119 research outputs found
A Search for EUV Emission from Comets with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS)
We have obtained EUV spectra between 90 and 255 \AA of the cometsC/2002 T7
(LINEAR), C/2001 Q4 (NEAT), and C/2004 Q2 (Machholz) near their perihelion
passages in 2004 with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS).
We obtained contemporaneous data on Comet NEAT Q4 with the X-ray
Observatory ACIS instrument, marking the first simultaneous EUV and X-ray
spectral observations of a comet. The total CHIPS/EUV observing times were 337
ks for Q4, 234 ks for T7, and 483 ks for Machholz and for both CHIPS and
we calculate we have captured all the comet flux in the instrument
field of view. We set upper limits on solar wind charge exchange emission lines
of O, C, N, Ne and Fe occurring in the spectral bandpass of CHIPS. The spectrum
of Q4 obtained with can be reproduced by modeling emission lines of
C, N O, Mg, Fe, Si, S, and Ne solar wind ions. The measured X-ray emission line
intensities are consistent with our predictions from a solar wind charge
exchange model. The model predictions for the EUV emission line intensities are
determined from the intensity ratios of the cascading X-ray and EUV photons
arising in the charge exchange processes. They are compatible with the measured
limits on the intensities of the EUV lines. For comet Q4, we measured a total
X-ray flux of 3.7 ergs cm s, and derive from
model predictions a total EUV flux of 1.5 erg cm
s. The CHIPS observations occurred predominantly while the satellite was
on the dayside of Earth. For much of the observing time, CHIPS performed
observations at smaller solar angles than it was designed for and EUV emission
from the Sun scattered into the instrument limited the sensitivity of the EUV
measurements.Comment: 28 pages total, 4 tables, 7 figures. Accepted by The Astrophysical
Journa
Ten-fold spectral resolution boosting using TEDI at the Mt. Palomar NIR Triplespec spectrograph
An optical technique called "interferometric spectral reconstruction" (ISR) is capable of increasing a spectrograph's resolution and stability by large factors, well beyond its classical limits. We have demonstrated a 6- to 11-fold increase in the Triplespec effective spectral resolution (R=2,700) to achieve R=16,000 at 4100 cm-^(1) to 30,000 at 9600 cm^(-1) by applying special Fourier processing to a series of exposures with different delays (optical path differences) taken with the TEDI interferometer and the near-infrared Triplespec spectrograph at the Mt. Palomar Observatory 200 inch telescope. The TEDI is an externally dispersed interferometer (EDI) used for Doppler radial velocity measurements on M-stars, and now also used for ISR. The resolution improvement is observed in both stellar and telluric features simultaneously over the entire spectrograph bandwidth (0.9-2.45 ÎŒm). By expanding the delay series, we anticipate achieving resolutions of R=45,000 or more. Since the delay is not continuously scanned, the technique is advantageous for measuring time-variable phenomena or in varying conditions (e.g. planetary fly-bys). The photon limited signal to noise ratio can be 100 times better than a classic Fourier Transform Spectrometer (FTS) due to the benefit of dispersion
The Three Dimensional Structure of EUV Accretion Regions in AM Herculis Stars: Modeling of EUV Photometric and Spectroscopic Observations
We have developed a model of the high-energy accretion region for magnetic
cataclysmic variables and applied it to {\it Extreme Ultraviolet Explorer}
observations of 10 AM Herculis type systems. The major features of the EUV
light curves are well described by the model. The light curves exhibit a large
variety of features such as eclipses of the accretion region by the secondary
star and the accretion stream, and dips caused by material very close to the
accretion region. While all the observed features of the light curves are
highly dependent on viewing geometry, none of the light curves are consistent
with a flat, circular accretion spot whose lightcurve would vary solely from
projection effects. The accretion region immediately above the WD surface is a
source of EUV radiation caused by either a vertical extent to the accretion
spot, or Compton scattering off electrons in the accretion column, or, very
likely, both. Our model yields spot sizes averaging 0.06 R, or the WD surface area, and average spot heights of 0.023
R. Spectra extracted during broad dip phases are softer than spectra
during the out-of-dip phases. This spectral ratio measurement leads to the
conclusion that Compton scattering, some absorption by a warm absorber,
geometric effects, an asymmetric temperature structure in the accretion region
and an asymmetric density structure of the accretion columnare all important
components needed to fully explain the data. Spectra extracted at phases where
the accretion spot is hidden behind the limb of the WD, but with the accretion
column immediately above the spot still visible, show no evidence of emission
features characteristic of a hot plasma.Comment: 30 Pages, 11 Figure
Comprehensive Measurements of the Volume-phase Holographic Gratings for the Dark Energy Spectroscopic Instrument
The Dark Energy Spectroscopic Instrument (DESI) is a Stage IV ground-based dark energy experiment that will be employed on the Mayall 4 m Telescope to study the expansion history of the universe. In the era of massively multiplexed fiber-fed spectrographs, DESI will push the boundaries of fiber spectroscopy with a design capable of taking 5000 simultaneous spectra over 360 to 980 nm. The instrument utilizes a suite of three-channel spectrographs, where volume-phase holographic (VPH) gratings provide dispersions. Thirty-six VPH gratings were produced and their performances were evaluated at the Lawrence Berkeley National Laboratory. We present the design and the evaluation tests for the production run of the VPH gratings, verifying the incidence angle, area-weighted efficiency, and wavefront errors (WFEs). We also present the specialized test set-up developed on-site to assess the grating performances. Measurements of the VPH gratings show high consistency in area-weighted efficiency to within an rms of 2% for the red and near-infrared and 6.2% for the blue gratings. Measured WFEs also showed high consistency per bandpass. Comprehensive evaluations show that the VPH gratings meet DESI performance requirements and have been approved for integration
Implications of the SPEAR FUV Maps on Our Understanding of the ISM
The distribution of a low-density transition temperature (10^4.5 - 10^5.5 K)
gas in the interstellar medium conveys the character and evolution of diffuse
matter in the Galaxy. This difficult to observe component of the ISM emits
mainly in the far-ultraviolet (FUV) (912-1800 {\AA}) band. We describe spectral
maps of FUV emission lines from the highly ionized species CIV and OVI likely
to be the dominant cooling mechanisms of transition temperature gas in the ISM.
