In this paper, we propose a novel method using artificial neural networks(ANNs) to reconstruct the global 21 cm signal from measurements of the 21 cm power spectrum. The 21 cm global signal provides crucial informatio...In this paper, we propose a novel method using artificial neural networks(ANNs) to reconstruct the global 21 cm signal from measurements of the 21 cm power spectrum. The 21 cm global signal provides crucial information on cosmic evolution from the Dark Ages through cosmic dawn and the Epoch of Reionization. Single-dish telescopes directly measure the global signal, whereas interferometric experiments primarily measure spatial fluctuations,represented by the 21 cm power spectrum. While no direct mathematical relationship exists between these two observables-since they probe fundamentally independent Fourier modes-they are indirectly linked through their common dependence on underlying astrophysical and cosmological parameters. The ANN effectively learns this implicit, model-dependent relationship, enabling it to predict the global signal from the power spectrum. We demonstrate that the ANN accurately recovers the global 21 cm signal across a broad redshift range(z = 7.5–35)even under realistic observational noise scenarios corresponding to SKA-1 observations. The reconstruction accuracy depends significantly on the spatial scales(wavenumber k) included, with larger-scale modes yielding better results due to their stronger sensitivity to global astrophysical processes. Although the ANN method does not provide a model-independent verification of anomalous observations(e.g., the Experiment to Detect the Global Epoch of Reionization Signature absorption trough), it offers a computationally efficient and robust tool to infer the global signal within the context of standard astrophysical and cosmological models.展开更多
Detection and mitigation of radio frequency interference （RFI） is the first and also the key step for data processing in radio observations, especially for ongoing low frequency radio experiments towards the detecti...Detection and mitigation of radio frequency interference （RFI） is the first and also the key step for data processing in radio observations, especially for ongoing low frequency radio experiments towards the detection of the cosmic dawn and epoch of reionization （EoR）. In this paper we demonstrate the tech- nique and efficiency of RFI identification and mitigation for the 21 Centimeter Array （21CMA）, a radio interferometer dedicated to the statistical measurement of EoR. For terrestrial, man-made RFI, we concen- trate mainly on a statistical approach by identifying and then excising non-Gaussian signatures, in the sense that the extremely weak cosmic signal is actually buried under thermal and therefore Gaussian noise. We also introduce the so-called visibility correlation coefficient instead of conventional visibility, which allows a further suppression of rapidly time-varying RFI. Finally, we briefly discuss removals of the sky RFI, the leakage of sidelobes from off-field strong radio sources with time-invariant power and a featureless spec- trum. It turns out that state of the art technique should allow us to detect and mitigate RFI to a satisfactory level in present low frequency interferometer observations such as those acquired with the 21CMA, and the accuracy and efficiency can be greatly improved with the employment of low-cost, high-speed computing facilities for data acquisition and processing.展开更多
Extracting the neutral hydrogen （HI） signal is a great challenge for cosmological 21 cm experiments; both the astrophysical foregrounds and receiver noise are typically several orders of magnitude greater than the 2...Extracting the neutral hydrogen （HI） signal is a great challenge for cosmological 21 cm experiments; both the astrophysical foregrounds and receiver noise are typically several orders of magnitude greater than the 21 cm signal. However, the different properties of the 21 cm signal, foreground and noise can be exploited to separate these components. The foregrounds are generally smooth or correlated over the frequency space along a line of sight （LOS）, while both the 21 cm signal and noise vary stochastically along the same LoS. The foreground can be removed by filtering out the smooth component in frequency space. The receiver noise is basically uncorrelated for observations at different times, hence for surveys it is also uncorrelated in different directions, while the 21 cm signal, which traces the large scale structure, is correlated up to certain scales. In this exercise, we apply Wiener filters in frequency and angular space to extract the 21 cm signals. We found that the method works well. Inaccurate knowl- edge about the beam could degrade the reconstruction, but the overall result is still good, showing that the method is fairly robust.展开更多
We present a conceptual design study of external calibrators in the 21 cm experiment towards detecting the globally averaged radiation of the epoch of reionization(EoR).Employment of external calibrator instead of int...We present a conceptual design study of external calibrators in the 21 cm experiment towards detecting the globally averaged radiation of the epoch of reionization(EoR).Employment of external calibrator instead of internal calibrator commonly used in current EoR experiments allows removing instrumental effects such as beam pattern,receiver gain and instability of the system if the conventional three-position switch measurements are implemented in a short time interval.Furthermore,in the new design the antenna system is placed in an underground anechoic chamber with an open/closing ceiling to maximally reduce the environmental effect such as RFI and ground radiation/reflection.It appears that three of the four external calibrators proposed in this paper,including two indoor artificial transmitters and one outdoor celestial radiation(the Galactic polarization),fail to meet our purpose.Diurnal motion of the Galactic diffuse emission turns out to be the most probable source as an external calibrator,for which we have discussed the observational strategy and the algorithm of extracting the EoR signal.展开更多
