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rss_1.0 Recursos de colección

Caltech Authors (99,106 recursos)
Repository of works by Caltech published authors.

Type = Report or Paper

Mostrando recursos 1 - 20 de 2,710

1. The Spitzer mid-infrared AGN survey. II-the demographics and cosmic evolution of the AGN population - Lacy, Mark; Ridgway, Susan E.; Sajina, Anna; Petric, Andreea; Gates, Elinor L.; Urritia, Tanya; Storrie-Lombardi, Lisa J.
We present luminosity functions derived from a spectroscopic survey of AGN selected from Spitzer Space Telescope imaging surveys. Selection in the mid-infrared is significantly less affected by dust obscuration. We can thus compare the luminosity functions of the obscured and unobscured AGN in a more reliable fashion than by using optical or X-ray data alone. We find that the AGN luminosity function can be well described by a broken power-law model in which the break luminosity decreases with redshift. At high redshifts (z>1.6), we find significantly more AGN at a given bolometric luminosity than found by either optical quasar surveys...

2. On-Shell Structures of MHV Amplitudes Beyond the Planar Limit - Arkani-Hamed, Nima; Bourjaily, Jacob L.; Cachazo, Freddy; Postnikov, Alexander; Trnka, Jaroslav
We initiate an exploration of on-shell functions in N=4 SYM beyond the planar limit by providing compact, combinatorial expressions for all leading singularities of MHV amplitudes and showing that they can always be expressed as a positive sum of differently ordered Parke-Taylor tree amplitudes. This is understood in terms of an extended notion of positivity in G(2,n), the Grassmannian of 2-planes in n dimensions: a single on-shell diagram can be associated with many different "positive" regions, of which the familiar positive region associated with planar diagrams is just one example. The decomposition into Parke-Taylor factors is simply a "triangulation" of...

3. Sparse Phase Retrieval: Uniqueness Guarantees and Recovery Algorithms - Jaganathan, Kishore; Oymak, Samet; Hassibi, Babak
The problem of signal recovery from its Fourier transform magnitude, or equivalently, autocorrelation, is of paramount importance in various fields of engineering and has been around for over 100 years. In order to achieve this, additional structure information about the signal is necessary. In this work, we first provide simple and general conditions, which when satisfied, allow unique recovery almost surely. In particular, we focus our attention on sparse signals and show that most O(n)-sparse signals, i.e., signals with O(n) non-zero components, have distinct Fourier transform magnitudes (up to time-shift, time-reversal and global sign). Our results are a significant improvement over the existing identifiability...

4. Simple Bounds for Noisy Linear Inverse Problems with Exact Side Information - Oymak, Samet; Thrampoulidis, Christos; Hassibi, Babak
This paper considers the linear inverse problem where we wish to estimate a structured signal x_0 from its corrupted observations. When the problem is ill-posed, it is natural to associate a convex function f(·) with the structure of the signal. For example, ℓ_1 norm can be used for sparse signals. To carry out the estimation, we consider two well-known convex programs: 1) Second order cone program (SOCP), and, 2) Lasso. Assuming Gaussian measurements, we show that, if precise information about the value f(x_0) or the ℓ_2-norm of the noise is available, one can do a particularly good job at estimation. In...

5. Exact synthesis of single-qubit unitaries over Clifford-cyclotomic gate sets - Forest, Simon; Gosset, David; Kliuchnikov, Vadym; McKinnon, David
We generalize an efficient exact synthesis algorithm for single-qubit unitaries over the Clifford+T gate set which was presented by Kliuchnikov, Maslov and Mosca. Their algorithm takes as input an exactly synthesizable single-qubit unitary--one which can be expressed without error as a product of Clifford and T gates--and outputs a sequence of gates which implements it. The algorithm is optimal in the sense that the length of the sequence, measured by the number of T gates, is smallest possible. In this paper, for each positive even integer n we consider the "Clifford-cyclotomic" gate set consisting of the Clifford group plus a...

