## Recursos de colección

#### Caltech Authors (144.724 recursos)

Repository of works by Caltech published authors.

Status = Submitted

1. #### Multi-epoch analysis of the X-ray spectrum of the active galactic nucleus in NGC 5506

Sun, Shangyu; Guainazzi, Matteo; Ni, Qingling; Wang, Jingchun; Qian, Chenyang; Shi, Fangzheng; Wang, Yu; Bambi, Cosimo
We present a multi-epoch X-ray spectroscopy analysis of the nearby narrow-line Seyfert I galaxy NGC 5506. For the first time, spectra taken by Chandra, XMM-Newton, Suzaku, and NuSTAR - covering the 2000-2014 time span - are analyzed simultaneously, using state-of-the-art models to describe reprocessing of the primary continuum by optical thick matter in the AGN environment. The main goal of our study is determining the spin of the supermassive black hole (SMBH). The nuclear X-ray spectrum is photoelectrically absorbed by matter with column density $\simeq 3 \times 10^{22}$~cm$^{-2}$. A soft excess is present at energies lower than the photoelectric cut-off. Both photo-ionized and collisionally ionized components are required to...

2. #### Exploring the locally low dimensional structure in solving random elliptic PDEs

Hou, Thomas Y.; Li, Qin; Zhang, Pengchuan
We propose a stochastic multiscale finite element method (StoMsFEM) to solve random elliptic partial differential equations with a high stochastic dimension. The key idea is to simultaneously upscale the stochastic solutions in the physical space for all random samples and explore the low stochastic dimensions of the stochastic solution within each local patch. We propose two effective methods to achieve this simultaneous local upscaling. The first method is a high order interpolation method in the stochastic space that explores the high regularity of the local upscaled quantities with respect to the random variables. The second method is a reduced-order method...

3. #### A sparse decomposition of low rank symmetric positive semi-definite matrices

Hou, Thomas Y.; Li, Qin; Zhang, Pengchuan
Suppose that A∈R^(N×N) is symmetric positive semidefinite with rank K ≤ N. Our goal is to decompose A into K rank-one matrices ∑^K_k=1gkg^T_k where the modes {gk}^K_(k=1) are required to be as sparse as possible. In contrast to eigen decomposition, these sparse modes are not required to be orthogonal. Such a problem arises in random field parametrization where A is the covariance function and is intractable to solve in general. In this paper, we partition the indices from 1 to N into several patches and propose to quantify the sparseness of a vector by the number of patches on which...

4. #### NuSTAR Hard X-ray Observations of the Energetic Millisecond Pulsars PSR B1821-24, PSR B1937+21, and PSR J0218+4232

Gotthelf, E. V.; Bogdanov, S.
We present Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray timing and spectroscopy of the three exceptionally energetic rotation-powered millisecond pulsars PSRs B1821-24, B1937+21, and J0218+4232. By correcting the NuSTAR on-board clock for phase and frequency drifts between observation intervals, we are able to recover the intrinsic hard X-ray pulse profiles of all three pulsars with a resolution down to < 15 microsec. The substantial reduction of background emission relative to previous broad-band X-ray observations allows us to detect for the first time pulsed emission up to ~50 keV, ~20 keV, and ~25 keV, for the three pulsars, respectively. We conduct phase-resolved spectroscopy in the 0.5-79 keV range for all...

5. #### The infrared to X-ray correlation spectra of unobscured type 1 active galactic nuclei

García-Bernete, I.; Ramos Almeida, C.; Landt, H.; Ward, M. J.; Baloković, M.; Acosta-Pulido, J. A.
We use new X-ray data obtained with the Nuclear Spectroscopic Telescope Array (NuSTAR), near-infrared (NIR) fluxes, and mid-infrared (MIR) spectra of a sample of 24 unobscured type 1 active galactic nuclei (AGN) to study the correlation between various hard X-ray bands between 3 and 80 keV and the infrared (IR) emission. The IR to X-ray correlation spectrum (IRXCS) shows a maximum at ~15-20 micron, coincident with the peak of the AGN contribution to the MIR spectra of the majority of the sample. There is also a NIR correlation peak at ~2 micron, which we associate with the NIR bump observed in some type 1 AGN at ~1-5 micron...

6. #### NuSTAR and Swift observations of the ultraluminous X-ray source IC 342 X-1 in 2016: witnessing spectral evolution

Shidatsu, Megumi; Ueda, Yoshihiro; Fabrika, Sergei
We report on an X-ray observing campaign of the ultraluminous X-ray source IC 342 X-1 with NuSTAR and Swift in 2016 October, in which we captured the very moment when the source showed spectral variation. The Swift/XRT spectrum obtained in October 9--11 has a power-law shape and is consistent with those observed in the coordinated XMM-Newton and NuSTAR observations in 2012. In October 16--17, when the 3--10 keV flux became $\approx$4 times higher, we performed simultaneous NuSTAR and Swift observations. In this epoch, the source showed a more round-shaped spectrum like that seen with ASCA 23 years ago. Thanks to the wide energy coverage and high sensitivity of...

7. #### The high-Eddington NLS1 Ark 564 has the coolest corona

Kara, E.; Garcia, J. A.; Lohfink, A.; Fabian, A. C.; Reynolds, C. S.; Tombesi, F.; Wilkins, D. R.
Ark 564 is an archetypal Narrow line Seyfert 1 that has been well observed in soft X-rays from 0.3-10 keV, revealing a steep spectrum, strong soft excess, iron K emission line and dramatic variability on the order of hours. Because of its very steep spectrum, observations of the source above 10 keV have been sparse. We report here on the first NuSTAR observation of Ark 564. The source was observed for 200 ks with NuSTAR, 50 ks of which were concurrent with Suzaku observations. NuSTAR and Suzaku observed a dramatic flare, in which the hard emission is clearly delayed with respect to the soft emission, consistent with previous...

