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  • ASTROPHYSICS  (80)
  • 1
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    ROSAT PSPC spectra of six Seyfert 1 galaxies (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: We present six ROSAT Position-Sensitive Proportional Counter (PSPC) observations of Seyfert 1 galaxies chosen to have low Galactic line-of-sight absorption (N(sub H) approx. = 10(exp 20)/sq cm). As expected, it is found that all of these sources possess significantly steeper spectra below approximately 1 keV, than that observed at higher X-ray energies. In addition we find evidence for soft X-ray spectral features, which are best parameterized as line emission at approximately 0.63 keV in NGC7469 and approximately 0.75 keV in ESO198-G24. We examine these results in the light of the accuracy of the PSPC spectral calibration. We suggest that Seyfert galaxies are a class of object whose spectra are similar to the diffuse X-ray background in the soft X-ray band. We suggest these data provide the first direct evidence that the sources which dominate the background differ as a function of energy.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 13; 12; p. (12)387-(12)390
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  • 2
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    Discovery and implications of very low metal abundances in NGC 1404 and NGC 4374 (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We present our analysis of ASCA PV phase observations of the elliptical galaxies NGC 1404 and NGC 4374 (M84). The average metallicities in the hot gas derived from the SIS spectra are exceptionally low, Z approximately 0.15 solar, while the temperatures are 'typical,' kT approximately 0.75 keV. We also place upper limits on intrinsic column densities. The low abundances lend support to the theory of Fe enrichment of intracluster media by protogalactic Type II supernova-driven winds and raise the possibility of a fundamental connection between baryon fraction, dissipation, and abundances in elliptical galaxies.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 436; 1; p. L75-L78
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  • 3
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    The X-ray variability of NGC 6814 - Power spectrum (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The existence of the periodic component seen in NGC 6814 with Exosat at 12,000 +/- 100 s is confirmed by a power spectrum and folded light curve analysis of unevenly sampled Ginga data. A comparison of the power spectra produced from simulated light curves with that observed enables the intrinsic shape of the power spectrum of the source to be determined despite the distortions introduced by the window function. The best estimate for the period is 12,132 +/- 3 s, where the error is that derived from simulations. An upper limit to the rate of change of period of about 10 exp -9 is inferred if the light curves are truly phase-coherent, but as this is not required by the data, the conservative upper limit is not greater than 5 x 10 exp -7. The large amount of power in the periodic component and its stability both suggest occultation of the source as its origin.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 400; 1; p. 138-152.
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  • 4
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    Resolving the iron K line in Cygnus X-2 - An observation with BBXRT (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The Astro-1 Space Shuttle payload's Broad Band X-ray telescope has been used to obtain high-quality, moderate-resolution spectroscopy of Cygnus X-2 which allow the resolving of the physical width of the 6.7 keV Fe K-alpha feature with a factor-of-4 energy resolution improvement over past experiments. Three possible sites are noted for the Fe K-alpha emission: the accretion disk, its corona, and the source itself. It is judged that reflection from the accretion disk can generate a line of the observed energy, width, and equivalent width, provided that the disk surface is highly ionized.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 410; 2; p. 796-802.
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  • 5
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    ROSAT Position Sensitive Proportional Counter spectra of six Seyfert 1 galaxies (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We present the results from ROSAT Position Sensitive Proportional Counter observations of six Seyfert 1 galaxies in the soft (0.1-2.0 keV) X-ray band. The sources (Mrk 335, ESO 198-G24, ESO 141-G55, Mrk 509, NGC 7469, and MCG-2-58-22) were chosen to have low absorbing column densities along the line of sight. As expected, it is found that all the sources possess significantly steeper spectra below about 1 keV than observed at higher X-ray energies. Assuming a simple absorbed power-law spectral model, the mean (photon) spectral index for the sample is Gamma = 2.38 +/- 0.25, compared to the canonical 1.7 typically observed in the 2-10 keV band. Furthermore, we find strong evidence for soft X-ray spectral features in half the sources. In NGC 7469 and ESO 198-G24, we find that the addition of a narrow emission line or an absorption edge to the underlying continuum is a significant improvement to the parameterization of the spectra. Mrk 335 also shows evidence for spectral complexity, but from these data it is not possible to unambiguously distinguish between an absorption edge and a steepening of the spectrum at low energies. We examine these results in the light of the accuracy of the PSPC spectral calibration.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 412; 1; p. 72-81.
