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  • 1
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    Statistical uncertainties in temperature diagnostics for hot coronal plasma using the ASCA SIS (1994)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: Statistical uncertainties in determining the temperatures of hot (0.5-10 keV) coronal plasmas are investigated. The statistical presicion of various spectral temperature diagnostics is established by analyzing synthetic ASCA solid-state imaging spectrometer (SIS) CCD spectra. The diagnostics considered are the ratio of hydrogen-like to helium-like line complexes of Z greater than or = 14 elements, line-free portions of the continuum, and the entire spectrum. While fits to the entire spectrum yield the highest statistical precision, it is argued that fits to the line-free continuum are less susceptible to atomic data uncertainties but lead to a modest increase in statistical uncertainty over full spectral fits. Temperatures deduced from line ratios can have similar accuracy, but only over a narrow range of temperatures. Convenient estimates of statistical accuracies for the various temperature diagnostics are provided which may be used in planning ASCA SIS observations.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 435; 2; p. L149-L152
    Format: text
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  • 2
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    On the Nature of the Bright Short-Period X-Ray Source in the Circinus Galaxy Field (2004)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The spectrum and light curve of the bright X-ray source CG X-1 in the field of the Circinus galaxy are reexamined. Previous analyses have concluded that the source is an accreting black hole of mass 〉 or approx. 50 solar masses although it has been noted that the light curve resembles that of an AM Herculis system. Here we show that the short period and an assumed main-sequence companion constrain the mass of the companion to less than 1 solar mass. Furthermore, a possible eclipse seen during one of the Chandra observations and a subsequent XMM-Newton observation constrain the mass of the compact object to less than 60 solar masses. If such a system lies in the Circinus galaxy, then the accreting object must either radiate anisotropically or strongly violate the Eddington limit. Even if the emission is beamed, then the companion star that intercepts this flux during eclipse will be driven out of thermal equilibrium and evaporate within approx. 10(exp 3) yr. We find that the observations cannot rule out an AM Herculis system in the Milky Way and that such a system can account for the variations seen in the light curve.
    Keywords: Astronomy
    Type: The Astrophysical Journal; Vol. 605; 360-367
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  • 3
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    Accretion geometry of synchronously rotating magnetic cataclysmic variables: Theories versus observations (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We review the magnetic locking of the white dwarf and the companion star in AM Herculis-type binaries for various white dwarf magnetic field structures, and compare the theoretical results with observations. The model in which the white dwarf has a dipole plus a quadrupole field is found to be in closest agreement with the recent observations.
    Keywords: ASTROPHYSICS
    Type: Astronomical Society of Australia, Proceedings (ISSN 0066-9997); 11; 2; p. 198-200
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  • 4
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    Dust scattering in the high-state eclipse of Cen X-3 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Exosat data from two high-state eclipses of Cen X-3 are analyzed for the effects of dust scattering. The light curves show bright (2 per cent of the out-of-eclipse flux), smoothly varying emission which reaches its minimum after mideclipse. Of this emission, the soft flux falls monotonically with orbital phase. This soft component with dust scattering is identified and spectral models are fitted to the asymmetric emission, that is, to spectra obtained by subtracting from a spectrum at a given orbital phase before phase zero the corresponding spectrum from the equal and opposite phase after phase zero. It is found that the model of Molnar and Mauche, used to fit dust scattering in Cyg X-3, can explain the observed spectra and yields values for the distance to Cen X-3 and for the size of the dust grains of 5.4 kpc and 0.33 micron, consistent with previous observations in other wavebands.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 251; 76-83
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  • 5
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    The absence of rapid X-ray variability in active galaxies (1983)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: (Previously announced in STAR as N82-30204)
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 264
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  • 6
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    The absence of rapid X-ray variability in active galaxies (1982)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Variations on time scales ranging from minutes to several hours in the X-ray flux from 54 observations of 38 active galaxies are identified. The sample is composed mostly of Seyfert I galaxies but also includes radio galaxies, NELG's BL Lacs and 3C 273. Only NGC 6814 varied on time scales as short as 100 sec. No other source was observed to vary with a time scale of less than 12 hours. Large amplitude short term variations are not a characteristic of the X-ray emission from active galaxies. Upper limits on sigma sub I/I ranged from 2% for Cen A, 5% for NGC 4151, to approximately 20% for sources giving 1 ct/sec in the detector. Three objects NGC 3227, NGC 4151 and MCG 5-23-16 show variability consistent with a time scale of approximately 1 day. Ways to reconcile the rapid variability seen for NGC 6814 (and NGC 4051) with the general stability observed for the other objects are considered.
