oalib

Publish in OALib Journal

ISSN: 2333-9721

APC: Only $99

Submit

Any time

2019 ( 1 )

2018 ( 18 )

2017 ( 15 )

2016 ( 33 )

Custom range...

Search Results: 1 - 10 of 8995 matches for " Anne Lahteenmaki "
All listed articles are free for downloading (OA Articles)
Page 1 /8995
Display every page Item
Multi-waveband Emission Maps of Blazars
Alan Marscher,Svetlana G. Jorstad,Valeri M. Larionov,Margo F. Aller,Anne Lahteenmaki
Physics , 2010, DOI: 10.1007/s12036-011-9013-8
Abstract: We are leading a comprehensive multi-waveband monitoring program of 34 gamma-ray bright blazars designed to locate the emission regions of blazars from radio to gamma-ray frequencies. The "maps" are anchored by sequences of images in both total and polarized intensity obtained with the VLBA at an angular resolution of ~ 0.1 milliarcseconds. The time-variable linear polarization at radio to optical wavelengths and radio to gamma-ray light curves allow us to specify the locations of flares relative to bright stationary features seen in the images and to infer the geometry of the magnetic field in different regions of the jet. Our data reveal that some flares occur simultaneously at different wavebands and others are only seen at some of the frequencies. The flares are often triggered by a superluminal knot passing through the stationary "core" on the VLBA images. Other flares occur upstream or even parsecs downstream of the core.
Multiwavelength Spectral Studies Of Fermi-LAT Blazars
Manasvita Joshi,Alan Marscher,Svetlana Jorstad,Markus Boettcher,Ivan Agudo,Valeri Larionov,Margo Aller,Mark Gurwell,Anne Lahteenmaki
Physics , 2011,
Abstract: We present multiwavelength spectral analyses of two Fermi-LAT blazars, OJ 287 and 3C 279, that are part of the Boston University multiwaveband polarization program. The data have been compiled from observations with Fermi, RXTE, the VLBA, and various ground-based optical and radio telescopes. We simulate the dynamic spectral energy distributions (SEDs) within the framework of a multi-slice, time-dependent leptonic jet model for blazars, with radiation feedback, in the internal shock scenario. We use the physical jet parameters obtained from the VLBA monitoring to guide our modeling efforts. We discuss the role of intrinsic parameters and the interplay between synchrotron and inverse Compton radiation processes responsible for producing the resultant SEDs.
Gamma-ray emission region located in the parsec scale jet of OJ287
Ivan Agudo,Svetlana G. Jorstad,Alan P. Marscher,Valeri M. Larionov,Jose L. Gomez,Anne Lahteenmaki,Mark Gurwell,Paul S. Smith,Helmut Wiesemeyer,Clemens Thum,Jochen Heidt
Physics , 2011, DOI: 10.1088/1742-6596/355/1/012032
Abstract: We report on the location of the gamma-ray emission region in flares of the BL Lacertae object OJ287 at >14pc from the central supermassive black hole. We employ data from multi-spectral range (total flux and linear polarization) monitoring programs combined with sequences of ultra-high-resolution 7mm VLBA images. The correlation between the brightest gamma-ray and mm flares is found to be statistically significant. The two gamma-ray peaks, detected by Fermi-LAT, that we report here happened at the rising phase of two exceptionally bright mm flares accompanied by sharp linear polarization peaks. The VLBA images show that these mm flares in total flux and polarization degree occurred in a jet region at >14pc from the innermost jet region. The time coincidence of the brighter gamma-ray flare and its corresponding mm linear polarization peak evidences that both the gamma-ray and mm outbursts occur >14pc from the central black hole. We find two sharp optical flares occurring at the peak times of the two reported gamma-ray flares. This is interpreted as the gamma-ray flares being produced by synchrotron self-Compton scattering of optical photons from the flares triggered by the interaction of moving knots with a stationary conical shock in the jet.
VLBI and Single Dish Monitoring of 3C84 in the Period of 2009-2011
