Cross-correlations between 21 cm, X-ray and infrared backgrounds

The history of the cosmological reionization is still unclear. Two ionizing sources, stars and QSOs, are believed to play important roles during this epoch. Besides the 21 cm signals, the infrared emission from Pop Ⅲ stars and X-ray photons from QSOs can be powerful probes of the reionization. Here we present a cross-correlation study of the 21 cm, infrared and X-ray backgrounds. The advantage of doing such cross-correlations is that we could highlight the correlated signals and eliminate irrelevant fore-grounds. We develop a shell model to describe the 21 cm signals and find that PopⅢ stars can provide higher 21 cm signals than QSOs. Using the ROSAT data for X-ray and AKARI data for infrared, we predict various cross power spectra analytically and dis-cuss prospects for detecting these cross-correlation signals in future low frequency radio surveys. We find that, although these cross-correlational signals have distinct features, so far, they have been difficult to detect due to the high noise of the soft X-ray and infrared backgrounds given by ROSAT and AKARI.     

Spike emission at 21 cm wavelength in post-flare loops

Electron-cyclotron maser instability produced by accelerated energetic electrons is today's most favoured process for spike emission on ultra-short time scales at dm wavelengths. The process of electron acceleration occurs primarily in pre-impulsive and impulsive phases. We present observations of spike bursts at 21 cm in a limb event that occurred in AR 5629 on 1989 August 17. Optical observations of the event from Beijing Astronomical Observatory show coronal loops that interact just before the spike groups appeared. This implies that additional particle acceleration might be produced by the interaction between emerging loops and original large loops, and become the source of the energetic electrons responsible for the spike emission appearing in the descending phase. Some important parameters are deduced.Paper presented at the 4th CESRA workshop in Ouranopolis (Greece) 1991.     

Foreground removal of 21 cm fluctuation with multifrequency fitting

The 21 centimeter (21 cm) line emission from neutral hydrogen in the inter-galactic medium (IGM) at high redshifts is strongly contaminated by foreground sources such as the diffuse Galactic synchrotron emission and free-free emission from the Galaxy, as well as emission from extragalactic radio sources, thus making its observation very complicated. However, the 21 cm signal can be recovered through its structure in fre-quency space, as the power spectrum of the foreground contamination is expected to be smooth over a wide band in frequency space while the 21 cm fluctuations vary signifi-cantly. We use a simple polynomial fitting to reconstruct the 21 cm signal around four frequencies 50, 100, 150 and 200 MHz with an especially small channel width of 20 kHz. Our calculations show that this multifrequency fitting approach can effectively recover the 21 cm signal in the frequency range 100 ～ 200 MHz. However, this method doesn't work well around 50 MHz because of the low intensity of the 21 cm signal at this frequency. We also show that the fluctuation of detector noise can be suppressed to a very low level by taking long integration times, which means that we can reach a sensitivity of ≈ 10 mK at 150 MHz with 40 antennas in 120 hours of observations.     

Foreground removal of 21 cm fluctuation with multifrequency fitting

The 21 centimeter （21 cm） line emission from neutral hydrogen in the intergalactic medium （IGM） at high redshifts is strongly contaminated by foreground sources such as the diffuse Galactic synchrotron emission and free-free emission from the Galaxy, as well as emission from extragalactic radio sources, thus making its observation very complicated. However, the 21 cm signal can be recovered through its structure in frequency space, as the power spectrum of the foreground contamination is expected to be smooth over a wide band in frequency space while the 21 cm fluctuations vary significantly. We use a simple polynomial fitting to reconstruct the 21 cm signal around four frequencies 50, 100, 150 and 200 MHz with an especially small channel width of 20 kHz. Our calculations show that this multifrequency fitting approach can effectively recover the 21 cm signal in the frequency range 100 - 200 MHz. However, this method doesn＇t work well around 50 MHz because of the low intensity of the 21 cm signal at this frequency. We also show that the fluctuation of detector noise can be suppressed to a very low level by taking long integration times, which means that we can reach a sensitivity of ≈ 10 mK at 150 MHz with 40 antennas in 120 hours of observations.     