    Keywords: ASTROPHYSICS
    Type: NASA-TM-83966 , NAS 1.15:83966
    Format: application/pdf
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  • 7
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    Rapid X-Ray Variability of Active Galaxies (1983)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Active galactic nuclei are luminous sources of X-rays. The thesis that the X-rays are generated within 10 gravitational radii from the central object is tested. A very sensitive search for rapid ( 1 day) X-ray variability from active galaxies was made.
    Keywords: ASTROPHYSICS
    Type: NASA-TM-85101 , NAS 1.15:85101
    Format: application/pdf
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  • 8
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    Observations of NGC 4151 at 2 keV to 2 MeV from HEAO 1 (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Broad-range X-ray and gamma-ray spectra of the Seyfert galaxy NGC 4151 obtained by the UCSD/MIT and GSFC/CIT detectors of HEAO 1 are presented. For two out of the three observational epochs the best-fit model is a power law with photon index 1.6 + or - 0.1 in the 2 keV-2 MeV spectral range. The third observation, in June, 1978, when the intensity was a factor of 2 higher, requires a change above 50 keV from a similar power law to a steeper power law or an exponential. Through comparisons with previous observations, a weak correlation is found between the high-energy spectral index and the flux level. Comparison of these data and other X-ray and gamma-ray observations with the spectra expected from several possible emission mechanisms supports a synchrotron-self-Compton model using infrared target photons or a Compton-scattering process involving X-rays of less than 10 keV energies.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 279; 555-562
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  • 9
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    A Study of the X-ray Source Population in the Dwarf Galaxy NGC 6822 (2003)
    In:  Other Sources
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    Publication Date: 2019-07-18
    Description: The dlrr galaxy NGC 6822 is a distant member of the Local Group. It is a site of recent star formation, rich in HII regions and OB associations, as well as containing an older globular cluster population. We present results of a deep Chandra observation of NGC 6822. The brightest source is extended and most likely a SNR. In addition to spectral analysis of the brightest sources, we extend the luminosity function down to the 10(sup)35 erg/s range.
    Keywords: Astronomy
    Type: American Astronomical Society; May 25, 2003 - May 29, 2003; Nashville, TN; United States
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  • 10
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    Chandra Observations of the Anomalous X-ray Pulsar 4U 0142+61 (2002)
    In:  Other Sources
    Details
    Publication Date: 2019-07-18
    Description: We present X-ray imaging, timing, and phase resolved spectroscopy of the anomalous X-ray pulsar 4U 0142+61 using the Chandra X-ray Observatory. The spectrum is well described by a power law plus blackbody model with Gamma = 3.35(2), kT=0.458(3) keV, and N-H = 0.91(2) x 10(exp 22)/sq cm); we find no significant evidence for spectral features (0.5 - 7.0 keV). Time resolved X-ray spectroscopy shows evidence for evolution in phase in either Gamma, or kT or some combination thereof as a function of pulse phase. We derive a precise X-ray position for the source and determine its spin period, P=8.68866(30) s. We have detected emission beyond 4 arcsec from the central source and extending beyond 100 arcsec, likely due to dust scattering in the interstellar medium.
    Keywords: Astronomy
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