## Recursos de colección

#### Caltech Authors (132.208 recursos)

Repository of works by Caltech published authors.

Type = Report or Paper

1. #### Improved Multi-Class Cost-Sensitive Boosting via Estimation of the Minimum-Risk Class

Appel, Ron; Burgos-Artizzu, Xavier P.; Perona, Pietro
We present a simple unified framework for multi-class cost-sensitive boosting. The minimum-risk class is estimated directly, rather than via an approximation of the posterior distribution. Our method jointly optimizes binary weak learners and their corresponding output vectors, requiring classes to share features at each iteration. By training in a cost-sensitive manner, weak learners are invested in separating classes whose discrimination is important, at the expense of less relevant classification boundaries. Additional contributions are a family of loss functions along with proof that our algorithm is Boostable in the theoretical sense, as well as an efficient procedure for growing decision trees for use as weak learners. We evaluate our method...

2. #### A deep X-ray view of the bare AGN Ark 120. II. Evidence for Fe K emission transients

Nardini, E.; Porquet, D.; Reeves, J. N.; Braito, V.; Lobban, A.; Matt, G.
We report on the results from a large observational campaign on the bare Seyfert galaxy Ark 120, jointly carried out in 2014 with XMM-Newton, Chandra, and NuSTAR. The fortunate line of sight to this source, devoid of any significant absorbing material, provides an incomparably clean view to the nuclear regions of an active galaxy. Here we focus on the analysis of the iron fluorescence features, which form a composite emission pattern in the 6-7 keV band. The prominent Kα line from neutral iron at 6.4 keV is resolved in the Chandra High-Energy Transmission Grating spectrum to a full-width at half maximum of 4700^(+2700)_(-1500) km s^(-1), consistent with an...

3. #### An in-depth study of a neutron star accreting at low Eddington rate: On the possibility of a truncated disk and an outflow

Degenaar, N.; Pinto, C.; Miller, J. M.; Wijnands, R.; Altamirano, D.; Paerels, F.; Fabian, A. C.; Chakrabarty, D.
Due to observational challenges our knowledge of low-level accretion flows around neutron stars is limited. We present Nustar, Swift and Chandra observations of the low-mass X-ray binary IGR J17062-6143, which has been persistently accreting at ≃0.1 per cent of the Eddington limit since 2006. Our simultaneous Nustar/Swift observations show that the 0.5-79 keV spectrum can be described by a combination of a power law with a photon index of Γ≃2, a black body with a temperature of kT_(bb)≃0.5 keV (presumably arising from the neutron star surface), and disk reflection. Modeling the reflection spectrum suggests that the inner accretion disk was located at R_(in)≳100 GM/c^2 (≳225 km) from the neutron...

4. #### Transfer of Energy and Materials in Fluid System

Sage, B. H.; Standart, G. C.; Billman, G. W.; Corcoran, W. H.; Hough, E. W.
Foreword: In a large number of the situations with which an engineer is concerned in the process industries, transfers of energy, momentum and material in and between phases of flowing fluids are of importance. It behooves one interested in process development and design to become familiar with the fundamentals of such transfer processes in order that the rates of transfer may be predicted with reasonable accuracy on the basis of available facts describing the situation. The work of J. Boussinesq and Osborne Reynolds in the early part of the nineteenth century was one of the most outstanding early contributions to this subject. They...

5. #### A Hoseus Banjo Restoration

Politzer, David
Intrigued by the sound of another recently restored example, I attempted to bring a sadly abused, bottom-of-the-line, Hoseus-equipped banjo up to playable condition. Reminders, lessons learned, and the joy of (albeit crude) handiwork made it well- worth the purchase price. The actual sound and physics of the Hoseus contraption remain hidden in the complex interaction of the various parts, as demonstrated by the accompanying sound samples.

