Boston University

About: Boston University is a education organization based out in Boston, Massachusetts, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 48688 authors who have published 119622 publications receiving 6276020 citations. The organization is also known as: BU & Boston U.

The spectral energy distribution of fermi bright blazars

Abstract: We have conducted a detailed investigation of the broadband spectral properties of the gamma-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/gamma-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these gamma-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual log nu-log nu F-nu representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low-and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, alpha(ro), and optical to X-ray, alpha(ox), spectral slopes) and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency (nu(S)(peak)) is positioned between 10(12.5) and 10(14.5) Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 10(13) and 10(17) Hz in featureless BL Lacertae objects. We find that the gamma-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter gamma-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum gamma-ray sources, the correlation between nu(S)(peak) and gamma-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars.

Learning to Contract: Evidence from the Personal Computer Industry

Abstract: Organizational forms involving more detailed contracts than are found in traditional spot market exchanges appear to be increasingly prevalent. There has been relatively little analysis, however, of the extent to which firms learn how to use contracts to manage their interfirm relationships over time. In this paper, we conduct a detailed case study of a time series of 11 contracts concluded during 1989-1997 between the same two partners, both of whom participate in the personal computer industry, to explore whether and how firms learn to contract. We find many changes to the structure of the contracts that cannot be fully explained by changes in the assets at risk in the relationship, and evidence that these changes are largely the result of processes in which the firms were learning how to work together, including learning how to contract with each other. The nature of this learning appears to have been quite incremental and local, that is, not very far sighted. We suggest how and when contracts might serve as repositories for knowledge about how to govern collaborations, and suggest some boundary conditions for this phenomenon. Our findings also provide implications for the debate about whether contracts have a positive or negative effect on interorganizational trust. We conclude with suggestions for future research.

Human Toll-Like Receptors Mediate Cellular Activation by Mycobacterium tuberculosis

Abstract: Recent studies have implicated a family of mammalian Toll-like receptors (TLR) in the activation of macrophages by Gram-negative and Gram-positive bacterial products. We have previously shown that different TLR proteins mediate cellular activation by the distinct CD14 ligands Gram-negative bacterial LPS and mycobacterial glycolipid lipoarabinomannan (LAM). Here we show that viable Mycobacterium tuberculosis bacilli activated both Chinese hamster ovary cells and murine macrophages that overexpressed either TLR2 or TLR4. This contrasted with Gram-positive bacteria and Mycobacterium avium, which activated cells via TLR2 but not TLR4. Both virulent and attenuated strains of M. tuberculosis could activate the cells in a TLR-dependent manner. Neither membrane-bound nor soluble CD14 was required for bacilli to activate cells in a TLR-dependent manner. We also assessed whether LAM was the mycobacterial cell wall component responsible for TLR-dependent cellular activation by M. tuberculosis. We found that TLR2, but not TLR4, could confer responsiveness to LAM isolated from rapidly growing mycobacteria. In contrast, LAM isolated from M. tuberculosis or Mycobacterium bovis bacillus Calmette-Guerin failed to induce TLR-dependent activation. Lastly, both soluble and cell wall-associated mycobacterial factors were capable of mediating activation via distinct TLR proteins. A soluble heat-stable and protease-resistant factor was found to mediate TLR2-dependent activation, whereas a heat-sensitive cell-associated mycobacterial factor mediated TLR4-dependent activation. Together, our data demonstrate that Toll-like receptors can mediate cellular activation by M. tuberculosis via CD14-independent ligands that are distinct from the mycobacterial cell wall glycolipid LAM.