6. The Cost of an Epidemic over a Complex Network: A Random Matrix Approach - Bose, Subhonmesh; Bodine-Baron, Elizabeth; Hassibi, Babak; Wierman, Adam
In this paper we quantify the total economic impact of an epidemic over a complex network using tools from random matrix theory. Incorporating the direct and indirect costs of infection, we calculate the disease cost in the large graph limit for an SIS (Susceptible - Infected - Susceptible) infection process. We also give an upper bound on this cost for arbitrary finite graphs and illustrate both calculated costs using extensive simulations on random and real-world networks. We extend these calculations by considering the total social cost of an epidemic, accounting for both the immunization and disease costs for various immunization strategies and determining the optimal immunization. Our work focuses...

7. The Squared-Error of Generalized LASSO: A Precise Analysis - Oymak, Samet; Thrampoulidis, Christos; Hassibi, Babak
We consider the problem of estimating an unknown signal x_0 from noisy linear observations y = Ax_0 + z ∈ ℝ^m. In many practical instances, x_0 has a certain structure that can be captured by a structure inducing convex function f(•). For example, ℓ_1 norm can be used to encourage a sparse solution. To estimate x_0 with the aid of f(•), we consider the well-known LASSO method and provide sharp characterization of its performance. Our study falls under a generic framework, where the entries of the measurement matrix A and the noise vector z have zero-mean normal distributions with variances 1 and σ^2,...

8. A Tight Version of the Gaussian min-max theorem in the Presence of Convexity - Thrampoulidis, Christos; Oymak, Samet; Hassibi, Babak
Gaussian comparison theorems are useful tools in probability theory; they are essential ingredients in the classical proofs of many results in empirical processes and extreme value theory. More recently, they have been used extensively in the analysis of underdetermined linear inverse problems. A prominent role in the study of those problems is played by Gordon's Gaussian min-max theorem. It has been observed that the use of the Gaussian min-max theorem produces results that are often tight. Motivated by recent work due to M. Stojnic, we argue explicitly that the theorem is tight under additional convexity assumptions. To illustrate the usefulness of the result we provide an application example from...

9. NuSTAR Observations of X-Ray Binaries - Tomsick, John A.; Bellm, Eric C.; Bhalerao, Varun; Chakrabarty, Deepto; Fuerst, F.; Harrison, Fiona A.; Miyasaka, Hiromasa; Tendulkar, Shriharsh; King, Ashlee; Miller, John M.; Natalucci, Lorenzo; Stern, Daniel
As of 2014 August, the Nuclear Spectroscopic Telescope Array (NuSTAR) had observed ~30 X-ray binaries either as part of the planned program, as targets of opportunity, or for instrument calibration. The main science goals for the observations include probing the inner part of the accretion disk and constraining black hole spins via reflection components, providing the first observations of hard X-ray emission from quiescent Low Mass X-ray Binaries (LMXBs), measuring cyclotron lines from accreting pulsars, and studying type I X-ray bursts from neutron stars. Here, we describe the science objectives in more depth and give an overview of the NuSTAR...

10. On the role of large elastic deformation in fracture initiation -- the cases of plane stress and plane strain - Chu, Billie
In recent years there has been an increase in interest in the problems of strength and deformation of highly elastic materials. A number of simpler cases which have been solved indicate some markedly different characteristics from their counterparts in infinitesimal elasticity. References 1 and 2 discuss these cases in some detail. It seems natural therefore to ask whether in the problem areas of fracture in which the application of the infinitesimal theory of elasticity has met with some success in brittle elastic materials an equally marked difference in behavior would result if the possibility of large strains were included in...

11. Ordovician-Silurian volcanogenic massive sulfide deposits on the southern Prince of Wales Island and the barrier islands, southeastern Alaska - Gehrels, George E.; Berg, Henry C.; Saleeby, Jason B.
Several pyritic massive sulfide deposits have been recognized in an Ordovician-Silurian volcano-plutonic complex in the southern Prince of Wales Island region (Fig. 1). These deposits have been studied as part of a U.S. Geological Survey-California Institute of Technology investigation into the geologic and mineralization history of southern Prince of Wales Island (south of 55° North Latitude; Fig. 1). This report describes the geologic setting of the deposits and presents preliminary chemical analyses of the mineralization.