高红移类星体的21cm信号研究

利用高精度数值模拟的结果修正了高红移类星体(QSO)电离和加热中性气体的理论模拟,并预测了500m口径球面射电望远镜(Five-hundred-meter Aperture Spherical Telescope, FAST)观测QSO周围21 cm辐射的信号特征以及信噪比,得出如下结论:(1)红移z=8且光度与ULAS J1120+0641一致的QSO在FAST望远镜视场下的21 cm频率谱有完整的HII区,但在z=10且较低QSO光度下的21 cm频率谱仅有一个小的凹陷;(2)光子有限旅行时间(Finite Light Travel Time, FLTT)效应明显改变高红移QSO的21 cm频率谱,使得较低频率端的过渡曲线明显比较高频率端陡;(3)使用FAST望远镜观测高红移QSO的21 cm辐射的信噪比非常高,最高可达～12,因此可以很好地识别HII区和FLTT效应.     

Imaging the Sun at 21 cm: Budgetting the S-component

On fourteen days in July and August 1992 and June 1993, we used the 7-element synthesis radio telescope at the Dominion Radio Astrophysical Observatory to make full-disc, arc-min resolution images of the Sun at 21 cm, with the objective of budgetting the contributions to the slowly-varying component of solar radio emission. This instrument has the advantage that the mapping field at this wavelength is about 2.5° wide. However, it has also the severe disadvantage that with only 12 hours to record each image, the brightness distribution is severely undersampled. This difficulty, along with solar rotation and declination motion during each observation, required development of special image correction procedures.The two observing sessions spanned about 25% of the activity range over a solar cycle. Over this range, comparable contributions to the slowly-varying component came from active region sources and a weak emission, widely-distributed over the solar disk. Both these contributions are correlated with the total photospheric magnetic flux and with the 10.7 cm flux.     

21cm波段太阳爆发中的精细结构

本文简要介绍了云南天文台新建立的21cm波段快速(不中断采样)记录系统,及使用该系统观测到的快速尖峰事件。系统得到的太阳分米波段爆发中不同时间尺度的精细结构与Stu-rroch等提出的太阳耀斑中能量释放的模式相一致。     

Study on 21 cm Signal of High Redshift Quasars

We use the results of high resolution numerical simulations to fix the theoretical model of high redshift quasars (QSOs) ionizing and heating neutral gas, and predict the signal characteristics and signal/noise (S/N) ratios for Five-hundred-meter Aperture Spherical Telescope (FAST) to observe the 21 cm radiation around QSO. Our conclusions are summarized as: (1) the 21 cm frequency spectrum under the FAST field has full HII region for the QSO at $z=8$ and with luminosity similar to ULAS J1120+0641, while has only a small trough for the QSO at $z=10$ and with low luminosity; (2) the effect of Finite Light Travel Time (FLTT) obviously changes the 21 cm frequency spectrum of high redshift QSOs, makes the transition curve at lower frequency much steeper than that at higher frequency; (3) the FAST telescope can observe the 21 cm radiation of high redshift QSO with very high S/N ratio as high as $～12$, thus can identify the HII regions and the FLTT effect.     

Particle decay in the early Universe: predictions for 21 cm

The influence of ultra-high energy cosmic rays (UHECRs) and decaying dark matter particles on the emission and absorption characteristics of neutral hydrogen in 21 cm at redshifts   z = 10–50  is considered. In the presence of UHECRs, 21 cm can be seen in absorption with the brightness temperature   T _b=−(5–10) mK  in the range   z = 10–30  . Decaying particles can stimulate a 21-cm signal in emission with   T _b∼ 50–60 mK  at   z = 50  and   T _b≃ 10 mK  at   z ∼ 20  . Characteristics of the fluctuations of the brightness temperature, in particular its power spectrum, are also calculated. The maps of the power spectrum of the brightness temperature on the plane wavenumber redshift are shown to be sensitive to the parameters of UHECRs and decaying dark matter. Observational possibilities to detect manifestations of UHECRs and/or decaying particles in 21 cm with the future radio telescopes (LOFAR, 21CMA and SKA), and to distinguish contributions from them, are briefly discussed.     