The bubble size distribution of ionized hydrogen regions probes information about the morphology of HⅡbubbles during reionization.Conventionally,the HⅡbubble size distribution can be derived from the tomographic ima...The bubble size distribution of ionized hydrogen regions probes information about the morphology of HⅡbubbles during reionization.Conventionally,the HⅡbubble size distribution can be derived from the tomographic imaging data of the redshifted 21 cm signal from the epoch of reionization,which,however,is observationally challenging even for upcoming large radio interferometer arrays.Given that these interferometers promise to measure the 21 cm power spectrum accurately,we propose a new method,which is based on artificial neural networks,to reconstruct the HⅡbubble size distribution from the 21 cm power spectrum.We demonstrate that reconstruction from the 21 cm power spectrum can be almost as accurate as being directly measured from the imaging data with fractional error■10%,even with thermal noise at the sensitivity level of the Square Kilometre Array.Nevertheless,the reconstruction implicitly exploits the modeling in reionization simulations,and hence the recovered HⅡbubble size distribution is not an independent summary statistic from the power spectrum,and should be used only as an indicator for understanding HⅡbubble morphology and its evolution.展开更多
Observations are beginning to constrain the history of the epoch of reionization(EoR).Modeling the reionization process is indispensable to interpret the observations,to infer the properties of ionizing sources,and to...Observations are beginning to constrain the history of the epoch of reionization(EoR).Modeling the reionization process is indispensable to interpret the observations,to infer the properties of ionizing sources,and to probe the various astrophysical processes from the observational data.Here we present an improved version of the seminumerical simulation islandFAST,by incorporating inhomogeneous recombinations and a corresponding inhomogeneous ionizing background,and simulate the reionization process of neutral islands during the late EoR.We find that the islands are more fragmented in models with inhomogeneous recombinations than the case with a homogeneous recombination number.In order to investigate the effects of basic assumptions in the reionization modeling,we compare the results from islandFAST with those from 21cmFAST for the same assumptions on the ionizing photon sources and sinks,to find how the morphology of the ionization field and the reionization history depend on the different treatments of these two models.Such systematic bias should be noted when interpreting the upcoming observations.展开更多
The cross-correlation between the high-redshift 21 cm background and the Soft X-ray Background （SXB） of the Universe may provide an additional probe of the Epoch of Reionization. Here we use semi-numerical simulatio...The cross-correlation between the high-redshift 21 cm background and the Soft X-ray Background （SXB） of the Universe may provide an additional probe of the Epoch of Reionization. Here we use semi-numerical simulations to create 21 cm and soft X-ray intensity maps and construct their cross power spectra. Our results indicate that the cross power spectra are sensitive to the thermal and ionizing states of the intergalactic medium （IGM）. The 21 cm background correlates positively to the SXB on large scales during the early stages of the reionization. However as the reionization develops, these two back- grounds turn out to be anti-correlated with each other when more than - 15% of the IGM is ionized in a warm reionization scenario. The anti-correlated power reaches its maximum when the neutral fraction declines to 0.2-0.5. Hence, the trough in the cross power spectrum might be a useful tool for tracing the growth of HII regions during the middle and late stages of the reionization. We estimate the detectability of the cross power spectrum based on the abilities of the Square Kilometre Array and the Wide Field X-ray Telescope （WFXT）, and find that to detect the cross power spectrum, the pixel noise of X-ray images has to be at least 4 orders of magnitude lower than that of the WFXT deep survey.展开更多
