# Dimitrios Psaltis - Steward Observatory and Department of Astronomy, University of Arizona

## Contact Details

NameDimitrios Psaltis |
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AffiliationSteward Observatory and Department of Astronomy, University of Arizona |
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CitySun City |
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CountryUnited States |
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## Pubs By Year |
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## External Links |
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## Pub CategoriesHigh Energy Astrophysical Phenomena (43) General Relativity and Quantum Cosmology (19) Instrumentation and Methods for Astrophysics (8) Nuclear Theory (7) Astrophysics of Galaxies (7) Cosmology and Nongalactic Astrophysics (5) Solar and Stellar Astrophysics (5) Earth and Planetary Astrophysics (2) High Energy Physics - Theory (1) |

## Publications Authored By Dimitrios Psaltis

The observable quantities that carry the most information regarding the structures of the images of black holes in the interferometric observations with the Event Horizon Telescope are the closure phases along different baseline triangles. We use long time span, high cadence, GRMHD+radiative transfer models of Sgr A$^*$ to investigate the expected variability of closure phases in such observations. We find that, in general, closure phases along small baseline triangles show little variability, except in the cases when one of the triangle vertices crosses one of a small regions of low visibility amplitude. Read More

The Event Horizon Telescope is a global very-long baseline interferometer capable of probing potential deviations from the Kerr metric, which is believed to provide the unique description of astrophysical black holes. Here we report an updated constraint on the quadrupolar deviation of Sagittarius A* within the context of a radiatively inefficient accretion flow model in a quasi-Kerr background. We also simulate near-future constraints obtainable by the forthcoming eight-station array and show that in this model already a one-day observation can measure the spin magnitude to within $0. Read More

The need for a consistent quantum evolution for black holes has led to proposals that their semiclassical description is modified not just near the singularity, but at horizon or larger scales. If such modifications extend beyond the horizon, they influence regions accessible to distant observeration. Natural candidates for these modifications behave like metric fluctuations, with characteristic length and time scales set by the horizon radius. Read More

The increasing number and precision of measurements of neutron star masses, radii, and, in the near future, moments of inertia offer the possibility of precisely determining the neutron star equation of state. One way to facilitate the mapping of observables to the equation of state is through a parametrization of the latter. We present here a generic method for optimizing the parametrization of any physically allowed EoS. Read More

A precise moment of inertia measurement for PSR J0737-3039A in the double pulsar system is expected within the next five years. We present here a new method of mapping the anticipated measurement of the moment of inertia directly into the neutron star structure. We determine the maximum and minimum values possible for the moment of inertia of a neutron star of a given radius based on physical stability arguments, assuming knowledge of the equation of state only at densities below the nuclear saturation density. Read More

Significant X-ray variability and flaring has been observed from Sgr A* but is poorly understood from a theoretical standpoint. We perform GRMHD simulations that take into account a population of non-thermal electrons with energy distributions and injection rates that are motivated by PIC simulations of magnetic reconnection. We explore the effects of including these non-thermal electrons on the predicted broadband variability of Sgr A* and find that X-ray variability is a generic result of localizing non-thermal electrons to highly magnetized regions, where particles are likely to be accelerated via magnetic reconnection. Read More

**Authors:**Vincent L. Fish, Michael D. Johnson, Sheperd S. Doeleman, Avery E. Broderick, Dimitrios Psaltis, Ru-Sen Lu, Kazunori Akiyama, Walter Alef, Juan Carlos Algaba, Keiichi Asada, Christopher Beaudoin, Alessandra Bertarini, Lindy Blackburn, Ray Blundell, Geoffrey C. Bower, Christiaan Brinkerink, Roger Cappallo, Andrew A. Chael, Richard Chamberlin, Chi-Kwan Chan, Geoffrey B. Crew, Jason Dexter, Matt Dexter, Sergio A. Dzib, Heino Falcke, Robert Freund, Per Friberg, Christopher H. Greer, Mark A. Gurwell, Paul T. P. Ho, Mareki Honma, Makoto Inoue, Tim Johannsen, Junhan Kim, Thomas P. Krichbaum, James Lamb, Jonathan León-Tavares, Abraham Loeb, Laurent Loinard, David MacMahon, Daniel P. Marrone, James M. Moran, Monika Mościbrodzka, Gisela N. Ortiz-León, Tomoaki Oyama, Feryal Özel, Richard L. Plambeck, Nicolas Pradel, Rurik A. Primiani, Alan E. E. Rogers, Katherine Rosenfeld, Helge Rottmann, Alan L. Roy, Chester Ruszczyk, Daniel L. Smythe, Jason SooHoo, Justin Spilker, Jordan Stone, Peter Strittmatter, Remo P. J. Tilanus, Michael Titus, Laura Vertatschitsch, Jan Wagner, John F. C. Wardle, Jonathan Weintroub, David Woody, Melvyn Wright, Paul Yamaguchi, André Young, Ken H. Young, J. Anton Zensus, Lucy M. Ziurys

The Galactic Center black hole Sagittarius A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Read More

**Authors:**Anna L. Watts, Nils Andersson, Deepto Chakrabarty, Marco Feroci, Kai Hebeler, Gianluca Israel, Frederick K. Lamb, M. Coleman Miller, Sharon Morsink, Feryal Özel, Alessandro Patruno, Juri Poutanen, Dimitrios Psaltis, Achim Schwenk, Andrew W. Steiner, Luigi Stella, Laura Tolos, Michiel van der Klis

One of the primary science goals of the next generation of hard X-ray timing instruments is to determine the equation of state of the matter at supranuclear densities inside neutron stars, by measuring the radius of neutron stars with different masses to accuracies of a few percent. Three main techniques can be used to achieve this goal. The first involves waveform modelling. Read More

