Purpose: To investigate the feasibility of accurate quantification of concentrations of mixtures of K‐edge contrast agents in spectral Computed Tomography (CT) imaging based on a novel maximum ...likelihood based method. Methods: Calcium, gold, and iodine solutions of various concentrations were prepared in vials and placed in phantoms for imaging at 100 kVp with a Cadmium‐Zinc‐Telluride (CZT) photon counting detector. The linear attenuation maps were reconstructed in 5 energy bins, whose boundaries were 22, 34, 60, 80, and 100 keV. A novel maximum likelihood‐based algorithm for material decomposition was developed. Summing over the energy bins, on a pixel‐by‐pixel basis, this procedure takes into account the reconstructed linear attenuation coefficient, its associated noise, and the tabulated mass attenuation coefficient of each material of interest, to determine a cost function. The unknown parameters to be fitted by optimization by the MINUIT optimizer (from CERN) of the cost function are the concentrations of the materials of interest at each pixel. The result is a density map image for upper limits on the concentration of each material. Results: Multiple material decomposition of the spectral CT phantoms according to this method yields reliable estimates of concentrations. The upper limits on concentrations are linearly correlated with known concentrations. Conclusions: This study indicates that the problem of decomposition of multiple materials of interest may be facilitated by optimization in the image domain, based on linear attenuation values, and using log‐likelihood and upper limit techniques. This technique can be used for analysis of the spectral CT data.
We present the second catalog of high-energy gamma -ray sources detected by the Large Area Telescope (LAT), the primary science instrument on the Fermi Gamma-ray Space Telescope (Fermi), derived from ...data taken during the first 24 months of the science phase of the mission, which began on 2008 August 4. Source detection is based on the average flux over the 24 month period. The second Fermi -LAT catalog (2FGL) includes source location regions, defined in terms of elliptical fits to the 95% confidence regions and spectral fits in terms of power-law, exponentially cutoff power-law, or log-normal forms. Also included are flux measurements in five energy bands and light curves on monthly intervals for each source. Twelve sources in the catalog are modeled as spatially extended. We provide a detailed comparison of the results from this catalog with those from the first Fermi-LAT catalog (1FGL). Although the diffuse Galactic and isotropic models used in the 2FGL analysis are improved compared to the 1FGL catalog, we attach caution flags to 162 of the sources to indicate possible confusion with residual imperfections in the diffuse model. The 2FGL catalog contains 1873 sources detected and characterized in the 100 MeV to 100 GeV range of which we consider 127 as being firmly identified and 1171 as being reliably associated with counterparts of known or likely gamma -ray-producing source classes.
The gamma -ray sky >100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse ...emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi Large Area Telescope (Fermi-LAT) mission and compare with models of the diffuse gamma -ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas, and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the X sub(CO) factor, the ratio between integrated CO-line intensity and H sub(2) column density, the fluxes and spectra of the gamma -ray point sources from the first Fermi-LAT catalog, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as gamma -rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum-likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but underpredict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point-source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi-LAT studies of this region that used different analysis methods than employed in this paper.
The second catalog of active galactic nuclei (AGNs) detected by the Fermi Large Area Telescope (LAT) in two years of scientific operation is presented. The second LAT AGN catalog (2LAC) includes 1017 ...Delta *g-ray sources located at high Galactic latitudes (|b| > 10?) that are detected with a test statistic (TS) greater than 25 and associated statistically with AGNs. However, some of these are affected by analysis issues and some are associated with multiple AGNs. Consequently, we define a Clean Sample which includes 886 AGNs, comprising 395 BL Lacertae objects (BL Lac objects), 310 flat-spectrum radio quasars (FSRQs), 157 candidate blazars of unknown type (i.e., with broadband blazar characteristics but with no optical spectral measurement yet), 8 misaligned AGNs, 4 narrow-line Seyfert 1 (NLS1s), 10 AGNs of other types, and 2 starburst galaxies. Where possible, the blazars have been further classified based on their spectral energy distributions (SEDs) as archival radio, optical, and X-ray data permit. While almost all FSRQs have a synchrotron-peak frequency <1014 Hz, about half of the BL Lac objects have a synchrotron-peak frequency >1015 Hz. The 2LAC represents a significant improvement relative to the first LAT AGN catalog (1LAC), with 52% more associated sources. The full characterization of the newly detected sources will require more broadband data. Various properties, such as Delta *g-ray fluxes and photon power-law spectral indices, redshifts, Delta *g-ray luminosities, variability, and archival radio luminosities and their correlations are presented and discussed for the different blazar classes. The general trends observed in 1LAC are confirmed.
Recent detections of the starburst galaxies M82 and NGC 253 by gamma-ray telescopes suggest that galaxies rapidly forming massive stars are more luminous at gamma-ray energies compared to their ...quiescent relatives. Building upon those results, we examine a sample of 69 dwarf, spiral, and luminous and ultraluminous infrared galaxies at photon energies 0.1-100 GeV using 3 years of data collected by the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (Fermi). Measured fluxes from significantly detected sources and flux upper limits for the remaining galaxies are used to explore the physics of cosmic rays in galaxies. We find further evidence for quasi-linear scaling relations between gamma-ray luminosity and both radio continuum luminosity and total infrared luminosity which apply both to quiescent galaxies of the Local Group and low-redshift starburst galaxies (conservative P-values > ~0.05 accounting for statistical and systematic uncertainties). The normalizations of these scaling relations correspond to luminosity ratios of log (L sub(0.1,100 GeV)/L sub(1.4 GHz)) = 1.7 + or - 0.1 sub((statistical)) + or - 0.2 sub((dispersion)) and log (L sub(0.1-100 GeV)/L sub(8,1000 mu m)) = -4.3 + or - 0.1 sub((statistical)) + or - 0.2 sub((dispersion)) for a galaxy with a star formation rate of 1 M sub(middot in circle) yr super(-1), assuming a Chabrier initial mass function. Using the relationship between infrared luminosity and gamma-ray luminosity, the collective intensity of unresolved star-forming galaxies at redshifts 0 > z > 2.5 above 0.1 GeV is estimated to be 0.4-2.4 x 10 super(-6) ph cm super(-2) s super(-1) sr super(-1) (4%-23% of the intensity of the isotropic diffuse component measured with the LAT). We anticipate that ~10 galaxies could be detected by their cosmic-ray-induced gamma-ray emission during a 10 year Fermi mission.
PDF
