Progenitor constraints on the Type-Ia supernova SN2011fe from pre-explosion Hubble Space Telescope HeII narrow-band observations

We present Hubble Space Telescope (HST) imaging observations of the site of the Type-Ia supernova SN2011fe in the nearby galaxy M101, obtained about one year prior to the event, in a narrow band centred on the HeII 4686 \AA{} emission line. In a "single-degenerate" progenitor scenario, the hard photon flux from an accreting white dwarf (WD), burning hydrogen on its surface over $\sim1$ Myr should, in principle, create a HeIII Str\"{o}mgren sphere or shell surrounding the WD. Depending on the WD luminosity, the interstellar density, and the velocity of an outflow from the WD, the HeIII region could appear unresolved, extended, or as a ring, with a range of possible surface brightnesses. We find no trace of HeII 4686 \AA{} line emission in the HST data. Using simulations, we set $2\sigma$ upper limits on the HeII 4686 \AA{} luminosity of $L_{\rm HeII} < 3.4 \times 10^{34}$ erg s$^{-1}$ for a point source, corresponding to an emission region of radius $r < 1.8$ pc. The upper limit for an extended source is $L_{\rm HeII} < 1.7 \times 10^{35}$ erg s$^{-1}$, corresponding to an extended region with $r\sim11$ pc. The largest detectable shell, given an interstellar-medium density of 1 cm$^{-3}$, has a radius of $\sim6$ pc. Our results argue against the presence, within the $\sim10^5$ yr prior to the explosion, of a supersoft X-ray source of luminosity $L_{\rm bol} \ge 3 \times 10^{37}$ erg s$^{-1}$, or of a super-Eddington accreting WD that produces an outflowing wind capable of producing cavities with radii of 2-6 pc.Comment: Accepted by MNRAS Letters; revised version following referee report and readers' comment

A survey for large image-separation lensed quasars

The statistics of gravitationally lensed quasars with multiple images in the 0.1''-7'' range have been measured in various surveys. Little is known, however, about lensed-quasar statistics at larger image separations, which probe masses on the scale of galaxy clusters. We extend the results of the HST Snapshot Survey for Lensed Quasars to the 7''-50'' range for a sub-sample of 76 quasars that is free of known selection effects. Using a combination of multicolor photometry and spectroscopy, we show that none of the point sources in the entire field of view of the HST observations of these quasars are lensed images. Large-separation quasar lensing is therefore not common. We carry out a detailed calculation of the expected statistics of large-separation lensing for this quasar sample, incorporating realistic input for the mass profiles and mass function of galaxy clusters. We find that the observational null results are consistent with the expected effect of galaxy clusters, even if these have existed in their present form and number since z of about 2. The rarity of large-separation lensed quasars can rule out some extreme scenarios, e.g. that the mass-function of clusters has been severely underestimated, or that large mass concentrations that are not associated with galaxies (i.e. ``failed'' clusters) are common. The rareness of wide lensing also sets limits on the cosmological constant that are independent of limits derived from galaxy lensing. The lensing statistics of larger quasar samples can probe the structure, number, and evolution of clusters, as well as the geometry of space.Comment: LaTex, ApJ, submitte

Quantifying dynamical spillover in co-evolving multiplex networks

Multiplex networks (a system of multiple networks that have different types of links but share a common set of nodes) arise naturally in a wide spectrum of fields. Theoretical studies show that in such multiplex networks, correlated edge dynamics between the layers can have a profound effect on dynamical processes. However, how to extract the correlations from real-world systems is an outstanding challenge. Here we provide a null model based on Markov chains to quantify correlations in edge dynamics found in longitudinal data of multiplex networks. We use this approach on two different data sets: the network of trade and alliances between nation states, and the email and co-commit networks between developers of open source software. We establish the existence of "dynamical spillover" showing the correlated formation (or deletion) of edges of different types as the system evolves. The details of the dynamics over time provide insight into potential causal pathways

A STRINGENT CONSTRAINT ON ALTERNATIVES TO A MASSIVE BLACK HOLE AT THE CENTER OF NGC 4258

There is now dynamical evidence for massive dark objects at the center of several galaxies, but suggestions that these are supermassive black holes are based only on indirect astrophysical arguments. The recent unprecedented measurement of the rotation curve of maser emission sources at the center of NGC 4258, and the remarkable discovery that it is Keplerian to high precision, provides us a unique opportunity for testing alternatives to a BH (e.g., a massive cluster of stellar remnants, brown dwarfs, low-mass stars, or halo dark matter). We use a conservative upper limit on the systematic deviation from a Keplerian rotation curve to constrain the mass distribution at the galaxy center. Based on evaporation and physical collision time-scale arguments, we show that a central cluster is ruled out, *unless* the cluster consists of *extremely* dense objects with mass less than about 0.05 solar masses (e.g., low mass BHs or elementary particles). Since both of these dynamically-allowed systems are very improbable for other astrophysical reasons, we conclude that a central dense cluster at the center of NGC 4258 is *very* improbable, thus leaving the alternative possibility of a massive BH. We also show that the mass of the BH must be at least 98% of the mass enclosed within the inner edge of the masering disk (3.6*10^7 solar masses). A substantial contribution to that mass from a density cusp in the background mass distribution is excluded.Comment: Submitted to ApJ (Letters) on March 15, 1995. 11 pages including 1 figure; uuencoded, compressed postscript