8. #### A NuSTAR Observation of the Gamma-ray Emitting Millisecond Pulsar PSR J1723-2837

Kong, A. K. H.; Hui, C. Y.; Takata, J.; Li, K. L.; Tam, P. H. T.
We report on the first NuSTAR observation of the gamma-ray emitting millisecond pulsar binary PSR J1723-2837. X-ray radiation up to 79 keV is clearly detected and the simultaneous NuSTAR and Swift spectrum is well described by an absorbed power-law with a photon index of ~1.3. We also find X-ray modulations in the 3-10 keV, 10-20 keV, 20-79 keV, and 3-79 keV bands at the 14.8-hr binary orbital period. All these are entirely consistent with previous X-ray observations below 10 keV. This new hard X-ray observation of PSR J1723-2837 provides strong evidence that the X-rays are from the intrabinary shock via an interaction between the pulsar wind and the...

9. #### Cosmic Equilibration: A Holographic No-Hair Theorem from the Generalized Second Law

Carroll, Sean M.; Chatwin-Davies, Aidan
In a wide class of cosmological models, a positive cosmological constant drives cosmological evolution toward an asymptotically de Sitter phase. Here we connect this behavior to the increase of entropy over time, based on the idea that de Sitter space is a maximum-entropy state. We prove a cosmic no-hair theorem for Robertson-Walker and Bianchi I spacetimes by assuming that the generalized entropy of a Q-screen ("quantum" holographic screen), in the sense of the cosmological version of the Generalized Second Law conjectured by Bousso and Engelhardt, increases up to a finite maximum value, which we show coincides with the de Sitter horizon entropy. We do not use the Einstein...

10. #### Reliability of Critical Infrastructure Networks: Challenges

Zuev, Konstantin M.; Beer, Michael
Critical infrastructures form a technological skeleton of our world by providing us with water, food, electricity, gas, transportation, communication, banking, and finance. Moreover, as urban population increases, the role of infrastructures become more vital. In this paper, we adopt a network perspective and discuss the ever growing need for fundamental interdisciplinary study of critical infrastructure networks, efficient methods for estimating their reliability, and cost-effective strategies for enhancing their resiliency. We also highlight some of the main challenges arising on this way, including cascading failures, feedback loops, and cross-sector interdependencies.

11. #### Insight Into "Changing-Look" AGN Mrk 1018 from the Fe Kα Line: The Reprocessing Gas Has Yet to Fully Respond to the Fading of the AGN

LaMassa, Stephanie M.; Yaqoob, Tahir; Kilgard, Roy
Mrk 1018 is a “changing-look” AGN whose optical spectrum transitioned from a Type 1.9 to Type 1 between 1979 and 1984 and back to a Type 1.9 in 2015. This latest transition was accompanied by a decrease in X-ray flux. We analyze the Chandra spectra from 2010 and 2016 and NuSTAR spectra from 2016, carefully treating pile-up in the Chandra spectrum from 2010 and self-consistently modeling absorption, reflection, and Fe Kα line emission in the X-ray spectra from 2016. We demonstrate that while the 2-10 keV X-ray flux decreased by an order of magnitude (1.46+0.10 −0.13 × 10−11 erg s−1 cm−2 to 1.31+0.09 −0.04×10−12 erg s−1 cm−2 ), the Fe Kα equivalent...

12. #### The Spin of the Supermassive Black Hole in MCG-05-23-16

Wang, Yu; Ghasemi-Nodehi, M.; Guainazzi, Matteo; Bambi, Cosimo

19. #### Experimental metrology to obtain thermal phonon transmission coefficients at solid interfaces

Hua, Chengyun; Chen, Xiangwen; Ravichandran, Navaneetha K.; Minnich, Austin J.
Interfaces play an essential role in phonon-mediated heat conduction in solids, impacting applications ranging from thermoelectric waste heat recovery to heat dissipation in electronics. From the microscopic perspective, interfacial phonon transport is described by transmission coefficients that link vibrational modes in the materials composing the interface. However, direct experimental determination of these coefficients is challenging because most experiments provide a mode-averaged interface conductance that obscures the microscopic detail. Here, we report a metrology to extract thermal phonon transmission coefficients at solid interfaces using ab-initio phonon transport modeling and a thermal characterization technique, time-domain thermoreflectance. In combination with transmission electron microscopy characterization...

20. #### Quantum phases of disordered three-dimensional Majorana-Weyl fermions

Wilson, Justin H.; Pixley, J. H.; Goswami, Pallab; Das Sarma, S.
The gapless Bogoliubov-de Gennes (BdG) quasiparticles of a clean three dimensional spinless p_x + ip_y superconductor provide an intriguing example of a thermal Hall semimetal (ThSM) phase of Majorana-Weyl fermions; such a phase can support a large anomalous thermal Hall conductivity and protected surface Majorana-Fermi arcs at zero energy. We study the effects of quenched disorder on such a gapless topological phase by carrying out extensive numerical and analytical calculations on a lattice model for a disordered, spinless p_x + ip_y superconductor. Using the kernel polynomial method, we compute both average and typical density of states for the BdG quasiparticles,...

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