148 research outputs found
Images and Spectral Performance of WFC3 Interference Filters
The Wide Field Camera 3 (WFC3) is a panchromatic imager that will be deployed in the Hubble Space Telescope (HST). The mission of the WFC3 is to enhance HST1s imaging capability in the ultraviolet, visible and near-infrared spectral regions. Together with a wavelength coverage spanning 2000A to 1.7 micron, the WFC3 high sensitivity, high spatial resolution, and large field-of-view provide the astronomer with an unprecedented set of tools for exploring all types of exciting astrophysical terrain and for addressing many key questions in astronomy today. The filter compliment, which includes broad, medium, and narrow band filters, naturally reflects the diversity of astronomical programs to be targeted with WFC3. The WFC3 holds 61 UVIS filters elements, 14 IR filters, and 3 dispersive elements. During ground testing, the majority of the UVIS filters were found to exhibit excellent performance consistent with or exceeding expectations; however, a subset of filters showed considerable ghost images; some with relative intensity as high as 10-15%. Replacement filters with band-defining coatings that substantially reduce these ghost images were designed and procured. A state-of-the-art characterization setup was developed to measured the intensity of ghost images, focal shift, wedge direction , transmitted uniformity and surface feature of filters that could effect uniform flat field images. We will report on this new filter characterization methods, as well as the spectral performance measurements of the in-band transmittance and blocking
Intrinsic Absorption in the Spectrum of NGC 7469: Simultaneous Chandra, FUSE, and STIS Observations
We present simultaneous X-ray, far-ultraviolet, and near-ultraviolet spectra
of the Seyfert 1 galaxy NGC 7469 obtained with the Chandra X-Ray Observatory,
the Far Ultraviolet Spectroscopic Explorer, and the Space Telescope Imaging
Spectrograph on the Hubble Space Telescope. Previous non-simultaneous
observations of this galaxy found two distinct UV absorption components, at
-560 and -1900 km/s, with the former as the likely counterpart of the X-ray
absorber. We confirm these two absorption components in our new UV
observations, in which we detect prominent O VI, Ly alpha, N V, and C IV
absorption. In our Chandra spectrum we detect O VIII emission, but no
significant O VIII or O VII absorption. We also detect a prominent Fe K alpha
emission line in the Chandra spectrum, as well as absorption due to
hydrogen-like and helium-like neon, magnesium, and silicon at velocities
consistent with the -560 km/s UV absorber. The FUSE and STIS data reveal that
the H I and C IV column densities in this UV- and X-ray- absorbing component
have increased over time, as the UV continuum flux decreased. We use measured H
I, N V, C IV, and O VI column densities to model the photoionization state of
both absorbers self-consistently. We confirm the general physical picture of
the outflow in which the low velocity component is a highly ionized, high
density absorber with a total column density of 10^20 cm^-2, located near the
broad emission line region, although due to measurable columns of N V and C IV,
we assign it a somewhat smaller ionization parameter than found previously,
U~1. The high velocity UV component is of lower density, log N=18.6, and likely
resides farther from the central engine as we find its ionization parameter to
be U=0.08.Comment: Minor correction to abstract; STScI eprint #1683; 50 pages, incl. 19
figures, 4 tables; Accepted to Ap
WFC3 Calibration and Data Processing
Wide Field Camera 3 (WFC3), a panchromatic imager being developed for the Hubble Space Telescope (HST), is now fully integrated and over the past year has completed first rounds of extensive ground testing at Goddard Space Flight Center (GSFC), in both ambient and thermal-vacuum test environments. This report summarizes the results of those tests and describes the pipeline processing methods that will be used to calibrate WFC3 data. WFC3 is designed to ensure that the superb imaging performance of HST is maintained through the end of the mission and takes advantage of recent developments in detector technology to provide new and unique capabilities for HST. WFC3 contains ultraviolet/visible (UVIS) and near-infrared (IR) imaging channels, offering high sensitivity and wide field of view over the broadest wavelength range of any HST instrument. It is slated to replace the current Wide Field and Planetary Camera 2 during Servicing Mission 4. The WFC3 UVIS channel is based on elements from the Advanced Camera for Surveys (ACS)Wide Field Camera (WFC), with a 4096x4096 pixel Marconi CCD covering a 160x160 arcsecond field of view. The WFC3 UVIS channel is optimized for maximum sensitivity in the near-UV and contains a complement of 48 spectral filters and a grism. The WFC3 IR channel uses a 1024x1024 pixel HgCdTe Hawaii-1R detector array covering a 135x135 arcsecond field of view. The array sensitivity is optimized in the 0.8-1.7micron spectral range. The IR channel accomodates 15 filters and 2 grisms for slitless spectroscopy
Quasars and the Big Blue Bump
We investigate the ultraviolet-to-optical spectral energy distributions
(SEDs) of 17 active galactic nuclei (AGNs) using quasi-simultaneous
spectrophotometry spanning 900-9000 Angstrom (rest frame). We employ data from
the Far Ultraviolet Spectroscopic Explorer (FUSE), the Hubble Space Telescope
(HST), and the 2.1-meter telescope at Kitt Peak National Observatory (KPNO).
