X-ray Spectroscopy of Accreting White Dwarfs

The advent of the grating spectrometers onboard Chandra and XMM-Newton opened up a new era in plasma diagnostics of compact binaries. High resolution spectroscopy using these spectrometers is of particular use in investigating accretion plasmas in cataclysmic variables (CVs) because they show a wealth of emission lines owing to their optically thin thermal nature. In this review, I present recent progress on density measurements of the plasma in magnetic CVs by means of He-like triplet and iron L lines, and the outcome of line velocity measurements in the dwarf nova SS Cygni in outburst, to demonstrate the potential power of high resolution spectroscopy to elucidate the geometry of the plasma. In the end, our expectations for the Soft X-ray Spectrometer onboard the forthcoming X-ray mission Astro-H are summarized.     

High Velocity Outflows in AGNs Observed with Gratings

Several Active Galactic Nuclei (AGNs) have been reported to harbor fast outflows exceeding 10,000 km?s^?1, which are detected mostly in their low-resolution CCD X-ray spectra. Only few, however, were detected with high-resolution grating spectrometers. Most of the grating detected outflows have been observed multiple times. In these cases, the absorption spectrum changes are indicating that variability is common among these high-velocity winds. In this paper we revisit the grating observations of PG?1211+143, and PDS?456, and report preliminary results on 4C?74.26. We discuss the spectral variability of the first two sources and its implications to the outflows.     

X-ray emission from isolated hot white dwarfs

Hot white dwarfs are objects that copiously emit in the Extreme Ultraviolet and soft X-ray range. They are the brightest sources seen in the Low Energy Telescope of EXOSAT, with countrates up to 25 cnts/s. in contrast to their optical and UV spectrum the total flux and spectral distribution at soft X-ray energies are highly sensitive to the effective temperature, structure and elemental composition of the dwarf's atmosphere. The imaging soft X-ray experiments onboard EXOSAT cover with large sensitivity the spectral region where the peak of emission of hot white dwarfs is expected to occur.I here review some of the (preliminary) results obtained so far with broadband X-ray photometry on a dozen or so white dwarfs, and some of the high-resolution spectra obtained for three white dwarfs with the grating spectrometers.     

Characterization and Calibration of the CheMin Mineralogical Instrument on Mars Science Laboratory

A principal goal of the Mars Science Laboratory (MSL) rover Curiosity is to identify and characterize past habitable environments on Mars. Determination of the mineralogical and chemical composition of Martian rocks and soils constrains their formation and alteration pathways, providing information on climate and habitability through time. The CheMin X-ray diffraction (XRD) and X-ray fluorescence (XRF) instrument on MSL will return accurate mineralogical identifications and quantitative phase abundances for scooped soil samples and drilled rock powders collected at Gale Crater during Curiosity’s 1-Mars-year nominal mission. The instrument has a Co X-ray source and a cooled charge-coupled device (CCD) detector arranged in transmission geometry with the sample. CheMin’s angular range of 5^° to 50^° 2θ with <0.35^° 2θ resolution is sufficient to identify and quantify virtually all minerals. CheMin’s XRF requirement was descoped for technical and budgetary reasons. However, X-ray energy discrimination is still required to separate Co?Kα from Co?Kβ and Fe?Kα photons. The X-ray energy-dispersive histograms (EDH) returned along with XRD for instrument evaluation should be useful in identifying elements Z>13 that are contained in the sample. The CheMin XRD is equipped with internal chemical and mineralogical standards and 27 reusable sample cells with either Mylar^? or Kapton^? windows to accommodate acidic-to-basic environmental conditions. The CheMin flight model (FM) instrument will be calibrated utilizing analyses of common samples against a demonstration-model (DM) instrument and CheMin-like laboratory instruments. The samples include phyllosilicate and sulfate minerals that are expected at Gale crater on the basis of remote sensing observations.     

Spectral variability of scorpius X-1, as observed with EXOSAT

EXOSAT measurements of Sco X-1 have been carried out with the 500 l/mm and 1000 l/mm grating spectrometers and simultaneously with the gas scintillation proportional counter (GSPC) on two occasions in August 1983. The first observing period, part of the instrument performance verification phase, on August 7 and 8, 1983, lasted for about twenty two hours and showed the source flaring during the first six hours. During the second observing period, August 29 and 30, 1983, the source was again active during part of the time. The lightcurves of the spectrometers and the GSPC show some correlation during the flaring activity. The grating spectra show clear emission features between 12 and 20 Å, which vary with time. When fitting the observed spectra to a thermal bremsstrahlungs model (exponential with gaunt factor) together with absorption by neutral matter, a reduced abundance of oxygen in the absorbing matter with respect to Solar is required.     

