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Author: Brice Michael Williams Publisher: ISBN: Category : Astrophysics Languages : en Pages : 0
Book Description
The latest data release from the Fermi Gamma-Ray Space Telescope (the Fourth Fermi LAT Catalog) has more than 50% of the detected galactic sources yet to be identified. In an attempt to uncover the nature of some of these unknown sources, we observed thirteen sources within the galactic plane using the Chandra X-Ray Observatory (CXO). Analysis was performed using Chandra Interactive Analysis of Observations (CIAO) software in order to determine if any of these Fermi sources also emitted in the X-ray spectrum. We performed spectral analysis of seven X-ray sources, which had higher quality data. We also found fainter X-ray sources that lied within the Fermi radii and performed X-ray analysis on them. We also analyzed the available multi-wavelength data to shed light on the nature of these high energy sources.
Author: Brice Michael Williams Publisher: ISBN: Category : Astrophysics Languages : en Pages : 0
Book Description
The latest data release from the Fermi Gamma-Ray Space Telescope (the Fourth Fermi LAT Catalog) has more than 50% of the detected galactic sources yet to be identified. In an attempt to uncover the nature of some of these unknown sources, we observed thirteen sources within the galactic plane using the Chandra X-Ray Observatory (CXO). Analysis was performed using Chandra Interactive Analysis of Observations (CIAO) software in order to determine if any of these Fermi sources also emitted in the X-ray spectrum. We performed spectral analysis of seven X-ray sources, which had higher quality data. We also found fainter X-ray sources that lied within the Fermi radii and performed X-ray analysis on them. We also analyzed the available multi-wavelength data to shed light on the nature of these high energy sources.
Author: National Aeronautics and Space Adm Nasa Publisher: ISBN: 9781729025710 Category : Languages : en Pages : 38
Book Description
As was the case for COS B, the majority of high-energy (greater than 100 MeV) gamma-ray sources detected by the EGRET instrument on GRO are not immediately identifiable with cataloged objects at other wavelengths. These persistent gamma-ray sources are, next to the gamma-ray bursts, the least understood objects in the universe. Even a rudimentary understanding of their nature awaits identifications and follow-up work at other wavelengths to tell us what they are. The as yet unidentified sources are potentially the most interesting, since they may represent unrecognized new classes of astronomical objects, such as radio-quiet pulsars or new types of active galactic nuclei (AGN's). This two-year investigation is intended to support the analysis, correlation, and theoretical interpretation of data that we are obtaining at x ray, optical, and radio wavelengths in order to render the gamma-ray data interpretable. According to plan, in the first year concentration was on the identification and study of Geminga. The second year will be devoted to studies of similar unidentified gamma-ray sources which will become available in the first EGRET catalogs. The results obtained so far are presented in the two papers which are reproduced in the Appendix. In these papers, we discuss the pulse profiles of Geminga, the geometry and efficiency of the magnetospheric accelerator, the distance to Geminga, the implications for theories of polar cap heating, the effect of the magnetic field on the surface emission and environment of the neutron star, and possible interpretations of a radio-quiet Geminga. The implications of the other gamma-ray pulsars which were discovered to have high gamma-ray efficiency are also discussed, and the remaining unidentified COS B sources are attributed to a population of efficient gamma-ray sources, some of which may be radio quiet. Halpern, Jules P. Unspecified Center NAG5-2051...