The Event Horizon Telescope (EHT) is a millimeter-wavelength, very-long-baseline interferometry (VLBI) experiment that is capable of observing black holes with horizon-scale resolution. Early observations have revealed variable horizon-scale emission in the Galactic Center black hole, Sagittarius A* (Sgr A*). Comparing such observations to time-dependent general relativistic magnetohydrodynamic (GRMHD) simulations requires statistical tools that explicitly consider the variability in both the data and the models. Read More

Synthesis imaging of the black hole in the center of the Milky Way, Sgr A*, with the Event Horizon Telescope (EHT) rests on the assumption of a stationary image. We explore the limitations of this assumption using high-cadence GRMHD simulations of Sgr A*. We employ analytic models that capture the basic characteristics of the images to understand the origin of the variability in the simulated visibility amplitudes. Read More

The Neutron-star Interior Composition Explorer (NICER) is an X-ray astrophysics payload that will be placed on the International Space Station. Its primary science goal is to measure with high accuracy the pulse profiles that arise from the non-uniform thermal surface emission of rotation-powered pulsars. Modeling general relativistic effects on the profiles will lead to measuring the radii of these neutron stars and to constraining their equation of state. Read More

**Authors:**Michael D. Johnson, Vincent L. Fish, Sheperd S. Doeleman, Daniel P. Marrone, Richard L. Plambeck, John F. C. Wardle, Kazunori Akiyama, Keiichi Asada, Christopher Beaudoin, Lindy Blackburn, Ray Blundell, Geoffrey C. Bower, Christiaan Brinkerink, Avery E. Broderick, Roger Cappallo, Andrew A. Chael, Geoffrey B. Crew, Jason Dexter, Matt Dexter, Robert Freund, Per Friberg, Roman Gold, Mark A. Gurwell, Paul T. P. Ho, Mareki Honma, Makoto Inoue, Michael Kosowsky, Thomas P. Krichbaum, James Lamb, Abraham Loeb, Ru-Sen Lu, David MacMahon, Jonathan C. McKinney, James M. Moran, Ramesh Narayan, Rurik A. Primiani, Dimitrios Psaltis, Alan E. E. Rogers, Katherine Rosenfeld, Jason SooHoo, Remo P. J. Tilanus, Michael Titus, Laura Vertatschitsch, Jonathan Weintroub, Melvyn Wright, Ken H. Young, J. Anton Zensus, Lucy M. Ziurys

Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1. Read More

One avenue for testing the no-hair theorem is obtained through timing a pulsar orbiting close to a black hole and fitting for quadrupolar effects on the time-of-arrival of pulses. If deviations from the Kerr quadrupole are measured, then the no-hair theorem is invalidated. To this end, we derive an expression for the light travel time delay for a pulsar orbiting in a black-hole spacetime described by the Butterworth-Ipser metric, which has an arbitrary spin and quadrupole moment. Read More

**Authors:**Henric S. Krawczynski

^{1}, Daniel Stern

^{2}, Fiona A. Harrison

^{3}, Fabian F. Kislat

^{4}, Anna Zajczyk

^{5}, Matthias Beilicke

^{6}, Janie Hoormann

^{7}, Qingzhen Guo

^{8}, Ryan Endsley

^{9}, Adam R. Ingram

^{10}, Hiromasa Miyasaka

^{11}, Kristin K. Madsen

^{12}, Kim M. Aaron

^{13}, Rashied Aminia

^{14}, Matthew G. Baring

^{15}, Banafsheh Beheshtipour

^{16}, Arash Bodaghee

^{17}, Jeffrey Booth

^{18}, Chester Borden

^{19}, Markus Boettcher

^{20}, Finn E. Christensen

^{21}, Paolo S. Coppi

^{22}, Ramanath Cowsik

^{23}, Shane Davis

^{24}, Jason Dexter

^{25}, Chris Done

^{26}, Luis A. Dominguez

^{27}, Don Ellison

^{28}, Robin J. English

^{29}, Andrew C. Fabian

^{30}, Abe Falcone

^{31}, Jeffrey A. Favretto

^{32}, Rodrigo Fernandez

^{33}, Paolo Giommi

^{34}, Brian W. Grefenstette

^{35}, Erin Kara

^{36}, Chung H. Lee

^{37}, Maxim Lyutikov

^{38}, Thomas Maccarone

^{39}, Hironori Matsumoto

^{40}, Jonathan McKinney

^{41}, Tatehiro Mihara

^{42}, Jon M. Miller

^{43}, Ramesh Narayan

^{44}, Lorenzo Natalucci

^{45}, Feryal Oezel

^{46}, Michael J. Pivovaroff

^{47}, Steven Pravdo

^{48}, Dimitrios Psaltis

^{49}, Takashi Okajima

^{50}, Kenji Toma

^{51}, William W. Zhang

^{52}

**Affiliations:**

^{1}Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences,

^{2}Jet Propulsion Laboratory,

^{3}California Institute of Technology, Cahill Center for Astronomy and Astrophysics,

^{4}Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences,

^{5}Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences,

^{6}Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences,

^{7}Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences,

^{8}Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences,

^{9}Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences,

^{10}Anton Pannekoek Institute for Astronomy,

^{11}California Institute of Technology, Cahill Center for Astronomy and Astrophysics,

^{12}California Institute of Technology, Cahill Center for Astronomy and Astrophysics,

^{13}Jet Propulsion Laboratory,

^{14}Jet Propulsion Laboratory,

^{15}Rice University, Department of Physics and Astronomy,

^{16}Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences,

^{17}Georgia College, Department of Chemistry, Physics, and Astronomy,

^{18}Jet Propulsion Laboratory,

^{19}Jet Propulsion Laboratory,

^{20}North-West University, Centre for Space Research,

^{21}Technical University of Denmark, DTU Space, National Space Institute,

^{22}Yale University, Department of Astronomy,

^{23}Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences,

^{24}University of Virginia, Department of Astronomy,

^{25}MPI for Extraterrestrial Physics Garching,

^{26}Durham University, Centre for Extragalactic Astronomy, Department of Physics,

