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Caltech Authors (167.915 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 1.283

  1. A tale of two periods: determination of the orbital ephemeris of the super-Eddington pulsar NGC 7793 P13

    Fürst, F.; Walton, D. J.; Heida, M.; Harrison, F. A.; Barret, D.; Brightman, M.; Fabian, A. C.; Middleton, M. J.; Pinto, C.; Rana, V.; Tramper, F.; Webb, N.; Kretschmar, P.
    We present a timing analysis of multiple XMM-Newton and NuSTAR observations of the ultra-luminous pulsar NGC 7793 P13 spread over its 65d variability period. We use the measured pulse periods to determine the orbital ephemeris, confirm a long orbital period with P_orb = 63.9 (+0.5,-0.6) d, and find an eccentricity of e <= 0.15. The orbital signature is imprinted on top of a secular spin-up, which seems to get faster as the source becomes brighter. We also analyse data from dense monitoring of the source with Swift and find an optical photometric period of 63.9 +/- 0.5 d and an X-ray flux period of 66.8 +/- 0.4 d....

  2. Radio/X-ray monitoring of the broad-line radio galaxy 3C 382. High-energy view with XMM-Newton and NuSTAR

    Ursini, F.; Petrucci, P.-O.; Matt, G.; Bianchi, S.; Cappi, M.; Dadina, M.; Grandi, P.; Torresi, E.; Ballantyne, D. R.; De Marco, B.; De Rosa, A.; Giroletti, M.; Malzac, J.; Marinucci, A.; Middei, R.; Ponti, G.; Tortosa, A.
    We present the analysis of five joint XMM-Newton/NuSTAR observations, 20 ks each and separated by 12 days, of the broad-line radio galaxy 3C 382. The data were obtained as part of a campaign performed in September-October 2016 simultaneously with VLBA. The radio data and their relation with the X-ray ones will be discussed in a following paper. The source exhibits a moderate flux variability in the UV/X-ray bands, and a limited spectral variability especially in the soft X-ray band. In agreement with past observations, we find the presence of a warm absorber, an iron K$\alpha$ line with no associated Compton reflection hump, and a variable soft excess well...

  3. Recurring obscuration in NGC 3783

    Kaastra, J. S.; Mehdipour, M.; Behar, E.; Bianchi, S.; Branduardi-Raymont, G.; Brenneman, L.; Cappi, M.; Costantini, E.; De Marco, B.; di Gesu, L.; Ebrero, J.; Kriss, G. A.; Mao, J.; Peretz, U.; Petrucci, P.-O.; Ponti, G.; Walton, D. J.
    Obscuration of the continuum emission from active galactic nuclei by streams of gas with relatively high velocity (> 1000 km/s) and column density (>3E25 per m^2) has been seen in a few Seyfert galaxies. This obscuration has a transient nature. In December 2016 we have witnessed such an event in NGC 3783. The frequency and duration of these obscuration events is poorly known. Here we study archival data of NGC 3783 in order to constrain this duty cycle. We use archival Chandra/NuSTAR spectra taken in August 2016. We also study the hardness ratio of all Swift XRT spectra taken between 2008-2017. In August 2016, NGC 3783 also showed...

  4. Search for dark matter decay of the free neutron from the UCNA experiment: n → χ + e^+e^−

    Sun, X.; Blatnik, M.; Carr, R.; Filippone, B. W.; Hickerson, K. P.; Martin, J. W.; Mendenhall, M. P.; Pérez Galván, A.; Picker, R.; Slutsky, S.; Swank, C.; Wei, W.
    It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle (χ) could account for the discrepancy in the neutron lifetime observed in experiments that use two different measurement techniques. One of the possible final states discussed includes a single χ along with an e^+e^− pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with ∼4π acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). The summed kinetic energy (E_(e^+e^−)) from such events is...

  5. Cavallo’s Multiplier for in situ Generation of High Voltage

    Clayton, Steven M.; Ito, Takeyasu M.; Ramsey, John C.; Wei, Wanchun; Blatnik, Marie A.; Filippone, Bradley W.; Seidel, George M.
    A classic electrostatic induction machine, Cavallo's multiplier, is suggested for in situ production of very high voltage in cryogenic environments. The device is suitable for generating a large electrostatic field under conditions of very small load current. Operation of the Cavallo multiplier is analyzed, with quantitative description in terms of mutual capacitances between electrodes in the system. A demonstration apparatus was constructed, and measured voltages are compared to predictions based on measured capacitances in the system. The simplicity of the Cavallo multiplier makes it amenable to electrostatic analysis using finite element software, and electrode shapes can be optimized to take...