Author: Stephen Kerby Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
The most recent catalogue of Fermi-LAT gamma-ray point sources includes over 4000 associated sources, most of which are pulsars or blazars. However, over 2000 sources are "unassociated", defying confident classification or identification based on gamma-ray data alone. Our understanding of the high-energy sky is systematically incomplete while these unassociated sources remain unexamined. An ongoing search for low-energy counterparts using the Neil Gehrels Swift Observatory has identified over three hundred X-ray/UV/optical counterparts to Fermi unassociated sources and placed limits on the maximum X-ray flux for almost a thousand more; these discoveries facilitate spatial localization and broadband SED analysis of the unassociated sources. Analyzing Swift-XRT and -UVOT data for these counterparts, we conduct machine learning classification to identify likely blazars and pulsars from this sample. Between random forest, neural network, and support vector machine classifiers, we find that the unassociated sources with low-energy counterparts are mostly dimmer blazars while also discovering several unusual sources and new gamma-ray pulsars. Using Fermi, Swift, and archival IR and radio data, we create a sample of 106 new blazars; via another machine learning program, we show that these dimmer blazars tend to have higher synchrotron peak frequencies like BL Lac objects. These findings align with the predictions of the blazar sequence, extending and confirming that observational framework with a totally new sample. While the dim blazars in our new sample confirm the predictions of the blazar sequence, many of the unassociated targets without Swift-XRT detections may be even dimmer blazars, especially those at high galactic latitude. Furthermore, several sources are not well-explained by either pulsar or blazar models, or are coincident with stars or known galactic binaries. In-depth examination of these unique objects shows that - like the associated sources - the unassociated sources contain rare minority classes ranging from low-mass X-ray binaries and cataclysmic variables to extreme Seyfert galaxies and star-forming regions.
Author: The Cta Consortium Publisher: World Scientific ISBN: 9813270101 Category : Science Languages : en Pages : 365
Book Description
This book summarizes the science to be carried out by the upcoming Cherenkov Telescope Array, a major ground-based gamma-ray observatory that will be constructed over the next six to eight years. The major scientific themes, as well as core program of key science projects, have been developed by the CTA Consortium, a collaboration of scientists from many institutions worldwide.CTA will be the major facility in high-energy and very high-energy photon astronomy over the next decade and beyond. CTA will have capabilities well beyond past and present observatories. Thus, CTA's science program is expected to be rich and broad and will complement other major multiwavelength and multimessenger facilities. This book is intended to be the primary resource for the science case for CTA and it thus will be of great interest to the broader physics and astronomy communities. The electronic version (e-book) is available in open access.
Author: Anton Dmytriiev Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Many questions related to the physics of jets of Active Galactic Nuclei remain open. A particular subclass of AGN, blazars, have a jet pointing towards the Earth. Such suitable orientation of the jet allows us to probe a rich variety of poorly understood physical phenomena related to relativistic outflows. Blazars show non-thermal emission, originating from the jet, which is highly variable across the entire electromagnetic spectrum, from radio frequencies to TeV gamma-rays. The energy flux can enhance by an order of magnitude on time-scales as short as minutes, a phenomenon referred to as a “flare”, and as long as months or even years. Despite the growing amount of available multi-wavelength (MWL) data, the origin and the physical mechanisms behind the frequently observed flaring events in blazars are still not well understood. Many attempts have been made to describe the flares with different emission models, but detailed properties of flux variation patterns (light curves) in different wavebands remain difficult to reproduce. In order to identify physical processes that are involved during blazar outbursts, I have developed a versatile radiative code, based on a time-dependent treatment of particle acceleration, escape and radiative cooling. The code computes time evolution of the distribution function of electrons in the blazar emitting zone and the spectrum of the Synchrotron Self-Compton (SSC) emission by these electrons. I applied the code to a giant MWL flare of the blazar Mrk 421, a representative of the BL Lacertae class, which is the brightest VHE flare ever detected from this source. In our approach, we consider the flare as a moderate perturbation of the quiescent state and search for interpretations with a minimum number of free parameters. As a result, I developed a novel physical scenario of the flaring activity that describes the data set, comprising spectra in the high state of the source in different energy ranges, and MWL light curves from the optical domain to the VHE gamma-ray band. In this scenario, the process initiating the outburst is the second-order Fermi acceleration of particles due to turbulence arising in the vicinity of the blazar stationary emission region. In this thesis, I also performed analysis of High Energy Stereoscopic System (H.E.S.S.) data of two giant flares of the blazar 3C 279, a representative of the Flat Spectrum Radio Quasars (FSRQ) class. Finally, I contributed to preparation of Cherenkov Telescope Array (CTA), which is a new-generation ground-based gamma-ray observatory, expected to start operations in 2022. The instrument, which is presently under development, will have greatly improved performance compared to currently operating Imaging Atmospheric Cherenkov Telescopes (IACTs), including unprecedented spectral coverage from a few tens of GeV to ~300 TeV. In the context of CTA, I performed simulations of the optical performance of the Gamma-Ray Cherenkov Telescope (GCT), one of the three proposed designs of Small-Size Telescopes (SST) for CTA. Also, using the observations of bright stars done by the telescope prototype installed on the site of Paris Observatory in Meudon, I studied the effect of micro-roughness of the telescope mirrors on the point spread function (PSF) and calculated the level of the mirror polishing quality required to optimize the performances.