12. Surface Operators - Gukov, Sergei
This is the seventh article in the collection of reviews "Exact results on N=2 supersymmetric gauge theories", ed. J.Teschner. It discusses an interesting class of observables localised on surfaces that attracts steadily growing attention. In the correspondence to conformal field theory some of these observables get related to a class of fields in two dimensions called degenerate fields. These fields satisfy differential equations that can be used to extract a lot of information on the correlation functions. Understanding the origin of these differential equations within gauge theory may help explaining the AGT-correspondence itself.

13. Logarithmic Singularities and Maximally Supersymmetric Amplitudes - Bern, Zvi; Herrmann, Enrico; Litsey, Sean; Stankowicz, James; Trnka, Jaroslav
The dual formulation of planar N = 4 super-Yang-Mills scattering amplitudes makes manifest that the integrand has only logarithmic singularities and no poles at infinity. Recently, Arkani-Hamed, Bourjaily, Cachazo and Trnka conjectured the same singularity properties hold to all loop orders in the nonplanar sector as well. Here we conjecture that to all loop orders these constraints give us the key analytic information contained in dual conformal symmetry. We also conjecture that to all loop orders, while N = 8 supergravity has poles at infinity, at least at four points it has only logarithmic singularities at finite locations. We provide nontrivial evidence for these conjectures. For the three-loop four-point...

14. SKA studies of nearby galaxies: star-formation, accretion processes and molecular gas across all environments - Beswick, R. J.; Brinks, E.; Pérez-Torres, M. A.; Richards, A. M. S.; Aalto, S.; Alberdi, A.; Argo, M. K.; van Bemmel, I.; Conway, J. E.; Dickinson, C.; Fenech, C. M.; Gray, M. D.; Klöckner, H.-R.; Murphy, E. J.; Muxlow, T. W. B.; Peel, M.; Rushton, A. P.; Schinnerer, E.
The SKA will be a transformational instrument in the study of our local Universe. In particular, by virtue of its high sensitivity (both to point sources and diffuse low surface brightness emission), angular resolution and the frequency ranges covered, the SKA will undertake a very wide range of astrophysical research in the field of nearby galaxies. By surveying vast numbers of nearby galaxies of all types with mJy sensitivity and sub-arcsecond angular resolutions at radio wavelengths, the SKA will provide the cornerstone of our understanding of star-formation and accretion activity in the local Universe. In this chapter we outline the...

15. Enabling the next generation of cm-wavelength studies of high-redshift molecular gas with the SKA - Wagg, Jeff; da Cunha, Elisabete; Carilli, Christopher L.; Walter, Fabian; Aravena, Manuel; Heywood, Ian; Hodge, Jacqueline; Murphy, Eric; Riechers, Dominik; Sargent, Mark; Wang, Ran
The Square Kilometre Array will be a revolutionary instrument for the study of gas in the distant Universe. SKA1 will have sufficient sensitivity to detect and image atomic 21 cm HI in individual galaxies at significant cosmological distances, complementing ongoing ALMA imaging of redshifted high-J CO line emission and far-infrared interstellar medium lines such as [CII] 157.7mm. At frequencies below ∼50 GHz, observations of redshifted emission from low-J transitions of CO, HCN, HCO^+, HNC, H_2O and CS provide insight into the kinematics and mass budget of the cold, dense star-forming gas in galaxies. In advance of ALMA band 1 deployment...

16. The star-formation history of the Universe with the SKA - Jarvis, Matt J.; Seymour, Nick; Afonso, Jose; Best, Philip; Beswick, Rob; Heywood, Ian; Huynh, Minh; Murphy, Eric; Prandoni, Isabella; Schinnerer, Eva; Simpson, Chris; Vaccari, Mattia; White, Sarah
Radio wavelengths offer the unique possibility of tracing the total star-formation rate in galaxies, both obscured and unobscured. As such, they may provide the most robust measurement of the star-formation history of the Universe. In this chapter we highlight the constraints that the SKA can place on the evolution of the star-formation history of the Universe, the survey area required to overcome sample variance, the spatial resolution requirements, along with the multi-wavelength ancillary data that will play a major role in maximising the scientific promise of the SKA. The required combination of depth and resolution means that a survey to trace the...