Hiroshi Nagai,Monica Orienti,Motoki Kino,Kenta Suzuki,Gabriele Giovannini,Akihiro Doi,Keiichi Asada,Marcello Giroletti,Jun Kataoka,Filippo D'Ammando,Makoto Inoue,Anne Lahteenmaki,Merja Tornikoski,Jonathan Leon-Tavares,Seiji Kameno,Uwe Bach
Physics , 2012, DOI: 10.1111/j.1745-3933.2012.01269.x
Abstract: The radio galaxy 3C 84 is a representative of gamma-ray-bright misaligned active galactic nuclei (AGNs) and one of the best laboratories to study the radio properties of the sub-pc jet in connection with the gamma-ray emission. In order to identify possible radio counterparts of the gamma-ray emissions in 3C 84, we study the change in structure within the central 1 pc and the light curve of sub-pc-size components C1, C2, and C3. We search for any correlation between changes in the radio components and the gamma-ray flares by making use of VLBI and single dish data. Throughout the radio monitoring spanning over two GeV gamma-ray flares detected by the {\it Fermi}-LAT and the MAGIC Cherenkov Telescope in the periods of 2009 April to May and 2010 June to August, total flux density in radio band increases on average. This flux increase mostly originates in C3. Although the gamma-ray flares span on the timescale of days to weeks, no clear correlation with the radio light curve on this timescale is found. Any new prominent components and change in morphology associated with the gamma-ray flares are not found on the VLBI images.
Locating the Gamma-ray Flaring Emission of Blazar AO 0235+164 in the Jet at Parsec Scales Through Multi Spectral Range Monitoring
Ivan Agudo,Alan P. Marscher,Svetlana G. Jorstad,Valeri M. Larionov,Jose L. Gomez,Anne Lahteenmaki,Paul S. Smith,Kari Nilsson,Anthony C. S. Readhead,Margo F. Aller,Jochen Heidt,Mark Gurwell,Clemens Thum,Ann E. Wehrle,Omar M. Kurtanidze
Physics , 2013,
Abstract: We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and gamma-ray wavelengths of the BL Lacertae object AO 0235+164 in 2008. We analyze the timing of multi-waveband variations in the flux and linear polarization, as well as changes in Very Long Baseline Array (VLBA) images at 7mm with ~0.15 milliarcsecond resolution. The association of the events at different wavebands is confirmed at high statistical significance by probability arguments and Monte-Carlo simulations. A series of sharp peaks in optical linear polarization, as well as a pronounced maximum in the 7mm polarization of a superluminal jet knot, indicate rapid fluctuations in the degree of ordering of the magnetic field. These results lead us to conclude that the outburst occurred in the jet both in the quasi-stationary core and in the superluminal knot, both at >12 parsecs downstream of the supermassive black hole. We interpret the outburst as a consequence of the propagation of a disturbance, elongated along the line of sight by light-travel time delays, that passes through a standing recollimation shock in the core and propagates down the jet to create the superluminal knot. The multi-wavelength light curves vary together on long time-scales (months/years), but the correspondence is poorer on shorter time-scales. This, as well as the variability of the polarization and the dual location of the outburst, agrees with the expectations of a multi-zone emission model in which turbulence plays a major role in modulating the synchrotron and inverse Compton fluxes.
Location of the Gamma-Ray Flaring Emission in the Parse-Scale Jet of the BL Lac Object AO 0235+164
Ivan Agudo,Alan P. Marscher,Svetlana G. Jorstad,Valeri M. Larionov,Jose L. Gomez,Anne Lahteenmaki,Paul S. Smith,Kari Nilsson,Anthony C. S. Readhead,Margo F. Aller,Jochen Heidt,Mark Gurwell,Clemens Thum,Ann E. Wehrle,Omar M. Kurtanidze
Physics , 2011, DOI: 10.1142/S2010194512004709
