Browsing by Subject "ultraviolet:"

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Discovery Of The Ultra-Bright Type II-L Supernova 2008es
(2009-01) Gezari, S.; Halpern, J. P.; Grupe, D.; Yuan, F.; Quimby, Robert; McKay, T.; Chamarro, D.; Sisson, M. D.; Akerlof, C.; Wheeler, J. Craig; Brown, Peter J.; Cenko, S. Bradley; Rau, A.; Djordjevic, J. O.; Terndrup, D. M.; Wheeler, J. Craig
We report the discovery by the Robotic Optical Transient Search Experiment (ROTSE-IIIb) telescope of SN 2008es, an overluminous supernova (SN) at z = 0.205 with a peak visual magnitude of -22.2. We present multiwavelength follow-up observations with the Swift satellite and several ground-based optical telescopes. The ROTSE-IIIb observations constrain the time of explosion to be 23 +/- 1 rest-frame days before maximum. The linear decay of the optical light curve, and the combination of a symmetric, broad Ha emission line profile with broad P Cygni H beta and Na lambda 5892 profiles, are properties reminiscent of the bright Type II-L SNe 1979C and 1980K, although SN 2008es is greater than 10 times more luminous. The host galaxy is undetected in pre-supernova Sloan Digital Sky Survey images, and similar to Type II-L SN 2005ap (the most luminous SN ever observed), the host is most likely a dwarf galaxy with M(r) > - 17. Swift Ultraviolet/Optical Telescope observations in combination with Palomar 60 inch photometry measure the spectral energy distribution of the SN from 200 to 800 nm to be a blackbody that cools from 14000 K at the time of the optical peak to 6400 K 65 days later. The inferred blackbody radius is in good agreement with the radius expected for the expansion speed measured from the broad lines (10000 km s(-1)). The bolometric luminosity at the optical peak is 2.8 x 10(44) erg s(-1), with a total energy radiated over the next 65 days of 5.6 x 10(50) erg. The exceptional luminosity of SN 2008es requires an efficient conversion of kinetic energy produced from the core-collapse explosion into radiation. We favor a model in which the large peak luminosity is a consequence of the core collapse of a progenitor star with a low-mass extended hydrogen envelope and a stellar wind with a density close to the upper limit on the mass-loss rate measured from the lack of an X-ray detection by the Swift X-Ray Telescope.
The Evolving Activity Of The Dynamically Young Comet C/2009 P1 (Garradd)
(2014-05) Bodewits, D.; Farnham, T. L.; A'Hearn, M. F.; Feaga, L. M.; McKay, A.; Schleicher, D. G.; Sunshine, J. M.; McKay, A.
We used the Ultraviolet-Optical Telescope on board Swift to observe the dynamically young comet C/2009 P1 (Garradd) from a heliocentric distance of 3.5 AU pre-perihelion until 4.0 AU outbound. At 3.5 AU pre-perihelion, comet Garradd had one of the highest dust-to-gas ratios ever observed, matched only by comet Hale-Bopp. The evolving morphology of the dust in its coma suggests an outburst that ended around 2.2 AU pre-perihelion. Comparing slit-based measurements and observations acquired with larger fields of view indicated that between 3 AU and 2 AU pre-perihelion a significant extended source started producing water in the coma. We demonstrate that this source, which could be due to icy grains, disappeared quickly around perihelion. Water production by the nucleus may be attributed to a constantly active source of at least 75 km(2), estimated to be > 20% of the surface. Based on our measurements, the comet lost 4 x 10(11) kg of ice and dust during this apparition, corresponding to at most a few meters of its surface. Even though this was likely not the comet's first passage through the inner solar system, the activity of Garradd was complex and changed significantly during the time it was observed.
HST-COS Observations Of AGNs. II. Extended Survey Of Ultraviolet Composite Spectra From 159 Active Galactic Nuclei
(2014-10) Stevans, Matthew L.; Shull, J. Michael; Danforth, Charles W.; Tilton, Evan M.; Stevans, Matthew L.
The ionizing fluxes from quasars and other active galactic nuclei (AGNs) are critical for interpreting their emissionline spectra and for photoionizing and heating the intergalactic medium. Using far-ultraviolet (FUV) spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we directly measure the rest-frame ionizing continua and emission lines for 159 AGNs at redshifts 0.001 < Z(AGN) < 1.476 and construct a composite spectrum from 475 to 1875 A. We identify the underlying AGN continuum and strong extreme ultraviolet (EUV) emission lines from ions of oxygen, neon, and nitrogen after masking out absorption lines from the HI Ly alpha forest, 7 Lyman-limit systems (N-HI, 10(17.2) Cm-2) and 214 partial Lyman-limit systems (14.5 < log N-HI < 17.2). The 159 AGNs exhibit a wide range of FUV/EUV spectral shapes, F-V, proportional to v(proportional to v)(4, typically with -2 <= alpha(v), <= 0 and no discernible continuum edges at 912 angstrom (HI) or 504 angstrom (He I). The composite rest-frame continuum shows a gradual break at lambda(br) approximate to 1000 angstrom, with mean spectral index alpha(v) = -0.83 +/- 0.09 in the FUV (1200-2000 angstrom) steepening to alpha(v), = -1.41 +/- 0.15 in the EUV (500-1000 angstrom). We discuss the implications of the UV flux turnovers and lack of continuum edges for the structure of accretion disks, AGN mass inflow rates, and luminosities relative to Eddington values.