Radiation-Driven Stellar Eruptions

Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor outbursts, while others are true SN events shrouded by material that was ejected earlier. Luminous Blue Variable stars (LBV’s) are traditionally cited in relation with giant eruptions, though this connection is not well established. After four decades of research, the fundamental causes of giant eruptions and LBV events remain elusive. This review outlines the basic relevant physics, with a brief summary of essential observational facts. Reasons are described for the spectrum and emergent radiation temperature of an opaque outflow. Proposed mechanisms are noted for instabilities in the star’s photosphere, in its iron opacity peak zones, and in its central region. Various remarks and conjectures are mentioned, some of them relatively unfamiliar in the published literature.

Comparison of SEDs of very massive radio-loud and radio-quiet AGN

ABSTRACT The main objective of this work is to establish and interpret the dominant spectral components and their differences in radio-loud (RL) and radio-quiet (RQ) AGN with very massive black holes, and accreting at moderate rates. Such a sample is selected from the Swift/BAT catalogue of AGN having determined optical spectra types and hosting black holes with masses $\rm \gt 10^{8.5}\, M_{\odot }$. We confirm our previous results, that radio loudness distribution of Swift/BAT AGN is bimodal and that radio galaxies are about two times X-ray louder than their RQ counterparts. We show that the average X-ray loudness (defined as a ratio of luminosity in the 14–195 keV band to that at 12 μm) of Type 1 and Type 2 AGN is very similar. This similarity holds for both RL and RQ subsamples and indicates negligible dependence of the observed X-ray luminosities on the inclination angle in both populations. In both the radiative output is dominated by mid-IR and hard X-ray components, and relatively weak UV luminosities indicate large amounts of dust in polar regions.

Preliminary analysis of the X-ray emission from the central regions of the Pictor A

Here we present some preliminary results of our analysis of the combined Chandra observations of the Pictor A radio galaxy. All the available Chandra data for the target, consisting of multiple pointings spanning over 15 years and amounting to the total exposure time of 464 ks, have been included in the analysis. We studied in detail the PSFs of the core region in the individual pointings, as well as the radial profile of the X-ray surface brightness of the source in the combined dataset, in order to discriminate between the radiative output of the unresolved core and the host galaxy. Based on these, we have performed spectral modeling of the active nucleus, constraining its variability.

GAMMA-RAY OBSERVATIONS OF GAMMA-RAY BINARIES

Gamma-ray binaries are binary systems that emit most of their radiative output above 1 MeV. Following the detection of five such systems in the past decade, they have been clearly established as a population of galactic GeV and TeV sources. In this review I discuss their recent gamma-ray observational results from Cherenkov telescopes and the Fermi satellite. A common trend has emerged in the high-energy spectra of several of these sources, with the detection of two separate components at GeV and TeV energies that cannot be explained as being emitted from a single region, and here I discuss a possible scenario giving rise to two separate acceleration locations in gamma-ray binaries.

NONTHERMAL EMISSIONS FROM SHOCKED SHELLS DRIVEN BY POWERFUL AGN JETS

We explore the emissions by accelerated electrons in shocked shells driven by jets in active galactic nuclei (AGNs). Focusing on powerful sources which host luminous quasars, the synchrotron radiation and inverse-Compton (IC) scattering of various photons that are mainly produced in the core are considered as radiation processes. We show that the radiative output is dominated by the IC emission for compact sources (≲ 30 kpc), whereas the synchrotron radiation is more important for larger sources. It is predicted that, for powerful sources (L j ~ 1047 ergs s -1), GeV – TeV gamma-rays produced via the IC emissions can be detected by the Fermi satellite and modern Cherenkov telescopes such as MAGIC, HESS and VERITAS if the source is compact.

Mechanisms for total and spectral solar irradiance variations

The total and spectral irradiance varies over short time scales, i.e. from days to months, and longer time scales from years to decades, centuries, and beyond. In this talk we review the current understanding of irradiance changes from days to decades. We present the current status of observations and discuss proposed reconstruction approaches to understand these variations. The main question that ultimately needs to be answered is what are the physical processes that could explain the enhanced heating of the photosphere, chromosphere, transition region, and corona, leading to a change in the solar radiative output at various wavelengths. As semi-empirical models allow us to reproduce the solar spectrum over a broad wavelength range, they offer a powerful tool to determine the energy necessary to heat certain layers and at the same time balance the radiative losses.

Commission 12: Solar Radiation & Structure

Commission 12 covers research on the internal structure and dynamics of the Sun, the “quiet” solar atmosphere, solar radiation and its variability, and the nature of relatively stable magnetic structures like sunspots, faculae and the magnetic network. There is considerable productive overlap with the other Commissions of Division II as investigations move progressively toward the fertile intellectual boundaries between traditional research disciplines. In large part, the solar magnetic field provides the linkage that connects these diverse themes. The same magnetic field that produces the more subtle variations of solar structure and radiative output over the 11 yr activity cycle is also implicated in rapid and often violent phenomena such as flares, coronal mass ejections, prominence eruptions, and episodes of sporadic magnetic reconnection.The last three years have again brought significant progress in nearly all the research endeavors touched upon by the interests of Commission 12. The underlying causes for this success remain the same: sustained advances in computing capabilities coupled with diverse observations with increasing levels of spatial, temporal and spectral resolution. It is all but impossible to deal with these many advances here in anything except a cursory and selective fashion. Thankfully, theLiving Reviews in Solar Physicsacute; has published several extensive reviews over the last two years that deal explicitly with issues relevant to the purview of Commission 12. The reader who is eager for a deeper and more complete understanding of some of these advances is directed tohttp://www.livingreviews.orgfor access to these articles.