Detecting primordial fluctuations from the cosmic dark ages requires extremely large low-frequency radio telescope arrays deployed on the far side of the Moon.The antenna of such an array must be lightweight,easily st...Detecting primordial fluctuations from the cosmic dark ages requires extremely large low-frequency radio telescope arrays deployed on the far side of the Moon.The antenna of such an array must be lightweight,easily storable and transportable,deployable on a large scale,durable,and capable of good electrical performance.A membrane antenna is an excellent candidate to meet these criteria.We study the design of a low-frequency membrane antenna for a lunar-based low-frequency(<30 MHz)radio telescope constructed from polyimide film widely used in aerospace applications,owing to its excellent dielectric properties and high stability as a substrate material.We first design and optimize an antenna in free space through dipole deformation and coupling principles,then simulate an antenna on the lunar surface with a simple lunar soil model,yielding an efficiency greater than 90%in the range of 12-19 MHz and greater than 10%in the range of 5-35 MHz.The antenna inherits the omni-directional radiation pattern of a simple dipole antenna in the 5-30 MHz frequency band,giving a large field of view and allowing detection of the 21 cm global signal when used alone.A demonstration prototype is constructed,and its measured electrical property is found to be consistent with simulated results using|S11|measurements.This membrane antenna can potentially fulfill the requirements of a lunar low-frequency array,establishing a solid technical foundation for future large-scale arrays for exploring the cosmic dark ages.展开更多
Quantitative assessment of natural internal variability and externally forced responses of Northern Hemisphere(NH)temperatures is necessary for understanding and attributing climate change signals during past warm and...Quantitative assessment of natural internal variability and externally forced responses of Northern Hemisphere(NH)temperatures is necessary for understanding and attributing climate change signals during past warm and cold periods.However,it remains a challenge to distinguish the robust internally generated variability from the observed variability.Here,largeensemble(70 member)simulations,Energy Balance Model simulation,temperature ensemble reconstruction,and three dominant external forcings(volcanic,solar,and greenhouse gas)were combined to estimate the internal variability of NH summer(June–August)temperatures over the past 2000 years(1–2000 CE).Results indicate that the Medieval Climate Anomaly was predominantly attributed to centennial-scale internal oscillation,accounting for an estimated 104%of the warming anomaly.In contrast,the Current Warm Period is influenced mainly by external forcing,contributing up to 90%of the warming anomaly.Internal temperature variability offsets cooling by volcanic eruptions during the Late Antique Little Ice Age.These findings have important implications for the attribution of past climate variability and improvement of future climate projections.展开更多
基金supported by the National SKA Program of China(No.2020SKA0110401)the National Natural Science Foundation of China(NSFC,grant No.12103044).
文摘In this paper, we propose a novel method using artificial neural networks(ANNs) to reconstruct the global 21 cm signal from measurements of the 21 cm power spectrum. The 21 cm global signal provides crucial information on cosmic evolution from the Dark Ages through cosmic dawn and the Epoch of Reionization. Single-dish telescopes directly measure the global signal, whereas interferometric experiments primarily measure spatial fluctuations,represented by the 21 cm power spectrum. While no direct mathematical relationship exists between these two observables-since they probe fundamentally independent Fourier modes-they are indirectly linked through their common dependence on underlying astrophysical and cosmological parameters. The ANN effectively learns this implicit, model-dependent relationship, enabling it to predict the global signal from the power spectrum. We demonstrate that the ANN accurately recovers the global 21 cm signal across a broad redshift range(z = 7.5–35)even under realistic observational noise scenarios corresponding to SKA-1 observations. The reconstruction accuracy depends significantly on the spatial scales(wavenumber k) included, with larger-scale modes yielding better results due to their stronger sensitivity to global astrophysical processes. Although the ANN method does not provide a model-independent verification of anomalous observations(e.g., the Experiment to Detect the Global Epoch of Reionization Signature absorption trough), it offers a computationally efficient and robust tool to infer the global signal within the context of standard astrophysical and cosmological models.
基金partially supported by the National Natural Science Foundation of China(Grant No.11433002)support by a Marsden Fund grant in New Zealand
文摘Detection and mitigation of radio frequency interference （RFI） is the first and also the key step for data processing in radio observations, especially for ongoing low frequency radio experiments towards the detection of the cosmic dawn and epoch of reionization （EoR）. In this paper we demonstrate the tech- nique and efficiency of RFI identification and mitigation for the 21 Centimeter Array （21CMA）, a radio interferometer dedicated to the statistical measurement of EoR. For terrestrial, man-made RFI, we concen- trate mainly on a statistical approach by identifying and then excising non-Gaussian signatures, in the sense that the extremely weak cosmic signal is actually buried under thermal and therefore Gaussian noise. We also introduce the so-called visibility correlation coefficient instead of conventional visibility, which allows a further suppression of rapidly time-varying RFI. Finally, we briefly discuss removals of the sky RFI, the leakage of sidelobes from off-field strong radio sources with time-invariant power and a featureless spec- trum. It turns out that state of the art technique should allow us to detect and mitigate RFI to a satisfactory level in present low frequency interferometer observations such as those acquired with the 21CMA, and the accuracy and efficiency can be greatly improved with the employment of low-cost, high-speed computing facilities for data acquisition and processing.
