A new rare type of supernovae: hypervelocity stellar collisions at galactic centers
When a binary star system is tidally disrupted by a supermassive black hole at a galactic nucleus, one star is ejected at a high speed while the other remains in a tightly bound orbit around the black hole. The cluster of tightly bound stars builds over time, eventually creating a steady state in which the rate of collisions between these stars is similar to the rate of capturing new stars. A large fraction of the collisions occur near the periapsis of the orbits around the black hole, where the kinetic energies are sufficient to generate an explosive disruption of the two stars involved. The typical flare brightens for several days, with a peak luminosity that is comparable to the lower-luminosity end of known supernovae. The explosion lightcurve is followed by a longer flare due to accretion of ejected matter onto the black hole. Dedicated searches in the near universe could observe several such "collisional-supernovae" per year.

title: In the beginning: the first sources of light and the reionization of the universe
abstract: The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at a redshift z =30 and reionized most of the hydrogen in the universe by z =7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.

title: Early formation and late merging of the giant galaxies
abstract: The most luminous galaxies in the present universe are found at the centers of the most massive dark matter halos: rich galaxy clusters. In the ΛCDM cosmology, such massive halo cores are present at redshift z = 6 with a comoving number density (as a function of mass interior to ~10 kpc) that is comparable to today's value. The identity of the matter in these central regions is, however, predicted to change as major mergers bring together stars and dark matter from initially well-separated subunits. We use N-body simulations to investigate how these mergers push preexisting matter outward in the dominant galaxy while preserving the inner density profile of collisionless matter. It appears that the central regions of large galaxies end up dominated by stars formed in a number of dense cores, well before the last major mergers. The density profile of collisionless matter (stars and dark matter combined) in these central regions appears to be stable and to have attractor-like behavior under merging. This suggests that the baryon loading associated with dissipative contraction and star formation may be erased as subsequent mergers drive the mass distribution back to a universal profile. Such suppression of the effects of baryon loading, along with the early assembly of mass concentrations, may help resolve some apparent challenges to the CDM model for structure formation.

title: 21 cm Intensity Mapping
abstract: Using the 21 cm line, observed all-sky and across the redshift range from 0 to 5, the large scale structure of the Universe can be mapped in three dimensions. This can be accomplished by studying specific intensity with resolution ~ 10 Mpc, rather than via the usual galaxy redshift survey. The data set can be analyzed to determine Baryon Acoustic Oscillation wavelengths, in order to address the question: 'What is the nature of Dark Energy?' In addition, the study of Large Scale Structure across this range addresses the questions: 'How does Gravity effect very large objects?' and 'What is the composition our Universe?' The same data set can be used to search for and catalog time variable and transient radio sources.

title: Constraints on the Nature of CID-42: Recoil Kick or Supermassive Black Hole Pair?
abstract: The galaxy CXOC J100043.1+020637, also known as CID-42, is a highly unusual object. An apparent galaxy merger remnant, it displays signatures of both an inspiraling, kiloparsecscale active galactic nucleus (AGN) pair and of a recoiling AGN with a kick velocity > 1300 km s −1 . Among recoiling AGN candidates, CID-42 alone has both spatial offsets (in optical and X-ray bands) and spectroscopic offsets. In order to constrain the relative likelihood of both scenarios, we develop models using hydrodynamic galaxy merger simulations coupled with radiative transfer calculations. Our gas-rich, major merger models are generally well matched to the galactic morphology and to the inferred stellar mass and star formation rate. We show that a recoiling supermassive black hole (SMBH) in CID-42 should be observable as an AGN at the time of observation. However, in order for the recoiling AGN to produce narrow-line emission, it must be observed shortly after the kick while it still inhabits a dense gaseous region, implying a large total kick velocity (vk > 2000 km s −1 ). For the dual AGN scenario, an unusually large broad-line offset is required, and the best match to the observed morphology requires a galaxy that is less luminous than CID-42. Further, the lack of X-ray emission from one of the two optical nuclei is not easily attributed to an intrinsically quiescent SMBH or to a Compton-thick galactic environment. While the current data do not allow either the recoiling or the dual AGN scenario for CID-42 to be excluded, our models highlight the most relevant parameters for distinguishing these possibilities with future observations. In particular, high-quality, spatially-resolved spectra that can pinpoint the origin of the broad and narrow line features will be critical for determining the nature of this unique source.

title: Signatures of Relativistic Helical Motion in the Rotation Measures of AGN Jets
abstract: Polarization has proved an invaluable tool for probing magnetic fields in relativistic jets. Maps of the intrinsic polarization vectors have provided the best evidence to date for uniform, toroidally dominated magnetic fields within jets. More recently, maps of the rotation measure (RM) in jets have for the first time probed the field geometry of the cool, moderately relativistic surrounding material. In most cases, clear signatures of toroidal magnetic field are detected, corresponding to gradients in RM profiles transverse to the jet. However, in many objects these profiles also display marked asymmetries which are difficult to explain in simple helical jet models. Furthermore, in some cases the RM profiles are strongly frequency and/or time dependent. Here we show that these features may be naturally accounted for by including relativistic helical motion in the jet model. In particular, we are able to reproduce bent RM profiles observed in a variety of jets, frequency dependent RM profile morphologies and even the time dependence of the RM profiles of knots in 3C 273. Finally, we predict that some sources may show reversals in their RM profiles at sufficiently high frequencies, depending upon the the ratio of the components of jet sheath velocity transverse and parallel to the jet. Thus, multi-frequency RM maps promise a novel way in which to probe the velocity structure of relativistic outflows.