Author: Publisher: ISBN: Category : Languages : en Pages : 54
Book Description
One of the unsolved mysteries of gamma-ray astronomy concerns the nature of the unidentified gamma-ray sources. Recently, using the Second Fermi LAT source catalog (2FGL) and the Wide-field Infrared Survey Explorer (WISE) archive, we discovered that the WISE counterparts of gamma-ray blazars, a class of active galactic nuclei, delineate a region (the WISE Gamma-ray Strip) in the 3-dimensional infrared color space well separated from the locus of the other astronomical objects. Based on this result, we built an association procedure to recognize if there areWISE blazar candidates within the positional uncertainty region of the unidentified gamma-ray sources. Here we report on our analysis of 2FGL J1823.8+4312, a gamma-ray active galactic nucleus of uncertain type associated with the X-ray source 1RXS J182418.7+430954 according to the 2FGL, to verify whether it is a blazar. Applying our association method we found two sources with IR colors typical of gamma-ray blazars, located within the 99.9% confidence region of 2FGL J1823.8+4312: WISE J182352.33+431452.5 and WISE J182409.25+431404.7. Then we searched in the Chandra, NVSS and SDSS archival observations for their counterparts. We discovered that WISE J182352.33+431452.5, our preferred gamma-ray blazar candidate according to our WISE association procedure, is detected in the optical and in the X-rays but not in the radio, making it extremely unusual if it is a blazar. Given its enigmatic spectral energy distribution, we considered the possibility that it is a 'radio faint blazar' or the prototype of a new class of extragalactic sources, our conclusion is independent of whether WISE J182352.33+431452.5 is the actual counterpart of 2FGL J1823.8+4312.
Author: E. VALTAOJA (Ed) Publisher: Cambridge University Press ISBN: 9780521413510 Category : Science Languages : en Pages : 484
Book Description
Blazars (also known as BL Lac objects), first discovered in 1978, are unusually energetic objects in the extragalactic universe. About 200 are known or suspected. They are radio sources with highly variable optical and radio emission, as well as high polarisation, and their optical spectra normally have no distinguishable features. It is generally accepted that they belong to that class of galaxies with active galactic nuclei, which are presumed to be driven by infall of matter to a supermassive black hole. In this book researchers give a complete summary of the observations of blazars and the theoretical interpretation. A comprehensive listing of confirmed and candidate objects is included. Mechanisms in which the variability can arise from shocks and relativistic jets are discussed. There are at least four different answers given to the question: what is a blazar? This book is a complete overview of the violent activity observed in these extreme active galactic nuclei.
Author: Brice Michael Williams
Author: Brice Michael Williams
Author: National Aeronautics and Space Adm Nasa
Author: Stephen Kerby
Author: Time-Life Books
Author: The Cta Consortium
Author: Dieter H. B. Hartmann
Author: Anton Dmytriiev
Author: 
Author: Sheetal Saxena
Author: E. VALTAOJA (Ed)