Search Publications
The Obscuring Starburst of NGC 6221 and Implications for the Hard X-Ray Background
Storchi-Bergmann, T.; Heckman, T. M.; Levenson, N. A. +2 more
We present NGC 6221 as a case study of ``X-ray-loud composite galaxies,'' which appear similar to starbursts at optical wavelengths and resemble traditional active galactic nuclei in X-rays. The net optical spectrum of NGC 6221 is properly characterized as a starburst galaxy, but in X-rays, NGC 6221 is similar to Seyfert 1 galaxies, exhibiting a p…
Intensity Ratios between the 2s2 1S0-2s2p 3P1 and 2s2p 1P1-2p2 1D2 Transitions in Be-like Ions as Electron Temperature Indicators for Solar Upper Atmosphere Plasmas
Landi, E.; Doschek, G. A.; Feldman, U. +1 more
We investigate the relative intensities of the two moderately bright Be-like 2s2 1S0-2s2p 3P1 and 2s2p 1P1-2p2 1D2 lines as a function of electron temperature. We show that the intensity ratios of the lines in the beryllium isoelectronic s…
Mg II Absorber Number Density at z~=0.05: Implications for ΩDLA Evolution
Churchill, Christopher W.
An unbiased sample of 147 quasar/active galactic nuclei spectra, obtained with the Faint Object Spectrograph on board the Hubble Space Telescope, has been searched for intervening Mg II λλ2796, 2803 absorbers over the redshift range 0<z<=0.15. The total redshift path searched is 18.8, with the survey being 80% complete to a 5 σ rest-frame eq…
The Stellar Content of NGC 6789, A Blue Compact Dwarf Galaxy in the Local Void
Greggio, Laura; Hopp, Ulrich; Schulte-Ladbeck, Regina E. +2 more
We find that NGC 6789 is the most nearby example of a blue compact dwarf galaxy known to date. With the help of the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope, we resolve NGC 6789 into over 15,000 point sources in the V and I bands. The young stars of NGC 6789 are found exclusively near the center of the galaxy. The red giant …
Dual Flows and Oscillations in the Sunspot Transition Region
Wilhelm, K.; Kjeldseth-Moe, O.; Brynildsen, N. +1 more
New knowledge about sunspot transition region lines that deviate significantly from a Gaussian shape are presented. Attention is given to ``multiple flows,'' a line profile phenomenon in which two or more distinct velocities are observed within the same spatial resolution element. We observe line profiles that are well represented by two Gaussian …
Relation between Thermal and Magnetic Properties of Active Regions as a Probe of Coronal Heating Mechanisms
Shibata, Kazunari; Yashiro, Seiji
We study the relation between thermal and magnetic properties of active regions in the corona observed with the soft X-ray telescope aboard Yohkoh. We derive the mean temperature and pressure of 64 mature active regions using the filter ratio technique, and examine the relationship of region size with temperature and pressure. We find that the tem…
Hubble Space Telescope Imaging in the Chandra Deep Field-South. I. Multiple Active Galactic Nucleus Populations
Marconi, A.; Gilli, R.; Tozzi, P. +9 more
We present preliminary results from imaging three Hubble Space Telescope (HST) Wide Field Planetary Camera 2 fields in V and I within the Chandra Deep Field-South (CDF-S). HST's sensitivity and resolution are sufficient to reveal optical counterparts for 24 of the 26 CDF-S X-ray sources detected in the 300 ks X-ray catalog and determine the morpho…
Hubble Space Telescope STIS Spectroscopy of VW Hydri during Early Quiescence following a Superoutburst
Szkody, Paula; Sion, Edward M.; Hubeny, Ivan +3 more
Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) observations of VW Hydri 2 and 7 days after the end of a superoutburst reveal a heated white dwarf with deep broad Lyα, narrow metallic absorption features and evidence of a hotter Keplerian-broadened component. We confirm the existence of enhanced abundances of odd-numbered nuclea…
Millihertz Optical/Ultraviolet Oscillations in 4U 1626-67: Evidence for a Warped Accretion Disk
O'Donoghue, Darragh; Chakrabarty, Deepto; Homer, Lee +1 more
We have detected large-amplitude 0.3-1.2 mHz quasi-periodic oscillations (QPOs) from the low-mass X-ray binary pulsar 4U 1626-67/KZ TrA, using UV photometry from the Hubble Space Telescope and ground-based optical photometry. These 1 mHz QPOs, which have coherence (ν/Δν)~8, are entirely distinct from the 130 mHz pulsar spin frequency, a previously…
Far Ultraviolet Spectroscopic Explorer Observations of Interstellar Gas toward the Small Magellanic Cloud Star Sk 108
Savage, B. D.; Shull, J. M.; Sembach, K. R. +10 more
We discuss the interstellar absorption lines found in Far Ultraviolet Spectroscopic Explorer spectra of the Wolf-Rayet binary Sk 108, which is located in the northeastern part of the main ``bar'' of the Small Magellanic Cloud. The spectra cover the 988-1187 Å wavelength range at a resolution of about 12,000 and a signal-to-noise ratio of 20-40. We…