal ground plane, and the beam directivity is sensitive to the electrical characteristics of the soil. In this paper, we use simulated observations with MIST to study how the detection of the global 21 cm signal from Cosmic Dawn is affected by the soil and the MIST beam directivity. We simulate observations using electromagnetic models of the directivity computed for single- and two-layer models of the soil. We test the recovery of the Cosmic Dawn signal with and without beam chromaticity correction applied to the simulated data. We find that our single-layer soil models enable a straightforward recovery of the signal even without chromaticity correction. Two-layer models increase the beam chromaticity and make the recovery more challenging. However, for the model in which the bottom soil layer has a lower electrical conductivity than the top layer, the signal can be recovered even without chromaticity correction. For the other two-layer models, chromaticity correction is necessary for the recovery of the signal, and the accuracy requirements for the soil parameters vary between models. These results will be used as a guideline to select observation sites that are favorable for the detection of the Cosmic Dawn signal.

Distinguishing the impact and signature of black holes from different origins in early cosmic history

ABSTRACT We semi-analytically model the effects of primordial black hole (PBH) accretion on the cosmic radiation background during the epoch of reionization (z ≳ 6). PBHs in the intergalactic medium (IGM) and haloes, where star formation can occur, are considered. For stars with a mass $\gtrsim 25 \rm \ {\rm M}_{\odot }$, formed in suitable host haloes, we assume they quickly burn out and form stellar-remnant black holes (SRBHs). These SRBHs, like PBHs, also accrete material, and are considered to have similar radiation feedback in the halo environment. To improve the background radiation estimation, we consider the impact of PBHs on structure formation, allowing a modified halo mass function. We consider the radiation feedback from a broad suite of black holes: PBHs, SRBHs, high-mass X-ray binaries, and supermassive black holes. The result shows that at z ≳ 30, the radiation background energy density is generated by PBHs accreting in the IGM, whereas at lower redshifts, halo accretion dominates. We also dissect the total power density by modelling the accretion spectral energy distribution across different wavebands. In the UV band, we find that for fPBH ≲ 10−3, the H-ionizing and Lyman-α fluxes from PBH accretion feedback do not violate existing constraints on the timing of reionization, and on the effective Wouthuysen–Field coupling of the 21-cm spin temperature of neutral hydrogen to the IGM kinetic temperature. However, in the X-ray band, with the same abundance, PBHs contribute significantly and could explain the unresolved part of the cosmic X-ray background.