6. #### NuSTAR observations of the supergiant X-ray pulsar IGR J18027-2016: accretion from the stellar wind and possible cyclotron absorption line

Lutovinov, Alexander A.; Tsygankov, Sergey S.; Postnov, Konstantin A.; Krivonos, Roman A.; Molkov, Sergey V.; Tomsick, John A.
We report on the first focused hard X-ray view of the absorbed supergiant system IGRJ18027-2016 performed with the NuSTAR observatory. The pulsations are clearly detected with a period of P_(spin)=139.866(1) s and a pulse fraction of about 50-60% at energies from 3 to 80 keV. The source demonstrates an approximately constant X-ray luminosity on a time scale of more than dozen years with an average spin-down rate of P≃6x10^(-10) s s^(-1). This behaviour of the pulsar can be explained in terms of the wind accretion model in the settling regime. The detailed spectral analysis at energies above 10 keV was performed for the first time and revealed a...

7. #### Dynamical Mass Ejection from Binary Neutron Star Mergers

Radice, David; Galeazzi, Filippo; Lippuner, Jonas; Roberts, Luke F.; Ott, Christian D.; Rezzolla, Luciano
We present fully general-relativistic simulations of binary neutron star mergers with a temperature and composition dependent nuclear equation of state. We study the dynamical mass ejection from both quasi-circular and dynamical-capture eccentric mergers. We systematically vary the level of our treatment of the microphysics to isolate the effects of neutrino cooling and heating and we compute the nucleosynthetic yields of the ejecta. We find that eccentric binaries can eject significantly more material than quasi-circular binaries and generate bright infrared and radio emission. In all our simulations the outflow is composed of a combination of tidally- and shock-driven ejecta, mostly distributed...

8. #### Finite-amplitude shear-Alfvén waves do not propagate in weakly magnetized collisionless plasmas

Squire, J.; Quataert, E.; Schekochihin, A. A.
It is shown that low-collisionality plasmas cannot support linearly polarized shear-Alfv\'en fluctuations above a critical amplitude δB_⊥/B_0∼β^(−1/2), where β is the ratio of thermal to magnetic pressure. Above this cutoff, a developing fluctuation will generate a pressure anisotropy that is sufficient to destabilize itself through the parallel firehose instability. This causes the wave frequency to approach zero, interrupting the fluctuation before any oscillation. The magnetic field lines rapidly relax into a sequence of angular zig-zag structures. Such a restrictive bound on shear-Alfv\'en-wave amplitudes has far-reaching implications for the physics of magnetized turbulence in the high-β conditions prevalent in many astrophysical...

9. #### Testing relativistic reflection and resolving outflows in PG 1211+143 with XMM-Newton and NuSTAR

Lobban, Andrew; Pounds, Ken; Vaughan, Simon; Reeves, James
We analyze the broad-band X-ray spectrum (0.3-50 keV) of the luminous Seyfert 1 / quasar PG 1211+143 - the archetypal source for high-velocity X-ray outflows - using near-simultaneous XMM-Newton and NuSTAR observations. We compare pure relativistic reflection models with a model including the strong imprint of photoionized emission and absorption from a high-velocity wind (Pounds16a,16b), finding a spectral fit that extrapolates well over the higher photon energies covered by NuSTAR. Inclusion of the high S/N XMM-Newton spectrum provides much tighter constraints on the model parameters, with a much harder photon index / lower reflection fraction compared to that from the NuSTAR data alone. We show that pure relativistic reflection...

10. #### NuSTAR and XMM-Newton observations of the ultraluminous X-ray source NGC 5643 X-1

Krivonos, Roman; Sazonov, Sergey
We present a high-quality hard X-ray spectrum of the ultraluminous X-ray source (ULX) NGC 5643 X-1 measured with NuSTAR in May-June 2014. We have obtained this spectrum by carefully separating the signals from the ULX and from the active nucleus of its host galaxy NGC 5643 located 0.8 arcmin away. Together with long XMM-Newton observations performed in July 2009 and August 2014, the NuSTAR data confidently reveal a high-energy cutoff in the spectrum of NGC 5643 X-1 above ~10 keV, which is a characteristic signature of ULXs. The NuSTAR and XMM-Newton data are consistent with the source having a constant luminosity ~1.5E40 erg/s (0.2-12 keV) in all but...