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  • 6
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    Broad-band X-ray telescope observations of the magnetic cataclysmic variable H0538 + 608 = BY Camelopardalis (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We report on BBXRT observations of the cataclysmic variable H0538 + 608 (BY Cam). The unexcelled energy resolution of BBXRT in the vicinity of the iron K line allows the line strength and energy to be constrained more tightly than has previously been possible. We are able to rule out a significant contribution from fluorescence on iron more neutral than Fe XVIII. Implications for the conditions in the accreting gas are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal - Part 1 (ISSN 0004-637X); 411; 2; p. 869-873.
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  • 7
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    Broad band X-ray Telescope observations of the hot interstellar media in NGC 1399 and NGC 4472 (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We present our analysis and interpretation of spatially resolved X-ray spectroscopy of the elliptical galaxies NGC 1399 and NGC 4472 obtained with the Broad Band X-Ray Telescope. The X-ray emission from both galaxies is dominated by thermal emission from a hot interstellar medium. The temperature of the plasma in NGC 1399 is 1.0-1.2 keV with a mild positive temperature gradient; for NGC 4472 the average temperature is 0.7-1.0 keV. The temperature of NGC 1399, when combined with other X-ray and optical data, implies that, if the gas is gravitationally confined, about 70 percent of the mass inside a radius of about 8.6 arcmin (38 kpc) is nonluminous. The derived metallicities and metallicity gradients are consistent with optically determined stellar metallicities, and allow surprisingly small upper limits to be placed on Type Ia supernovae rates in both galaxies. The implications of this for the gasdynamical evolution of the two galaxies are discussed. Limits on the oxygen-to-iron ratio and the line-of-sight column density are derived. There is a possible detection of cold gas in the line of sight to NGC 1399 that exceeds the Galactic value. Constraints on the contribution from a harder spectral component are placed, and likely candidates for the origin of this secondary component are evaluated.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 413; 2; p. 518-530.
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  • 8
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    Problems for the standard black hole/accretion disk models in Cygnus X-1? (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Archival EXOSAT and HEAO1-A2 data from Cyg X-1 show the 'high energy excess' above 10 keV seen in X-ray observations of AGN. Using a likelihood ratio test, we are for the first time able to distinguish conclusively in favor of Compton reflection rather than partial covering as the origin of the high energy excess. This supports the idea of an X-ray illuminated accretion disk in Cyg X-1, but the line equivalent width is smaller by a factor of 2-3 than that expected from such a disk. While the larger optical depth required for reflection as opposed to line emission admit the possibility of seeing line without reflection, the converse is not possible. To see a reflection spectrum, including the strong iron absorption edge, implies that strong iron emission must be observed as the line and edge are causally linked.
    Keywords: ASTROPHYSICS
    Type: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 196-199.
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  • 9
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    X-ray FE line studies (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We present the first broadband, moderate-resolution X-ray spectral observations of the Seyfert galaxies NGC 4151 and NGC 1068. The Seyfert 1 galaxy NGC 4151 is shown to have a complex X-ray spectrum with multiple components and a strong, intrinsically narrow Fe K emission line. The characteristics of the scattering medium of the Seyfert 2 galaxy NGC 1068 are determined using the very strong Fe line emission and the lack of features due to oxygen in the X-ray spectrum. The soft X-ray flux from Mrk 3 and Mrk 348 are consistent with that expected from electron scattered X-rays from an obscured Seyfert 1 nucleus. A survey of the X-ray properties of Seyfert 2 galaxies indicates that a substantial fraction may have column densities of about 10 exp 23, suggesting that NGC 1068 has an unusually thick obscuring torus.
    Keywords: ASTROPHYSICS
    Type: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 183-191.
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  • 10
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    The X-ray variability of AGN and the anomalous behavior of NGC 6814 (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The power spectra of the X-ray variable AGN are typically scale invariant with no characteristic timescale. The one strong exception to this is NGC 6814, where the EXOSAT data showed evidence for a periodic component at 12200 +/- 100 seconds. The power spectra of a GINGA lightcurve from this source, found using simulation techniques to account for the uneven sampling, also cannot be well fit by a single power law. A folded light curve analysis of GINGA data shows strong evidence for this periodic component. A second GINGA observation of this source taken one year later is consistent with the phenomena being completely periodic and phase coherent for 7 periods in the range of 12,110-12,145 seconds. Including the (optimistic) limits from the folding selects a period of 12130.39 +/- 0.05 seconds. Phase coherence is not maintained between this and the EXOSAT observations, as the structure of the folded light curves is very different. Thus the periodicity is long lived and stable, but phase coherence is only maintained on timescales of about 1 year.
    Keywords: ASTROPHYSICS
    Type: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 83-92.
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