Taking advantage of the short-wavelength coverage, we are able to study the
so-called "big blue bump," the region where the energy output peaks, in detail.
Most objects exhibit a spectral break around 1100 Angstrom. Although this
result is formally associated with large uncertainty for some objects, there is
strong evidence in the data that the far-ultraviolet spectral region is below
the extrapolation of the near-ultraviolet-optical slope, indicating a spectral
break around 1100 Angstrom. We compare the behavior of our sample to those of
non-LTE thin-disk models covering a range in black-hole mass, Eddington ratio,
disk inclination, and other parameters. The distribution of ultraviolet-optical
spectral indices redward of the break, and far-ultraviolet indices shortward of
the break, are in rough agreement with the models. However, we do not see a
correlation between the far-ultraviolet spectral index and the black hole mass,
as seen in some accretion disk models. We argue that the observed spectral
break is intrinsic to AGNs, although intrinsic reddening as well as
Comptonization can strongly affect the far-ultraviolet spectral index. We make
our data available online in digital format.Comment: 32 pages (10pt), 12 figures. Accepted for publication in Ap
Distance to the RR Lyrae Star V716 Monocerotis
We present high quality BVRI CCD photometry of the variable star V716
Monocerotis (= NSV 03775). We confirm it to be an RR Lyrae star of variability
type ab (i.e. a fundamental mode pulsator), and determine its metallicity
([Fe/H] = -1.33 +/- 0.25), luminosity (Mv = 0.80 +/- 0.06), and foreground
reddening (E(B-V) = 0.05-0.17) from the Fourier components of its light curve.
These parameters indicate a distance of 4.1 +/- 0.3 kpc, placing V716 Mon near
the plane of the Galaxy well outside the solar circle. This research was
conducted as part of the 1999 Research Experiences for Undergraduates (REU) and
Practicas de Investigacion en Astronomia (PIA) Programs at Cerro Tololo
Inter-American Observatory (CTIO).Comment: 9 pages including 2 figures and 2 tables; accepted by PAS
Properties and Origin of the High-Velocity Gas Toward the Large Magellanic Cloud
In the spectra of 139 early-type Large Magellanic Cloud (LMC) stars observed
with FUSE and with deep radio Parkes HI 21-cm observations along those stars,
we search for and analyze the absorption and emission from high-velocity gas at
+90<v<+175 km/s. The HI column density of the high-velocity clouds (HVCs) along
these sightlines ranges from <10^18.4 to 10^19.2 cm^-2. The incidence of the
HVC metal absorption is 70%, significantly higher than the HI emission
occurrence of 32%. We find that the mean metallicity of the HVC is [OI/HI] =
-0.51 (+0.12,-0.16). There is no strong evidence for a large variation in the
HVC metallicity, implying that thes e HVCs have a similar origin and are part
of the same complex. The mean and scatter of the HVC metallicities are more
consistent with the present-day LMC oxygen abundance than that of the Small
Magellanic Cloud or the Milky Way. We find that on average [SiII/OI] = +0.48
(+0.15,- 0.25) and [FeII/OI] = +0.33 (+0.14,-0.21), implying that the HVC
complex is dominantly ionized. The HVC complex has a multiphase structure with
a neutral (OI, FeII), weakly ionized (FeII, NII), and highly ionized (OVI)
components, and has evidence of dust but no molecules. All the observed
properties of the HVC can be explained by an energetic outflow from the LMC.
