2,050 research outputs found
On Generalized Additive Models for Representation of Solar EUV Irradiance
In the context of space weather forecasting, solar EUV irradiance specification is needed on multiple time scales, with associated uncertainty quantification for determining the accuracy of downstream parameters. Empirical models of irradiance often rely on parametric fits between irradiance in several bands and various solar indices. We build upon these empirical models by using Generalized Additive Models (GAMs) to represent solar irradiance. We apply the GAM approach in two steps: (a) A GAM is fitted between FISM2 irradiance and solar indices F10.7, Revised Sunspot Number, and the Lyman-α solar index. (b) A second GAM is fit to model the residuals of the first GAM with respect to FISM2 irradiance. We evaluate the performance of this approach during Solar Cycle 24 using GAMs driven by known solar indices as well as those forecasted 3 days ahead with an autoregressive modeling approach. We demonstrate negligible dependence of performance on solar cycle and season, and we assess the efficacy of the GAM approach across different wavelengths
Eigenvalue Estimation of Differential Operators
We demonstrate how linear differential operators could be emulated by a
quantum processor, should one ever be built, using the Abrams-Lloyd algorithm.
Given a linear differential operator of order 2S, acting on functions
psi(x_1,x_2,...,x_D) with D arguments, the computational cost required to
estimate a low order eigenvalue to accuracy Theta(1/N^2) is
Theta((2(S+1)(1+1/nu)+D)log N) qubits and O(N^{2(S+1)(1+1/nu)} (D log N)^c)
gate operations, where N is the number of points to which each argument is
discretized, nu and c are implementation dependent constants of O(1). Optimal
classical methods require Theta(N^D) bits and Omega(N^D) gate operations to
perform the same eigenvalue estimation. The Abrams-Lloyd algorithm thereby
leads to exponential reduction in memory and polynomial reduction in gate
operations, provided the domain has sufficiently large dimension D >
2(S+1)(1+1/nu). In the case of Schrodinger's equation, ground state energy
estimation of two or more particles can in principle be performed with fewer
quantum mechanical gates than classical gates.Comment: significant content revisions: more algorithm details and brief
analysis of convergenc
A TLE-based Algorithm for Correcting Empirical Model Densities during Geomagnetic Storms
Neutral densities increase up to 800% during geomagnetic storms. Satellite two-line element sets (TLEs) show increased orbital decay during geomagnetic storms from increased drag
The NuSTAR Extragalactic Surveys: unveiling rare, buried AGNs and detecting the contributors to the peak of the Cosmic X-ray Background
We report on the results of active galactic nuclei (AGNs) detection by NuSTAR
performed in three extragalactic survey fields (COSMOS, UDS, ECDFS) in three
hard bands, namely H1 (8-16 keV), H2 (16-24 keV) and VH (35-55 keV). The
aggregated area of the surveys is deg. While a large number of
sources is detected in the H1 band (72 at the level of reliability), the
H2 band directly probing close to the peak of the Cosmic X-ray Background (CXB)
returns four significant detections, and two tentative, although not
significant, detections are found in the VH band. All the sources detected
above 16 keV are also detected at lower energies. We compute the integral
number counts for sources in such bands, which show broad consistency with
population synthesis models of the CXB. We furthermore identify two
Compton-thick AGNs, one in the COSMOS field, associated with a hard and faint
Chandra source, and one in the UDS field, never detected in the X-ray band
before. Both sources are at the same redshift , which shifts their
Compton-hump into the H1 band, and were previously missed in the usually
employed NuSTAR bands, confirming the potential of using the H1 band to
discover obscured AGNs at in deep surveys.Comment: 12 pages, 5 figures, accepted for publication in The Astrophysical
Journa
An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources
We have studied the stellar and interstellar environments of two luminous
X-ray sources and five ultraluminous X-ray sources (ULXs) in order to gain
insight into their nature. Archival Hubble Space Telescope images were used to
identify the optical counterparts of the ULXs Ho IX X-1 and NGC 1313 X-2, and
to make photometric measurements of the local stellar populations of these and
the luminous source IC 10 X-1. We obtained high-dispersion spectroscopic
observations of the nebulae around these seven sources to search for He II
lambda-4686 emission and to estimate the expansion velocities and kinetic
energies of these nebulae. Our observations did not detect nebular He II
emission from any source, with the exception of LMC X-1; this is either because
we missed the He III regions or because the nebulae are too diffuse to produce
He II surface brightnesses that lie within our detection limit. We compare the
observed ionization and kinematics of the supershells around the ULXs Ho IX X-1
and NGC 1313 X-2 with the energy feedback expected from the underlying stellar
population to assess whether additional energy contributions from the ULXs are
needed. In both cases, we find insufficient UV fluxes or mechanical energies
from the stellar population; thus these ULXs may be partially responsible for
the ionization and energetics of their supershells. All seven sources we
studied are in young stellar environments and six of them have optical
counterparts with masses >~7 M_sun; thus, these sources are most likely
high-mass X-ray binaries.Comment: 30 pages, 9 figures. Numerous minor revisions, primarily to more
accurately cite earlier work by Pakull and Mirioni, and to correct
typographical errors. Removed a misleading sentence in the Introduction (re:
