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X -ray Po larimeter Sat ellite (XPoSat) is India’s first landmark mission dedicated to X-ray polarimetry, with the aim of measuring and studying X-rays emitted by bright astronomical objects such as black hole X-ray binaries, pulsar wind nebulae, and accretion-powered pulsars. Polar Satellite Launch Vehicle-C58 (PSLV-C58) launched the XPoSat mission on 1st January 2024, equipped with two significant, scientific instruments: XSPECT (X-ray Spectroscopy and Timing) and POLIX (POLarimeter Instrument in X-rays). With the launch of XPoSat, a new and important fourth dimension of polarization has been added. POLIX became the first in the world to provide measurements of polarization in 8–30 kilo electron Volt (keV) energy band. XSPECT is a spectroscopy payload responsible for providing timing and spectral information in 0.8–15 keV energy band of X-ray emissions from about 54 potential identified cosmic X-ray sources. Astronomical sources emitting X-rays are sites of strong gravity, and strong magnetic fields and have a variety of geometries for scattering, which are expected to give rise to polarization signatures in these sources. This article provides a comprehensive overview from mission specifications to mission design, mission planning, mission analysis, and mission operations aspects of spacecraft configuration, operations, and on-orbit operations of XPoSat mission with the science brought by the first-time flown payload in high energy bands, which will allow astronomers to explore materials under intense magnetic and gravitational fields. The challenges involved in planning and executing the mission operations with critical scenarios have also been highlighted.

AbstractThe MVN (Monitor Vsego Neba, all-sky monitor) experiment onboard the International Space Station for high-accuracy measurement of the cosmic X-ray background by the aperture modulation method being planned in 2022–2025 is considered. The technique that allows the measurement error within the MVN experiment to be minimized is described. Simulations of the main results of the experiment have been performed. They have shown that a record accuracy of measuring the cosmic X-ray background can be achieved. The main MVN systems are considered, and the results of the flight and ground-based experiments to measure the parameters of these systems are presented.

The article describes the MVN experiment (MVN – Monitor Vsego Neba in Russian transliteration) – All Sky Monitor onboard the ISS, which will start in 2021. The main scientific task of the experiment is to measure the cosmic X-ray background in energy range of 6 – 70 keV with unprecedented high absolute and spectral accuracy (about 1%). To reach this goal, Space Research Institute of the Russian Academy of Sciences has developed scientific payload called MVN. The main instrument of MVN is the SPIN-X1-MVN X-ray monitor, which is equipped with four CdTe based semiconductor detectors. The X-ray monitor will be mounted on the external surface of the ISS with zenith orientation. The structure of the monitor including the key systems (detectors, gears, thermal control system) is considered. Measuring of the cosmic X-ray background is carried out by the aperture modulation method described in detail in this article. Also we describe an optimal algorithm of the obtained in experiment data filtering, which will provide us with highest quality of the CXB measurement.

The Einstein Probe (EP) is an interdisciplinary mission of time-domain and X-ray astronomy. Equipped with a wide-field lobster-eye X-ray focusing imager, EP will discover cosmic X-ray transients and monitor the X-ray variability of known sources in 0.5–4 keV, at a combination of detecting sensitivity and cadence that is not accessible to the previous and current wide-field monitoring missions. EP can perform quick characterisation of transients or outbursts with a Wolter-I X-ray telescope onboard. In this paper, the science objectives of the EP mission are presented. EP is expected to enlarge the sample of previously known or predicted but rare types of transients with a wide range of timescales. Among them, fast extragalactic transients will be surveyed systematically in soft X-rays, which include γ -ray bursts and their variants, supernova shock breakouts, and the predicted X-ray transients associated with binary neutron star mergers. EP will detect X-ray tidal disruption events and outbursts from active galactic nuclei, possibly at an early phase of the flares for some. EP will monitor the variability and outbursts of X-rays from white dwarfs, neutron stars and black holes in our and neighbouring galaxies at flux levels fainter than those detectable by the current instruments, and is expected to discover new objects. A large sample of stellar X-ray flares will also be detected and characterised. In the era of multi-messenger astronomy, EP has the potential of detecting the possible X-ray counterparts of gravitational wave events, neutrino sources, and ultra-high energy γ -ray and cosmic ray sources. EP is expected to help advance the studies of extreme objects and phenomena revealed in the dynamic X-ray universe, and their underlying physical processes. Besides EP’s strength in time-domain science, its follow-up telescope, with excellent performance, will also enable advances in many areas of X-ray astronomy.

