Search

Byline: D. H. Lumb (1), A. Finoguenov (2), R. Saxton (3), B. Aschenbach (2), P. Gondoin (1), M. Kirsch (4), I. M. Stewart (3) Keywords: CCDs; X-ray astronomy; X-ray detectors; X-ray mirrors; XMM-Newton Abstract: We describe measurements of the mirror vignetting in the XMM-Newton Observatory made in-orbit, using observations of SNR G21.5-09 and SNR 3C58 with the EPIC imaging cameras. The instrument features that complicate these measurements are briefly described. We show the spatial and energy dependences of measured vignetting, outlining assumptions made in deriving the eventual agreement between simulation and measurement. Alternate methods to confirm these are described, including an assessment of source elongation with off-axis angle, the surface brightness distribution of the diffuse X-ray background, and the consistency of Coma cluster emission at different position angles. A synthesis of these measurements leads to a change in the XMM calibration data base, for the optical axis of two of the three telescopes, by in excess of 1 arcmin. This has a small but measureable effect on the assumed spectral responses of the cameras for on-axis targets. Author Affiliation: (1) Adv. Concepts &amp Science Payloads Office, European Space Agency, ESTEC, 2200AG, Noordwijk, the Netherlands (2) Max-Planck Institut fur Extraterrestrische Physik, Giessenbachstrasse 1, D-85748, Garching, Germany (3) XMM Survey Science Centre, Dept. of Physics &amp Astronomy, Leicester University, Leicester, LE1 7RH, U.K (4) XMM Science Ops. Centre, European Space Agency, Apartado, P.O. Box 50727, 28080, Madrid, Spain Article History: Registration Date: 15/09/2004

Byline: D. H. Lumb (1) Keywords: CCD detectors; CHANDRA; detector simulation; XMM; X-ray astronomy Abstract: Simulations of the XMM EPIC instruments are presented. They suggest the correct operating mode must be chosen to ensure that spectral analysis of the data is not compromised by 'pile-up' effects. The performanceis contrasted with the CHANDRA AXAF CCD Imaging Spectrometer, showing how the higher effective area but better over-sampling of its mirror response function in XMM, impacts this pile-up criteria. Targetsexceeding a flux of a few 10.sup.-12 ergs cm.sup.-2 s.sup.-1 will becompromised for spectral analysis in these CCD instruments, unlesscare is taken in defining correct instrument configurations. Somesimple methods of mitigating the effects are described, but this featureof X-ray pile-up warrants careful attention in calibration. Author Affiliation: (1) Space Science Department, European Space Agency ESTEC, Noordwijk, The Netherlands Article History: Registration Date: 07/10/2004

To investigate the formation and early evolution of stars, astronomers study the x-ray emission of T Tauri stars, which are young, solar-mass stars called pre-main sequence stars. Two Earth-orbiting x-ray satellites, the Rontgen X-ray Satellite (ROSAT) and the Advanced Satellite for Cosmology and Astrophysics (ASCA), have discovered x-ray emission from young protostars, called Class I objects. Many T Tauri stars were detected as x-ray sources by ROSAT. X-ray luminosity functions and correlations with other stellar parameters can be studied and used to investigate the x-ray emission mechanism. From the ROSAT data hundreds of T Tauri stars have been discovered, some of which are located outside regions of ongoing star formation. Stellar x-rays also irradiate circumstellar disks, regions where planets may form, so x-ray emission data from T Tauri stars may also be used to investigate the formation of planets., Stars are born during the collapse of gas and dust clouds, so the youngest protostars are best observed at infrared (IR) and millimeter wavelengths (1). Optical emission from the protostar [...]

