| 1. |
- Abdo, A. A., et al.
(författare)
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The spectral energy distribution of fermi bright blazars
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 716:1, s. 30-70
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Tidskriftsartikel (refereegranskat)abstract
- We have conducted a detailed investigation of the broadband spectral properties of the gamma-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/gamma-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these gamma-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual log nu-log nu F-nu representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low-and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, alpha(ro), and optical to X-ray, alpha(ox), spectral slopes) and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency (nu(S)(peak)) is positioned between 10(12.5) and 10(14.5) Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 10(13) and 10(17) Hz in featureless BL Lacertae objects. We find that the gamma-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter gamma-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum gamma-ray sources, the correlation between nu(S)(peak) and gamma-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars.
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| 2. |
- D'Ammando, F., et al.
(författare)
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Long-term monitoring of PKS 0537-441 with Fermi-LAT and multiwavelength observations
- 2013
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 431:3, s. 2481-2492
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Tidskriftsartikel (refereegranskat)abstract
- We report on multiwavelength observations of the blazar PKS 0537-441 (z = 0.896) obtained from microwaves through gamma-rays by Submillimeter Array, Rapid Eye Mounting, Automatic Telescope for Optical Monitoring (ATOM), Swift and Fermi mostly during 2008 August-2010 April. Strong variability has been observed in gamma-rays, with two major flaring episodes (2009 July and 2010 March) and a harder-when-brighter behaviour, quite common for flat spectrum radio quasars and low-synchrotron-peaked BL Lacertae objects (BL Lacs), in 2010 March. In the same way, the spectral energy distribution (SED) of the source cannot be modelled by a simple synchrotron self-Compton model, as opposed to many BL Lacs, but the addition of an external Compton component of seed photons from a dust torus is needed. The 230 GHz light curve showed an increase simultaneous with the gamma-ray one, indicating co-spatiality of the mm and gamma-ray emission region likely at large distance from the central engine. The low, average, and high activity SED of the source could be fit changing only the electron distribution parameters, but two breaks in the electron distribution are necessary. The ensuing extra spectral break, located at near-infrared (NIR)-optical frequencies, together with that in gamma-rays seem to indicate a common origin, most likely due to an intrinsic feature in the underlying electron distribution. An overall correlation between the gamma-ray band with the R band and K band has been observed with no significant time lag. On the other hand, when inspecting the light curves on short time-scales some differences are evident. In particular, flaring activity has been detected in NIR and optical bands with no evident gamma-ray counterparts in 2009 September and November. Moderate variability has been observed in X-rays with no correlation between flux and photon index. An increase of the detected X-ray flux with no counterpart at the other wavelengths has been observed in 2008 October, suggesting once more a complex correlation between the emission at different energy bands.
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| 3. |
- Gedalin, M., et al.
(författare)
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Growth of filaments and saturation of the filamentation instability
- 2010
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 17:3
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Tidskriftsartikel (refereegranskat)abstract
- The filamentation instability of counterstreaming beams is a nonresonant hydrodynamic-type instability whose growth rate is a smooth function of the wavelength (scale). As a result, perturbations with all unstable wavelengths develop, and the growth saturates due to the saturation of available current. For a given scale, the magnetic field at saturation is proportional to the scale. As a result, the instability develops in a nearly linear regime, where the unstable modes stop growing as soon as the saturation of the corresponding wavelength is reached. At each moment there exists a dominant scale of the magnetic field which is the scale that reached saturation at this particular time. The smaller scales do not disappear and can be easily distinguished in the current structure. The overall growth of the instability stops when the loss of the streaming ion energy because of deceleration is comparable to the initial ion energy.
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| 4. |
- Gedalin, M., et al.
(författare)
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Relativistic filamentary equilibria
- 2011
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Ingår i: Journal of Plasma Physics. - : Cambridge University Press. - 0022-3778 .- 1469-7807. ; 77, s. 193-205
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Tidskriftsartikel (refereegranskat)abstract
- Plasma filamentation is often encountered in collisionless shocks and inertial confinement fusion. We develop a general analytical description of the two-dimensional relativistic filamentary equilibrium and derive the conditions for existence of potential-free equilibria. A pseudopotential equation for the vector-potential is constructed for cold and relativistic Maxwellian distributions. The role of counter-streaming is explained. We present single current sheet and periodic current sheet solutions, and analyze the equilibria with electric potential. These solutions can be used to study linear and nonlinear evolution of the relativistic filamentation instability.
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| 5. |
- Calcagno, A., et al.
(författare)
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Determinants of darunavir cerebrospinal fluid concentrations: impact of once-daily dosing and pharmacogenetics
- 2012
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Ingår i: Aids. - 0269-9370. ; 26:12, s. 1529-1533
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Tidskriftsartikel (refereegranskat)abstract
- Objectives: To compare cerebrospinal fluid (CSF) darunavir and ritonavir concentrations in patients receiving darunavir/ritonavir 800/100 mg once daily or 600/100 mg twice daily. To determine the influence of single-nucleotide polymorphisms in the genes encoding for blood-brain barrier transporters (ABCB1 3435 C>T, ABCB1 1236 C>T, ABCB1 2677 G>T, SLCO1A2 38 A>G, SLCO1A2 516 A>C, ABCC2-24 G>A) on darunavir and ritonavir penetration into CSF. Design: Comparative pharmacokinetics study in patients. Methods: Plasma and CSF darunavir and ritonavir concentrations (2-26 h after drug intake) were determined by a validated HPLC coupled with mass spectrometry method in adults on darunavir-based combination antiretroviral therapy undergoing a lumbar puncture. Results: HIV-infected patients on once-daily darunavir/ritonavir had significantly lower CSF darunavir trough concentrations and CSF-to-plasma ratios than patients on darunavir/ritonavir twice-daily (10.7 versus 38.2 ng/ml and 0.32 versus 0.90%; P < 0.05). No significant effect of single-nucleotide polymorphisms in the genes encoding for blood-brain barrier transporters was noted apart from slightly higher CSF darunavir penetration in patients carrying OATP1A2 uncommon variants. Conclusions: This is the first study to compare darunavir CSF concentrations in patients taking the once-daily or the twice-daily dosage: our data show that darunavir and ritonavir dosing significantly affects not only CSF concentrations but also the extent of drug penetration into the CSF. Furthermore a minority of patients in the once-daily arm presented very low CSF concentration of potential concern for HIV control in the central nervous system. The relative importance of pharmacogenetics in influencing CSF darunavir pharmacokinetics deserves further clinical investigation. (C) 2012 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins
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