Abstract: We locate the gamma-ray and lower frequency emission in flares of the BL Lac object AO 0235+164 at >12pc in the jet of the source from the central engine. We employ time-dependent multi-spectral-range flux and linear polarization monitoring observations, as well as ultra-high resolution (~0.15 milliarcsecond) imaging of the jet structure at lambda=7mm. The time coincidence in the end of 2008 of the propagation of the brightest superluminal feature detected in AO 0235+164 (Qs) with an extreme multi-spectral-range (gamma-ray to radio) outburst, and an extremely high optical and 7mm (for Qs) polarization degree provides strong evidence supporting that all these events are related. This is confirmed at high significance by probability arguments and Monte-Carlo simulations. These simulations show the unambiguous correlation of the gamma-ray flaring state in the end of 2008 with those in the optical, millimeter, and radio regime, as well as the connection of a prominent X-ray flare in October 2008, and of a series of optical linear polarization peaks, with the set of events in the end of 2008. The observations are interpreted as the propagation of an extended moving perturbation through a re-collimation structure at the end of the jet's acceleration and collimation zone.
Location of gamma-ray Flare Emission in the Jet of the BL Lacertae Object OJ287 more than 14pc from the Central Engine
Ivan Agudo,Svetlana G. Jorstad,Alan P. Marscher,Valeri M. Larionov,Jose L. Gomez,Anne Lahteenmaki,Mark A. Gurwell,Paul S. Smith,Helmut Wiesemeyer,Clemens Thum,Jochen Heidt,Dmitriy A. Blinov,Francesca D. D'Arcangelo,Vladimir A. Hagen-Thorn,Daria A. Morozova,Elina Nieppola,Mar Roca-Sogorb,Gary D. Schmidt,Brian Taylor,Merja Tornikoski,Ivan S. Troitsky
Physics , 2010, DOI: 10.1088/2041-8205/726/1/L13
Abstract: We combine time-dependent multi-waveband flux and linear polarization observations with sub-milliarcsecond-scale polarimetric images at lambda=7mm of the BL Lacertae-type blazar OJ287 to locate the gamma-ray emission in prominent flares in the jet of the source >14pc from the central engine. We demonstrate a highly significant correlation between the strongest gamma-ray and millimeter-wave flares through Monte-Carlo simulations. The two reported gamma-ray peaks occurred near the beginning of two major mm-wave outbursts, each of which is associated with a linear polarization maximum at millimeter wavelengths. Our Very Long Baseline Array observations indicate that the two mm-wave flares originated in the second of two features in the jet that are separated by >14 pc. The simultaneity of the peak of the higher-amplitude gamma-ray flare and the maximum in polarization of the second jet feature implies that the gamma-ray and mm-wave flares are co-spatial and occur >14 pc from the central engine. We also associate two optical flares, accompanied by sharp polarization peaks, with the two gamma-ray events. The multi-waveband behavior is most easily explained if the gamma-rays arise from synchrotron self-Compton scattering of optical photons from the flares. We propose that flares are triggered by interaction of moving plasma blobs with a standing shock. The gamma-ray and optical emission is quenched by inverse Compton losses as synchrotron photons from the newly shocked plasma cross the emission region. The mm-wave polarization is high at the onset of a flare, but decreases as the electrons emitting at these wavelengths penetrate less polarized regions.
Connection between the Accretion Disk and Jet in the Radio Galaxy 3C 111
Ritaban Chatterjee,Alan P. Marscher,Svetlana G. Jorstad,Alex Markowitz,Elizabeth Rivers,Richard E. Rothschild,Ian M. McHardy,Margo F. Aller,Hugh D. Aller,Anne Lahteenmaki,Merja Tornikoski,Brandon Harrison,Iv'an Agudo,Jos'e L. Gomez,Brian W. Taylor,Mark Gurwell
Physics , 2011, DOI: 10.1088/0004-637X/734/1/43
Abstract: We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 111 between 2004 and 2010 at X-ray (2.4--10 keV), optical (R band), and radio (14.5, 37, and 230 GHz) wave bands, as well as multi-epoch imaging with the Very Long Baseline Array (VLBA) at 43 GHz. Over the six years of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. This shows a clear connection between the radiative state near the black hole, where the X-rays are produced, and events in the jet. The X-ray continuum flux and Fe line intensity are strongly correlated, with a time lag shorter than 90 days and consistent with zero. This implies that the Fe line is generated within 90 light-days of the