arXiv subjects

Eli Dwek

Publications and source records attributed to Eli Dwek.

87 records · Page 5Linked to original sources

arXiv2004

Simultaneous Constraints on the Spectrum of the Extragalactic Background Light and the Intrinsic Tev Spectra of Mrk 421, Mrk 501, and H1426+428

Very high energy (~ TeV) $γ$-rays from blazars are attenuated by photons from the extragalactic background light (EBL). Observations of blazars can therefore provide an ideal opportunity for determining the EBL intensity if their intrinsic spectrum is known. Conversely, knowledge of the EBL intensity can be used to determine the intrinsic blazar spectrum. Unfortunately, neither the EBL intensity nor the intrinsic blazar spectrum is known with high enough precision to accurately derive one quantity from the other. In this paper we use the most recent data on the EBL to construct twelve different realizations representing all possible permutations between EBL limits and the detections in the different wavelength regions. We use these realizations to explore the effects of the EBL on the inferred spectra of blazars. In particular, we show that the frequently cited "IR background-TeV gamma-ray crisis" does not exist, and derive the intrinsic spectra and peak energies of the blazars Mrk 421, 501 and H1426+428 for EBL realizations that give rise to physically viable intrinsic blazar spectra. We also show that the intrinsic spectrum of Mrk~421 during a period of intense flaring activity has a peak energy that seems to shift to higher energies at higher flux states. Finally, we also explore the effect of the uncertainties in the absolute calibration of the gamma-ray energies on derived TeV opacities and the intrinsic blazar spectra.

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arXiv2004

Galactic Center Extinction: Evidence for Metallic Needles in the General Interstellar Medium

The extinction curve derived from ISO mid-infrared (IR) observations of the Galactic center (GC) exhibits a surprisingly flat behavior in the ~ 3 to 8 micron region, contrary to the deep minimum expected from standard interstellar dust models consisting of bare silicate and graphite dust particles. We show that this extinction is likely caused by the presence of metallic needles in the interstellar medium (ISM) towards the Galactic center. If the needles contribute only to the 3 - 8 micron extinction, they must have a long wavelength cutoff at ~ 8 microns, and therefore a typical length over radius ratio of about 600, smaller than the 3000 aspect ratio determined for the needles in Cas A. Homogeneously distributed throughout the ISM, they comprise only a minor mass fraction of the ISM, with a needle-to-H mass ratio of \~5x10^{-6}, which is equivalent to 0.14% of the silicate dust mass. Their total ISM abundance can then be readily explained by the combined production in SNe and O-rich stellar outflows. It is currently unclear how universal the GC extinction law is. Local 2 - 5 micron extinction measurements seem to be consistent with the standard extinction law, suggesting a non uniform distribution of needles in the ISM. The GC observations show that metallic needles, in spite of their low abundance or non uniform distribution, can be the dominant source of opacity in the 3 - 8 micron wavelength region. However, expelled into the intergalactic medium, their abundance is too low to cause any dimming of cosmological sources, and their length is too short to make them a significant source of submillimeter emission.

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arXiv2004

The Detection of Cold Dust in Cas A: Evidence for the Formation of Metallic Needles in the Ejecta

Recently, Dunne et al. (2003) obtained 450 and 850 micron SCUBA images of CasA, and reported the detection of 2-4 M_sun of cold, 18K, dust in the remnant. Here we show that their interpretation of the observations faces serious difficulties. Their inferred dust mass is larger than the mass of refractory material in the ejecta of a 10 to 30 M_sun star. The cold dust model faces even more difficulties if the 170 micron observations of the remnant are included in the analysis, decreasing the cold dust temperature to ~ 8K, and increasing its mass to > 20 M_sun. We offer here a more plausible interpretation of their observation, in which the cold dust emission is generated by conducting needles with properties that are completely determined by the combined submillimeter and X-ray observations of the remnant. The needles consist of metallic whiskers with <1% of embedded impurities that may have condensed out of blobs of material that were expelled at high velocities from the inner metal-rich layers of the star in an asymmetric explosion. The needles are collisionally heated by the shocked gas to a temperature of 8K. Taking the destruction of needles into account, a dust mass of only 1E-4 to 1E-3M_sun is needed to account for the observed SCUBA emission. Aligned in the magnetic field, needles may give rise to observable polarized emission. The detection of submillimeter polarization will therefore offer definitive proof for a needle origin for the cold dust emission. Supernovae may still be proven to be important sources of interstellar dust, but the evidence is still inconclusive.