^{27}Jet Propulsion Laboratory,

^{28}North Carolina State University, Department of Physics,

^{29}Jet Propulsion Laboratory,

^{30}Cambridge, Institute of Astronomy, UK,

^{31}Penn State University, Department of Astronomy and Astrophysics,

^{32}Jet Propulsion Laboratory,

^{33}University of California, Berkeley, Department of Physics,

^{34}ASI Science Data Center, Italy,

^{35}California Institute of Technology, Cahill Center for Astronomy and Astrophysics,

^{36}Cambridge, Institute of Astronomy, UK,

^{37}Jet Propulsion Laboratory,

^{38}Purdue University, Department of Physics and Astronomy,

^{39}Texas Tech University, Physics Department,

^{40}Nagoya University, Center for Experimental Studies, Kobayashi-Maskawa Institute for the Origin of Particles and the Universe,

^{41}University of Maryland, Physics Department,

^{42}RIKEN,

^{43}Univ. of Michigan in Ann Arbor, Astronomy Dept,

^{44}Harvard-Smithsonian Center for Astrophysics,

^{45}Istituto di Astrofisica e Planetologia Spaziali, INAF,

^{46}Department of Astronomy/Steward Observatory,

^{47}Lawrence Livermore National Laboratory,

^{48}Jet Propulsion Laboratory,

^{49}Department of Astronomy/Steward Observatory,

^{50}NASA Goddard Space Flight Center,

^{51}Tohoku University, Astronomical Institute,

^{52}NASA Goddard Space Flight Center

This paper describes the Polarization Spectroscopic Telescope Array (PolSTAR), a mission proposed to NASA's 2014 Small Explorer (SMEX) announcement of opportunity. PolSTAR measures the linear polarization of 3-50 keV (requirement; goal: 2.5-70 keV) X-rays probing the behavior of matter, radiation and the very fabric of spacetime under the extreme conditions close to the event horizons of black holes, as well as in and around magnetars and neutron stars. Read More

We describe HEROIC, an upgraded version of the relativistic radiative post-processor code HERO described in a previous paper, but which now Includes Comptonization. HEROIC models Comptonization via the Kompaneets equation, using a quadratic approximation for the source function in the short characteristics radiation solver. It employs a simple form of accelerated lambda iteration to handle regions of high scattering opacity. Read More

**Affiliations:**

^{1}Arizona,

^{2}MPIfR,

^{3}MPIfR

The black hole in the center of the Milky Way, Sgr A*, has the largest mass-to-distance ratio among all known black holes in the Universe. This property makes Sgr A* the optimal target for testing the gravitational no-hair theorem. In the near future, major developments in instrumentation will provide the tools for high-precision studies of its spacetime via observations of relativistic effects in stellar orbits, in the timing of pulsars, and in horizon-scale images of its accretion flow. Read More

Data selection and the determination of systematic uncertainties in the spectroscopic measurements of neutron star radii from thermonuclear X-ray bursts have been the subject of numerous recent studies. In one approach, the uncertainties and outliers were determined by a data-driven Bayesian mixture model, whereas in a second approach, data selection was performed by requiring that the observations follow theoretical expectations. We show here that, due to inherent limitations in the data, the theoretically expected trends are not discernible in the majority of X-ray bursts even if they are present. Read More

We present a comprehensive study of spectroscopic radius measurements of twelve neutron stars obtained during thermonuclear bursts or in quiescence. We incorporate, for the first time, a large number of systematic uncertainties in the measurement of the apparent angular sizes, Eddington fluxes, and distances, in the composition of the interstellar medium, and in the flux calibration of X-ray detectors. We also take into account the results of recent theoretical calculations of rotational effects on neutron star radii, of atmospheric effects on surface spectra, and of relativistic corrections to the Eddington critical flux. Read More

Measuring neutron star radii with spectroscopic and timing techniques relies on the combination of multiple observables to break the degeneracies between the mass and radius introduced by general relativistic effects. Here, we explore a previously used frequentist and a newly proposed Bayesian framework to obtain the most likely value and the uncertainty in such a measurement. We find that, for the expected range of masses and radii and for realistic measurement errors, the frequentist approach suffers from biases that are larger than the accuracy in the radius measurement required to distinguish between the different equations of state. Read More

HERO (Hybrid Evaluator for Radiative Objects) is a 3D general relativistic radiative transfer code which has been tailored to the problem of analyzing radiation from simulations of relativistic accretion discs around black holes. HERO is designed to be used as a postprocessor. Given some fixed fluid structure for the disc (i. Read More

**Authors:**Chi-kwan Chan

^{1}, Dimitrios Psaltis

^{2}, Feryal Ozel

^{3}, Lia Medeiros

^{4}, Daniel Marrone

^{5}, Aleksander Sadowski

^{6}, Ramesh Narayan

^{7}

**Affiliations:**

^{1}Steward Observatory and Department of Astronomy, University of Arizona,

^{2}Steward Observatory and Department of Astronomy, University of Arizona,

^{3}Steward Observatory and Department of Astronomy, University of Arizona,

^{4}Department of Physics, UC Santa Barbara,

^{5}Steward Observatory and Department of Astronomy, University of Arizona,

^{6}MIT Kavli Institute for Astrophysics and Space Research,

^{7}Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics

**Category:**High Energy Astrophysical Phenomena

We explore the variability properties of long, high cadence GRMHD simulations across the electromagnetic spectrum using an efficient, GPU-based radiative transfer algorithm. We focus on both disk- and jet-dominated simulations with parameters that successfully reproduce the time-averaged spectral properties of Sgr A* and the size of its image at 1.3mm. Read More

We calculate the effects of spot size on pulse profiles of moderately rotating neutron stars. Specifically, we quantify the bias introduced in radius measurements from the common assumption that spots are infinitesimally small. We find that this assumption is reasonable for spots smaller than 10$^\circ$-18$^\circ$ and leads to errors that are $\le$10% in the radius measurement, depending on the location of the spot and the inclination of the observer. Read More

