581 research outputs found
Gamma-ray blazars: the view from AGILE
During the first 3 years of operation the Gamma-Ray Imaging Detector onboard
the AGILE satellite detected several blazars in a high gamma-ray activity: 3C
279, 3C 454.3, PKS 1510-089, S5 0716+714, 3C 273, W Comae, Mrk 421, PKS
0537-441 and 4C +21.35. Thanks to the rapid dissemination of our alerts, we
were able to obtain multiwavelength data from other observatories such as
Spitzer, Swift, RXTE, Suzaku, INTEGRAL, MAGIC, VERITAS, and ARGO as well as
radio-to-optical coverage by means of the GASP Project of the WEBT and the REM
Telescope. This large multifrequency coverage gave us the opportunity to study
the variability correlations between the emission at different frequencies and
to obtain simultaneous spectral energy distributions of these sources from
radio to gamma-ray energy bands, investigating the different mechanisms
responsible for their emission and uncovering in some cases a more complex
behaviour with respect to the standard models. We present a review of the most
interesting AGILE results on these gamma-ray blazars and their multifrequency
data.Comment: 25 pages, 10 figures, accepted for publication on Advances in Space
Research. Talk presented at the 38th COSPAR Scientific Assembly (Bremen,
Germany; July 18-25, 2010
Probabilistic Algorithmic Knowledge
The framework of algorithmic knowledge assumes that agents use deterministic
knowledge algorithms to compute the facts they explicitly know. We extend the
framework to allow for randomized knowledge algorithms. We then characterize
the information provided by a randomized knowledge algorithm when its answers
have some probability of being incorrect. We formalize this information in
terms of evidence; a randomized knowledge algorithm returning ``Yes'' to a
query about a fact \phi provides evidence for \phi being true. Finally, we
discuss the extent to which this evidence can be used as a basis for decisions.Comment: 26 pages. A preliminary version appeared in Proc. 9th Conference on
Theoretical Aspects of Rationality and Knowledge (TARK'03
Powerful high energy emission of the remarkable BL Lac object S5 0716+714
BL Lac objects of the intermediate subclass (IBLs) are known to emit a
substantial fraction of their power in the energy range 0.1--10 GeV. Detecting
gamma-ray emission from such sources provides therefore a direct probe of the
emission mechanisms and of the underlying powerhouse.
The AGILE gamma-ray satellite detected the remarkable IBL S5 0716+714 (z
\simeq 0.3) during a high state in the period from 2007 September - October,
marked by two very intense flares reaching peak fluxes of 200\times10^{-8} ph /
cm^2 s above 100 MeV, with simultaneous optical and X-ray observations. We
present here a theoretical model for the two major flares and discuss the
overall energetics of the source.
We conclude that 0716+714 is among the brightest BL Lac's ever detected at
gamma-ray energies. Because of its high power and lack of signs for ongoing
accretion or surrounding gas, the source is an ideal candidate to test the
maximal power extractable from a rotating supermassive black hole via the pure
Blandford-Znajek (BZ) mechanism. We find that during the 2007 gamma-ray flares
our source approached or just exceeded the upper limit set by BZ for a black
hole of mass 10^9 M_sunComment: 12 pages, 3 figure
The Agile Alert System For Gamma-Ray Transients
In recent years, a new generation of space missions offered great
opportunities of discovery in high-energy astrophysics. In this article we
focus on the scientific operations of the Gamma-Ray Imaging Detector (GRID)
onboard the AGILE space mission. The AGILE-GRID, sensitive in the energy range
of 30 MeV-30 GeV, has detected many gamma-ray transients of galactic and
extragalactic origins. This work presents the AGILE innovative approach to fast
gamma-ray transient detection, which is a challenging task and a crucial part
of the AGILE scientific program. The goals are to describe: (1) the AGILE
Gamma-Ray Alert System, (2) a new algorithm for blind search identification of
transients within a short processing time, (3) the AGILE procedure for
gamma-ray transient alert management, and (4) the likelihood of ratio tests
that are necessary to evaluate the post-trial statistical significance of the
results. Special algorithms and an optimized sequence of tasks are necessary to
reach our goal. Data are automatically analyzed at every orbital downlink by an
alert pipeline operating on different timescales. As proper flux thresholds are
exceeded, alerts are automatically generated and sent as SMS messages to
cellular telephones, e-mails, and push notifications of an application for
smartphones and tablets. These alerts are crosschecked with the results of two
pipelines, and a manual analysis is performed. Being a small scientific-class
mission, AGILE is characterized by optimization of both scientific analysis and
ground-segment resources. The system is capable of generating alerts within two
to three hours of a data downlink, an unprecedented reaction time in gamma-ray
astrophysics.Comment: 34 pages, 9 figures, 5 table
Towards a runtime verification framework for the Ada Programming Language
Runtime verification is an emerging discipline that investigates methods and tools to enable the verification of program properties during the execution of the application. The goal is to complement static analysis approaches, in particular when static verification leads to the explosion of states. Non-functional properties, such as the ones present in real-time systems are an ideal target for this kind of verification methodology, as are usually out of the range of the power and expressiveness of classic static analyses. In this paper, we present a framework that allows real-time programs written in Ada to be augmented with runtime verification capabilities. Our framework provides the infrastructures which is needed to instrument the code with runtime monitors. These monitors are responsible for observing the system and reaching verdicts about whether its behavior is compliant with its non-functional properties. We also sketch a contract language to extend the one currently provided by Ada, with the long term goal of having an elegant way in which runtime monitors can be automatically synthesized and instrumented into the target systems. The usefulness of the proposed approach is demonstrated by showing its use for an application scenario.This work was partially supported by Portuguese National Funds through FCT (Portuguese Founda- tion for Science and Technology) and by ERDF (European Regional Develop- ment Fund) through COMPETE (Operational Programme ’Thematic Factors of Competitiveness’), within projects FCOMP-01-0124-FEDER-037281 (CISTER), FCOMP-01-0124-FEDER-015006 (VIPCORE) and FCOMP-01-0124-FEDER- 020486 (AVIACC); and by FCT and EU ARTEMIS JU, within project ARTEMIS/0003/2012, JU grant nr. 333053 (CONCERTO)
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