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Javadi B.,University of Melbourne | Kondo D.,French Institute for Research in Computer Science and Automation | Vincent J.-M.,Joseph Fourier University | Anderson D.P.,Uc Berkeley Space Science Laboratory
IEEE Transactions on Parallel and Distributed Systems | Year: 2011

In the age of cloud, Grid, P2P, and volunteer distributed computing, large-scale systems with tens of thousands of unreliable hosts are increasingly common. Invariably, these systems are composed of heterogeneous hosts whose individual availability often exhibit different statistical properties (for example stationary versus nonstationary behavior) and fit different models (for example exponential, Weibull, or Pareto probability distributions). In this paper, we describe an effective method for discovering subsets of hosts whose availability have similar statistical properties and can be modeled with similar probability distributions. We apply this method with about 230,000 host availability traces obtained from a real Internet-distributed system, namely SETI@home. We find that about 21 percent of hosts exhibit availability, that is, a truly random process, and that these hosts can often be modeled accurately with a few distinct distributions from different families. We show that our models are useful and accurate in the context of a scheduling problem that deals with resource brokering. We believe that these methods and models are critical for the design of stochastic scheduling algorithms across large systems where host availability is uncertain. © 2011 IEEE. Source


Wik D.R.,NASA | Wik D.R.,Johns Hopkins University | Lehmer B.D.,NASA | Lehmer B.D.,Johns Hopkins University | And 23 more authors.
Astrophysical Journal | Year: 2014

Prior to the launch of NuSTAR, it was not feasible to spatially resolve the hard (E > 10 keV) emission from galaxies beyond the Local Group. The combined NuSTAR data set, comprised of three ∼165 ks observations, allows spatial characterization of the hard X-ray emission in the galaxy NGC 253 for the first time. As a follow up to our initial study of its nuclear region, we present the first results concerning the full galaxy from simultaneous NuSTAR, Chandra, and Very Long Baseline Array monitoring of the local starburst galaxy NGC 253. Above ∼10 keV, nearly all the emission is concentrated within 100″ of the galactic center, produced almost exclusively by three nuclear sources, an off-nuclear ultraluminous X-ray source (ULX), and a pulsar candidate that we identify for the first time in these observations. We detect 21 distinct sources in energy bands up to 25 keV, mostly consisting of intermediate state black hole X-ray binaries. The global X-ray emission of the galaxy - dominated by the off-nuclear ULX and nuclear sources, which are also likely ULXs - falls steeply (photon index ≳3) above 10 keV, consistent with other NuSTAR-observed ULXs, and no significant excess above the background is detected at E > 40 keV. We report upper limits on diffuse inverse Compton emission for a range of spatial models. For the most extended morphologies considered, these hard X-ray constraints disfavor a dominant inverse Compton component to explain the γ-ray emission detected with Fermi and H.E.S.S. If NGC 253 is typical of starburst galaxies at higher redshift, their contribution to theE > 10 keV cosmic X-ray background is <1%. © 2014. The American Astronomical Society. All rights reserved. Source


Ptak A.,NASA | Ptak A.,Johns Hopkins University | Hornschemeier A.,NASA | Hornschemeier A.,Johns Hopkins University | And 25 more authors.
Astrophysical Journal | Year: 2015

We report on simultaneous observations of the local starburst system Arp 299 with NuSTAR and Chandra, which provides the first resolved images of this galaxy up to energies of ∼45 keV. Fitting the 3-40 keV spectrum reveals a column density of N H ∼ 4 × 1024 cm-2, characteristic of a Compton-thick active galactic nucleus (AGN), and a 10-30 keV luminosity of 1.2 × 1043 erg s-1. The hard X-rays detected by NuSTAR above 10 keV are centered on the western nucleus, Arp 299-B, which previous X-ray observations have shown to be the primary source of neutral Fe-K emission. Other X-ray sources, including Arp 299-A, the eastern nucleus also thought to harbor an AGN, as well as X-ray binaries, contribute ≲ 10% to the 10-20 keV emission from the Arp 299 system. The lack of significant emission above 10 keV other than that attributed to Arp 299-B suggests that: (1) any AGN in Arp 299-A must be heavily obscured (N H > 1024 cm-2) or have a much lower luminosity than Arp 299-B and (2) the extranuclear X-ray binaries have spectra that cut-off above ∼10 keV. Such soft spectra are characteristic of ultraluminous X-ray sources observed to date by NuSTAR. © 2015. The American Astronomical Society. All rights reserved.. Source