Many techniques for measuring neutron star radii rely on absolute flux measurements in the X-rays. As a result, one of the fundamental uncertainties in these spectroscopic measurements arises from the absolute flux calibrations of the detectors being used. Using the stable X-ray burster, GS 1826-238, and its simultaneous observations by Chandra HETG/ACIS-S and RXTE/PCA as well as by XMM-Newton EPIC-pn and RXTE/PCA, we quantify the degree of uncertainty in the flux calibration by assessing the differences between the measured fluxes during bursts. Read More

The current effort to test General Relativity employs multiple disparate formalisms for different observables, obscuring the relations between laboratory, astrophysical and cosmological constraints. To remedy this situation, we develop a parameter space for comparing tests of gravity on all scales in the universe. In particular, we present new methods for linking cosmological large-scale structure, the Cosmic Microwave Background and gravitational waves with classic PPN tests of gravity. Read More

The half opening angle of a Kerr black-hole shadow is always equal to (5+-0.2)GM/Dc^2, where M is the mass of the black hole and D is its distance from the Earth. Therefore, measuring the size of a shadow and verifying whether it is within this 4% range constitutes a null hypothesis test of General Relativity. Read More

**Authors:**Chi-Kwan Chan

^{1}, Dimitrios Psaltis

^{2}, Feryal Ozel

^{3}, Ramesh Narayan

^{4}, Aleksander Sadowski

^{5}

**Affiliations:**

^{1}Steward Observatory and Department of Astronomy, University of Arizona,

^{2}Steward Observatory and Department of Astronomy, University of Arizona,

^{3}Steward Observatory and Department of Astronomy, University of Arizona,

^{4}Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics,

^{5}MIT Kavli Institute for Astrophysics and Space Research

**Category:**High Energy Astrophysical Phenomena

Recent advances in general relativistic magnetohydrodynamic simulations have expanded and improved our understanding of the dynamics of black-hole accretion disks. However, current simulations do not capture the thermodynamics of electrons in the low density accreting plasma. This poses a significant challenge in predicting accretion flow images and spectra from first principles. Read More

Observations of the black hole in the center of the Milky Way with the Event Horizon Telescope at 1.3 mm have revealed a size of the emitting region that is smaller than the size of the black-hole shadow. This can be reconciled with the spectral properties of the source, if the accretion flow is seen at a relatively high inclination (50-60 degrees). Read More

The image of the emission surrounding the black hole in the center of the Milky Way is predicted to exhibit the imprint of general relativistic (GR) effects, including the existence of a shadow feature and a photon ring of diameter ~50 microarcseconds. Structure on these scales can be resolved by millimeter-wavelength very long baseline interferometry (VLBI). However, strong-field GR features of interest will be blurred at lambda >= 1. Read More

We calculate the rotational broadening in the observed thermal spectra of neutron stars spinning at moderate rates in the Hartle-Thorne approximation. These calculations accurately account for the effects of the second-order Doppler boosts as well as for the oblate shapes and the quadrupole moments of the neutron stars. We find that fitting the spectra and inferring the bolometric fluxes under the assumption that a star is not rotating causes an underestimate of the inferred fluxes and, thus, radii. Read More

**Authors:**Deepto Chakrabarty

^{1}, John A. Tomsick

^{2}, Brian W. Grefenstette

^{3}, Dimitrios Psaltis

^{4}, Matteo Bachetti

^{5}, Didier Barret

^{6}, Steven E. Boggs

^{7}, Finn E. Christensen

^{8}, William W. Craig

^{9}, Felix Fuerst

^{10}, Charles J. Hailey

^{11}, Fiona A. Harrison

^{12}, Victoria A. Kaspi

^{13}, Jon M. Miller

^{14}, Michael A. Nowak

^{15}, Vikram Rana

^{16}, Daniel Stern

^{17}, Daniel R. Wik

^{18}, Joern Wilms

^{19}, William W. Zhang

^{20}

**Affiliations:**

^{1}MIT,

^{2}Berkeley,

^{3}Caltech,

^{4}Arizona,

^{5}IRAP/Toulouse,

^{6}IRAP/Toulouse,

^{7}Berkeley,

^{8}DTU Space,

^{9}LLNL/Berkeley,

^{10}Caltech,

^{11}Columbia,

^{12}Caltech,

^{13}McGill,

^{14}Michigan,

^{15}MIT,

^{16}Caltech,

^{17}JPL/Caltech,

^{18}NASA/GSFC,

^{19}Erlangen-Nuremberg,

^{20}NASA/GSFC

**Category:**High Energy Astrophysical Phenomena

The low-mass X-ray binary Cen X-4 is the brightest and closest (<1.2 kpc) quiescent neutron star transient. Previous 0. Read More

Black hole binaries exhibit a wide range of variability phenomena, from large-scale state changes to broadband noise and quasi-periodic oscillations, but the physical nature of much of this variability is poorly understood. We examine the variability properties of three GRMHD simulations of thin accretion disks around black holes of varying spin, producing light curves and power spectra as would be seen by observers. We find that the simulated power spectra show a broad feature at high frequency, which increases in amplitude with the inclination of the observer. Read More

**Affiliations:**

^{1}Perimeter Institute for Theoretical Physics,

^{2}Perimeter Institute for Theoretical Physics,

^{3}Harvard University,

^{4}University of Arizona

The advent of the Event Horizon Telescope (EHT), a millimeter-wave very-long baseline interferometric array, has enabled spatially-resolved studies of the sub-horizon-scale structure for a handful of supermassive black holes. Among these, the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), presents the largest angular cross section. Thus far, these studies have focused upon measurements of the black hole spin and the validation of low-luminosity accretion models. Read More

Modeling the amplitudes and shapes of the X-ray pulsations observed from hot, rotating neutron stars provides a direct method for measuring neutron-star properties. This technique constitutes an important part of the science case for the forthcoming NICER and proposed LOFT X-ray missions. In this paper, we determine the number of distinct observables that can be derived from pulse profile modeling and show that using only bolometric pulse profiles is insufficient for breaking the degeneracy between inferred neutron-star radius and mass. Read More