Neutron stars and black holes can be studied by observation of the radiation produced as matter falls into their gravitational fields. X-ray binaries, which are systems consisting of a neutron star or black hole and a companion gaseous star, produce radiation in this manner. Recently, oscillations at frequencies near 1000 cycles per second have been detected from x-ray binaries. These oscillations are likely produced in regions of very strong gravitational fields within a few tens of kilometers of the compact star. The oscillations have been interpreted as evidence for the existence of an innermost stable orbit near a compact star, a key prediction of general relativity theory. The study of x-ray binaries has also advanced the search for definitive evidence of black holes. Recent developments in our understanding of accretion flows in x-ray binaries have provided evidence for the existence of event horizons in x-ray binaries thought to contain black holes., Neutron stars and stellar-mass black holes are born in the deaths of gaseous stars, when exhaustion of the nuclear energy of a star causes it to collapse under the pull [...]

During its close approach to Earth, comet C/Hyakutake 1996 B2 was observed at extreme ultraviolet and x-ray wavelengths with the Rontgen X-ray Satellite and Rossi X-ray Timing Explorer. The emission morphology was symmetric with respect to a vector from the comet's nucleus toward the sun, but not symmetric around the direction of motion of the comet with respect to interplanetary dust. A slowly varying emission and a large impulsive event that varied on time scales of 1 to 2 hours were observed. An interaction between the comet and the solar wind/solar magnetic field seems to be the most likely mechanism for the observed emission., The close perigee [minimum geocentric distance δ = 0.102 AU (astronomical unit)] of the long-period comet C/Hyakutake 1996 B2 at universal time (UT) 25.3 March 1996 presented a unique opportunity [...]

Observations made with the x-ray satellite ROSAT (Roentgen Satellite) have produced the first spatially resolved x-ray image of a corona around a star like our sun. The star is the secondary in the eclipsing binary system α Coronae Borealis (CrB), which consists of one star of spectral type AOV and one of type G5V. The x-ray light curve of α CrB shows a total x-ray eclipse during secondary optical minimum, with the G star behind the A star. The totality of the eclipse demonstrates that the A-type component in α CrB is x-ray dark and that the x-ray flux arises exclusively from the later-type companion. The x-ray eclipse ingress and egress are highly asymmetric compared with the optical eclipse, indicating a highly asymmetric x-ray intensity distribution on the surface of the G star. From a detailed modeling of the ingress and egress of the x-ray light curve, an eclipse map of the G star was constructed by a method based on an optimization by simulated annealing., Soft x-ray observations with the Einstein and ROSAT satellites in the wavelength range 6 to 100 Å have shown the ubiquitous presence of hot, tenuous coronae around almost all types [...]

THE 1/4-keV COSMIC SOFT X-RAY diffuse background (SXRB) was first detected over two decades ago [1, 2], yet the spatial distribution of the source or sources responsible for this emission [...]

Photometry and spectroscopy of the object Cha H[Alpha] 1, located in the Chamaeleon I star-forming cloud, show that it is a ~[10.sup.6]-year-old brown dwarf with spectral type M7.5 to M8 and 0.04 ± 0.01 solar masses. Quiescent x-ray emission was detected in a 36-kilosecond observation with 31.4 ± 7.7 x-ray photons, obtained with the Rontgen Satellite (ROSAT), with 9 Σ detection significance. This corresponds to an x-ray luminosity of 2.57 x [10.sup.28] ergs per second and an x-ray to bolometric luminosity ratio of [10.sup.-3.44]. These are typical values for late M-type stars. Because the interior of brown dwarfs may be similar to that of convective late-type stars, which are well-known x-ray sources, x-ray emission from brown dwarfs may indicate magnetic activity., A brown dwarf (BD) is an object with a mass [is less than or equal to] 0.08 solar masses (M??), which is insufficient to sustain nuclear fusion of hydrogen (1). [...]

An X-ray afterglow was discovered by the Beppo-SAX satellite following the gamma-ray burst on Feb 28 1997. A substantial proportion of the gamma-ray burst's total energy was contained in the X-ray transient. A multi-wavelength observational campaign was inititated by the quick detection of the gamma-ray burst that led to the discovery of a fading optical transient. The Beppo-SAX measurement explains the link between the location capability of radio, optical and X-ray astronomy and the gamma-ray phenomenology.