11. #### Search for gamma-ray emission from AE Aquarii with seven years of FERMI-LAT observations

Li, Jian; Torres, Diego F.; Rea, Nanda; de Ona Wilhelmi, Emma; Papitto, Alessandro; Hou, Xian; Mauche, Christopher W.
AE Aquarii (AE Aqr) is a cataclysmic binary hosting one of the fastest rotating (P$_{\rm spin}$ = 33.08 s) white dwarfs known. Based on seven years of Fermi Large Area Telescope (LAT) Pass 8 data, we report on a deep search for gamma-ray emission from AE Aqr. Using X-ray observations from ASCA, XMM-Newton, Chandra, Swift, Suzaku, and NuSTAR, spanning 20 years, we substantially extend and improve the spin ephemeris of AE Aqr. Using this ephemeris, we searched for gamma-ray pulsations at the spin period of the white dwarf. No gamma-ray pulsations were detected above 3 $\sigma$ significance. Neither phase-averaged gamma-ray emission nor gamma-ray variability of AE Aquarii is...

12. #### Pseudomagnetic fields for sound at the nanoscale

Brendel, Christian; Peano, Vittorio; Painter, Oskar; Marquardt, Florian
There is a growing effort in creating chiral transport of sound waves. However, most approaches so far are confined to the macroscopic scale. Here, we propose a new approach suitable to the nanoscale which is based on pseudomagnetic fields. These fields are the analogon for sound of the pseudomagnetic field for electrons in strained graphene. In our proposal, they are created by simple geometrical modifications of an existing and experimentally proven phononic crystal design, the snowflake crystal. This platform is robust, scalable, and well-suited for a variety of excitation and readout mechanisms, among them optomechanical approaches.

13. #### Generalized nonreciprocity in an optomechanical circuit via synthetic magnetism and reservoir engineering

Fang, Kejie; Luo, Jie; Metelmann, Anja; Matheny, Mathew H.; Marquardt, Florian; Clerk, Aashish A.; Painter, Oskar
Synthetic magnetism has been used to control charge neutral excitations for applications ranging from classical beam steering to quantum simulation. In optomechanics, radiation-pressure-induced parametric coupling between optical (photon) and mechanical (phonon) excitations may be used to break time-reversal symmetry, providing the prerequisite for synthetic magnetism. Here we design and fabricate a silicon optomechanical circuit with both optical and mechanical connectivity between two optomechanical cavities. Driving the two cavities with phase-correlated laser light results in a synthetic magnetic flux, which in combination with dissipative coupling to the mechanical bath, leads to nonreciprocal transport of photons with 35dB of isolation. Additionally, optical pumping with blue-detuned light manifests as a particle non-conserving interaction between...

14. #### BPS counting for knots and combinatorics on words

Kucharski, Piotr; Sułkowski, Piotr
We discuss relations between quantum BPS invariants defined in terms of a product decomposition of certain series, and difference equations (quantum A-polynomials) that annihilate such series. We construct combinatorial models whose structure is encoded in the form of such difference equations, and whose generating functions (Hilbert-Poincaré series) are solutions to those equations and reproduce generating series that encode BPS invariants. Furthermore, BPS invariants in question are expressed in terms of Lyndon words in an appropriate language, thereby relating counting of BPS states to the branch of mathematics referred to as combinatorics on words. We illustrate these results in the framework of colored extremal knot polynomials: among others we determine dual...

15. #### In Situ Formation and Dynamical Evolution of Hot Jupiter Systems

Batygin, Konstantin; Bodenheimer, Peter H.; Laughlin, Gregory P.
Hot Jupiters, giant extrasolar planets with orbital periods shorter than ~10 days, have long been thought to form at large radial distances, only to subsequently experience long-range inward migration. Here, we propose that in contrast with this picture, a substantial fraction of the hot Jupiter population formed in situ via the core accretion process. We show that under conditions appropriate to the inner regions of protoplanetary disks, rapid gas accretion can be initiated by Super-Earth type planets, comprising 10-20 Earth masses of refractory composition material. An in situ formation scenario leads to testable consequences, including the expectation that hot Jupiters...

