Compact X-ray Sources in Nearby Galaxy Nuclei

We have found compact, near-nuclear X-ray sources in 21 (54\%) of a complete sample of 39 nearby face-on spiral and elliptical galaxies with available ROSAT HRI data. ROSAT X-ray luminosities (0.2 $-$ 2.4 keV) of these compact X-ray sources are $\sim$10$^{37}$$-$10$^{40}$ erg~s$^{-1}$. The mean displacement between the location of the compact X-ray source and the optical photometric center of the galaxy is $\sim$390 pc. ASCA spectra of six of the 21 galaxies show the presence of a hard component with relatively steep ($\Gamma \approx$ 2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the data from the spiral galaxies well, suggesting that the X-ray objects in these galaxies may be similar to a black hole candidate (BHC) in its soft (high) state. ASCA data from the elliptical galaxies indicate that hot (kT $\approx$ 0.7 keV) gas dominates the emission. The fact that the spectral slope of the spiral galaxy sources is steeper than in normal type 1 active galactic nuclei (AGNs) and that relatively low absorbing columns (N$_H \approx$ 10$^{21}$ cm$^{-2}$) were found to the power-law component indicates that these objects are somehow geometrically and/or physically different from AGNs in normal active galaxies. The X-ray sources in the spiral galaxies may be BHCs, low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs or AGNs) to be $\sim$10$^2$$-$10$^3$ M$_\odot$. The X-ray sources in the elliptical galaxies may be BHCs, AGNs or young X-ray supernova also.Comment: 4 pages, TeX, two postscript figures, to be published in proceedings of 32nd COSPAR Session E1.2 (1998 July 15-17 Nagoya) "The AGN-Normal Galaxy Connection

The Statistical Properties of Galaxies Containing ULXs

We present a statistical analysis of the properties of galaxies containing ultraluminous X-ray objects (ULXs). Our primary goal is to establish the fraction of galaxies containing a ULX as a function of ULX luminosity. Our sample is based on ROSAT HRI observations of galaxies. We find that ~ 12% of galaxies contain at least one ULX with L_X > 10^39 erg/s and ~ 1% of galaxies contain at least one ULX with L_X > 10^40 erg/s. These ULX frequencies are lower limits since ROSAT HRI observation would miss absorbed ULXs (i.e., with N_H >~ 10^21 cm^-2) and those within ~ 10" of the nucleus (due to the positional error circle of the ROSAT HRI). The Hubble type distribution of galaxies with a ULX differs significantly from the distribution of types for nearby RC3 galaxies, but does not differ significantly from the galaxy type distribution of galaxies observed by the HRI in general. We find no increase in the mean FIR luminosity or FIR / K band luminosity ratio for galaxies with a ULX relative to galaxies observed by the HRI in general, however this result is also most likely biased by the soft bandpass of the HRI and the relatively low number of high SFR galaxies observed by the HRI with enough sensitivity to detect a ULX.Comment: Accepted by Apj. 5 pages with 4 figures formatted using emulateapj. Version with just b/w figures available at http://www.pha.jhu.edu/~ptak/paper

A Proposal to Measure the Quasiparticle Poisoning Time of Majorana Bound States

We propose a method of measuring the fermion parity lifetime of Majorana fermion modes due to quasiparticle poisoning. We model quasiparticle poisoning by coupling the Majorana modes to electron reservoirs, explicitly breaking parity conservation in the system. This poisoning broadens and shortens the resonance peak associated with Majorana modes. In a two lead geometry, the poisoning decreases the correlation in current noise between the two leads from the maximal value characteristic of crossed Andreev reflection. The latter measurement allows for calculation of the poisoning rate even if temperature is much higher than the resonance width.Comment: 5 pages, 5 figure

Teaching and Learning in Large Classes

There are several crucial questions one might wish to ask about large classes. The first is: Should we be teaching large classes at ISU? This is an important question because it involves time and resource issues, as well as issues related to student engagement and learning. I\u27m not going to address this question- it\u27s a question for those higher up on the food chain\u27\u27 than I. The second question is: Do we teach large classes here at ISU? The answer to that question is, of course, a resounding \u27\u27yes., That leads to a much more difficult question to answer, and the one that I actually wish to address in this article