133 research outputs found
Stability of the Infrared Array Camera for the Spitzer Space Telescope
We present an analysis of the stability of the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope over the first 4.5 years of in-flight operations. IRAC consists of two InSb and two Si:As 256x256 imaging arrays with passbands centered on 3.6, 4.5. 5.8 and 8.0 microns. Variations in photometric stability, read noise, dark offsets, pixel responsivity and number of hot and noisy pixels for each detector array are trended with time. To within our measurement uncertainty, the performance of the IRAC arrays has not changed with time. The most significant variation is that number of hot pixels in the 8 micron array has increased linearly with time at a rate of 60 pixels per year. We expect that the 3.6 and 4.5 micron arrays should remain stable during the post-cryogenic phase of the Spitzer mission. We will briefly discuss some science that is enabled by the excellent stability of IRAC
Calibration and data quality of warm IRAC
We present an overview of the calibration and properties of data from the IRAC instrument aboard the Spitzer Space Telescope taken after the depletion of cryogen. The cryogen depleted on 15 May 2009, and shortly afterward a two-month- long calibration and characterization campaign was conducted. The array temperature and bias setpoints were revised on 19 September 2009 to take advantage of lower than expected power dissipation by the instrument and to improve sensitivity. The final operating temperature of the arrays is 28.7 K, the applied bias across each detector is 500 mV and the equilibrium temperature of the instrument chamber is 27.55 K. The final sensitivities are essentially the same as the cryogenic mission with the 3.6 Ī¼m array being slightly less sensitive (10%) and the 4.5 Ī¼m array within 5% of the cryogenic sensitivity. The current absolute photometric uncertainties are 4% at 3.6 and 4.5 Ī¼m, and better than milli-mag photometry is achievable for long-stare photometric observations. With continued analysis, we expect the absolute calibration to improve to the cryogenic value of 3%. Warm IRAC operations fully support all science that was conducted in the cryogenic mission and all currently planned warm science projects (including Exploration Science programs). We expect that IRAC will continue to make ground-breaking discoveries in star formation, the nature of the early universe, and in our understanding of the properties of exoplanets
Structure and Colors of Diffuse Emission in the Spitzer Galactic First Look Survey
We investigate the density structure of the interstellar medium using new
high-resolution maps of the 8 micron, 24 micron, and 70 micron surface
brightness towards a molecular cloud in the Gum Nebula, made as part of the
Spitzer Space Telescope Galactic First Look Survey. The maps are correlated
with 100 micron images measured with IRAS. At 24 and 70 micron, the spatial
power spectrum of surface brightness follows a power law with spectral index
-3.5. At 24 micron, the power law behavior is remarkably consistent from the
0.2 degree size of our maps down to the 5 arcsecond spatial resolution. Thus,
the structure of the 24 micron emission is self-similar even at milliparsec
scales. The combined power spectrum produced from Spitzer 24 micron and IRAS 25
micron images is consistent with a change in the power law exponent from -2.6
to -3.5. The decrease may be due to the transition from a two-dimensional to
three-dimensional structure. Under this hypothesis, we estimate the thickness
of the emitting medium to be 0.3 pc.Comment: 13 Pages, 3 Figures, to be published in Astrophysical Journal
Supplement Series (Spitzer Special Issue), volume 154. Uses aastex v5.