Kinematics of Supernova Remnants: Status of X-Ray Observations

A supernova (SN) explosion drives stellar debris into the circumstellar material (CSM) filling a region on a scale of parsecs with X-ray emitting plasma. The velocities involved in supernova remnants (SNRs), thousands of km?s^?1, can be directly measured with medium and high-resolution X-ray spectrometers and add an important dimension to our understanding of the last stages of the progenitor, the explosion mechanism, and the physics of strong shocks. After touching on the ingredients of SNR kinematics, I present a summary of the still-growing measurement results from SNR X-ray observations. Given the advances in 2D/3D hydrodynamics, data analysis techniques, and especially X-ray instrumentation, it is clear that our view of SNRs will continue to deepen in the decades ahead.     

Energy Release and Particle Acceleration in Flares: Summary and Future Prospects

RHESSI measurements relevant to the fundamental processes of energy release and particle acceleration in flares are summarized. RHESSI??s precise measurements of hard X-ray continuum spectra enable model-independent deconvolution to obtain the parent electron spectrum. Taking into account the effects of albedo, these show that the low energy cut-off to the electron power-law spectrum is typically ?tens of keV, confirming that the accelerated electrons contain a large fraction of the energy released in flares. RHESSI has detected a high coronal hard X-ray source that is filled with accelerated electrons whose energy density is comparable to the magnetic-field energy density. This suggests an efficient conversion of energy, previously stored in the magnetic field, into the bulk acceleration of electrons. A new, collisionless (Hall) magnetic reconnection process has been identified through theory and simulations, and directly observed in space and in the laboratory; it should occur in the solar corona as well, with a reconnection rate fast enough for the energy release in flares. The reconnection process could result in the formation of multiple elongated magnetic islands, that then collapse to bulk-accelerate the electrons, rapidly enough to produce the observed hard X-ray emissions. RHESSI??s pioneering ??-ray line imaging of energetic ions, revealing footpoints straddling a flare loop arcade, has provided strong evidence that ion acceleration is also related to magnetic reconnection. Flare particle acceleration is shown to have a close relationship to impulsive Solar Energetic Particle (SEP) events observed in the interplanetary medium, and also to both fast coronal mass ejections and gradual SEP events. New instrumentation to provide the high sensitivity and wide dynamic range hard X-ray and ??-ray measurements, plus energetic neutral atom (ENA) imaging of SEPs above ??2 R_??, will enable the next great leap forward in understanding particle acceleration and energy release is large solar eruptions??solar flares and associated fast coronal mass ejections (CMEs).     

振幅矢量法与半波带法分析光栅衍射的比较

结合教学实际情况，为了在有限的学时内使学生能较好地理解光栅衍射条纹分布特点，通过振幅矢量法与半波带法分析光栅衍射的比较，提出用半波带法分析光栅衍射是可行的简化方法。因为在学习光栅衍射之前学生已学习了用半波带法分析单缝衍射，所以用半波带法分析光栅衍射，能够使学生较易于理解。     

Alice: The rosetta Ultraviolet Imaging Spectrograph

We describe the design, performance and scientific objectives of the NASA-funded ALICE instrument aboard the ESA Rosetta asteroid flyby/comet rendezvous mission. ALICE is a lightweight, low-power, and low-cost imaging spectrograph optimized for cometary far-ultraviolet (FUV) spectroscopy. It will be the first UV spectrograph to study a comet at close range. It is designed to obtain spatially-resolved spectra of Rosetta mission targets in the 700–2050 Å spectral band with a spectral resolution between 8 Å and 12 Å for extended sources that fill its ～0.05^ × 6.0^ field-of-view. ALICE employs an off-axis telescope feeding a 0.15-m normal incidence Rowland circle spectrograph with a toroidal concave holographic reflection grating. The microchannel plate detector utilizes dual solar-blind opaque photocathodes (KBr and CsI) and employs a two-dimensional delay-line readout array. The instrument is controlled by an internal microprocessor. During the prime Rosetta mission, ALICE will characterize comet 67P/Churyumov-Gerasimenko's coma, its nucleus, and nucleus/coma coupling; during cruise to the comet, ALICE will make observations of the mission's two asteroid flyby targets and of Mars, its moons, and of Earth's moon. ALICE has already successfully completed the in-flight commissioning phase and is operating well in flight. It has been characterized in flight with stellar flux calibrations, observations of the Moon during the first Earth fly-by, and observations of comet C/2002 T7 (LINEAR) in 2004 and comet 9P/Tempel 1 during the 2005 Deep Impact comet-collision observing campaign.     

