Probing the first stars and black holes in the early Universe with the Dark Ages Radio Explorer (DARE)

A concept for a new space-based cosmology mission called the Dark Ages Radio Explorer (DARE) is presented in this paper. DARE’s science objectives include: (1) When did the first stars form? (2) When did the first accreting black holes form? (3) When did Reionization begin? (4) What surprises does the end of the Dark Ages hold (e.g., Dark Matter decay)? DARE will use the highly-redshifted hyperfine 21-cm transition from neutral hydrogen to track the formation of the first luminous objects by their impact on the intergalactic medium during the end of the Dark Ages and during Cosmic Dawn (redshifts z = 11–35). It will measure the sky-averaged spin temperature of neutral hydrogen at the unexplored epoch 80–420 million years after the Big Bang, providing the first evidence of the earliest stars and galaxies to illuminate the cosmos and testing our models of galaxy formation. DARE’s approach is to measure the expected spectral features in the sky-averaged, redshifted 21-cm signal over a radio bandpass of 40–120 MHz. DARE orbits the Moon for a mission lifetime of 3 years and takes data above the lunar farside, the only location in the inner solar system proven to be free of human-generated radio frequency interference and any significant ionosphere. The science instrument is composed of a low frequency radiometer, including electrically-short, tapered, bi-conical dipole antennas, a receiver, and a digital spectrometer. The smooth frequency response of the antennas and the differential spectral calibration approach using a Markov Chain Monte Carlo technique will be applied to detect the weak cosmic 21-cm signal in the presence of the intense solar system and Galactic foreground emissions.     

基于DCT-CS的脉冲星周期超快速估计方法

受快速傅里叶变换（fast Fourier transform，FFT）的影响，基于FFT和压缩感知（compressive sensing, CS）的脉冲星周期快速估计算法的计算量大。为进一步减小计算量并提高计算精度，利用离散余弦变换（discrete cosine transform，DCT)取代FFT，提出了一种基于DCT-CS的脉冲星周期超快速估计算法。在该方法中，利用DCT提取脉冲星信号的低频部分构建低频DCT矩阵；构建畸变轮廓字典并获取累积轮廓；提出了利用最大值超分辨率稀疏恢复估计脉冲星周期的方法。仿真结果表明，DCT-CS的脉冲星周期估计精度达到了3.82×10-12 s，计算时间达到了9.31 ms。与FFT-CS相比，周期估计精度提高了约16%，计算时间缩短了约37.5%，实现了实时高精度的脉冲星周期估计。     

Soft X-ray observations of helmet structures from SXT/Yohkoh

We present soft X-ray observations of helmet structures in solar active regions obtained from SXT/Yohkoh. These helmet structures are observed to form in the flare decay phase and to be associated with active region loop interactions. Their morphology is similar to the much larger scale helmet streamers that appear in the outer corona as shown in optical images of solar eclipse. The observed X-ray helmet structures appear to be in quasi-equilibrium with lifetimes greater than the MHD time scale. Using the filter ratio method for the X-ray observations, we find that the cusp region has lower temperature and higher density than that in the stalk region above it. The plasma pressure in the cusp region is about the same or slightly higher than that in the stalk region.     

Indicators of black-hole mass and Eddington accretion ratio from QSO X-ray and UV spectra

The evolution of luminous QSOs is linked to the evolution of massive galaxies. We know this because the relic black-holes found locally have masses dependent on the properties of the host galaxy’s bulge. An important way to explore this evolution would be to measure dependences of black-hole masses and Eddington accretion ratios over a range of redshifts, i.e., with cosmological age. For low redshift QSOs (and their lower luminosity Seyfert galaxy counterparts) it has been possible to infer black-hole masses from the luminosities and velocity dispersions of their host-galaxy bulges. These masses agree with those virial black-hole masses calculated from the Doppler widths of the broad Hβ emission lines. The latter method can then be extended to more distant and luminous QSOs, up to redshifts of 0.6 with ground-based optical observations. We discuss ways to extend these explorations to higher redshifts – up to 3 using the widths of QSOs’ broad UV emission lines, and in principle, and to redshifts near 4 from ground-based infrared observations of rest-frame Hβ at 2.5 μm. We discuss the possibility of investigating the accretion history of the higher redshift QSOs using measures of Eddington accretion ratio – the soft X-ray spectral index and the eigenvectors of Principal Components Analyses of QSOs’ UV emission-line spectra.     

X-Ray Emission from Flare Collapsing Trap

An acceleration process in the collapsing magnetic trap, formed in the flare with cusp magnetic field topology, is described. Computations show that high-energy electrons are accumulated in the central part of the collapsing magnetic trap due to an increase of their pitch angles. The effect explains in a natural way the formation of X-ray loop-top sources. Then, using the model with the collapsing trap and considering only the adiabatic heating process, a possible explanation for the motion of the X-ray loop-top source observed at the beginning of some cusp-type flares is presented.     

Overview of QPOs in neutron-star low-mass X-ray binaries

Some aspects of the rapid X-ray variability of low magnetic-field neutron stars in low-mass X-ray binaries are briefly summarized.     

Evolution of binaries with chemically peculiar primaries

The chemically peculiar (CP) stars are classified into subgroups based on the type of peculiarities. A significant fraction of these are known to be binaries. The faster evolution of the massive component leads to a white dwarf or a neutron star. Further evolution of the binary is analysed taking into consideration, the orbital parameters, effect of magnetic field, spectroscopic peculiarities and finally the statistics of CP binaries and Low Mass X-ray Binaries (LMXB).

The possible consequences of the evolution to lead to the formation of Magnetic Cataclysmic Variables (MCV) and LMXB are discussed.     

Near infrared studies of the massive X-ray binary 2S 0114+65

We present near-infrared spectroscopy of the massive X-ray binary 2S 0114+650. These observations covering the spectral range 1.08–2.35 μm span the region where Paschen and Brackett series recombination lines of hydrogen are expected to be seen, namely, Paβ , Brγ and Br 10–17 lines. The absence of any of these lines in emission supports earlier inferences that the optical component in 2S 0114+650 is unlikely to be a Be star but rather a B type supergiant. Near-IR photometry gives J = 8.78, H = 8.53 and K = 7.96; these values show marginal variations from earlier reported measurements.     

Orbital evolution studies of two HMXB pulsars

High mass X-ray binary (HMXB) pulsars are of two types, persistent and transient. 4U1538−52 is a persistent HMXB whose orbit was previously measured to be circular but the RXTE observations revealed an eccentric orbit. We observed this system with RXTE-PCA in August 2003 and our timing analysis supports the eccentric orbit of the system. However, we do not find any evidence for orbital evolution.

Rotational and tidal interactions between the stars of a closed binary system result in apsidal motion which can be measured in systems with eccentric orbit. 4U0115+63 is a Be-transient HMXB whose eccentric orbit was well-determined during its 1978 outburst. We report preliminary results from analysis of data obtained during the 1999 outburst of this source with the RXTE-PCA.     

Effects of the photo-ionized absorber on the continuum spectra of EXO 0748–676

We report on the analysis of XMM-Newton archival data of EXO 0748–676. We studied changes of the continuum spectra due to the presence of photo-ionized plasma on the line of sight. We show that the ionization degree of the plasma could change largely during the X-ray bursts and the dips. These changes can significantly modify the soft-band spectrum, which was in fact observed from EXO 0748–676. We discuss the effect of the photo-ionized plasma on the continuum spectra in comparison with a frequently used model such as partial covering absorption.