Keywords

In observational cosmology, a supernova Ia is used as a standard candle in order to extend the Hubble diagram to a higher redshift range. Astrophysicists found that the observed brightness of high redshift supernovae Ia is dimmer than expected. This dimming effect is considered observational evidence for the existence of dark energy in the universe. It should be noted that this conclusion is based on an assumption that the mass density of the cosmic plasma is very small. Therefore, the dimming effect caused by the Compton scattering of free electrons in cosmic plasma can be neglected. X-ray observations suggest that the mass density of the cosmic plasma may be very large. In theory, the observed dimming effect of high redshift supernovae Ia may be caused by the Compton scattering of free electrons in the cosmic plasma. In this paper it will be shown that this idea is reasonable. Therefore, there is no need to introduce the confusing concept of dark energy into cosmology.

The feeding and feedback processes at the vicinity of a supermassive black hole (BH) are essential for our understanding of the connection between supermassive BH and its host galaxy. In this work, we provide a detailed investigation, both observational and theoretical, on the diffuse (∼2''–20'', ∼0.08–0.8 pc) X-ray emission around Sgr A*. Over two-decade Chandra observations are gathered to obtain highest signal-to-noise to date. We find that, the line center of iron lines of the outer 8''–18'' region, ${\epsilon }_{{\rm{c}}}={6.65}_{-0.03}^{+0.02}\,\mathrm{keV}$, is comparable to that (${\epsilon }_{{\rm{c}}}={6.60}_{-0.03}^{+0.05}\,\mathrm{keV}$) of the inner 2''–5'' region. This is somewhat unexpected, since the gas temperature decreases further away from the central BH. Based on a dynamical inflow–outflow model that considers the gas feeding by stellar winds from Wolf–Rayet stars, we calculate the X-ray spectrum based on both the conventional collisional ionization equilibrium (CIE) assumption, and the newly developed non-equilibrium ionization (NEI) assumption. We find that, theoretically gases within ∼8''–10'' remain in a CIE state, outside of this radius they will be in the NEI state. A comparison of the properties of ∼6.6 keV iron lines between CIE and NEI is addressed. Interestingly, the NEI interpretation of outer region is supported by the Chandra line center _c measurements of this region.

The space environment background of various particle fluxes of the Hard X-ray Imager (HXI), one of the payloads of the Advanced Space-based Solar Observatory (ASO-S) spacecraft, is investigated and presented. Different approaches are used to obtain the input information on various space environment particles (protons, alpha particles, electrons, positrons, neutrons, and photons). Some special regions (SAA and radiation belt) are also taken into account. The findings indicate that electrons are the primary background source in the radiation belt. Due to the large background flux generated by electrons, HXI cannot effectively observe solar flares in the radiation belt. Outside the radiation belt, primary protons and albedo photons are the main sources of background at low and high magnetic latitudes respectively. The statistical analysis of the flare and background spectra shows that the errors of the flare energy spectrum observation are mainly concentrated in the high energy band, and the detector still has a certain spectrum observation capability for flares of C-class and below in the low energy band of the non-radiation belt. The imaging observation of flares of C-class and below is significantly affected by the accuracy of background subtraction. The energy band with the best signal-to-noise ratio is from 10 to 50 keV, which can be used to monitor the formation and class of flares.