The effect of gravitational-wave recoil on the demography of massive black holes

The coalescence of massive black hole (MBH) binaries following galaxy mergers is one of the main sources of low-frequency gravitational radiation. A higher-order relativistic phenomenon, the recoil as a result of the non-zero net linear momentum carried away by gravitational waves, may have interesting consequences for the demography of MBHs at the centers of galaxies. We study the dynamics of recoiling MBHs and its observational consequences. The ``gravitational rocket'' may: i) deplete MBHs from late-type spirals, dwarf galaxies, and stellar clusters; ii) produce off-nuclear quasars, including unusual radio morphologies during the recoil of a radio-loud source; and iii) give rise to a population of interstellar and intergalactic MBHs.Comment: emulateapj, 5 pages, 2 figures, to appear in the ApJ Letter

Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei. II. A Homogeneous Analysis of a Large Reverberation-Mapping Database

We present improved black hole masses for 35 active galactic nuclei (AGNs) based on a complete and consistent reanalysis of broad emission-line reverberation-mapping data. From objects with multiple line measurements, we find that the highest precision measure of the virial product is obtained by using the cross-correlation function centroid (as opposed to the cross-correlation function peak) for the time delay and the line dispersion (as opposed to full width half maximum) for the line width and by measuring the line width in the variable part of the spectrum. Accurate line-width measurement depends critically on avoiding contaminating features, in particular the narrow components of the emission lines. We find that the precision (or random component of the error) of reverberation-based black hole mass measurements is typically around 30%, comparable to the precision attained in measurement of black hole masses in quiescent galaxies by gas or stellar dynamical methods. Based on results presented in a companion paper by Onken et al., we provide a zero-point calibration for the reverberation-based black hole mass scale by using the relationship between black hole mass and host-galaxy bulge velocity dispersion. The scatter around this relationship implies that the typical systematic uncertainties in reverberation-based black hole masses are smaller than a factor of three. We present a preliminary version of a mass-luminosity relationship that is much better defined than any previous attempt. Scatter about the mass-luminosity relationship for these AGNs appears to be real and could be correlated with either Eddington ratio or object inclination.Comment: 61 pages, including 8 Tables and 16 Figures. Accepted for publication in The Astrophysical Journa

The expansion of thymopoiesis in neonatal mice is dependent on expression of high mobility group a 2 protein (Hmga2).

Cell number in the mouse thymus increases steadily during the first two weeks after birth. It then plateaus and begins to decline by seven weeks after birth. The factors governing these dramatic changes in cell production are not well understood. The data herein correlate levels of High mobility group A 2 protein (Hmga2) expression with these temporal changes in thymopoiesis. Hmga2 is expressed at high levels in murine fetal and neonatal early T cell progenitors (ETP), which are the most immature intrathymic precursors, and becomes almost undetectable in these progenitors after 5 weeks of age. Hmga2 expression is critical for the initial, exponential expansion of thymopoiesis, as Hmga2 deficient mice have a deficit of ETPs within days after birth, and total thymocyte number is repressed compared to wild type littermates. Finally, our data raise the possibility that similar events occur in humans, because Hmga2 expression is high in human fetal thymic progenitors and falls in these cells during early infancy

Reverberation Mapping and the Physics of Active Galactic Nuclei

Reverberation-mapping campaigns have revolutionized our understanding of AGN. They have allowed the direct determination of the broad-line region size, enabled mapping of the gas distribution around the central black hole, and are starting to resolve the continuum source structure. This review describes the recent and successful campaigns of the International AGN Watch consortium, outlines the theoretical background of reverberation mapping and the calculation of transfer functions, and addresses the fundamental difficulties of such experiments. It shows that such large-scale experiments have resulted in a ``new BLR'' which is considerably different from the one we knew just ten years ago. We discuss in some detail the more important new results, including the luminosity-size-mass relationship for AGN, and suggest ways to proceed in the near future.Comment: Review article to appear in Astronomical Time Series, Proceedings of the Wise Observatory 25th Ann. Symposium. 24 pages including 7 figure