Recursos de colección
Caltech Authors (167.915 recursos)
Repository of works by Caltech published authors.
Status = Submitted
Repository of works by Caltech published authors.
Status = Submitted
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....
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...
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...
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...
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...
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....
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...
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...
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...
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.
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.
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.
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...
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...
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...
Lupini, Martino
We present an introductory survey to first order logic for metric structures and its applications to C*-algebras.
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...
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.
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.
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...