Obscuration model of Variability in AGN

There are strong suggestions that the disk-like accretion flow onto massive black hole in AGN is disrupted in its innermost part (10-100 Rg), possibly due to the radiation pressure instability. It may form a hot optically thin quasi spherical (ADAF) flow surrounded by or containing denser clouds due to the disruption of the disk. Such clouds might be optically thick, with a Thompson depth of order of 10 or more. Within the frame of this cloud scenario (Collin-Souffrin et al. 1996, Czerny & Dumont 1998), obscuration events are expected and the effect would be seen as a variability. We consider expected random variability due to statistical dispersion in location of clouds along the line of sight for a constant covering factor. We discuss a simple analytical toy model which provides us with the estimates of the mean spectral properties and variability amplitude of AGN, and we support them with radiative transfer computations done with the use of TITAN code of Dumont, Abrassart & Collin (1999) and NOAR code of Abrassart (1999).Comment: to appear in Proc. of 5th Compton Symposium on Gamma-Ray Astronomy and Astrophysic

A VLBA Search for a Stimulated Recombination Line from the Accretion Region in NGC1275

The radio source 3C84, in NGC1275, has a two sided structure on parsec scales. The northern feature, presumed to be associated with a jet moving away from the Earth, shows strong evidence for free-free absorption. The ionized gas responsible for that absorption would be a source of detectable stimulated recombination line emission for a wide range of physical conditions. The VLBA has been used to search for the H65$\alpha$ hydrogen recombination line. The line is only expected to be seen against the northern feature which contains a small fraction of the total radio flux density. This spatial discrimination significantly aids the search for a weak line. No line was seen, with upper limits of roughly 15% of the continuum over a velocity range of 1486 km/s with resolutions up to 6.6 km/s. In the absence of a strong radiation field, this would imply that the free-free absorbing gas has a wide velocity width, is moving rapidly relative to the systemic velocity, or is concentrated in a thin, high density structure. All of these possibilities are reasonably likely close to an AGN. However, in the intense radiation environment of the AGN, even considering only the radiation we actually observe passing through the free-free absorbing gas, the non-detection is probably assured by a combination of saturation and radiation damping.Comment: 14 pages with 4 postscript figures. Accepted for publication in the April 2003 Astronomical Journa

Investigating the noise residuals around the gravitational wave event GW150914

We use the Pearson cross-correlation statistic proposed by Liu and Jackson, and employed by Creswell et al., to look for statistically significant correlations between the LIGO Hanford and Livingston detectors at the time of the binary black hole merger GW150914. We compute this statistic for the calibrated strain data released by LIGO, using both the residuals provided by LIGO and using our own subtraction of a maximum-likelihood waveform that is constructed to model binary black hole mergers in general relativity. To assign a significance to the values obtained, we calculate the cross-correlation of both simulated Gaussian noise and data from the LIGO detectors at times during which no detection of gravitational waves has been claimed. We find that after subtracting the maximum likelihood waveform there are no statistically significant correlations between the residuals of the two detectors at the time of GW150914.Comment: 14 pages, 7 figures. Minor text and figure changes in final v3. Notebooks for generating the results are available at https://github.com/gwastro/gw150914_investigatio

Center of mass rotation and vortices in an attractive Bose gas

The rotational properties of an attractively interacting Bose gas are studied using analytical and numerical methods. We study perturbatively the ground state phase space for weak interactions, and find that in an anharmonic trap the rotational ground states are vortex or center of mass rotational states; the crossover line separating these two phases is calculated. We further show that the Gross-Pitaevskii equation is a valid description of such a gas in the rotating frame and calculate numerically the phase space structure using this equation. It is found that the transition between vortex and center of mass rotation is gradual; furthermore the perturbative approach is valid only in an exceedingly small portion of phase space. We also present an intuitive picture of the physics involved in terms of correlated successive measurements for the center of mass state.Comment: version2, 17 pages, 5 figures (3 eps and 2 jpg

Blueprint for a high-performance biomaterial: full-length spider dragline silk genes.

Spider dragline (major ampullate) silk outperforms virtually all other natural and manmade materials in terms of tensile strength and toughness. For this reason, the mass-production of artificial spider silks through transgenic technologies has been a major goal of biomimetics research. Although all known arthropod silk proteins are extremely large (>200 kiloDaltons), recombinant spider silks have been designed from short and incomplete cDNAs, the only available sequences. Here we describe the first full-length spider silk gene sequences and their flanking regions. These genes encode the MaSp1 and MaSp2 proteins that compose the black widow's high-performance dragline silk. Each gene includes a single enormous exon (>9000 base pairs) that translates into a highly repetitive polypeptide. Patterns of variation among sequence repeats at the amino acid and nucleotide levels indicate that the interaction of selection, intergenic recombination, and intragenic recombination governs the evolution of these highly unusual, modular proteins. Phylogenetic footprinting revealed putative regulatory elements in non-coding flanking sequences. Conservation of both upstream and downstream flanking sequences was especially striking between the two paralogous black widow major ampullate silk genes. Because these genes are co-expressed within the same silk gland, there may have been selection for similarity in regulatory regions. Our new data provide complete templates for synthesis of recombinant silk proteins that significantly improve the degree to which artificial silks mimic natural spider dragline fibers

The structure and radiation spectra of illuminated accretion discs in AGN. I. Moderate illumination

We present detailed computations of the vertical structure of an accretion disc illuminated by hard X-ray radiation with the code {\sc titan-noar} suitable for Compton thick media. The energy generated via accretion is dissipated partially in the cold disc as well as in the X-ray source. We study the differences between the case where the X-ray source is in the form of a lamp post above the accretion disc and the case of a heavy corona. We consider radiative heating via Comptonization together with heating via photo-absorption on numerous heavy elements as carbon, oxygen, silicon, iron. The transfer in lines is precisely calculated. A better description of the heating/cooling through the inclusion of line transfer, a correct description of the temperature in the deeper layers, a correct description of the entire disc vertical structure, as well as the study of the possible coronal pressure effect, constitute an improvement in comparison to previous works. We show that exact calculations of hydrostatic equilibrium and determination of the disc thickness has a crucial impact on the optical depth of the hot illuminated zone. We assume a moderate illumination where the viscous flux equals the X-ray radiation flux. A highly ionized skin is created in the lamp post model, with the outgoing spectrum containing many emission lines and ionization edges in emission or absorption in the soft X-ray domain, as well as an iron line at $\sim 7$ keV consisting of a blend of low ionization line from the deepest layers and hydrogen and helium like resonance line from the upper layers, and almost no absorption edge, contrary to the case of a slab of constant density.A full heavy corona completely suppresses the highly ionized zone on the top of the accretion disc and in such case the spectrum is featureless.Comment: 16 pages, 20 figures, corrected two sentences, accepted by MNRA