arXiv subjects

L. Greenhill

Publications and source records attributed to L. Greenhill.

5 recordsLinked to original sources

arXiv2012

The Hubble constant and new discoveries in cosmology

We report the outcome of a 3-day workshop on the Hubble constant (H_0) that took place during February 6-8 2012 at the Kavli Institute for Particle Astrophysics and Cosmology, on the campus of Stanford University. The participants met to address the following questions. Are there compelling scientific reasons to obtain more precise and more accurate measurements of H_0 than currently available? If there are, how can we achieve this goal? The answers that emerged from the workshop are (1) better measurements of H_0 provide critical independent constraints on dark energy, spatial curvature of the Universe, neutrino physics, and validity of general relativity, (2) a measurement of H_0 to 1% in both precision and accuracy, supported by rigorous error budgets, is within reach for several methods, and (3) multiple paths to independent determinations of H_0 are needed in order to access and control systematics.

astro-ph.CO↗

arXiv2009

X-ray vs. water maser emission in AGN

Correlations between X-ray and water maser emission in AGN have been recently reported. However, the lack of systematic studies affects the confidence level of these results. In the following, we introduce a project aimed at studying all the water maser sources believed to be associated with AGN activity through X-ray data obtained with the XRT and BAT instruments on-board the Swift satellite. Preliminary results of this work indicate a promising rate of XRT detections allowing us to refine follow-up observing strategies focused on investigating the nuclei of individual galaxies and deriving, on statistical basis, the main characteristics of water maser hosts. In addition, a cross-correlation between our sample and the BAT 22-months all-sky survey provides an exceptionally high detection rate at hard X-ray energies when compared to other AGN-related catalogs.

astro-ph.CO↗

arXiv2009

GPUs for data processing in the MWA

The MWA is a next-generation radio interferometer under construction in remote Western Australia. The data rate from the correlator makes storing the raw data infeasible, so the data must be processed in real-time. The processing task is of order ~10 TFLOPS. The remote location of the MWA limits the power that can be allocated to computing. We describe the design and implementation of elements of the MWA real-time data processing system which leverage the computing abilities of modern graphics processing units (GPUs). The matrix algebra and texture mapping capabilities of GPUs are well suited to the majority of tasks involved in real-time calibration and imaging. Considerable performance advantages over a conventional CPU-based reference implementation are obtained.

astro-ph.IM↗

arXiv2007

A deep Chandra look at the low L_B elliptical NGC821: X-ray binaries, a galactic wind and emission at the nucleus

The relatively nearby (distance=24.1 Mpc) elliptical galaxy NGC821, hosting a central massive black hole but inactive at all wavelengths, was observed with Chandra for a total exposure of 230 ksec, to search for nuclear emission and gas available for accretion. Within its optical image, 41 sources were detected, with spectral properties typical of low mass X-ray binaries (LMXBs). The fractions of LMXBs in the field and in globular clusters were determined, together with their X-ray luminosity function (XLF) down to L(0.3-8 keV)=2\times 10^{37} erg/s. At the galactic center a source of L(0.3-8 keV)=6\times 10^{38} erg/s was detected for the first time, slightly extended. Its spectral shape is quite hard (Γ=1.49^{+0.14}_{-0.13}), without intrinsic absorption. It is surrounded by three sources with spectral shape typical of LMXBs and luminosities on the brightest end of the XLF. One is consistent with being pointlike; the others could be the superposition of few point sources and/or truly diffuse emission, with one resembling a jet-like feature. Diffuse emission was detected out to R\sim 30'', and comes mostly from unresolved LMXBs, with a minor contribution from other types of stellar sources. Different lines of investigation consistently provide no evidence for hot gas. Hydrodynamical simulations show that stellar mass losses are driven out of NGC821 in a wind sustained by type Ia supernovae, but also hot accreting gas within a very small inner region. A companion paper presents further observational results from $Spitzer$ and the VLA, and possible accretion modalities for this central massive black hole.

astro-ph↗

arXiv2006

A new Cepheid distance to the maser-host galaxy NGC 4258 and its implications for the Hubble Constant

We present initial results from a time-series BVI survey of two fields in NGC 4258 using the Advanced Camera for Surveys onboard the Hubble Space Telescope. This galaxy was selected because of its accurate maser-based distance, which is anticipated to have a total uncertainty of ~3%. The goal of the HST observations is to provide an absolute calibration of the Cepheid Distance Scale and to measure its dependence on chemical abundance (the so-called "metallicity effect"). We carried out observations of two fields at different galactocentric distances with a mean abundance difference of 0.5 dex. We discovered a total of 281 Cepheids with periods ranging from 4 to 45 days (the duration of our observing window). We determine a Cepheid distance modulus for NGC 4258 (relative to the LMC) of 10.88 +- 0.04 (random) +- 0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies μ(LMC)=18.41 +- 0.10 (r) +- 0.13 (s) mag or D(LMC)= 48.1 +- 2.3 (r) +- 2.9 (s) kpc. We measure a metallicity effect of γ=-0.29 +- 0.09 (r) +- 0.05 (s) mag/dex. We see no evidence for a variation in the slope of the Period-Luminosity relation as a function of abundance. We estimate a Hubble Constant of H_0= 74 +- 3 (r) +- 6 (s) km/s Mpc using a recent sample of 4 well-observed type Ia SNe and our new calibration of the Cepheid Distance Scale. It may soon be possible to measure the value of H_0 with a total uncertainty of 5%, with consequent improvement in the determination of the equation of state of dark energy.

astro-ph↗