The instability resulting from the relative motion of newly picked-up cometary photoions and the solar wind generates lower hybrid waves that are capable of accelerating electrons to the keV range of energies. These electrons may be responsible for the x-rays from comet C/Hyakutake 1996 B2 seen by the Rontgen X-ray Satellite. The inferred x-ray photon power depends on the electron energy, with keV electrons providing values of photon power two orders of magnitude greater than 100 eV electrons. These observations and in particular, spectral resolution of the x-rays, should provide more insight into the composition of the comet., Observations by the Rontgen X-ray Satellite (ROSAT) on 26 to 28 March 1996 of x-ray emission from comet C/Hyakutake 1996 b2 (1) appear to have been a surprise to most [...]

An astronomical source GRO J1744 - 28 is a source of coherent, hard X-ray bursts which have a period of 467 milliseconds. The object is most likely a magnetized neutron star which is gathering gas from a low-mass companion star. Observations show that the pulsation rate is increasing and this indicates the formation of an accretion disk. The neutron star is spinning faster due to transfer of matter from the disk to the star.

Among the great astronomical discoveries of the 1960s was the totally unexpected finding by Giacconi et al. (1) of a bright x-ray star, now known as Scorpio X-1. Because cosmic [...]

During the last 30 years x-ray astronomy has become one of the cornerstones of observational astrophysics. At x-ray wavelengths, we see the 'hot universe'--objects at temperatures of millions to billions [...]

The Galactic plane is a strong emitter of hard x-rays (2 to 10 kiloelectron volts), and the emission forms a narrow continuous ridge. The currently known hard x-ray sources are far too few to explain the ridge x-ray emission, and the fundamental question of whether the ridge emission is ultimately resolved into numerous dimmer discrete sources or truly diffuse emission has not yet been settled. In order to obtain a decisive answer, using the Chandra X-ray Observatory, we carried out the deepest hard x-ray survey of a Galactic plane region that is devoid of known x-ray point sources. We detected at least 36 new hard x-ray point sources in addition to strong diffuse emission within a 17' by 17' field of view. The surface density of the point sources is comparable to that at high Galactic latitudes after the effects of Galactic absorption are considered. Therefore, most of these point sources are probably extragalactic, presumably active galaxies seen through the Galactic disk. The Galactic ridge hard x-ray emission is diffuse, which indicates omnipresence within the Galactic plane of a hot plasma, the energy density of which is more than one order of magnitude higher than any other substance in the interstellar space., The Galactic ridge x-ray emission exhibits emission lines from highly ionized heavy elements such as Si, S, and Fe; hence, it may be considered to originate from optically thin hot [...]

If you spin in your office chair while holding a full cup of coffee, centrifugal force is likely to leave you wet -- even if the entire office building and [...]

The soft x-ray spectra of the Virgo and Coma clusters cannot be modeled by emission from the well-known hot intracluster medium attenuated by photoelectric absorption along the line of sight in our galaxy. If the excess soft emission is due to thermal emission in the clusters from gas at about 1 million kelvin, then the gas should be rapidly cooling. The high rate of cooling poses problems for the origin of the gas and its sink. A model in which the component is due to turbulent mixing layers around clouds scattered throughout the intracluster medium may explain the excess emission. The gas in the mixing layer is reheated after cooling, so that the total mass remains relatively small., Lieu et al. (1) report excess soft x-ray emission in the Virgo and Coma clusters, greater than that expected from the known hot intracluster medium in those clusters. The excess [...]

The Hague--You wouldn't want to get too close to a quasar, but astronomers would love to get a better view of these enigmatic beasts. Seen only in the farthest reaches [...]

The diversity of physics was much in evidence when 1500 physicists assembled in Washington, D.C., last week for the spring meeting of the American Physical Society (APS). But so was [...]