17. Short-time transport properties of bidisperse suspensions and porous media: a Stokesian Dynamics study - Wang, Mu; Brady, John F.
We present a comprehensive computational study of the short-time transport properties of bidisperse neutral colloidal suspensions and the corresponding porous media. Our study covers bidisperse particle size ratios up to 4, and total volume fractions up to and beyond the monodisperse hard-sphere close packing limit. The many-body hydrodynamic interactions are computed using conventional Stokesian Dynamics (SD) via a Monte-Carlo approach. We address suspension properties including the short-time translational and rotational self-diffusivities, the instantaneous sedimentation velocity, the wavenumber-dependent partial hydrodynamic functions, and the high-frequency shear and bulk viscosities; and porous media properties including the permeability and the translational and rotational hindered diffusivities. We carefully compare the SD computations with existing theoretical and...

18. Direct Detection Constraints on Dark Photon Dark Matter - An, Haipeng; Pospelov, Maxim; Pradler, Josef; Ritz, Adam
Dark matter detectors built primarily to probe elastic scattering of WIMPs on nuclei are also precise probes of light, weakly coupled particles that may be absorbed by the detector material. In this paper, we derive constraints on the minimal model of dark matter comprised of long-lived vector states V (dark photons) in the 0.01-100 keV mass range. The absence of an ionization signal in direct detection experiments such as XENON10 and XENON100 places a very strong constraint on the dark photon mixing angle, down to O(10−15), assuming that dark photons comprise the dominant fraction of dark matter. This sensitivity to...

19. ALMA Imaging of Gas and Dust in a Galaxy Protocluster at Redshift 5.3: [CII] Emission in "Typical" Galaxies and Dusty Starbursts ≈1 Billion Years after the Big Bang - Riechers, Dominik A.; Carilli, Christopher L.; Capak, Peter L.; Scoville, Nicholas Z.; Smolčić, Vernesa; Schinnerer, Eva; Yun, Min; Cox, Pierre; Bertoldi, Frank; Karim, Alexander; Yan, Lin
We report interferometric imaging of [CII](^2P_(3/2)→^2P_(1/2)) and OH(^2II_(1/2) J=3/2→1/2) emission toward the center of the galaxy protocluster associated with the z=5.3 submillimeter galaxy (SMG) AzTEC-3, using the Atacama Large (sub)Millimeter Array (ALMA). We detect strong [CII], OH, and rest-frame 157.7μm continuum emission toward the SMG. The [CII](^2P_(3/2)→^2P_(1/2)) emission is distributed over a scale of 3.9 kpc, implying a dynamical mass of 9.7×10^(10)M_⊙, and a star formation rate (SFR) surface density of ∑_(SFR)=530M_⊙ yr^(−1) kpc^(−2). This suggests that AzTEC-3 forms stars at ∑_(SFR) approaching the Eddington limit for radiation pressure supported disks. We find that the OH emission is slightly blueshifted relative to the...

20. Patchy Accretion Disks in Ultraluminous X-ray Sources - Miller, J. M.; Bachetti, M.; Barret, D.; Harrison, F. A.; Fabian, A. C.; Webb, N. A.; Walton, D. J.; Rana, V.
The X-ray spectra of the most extreme ultra-luminous X-ray sources – those with L ≥ 10^(40) erg s^(-1) – remain something of a mystery. Spectral roll-over in the 5–10 keV band was originally detected in in the deepest XMM-Newton observations of the brightest sources; this is confirmed in subsequent NuSTAR spectra. This emission can be modeled via Comptonization, but with low electron temperatures (kT_e ≃2 keV) and high optical depths (T ≃ 10) that pose numerous difficulties. Moreover, evidence of cooler thermal emission that can be fit with thin disk models persists, even in fits to joint XMM-Newton and NuSTAR observations....

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