37,666 research outputs found

    Compton X-ray and Gamma-ray Emission from Extended Radio Galaxies

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    The extended lobes of radio galaxies are examined as sources of X-ray and gamma-ray emission via inverse Compton scattering of 3K background photons. The Compton spectra of two exemplary examples, Fornax A and Centaurus A, are estimated using available radio measurements in the ~10's MHz - 10's GHz range. For average lobe magnetic fields of >~0.3-1 micro-G, the lobe spectra are predicted to extend into the soft gamma-rays making them likely detectable with the GLAST LAT. If detected, their large angular extents (~1 deg and 8 deg) will make it possible to ``image'' the radio lobes in gamma-rays. Similarly, this process operates in more distant radio galaxies and the possibility that such systems will be detected as unresolved gamma-ray sources with GLAST is briefly considered.Comment: 3 pages, one 3 panel figure. Proceedings of the First GLAST Symposium. (The actual submitted version is 2 pages with references used more sparingly.

    Panchromatic Views of Large-scale Extragalactic Jets

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    Highlights of recent observations of extended jets in AGN are presented. Specifically, we discuss new spectral constraints enabled by Spitzer, studies of the highest-redshift (z~4) radio/X-ray quasar jets, and a new VLBA detection of superluminal motion in the M87 jet associated with a recent dramatic X-ray outburst. Expanding on the title, inverse Compton emission from extended radio lobes is considered and a testable prediction for the gamma-ray emission in one exemplary example is presented. Prospects for future studies with ALMA and low-frequency radio interferometers are briefly described.Comment: 4 pgs, 3 figs, To appear in "The Central Engine of Active Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP

    Fermi Discovers a New Population of Gamma-ray Novae

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    Novae had not been widely considered as high-energy (>100 MeV) gamma-ray sources before the launch of the Fermi Large Area Telescope (LAT). In March 2010, the LAT made the first gamma-ray detection of a nova in the symbiotic binary V407 Cygni. The LAT observations uniquely probed the high-energy particle acceleration mechanism in the environs of the V407 Cyg binary system consisting of a white dwarf and its red giant companion. Subsequently in June 2012, two more novae were detected with the LAT, Nova Sco 2012 and Nova Mon 2012, thus heralding novae as a new gamma-ray source class. For Nova Mon 2012, the gamma-ray transient source was discovered first, followed by the optical confirmation of the nova, showcasing how novae can be found with the LAT independently from traditional optical searches. We discuss the LAT detected gamma-ray novae together with observational limits on other optical novae over the first four years of the Fermi mission and reconsider the possible high-energy gamma-ray production mechanisms in novae in light of the new detections.Comment: 2012 Fermi Symposium proceedings - eConf C121028, 6 pages, 4 figures. Corrects a typo in v1 for Nova Mon 2012 start data in Fig 2 caption: June 9 -> June 1

    The role of social media in the collaboration, interaction, co-creation and co-delivery of a social venture in an uncertain conflict environment

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    This research case study examines the creation and development of a bottom-up social enterprise immediately after the outbreak of a civil war in an Arab country by a group of young patriots in the aftermath of the Arab Spring uprisings in the 2010s. Drawing on Linders’ (2012) model of social action lifecycle, our study examines how different actors become involved and co-created a socially entrepreneurial venture (SEV), how these actors contributed to the coproduction and co-delivery of the social actions (values) over time, and how social media play roles in these processes. Drawing from the empirical evidences of citizen co-production within the existing literature, we found that SEVs operate in very different ways in which the role of social media is critical, both from their equivalents operating in a penurious but stable environment, and those intending to pursue political action within a conflict situation. In our case study, social media was not intended for mass-mobilisation, but for selected mobilisation amongst those within the network. This is due to the insecure environment and the fear of infiltration from opposing parties. We also examine how new actors were, over time, being carefully screened and selected, and potential harmful existing players being excluded, which in turn contributed to the evolving nature of the social enterprises

    Language as a Geometry in Wittgenstein"s Tractatus

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    In TLP 4.011, while admitting that propositions expressed by the phonetic notation, or the alphabet, just like the written notes of a piece of music, do not seem at first sight to be pictures of what they represent, the Tractatus insists that those "sign-languages" (that is, the phonetic notation and the written musical notes) prove to be pictures of what they represent (that is, our speech and the piece of music, respectively) "even in the ordinary sense". (TLP 4.016 also says that "alphabetic script developed out of [hieroglyphic script] without losing what was essential to depiction".) So, contrary to the view of some commentators (e.g. Pears 1987, 115-121), instead of making an analogy here, the Tractatus holds that a proposition is a picture literally. How can a proposition be a picture literally

    VLA Observations of a Complete Sample of Radio Loud Quasars between redshifts 2.5 and 5.28: I. high-redshift sample summary and the radio images

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    We present high resolution (arcsecond or better) observations made with the Karl G. Jansky Very Large Array of 123 radio-loud quasars with redshifts in the range 2.5z5.282.5 \leq z \leq 5.28 that form a complete flux limited sample (70\geq 70 mJy at 1.4 GHz or 5 GHz). Where possible, we used previous high resolution VLA observations (mainly A array at 1.4, 5 and 8 GHz) from the NRAO archive and re-imaged them (43 sources). For the remainder, new observations were made in the A array at 1.4 and 5 GHz. We show images of the 61 resolved sources, and list structural properties of all of them. Optical data from the SDSS are available for nearly every source. This work represents a significant increase in the number of high redshift quasars with published radio structures, and will be used to study the properties and evolution of luminous radio sources in the high redshift universe