The diffuse hard X-ray emission that fills the Galactic center, bulge, and ridge is believed to arise from unresolved populations of X-ray binary systems. However, the identity of the dominant class of accreting objects in each region remains unclear. Recent studies of Fe line properties and the low-energy (<10 keV) X-ray continuum of the bulge indicate a major population fraction of nonmagnetic cataclysmic variables (CVs), in particular quiescent dwarf novae (DNe). This is in contrast to previous high-energy (>10 keV) X-ray measurements of the bulge and ridge, which indicate a dominant population of magnetic CVs, in particular intermediate polars. In addition, NuSTAR broadband measurements have uncovered a much heavier intermediate polar population in the central ∼100 pc than previously assumed, raising the possibility that some fraction of this population extends further from the center. Here we use NuSTAR's large aperture for unfocused photons and its broadband X-ray range to probe the diffuse continuum of the inner ∼1°–3° of the Galactic bulge. This allows us to constrain possible multitemperature components of the spectrum, such as could indicate a mixture of soft and hard populations. Our emissivity is consistent with previous hard X-ray measurements in the bulge and ridge, with the diffuse X-ray luminosity tracing the stellar mass. The spectrum is well described by a single-temperature thermal plasma with kT ≈ 8 keV, with no significant emission above 20 keV. This supports that the bulge is dominated by quiescent DNe; we find no evidence of a significant intermediate polar population in the hard X-ray band.

We present here a combined analysis of four high spectral resolution observations of the Diffuse X-ray Background, made using the University of Wisconsin-Madison/Goddard Space Flight Center X-ray Quantum Calorimeter sounding rocket payload. The observed spectra support the existence of a ∼0.1 keV Local Hot Bubble and a ∼0.2 keV Hot Halo, with discrepancies between repeated observations compatible with expected contributions of time-variable emission from Solar Wind Charge Exchange. An additional component of ∼0.9 keV emission observed only at low galactic latitudes can be consistently explained by unresolved dwarf M stars.

We study the coherence of the near-infrared and X-ray background fluctuations and the X-ray spectral properties of the sources producing it. We use data from multiple Spitzer and Chandra surveys, including the UDS/SXDF surveys, the Hubble Deep Field North, the EGS/AEGIS field, the Chandra Deep Field South, and the COSMOS surveys, comprising ∼2275 Spitzer/IRAC hours and ∼16 Ms of Chandra data collected over a total area of ∼1 deg². We report an overall ∼5σ detection of a cross-power signal on large angular scales >20'' between the 3.6 and 4.5 μm and the X-ray bands, with the IR versus [1–2] keV signal detected at 5.2σ. The [0.5–1] and [2–4] keV bands are correlated with the infrared wavelengths at a ∼1–3σ significance level. The hardest X-ray band ([4–7] keV) alone is not significantly correlated with any infrared wavelengths due to poor photon and sampling statistics. We study the X-ray spectral energy distribution of the cross-power signal. We find that its shape is consistent with a variety of source populations of accreting compact objects, such as local unabsorbed active galactic nuclei or high-z absorbed sources. We cannot exclude that the excess fluctuations are produced by more than one population. Because of poor statistics, the current relatively broad photometric bands employed here do not allow distinguishing the exact nature of these compact objects or if a fraction of the fluctuations have instead a local origin.

As matter accretes onto the central supermassive black holes in active galactic nuclei (AGNs), X-rays are emitted. We present a population synthesis model that accounts for the summed X-ray emission from growing black holes; modulo the efficiency of converting mass to X-rays, this is effectively a record of the accreted mass. We need this population synthesis model to reproduce observed constraints from X-ray surveys: the X-ray number counts, the observed fraction of Compton-thick AGNs [log (N_H/cm⁻²) > 24], and the spectrum of the cosmic X-ray background (CXB), after accounting for selection biases. Over the past decade, X-ray surveys by XMM-Newton, Chandra, NuSTAR, and Swift-BAT have provided greatly improved observational constraints. We find that no existing X-ray luminosity function (XLF) consistently reproduces all these observations. We take the uncertainty in AGN spectra into account and use a neural network to compute an XLF that fits all observed constraints, including observed Compton-thick number counts and fractions. This new population synthesis model suggests that, intrinsically, 50% ± 9% (56% ± 7%) of all AGNs within z ≃ 0.1 (1.0) are Compton-thick.