The Low-Energy X-ray Polarization Detector (LPD) is one of the payloads in the POLAR-2 experiment, designed as an external payload for the China Space Station deployment in early 2026. LPD is specifically designed to observe the polarization of gamma-ray burst (GRB) prompt emission in the energy range of 2–10 keV, with a wide field of view (FoV) of 90° in preliminary design. This observation is achieved using an array of X-ray photoelectric polarimeters based on gas pixel detectors. Due to the wide FoV configuration, the in-orbit background count rate in the soft X-ray range is high, while GRBs themselves also exhibit high flux in this energy band. In order to assess the contribution of various background components to the total count rate, we conducted detailed simulations using the GEANT4 C++ package. Our simulations encompassed the main interactions within the instrument materials and provided insights into various background components within the wide-FoV scheme. The simulation results reveal that among the background components, the primary contributors are the cosmic X-ray background (CXB) and bright X-ray sources. The total background count rate of LPD, after applying the charged particle background rejection algorithm, is approximately 0.55 counts cm–2 s–1 on average, and it varies with the detector’s orbit and pointing direction. Furthermore, we performed comprehensive simulations and comparative analyses of the CXB and X-ray bright sources under different FoVs and detector pointings. These analyses provide valuable insights into the background characteristics for soft X-ray polarimeter with wide FoV.

We analyzed the unexposed to the sky (outFOV) region of the MOS2 detector on board XMM-Newton covering 15 yr of data amounting to 255 Ms. We show convincing evidence that the origin of the unfocused background in XMM-Newton is due to energetic protons, electrons, and hard X-ray photons. Galactic cosmic rays are the main contributors as shown by the tight correlation (2.6% of total scatter) with the 1 GeV proton data of the SOHO EPHIN detector. Tight correlations are found with a proxy of the Chandra background rate, revealing the common source of background for detectors in similar orbits, and with the data of the EPIC Radiation Monitor, only when excluding Solar energetic particle events. The entrance to the outer electron belts is associated with a sudden increase in the outFOV MOS2 rate and a spectral change. These facts support the fact that MeV electrons can generate an unfocused background signal. The correlation between MOS2 outFOV data and the SOHO EPHIN data reveals a term constant in time and isotropic, similar to the one found in the study of the pn data. The most plausible origin of this component is hard unfocused X-ray photons of the cosmic X-ray background Compton scattering in the detector as supported by the strength of the signal in the two detectors with different thicknesses. Based on this physical understanding, a particle radiation monitor on board the Advanced Telescope for High Energy Astrophysics has been proposed and it is currently under study. It will be able to track different species with the necessary accuracy and precision to guarantee the challenging requirement of 2% reproducibility of the background.

The Low Energy X-ray telescope (LE) is one of the three main instruments of the Insight-Hard X-ray Modulation Telescope ( Insight- HXMT) . It is equipped with Swept Charge Device (SCD) sensor arrays with a total geometrical area of 384 cm and an energy band from 0.7 to 13 keV. In order to evaluate the particle induced X-ray background and the cosmic X-ray background simultaneously, LE adopts collimators to define four types of Field Of Views (FOVs), i.e., 1.6°×6°, 4°×6°, 50°-60°×2°-6° and the blocked ones which block the X-ray by an aluminum cover. LE is constituted of three detector boxes (LEDs) and an electric control box (LEB) and achieves a good energy resolution of 140 eV@5.9 keV, an excellent time resolution of 0.98 ms, as well as an extremely low pileup (<1%@18000 cts/s). Detailed performance tests and calibration on the ground have been performed, including energy-channel relation, energy response, detection efficiency and time response.