The solar flare plasma: Observation and interpretation

In the past several years, X-ray observations of the Sun made from rockets and satellites have demonstrated the existence of high temperature (20 × 10⁶ – 100 × 10⁶ K), low density plasmas associated with solar flare phenomena. In the hard X-ray range ( < 1 ), spectra of the flaring plasma have been obtained using proportional and scintillation counter detectors. It is possible from these data to determine the evolution of the hard X-ray flare spectrum as the burst progresses; and by assuming either a non-thermal or thermal (Maxwellian) electron distribution function, characteristic plasma parameters such as emission measure and temperature (for a thermal interpretation) can be determined. Thermal interpretations of hard X-ray data require temperatures of 100 × 10⁶ K.In contrast, the soft X-ray flare spectrum (1 <<30 ) exhibits line emission from hydrogen-like and helium-like ions, e.g. Ne, Mg, Al, Si,... Fe, that indicates electron energies more characteristic of temperatures of 20 × 10⁶ K. Furthermore, line intensity ratios obtained during the course of an event show that the flare plasma can only be described satisfactorily by assuming a source composed of several different temperature regions; and that the emission measures and temperatures of these regions appear to change as the flare evolves. Temperatures are determined from line ratios of hydrogen-like to helium-like ions for a number of different elements, e.g., S, Si, and Mg, and from the slope of the X-ray continuum which is assumed to be due to free-free and free-bound emission. There is no obvious indication in soft X-ray flare spectra of non-thermal processes, although accurate continuum measurements are difficult with the data obtained to date because of higher order diffraction effects due to the use of crystal spectrometers.Soft X-ray flare spectra also show satellite lines of the hydrogen-like and helium-like ions, notably the 1s ²2s ² S-1s2s2p ² P transition of the lithium-like ion, and support the contention that in low density plasmas these lines are formed by dielectronic recombination to the helium-like ion. Also, series of allowed transitions of hydrogen-like and helium-like ions are strong, e.g., the Lyman series of S up to Lyman-, and ratios of the higher member lines to the Lyman- line can be compared with theoretical calculations of the relative line strengths obtained by assuming various processes of line formation.This review will discuss the X-ray spectrum of solar flares from 250 keV to 0.4 keV, but will be primarily concerned with the soft X-ray spectrum and the interpretation of emission lines and continuum features that lie in this spectral range.     

Measuring Black Hole Spin Using X-Ray Reflection Spectroscopy

I review the current status of X-ray reflection (a.k.a. broad iron line) based black hole spin measurements. This is a powerful technique that allows us to measure robust black hole spins across the mass range, from the stellar-mass black holes in X-ray binaries to the supermassive black holes in active galactic nuclei. After describing the basic assumptions of this approach, I lay out the detailed methodology focusing on “best practices” that have been found necessary to obtain robust results. Reflecting my own biases, this review is slanted towards a discussion of supermassive black hole (SMBH) spin in active galactic nuclei (AGN). Pulling together all of the available XMM-Newton and Suzaku results from the literature that satisfy objective quality control criteria, it is clear that a large fraction of SMBHs are rapidly-spinning, although there are tentative hints of a more slowly spinning population at high (M>5×10⁷ M _⊙) and low (M<2×10⁶ M _⊙) mass. I also engage in a brief review of the spins of stellar-mass black holes in X-ray binaries. In general, reflection-based and continuum-fitting based spin measures are in agreement, although there remain two objects (GRO J1655–40 and 4U 1543–475) for which that is not true. I end this review by discussing the exciting frontier of relativistic reverberation, particularly the discovery of broad iron line reverberation in XMM-Newton data for the Seyfert galaxies NGC 4151, NGC 7314 and MCG–5-23-16. As well as confirming the basic paradigm of relativistic disk reflection, this detection of reverberation demonstrates that future large-area X-ray observatories such as LOFT will make tremendous progress in studies of strong gravity using relativistic reverberation in AGN.     

Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares

High-energy X-rays and ??-rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at energies above several tens of keV has been the collisional thick-target model. After the launch of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed.     

High and medium resolution spectroscopy of the X-ray transient 4U1543-47

Preliminary results of an EXOSAT observation of the transient X-ray source 4U1543-47 are presented. The source was observed in August 1983, during a high state, following a Tenma alert that the source was again active. Results from the GSPC and the LE 1000 l/mm grating are presented. The spectrum is complex, but in the 2–10 keV energy range can be well described by a Comptonised thermal distribution. Extrapolating the same model into the lower energy band of the grating requires an absorption column density equivalent to 2 × 10²¹ H cm². A marked under-abundance of Oxygen and overabundance of Nitrogen, along with a strong, unidentified line feature at 9.8 Å, are necessary to model the grating data.     

Rosina – Rosetta Orbiter Spectrometer for Ion and Neutral Analysis

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) will answer important questions posed by the mission’s main objectives. After Giotto, this will be the first time the volatile part of a comet will be analyzed in situ. This is a very important investigation, as comets, in contrast to meteorites, have maintained most of the volatiles of the solar nebula. To accomplish the very demanding objectives through all the different phases of the comet’s activity, ROSINA has unprecedented capabilities including very wide mass range (1 to >300 amu), very high mass resolution (m/Δ m > 3000, i.e. the ability to resolve CO from N₂ and ¹³C from ¹²CH), very wide dynamic range and high sensitivity, as well as the ability to determine cometary gas velocities, and temperature. ROSINA consists of two mass spectrometers for neutrals and primary ions with complementary capabilities and a pressure sensor. To ensure that absolute gas densities can be determined, each mass spectrometer carries a reservoir of a calibrated gas mixture allowing in-flight calibration. Furthermore, identical flight-spares of all three sensors will serve for detailed analysis of all relevant parameters, in particular the sensitivities for complex organic molecules and their fragmentation patterns in our electron bombardment ion sources.     

Black Hole Spin via Continuum Fitting and the Role of Spin in Powering Transient Jets

The spins of ten stellar black holes have been measured using the continuum-fitting method. These black holes are located in two distinct classes of X-ray binary systems, one that is persistently X-ray bright and another that is transient. Both the persistent and transient black holes remain for long periods in a state where their spectra are dominated by a thermal accretion disk component. The spin of a black hole of known mass and distance can be measured by fitting this thermal continuum spectrum to the thin-disk model of Novikov and Thorne; the key fit parameter is the radius of the inner edge of the black hole’s accretion disk. Strong observational and theoretical evidence links the inner-disk radius to the radius of the innermost stable circular orbit, which is trivially related to the dimensionless spin parameter a _? of the black hole (|a _?|<1). The ten spins that have so far been measured by this continuum-fitting method range widely from a _?≈0 to a _?>0.95. The robustness of the method is demonstrated by the dozens or hundreds of independent and consistent measurements of spin that have been obtained for several black holes, and through careful consideration of many sources of systematic error. Among the results discussed is a dichotomy between the transient and persistent black holes; the latter have higher spins and larger masses. Also discussed is recently discovered evidence in the transient sources for a correlation between the power of ballistic jets and black hole spin.     

The Rosat mission

The primary scientific objective of the ROSAT mission is to perform the first all sky survey with an imaging X-ray telescope leading to an improvement in sensitivity by several orders of magnitude compared with previous surveys. Consequently a large number of new sources (> 10⁵) will be discovered and located with an accuracy of 1 arcmin. After completion of the survey which will take about half a year the instrument will be used for detailed observations of selected targets.The X-ray telescope consists of a fourfold nested Wolter type I mirror system with 80 cm aperture and 240 cm focal length, and three focal plane detectors. In the baseline version these will be imaging proportional counters (0.1 – 2 keV) providing a field of view of 2⁰ × 2⁰.     