  6. System-level studies of a cell-free transcription-translation platform for metabolic engineering

    Wu, Yong Y.; Sato, Hirokazu; Huang, Hongjun; Culler, Stephanie J.; Khandurina, Julia; Nagarajan, Harish; Yang, Tae Hoon; Van Dien, Stephen; Murray, Richard M.
    Current methods for assembling biosynthetic pathways in microorganisms require a process of repeated trial and error and have long design-build-test cycles. We describe the use of a cell-free transcription-translation (TX-TL) system as a biomolecular breadboard for the rapid engineering of the 1,4-butanediol (BDO) pathway. We demonstrate the reliability of TX-TL as a platform for engineering biological systems by undertaking a careful characterization of its transcription and translation capabilities and provide a detailed analysis of its metabolic output. Using TX-TL to survey the design space of the BDO pathway enables rapid tuning of pathway enzyme expression levels for improved product yield....

  7. A curious case of the accretion-powered X-ray pulsar GX 1+4

    Jaisawal, Gaurava K.; Naik, Sachindra; Gupta, Shivangi; Chenevez, J.; Epili, Prahlad
    We present detailed spectral and timing studies using a NuSTAR observation of GX 1+4 in October 2015 during an intermediate intensity state. The measured spin period of 176.778 s is found to be one of the highest values since its discovery. In contrast to a broad sinusoidal-like pulse profile, a peculiar sharp peak is observed in profiles below ~25 keV. The profiles at higher energies are found to be significantly phase-shifted compared to the soft X-ray profiles. Broadband energy spectra of GX 1+4, obtained from NuSTAR and Swift observations, are described with various continuum models. Among these, a two component model consisting of a bremsstrahlung and a blackbody...

  8. Accurate design of translational output by a neural network model of ribosome distribution

    Tunney, Robert J.; McGlincy, Nicholas J.; Graham, Monica E.; Naddaf, Nicki; Pachter, Lior; Lareau, Liana
    Synonymous codon choice can have dramatic effects on ribosome speed, RNA stability, and protein expression. Ribosome profiling experiments have underscored that ribosomes do not move uniformly along mRNAs, exposing a need for models of coding sequences that capture the full range of empirically observed variation. We present a method, Iχnos, that models this variation in translation elongation using a feedforward neural network to predict the translation elongation rate at each codon as a function of its sequence neighborhood. Our approach revealed sequence features affecting translation elongation and quantified the impact of large technical biases in ribosome profiling. We applied our...

  9. Combinatorial Alloying Improves Bismuth Vanadate Photoanodes via Reduced Monoclinic Distortion

    Newhouse, P. F.; Guevarra, D.; Umehara, M.; Reyes-Lillo, S. E.; Zhou, L.; Boyd, D. A.; Suram, S. K.; Haber, J. A.; Neaton, J. B.; Gregoire, J. M.
    Improving the efficiency of solar-power oxygen evolution is both critical for development of solar fuels technologies and challenging due to the broad set of properties required of a solar fuels photoanode. Bismuth vanadate, in particular the monoclinic clinobisvanite phase, has received substantial attention and has exhibited the highest radiative efficiency among metal oxides with a band gap in the visible range. Efforts to further improve its photoelectrochemical performance have included alloying one or more metals onto the Bi and/or V sites, with progress on this frontier stymied by the difficulty in computational modelling of substitutional alloys and the high dimensionality...

  10. Actions of trees on semigroups, and an infinitary Gowers-Hales-Jewett Ramsey theorem

    Lupini, Martino
    We introduce the notion of (Ramsey) action of a tree on a (filtered) semigroup. We then prove in this setting a general result providing a common generalization of the infinitary Gowers Ramsey theorem for multiple tetris operations, the infinitary Hales--Jewett theorems (for both located and nonlocated words), and the Farah--Hindman--McLeod Ramsey theorem for layered actions on partial semigroups. We also establish a polynomial version of our main result, recovering the polynomial Milliken--Taylor theorem of Bergelson--Hindman--Williams as a particular case. We present applications of our Ramsey-theoretic results to the structure of delta sets in amenable groups.

  11. Boundary representations of operator spaces, and compact rectangular matrix convex sets

    Fuller, Adam H.; Hartz, Michael; Lupini, Martino
    We initiate the study of matrix convexity for operator spaces. We define the notion of compact rectangular matrix convex set, and prove the natural analogs of the Krein-Milman and the bipolar theorems in this context. We deduce a canonical correspondence between compact rectangular matrix convex sets and operator spaces. We also introduce the notion of boundary representation for an operator space, and prove the natural analog of Arveson's conjecture: every operator space is completely normed by its boundary representations. This yields a canonical construction of the triple envelope of an operator space.

  12. Games orbits play and obstructions to Borel reducibility

    Lupini, Martino; Panagiotopoulos, Aristotelis
    We introduce a new game-theoretic approach to anti-classification results for orbit equivalence relations. Within this framework, we give a short conceptual proof of Hjorth's turbulence theorem. We also introduce a new dynamical criterion providing an obstruction to classification by orbits of CLI groups. We apply this criterion to the relation of equality of countable sets of reals, and the relations of unitary conjugacy of unitary and selfadjoint operators on the separable infinite-dimensional Hilbert space.

