Engineering topics

Weaver, Kimberly A.

Publications and source records attributed to Weaver, Kimberly A..

8 records

NASA NTRS2014

Goddard's Astrophysics Science Division Annual Report 2013

The Astrophysics Science Division (ASD) at Goddard Space Flight Center (GSFC) is one of the largest and most diverse astrophysical organizations in the world, with activities spanning a broad range of topics in theory, observation, and mission and technology development. Scientific research is carried out over the entire electromagnetic spectrum from gamma rays to radio wavelengths as well as particle physics and gravitational radiation. Members of ASD also provide the scientific operations for two orbiting astrophysics missions Fermi Gamma-ray Space Telescope and Swift as well as the Science Support Center for Fermi. A number of key technologies for future missions are also under development in the Division, including X-ray mirrors, space-based interferometry, high contrast imaging techniques to search for exoplanets, and new detectors operating at gamma-ray, X-ray, ultraviolet, infrared, and radio wavelengths. The overriding goals of ASD are to carry out cutting-edge scientific research, provide Project Scientist support for spaceflight missions, implement the goals of the NASA Strategic Plan, serve and support the astronomical community, and enable future missions by conceiving new concepts and inventing new technologies.

ultraviolet astrophysics↗

NASA NTRS2003

Fe XXV and Fe XXVI Diagnostics of the Black Hole and Accretion Disk in Active Galaxies: Chandra Time-Resolved Spectroscopy of NGC 7314

We report the detection of Fe xxv and Fe XXVI Ka emission lines from a Chandra High Energy Grating Spectrometer (HETGS) observation of the narrow-line Seyfert 1 galaxy NGC 7314, made simultaneously with RXTE. The lines are redshifted (cz approximately 1500 kilometers per second) relative to the systemic velocity and unresolved by the gratings. We argue that the lines originate in a near face-on (less than 7 deg) disk having a radial line emissivity flatter than r(exp -2). Line emission from ionization states of Fe in the range approximately Fe I a up to Fe XXVI is observed. The ionization balance of Fe responds to continuum variations on timescales less than 12.5 ks, supporting an origin of the lines close to the X-ray source. We present additional, detailed diagnostics from this rich data set. These results identify NGC 7314 as a key source to study in the future if we are to pursue reverberation mapping of space-time near black-hole event horizons. This is because it is first necessary to understand the ionization structure of accretion disks and the relation between the X-ray continuum and Fe Ka line emission. However, we also describe how our results are suggestive of a means of measuring black-hole spin without a knowledge of the relation between the continuum and line emission. Finally, these data emphasize that one can study strong gravity with narrow (as opposed to very broad) disk lines. In fact narrow lines offer higher precision, given sufficient energy resolution.

Yaqoob, Tahir↗

NASA NTRS2002

The X-ray Reflectors in the Nucleus of the Seyfert Galaxy NGC 1068

Based on observations of the Seyfert nucleus in NGC 1068 with ASCA, RXTE and BeppoSAX, we report the discovery of a flare (increase in flux by a factor of approximately 1.6) in the 6.7 keV Fe K line component between observations obtained four months apart, with no significant change in the other (6.21, 6.4, and 6.97 keV) Fe Kalpha line components. During this time, the continuum flux decreased by approximately 20%. The RXTE spectrum requires an Fe K absorption edge near 8.6 keV (Fe XXIII- XXV). The spectral data indicate that the 2-10 keV continuum emission is dominated (approximately 2/3 of the luminosity) by reflection from a previously unidentified region of warm, ionized gas located approximately or less than 0.2 pc from the AGN. The remaining approximately 1/3 of the observed X-ray emission is reflected from optically thick, neutral gas. The coronal gas in the inner Narrow-Line Region (NLR) and/or the cold gas at the inner surface of the obscuring 'torus' are possible cold reflectors. The inferred properties of the warm reflector are: size (diameter) approximately or less than 0.2 pc, gas density n approximately or greater than 10(exp 5.5)/cu cm, ionization parameter xi is approximately 10(exp 3.5) erg cm s(exp -1), and covering fraction 0.003 (L(sub 0)/ 10(exp 43.5) erg s(exp -1)(exp -1) less than (omega/4pi) less than 0.024 (L(sub 0)/ 10(exp 43.5) erg s(exp -1) (exp -1) where L(sub 0) is the intrinsic 2-10 keV X-ray luminosity of the AGN. We suggest that the warm reflector gas is the source of the (variable) 6.7 keV Fe line emission, and the 6.97 keV Fe line emission. The 6.7 keV line flare is assumed to be due to an increase in the emissivity of the warm reflector gas from a decrease (by 20-30%) in L(sub 0). The properties of the warm reflector are most consistent with an intrinsically X-ray weak AGN with L(sub 0) approximately equals 10(exp 43.0) erg s(exp -1). The optical and UV emission that scatters from the warm reflector into our line of sight is required to suffer strong extinction, which can be reconciled if the line-of-sight skims the outer surface of the torus. Thermal bremsstrahlung radio emission from the warm reflector may be detectable in VLBA radio maps of the NGC 1068 nucleus.

