A Comprehensive Study of GRB 070125, A Most Energetic Gamma-Ray Burst

We present a comprehensive multiwavelength analysis of the bright, long-duration gamma-ray burst GRB 070125, comprised of observations in gamma-ray, X-ray, optical, millimeter, and centimeter wave bands. Simultaneous fits to the optical and X-ray light curves favor a break on day 3.78, which we interpret as the jet break from a collimated outflow. Independent fits to optical and X-ray bands give similar results in the optical bands but shift the jet break to around day 10 in the X-ray light curve. We show that for the physical parameters derived for GRB 070125, inverse Compton scattering effects are important throughout the afterglow evolution. While inverse Compton scattering does not affect radio and optical bands, it may be a promising candidate to delay the jet break in the X-ray band. Radio light curves show rapid flux variations, which are interpreted as due to interstellar scintillation and used to derive an upper limit of 2.4 × 10 17 cm on the radius of the fireball in the lateral expansion phase of the jet. Radio light curves and spectra suggest a high synchrotron self-absorption frequency indicative of the afterglow shock wave moving in a dense medium. Our broadband modeling favors a constant density profile for the circumburst medium over a windlike profile (R -2). However, keeping in mind the uncertainty of the parameters, it is difficult to unambiguously distinguish between the two density profiles. Our broadband fits suggest that GRB 070125 is a burst with high radiative efficiency (>60%).R. A. C. was supported in part by NASA grant NNG06GJ33G

The structure and emission model of the relativistic jet in the quasar 3C279 inferred from radio to high-energy γ-ray observations in 2008-2010

We present time-resolved broadband observations of the quasar 3C279 obtained from multi-wavelength campaigns conducted during the first two years of the Fermi Gamma-ray Space Telescope mission. While investigating the previously reported γ-ray/optical flare accompanied by a change in optical polarization, we found that the optical emission appears to be delayed with respect to the γ-ray emission by about 10days. X-ray observations reveal a pair of "isolated" flares separated by 90 days, with only weak γ-ray/optical counterparts. The spectral structure measured by Spitzer reveals a synchrotron component peaking in the mid-infrared band with a sharp break at the far-infrared band during the γ-ray flare, while the peak appears in the millimeter (mm)/submillimeter (sub-mm) band in the low state. Selected spectral energy distributions are fitted with leptonic models including Comptonization of external radiation produced in a dusty torus or the broad-line region. Adopting the interpretation of the polarization swing involving propagation of the emitting region along a curved trajectory, we can explain the evolution of the broadband spectra during the γ-ray flaring event by a shift of its location from 1pc to 4pc from the central black hole. On the other hand, if the γ-ray flare is generated instead at sub-pc distance from the central black hole, the far-infrared break can be explained by synchrotron self-absorption. We also model the low spectral state, dominated by the mm/sub-mm peaking synchrotron component, and suggest that the corresponding inverse-Compton component explains the steady X-ray emission. © 2012. The American Astronomical Society. All rights reserved.

A VLBI view on the extreme scintillator J1819+3845

Scintillation of flat spectrum radio sources provides a unique instrument to study the characteristics of the electron density distribution throughout the interstellar medium as well as to study the microarcsecond structure of compact radio objects. During the course of its extreme activity, the quasar J1819+3845 has been observed with a number of VLBI arrays. The fast variability, up to 600% in less than one hour, has made it very difficult to image the source. The variability has now stopped and we will present scintillation-free VLBI images of J1819+3548 taken with the EVN (2008) and VLBA (2007). We will compare the quiescent phase of J1819+3845 with images of the source during its extreme variability (observed with global and space VLBI). A description of the possible scattering screen and implications for the source internal structure will be presented.Space EngineeringAerospace Engineerin

Synchrotron Radiation and Faraday Rotation

Synchrotron radiation and its degree of linear polarization are powerful tracers of magnetic fields that are illuminated by cosmic ray electrons. Faraday rotation of the linearly polarized radiation is induced by intervening line-of-sight magnetic fields that are embedded in ionized plasmas. For decades these observational tools have been workhorses for utilizing radio telescopes to study magnetic fields inside and outside the Milky Way. In the modern era they are becoming still more powerful with the advent of broadband receiving systems on the current generation of radio telescopes. In the study of Faraday rotation in particular, rapid development is taking place in techniques to confidently recover the physical conditions of the magnetoionized medium in the Milky Way and extragalactic objects

Science with the Australian Square Kilometre Array Pathfinder

The future of centimetre and metre-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science fo