**Authors:**Vincent Fish, Walter Alef, James Anderson, Keiichi Asada, Alain Baudry, Avery Broderick, Chris Carilli, Francisco Colomer, John Conway, Jason Dexter, Sheperd Doeleman, Ralph Eatough, Heino Falcke, Sándor Frey, Krisztina Gabányi, Roberto Gálvan-Madrid, Charles Gammie, Marcello Giroletti, Ciriaco Goddi, Jose L. Gómez, Kazuhiro Hada, Michael Hecht, Mareki Honma, Elizabeth Humphreys, Violette Impellizzeri, Tim Johannsen, Svetlana Jorstad, Motoki Kino, Elmar Körding, Michael Kramer, Thomas Krichbaum, Nadia Kudryavtseva, Robert Laing, Joseph Lazio, Abraham Loeb, Ru-Sen Lu, Thomas Maccarone, Alan Marscher, Iván Mart'ı-Vidal, Carlos Martins, Lynn Matthews, Karl Menten, Jon Miller, James Miller-Jones, Félix Mirabel, Sebastien Muller, Hiroshi Nagai, Neil Nagar, Masanori Nakamura, Zsolt Paragi, Nicolas Pradel, Dimitrios Psaltis, Scott Ransom, Luis Rodríguez, Helge Rottmann, Anthony Rushton, Zhi-Qiang Shen, David Smith, Benjamin Stappers, Rohta Takahashi, Andrea Tarchi, Remo Tilanus, Joris Verbiest, Wouter Vlemmings, R. Craig Walker, John Wardle, Kaj Wiik, Erik Zackrisson, J. Anton Zensus

An international consortium is presently constructing a beamformer for the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile that will be available as a facility instrument. The beamformer will aggregate the entire collecting area of the array into a single, very large aperture. The extraordinary sensitivity of phased ALMA, combined with the extremely fine angular resolution available on baselines to the Northern Hemisphere, will enable transformational new very long baseline interferometry (VLBI) observations in Bands 6 and 7 (1. Read More