X-ray photoionization by ULXs). Accepted for publication in The Astrophysical
Journal. Figures have been reduced in resolution for space requirements;
full-resolution figures may be requested by email to [email protected]
An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources
We have studied the stellar and interstellar environments of two luminous
X-ray sources and five ultraluminous X-ray sources (ULXs) in order to gain
insight into their nature. Archival Hubble Space Telescope images were used to
identify the optical counterparts of the ULXs Ho IX X-1 and NGC 1313 X-2, and
to make photometric measurements of the local stellar populations of these and
the luminous source IC 10 X-1. We obtained high-dispersion spectroscopic
observations of the nebulae around these seven sources to search for He II
lambda-4686 emission and to estimate the expansion velocities and kinetic
energies of these nebulae. Our observations did not detect nebular He II
emission from any source, with the exception of LMC X-1; this is either because
we missed the He III regions or because the nebulae are too diffuse to produce
He II surface brightnesses that lie within our detection limit. We compare the
observed ionization and kinematics of the supershells around the ULXs Ho IX X-1
and NGC 1313 X-2 with the energy feedback expected from the underlying stellar
population to assess whether additional energy contributions from the ULXs are
needed. In both cases, we find insufficient UV fluxes or mechanical energies
from the stellar population; thus these ULXs may be partially responsible for
the ionization and energetics of their supershells. All seven sources we
studied are in young stellar environments and six of them have optical
counterparts with masses >~7 M_sun; thus, these sources are most likely
high-mass X-ray binaries.Comment: 30 pages, 9 figures. Numerous minor revisions, primarily to more
accurately cite earlier work by Pakull and Mirioni, and to correct
typographical errors. Removed a misleading sentence in the Introduction (re:
X-ray photoionization by ULXs). Accepted for publication in The Astrophysical
Journal. Figures have been reduced in resolution for space requirements;
full-resolution figures may be requested by email to [email protected]
A Hard Look at NGC 5347: Revealing a Nearby Compton-thick AGN
Current measurements show that the observed fraction of Compton-thick (CT) active galactic nuclei (AGN) is smaller than the expected values needed to explain the cosmic X-ray background. Prior fits to the X-ray spectrum of the nearby Seyfert-2 galaxy NGC 5347 (z = 0.00792, D = 35.5 Mpc ) have alternately suggested a CT and Compton-thin source. Combining archival data from Suzaku, Chandra, andâmost importantlyânew data from NuSTAR, ... See full text for complete abstrac
Phenomenological analysis of quantum collapse as source of the seeds of cosmic structure
The standard inflationary version of the origin of the cosmic structure as
the result of the quantum fluctuations during the early universe is less than
fully satisfactory as has been argued in [A. Perez, H. Sahlmann, and D.
Sudarsky, Class. Quantum Grav., 23, 2317, (2006)]. A proposal is made there of
a way to address the shortcomings by invoking a process similar to the collapse
of the quantum mechanical wave function of the various modes of the inflaton
field. This in turn was inspired on the ideas of R. Penrose about the role that
quantum gravity might play in bringing about such breakdown of the standard
unitary evolution of quantum mechanics. In this paper we study in some detail
the two schemes of collapse considered in the original work together with an
alternative scheme, which can be considered as "more natural" than the former
two. The new scheme, assumes that the collapse follows the correlations
indicated in the Wigner functional of the initial state. We end with
considerations regarding the degree to which the various schemes can be
expected to produce a spectrum that resembles the observed one.Comment: 18 pages, 9 figure
Knowledge of Objective 'Oughts': Monotonicity and the New Miners Puzzle
In the classic Miners case, an agent subjectively ought to do what they know is objectively wrong. This case shows that the subjective and objective âoughtsâ are somewhat independent. But there remains a powerful intuition that the guidance of objective âoughtsâ is more authoritativeâso long as we know what they tell us. We argue that this intuition must be given up in light of a monotonicity principle, which undercuts the rationale for saying that objective âoughtsâ are an authoritative guide for agents and advisors
Interstitials, Vacancies, and Supersolid Order in Vortex Crystals
Interstitials and vacancies in the Abrikosov phase of clean Type II
superconductors are line imperfections, which cannot extend across macroscopic
equilibrated samples at low temperatures. We argue that the entropy associated
with line wandering nevertheless can cause these defects to proliferate at a
sharp transition which will exist if this occurs below the temperature at which
the crystal actually melts. Vortices are both entangled and crystalline in the
resulting ``supersolid'' phase, which in a dual ``boson'' analog system is
closely related to a two-dimensional quantum crystal of He with
interstitials or vacancies in its ground state. The supersolid {\it must} occur
for , where is the decoupling field above which
vortices begin to behave two-dimensionally. Numerical calculations show that
interstitials, rather than vacancies, are the preferred defect for , and allow us to estimate whether proliferation also
occurs for B\,\lot\,B_\times.The implications of the supersolid phase for
transport measurements, dislocation configurations and neutron diffraction are
discussed.Comment: 53 pages and 15 figures, available upon request, written in plain TE
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