基金supported by the Ministry of Science and Technology(2016YFE0100300)the National Natural Science Foundation of China(Grant Nos.11473044,U1501501,U1631118 and 11633004)+1 种基金the Chinese Academy of Sciences(QYZDJ-SSW-SLH017)support by the CSC Cai Yuanpei grant
文摘Extracting the neutral hydrogen （HI） signal is a great challenge for cosmological 21 cm experiments; both the astrophysical foregrounds and receiver noise are typically several orders of magnitude greater than the 21 cm signal. However, the different properties of the 21 cm signal, foreground and noise can be exploited to separate these components. The foregrounds are generally smooth or correlated over the frequency space along a line of sight （LOS）, while both the 21 cm signal and noise vary stochastically along the same LoS. The foreground can be removed by filtering out the smooth component in frequency space. The receiver noise is basically uncorrelated for observations at different times, hence for surveys it is also uncorrelated in different directions, while the 21 cm signal, which traces the large scale structure, is correlated up to certain scales. In this exercise, we apply Wiener filters in frequency and angular space to extract the 21 cm signals. We found that the method works well. Inaccurate knowl- edge about the beam could degrade the reconstruction, but the overall result is still good, showing that the method is fairly robust.
基金supported by the Key Projects of Frontier Science of Chinese Academy of Sciences(Grant No.QYZDY-SSW-SLH022)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB23000000)the National Key R&D Program of China(Grant No.2018YFA0404601)。
文摘We present a conceptual design study of external calibrators in the 21 cm experiment towards detecting the globally averaged radiation of the epoch of reionization(EoR).Employment of external calibrator instead of internal calibrator commonly used in current EoR experiments allows removing instrumental effects such as beam pattern,receiver gain and instability of the system if the conventional three-position switch measurements are implemented in a short time interval.Furthermore,in the new design the antenna system is placed in an underground anechoic chamber with an open/closing ceiling to maximally reduce the environmental effect such as RFI and ground radiation/reflection.It appears that three of the four external calibrators proposed in this paper,including two indoor artificial transmitters and one outdoor celestial radiation(the Galactic polarization),fail to meet our purpose.Diurnal motion of the Galactic diffuse emission turns out to be the most probable source as an external calibrator,for which we have discussed the observational strategy and the algorithm of extracting the EoR signal.
基金supported by the National SKA Program of China(Grant No.2020SKA0110401)NSFC(Grant Nos.12103044,11821303 and 11850410429)+1 种基金National Key R&D Program of China(Grant No.2018YFA0404502)supported in part by grants from Yunnan University。
文摘The bubble size distribution of ionized hydrogen regions probes information about the morphology of HⅡbubbles during reionization.Conventionally,the HⅡbubble size distribution can be derived from the tomographic imaging data of the redshifted 21 cm signal from the epoch of reionization,which,however,is observationally challenging even for upcoming large radio interferometer arrays.Given that these interferometers promise to measure the 21 cm power spectrum accurately,we propose a new method,which is based on artificial neural networks,to reconstruct the HⅡbubble size distribution from the 21 cm power spectrum.We demonstrate that reconstruction from the 21 cm power spectrum can be almost as accurate as being directly measured from the imaging data with fractional error■10%,even with thermal noise at the sensitivity level of the Square Kilometre Array.Nevertheless,the reconstruction implicitly exploits the modeling in reionization simulations,and hence the recovered HⅡbubble size distribution is not an independent summary statistic from the power spectrum,and should be used only as an indicator for understanding HⅡbubble morphology and its evolution.
基金supported by the National Key R&D Program of China No.2018YFE0120800the National Natural Science Foundation of China grant No.11973047+1 种基金the National Key R&D Program of China No.2022YFF0504300the National SKA Program of China Nos.2020SKA0110401,and 2020SKA0110402。
文摘Observations are beginning to constrain the history of the epoch of reionization(EoR).Modeling the reionization process is indispensable to interpret the observations,to infer the properties of ionizing sources,and to probe the various astrophysical processes from the observational data.Here we present an improved version of the seminumerical simulation islandFAST,by incorporating inhomogeneous recombinations and a corresponding inhomogeneous ionizing background,and simulate the reionization process of neutral islands during the late EoR.We find that the islands are more fragmented in models with inhomogeneous recombinations than the case with a homogeneous recombination number.In order to investigate the effects of basic assumptions in the reionization modeling,we compare the results from islandFAST with those from 21cmFAST for the same assumptions on the ionizing photon sources and sinks,to find how the morphology of the ionization field and the reionization history depend on the different treatments of these two models.Such systematic bias should be noted when interpreting the upcoming observations.
