Black hole hunting in the Andromeda Galaxy

We present a new technique for identifying stellar mass black holes in low mass X-ray binaries (LMXBs), and apply it to XMM-Newton observations of M31. We examine X-ray time series variability seeking power density spectra (PDS) typical of LMXBs accreting at a low accretion rate (which we refer to as Type A PDS); these are very similar for black hole and neutron star LMXBs. Galactic neutron star LMXBs exhibit Type A PDS at low luminosities (~10^36--10^37 erg/s) while black hole LMXBs can exhibit them at luminosities >10^38 erg/s. We propose that Type A PDS are confined to luminosities below a critical fraction of the Eddington limit, $l_c$ that is constant for all LMXBs; we have examined asample of black hole and neutron star LMXBs and find they are all consistent with $l_c$ = 0.10+/-0.04 in the 0.3--10 keV band. We present luminosity and PDS data from 167 observations of X-ray binaries in M31 that provide strong support for our hypothesis. Since the theoretical maximum mass for a neutron star is \~3.1 M_Sun, we therefore assert that any LMXB that exhibits a Type A PDS at a 0.3--10 keV luminosity greater than 4 x 10^37 erg/s is likely to contain a black hole primary. We have found eleven new black hole candidates in M31 using this method. We focus on XMM-Newton observations of RX J0042.4+4112, an X-ray source in M31 and find the mass of the primary to be 7+/-2 M_Sun, if our assumptions are correct. Furthermore, RX J0042.4+4112 is consistently bright in \~40 observations made over 23 years, and is likely to be a persistently bright LMXB; by contrast all known Galactic black hole LMXBs are transient. Hence our method may be used to find black holes in known, persistently bright Galactic LMXBs and also in LMXBs in other galaxies.Comment: 6 Pages, 6 figures. To appear in the conference proceedings of "Interacting Binaries: Accretion, Evolution and Outcomes" (Cefalu, July 4-10 2004

Evolution of correlation strength in KxFe(2-y)Se2 superconductor doped with S

We report the evolution of thermal transport properties of iron-based superconductor K$_x$Fe$_{2-y}$Se$_2$ with sulfur substitution at Se sites. Sulfur doping suppresses the superconducting $T_c$ as well as the Seebeck coefficient. The Seebeck coefficient of all crystals in the low temperature range can be described very well by diffusive thermoelectric response model. The zero-temperature extrapolated value of Seebeck coefficient divided by temperature $S/T$ gradually decreases from $-0.48 \mu V/K^2$ to a very small value $\sim$ 0.03 $\mu$V/K$^2$ where $T_c$ is completely suppressed. The normal state electron Sommerfeld term ($\gamma_n$) of specific heat also decreases with the increase of sulfur content. The dcrease of $S/T$ and $\gamma_n$ reflects a suppression of the density of states at the Fermi energy, or a change in the Fermi surface that would induce the suppression of correlation strength.Comment: 5 Pages, 4 figures, 1 Table; submitted to Physical Review

Systems, interactions and macrotheory

A significant proportion of early HCI research was guided by one very clear vision: that the existing theory base in psychology and cognitive science could be developed to yield engineering tools for use in the interdisciplinary context of HCI design. While interface technologies and heuristic methods for behavioral evaluation have rapidly advanced in both capability and breadth of application, progress toward deeper theory has been modest, and some now believe it to be unnecessary. A case is presented for developing new forms of theory, based around generic “systems of interactors.” An overlapping, layered structure of macro- and microtheories could then serve an explanatory role, and could also bind together contributions from the different disciplines. Novel routes to formalizing and applying such theories provide a host of interesting and tractable problems for future basic research in HCI

Thermoelectric studies of KxFe2-ySe2: weakly correlated superconductor

We report thermal transport measurement of KxFe2-ySe2 superconducting single crystal. Significant peak anomaly in thermal conductivity is observed at nearly TC/2 indicating a large phonon mean-free-path in the superconducting state. The zero-temperature extrapolated thermoelectric power is smaller than the value in typical strongly correlated superconductors, implying large normalized Fermi temperature. In contrast to other iron superconductors, thermoelectric power in our sample does not exhibit significant anomalies. These findings indicate that KxFe2-ySe2 is a weakly or intermediately correlated superconductor without significant Fermi surface nesting.Comment: Revised version, 5 pages, 5 figures, Will appear in Physical Review

Cumulative luminosity functions of the X-ray point source population in M31

We present preliminary results from a detailed analysis of the X-ray point sources in the XMM-Newton survey of M31. These sources are expected to be mostly X-ray binaries. We have so far studied 225 of the 535 sources found by automated source detection. Only sources which were present in all three EPIC images were considered. X-ray binaries are identified by their energy spectrum and power density spectrum. Unlike in other surveys we have obtained source luminosities from freely fit emission models. We present uncorrected luminosity functions of the sources analysed so far.Comment: 2 pages, 1 figure. To appear in proceedings of IAUS23