This is the first example of a large (>10^6 M_sun) HVC complex that is linked
to stellar feedback occurring in a dwarf spiral galaxy.Comment: Accepted for publication in the Ap
Hubble Space Telescope Spectroscopy of the Balmer lines in Sirius B
Sirius B is the nearest and brightest of all white dwarfs, but it is very
difficult to observe at visible wavelengths due to the overwhelming scattered
light contribution from Sirius A. However, from space we can take advantage of
the superb spatial resolution of the Hubble Space Telescope to resolve the A
and B components. Since the closest approach in 1993, the separation between
the two stars has become increasingly favourable and we have recently been able
to obtain a spectrum of the complete Balmer line series for Sirius B using
HST?s Space Telescope Imaging Spectrograph (STIS). The quality of the STIS
spectra greatly exceed that of previous ground-based spectra, and can be used
to provide an important determination of the stellar temperature (Teff =
25193K) and gravity (log g = 8.556). In addition we have obtained a new, more
accurate, gravitational red-shift of 80.42 +/- 4.83 km s-1 for Sirius B.
Combining these results with the photometric data and the Hipparcos parallax we
obtain new determinations of the stellar mass for comparison with the
theoretical mass-radius relation. However, there are some disparities between
the results obtained independently from log g and the gravitational redshift
which may arise from flux losses in the narrow 50x0.2arcsec slit. Combining our
measurements of Teff and log g with the Wood (1995) evolutionary mass-radius
relation we get a best estimate for the white dwarf mass of 0.978 M. Within the
overall uncertainties, this is in agreement with a mass of 1.02 M obtained by
matching our new gravitational red-shift to the theoretical M/R relation.Comment: 11 pages, 6 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
X-Ray/Ultraviolet Observing Campaign of the Markarian 279 Active Galactic Nucleus Outflow: a close look at the absorbing/emitting gas with Chandra-LETGS
We present a Chandra-LETGS observation of the Seyfert 1 galaxy Mrk 279. This
observation was carried out simultaneously with HST-STIS and FUSE, in the
context of a multiwavelength study of this source. The Chandra pointings were
spread over ten days for a total exposure time of ~360 ks. The spectrum of
Mrk279 shows evidence of broad emission features, especially at the wavelength
of the OVII triplet. We quantitatively explore the possibility that this
emission is produced in the broad line region (BLR). We modeled the broad UV
emission lines seen in the FUSE and HST-STIS spectra following the ``locally
optimally emitting cloud" approach. We find that the X-ray lines luminosity
derived from the best fit BLR model can match the X-ray features, suggesting
that the gas producing the UV lines is sufficient to account also for the X-ray
emission. The spectrum is absorbed by ionized gas whose total column density is
~5x10^{20} cm^{-2}. The absorption spectrum can be modeled by two distinct gas
components (log xi ~ 0.47 and 2.49, respectively) both showing a significant
outflow velocity. However, the data allow also the presence of intermediate
ionization components. The distribution of the column densities of such extra
components as a function of the ionization parameter is not consistent with a
continuous, power law-like, absorber, suggesting a complex structure for the
gas outflow for Mrk 279 (abridged).Comment: 16 pages, 12 figures. To appear in A&
Characterizing Transition Temperature Gas in the Galactic Corona
We present a study of the properties of the transition temperature (T~10^5 K)
gas in the Milky Way corona, based on measurements of OVI, NV, CIV, SiIV and
FeIII absorption lines seen in the far ultraviolet spectra of 58 sightlines to
extragalactic targets, obtained with Far-Ultraviolet Spectroscopic Explorer
(FUSE) and Space Telescope Imaging Spectrograph. In many sightlines the
Galactic absorption profiles show multiple components, which are analyzed
separately. We find that the highly-ionized atoms are distributed irregularly
in a layer with a scaleheight of about 3 kpc, which rotates along with the gas
in the disk, without an obvious gradient in the rotation velocity away from the
Galactic plane. Within this layer the gas has randomly oriented velocities with
a dispersion of 40-60 km/s. On average the integrated column densities are log
N(OVI)=14.3, log N(NV)=13.5, log N(CIV)=14.2, log N(SiIV)=13.6 and log
N(FeIII)=14.2, with a dispersion of just 0.2 dex in each case. In sightlines
around the Galactic Center and Galactic North Pole all column densities are
enhanced by a factor ~2, while at intermediate latitudes in the southern sky
there is a deficit in N(OVI) of about a factor 2, but no deficit for the other
ions. We compare the column densities and ionic ratios to a series of
theoretical predictions: collisional ionization equilibrium, shock ionization,
conductive interfaces, turbulent mixing, thick disk supernovae, static
non-equilibrium ionization (NIE) radiative cooling and an NIE radiative cooling
model in which the gas flows through the cooling zone. None of these models can
fully reproduce the data, but it is clear that non-equilibrium ionization
radiative cooling is important in generating the transition temperature gas.Comment: 99 pages, 11 figures, with appendix on Cooling Flow model; only a
sample of 5 subfigures of figure 2 included - full set of 69 available
through Ap
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