The maps were obtained using an orbital spectrometer, SPEAR, that was launched
in 2003 and has observed the FUV sky with a spectral resolution of \sim 550 and
an angular resolution of 10'. We compare distribution of flux in these maps
with three basic models of the distribution of transition temperature gas. We
find that the median distribution of CIV and OVI emission is consistent with
the spatial distribution and line ratios expected from a McKee-Ostriker (MO)
type model of evaporative interfaces. However, the intensities are a factor of
three higher than would be expected at the MO preferred parameters. Some high
intensity regions are clearly associated with supernova remnants and
superbubble structures. Others may indicate regions where gas is cooling
through the transition temperature.Comment: 6 pages, 2 figures, presented at "The Local Bubble and Beyond II,"
April 200
Measuring the Boundary Layer and Inner Accretion Disk Temperatures for WX Ceti During Superoutburst
We obtained EUV photometry, optical spectroscopy, and multi-color optical
photometry for WX Cet during its 1998 November superoutburst. WX Cet is only
the second short-period, low mass transfer CV (TOAD) to ever be observed in the
EUV. Our determined superhump period is consistent with that found by Kato et
al. (0.059 d) and we confirm that superhumps are grey in the optical. The
optical spectra provide direct evidence that the line emission region is
optically thick and our multi-wavelength photometric measurements are used to
determine the inner accretion disk and boundary layer temperatures during
superoutburst. Using a determined distance to WX Cet of ~130 pc, we find
T= 21,000K and T_{BL}~72,500K. Both values are in good agreement with
that expected by models of the superoutburst continuum being produced by the
inner disk and boundary layer.Comment: Accepted in PASP - July 2002 issu
XMM-Newton observations of the X-ray soft polar QS Telescopii
Context. On the basis of XMM-Newton observations, we investigate the energy
balance of selected magnetic cataclysmic variables, which have shown an extreme
soft-to-hard X-ray flux ratio in the ROSAT All-Sky Survey.
Aims. We intend to establish the X-ray properties of the system components,
their flux contributions, and the accretion geometry of the X-ray soft polar QS
Tel. In the context of high-resolution X-ray analyses of magnetic cataclysmic
variables, this study will contribute to better understanding the accretion
processes on magnetic white dwarfs.
Methods. During an intermediate high state of accretion of QS Tel, we have
obtained 20 ks of XMM-Newton data, corresponding to more than two orbital
periods, accompanied by simultaneous optical photometry and phase-resolved
spectroscopy. We analyze the multi-wavelength spectra and light curves and
compare them to former high- and low-state observations.
Results. Soft emission at energies below 2 keV dominates the X-ray light
curves. The complex double-peaked maxima are disrupted by a sharp dip in the
very soft energy range (0.1-0.5 keV), where the count rate abruptly drops to
zero. The EPIC spectra are described by a minimally absorbed black body at 20
eV and two partially absorbed MEKAL plasma models with temperatures around 0.2
and 3 keV. The black-body-like component arises from one mainly active, soft
X-ray bright accretion region nearly facing the mass donor. Parts of the plasma
emission might be attributed to the second, virtually inactive pole. High
soft-to-hard X-ray flux ratios and hardness ratios demonstrate that the
high-energy emission of QS Tel is substantially dominated by its X-ray soft
component.Comment: Accepted for publication in Astronomy and Astrophysics. 7 pages, 4
figures, 2 table
A model for the optical high state light curve of AM Herculis
We present a simple quantitative model that can describe the photometric B
and V band light curves of AM Herculis obtained during a high state. The
double-humped shape of the V band light curve is dominated by cyclotron
emission from a region at the main accreting pole with an area of ~5x10e16 cm^2
and sustaining an inflow of ~0.06 g/cm^2/s. The almost unmodulated B band is
dominated by emission from the accretion stream. The contribution of the heated
white dwarf to the optical emission is small in the B band, but comparable to
that of the accretion stream in the V band. The emission of the secondary star
is negligible both in B and in V.Comment: 6 pages A&A-Latex, 6 Figures, accepted for publication in A&
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