source of the X-ray continuum. The power spectral density function of X-ray variations contains a break, with steeper slope at shorter timescales. The break timescale of 13 (+12,-6) days is commensurate with scaling according to the mass of the central black hole based on observations of Seyfert galaxies and black hole X-ray binaries (BHXRBs). The data are consistent with the standard paradigm, in which the X-rays are predominantly produced by inverse Compton scattering of thermal optical/UV seed photons from the accretion disk by a distribution of hot electrons --- the corona --- situated near the disk. Most of the optical emission is generated in the accretion disk due to reprocessing of the X-ray emission. The relationships that we have uncovered between the accretion disk and the jet in 3C 111, as well as in the FR I radio galaxy 3C 120 in a previous paper, support the paradigm that active galactic nuclei and Galactic BHXRBs are fundamentally similar, with characteristic time and size scales proportional to the mass of the central black hole
The Occurrence of Cryptosporidium and Giardia Infections Among Patients Reporting Diarrheal Disease in Chobe District, Botswana  [PDF]
Kathleen Anne Alexander, Joel Herbein, Anne Zajac
Advances in Infectious Diseases (AID) , 2012, DOI: 10.4236/aid.2012.24023
Abstract: Diarrheal disease is a chronic public health problem in Chobe District, Botswana. Acute diarrheal outbreaks occur annually with a bimodal seasonal pattern coinciding with major hydrological phenomena. Information is lacking regarding the etiology of reoccurring outbreaks. Cryptosporidium and Giardia are recognized as important waterborne causes of diarrheal disease with Cryptosporidium transmission potentially involving zoonotic reservoirs. In Chobe District, municipal water is obtained from the Chobe River after it exits the Chobe National Park where high concentrations of wildlife occur. Using the Giardia/Cryptosporidium QUIK CHEK rapid antigen cartridge test, we evaluate the occurrence of Giardia and Cryptosporidium among patients (n = 153) presenting to medical facilities in Chobe District with diarrhea (August 2011-July 2012). Cryptosporidium was only identified in children less than two years of age (10%, n = 42, 95% CI 3% - 23%), with positive cases occurring during diarrheal outbreak periods. Infections were not identified in adults despite the high level of human immunodeficiency virus infection/acquired immunodeficiency syndrome (HIV/AIDS) in the region (n = 117). Although not significant, cases were predominately identified in the wet season outbreak (p = 0.94, 25%, n = 12, 95% CI 5% - 57%, dry season outbreaks 3%, n = 30, CI 95% 0% - 17%). Giardia infections occurred across age groups and seasons (7%, n = 153, 95% CI 4% - 10%). This is the first published report of human infections with Cryptosporidium and Giardia in this area of Africa. This study suggests that Cryptosporidium may be a potentially important cause of diarrheal disease in children less than 2 years of age in this region. Further research is required to identify pathogen transmission and persistence dynamics and public health implications, particularly the role of HIV/AIDS and vulnerability to waterborne disease.
3e tunneling processes in a superconducting single-electron tunneling transistor
P. Hadley,E. Delvigne,E. H. Visscher,S. Lahteenmaki,J. E. Mooij
Physics , 1998, DOI: 10.1103/PhysRevB.58.15317
Abstract: A current due to a tunneling event that involves three times the charge of an electron was observed in the current - voltage characteristics of a superconducting single-electron tunneling transistor. In this tunnel event, a Cooper pair tunnels through one tunnel barrier simultaneously with a quasiparticle that tunnels through a second tunnel barrier which is about 0.5 microns distant from the first tunnel barrier. This current was observed in a bias regime where current flow due to sequential quasiparticle tunneling is forbidden due to the Coulomb blockade.
Page 1 /8995
Display every page Item


Home
Copyright © 2008-2017 Open Access Library. All rights reserved.