SYSTEMATIC ERRORS IN THE HUBBLE CONSTANT BASED UPON MEASUREMENT OF THE SUNYAEV-ZELDOVICH EFFECT
Values of the Hubble constant reported to date that are based upon measurement of the Sunyaev-Zeldovich (SZ) effect in clusters of galaxies are systematically lower than those derived by other methods (e.g., Cepheid variable stars or the Tully-Fisher relation). We investigate the possibility that systematic errors may be introduced into the analysis by the generally adopted assumptions that observed clusters are in hydrostatic equilibrium, are spherically symmetric, and are isothermal. We construct self-consistent theoretical models of merging clusters of galaxies, using hydrodynamic/N-body simulations. We then compute the magnitude of H0 derived from the SZ effect at different times and at different projection angles, both from first principles and by applying each of the standard assumptions used in the interpretation of observations. Our results indicate that the assumption of isothermality in the evolving clusters can result in H0 being underestimated by 10%-30%, depending upon both epoch and projection angle. Moreover, use of the projected, emission-weighted temperature profile under the assumption of spherical symmetry does not significantly improve the situation except in the case of more extreme mergers (i.e., those involving relatively gas-rich subclusters). Although less significant, we find that asphericity in the gas density can also result in a 15% error in H0. If the cluster is prolate (as is generally the case for on-axis, or nearly on-axis, mergers) and viewed along its major axis, H0 will be systematically underestimated. More extreme off-axis mergers may result in oblate merger remnants, which, when viewed nearly face-on, may result in an overestimation of H0. A similar effect is noted when viewing a prolate distribution along a line of sight that is nearly perpendicular to its major axis. In both cases the potential overestimation occurs only when the remnant is viewed within 15°-30° of face-on. Bulk gas motions and the kinematic SZ effect do not appear to be significant except for a brief period during the very early stages of a merger. Our study shows that the most meaningful SZ measurement will be accompanied by high-resolution temperature data and a detailed dynamical modeling of the observed system. In lieu of this, a large sample selected to avoid dynamically evolving systems is preferred.

title: OMCat: Catalog of Serendipitous Sources Detected with the XMM-Newton Optical Monitor
abstract: ABSTRACT.The Optical Monitor Catalog of serendipitous sources (OMCat) contains entries for every source detected in the publicly available XMM-Newton Optical Monitor (OM) images taken in either the imaging or “fast” modes. Since the OM is coaligned and records data simultaneously with the X-ray telescopes on XMM-Newton, it typically produces images in one or more near-UV or optical bands for every pointing of the observatory. As of the beginning of 2006, the public archive had covered roughly 0.5% of the sky in 2950 fields. The OMCat is not dominated by sources previously undetected at other wavelengths; the bulk of the objects have optical counterparts. However, the OMCat can be used to extend optical or X-ray spectral energy distributions for known objects into the ultraviolet, to study at higher angular resolution objects detected with GALEX, or to find high-Galactic-latitude objects of interest for UV spectroscopy.

title: A search for X-ray emission associated with the great attractor
abstract: A search is reported for X-ray emission associated with the object, dubbed the 'great attractor', which has been postulated as the cause of the coherent deviations from the Hubble flow which are observed in nearby parts of the universe. The hypothesis that a substantial fraction of the dynamical mass of the great attractor exists in the form of rich clusters of galaxies is ruled out. The possibility is considered that a substantial fraction of the mass of the great attractor could exist in a form which would increase the apparently diffuse X-ray surface brightness. The observational upper limit to the diffuse X-ray sky surface brightness enhancement in the general direction of the great attractor allows interesting limits to be set on the fraction of the dynamical mass of the great attractor which can exist in the form of X-ray-luminous material. 43 refs.

title: The XMM-Newton View of the Nucleus of NGC 4261
abstract: We present the first results from an XMM-Newton observation of the FRI galaxy NGC 4261, which harbors a supermassive black hole and a low-ionization nuclear emission-line region (LINER). Here we focus on the X-ray properties of the nucleus, using the EPIC pn data. The 0.6-10 keV continuum in best fitted by a thermal component with kT ~0.7 keV, plus a power law with photon index ~1.4, absorbed by a column density NH~4e22 cm-2. An unresolved Fe K emission line with EW 280 eV is detected at 7 keV. We also detect, for the first time, short-term flux variability from the nucleus, on a timescale of 3-5 ks. The short-term variations rule out an ADAF as the only production mechanism of the X-ray continuum. Instead, we argue that the inner jet contributes to the emission in the X-ray band.

title: X ray emission from active galactic nuclei
abstract: The advantages of X-ray astronomy for studying active galactic nuclei (AGN) are discussed. If the X-ray emission from AGN arises from a region close to the central energy source X-ray observations may provide the best constraints on the central engine. The shape of the X-ray continuum gives information about the mechanism for photon generation; X-ray time variability data can constrain the size and mass of the continuum source; and X-ray occultation data gives constraints on the relative sizes of the continuum source and the intervening absorbing material. Since a fair fraction of the total energy of an AGN is emitted at X-ray wavelengths, direct measurement of the amount and spectral form of this radiation is important for modeling of the optically emitting clouds.

title: Abell 548: An X-ray and optical analysis of substructure

title: Does Pulsar B1757–24 Have a Fallback Disk?
abstract: Radio pulsars are thought to spin down primarily as a result of torque from magnetic dipole radiation (MDR) emitted by the time-varying stellar magnetic field as the star rotates. This assumption yields a characteristic age for a pulsar, which has generally been assumed to be comparable to the actual age. Recent observational limits on the proper motion of pulsar B1757-24, however, revealed that the actual age (>39 kyr) of this pulsar is much greater than its MDR characteristic age (16 kyr), calling into question the assumption of pure MDR spin-down for this and other pulsars. To explore the possible cause of this discrepancy, we consider a scenario in which the pulsar acquired an accretion disk from supernova ejecta and the subsequent spin-down occurred under the combined action of MDR and accretion torques. A simplified model of the accretion torque involving a constant mass inflow rate at the pulsar magnetosphere can explain the age and period derivative of the pulsar for reasonable values of the pulsar magnetic field and inflow rate. We discuss testable predictions of this model.