16. #### The IPAC Image Subtraction and Discovery Pipeline for the intermediate Palomar Transient Factory

Masci, Frank J.; Laher, Russ; Rebbapragada, Umaa; Doran, Gary; Miller, Adam; Bell, Eric; Kasliwal, Mansi M.; Ofek, Eran; Surace, Jason; Shupe, David; Grillmair, Carl J.; Jackson, Ed; Barlow, Tom A.; Yan, Lin; Cao, Yi; Cenko, S. Bradley; Storrie-Lombardi, Lisa; Helou, George; Prince, Thomas A.; Kulkarni, Shrinivas R.
We describe the near real-time transient-source discovery engine for the intermediate Palomar Transient Factory (iPTF), currently in operations at the Infrared Processing and Analysis Center (IPAC), Caltech. We coin this system the IPAC/iPTF Discovery Engine (or IDE). We review the algorithms used for PSF-matching, image subtraction, detection, photometry, and machine-learned (ML) vetting of extracted transient candidates. We also review the performance of our ML classifier. For a limiting signal-to-noise ratio of 4 in relatively unconfused regions, "bogus" candidates from processing artifacts and imperfect image subtractions outnumber real transients by ~ 10:1. This can be considerably higher for image data with inaccurate astrometric and/or PSF-matching solutions. Despite this occasionally high contamination...

17. #### Positivity of Curvature-Squared Corrections in Gravity

Cheung, Clifford; Remmen, Grant N.
We study the Gauss-Bonnet (GB) term as the leading higher-curvature correction to pure Einstein gravity. Assuming a tree-level ultraviolet completion free of ghosts or tachyons, we prove that the GB term has a nonnegative coefficient in dimensions greater than four. Our result follows from unitarity of the spectral representation for a general ultraviolet completion of the GB term.

18. #### Super-quantum curves from super-eigenvalue models

Ciosmak, Paweł; Hadasz, Leszek; Manabe, Masahide; Sułkowski, Piotr
In modern mathematical and theoretical physics various generalizations, in particular supersymmetric or quantum, of Riemann surfaces and complex algebraic curves play a prominent role. We show that such supersymmetric and quantum generalizations can be combined together, and construct supersymmetric quantum curves, or super-quantum curves for short. Our analysis is conducted in the formalism of super-eigenvalue models: we introduce $\beta$-deformed version of those models, and derive differential equations for associated $\alpha/\beta$-deformed super-matrix integrals. We show that for a given model there exists an infinite number of such differential equations, which we identify as super-quantum curves, and which are in one-to-one correspondence with, and have the structure of, super-Virasoro singular vectors. We...

19. #### The Verlinde formula for Higgs bundles

Andersen, Jørgen Ellegaard; Gukov, Sergei; Pei, Du
We propose and prove the Verlinde formula for the quantization of the Higgs bundle moduli spaces and stacks for any simple and simply-connected group. This generalizes the equivariant Verlinde formula for the case of SU(n) proposed previously by the second and third author. We further establish a Verlinde formula for the quantization of parabolic Higgs bundle moduli spaces and stacks.

20. #### High-energy monitoring of NGC 4593 with XMM-Newton and NuSTAR. X-ray spectral analysis

Ursini, F.; Petrucci, P.-O.; Matt, G.; Bianchi, S.; Cappi, M.; De Marco, B.; De Rosa, A.; Malzac, J.; Marinucci, A.; Ponti, G.; Tortosa, A.
We present results from a joint Xmm-Newton/NuSTAR monitoring of the Seyfert 1 NGC 4593, consisting of 5x20 ks simultaneous observations spaced by two days, performed in January 2015. The source is variable, both in flux and spectral shape, on time-scales down to a few ks and with a clear softer-when-brighter behaviour. In agreement with past observations, we find the presence of a warm absorber well described by a two-phase ionized outflow. The source exhibits a cold, narrow and constant Fe K alpha line at 6.4 keV, and a broad component is also detected. The broad-band (0.3-79 keV) spectrum is well described by a primary power law with Gamma=1.6-1.8...

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