By characterizing the contribution of stray light to large data sets from the CXB Measurement X-ray observatory collected over 2012–2017, we report a measurement of the cosmic X-ray background (CXB) in the 3–20 keV energy range. These data represent ∼20% sky coverage while avoiding Galactic ridge X-ray emission and are less weighted by deep survey fields than previous measurements with CXB Measurement. Images in narrow energy bands are stacked in detector space and spatially fit with a model representing the stray light and uniform pattern expected from the CXB and the instrumental background, respectively. We establish baseline flux values from Earth-occulted data and validate the fitting method on stray-light observations of the Crab, which further serve to calibrate the resulting spectra. We present independent spectra of the CXB with the focal plane module FPMA and FPMB detector arrays, which are in excellent agreement with the canonical characterization by HEAO 1 and are 10% lower than most subsequent measurements: F3−20keVFPMA=2.63×10−11ergs−1cm−2deg−2 and F3–20keVFPMB=2.58×10−11ergs−1cm−2deg−2 . We discuss these results in light of previous measurements of the CXB and consider the impact of systematic uncertainties on our spectra.

The cosmic X-ray background (CXB) is dominated by the obscured and unobscured coronal light of active galactic nuclei (AGN). At energies below 10 keV, the CXB can be well explained by models taking into account the known AGN and the observed distribution of their obscuring, line-of-sight column densities, NH,l.o.s. However, at energies around the Compton reflection hump (~30 keV), the models fall short of the data. This suggests the existence of a population of as yet undetected Compton-thick (CT) AGN (NH,l.o.s > 1.5 × 1024 cm−2) whose X-ray spectra are dominated by the light that has been reprocessed by the obscuring material. In this work, we continue the effort to find and catalog all local (z < 0.05) CT AGN. To this end, we obtained soft X-ray data with Chandra for six local BAT detected sources lacking ROSAT (0.1–2.4 keV) counterparts, indicating potential obscuration. We fit their spectra with Bayesian and least squares methods using two different models, borus02 and UXCLUMPY. We compare the results of the different models and methods and find that the NH,l.o.s is consistently measured in each case. Three of the sources were also observed with XMM-Newton, allowing the opportunity to search for variability in soft X-ray flux or NH,l.o.s. From this sample, we find one strong CT candidate (NGC 5759) and one weaker CT candidate (CGCG 1822.3+2053). Furthermore, we find tentative evidence of NH,l.o.s variability in 2MASX J17253053–4510279, which has NH,l.o.s < 1022 cm−2.

The circumgalactic medium (CGM) plays a crucial role in galaxy evolution as it fuels star formation, retains metals ejected from the galaxies, and hosts gas flows in and out of galaxies. For Milky Way–type and more-massive galaxies, the bulk of the CGM is in hot phases best accessible at X-ray wavelengths. However, our understanding of the CGM remains largely unconstrained due to its tenuous nature. A promising way to probe the CGM is via X-ray absorption studies. Traditional absorption studies utilize bright background quasars, but this method probes the CGM in a pencil beam, and, due to the rarity of bright quasars, the galaxy population available for study is limited. Large-area, high spectral resolution X-ray microcalorimeters offer a new approach to exploring the CGM in emission and absorption. Here, we demonstrate that the cumulative X-ray emission from cosmic X-ray background sources can probe the CGM in absorption. We construct column density maps of major X-ray ions from the Magneticum simulation and build realistic mock images of nine galaxies to explore the detectability of X-ray absorption lines arising from the large-scale CGM. We conclude that the O VII absorption line is detectable around individual massive galaxies at the 3σ–6σ confidence level. For Milky Way–type galaxies, the O VII and O VIII absorption lines are detectable at the ∼ 6σ and ∼ 3σ levels even beyond the virial radius when coadding data from multiple galaxies. This approach complements emission studies, does not require additional exposures, and will allow for probing the baryon budget and the CGM at the largest scales.