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arXiv2003

New Interstellar Dust Models Consistent with Extinction, Emission, and Abundance Constraints

We present new interstellar dust models which have been derived by simultaneously fitting the far-ultraviolet to near-infrared extinction, the diffuse infrared (IR) emission and, unlike previous models, the elemental abundance constraints on the dust for different interstellar medium abundances, including solar, F and G star, and B star abundances. The fitting problem is a typical ill-posed inversion problem, in which the grain size distribution is the unknown, which we solve by using the method of regularization. The dust model contains various components: PAHs, bare silicate, graphite, and amorphous carbon particles, as well as composite particles containing silicate, organic refractory material, water ice, and voids. The optical properties of these components were calculated using physical optical constants. As a special case, we reproduce the Li & Draine (2001) results, however their model requires an excessive amount of silicon, magnesium, and iron to be locked up in dust: about 50 ppm (atoms per million of H atoms), significantly more than the upper limit imposed by solar abundances of these elements, about 34, 35, and 28 ppm, respectively. A major conclusion of this paper is that there is no unique interstellar dust model that simultaneously fits the observed extinction, diffuse IR emission, and abundances constraints.

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arXiv2003

Probing Interstellar Dust Models Through SAXS (Small Angle X-Ray Scattering)

A viable interstellar dust model - characterized by the composition, morphology, and size distribution of the dust grains and by the abundance of the different elements locked up in the dust - should fit all observational constraints arising primarily from the interactions of the dust with incident radiation or the ambient gas. In spite of the many different manifestations of these interactions, we still lack a comprehensive dust model that is consistent with {\it all} the observational constraints. An important advance towards the construction of such a model was recently made by Zubko, Dwek, and Arendt (2003, ZDA03) who, for the first time, included the average interstellar extinction, the diffuse infrared emission, {\it and} the interstellar abundances as explicit constraints in the construction of models consisting of astrophysical dust particles without any predetermined functional form for their grain size distribution. The results showed the existence of many distinct dust models that satisfy a basic set of observational constraints. X-ray halos, produced primarily by small angle scattering off large dust particles along the line of sight to bright X-ray sources, probe dust properties largely inaccessible at other wavelengths. In this contribution we briefly review the microscopic physics and the macroscopic effects that determine the intensity and spatial profile of X-ray halos. Focusing on the X-ray halo around the bright source GX~13+1, we show that halo observations can be used to discriminate between the different, currently viable, ZDA03 dust models. X-ray halos may therefore be an essential constraint that need to be explicitly included in all future dust models.

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arXiv2002

An Empirical Decomposition of Near-IR Emission into Galactic and Extragalactic Components

We decompose the COBE/DIRBE observations of the near-IR sky brightness (minus zodiacal light) into Galactic stellar and interstellar medium (ISM) components and an extragalactic background. This empirical procedure allows us to estimate the 4.9 micron cosmic infrared background (CIB) as a function of the CIB intensity at shorter wavelengths. A weak indication of a rising CIB intensity at wavelengths > 3.5 micron hints at interesting astrophysics in the CIB spectrum, or warns that the foreground zodiacal emission may be incompletely subtracted. Subtraction of only the stellar component from the zodiacal-light-subtracted all-sky map reveals the clearest 3.5 micron ISM emission map, which is found to be tightly correlated with the ISM emission at far-IR wavelengths.

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arXiv2002

The Cosmic Radio and Infrared Backgrounds Connection

We use the radio-infrared (IR) flux correlation bet ween star-forming galaxies in the local universe to derive a simple analytical expression between the intensity of the IR background and the brightness t emperature of the radio background. This relation i s insensitive to the star formation history of the galaxies that produce the cosmic IR background (CIB). We use the observed CIB intensity to constrain t he cosmic star formation history, and the relation between the CIB and the cosmic radio background (CR B) to constrain the relative contribution of star-f orming galaxies to the CRB. Current limits on the C IB intensity predict a 178 MHz brightness temperature of ~18+-9 K, about half of the 37+-8 K inferred for an isotropic radio component. This suggests that s tar-forming galaxies and AGN contribute about equal ly to the CRB intensity at that frequency.

