We present a spectral analysis of the Crab Nebula obtained with the Chandra X-ray observatory. The X-ray spectrum is characterized by a power law whose index varies across the nebula. The variation ...can be discussed in terms of the particle injection from the pulsar in two different directions: the equatorial plane containing the torus and the symmetry axis along the jet. In the equatorial plane, spectra within the torus are the hardest, with a photon index alpha ~ 1.9, and are almost independent of the surface brightness. At the periphery of the torus, the spectrum gradually softens in the outer, lower surface brightness regions, up to alpha ~ 3.0. This indicates that synchrotron losses become significant to X-ray-emitting particles at the outer boundary of the torus. We discuss the nature of the torus, incorporating information from observations at other wavelengths. Spectral variations are also seen within the southern jet. The core of the jet is the hardest, with alpha ~ 2.0, and the outer sheath surrounding the core becomes softer with alpha up to 2.5 at the outermost part. Based on the similarity between the spectra of the jet core and the torus, we suggest that the electron spectra of the particles injected from the pulsar are also similar in these two different directions. The brightness ratio between the near and far sides of the torus can be explained by Doppler boosting and relativistic aberration; however, the observed ratio cannot be derived from the standard weakly magnetized pulsar wind model. We also found a site where an optical filament comprising supernova ejecta is absorbing the soft X-ray emission (<2 keV).
The Chandra X-Ray Observatory observed the Crab Nebula and pulsar during orbital calibration. Zeroth-order images with the High-Energy Transmission Grating (HETG) readout by the Advanced CCD Imaging ...Spectrometer spectroscopy array (ACIS-S) show a striking richness of X-ray structure at a resolution comparable to that of the best ground-based visible-light observations. The HETG-ACIS-S images reveal, for the first time, an X-ray inner ring within the X-ray torus, the suggestion of a hollow-tube structure for the torus, and X-ray knots along the inner ring and (perhaps) along the inward extension of the X-ray jet. Although complicated by instrumental effects and the brightness of the Crab Nebula, the spectrometric analysis shows systematic variations of the X-ray spectrum throughout the nebula.
The Cradle of the Solar System Hester, J. Jeff; Desch, Steven J.; Healy, Kevin R. ...
Science (American Association for the Advancement of Science),
05/2004, Letnik:
304, Številka:
5674
Journal Article
Recenzirano
Like most low-mass stars, the Sun is formed in a high-mass star-forming region where one or more stars went supernova. Hester et al discuss the recent studies on the star formation of the solar ...system.
The Photometric Performance and Calibration of WFPC2 Holtzman, Jon A.; Burrows, Christopher J.; Casertano, Stefano ...
Publications of the Astronomical Society of the Pacific,
11/1995, Letnik:
107, Številka:
717
Journal Article
Recenzirano
Odprti dostop
We discuss the photometric performance and calibration of the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). The stability and accuracy of WFPC2 photometric measurements ...is discussed, with particular attention given to charge-transfer efficiency (CTE) effects, contamination effects in the ultraviolet (UV), and flat-field accuracy and normalization. Observational data are presented from both WFPC2 observations and ground observations using a system similar to that flown. WFPC2 photometric systems are defined both for the ground and flight systems. Transformations between these systems and the Landolt UBVRI system are presented. These transformations are sensitive to details in the spectra being transformed, and these sensitivities are quantified and discussed. On-orbit observations are used to revise the prelaunch estimates of response curves to best match synthetic photometry results with observations, and the accuracy of the resulting synthetic photometry is discussed. Synthetic photometry is used to determine zero points and transformations for all of the flight filters, and also to derive interstellar extinction values for the WFPC2 system. Using stellar interior and atmosphere models, isochrones in the WFPC2 system are calculated and compared with several observations.
Far-UV images of Jupiter from the Hubble Space Telescope Wide Field Planetary Camera 2 reveal polar auroral emissions at 300 kilometer resolution and three times higher sensitivity than previously ...achieved.
The BvgAS signal transduction system controls the expression of at least three distinct phenotypic phases that lie along a continuum of gene expression states. The Bvg+ phase is characterized by the ...expression of adhesins and toxins, whereas the Bvg− phase is characterized by motility in Bordetella bronchiseptica and the expression of vrg loci in Bordetella pertussis. The Bvg‐intermediate (Bvgi) phase is characterized by the absence of Bvg‐repressed phenotypes, the expression of some, but not all, Bvg‐activated virulence factors and the presence of a recently discovered set of antigens and phenotypes that are unique to this phase. We report here the transcriptional regulation of bipA, the first‐identified Bvgi phase gene. We have mapped the bipA promoter and identified numerous BvgA binding sites in the transcriptional control region. Based on these data, we present a model in which phase‐dependent expression of bipA results from the spatial distribution and relative affinities of multiple BvgA binding sites relative to the start site of transcription.
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