Cross-correlation of 21 cm and soft X-ray backgrounds during the epoch of reionization

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 backgrounds 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.     

Brightness distribution at λ = 3 and 21 cm near the solar north pole

The brightness distribution of the Sun near the north pole at = 3 and 21 cm is obtained using the results of the partial eclipse of February 25, 1971. The solution obtained by expanding B (r) in orthogonal polynomials does not show any brightening at either frequency, but a sensible darkening. If this result is explained by the presence of spicules, the coronal emission must be negligible with respect to the emission coming from the transition region. This is possible, at = 21 cm, only if we assume the presence of a coronal hole near the pole.     

Fine structures in solar radio bursts at a 21 cm wavelength and pulsation modulation

The solar burst of 13 July, 1986 at 21 cm wavelength was recorded with a time constant of 8 ms. In the course of the burst lasting for about 40 min there appeared distinct stages of the burst's evolution. They consisted of the pattern of energy release in flares which was proposed by Sturrock et al. (1984). There were pulsations with periodicities of 0.178 and 4 s superimposed on the flux density. The pulsations were quasiperiodical with features of almost unchanged mean periods. The relative amplitude of the pulsation modulation changed with the phase of the burst; in general, it reached 10–20% in the rising phase. The possible mechanisms of pulsations are discussed and some plasma parameters of oscillation sources are deduced.     

GMRT observations of interstellar clouds in the 21cm line of atomic hydrogen

Nearby interstellar clouds with high (|ν|≥10km s⁻¹) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption) in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds. The mean optical depth of these detections is ∼0.09 and FWHM is ∼10km s⁻¹, consistent with absorption arising from CNM clouds.     

21cm波段ms级太阳射电爆发形态分析

本文利用10米天线上21cmms级连续无间隙快速记录系统,在第22周太阳峰年期间观测到大量的分米波段的太阳射电尖峰辐射的快速活动资料,对这些资料进行分析、比对发现尖峰辐射的快速活动至少包括：毛刺型、缓变型、缓变脉冲型、脉冲型、开关型．同时也对太阳射电尖峰辐射与其它共生现象进行分析,并对尖峰辐射的形态进行了简单的讨论．     

S-Component Sources at 21 cm Wavelength in the Rising Phase of Cycle 23

The Synthesis Radio Telescope at the Dominion Radio Astrophysical Observatory is being used to make a long-term study of the relationship between sources of the slowly-varying component at 21-cm wavelength and the active regions with which they are associated. Although the arc-minute angular resolution of the radio telescope is insufficient to map individual active regions, the unique capability of this radio telescope to map the whole solar disc in a single operation makes it useful for identifying and measuring sources of the slowly-varying component, which at this wavelength are largely due to free–free thermal emission from plasma concentrations trapped in active region magnetic fields. The brightness temperatures of the emission are higher in large, complex active regions, suggesting the plasma slab producing the emission is thicker and/or denser in such regions. We find also that the relationship between the brightness temperature and the size and complexity of the host region is a function of the phase of the 10–13-year solar activity cycle.     

宇宙再电离时期的kSZ效应和X射线背景与21 cm信号的互相关研究

正宇宙再电离是宇宙从黑暗时期到完全电离过渡的重要阶段,也是宇宙学研究的一个非常重要的课题,但是目前为止人们对宇宙再电离仍然缺乏足够精确的观测,其中最大的问题是微弱的有效信号往往淹没于巨大的前景噪声中因而很难提取出来.本工作中研究了宇宙再电离时代的动力学苏尼阿耶夫-泽尔多维奇效应(Kinetic Sunyaev-Zel’dovich, k SZ)、X射线背景以及与中性氢的21 cm信息的互相     