**Authors:**Kirpal Nandra

^{1}, Didier Barret

^{2}, Xavier Barcons

^{3}, Andy Fabian

^{4}, Jan-Willem den Herder

^{5}, Luigi Piro

^{6}, Mike Watson

^{7}, Christophe Adami

^{8}, James Aird

^{9}, Jose Manuel Afonso

^{10}, Dave Alexander

^{11}, Costanza Argiroffi

^{12}, Lorenzo Amati

^{13}, Monique Arnaud

^{14}, Jean-Luc Atteia

^{15}, Marc Audard

^{16}, Carles Badenes

^{17}, Jean Ballet

^{18}, Lucia Ballo

^{19}, Aya Bamba

^{20}, Anil Bhardwaj

^{21}, Elia Stefano Battistelli

^{22}, Werner Becker

^{23}, Michaël De Becker

^{24}, Ehud Behar

^{25}, Stefano Bianchi

^{26}, Veronica Biffi

^{27}, Laura Bîrzan

^{28}, Fabrizio Bocchino

^{29}, Slavko Bogdanov

^{30}, Laurence Boirin

^{31}, Thomas Boller

^{32}, Stefano Borgani

^{33}, Katharina Borm

^{34}, Nicolas Bouché

^{35}, Hervé Bourdin

^{36}, Richard Bower

^{37}, Valentina Braito

^{38}, Enzo Branchini

^{39}, Graziella Branduardi-Raymont

^{40}, Joel Bregman

^{41}, Laura Brenneman

^{42}, Murray Brightman

^{43}, Marcus Brüggen

^{44}, Johannes Buchner

^{45}, Esra Bulbul

^{46}, Marcella Brusa

^{47}, Michal Bursa

^{48}, Alessandro Caccianiga

^{49}, Ed Cackett

^{50}, Sergio Campana

^{51}, Nico Cappelluti

^{52}, Massimo Cappi

^{53}, Francisco Carrera

^{54}, Maite Ceballos

^{55}, Finn Christensen

^{56}, You-Hua Chu

^{57}, Eugene Churazov

^{58}, Nicolas Clerc

^{59}, Stephane Corbel

^{60}, Amalia Corral

^{61}, Andrea Comastri

^{62}, Elisa Costantini

^{63}, Judith Croston

^{64}, Mauro Dadina

^{65}, Antonino D'Ai

^{66}, Anne Decourchelle

^{67}, Roberto Della Ceca

^{68}, Konrad Dennerl

^{69}, Klaus Dolag

^{70}, Chris Done

^{71}, Michal Dovciak

^{72}, Jeremy Drake

^{73}, Dominique Eckert

^{74}, Alastair Edge

^{75}, Stefano Ettori

^{76}, Yuichiro Ezoe

^{77}, Eric Feigelson

^{78}, Rob Fender

^{79}, Chiara Feruglio

^{80}, Alexis Finoguenov

^{81}, Fabrizio Fiore

^{82}, Massimiliano Galeazzi

^{83}, Sarah Gallagher

^{84}, Poshak Gandhi

^{85}, Massimo Gaspari

^{86}, Fabio Gastaldello

^{87}, Antonis Georgakakis

^{88}, Ioannis Georgantopoulos

^{89}, Marat Gilfanov

^{90}, Myriam Gitti

^{91}, Randy Gladstone

^{92}, Rene Goosmann

^{93}, Eric Gosset

^{94}, Nicolas Grosso

^{95}, Manuel Guedel

^{96}, Martin Guerrero

^{97}, Frank Haberl

^{98}, Martin Hardcastle

^{99}, Sebastian Heinz

^{100}, Almudena Alonso Herrero

^{101}, Anthony Hervé

^{102}, Mats Holmstrom

^{103}, Kazushi Iwasawa

^{104}, Peter Jonker

^{105}, Jelle Kaastra

^{106}, Erin Kara

^{107}, Vladimir Karas

^{108}, Joel Kastner

^{109}, Andrew King

^{110}, Daria Kosenko

^{111}, Dimita Koutroumpa

^{112}, Ralph Kraft

^{113}, Ingo Kreykenbohm

^{114}, Rosine Lallement

^{115}, Giorgio Lanzuisi

^{116}, J. Lee

^{117}, Marianne Lemoine-Goumard

^{118}, Andrew Lobban

^{119}, Giuseppe Lodato

^{120}, Lorenzo Lovisari

^{121}, Simone Lotti

^{122}, Ian McCharthy

^{123}, Brian McNamara

^{124}, Antonio Maggio

^{125}, Roberto Maiolino

^{126}, Barbara De Marco

^{127}, Domitilla de Martino

^{128}, Silvia Mateos

^{129}, Giorgio Matt

^{130}, Ben Maughan

^{131}, Pasquale Mazzotta

^{132}, Mariano Mendez

^{133}, Andrea Merloni

^{134}, Giuseppina Micela

^{135}, Marco Miceli

^{136}, Robert Mignani

^{137}, Jon Miller

^{138}, Giovanni Miniutti

^{139}, Silvano Molendi

^{140}, Rodolfo Montez

^{141}, Alberto Moretti

^{142}, Christian Motch

^{143}, Yaël Nazé

^{144}, Jukka Nevalainen

^{145}, Fabrizio Nicastro

^{146}, Paul Nulsen

^{147}, Takaya Ohashi

^{148}, Paul O'Brien

^{149}, Julian Osborne

^{150}, Lida Oskinova

^{151}, Florian Pacaud

^{152}, Frederik Paerels

^{153}, Mat Page

^{154}, Iossif Papadakis

^{155}, Giovanni Pareschi

^{156}, Robert Petre

^{157}, Pierre-Olivier Petrucci

^{158}, Enrico Piconcelli

^{159}, Ignazio Pillitteri

^{160}, C. Pinto

^{161}, Jelle de Plaa

^{162}, Etienne Pointecouteau

^{163}, Trevor Ponman

^{164}, Gabriele Ponti

^{165}, Delphine Porquet

^{166}, Ken Pounds

^{167}, Gabriel Pratt

^{168}, Peter Predehl

^{169}, Daniel Proga

^{170}, Dimitrios Psaltis

^{171}, David Rafferty

^{172}, Miriam Ramos-Ceja

^{173}, Piero Ranalli

^{174}, Elena Rasia

^{175}, Arne Rau

^{176}, Gregor Rauw

^{177}, Nanda Rea

^{178}, Andy Read

^{179}, James Reeves

^{180}, Thomas Reiprich

^{181}, Matthieu Renaud

^{182}, Chris Reynolds

^{183}, Guido Risaliti

^{184}, Jerome Rodriguez

^{185}, Paola Rodriguez Hidalgo

^{186}, Mauro Roncarelli

^{187}, David Rosario

^{188}, Mariachiara Rossetti

^{189}, Agata Rozanska

^{190}, Emmanouil Rovilos

^{191}, Ruben Salvaterra

^{192}, Mara Salvato

^{193}, Tiziana Di Salvo

^{194}, Jeremy Sanders

^{195}, Jorge Sanz-Forcada

^{196}, Kevin Schawinski

^{197}, Joop Schaye

^{198}, Axel Schwope

^{199}, Salvatore Sciortino

^{200}, Paola Severgnini

^{201}, Francesco Shankar

^{202}, Debora Sijacki

^{203}, Stuart Sim

^{204}, Christian Schmid

^{205}, Randall Smith

^{206}, Andrew Steiner

^{207}, Beate Stelzer

^{208}, Gordon Stewart

^{209}, Tod Strohmayer

^{210}, Lothar Strüder

^{211}, Ming Sun

^{212}, Yoh Takei

^{213}, V. Tatischeff

^{214}, Andreas Tiengo

^{215}, Francesco Tombesi

^{216}, Ginevra Trinchieri

^{217}, T. G. Tsuru

^{218}, Asif Ud-Doula

^{219}, Eugenio Ursino

^{220}, Lynne Valencic

^{221}, Eros Vanzella

^{222}, Simon Vaughan

^{223}, Cristian Vignali

^{224}, Jacco Vink

^{225}, Fabio Vito

