177 research outputs found
Astro 2020 Science White Paper: Time Domain Studies of Neutron Star and Black Hole Populations: X-ray Identification of Compact Object Types
What are the most important conditions and processes governing the growth of
stellar-origin compact objects? The identification of compact object type as
either black hole (BH) or neutron star (NS) is fundamental to understanding
their formation and evolution. To date, time-domain determination of compact
object type remains a relatively untapped tool. Measurement of orbital periods,
pulsations, and bursts will lead to a revolution in the study of the
demographics of NS and BH populations, linking source phenomena to accretion
and galaxy parameters (e.g., star formation, metallicity). To perform these
measurements over sufficient parameter space, a combination of a wide-field
(>5000 deg^2) transient X-ray monitor over a dynamic energy range (~1-100 keV)
and an X-ray telescope for deep surveys with <5 arcsec PSF half-energy width
(HEW) angular resolution are required. Synergy with multiwavelength data for
characterizing the underlying stellar population will transform our
understanding of the time domain properties of transient sources, helping to
explain details of supernova explosions and gravitational wave event rates.Comment: 9 pages, 2 figures. Submitted to the Astro2020 Decadal Surve
Resolving the cosmic X-ray background with a next-generation high-energy X-ray observatory
The cosmic X-ray background (CXB), which peaks at an energy of ~30 keV, is
produced primarily by emission from accreting supermassive black holes (SMBHs).
The CXB therefore serves as a constraint on the integrated SMBH growth in the
Universe and the accretion physics and obscuration in active galactic nuclei
(AGNs). This paper gives an overview of recent progress in understanding the
high-energy (>~10 keV) X-ray emission from AGNs and the synthesis of the CXB,
with an emphasis on results from NASA's NuSTAR hard X-ray mission. We then
discuss remaining challenges and open questions regarding the nature of AGN
obscuration and AGN physics. Finally, we highlight the exciting opportunities
for a next-generation, high-resolution hard X-ray mission to achieve the
long-standing goal of resolving and characterizing the vast majority of the
accreting SMBHs that produce the CXB.Comment: Science White paper submitted to Astro2020 Decadal Survey; 5 pages, 3
figures, plus references and cover pag
Role of interface region on the optoelectronic properties of silicon nanocrystals embedded in SiO2
Light emitting silicon nanocrystals embedded in SiO2 have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered TEM analysis and by ab-initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding SiO2 is not sharp: an intermediate region of amorphous nature and of variable composition links the crystalline Si with the amorphous stoichiometric SiO2. This region plays an active role in the light emission process
Detectability subspaces and observer synthesis for two-dimensional systems
The notions of input-containing and detectability subspaces are developed within the context of observer synthesis for two-dimensional (2-D) Fornasini-Marchesini models. Specifically, the paper considers observers which asymptotically estimate the local state, in the sense that the error tends to zero as the reconstructed local state evolves away from possibly mismatched boundary values, modulo a detectability subspace. Ultimately, the synthesis of such observers in the absence of explicit input information is addressed
A geometric theory for 2-D systems including notions of stabilisability
In this paper we consider the problem of internally and externally stabilising controlled invariant and output-nulling subspaces for two-dimensional (2-D) Fornasini–Marchesini models, via static feedback. A numerically tractable procedure for computing a stabilising feedback matrix is developed via linear matrix inequality techniques. This is subsequently applied to solve, for the first time, various 2-D disturbance decoupling problems subject to a closed-loop stability constraint
Geometric techniques for implicit two-dimensional systems
Geometric tools are developed for two-dimensional (2-D) models in an implicitFornasini–Marchesini form. In particular, the structural properties of controlled and conditionedinvariance are defined and studied. These properties are investigated in terms ofquarter-plane causal solutions of the implicit model given compatible boundary conditions.The definitions of controlled and conditioned invariance introduced, along with the correspondingoutput-nulling and input-containing subspaces, are shown to be richer than theone-dimensional counterparts. The analysis carried out in this paper establishes necessaryand sufficient conditions for the solvability of 2-D disturbance decoupling problems andunknown-input observation problems. The conditions obtained are expressed in terms ofoutput-nulling and input-containing subspaces, which can be computed recursively in a finitenumber of steps
Black Holes and Neutron Stars in Nearby Galaxies: Insights from NuSTAR
Nearby galaxy surveys have long classified X-ray binaries (XRBs) by the mass
category of their donor stars (high-mass and low-mass). The NuSTAR observatory,
which provides imaging data at E keV, has enabled the classification of
extragalactic XRBs by their compact object type: neutron star (NS) or black
hole (BH). We analyzed NuSTAR/Chandra/XMM-Newton observations from a
NuSTAR-selected sample of 12 galaxies within 5 Mpc having stellar masses
() and star formation rates (SFR)
yr. We detect 128 NuSTAR sources to a
sensitivity of erg s. Using NuSTAR color-intensity and
color-color diagrams we classify 43 of these sources as candidate NS and 47 as
candidate BH. We further subdivide BH by accretion states (soft, intermediate,
and hard) and NS by weak (Z/Atoll) and strong (accreting pulsar) magnetic
field. Using 8 normal (Milky Way-type) galaxies in the sample, we confirm the
relation between SFR and galaxy X-ray point source luminosity in the 4-25 and
12-25 keV energy bands. We also constrain galaxy X-ray point source luminosity
using the relation , finding
agreement with previous work. The XLF of all sources in the 4-25 and 12-25 keV
energy bands matches with the slope for high-mass XRBs. We find
that NS XLFs suggest a decline beginning at the Eddington limit for a 1.4
NS, whereas the BH fraction shows an approximate monotonic increase
in the 4-25 and 12-25keV energy bands. We calculate the overall ratio of BH to
NS to be for 4-25 keV and for 12-25 keV.Comment: 38 pages, 12 figures, 8 tables. ApJ, in pres
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The MOSDEF Survey: The Metallicity Dependence of X-Ray Binary Populations at z ∼ 2
Population synthesis models predict that high-mass X-ray binary (HMXB) populations produced in low-metallicity environments should be more X-ray luminous, a trend supported by studies of nearby galaxies. This trend may be responsible for the observed increase of the X-ray luminosity (L X) per star formation rate (SFR) with redshift due to the decrease of metallicity (Z) at fixed stellar mass as a function of redshift. To test this hypothesis, we use a sample of 79 z ∼ 2 star-forming galaxies with oxygen abundance measurements from the MOSDEF survey, which obtained rest-frame optical spectra for ∼1500 galaxies in the CANDELS fields at 1.37 < z < 3.80. Using Chandra data from the Chandra AEGIS-X Deep, Chandra Deep Field North, and Chandra Deep Field South surveys, we stack the X-ray data at the galaxy locations in bins of redshift and Z because the galaxies are too faint to be individually detected. In agreement with previous studies, the average L X/SFR of our z ∼ 2 galaxy sample is enhanced by ≈0.4-0.8 dex relative to local HMXB L X-SFR scaling relations. Splitting our sample by Z, we find that L X/SFR and Z are anticorrelated with 97% confidence. This observed Z dependence for HMXB-dominated galaxies is consistent with both the local L X-SFR-Z relation and a subset of population synthesis models. Although the statistical significance of the observed trends is weak owing to the low X-ray statistics, these results constitute the first direct evidence connecting the redshift evolution of L X/SFR and the Z dependence of HMXBs
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