The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma-ray telescopes. Although small-scale variations in the nebula are well known, since the start of science operations of
Remote observations with the Chandra X-ray Observatory and the XMM-Newton Observatory have shown that the jovian system is a source of X-rays with a rich and complicated structure. The planet's polar auroral zones and its disk are both powerful sources of X-ray emission. Chandra observations revealed X-ray emission from the Io plasma torus and from the Galilean moons Io, Europa, and possibly Ganymede. The emission from the moons is due to bombardment of their surfaces by highly energetic magnetospheric protons, and oxygen and sulfur ions. These ions excite atoms in their surfaces leading to fluorescent X-ray emission lines. These lines are produced against an intense background continuum, including bremsstrahlung radiation from surface interactions of primary magnetospheric and secondary electrons. Although the X-ray emission from the Galilean moons is faint when observed from Earth orbit, an imaging X-ray spectrometer in orbit around one or more of these moons, operating from 200 eV to 8 keV with 150 eV energy resolution, would provide a detailed mapping of the elemental composition in their surfaces. Surface resolution of 40 m for small features could be achieved in a 100-km orbit around one moon while also remotely imaging surfaces of other moons and Jupiter's upper atmosphere at maximum regional resolutions of hundreds of kilometers. Due to its relatively more benign magnetospheric radiation environment, its intrinsic interest as the largest moon in the Solar System, and its mini-magnetosphere, Ganymede would be the ideal orbital location for long-term observational studies of the jovian system. Here we describe the physical processes leading to X-ray emission from the surfaces of Jupiter's moons and the properties required for the technique of imaging X-ray spectroscopy to map the elemental composition of their surfaces, as well as studies of the X-ray emission from the planet's aurora and disk and from the Io plasma torus.
The energy spectra and fluxes of atmospheric neutrons and gamma rays from 8 to 60 MeV and from 2 to 10 MeV, respectively, have been measured with a time‐of‐flight (TOF) system using organic liquid scintillators at balloon altitudes (3–5 g/cm²) over Palestine, Texas (λ=42°N). The TOF detector consisted of two banks of six cylindrical NE213 liquid scintillators, each being 7.5 cm in diameter by 7.5 cm long, separated by 0.75 m. A TOF system was used to measure upward and downward moving neutrons and gamma rays. Pulse shape discrimination is included on each NE213 scintillator to discriminate between neutrons and gamma rays. The measured neutron leakage current is 1.9 × 10−3 from 8 ≤En <18 MeV, 1.5 × 10−3 from 18 ≤En <30 MeV, and 9.5 × 10−4 neutron/cm² s MeV from 30 ≤En ≤60 MeV. The total downward gamma ray flux from 2 to 6 MeV at 3.5 g cm−2 is given by 0.012E−1.5 photon/cm² s sr MeV. The upward moving gamma ray flux at 4 g cm−2 from 2 to 10 MeV is 0.063E−1.8 photon/cm² s sr Mev.
The Crab pulsar and its nebula are among the most studied astrophysical systems, and constitute one of the most promising environments where high energy processes and particle acceleration can be investigated. They are the only objects for which previous X-ray polarisation has been reported. We present here the first Imaging X-ray Polarimetry Explorer (IXPE) observation of the Crab pulsar and nebula. The total pulsar pulsed emission in the [2-8] keV energy range is unpolarised. Significant polarisation up to 15% is detected only in the core of the main peak. The nebula has a total space integrated polarised degree of 20% and polarisation angle of 145 • . The polarised maps show a large variation in the local polarisation, and regions with polarised degree up to 45-50%. The polarisation pattern suggests a predominantly toroidal magnetic field.
Four NASA Science and Technology Definition Teams have been convened in order to develop and study four mission concepts to be evaluated by the upcoming 2020 Decadal Survey. The Lynx x-ray surveyor mission is one of these four large missions. Lynx will couple fine angular resolution (<0.5 arcsec HPD) x-ray optics with large effective area (∼2 m 2 at 1 keV), thus enabling exploration within a unique scientific parameter space. One of the primary soft x-ray imaging instruments being baselined for this mission concept is the highdefinition x-ray imager, HDXI. This instrument would use a finely pixelated silicon sensor array to achieve fine angular resolution imaging over a wide field of view (∼22 × 22 arcmin). Silicon sensors enable large-format/ small-pixel devices, radiation tolerant designs, and high quantum efficiency across the entire soft x-ray bandpass. To fully exploit the large collecting area of Lynx (∼30× Chandra), with negligible or minimal x-ray event pile-up, the HDXI will be capable of much faster frame rates than current x-ray imagers. We summarize the planned requirements, capabilities, and development status of the HDXI instrument, and associated papers in this special edition will provide further details on some specific detector options. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
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