  13. Rokhlin dimension for compact quantum group actions

    Gardella, Eusebio; Kalantar, Mehrdad; Lupini, Martino
    We show that, for a given compact or discrete quantum group G, the class of actions of G on C*-algebras is first-order axiomatizable in the logic for metric structures. As an application, we extend the notion of Rokhlin property for G-C*-algebra, introduced by Barlak, Szabó, and Voigt in the case when G is second countable and coexact, to an arbitrary compact quantum group G. All the the preservations and rigidity results for Rokhlin actions of second countable coexact compact quantum groups obtained by Barlak, Szabó, and Voigt are shown to hold in this general context. As a further application, we...

  14. The Ramsey property for Banach spaces, Choquet simplices, and their noncommutative analogs

    Bartošová, Dana; López-Abad, Jordi; Lupini, Martino; Mbombo, Brice
    We show that the Gurarij space G and its noncommutative analog NG both have extremely amenable automorphism group. We also compute the universal minimal flows of the automorphism groups of the Poulsen simplex P and its noncommutative analogue NP. The former is P itself, and the latter is the state space of the operator system associated with NP. This answers a question of Conley and Törnquist. We also show that the pointwise stabilizer of any closed proper face of P is extremely amenable. Similarly, the pointwise stabilizer of any closed proper biface of the unit ball of the dual of...

  15. The complexity of conjugacy, orbit equivalence, and von Neumann equivalence of actions of nonamenable groups

    Gardella, Eusebio; Lupini, Martino
    Building on work of Popa, Ioana, and Epstein--Törnquist, we show that, for every nonamenable countable discrete group Γ, the relations of conjugacy, orbit equivalence, and von Neumann equivalence of free ergodic (or weak mixing) measure preserving actions of Γ on the standard atomless probability space are not Borel, thus answering questions of Kechris. This is an optimal and definitive result, which establishes a neat dichotomy with the amenable case, since any two free ergodic actions of an amenable group on the standard atomless probability space are orbit equivalent by classical results of Dye and Ornstein--Weiss. The statement about conjugacy solves...

  16. An invitation to model theory and C*-algebras

    Lupini, Martino
    We present an introductory survey to first order logic for metric structures and its applications to C*-algebras.

  17. Giant earthquakes on quiet faults governed by rheological transitions

    van den Ende, Martijn P. A.; Chen, Jianye; Ampuero, Jean-Paul; Niemeijer, André R.
    The apparent stochastic nature of earthquakes poses major challenges for earthquake forecasting attempts. Physical constraints on the seismogenic potential of major fault zones may greatly aid in improving seismic hazard assessments, but the mechanics of earthquake nucleation and rupture are obscured by the enormous complexity that natural faults display. In this study, we investigate the mechanisms behind giant earthquakes by employing a microphysically based seismic cycle simulator. This microphysical approach is directly based on the mechanics of friction as inferred from laboratory tests, and can explain a broad spectrum of fault slip behaviour. We show that exceptionally large, fault-spanning earthquakes...

  18. An intrinsic order-theoretic characterization of the weak expectation property

    Lupini, Martino
    We prove the following characterization of the weak expectation property for operator systems in terms of Wittstock's matricial Riesz separation property: an operator system S satisfies the weak expectation property if and only if M_q(S) satisfies the matricial Riesz separation property for every q∈N. This can be seen as the noncommutative analog of the characterization of simplex spaces among function systems in terms of the classical Riesz separation property.

  19. The Fraïssé limit of matrix algebras with the rank metric

    Anderson, Aaron; Lupini, Martino
    We realize the F_q-algebra M(F_q) studied by von Neumann and Halperin as the Fraïssé limit of the class of finite-dimensional matrix algebras over a finite field F_q equipped with the rank metric. We then provide a new Fraïssé-theoretic proof of uniqueness of such an object. Using the results of Carderi and Thom, we show that the automorphism group of Aut(F_q) is extremely amenable. We deduce a Ramsey-theoretic property for the class of algebras M(F_q), and provide an explicit bound for the quantities involved.

  20. Actions of nonamenable groups on Z-stable C*-algebras

    Gardella, Eusebio; Lupini, Martino
    We study strongly outer actions of discrete groups on C*-algebras in relation to (non)amenability. In contrast to related results for amenable groups, where uniqueness of strongly outer actions on the Jiang-Su algebra is expected, we show that uniqueness fails for all nonamenable groups, and that the failure is drastic. Our main result implies that if G contains a copy of the free group, then there exist uncountable many, non-cocycle conjugate strongly outer actions of G on any Jiang-Su stable tracial C*-algebra. Similar conclusions apply for outer actions on McDuff tracial von Neumann algebras. We moreover show that G is amenable...

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