Harrison F.A.,Mathematics and Astronomy | Boggs S.,Uc Berkeley Space Science Laboratory | Christensen F.,Technical University of Denmark | Craig W.,Uc Berkeley Space Science Laboratory | And 30 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer mission that will carry the first focusing hard X-ray (6 - 80 keV) telescope to orbit. NuSTAR will offer a factor 50 - 100 sensitivity improvement compared to previous collimated or coded mask imagers that have operated in this energy band. In addition, NuSTAR provides sub-arcminute imaging with good spectral resolution over a 12-arcminute field of view. After launch, NuSTAR will carry out a two-year primary science mission that focuses on four key programs: studying the evolution of massive black holes through surveys carried out in fields with excellent multiwavelength coverage, understanding the population of compact objects and the nature of the massive black hole in the center of the Milky Way, constraining the explosion dynamics and nucleosynthesis in supernovae, and probing the nature of particle acceleration in relativistic jets in active galactic nuclei. A number of additional observations will be included in the primary mission, and a guest observer program will be proposed for an extended mission to expand the range of scientific targets. The payload consists of two co-aligned depth-graded multilayer coated grazing incidence optics focused onto a solid state CdZnTe pixel detectors. To be launched in early 2012 on a Pegasus rocket into a low-inclination Earth orbit, NuSTAR largely avoids SAA passage, and will therefore have low and stable detector backgrounds. The telescope achieves a 10.14-meter focal length through on-orbit deployment of an extendable mast. An aspect and alignment metrology system enable reconstruction of the absolute aspect and variations in the telescope alignment resulting from mast exure during ground data processing. Data will be publicly available at GSFC's High Energy Archive Research Center (HEASARC) following validation at the science operations center located at Caltech. © 2010 SPIE. Source


Manning C.V.,Uc Berkeley Space Science Laboratory | Manning C.V.,NASA | Ma Y.,IGPP | Brain D.A.,Uc Berkeley Space Science Laboratory | And 2 more authors.
Icarus | Year: 2011

We develop a parametric fit to the results of a detailed magnetohydrodynamic (MHD) study of the response of ion escape rates (O+, O2+ and CO2+) to strongly varied solar forcing factors, as a way to efficiently extend the MHD results to different conditions. We then use this to develop a second, evolutionary model of solar forced ion escape. We treat the escape fluxes of ion species at Mars as proportional to the product of power laws of four factors - that of the EUV flux Reuv, the solar wind particle density Rρ, its velocity (squared) Rv2, and the interplanetary magnetic field pressure RB2, where forcing factors are expressed in units of the current epoch-averaged values. Our parametric model is: φ(i)=φ0(i)Reuvα(i)Rρβ(i)Rv2γ(i)RB2δ(i), where φ(i) is the escape flux of ion i. We base our study on the results of just six provided MHD model runs employing large forcing factor variations, and thus construct a successful, first-order parametric model of the MHD program. We perform a five-dimensional least squares fit of this power law model to the MHD results to derive the flux normalizations and the indices of the solar forcing factors. For O+, we obtain the values, 1.73×1024s-1, 0.782, 0.251, 0.382, and 0.214, for φ0, α, β, γ, and δ, respectively. For O2+, the corresponding values are 1.68×1024s-1, -0.393, 0.798, 0.967, and 0.533. For CO2+, they are 8.66×1022s-1, -0.427, 1.083, 1.214, and 0.690. The fit reproduces the MHD results to an average error of about 5%, suggesting that the power laws are broadly representative of the MHD model results. Our analysis of the MHD model shows that by itself an increase in REUV enhances O+ loss, but suppresses the escape of O2+ and CO2+, whereas increases in solar wind (i.e., in Rρ,Rv2, and RB2, with Reuv constant) favors the escape of heavier ions more than light ions. The ratios of escaping ions detectable at Mars today can be predicted by this parametric fit as a function of the solar forcing factors. We also use the parametric model to compute escape rates over martian history. This second parametric model expresses ion escape functions of one variable (per ion), φ(i)=φ0(i)(t/t0)-ξ(i). The ξ(i) are linear combinations of the epoch-averaged ion escape sensitivities, which are seen to increase with ion mass. We integrate the CO2+ and oxygen ion escape rates over time, and find that in the last 3.85Gyr, Mars would have lost about 25-0.19+85 mbars of CO2+, and 0.64-0.34+0.62m of water (from O+ and O2+) from ion escape. © 2010 Elsevier Inc. Source

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