Colbert, Edward J. M.↗

NASA NTRS2001

X-ray Properties of the Central kpc of AGN and Starbursts: The Latest News from Chandra

The X-ray properties of 15 nearby (v less than 3,000 km/s) galaxies that possess AGN (active galactic nuclei) and/or starbursts are discussed. Two-thirds have nuclear extended emission on scales from approx. 0.5 to approx. 1.5 kpc that is either clearly associated with a nuclear outflow or morphologically resembles an outflow. Galaxies that are AGN-dominated tend to have linear structures while starburst-dominated galaxies tend to have plume-like structures. Significant X-ray absorption is present in the starburst regions, indicating that a circumnuclear starburst is sufficient to block an AGN at optical wavelengths. Galaxies with starburst activity possess more X-ray point sources within their central kpc than non-starbursts. Many of these sources are more luminous than typical X-ray binaries. The Chandra results are discussed in terms of the starburst-AGN connection, a revised unified model for AGN, and possible evolutionary scenarios.

Weaver, Kimberly A.↗

NASA NTRS1997

A Robust Test of the Unified Model for Seyfert Galaxies with Implications for the Starburst Phenomenon

My research involves detailed analysis of X-ray emission from Active Galactic Nuclei (AGN). For over a decade, the paradigm for AGN has rested soundly on the unified model hypothesis, which posits that the only difference between broad-line objects (e.g., Type 1 Seyfert galaxies) and narrow-line objects (e.g., Type 2 Seyferts) is that in the former case our line of sight evades toroidal obscuration surrounding the nucleus, while in the latter, our line of sight is blocked by the optically thick torus. It is well established that some Seyfert 2s contain Seyfert I nuclei (i.e., a hidden broad line region), but whether or not all Seyfert 2s contain obscured Seyfert 1 nuclei or whether some Seyfert 2s are intrinsically Seyfert 2s is not known. Optical, IR, and UV surveys are not appropriate to examine this hypothesis because such emissions are either anisotropic or subject to the effects of obscuration, and thus depend strongly on viewing angle. Hard X-rays, on the other hand, can penetrate gas with column densities as high as 10( exp 24.5) cm(-2) and thus provide reliable, direct probes of the cores of heavily obscured AGN. Combining NASA archival data from the Advanced Satellite of Cosmology and Astrophysics (ASCA), the Rossi X-ray Timing Explorer (RXTE), and Rosat, I am accumulating X-ray data between 0.1 and 60 keV to produce a catalog of the broad-band X-ray spectral properties of Seyfert galaxies. These data will be used to perform concrete tests of the unified model, and (compared with similar data on Starbursts) to examine a possible evolutionary connection between Seyfert and Starburst galaxies.

Weaver, Kimberly A.↗

NASA NTRS1995

Extended soft X-ray emission in Seyfert galaxies: ROSAT HRI observations of NGC 3516, NGC 4151, and Markarian 3

We have used the ROSAT High Resolution Imager (HRI) to examine the distribution of soft X-rays in three nearby Seyfert galaxies with approximately 4 to 5 arcsecs FWHM spatial resolution. A feature of our analysis is an attempt to remove errors in the aspect solution using a method developed by one of us (J.M.). NGC 4151 shows resolved X-ray emission that is spatially correlated with the optical extended narrow-line region (ENLR), confirming the results obtained with the Einstein HRI by Elvis, Briel, & Henry. NGC 3516 is elongated along a position angle of approximately 40 to 220 deg, similar to the direction of the Z-shaped narrow-line region. MRK 3 is very faint in our HRI image and is probably spatially unresolved. We detect the faint X-ray source approximately 2 arcmins west of the MRK 3 nucleus previously found by Turner, Urry, & Mushotzky. We also detected the BL Lac object BL 1207 + 39 approximately 5 arcmins north-northwest of NGC 4151. This object appears spatially unresolved, but some excess X-ray emission may be observed in the azimuthally averaged radial brightness profile of BL 1207 + 39 between radii of 10 arcsecs and 30 arcsecs when compared to a calibration source. A much deeper image is necessary to confirm this result.

Morse, Jon A.↗

NASA NTRS1992

Hard X-ray tests of the unified model for an ultraviolet-detected sample of Seyfert 2 galaxies

An ultraviolet-detected sample of Seyfert 2 galaxies shows heavy photoelectric absorption in the hard X-ray band. The presence of UV emission combined with hard X-ray absorption argues strongly for a special geometry which must have the general properties of the Antonucci and Miller unified model. The observations of this sample are consistent with the picture in which the hard X-ray photons are viewed directly through the obscuring matter (molecular torus?) and the optical, UV, and soft X-ray continuum are seen in scattered light. The large range in X-ray column densities implies that there must be a large variation in intrinsic thicknesses of molecular tori, an assumption not found in the simplest of unified models. Furthermore, constraints based on the cosmic X-ray background suggest that some of the underlying assumptions of the unified model are wrong.

Mulchaey, John S.↗

NASA NTRS1991

A ubiquitous absorption feature in the X-ray spectra of BL Lacertae objects

The paper presents the broadband (0.5-20-keV) X-ray spectra of five X-ray bright BL Lac objects observed with the Einstein Observatory Solid State Spectrometer (SSS) and Monitor Proportional Counter (MPC) detectors. The combination of moderate energy resolution and broad spectral coverage makes it possible to confirm the presence of an absorption feature at an energy of 650 eV in the BL Lac object PKS 2155-304, originally reported by Canizares and Kruper (1984) based on higher resolution Einstein Objective Grating Spectrometer (OGS) data.

Madejski, Greg M.↗