Solar eclipse observations of a magnetic loop structure at 2, 3.2, 8.2 and 21 cm

During the total eclipse of 1980 February 16, eclipse slope curves at 2, 3.2, 8.2 and 21 cm were obtained as the Moon occulted an active region with a bipolar sunspot group. From the peaks in these curves and the heights of the effective emission layers, the cores of the radio sources at the four wavelengths are located. Connecting these sites gives a magnetic loop structure. Relations between the locations, sizes, fluxes of the radio sources and the intensities, extensions, sizes and shapes of the bipolar sunspot field are analysed. The result at 8.2 cm is similar to the loop structure observed with the VLA at 6 cm. The radio flux spectrum shows that the gyro-radiation dominates over the thermal bremsstrahlung in this active region.     

Evolution of Intergalactic Gas in the Neighborhood of Dwarf Galaxies and Its Manifestations in the HI 21 cm Line

Low-mass galaxies are known to have played the crucial role in the hydrogen reionization in the Universe. In this paper we investigate the contribution of soft x-ray radiation (E ~ 0.1–1 keV) from dwarf galaxies to hydrogen ionization during the initial reionization stages. The only possible sources of this radiation in the process of star formation in dwarf galaxies during the epochs preceding the hydrogen reionization epoch are hot intermediate-mass stars (M ~ 5–8 M_⊙) that entered the asymptotic giant branch (AGB) stage and massive x-ray binaries. We analyze the evolution of the intergalactic gas in the neighborhood of a dwarf galaxy with a total mass of 6 × 10⁸M_⊙ formed at the redshift of z ~ 15 and having constant star-formation rate of 0.01–0.1 M_⊙ yr^?1 over a starburst with a duration of up to 100 Myr. We show that the radiation from AGB stars heats intergalactic gas to above 100 K and ensures its ionization x_e ? 0.03 within about 4–10 kpc from the galaxy in the case of a star-formation rate of star formation 0.03–0.1 M_⊙ yr^?1, and that after the end of the starburst this region remains quasi-stationary over the following 200–300 Myr, i.e., until z ~ 7.5. Formation of x-ray binaries form in dwarf galaxies at z ~ 15 results in a 2–3 and 5–6 times greater size of the ionized and heated region compared to the case where ionization is produced by AGB stars exclusively, if computed with the “x-ray luminosity–star-formation rate” dependence (L_X ~ f_XSFR) factor f_X = 0.1 and f_X ~ 1, respectively. For f_X ? 0.03 the effect of x-ray binaries is smaller that that of AGB star population. Lyα emission, heating, and ionization of the intergalactic gas in the neighborhood of dwarf galaxies result in the excitation of the 21 cm HI line. We found that during the period of the starburst end at z ~11.5–12.5 the brightness temperature in the neighborhood of galaxies is 15–25 mK and the region where the brightness temperature remains close to its maximum has a size of about 12–30 kpc. Hence the epoch of the starburst end is most favorable for 21 cm HI line observations of dwarf galaxies, because at that time the size of the region of maximum brightness temperature is the greatest over the entire evolution of the dwarf galaxy. In the case of the sizes corresponding to almost 0.’1 for z ~ 12 regions with maximum emission can be detected with the Square Kilometre Array, which is currently under construction.     

GMRT detection of HI 21 cm associated absorption towards thez = 1.2 red quasar 3C 190

We report the GMRT detection of associated HI 21 cm-line absorption in thez = 1.1946 red quasar 3C 190. Most of the absorption is blue-shifted with respect to the systemic redshift. The absorption, at ～ 647.7 MHz, is broad and complex, spanning a velocity width of ～ 600 kms^?1. Since the core is self-absorbed at this frequency, the absorption is most likely towards the hotspots. Comparison of the radio and deep optical images reveal linear filaments in the optical which overlap with the brighter radio jet towards the south-west. We therefore suggest that most of the HI 21 cm-line absorption could be occurring in the atomic gas shocked by the south-west jet.