Reversible defects in natural killer and memory CD8 T cell lineages in interleukin 15-deficient mice
C57BL/6 mice genetically deficient in interleukin 15 (IL-15(-/-) mice) were generated by gene targeting. IL-15(-/-) mice displayed marked reductions in numbers of thymic and peripheral natural killer (NK) T cells, memory phenotype CD8+ T cells, and distinct subpopulations of intestinal intraepithelial lymphocytes (IELs). The reduction but not absence of these populations in IL-15(-/-) mice likely reflects an important role for IL-15 for expansion and/or survival of these cells. IL-15(-/-) mice lacked NK cells, as assessed by both immunophenotyping and functional criteria, indicating an obligate role for IL-15 in the development and functional maturation of NK cells. Specific defects associated with IL-15 deficiency were reversed by in vivo administration of exogenous IL-15. Despite their immunological defects, IL-15(-/-) mice remained healthy when maintained under specific pathogen-free conditions. However, IL-l5(-/-) mice are likely to have compromised host defense responses to various pathogens, as they were unable to mount a protective response to challenge with vaccinia virus. These data reveal critical roles for IL-15 in the development of specific lymphoid lineages. Moreover, the ability to rescue lymphoid defects in IL-15(-/-) mice by IL-15 administration represents a powerful means by which to further elucidate the biological roles of this cytokine
Stability of the Infrared Array Camera for the Spitzer Space Telescope
We present an analysis of the stability of the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope over the first 4.5 years of in-flight operations. IRAC consists of two InSb and two Si:As 256x256 imaging arrays with passbands centered on 3.6, 4.5. 5.8 and 8.0 microns. Variations in photometric stability, read noise, dark offsets, pixel responsivity and number of hot and noisy pixels for each detector array are trended with time. To within our measurement uncertainty, the performance of the IRAC arrays has not changed with time. The most significant variation is that number of hot pixels in the 8 micron array has increased linearly with time at a rate of 60 pixels per year. We expect that the 3.6 and 4.5 micron arrays should remain stable during the post-cryogenic phase of the Spitzer mission. We will briefly discuss some science that is enabled by the excellent stability of IRAC
In-flight performance and calibration of the Infrared Array Camera (IRAC) for the Spitzer Space Telescope
The Infrared Array Camera (IRAC) is one of three focal plane instruments on board the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 Ī¼m in two nearly adjacent fields of view. We summarize here the in-flight scientific, technical, and operational performance of IRAC
The Infrared Array Camera (IRAC) for the Spitzer Space Telescope
The Infrared Array Camera (IRAC) is one of three focal plane instruments in
the Spitzer Space Telescope. IRAC is a four-channel camera that obtains
simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 microns. Two nearly
adjacent 5.2x5.2 arcmin fields of view in the focal plane are viewed by the
four channels in pairs (3.6 and 5.8 microns; 4.5 and 8 microns). All four
detector arrays in the camera are 256x256 pixels in size, with the two shorter
wavelength channels using InSb and the two longer wavelength channels using
Si:As IBC detectors. IRAC is a powerful survey instrument because of its high
sensitivity, large field of view, and four-color imaging. This paper summarizes
the in-flight scientific, technical, and operational performance of IRAC.Comment: 7 pages, 3 figures. Accepted for publication in the ApJS. A higher
resolution version is at http://cfa-www.harvard.edu/irac/publication
Variability Timescale and Spectral Index of Sgr A* in the Near Infrared: Approximate Bayesian Computation Analysis of the Variability of the Closest Supermassive Black Hole
Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray
source associated with accretion onto the Galactic center black hole. We
present an analysis of the most comprehensive NIR variability dataset of Sgr A*
to date: eight 24-hour epochs of continuous monitoring of Sgr A* at 4.5 m
with the IRAC instrument on the Spitzer Space Telescope, 93 epochs of 2.18
m data from Naos Conica at the Very Large Telescope, and 30 epochs of 2.12
m data from the NIRC2 camera at the Keck Observatory, in total 94,929
measurements. A new approximate Bayesian computation method for fitting the
first-order structure function extracts information beyond current Fast Fourier
Transformation (FFT) methods of power spectral density (PSD) estimation. With a
combined fit of the data of all three observatories, the characteristic
coherence timescale of Sgr A* is minutes (
credible interval). The PSD has no detectable features on timescales down to
8.5 minutes ( credible level), which is the ISCO orbital frequency for a
dimensionless spin parameter . One light curve measured
simultaneously at 2.12 and 4.5 m during a low flux-density phase gave a
spectral index (). This
value implies that the Sgr A* NIR color becomes bluer during higher
flux-density phases. The probability densities of flux densities of the
combined datasets are best fit by log-normal distributions. Based on these
distributions, the Sgr A* spectral energy distribution is consistent with
synchrotron radiation from a non-thermal electron population from below 20 GHz
through the NIR.Comment: Accepted for publication in ApJ on May 30, 2018. A machine readable
version of the light curve data is included in the journal's online
publication. Version 2 includes proof correction
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