^{226}, Marta Volonteri

^{227}, Daniel Wang

^{228}, Natalie Webb

^{229}, Richard Willingale

^{230}, Joern Wilms

^{231}, Michael Wise

^{232}, Diana Worrall

^{233}, Andrew Young

^{234}, Luca Zampieri

^{235}, Jean In't Zand

^{236}, Silvia Zane

^{237}, Andreas Zezas

^{238}, Yuying Zhang

^{239}, Irina Zhuravleva

^{240}

**Affiliations:**

^{1}DE,

^{2}FR,

^{3}ES,

^{4}UK,

^{5}NL,

^{6}IT,

^{7}UK,

^{8}FR,

^{9}UK,

^{10}PT,

^{11}UK,

^{12}IT,

^{13}IT,

^{14}FR,

^{15}FR,

^{16}CH,

^{17}US,

^{18}FR,

^{19}IT,

^{20}JP,

^{21}IN,

^{22}IT,

^{23}DE,

^{24}BE,

^{25}IL,

^{26}IT,

^{27}IT,

^{28}NL,

^{29}IT,

^{30}US,

^{31}FR,

^{32}DE,

^{33}IT,

^{34}DE,

^{35}FR,

^{36}IT,

^{37}UK,

^{38}IT,

^{39}IT,

^{40}UK,

^{41}US,

^{42}US,

^{43}DE,

^{44}DE,

^{45}DE,

^{46}US,

^{47}IT,

^{48}CZ,

^{49}IT,

^{50}US,

^{51}IT,

^{52}IT,

^{53}IT,

^{54}ES,

^{55}ES,

^{56}DK,

^{57}US,

^{58}DE,

^{59}DE,

^{60}FR,

^{61}GR,

^{62}IT,

^{63}NL,

^{64}UK,

^{65}IT,

^{66}IT,

^{67}FR,

^{68}IT,

^{69}DE,

^{70}DE,

^{71}UK,

^{72}CZ,

^{73}US,

^{74}CH,

^{75}UK,

^{76}IT,

^{77}JP,

^{78}US,

^{79}UK,

^{80}FR,

^{81}FI,

^{82}IT,

^{83}IT,

^{84}CA,

^{85}UK,

^{86}IT,

^{87}IT,

^{88}DE,

^{89}GR,

^{90}DE,

^{91}IT,

^{92}US,

^{93}FR,

^{94}BE,

^{95}FR,

^{96}AT,

^{97}ES,

^{98}DE,

^{99}UK,

^{100}US,

^{101}ES,

^{102}FR,

^{103}SE,

^{104}ES,

^{105}NL,

^{106}NL,

^{107}UK,

^{108}CZ,

^{109}US,

^{110}UK,

^{111}FR,

^{112}FR,

^{113}US,

^{114}D,

^{115}FR,

^{116}GR,

^{117}US,

^{118}FR,

^{119}UK,

^{120}IT,

^{121}DE,

^{122}IT,

^{123}UK,

^{124}CA,

^{125}IT,

^{126}UK,

^{127}DE,

^{128}IT,

^{129}ES,

^{130}IT,

^{131}UK,

^{132}IT,

^{133}NL,

^{134}DE,

^{135}IT,

^{136}IT,

^{137}IT,

^{138}US,

^{139}ES,

^{140}IT,

^{141}ES,

^{142}IT,

^{143}FR,

^{144}BE,

^{145}FI,

^{146}IT,

^{147}US,

^{148}JP,

^{149}UK,

^{150}UK,

^{151}DE,

^{152}DE,

^{153}US,

^{154}UK,

^{155}GR,

^{156}IT,

^{157}US,

^{158}FR,

^{159}IT,

^{160}IT,

^{161}UK,

^{162}NL,

^{163}FR,

^{164}UK,

^{165}DE,

^{166}FR,

^{167}UK,

^{168}FR,

^{169}DE,

^{170}US,

^{171}US,

^{172}NL,

^{173}DE,

^{174}IT,

^{175}US,

^{176}DE,

^{177}BE,

^{178}IT,

^{179}UK,

^{180}UK,

^{181}DE,

^{182}FR,

^{183}US,

^{184}IT,

^{185}FR,

^{186}CA,

^{187}IT,

^{188}DE,

^{189}IT,

^{190}PL,

^{191}UK,

^{192}IT,

^{193}DE,

^{194}IT,

^{195}DE,

^{196}ES,

^{197}CH,

^{198}NL,

^{199}D,

^{200}IT,

^{201}IT,

^{202}FR,

^{203}UK,

^{204}IE,

^{205}DE,

^{206}US,

^{207}US,

^{208}IT,

^{209}UK,

^{210}US,

^{211}DE,

^{212}US,

^{213}JP,

^{214}FR,

^{215}IT,

^{216}US,

^{217}IT,

^{218}JP,

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^{220}NL,

^{221}US,

^{222}IT,

^{223}UK,

^{224}IT,

^{225}NL,

^{226}IT,

^{227}FR,

^{228}US,

^{229}FR,

^{230}UK,

^{231}DE,

^{232}NL,

^{233}UK,

^{234}UK,

^{235}IT,

^{236}NL,

^{237}UK,

^{238}GR,

^{239}DE,

^{240}US

This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Read More

**Authors:**Christian Motch, Jörn Wilms, Didier Barret, Werner Becker, Slavko Bogdanov, Laurence Boirin, Stéphane Corbel, Ed Cackett, Sergio Campana, Domitilla de Martino, Frank Haberl, Jean in't Zand, Mariano Méndez, Roberto Mignani, Jon Miller, Marina Orio, Dimitrios Psaltis, Nanda Rea, Jérôme Rodriguez, Agata Rozanska, Axel Schwope, Andrew Steiner, Natalie Webb, Luca Zampieri, Silvia Zane

**Category:**High Energy Astrophysical Phenomena

White dwarfs, neutron stars and stellar mass black holes are key laboratories to study matter in most extreme conditions of gravity and magnetic field. The unprecedented effective area of Athena+ will allow us to advance our understanding of emission mechanisms and accretion physics over a wide range of mass accretion rates, starting from lower and sub-luminous quiescent X-ray binaries up to super-Eddington ultra-luminous sources. Athena+ will measure stellar black hole spins in a much higher number of binaries than achievable now, opening the possibility to study how spin varies with black hole history. Read More

Using stellar structure calculations in the Hartle-Thorne approximation, we derive analytic expressions connecting the ellipticity of the stellar surface to the compactness, the spin angular momentum, and the quadrupole moment of the spacetime. We also obtain empirical relations between the compactness, the spin angular momentum, and the spacetime quadrupole. Our formulae reproduce the results of numerical calculations to within a few percent and help reduce the number of parameters necessary to model the observational appearance of moderately spinning neutron stars. Read More

**Affiliations:**

^{1}Arizona,

^{2}Arizona

**Category:**High Energy Astrophysical Phenomena

We present a new numerical algorithm for the calculation of pulse profiles from spinning neutron stars in the Hartle-Thorne approximation. Our approach allows us to formally take into account the effects of Doppler shifts and aberration, of frame dragging, as well as of the oblateness of the stellar surface and of its quadrupole moment. We confirm an earlier result that neglecting the oblateness of the neutron-star surface leads to ~5-30% errors in the calculated profiles and further show that neglecting the quadrupole moment of its spacetime leads to ~1-5% errors at a spin frequency of 600 Hz. Read More