基金supported by the National Natural Science Foundation of China(Grant Nos.11473031,11261140641 and 11173028)the 973 Program(Grant No.2013CB837900)
文摘The cross-correlation between the high-redshift 21 cm background and the Soft X-ray Background （SXB） of the Universe may provide an additional probe of the Epoch of Reionization. Here we use semi-numerical simulations to create 21 cm and soft X-ray intensity maps and construct their cross power spectra. Our results indicate that the cross power spectra are sensitive to the thermal and ionizing states of the intergalactic medium （IGM）. The 21 cm background correlates positively to the SXB on large scales during the early stages of the reionization. However as the reionization develops, these two back- grounds turn out to be anti-correlated with each other when more than - 15% of the IGM is ionized in a warm reionization scenario. The anti-correlated power reaches its maximum when the neutral fraction declines to 0.2-0.5. Hence, the trough in the cross power spectrum might be a useful tool for tracing the growth of HII regions during the middle and late stages of the reionization. We estimate the detectability of the cross power spectrum based on the abilities of the Square Kilometre Array and the Wide Field X-ray Telescope （WFXT）, and find that to detect the cross power spectrum, the pixel noise of X-ray images has to be at least 4 orders of magnitude lower than that of the WFXT deep survey.
基金We acknowledge the support of the National SKA program of China(2022SKA0110100,2022SKA0110101)the Natural Science Foundation of China(12273070,12203061,1236114814,12303004).
文摘Detecting primordial fluctuations from the cosmic dark ages requires extremely large low-frequency radio telescope arrays deployed on the far side of the Moon.The antenna of such an array must be lightweight,easily storable and transportable,deployable on a large scale,durable,and capable of good electrical performance.A membrane antenna is an excellent candidate to meet these criteria.We study the design of a low-frequency membrane antenna for a lunar-based low-frequency(<30 MHz)radio telescope constructed from polyimide film widely used in aerospace applications,owing to its excellent dielectric properties and high stability as a substrate material.We first design and optimize an antenna in free space through dipole deformation and coupling principles,then simulate an antenna on the lunar surface with a simple lunar soil model,yielding an efficiency greater than 90%in the range of 12-19 MHz and greater than 10%in the range of 5-35 MHz.The antenna inherits the omni-directional radiation pattern of a simple dipole antenna in the 5-30 MHz frequency band,giving a large field of view and allowing detection of the 21 cm global signal when used alone.A demonstration prototype is constructed,and its measured electrical property is found to be consistent with simulated results using|S11|measurements.This membrane antenna can potentially fulfill the requirements of a lunar low-frequency array,establishing a solid technical foundation for future large-scale arrays for exploring the cosmic dark ages.
基金This work was jointly funded by the National Natural Science Foundation of China(Grant Nos.41888101 and 42077406)the Strategic Priority Research Program of Chinese Academy of Sciences(No.XDB26020000)+1 种基金the Key Research Program of the Institute of Geology&Geophysics,CAS(No.IGGCAS-201905)Feng Shi is funded by the Youth Innovation Promotion Association CAS.
文摘Quantitative assessment of natural internal variability and externally forced responses of Northern Hemisphere(NH)temperatures is necessary for understanding and attributing climate change signals during past warm and cold periods.However,it remains a challenge to distinguish the robust internally generated variability from the observed variability.Here,largeensemble(70 member)simulations,Energy Balance Model simulation,temperature ensemble reconstruction,and three dominant external forcings(volcanic,solar,and greenhouse gas)were combined to estimate the internal variability of NH summer(June–August)temperatures over the past 2000 years(1–2000 CE).Results indicate that the Medieval Climate Anomaly was predominantly attributed to centennial-scale internal oscillation,accounting for an estimated 104%of the warming anomaly.In contrast,the Current Warm Period is influenced mainly by external forcing,contributing up to 90%of the warming anomaly.Internal temperature variability offsets cooling by volcanic eruptions during the Late Antique Little Ice Age.These findings have important implications for the attribution of past climate variability and improvement of future climate projections.