A search for counterparts of selected COS-B galactic gamma-ray sources: The X-ray survey and optical identifications

The Einstein X-ray Imaging Instruments have been used to explore, down to an unprecedented sensitivity, the X-ray behavior of 7 high-energy γ-ray sources discovered by the COS-B satellite. 32 low latitude (¦b¦ < 5°) IPC fields, mosaic-arranged to cover the few-square-degrees COS-B error circles, yielded 30 soft X-ray sources, the fluxes of which range from ～ 1/100 to few UFU, and no diffuse features. While the density of ～ 1 source/IPC field is consistent with the value found at higher latitudes, the percentage of ‘stellar’ identifications among these low-latitude sources is significantly higher than in non-galactic-biased samples. Unfortunately, the positional accuracy achieved with the IPC does not allow astronomical identification in the absence of obvious counterpart(s). However, after the exploratory coverage, the IPC data were used, when possible, to point out potentially interesting targets for the HRI instrument capable of an accuracy of ～ 3 arc sec. Due to the misfortunes which occurred to the Einstein satellite, this time-consuming process was feasible only in two cases: within the error circle of 2CG135 + 01, the radio variable star LSI61.303 was pinpointed by the HRI, while the HRI exposure of the brightest X-ray source discovered in 2CG 195 + 04 (Geminga) positioned a source in an empty POSS field. The latter case will be presented and the nature of the X- and γ(?)-ray source briefly discussed.     

The plasma instrumentation for the Galileo Mission

The plasma instrumentation (PLS) for the Galileo Mission comprises a nested set of four spherical-plate electrostatic analyzers and three miniature, magnetic mass spectrometers. The three-dimensional velocity distributions of positive ions and electrons, separately, are determined for the energy-per-unit charge (E/Q) range of 0.9 V to 52 kV. A large fraction of the 4-steradian solid angle for charged particle velocity vectors is sampled by means of the fan-shaped field-of-view of 160°, multiple sensors, and the rotation of the spacecraft spinning section. The fields-of-view of the three mass spectrometers are respectively directed perpendicular and nearly parallel and anti-parallel to the spin axis of the spacecraft. These mass spectrometers are used to identify the composition of the positive ion plasmas, e.g., H⁺, O⁺, Na⁺, and S⁺, in the Jovian magnetosphere. The energy range of these three mass spectrometers is dependent upon the species. The maximum temporal resolutions of the instrument for determining the energy (E/Q) spectra of charged particles and mass (M/Q) composition of positive ion plasmas are 0.5 s. Three-dimensional velocity distributions of electrons and positive ions require a minimum sampling time of 20 s, which is slightly longer than the spacecraft rotation period. The two instrument microprocessors provide the capability of inflight implementation of operational modes by ground-command that are tailored for specific plasma regimes, e.g., magnetosheath, plasma sheet, cold and hot tori, and satellite wakes, and that can be improved upon as acquired knowledge increases during the tour of the Jovian magnetosphere. Because the instrument is specifically designed for measurements in the environs of Jupiter with the advantages of previous surveys with the Voyager spacecraft, first determinations of many plasma phenomena can be expected. These observational objectives include field-aligned currents, three-dimensional ion bulk flows, pickup ions from the Galilean satellites, the spatial distribution of plasmas throughout most of the magnetosphere and including the magnetotail, and ion and electron flows to and from the Jovian ionosphere.     

He-like Ions as Practical Astrophysical Plasma Diagnostics: From Stellar Coronae to Active Galactic Nuclei

We review X-ray plasma diagnostics based on the line ratios of He-like ions. Triplet/singlet line intensities can be used to determine electronic temperature and density, and were first developed for the study of the solar corona. Since the launches of the X-ray satellites Chandra and XMM-Newton, these diagnostics have been extended and used (from C?v to Si?xiii) for a wide variety of astrophysical plasmas such as stellar coronae, supernova remnants, solar system objects, active galactic nuclei, and X-ray binaries. Moreover, the intensities of He-like ions can be used to determine the ionization process(es) at work, as well as the distance between the X-ray plasma and the UV emission source for example in hot stars. In the near future thanks to the next generation of X-ray satellites (e.g., Astro-H and IXO), higher-Z He-like lines (e.g., iron) will be resolved, allowing plasmas with higher temperatures and densities to be probed. Moreover, the so-called satellite lines that are formed closed to parent He-like lines, will provide additional valuable diagnostics to determine electronic temperature, ionic fraction, departure from ionization equilibrium and/or from Maxwellian electron distribution.