**Affiliations:**

^{1}Department of Astronomy, University of Arizona,

^{2}Department of Astronomy, University of Arizona,

^{3}Department of Astronomy, University of Arizona

We introduce GRay, a massively parallel integrator designed to trace the trajectories of billions of photons in a curved spacetime. This GPU-based integrator employs the stream processing paradigm, is implemented in CUDA C/C++, and runs on nVidia graphics cards. The peak performance of GRay using single precision floating-point arithmetic on a single GPU exceeds 300 GFLOP (or 1 nanosecond per photon per time step). Read More

**Affiliations:**

^{1}Arizona,

^{2}Harvard,

^{3}Harvard

Monitoring the orbits of stars around Sgr A* offers the possibility of detecting the precession of their orbital planes due to frame dragging, of measuring the spin and quadrupole moment of the black hole, and of testing the no-hair theorem. Here we investigate whether the deviations of stellar orbits from test-particle trajectories due to wind mass loss and tidal dissipation of the orbital energy compromise such measurements. We find that the effects of stellar winds are, in general, negligible. Read More

Neutron stars spinning at moderate rates (~300-600Hz) become oblate in shape and acquire a nonzero quadrupole moment. In this paper, we calculate profiles of atomic features from such neutron stars using a ray-tracing algorithm in the Hartle-Thorne approximation. We show that line profiles acquire cores that are much narrower than the widths expected from pure Doppler effects for a large range of observer inclinations. Read More

According to the no-hair theorem, astrophysical black holes are fully characterized by their masses and spins and are described by the Kerr metric. This theorem can be tested observationally by measuring (at least) three different multipole moments of the spacetimes of black holes. In this paper, we calculate the profiles of fluorescent iron lines emitted from the accretion flows around black hole candidates within a framework that allows us to perform the calculation as a function of its mass and spin as well as of a free parameter that measures potential deviations from the Kerr metric. Read More

The observed flat rotation curves of galaxies require either the presence of dark matter in Newtonian gravitational potentials or a significant modification to the theory of gravity at galactic scales. Detecting relativistic Doppler shifts and gravitational effects in the rotation curves offers a tool for distinguishing between predictions of gravity theories that modify the inertia of particles and those that modify the field equations. These higher-order effects also allow us in principle, to test whether dark matter particles obey the equivalence principle. Read More

**Affiliations:**

^{1}Arizona,

^{2}Arizona,

^{3}Harvard,

^{4}Arizona

We investigate the distribution of neutron star masses in different populations of binaries, employing Bayesian statistical techniques. In particular, we explore the differences in neutron star masses between sources that have experienced distinct evolutionary paths and accretion episodes. We find that the distribution of neutron star masses in non-recycled eclipsing high-mass binaries as well as of slow pulsars, which are all believed to be near their birth masses, has a mean of 1. Read More

**Authors:**Tim Johannsen

^{1}, Dimitrios Psaltis

^{2}, Stefan Gillessen

^{3}, Daniel P. Marrone

^{4}, Feryal Ozel

^{5}, Sheperd S. Doeleman

^{6}, Vincent L. Fish

^{7}

**Affiliations:**

^{1}Arizona,

^{2}Arizona,

^{3}MPE,

^{4}Arizona,

^{5}Arizona,

^{6}MIT Haystack,

^{7}MIT Haystack

Dynamical mass measurements to date have allowed determinations of the mass M and the distance D of a number of nearby supermassive black holes. In the case of Sgr A*, these measurements are limited by a strong correlation between the mass and distance scaling roughly as M ~ D^2. Future very-long baseline interferometric (VLBI) observations will image a bright and narrow ring surrounding the shadow of a supermassive black hole, if its accretion flow is optically thin. Read More

**Affiliations:**

^{1}Arizona

**Category:**High Energy Astrophysical Phenomena

Observations of stars and pulsars orbiting the black hole in the center of the Milky Way offer the potential of measuring not only the mass of the black hole but also its spin and quadrupole moment, thereby providing observational verification of the no-hair theorem. The relativistic effects that will allow us to measure these higher moments of the gravitational field, however, are very small and may be masked by drag forces that stars and pulsars experience orbiting within the hot, tenuous plasma that surrounds the black hole. The properties of this plasma at large distances from the central object have been measured using observations of the extended X-ray emission that surrounds the point source. Read More

We explore the evolution of the mass distribution of dust in collision-dominated debris disks, using the collisional code introduced in our previous paper. We analyze the equilibrium distribution and its dependence on model parameters by evolving over 100 models to 10 Gyr. With our numerical models, we confirm that systems reach collisional equilibrium with a mass distribution that is steeper than the traditional solution by Dohnanyi (1969). Read More

We develop a new numerical algorithm to model collisional cascades in debris disks. Because of the large dynamical range in particle masses, we solve the integro-differential equations describing erosive and catastrophic collisions in a particle-in-a-box approach, while treating the orbital dynamics of the particles in an approximate fashion. We employ a new scheme for describing erosive (cratering) collisions that yields a continuous set of outcomes as a function of colliding masses. Read More

**Affiliations:**

^{1}Arizona,

^{2}Arizona,

^{3}Arizona,

^{4}Arizona

A moderately spinning neutron star acquires an oblate shape and a spacetime with a significant quadrupole moment. These two properties affect its apparent surface area for an observer at infinity, as well as the lightcurve arising from a hot spot on its surface. In this paper, we develop a ray-tracing algorithm to calculate the apparent surface areas of moderately spinning neutron stars making use of the Hartle-Thorne metric. Read More

**Affiliations:**

^{1}Arizona,

^{2}Arizona

According to the no-hair theorem, astrophysical black holes are uniquely characterized by their masses and spins and are described by the Kerr metric. Several parametric deviations from the Kerr metric have been suggested to study observational signatures in both the electromagnetic and gravitational-wave spectra that differ from the expected Kerr signals. Due to the no-hair theorem, however, such spacetimes cannot be regular everywhere outside the event horizons, if they are solutions to the Einstein field equations; they are often characterized by naked singularities or closed time-like loops in the regions of the spacetime that are accessible to an external observer. Read More