title: Metastability of the false vacuum in a Higgs-seesaw model of dark energy
abstract: In a recently proposed Higgs-Seesaw model the observed scale of dark energy results from a metastable false vacuum energy associated with mixing of the standard model Higgs particle and a scalar associated with new physics at the GUT or Planck scale. Here we address the issue of how to ensure metastability of this state over cosmological time. We consider new tree-level operators, the presence of a thermal bath of hidden sector particles, and quantum corrections to the effective potential. We find that in the thermal scenario many additional light degrees of freedom are typically required unless coupling constants are somewhat fine-tuned. However quantum corrections arising from as few as one additional light scalar field can provide the requisite support. We also briefly consider implications of late-time vacuum decay for the perdurance of observed structures in the universe in this model.

title: Time-dependent Ionization in Radiatively Cooling Gas
abstract: We present new computations of the equilibrium and nonequilibrium cooling efficiencies and ionization states for low-density radiatively cooling gas containing the elements H, He, C, N, O, Ne, Mg, Si, S, and Fe. We present results for gas temperatures between 10^4 and 10^8 K, assuming dust-free and optically thin conditions, and no external radiation. For nonequilibrium cooling we solve the coupled time-dependent ionization and energy loss equations for a radiating gas cooling from an initially hot, ≳5 × 10^6 K, equilibrium state, down to 10^4 K. We present results for heavy element compositions ranging from 10^(-3) to 2 times the elemental abundances in the Sun. We consider gas cooling at constant density (isochoric) and at constant pressure (isobaric). We calculate the critical column densities and temperatures at which radiatively cooling clouds make the dynamical transition from isobaric to isochoric evolution. We construct ion ratio diagnostics for the temperature and metallicity in radiatively cooling gas. We provide numerical estimates for the maximal cloud column densities for which the gas remains optically thin to the cooling radiation. We present our computational results in convenient online figures and tables.

title: X-ray orbital modulations in intermediate polars
abstract: We present an analysis of 30 archival ASCA and RXTE X-ray observations of 16 intermediate polars to investigate the nature of their orbital modulation. We show that X-ray orbital modulation is widespread amongst these systems, but not ubiquitous as indicated by previous studies that included fewer objects. Only seven of the sixteen systems show a clearly statistically significant modulation depth whose amplitude decreases with increasing X-ray energy. Interpreting this as due to photoelectric absorption in material at the edge of an accretion disc would imply that such modulations are visible for all system inclination angles in excess of 60 degrees. However, it is also apparent that the presence of an X-ray orbital modulation can appear and disappear on a timescale of ∼years or months in an individual system. This may be evidence for the presence of a precessing, tilted accretion disc, as inferred in some low mass X-ray binaries.

title: General Relativistic Effects in the Core Collapse Supernova Mechanism
abstract: We apply our recently developed code for spherically symmetric, fully general relativistic (GR) Lagrangian hydrodynamics and multigroup flux-limited diffusion (MGFLD) neutrino transport to examine the effects of GR on the hydrodynamics and transport during collapse, bounce, and the critical shock reheating phase of core collapse supernovae. GR effects were examined by performing core collapse simulations from several precollapse models in the Newtonian limit, in a hybrid limit consisting of GR hydrodynamics and Newtonian transport, and in the fully GR limit. Comparisons of models computed with GR versus Newtonian hydrodynamics show that collapse to bounce takes slightly less time in the GR limit and that the shock propagates slightly farther out in radius before receding. After a secondary quasi-static rise in the shock radius, the shock radius declines considerably more rapidly in the GR simulations than in the corresponding Newtonian simulations. During the shock reheating phase, core collapse computed with GR hydrodynamics results in a substantially more compact structure from the center out to the stagnated shock, the shock radius being reduced by a factor of 2 after 300 ms for a 25 M model and 600 ms for a 15 M model, times being measured from bounce. The inflow speedmore » of material behind the shock is also increased by about a factor of 2 throughout most of the evolution as a consequence of GR hydrodynamics. Regarding neutrino transport, comparisons show that the luminosity and rms energy of any neutrino flavor during the shock reheating phase increases when switching from Newtonian to GR hydrodynamics. This arises from the close coupling of the hydrodynamics and transport and the effect of GR hydrodynamics to produce more compact core structures, hotter neutrinospheres at smaller radii. On additionally switching from Newtonian to GR transport, gravitational time dilation and redshift effects decrease the luminosities and rms energies of all neutrino flavors. This decrease is less in magnitude than the increase in neutrino luminosities and rms energies that arise when switching from Newtonian to GR hydrodynamics, with the result that a fully GR simulation gives higher neutrino luminosities and harder neutrino spectra than a fully Newtonian simulation of the same precollapse model.« less