37,727 research outputs found
Compton X-ray and Gamma-ray Emission from Extended Radio Galaxies
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
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
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
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
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
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 that form a complete flux limited sample ( 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
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