Previous radio and infrared observations have revealed an obscured region of high-mass star formation in Cygnus known as Onsala 2 (ON2). Within this region lies the optically revealed young stellar cluster Berkeley 87, which contains several OB stars and the rare oxygen-type Wolf–Rayet star WR 142. Previous radio studies of ON2 have also discovered masers and several H ii regions excited by embedded OB stars. Radio and Gaia parallaxes have now shown that the H ii regions are more distant than Berkeley 87. We summarize two Chandra X-ray observations of ON2, which detected more than 300 X-ray sources. Several optically identified stars in Berkeley 87 were detected, including massive OB stars and WR 142, the latter being a faint hard source whose X-ray emission likely arises in hot thermal plasma. Intense X-ray emission was detected near the compact H ii regions G75.77+0.34 and G75.84+0.40, consisting of numerous point sources and diffuse emission. Heavily absorbed X-ray sources and their near-IR counterparts that may be associated with the exciting OB stars of the H ii regions are identified. Shocked winds from embedded massive stars offer a plausible explanation of the diffuse emission. Young stellar object candidates in the ON2 region are identified using near-IR colors, but surprisingly few counterparts of X-ray sources have near-IR excesses typical of classical T Tauri stars.

We study the source-subtracted near-infrared and X-ray background fluctuations of the COSMOS field using data from the Spitzer SPLASH program (∼1272 hr) and Chandra COSMOS Legacy Survey (4.6 Ms). The new auto-power spectra of the cosmic infrared and X-ray background fluctuations reach maximum angular scales of ∼3000'' and ∼5000'', respectively. We measure the cross-power spectra between each infrared and X-ray band and calculate the mean power above 20''. We find that the soft X-ray band is correlated with 3.6 and 4.5 μm at ∼4σ significance level. The significance between hard X-ray and the 3.6 μm (4.5 μm) band is ∼2.2σ (∼3.8σ). The combined infrared (3.6 + 4.5 μm) data are correlated with the X-ray data in soft ([0.5–2] keV), hard ([2–7] keV), and broad ([0.5–7] keV) bands at ∼5.6σ, ∼4.4σ, and ∼6.6σ levels, respectively. We compare the new measurements with existing models for the contributions from known populations at z < 7, which are not subtracted. The model predictions are consistent with the measurements, but we cannot rule out contributions from other components, such as Direct Collapse Black Holes (DCBH). However, the stacked cross-power spectra, combining other available data, show excess fluctuations about an order of magnitude on average at ∼4σ confidence at scales within ∼300''. By studying the X-ray SED of the cross-power signal, assuming no significant variation from the infrared, we find that its shape is consistent with DCBHs.

We report a systematic search for an emission line around 3.5 keV in the spectrum of the cosmic X-ray background using a total of ∼10 Ms Chandra observations toward the COSMOS Legacy and Extended Chandra Deep Field South survey fields. We find marginal evidence of a feature at an energy of ∼3.51 keV with a significance of 2.5–3σ, depending on the choice of statistical treatment. The line intensity is best fit at (8.8 ± 2.9) × 10⁻⁷ ph cm⁻² s⁻¹ when using a simple Δχ² or ${10.2}_{-0.4}^{+0.2}\times {10}^{-7}$ ph cm⁻² s⁻¹ when Markov chain Monte Carlo is used. Based on our knowledge of Chandra and the reported detection of the line by other instruments, an instrumental origin for the line remains unlikely. We cannot, however, rule out a statistical fluctuation, and in that case our results provide a 3σ upper limit at 1.85 × 10⁻⁶ ph cm⁻² s⁻¹. We discuss the interpretation of this observed line in terms of the iron line background, S xvi charge exchange, as well as potentially being from sterile neutrino decay. We note that our detection is consistent with previous measurements of this line toward the Galactic center and can be modeled as the result of sterile neutrino decay from the Milky Way for the dark matter distribution modeled as a Navarro–Frenk–White profile. For this case, we estimate a mass m_ν ∼ 7.01 keV and a mixing angle sin²(2θ) = (0.83–2.75) × 10⁻¹⁰. These derived values are in agreement with independent estimates from galaxy clusters, the Galactic center, and M31.