Day 1058

Tuesday.


Nature
Positron annihilation signatures associated with the outburst of the micro quasar V404 Cygni
Siegert et al.

Micro quasars are stellar-mass BHs accreting matter from a companion star and ejecting plasma jets at almost the speed of light.  They are analogues of quasars that contain SMBHs of 1e6 to 1e10 Msun.  Accretion in micro quasars varies on much shorter timescales than in quasars and occasionally produces exceptionally bright X-ray flares.  How the flares are produced is unclear, as is the mechanism for launching the relativistic jets and their composition.  An emission line near 511 keV has ling been sought in the emission spectrum of micro quasars as evidence for the expected electron-positron plasma.  Transient high-energy spectral features have been reported in two objects, but their positron interpretation remains contentious.  Report observations of gamma-ray emission from the micro quasar V404 Cygni during a recent period of strong flairing activity.  The emission spectra around 511 keV shows clear signatures of variable positron annihilation, which implies a high rate of positron production.  This supports the earlier conjecture that micro quasars may be the main sources of the electron-positron plasma responsible for the bright diffuse emission of annihilation gamma-rays in the bulge region of our Galaxy.  Additionally, micro quasars could be the origin of the observed MeV continuum excess in the inner Galaxy.


1602.08503
Improving lognormal models for cosmological fields
Xavier, Abdallah, Joachimi

It is common practice in cosmology to model LSS observables as lognormal random fields, and this approach has been successfully applied in the past to the matter density and WL convergence fields separately.  Argue that this approach has fundamental limitations which prevent its use for jointly modeling these two fields since the lognormal distribution's shape can prevent certain correlations to be attainable.  Given the need of ongoing and future LSS surveys for fast joint simulations of clustering and WL, propose two ways of overcoming these limitations.  The first approach slightly distorts the PS of the fields using one of two algorithms that minimizes either the absolute or the fractional distortions.  The second one is by obtaining more accurate convergence marginal distributions, for which a fitting function is provided, by integrating the lognormal density along the line of sight.  The latter approach also provides a way to determine directly from theory the skewness of the convergence distribution and, therefore, the parameters for a lognormal fit.  Present the public code Full-sky Lognormal Astro-fields Simulation Kit (FLASK) which can make tomographic realizations on the sphere of an arbitrary number of correlated lognormal or Gaussian random fields by applying either of the two proposed solutions, and show that it can create joint simulations of clustering and lensing with sub-per-cent accuracy over relevant angular scales and redshift ranges.


1602.09010
A 14 $h^{-3}$ Gpc$^3$ study of cosmic homogeneity using BOSS DR12 quasar sample
Laurent et al

47,858 quasars over 5983 deg2 and 2.2<z<2.8.  Measure the count-in-sphere N(<r), i.e., the average number of objects around a given object, and its logarithmic derivative, the fractal correlation dimension D_2(r).  For a homogeneous distribution N(<r) ~ r^3 and D_2(r)=3.  With the simplest DD/RR estimator, these behaviors can be checked without determining an average density, which requires homogeneity to be defined.  Observe the predicted behavior over one decade in the radius r.  In particular, the fractal correlation dimension D_2 is found to be compatible with 3 on large scales at a high accuracy: 3-<D_2> < 6e-4 (2 sigma) over the range 250 <r< 1200 Mpc/h.  This result establishes homogeneity on large scales, therefore allowing to define a reliable average density.  Then introduce a more accurate estimator which requires a crude estimate of the average density, and find that the transition to homogeneity, described by N(<r) and D_2(r), quantitatively agrees with the LCDM prediction and that D_2 is still compatible with 3 at an even higher accuracy: 3-<D_2><1.5e-4 (2 sigma) over the range 250 <r< 1200 Mpc/h.