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arXiv2001

The Cosmic Infrared Background: Measurements and Implications

The cosmic infrared background records much of the radiant energy released by processes of structure formation that have occurred since the decoupling of matter and radiation following the Big Bang. In the past few years, data from the Cosmic Background Explorer mission provided the first measurements of this background, with additional constraints coming from studies of the attenuation of TeV gamma-rays. At the same time there has been rapid progress in resolving a significant fraction of this background with the deep galaxy counts at infrared wavelengths from the Infrared Space Observatory instruments and at submillimeter wavelengths from the Submillimeter Common User Bolometer Array instrument. This article reviews the measurements of the infrared background and sources contributing to it, and discusses the implications for past and present cosmic processes.

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arXiv2001

The Role of Dust in Producing the Cosmic Infrared Background

The extragalactic background light (EBL), exclusive of the cosmic microwave background, consists of the cumulative radiative output from all energy sources in the universe since the epoch of recombination. Most of this energy is released at ultraviolet and optical wavelengths. However, observations show that a significant fraction of the EBL falls in the 10 to 1000 micron wavelength regime. This provides conclusive evidence that we live in a dusty universe, since only dust can efficiently absorbs a significant fraction of the background energy and reemit it at infrared wavelengths. The general role of dust in forming the cosmic infrared background (CIB) is therefore obvious. However, its role in determining the exact spectral shape of the CIB is quite complex. The CIB spectrum depends on the microscopic physical properties of the dust, its composition, abundance, and spatial distribution relative to the emitting sources, and its response to evolutionary processes that can modify all the factors listed above. This paper will present a brief summary of the many ways dust affects the intensity and spectral shape of the cosmic infrared background. In an Appendix we present new limits on the mid-infrared intensity of the CIB using TeV gamma-ray observations of Mrk 501.

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arXiv2001

X-ray Halos and Large Grains in the Diffuse Interstellar Medium

Recent observations with dust detectors on board the interplanetary spacecraft Ulysses and Galileo have recorded a substantial flux of large interstellar grains with radii between 0.25 and 2.0 mu entering the solar system from the local interstellar cloud. The most commonly used interstellar grain size distribution is characterized by a a^-3.5 power law in grain radii a, and extends to a maximum grain radius of 0.25 mu. The extension of the interstellar grain size distribution to such large radii will have a major effect on the median grain size, and on the amount of mass needed to be tied up in dust for a given visual optical depth. It is therefore important to investigate whether this population of larger dust particles prevails in the general interstellar medium, or if it is merely a local phenomenon. The presence of large interstellar grains can be mainly inferred from their effect on the intensity and radial profiles of scattering halos around X-ray sources. In this paper we examine the grain size distribution that gives rise to the X-ray halo around Nova Cygni 1992. The results of our study confirm the need to extend the interstellar grain size distribution in the direction of this source to and possibly beyond 2.0 mu. The model that gives the best fit to the halo data is characterized by: (1) a grain size distribution that follows an a^-3.5 power law up to 0.50 mu, followed by an a^-4.0 extension from 0.50 mu to 2.0 mu; and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044 and 0.0022, respectively, consistent with solar abundances constraints. Additional observations of X-ray halos probing other spatial directions are badly needed to test the general validity of this result.

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arXiv1999

Analytical Approximations for Calculating the Escape and Absorption of Radiation in Clumpy Dusty Environments

We present analytical approximations for calculating the scattering, absorption and escape of nonionizing photons from a spherically symmetric two-phase clumpy medium, with either a central point source of isotropic radiation, a uniform distribution of isotropic emitters, or uniformly illuminated by external sources. The analytical approximations are based on the mega-grains model of two-phase clumpy media, as proposed by Hobson & Padman, combined with escape and absorption probability formulae for homogeneous media. The accuracy of the approximations is examined by comparison with 3D Monte Carlo simulations of radiative transfer, including multiple scattering. Our studies show that the combined mega-grains and escape/absorption probability formulae provide a good approximation of the escaping and absorbed radiation fractions for a wide range of parameters characterizing the medium. A realistic test is performed by modeling the absorption of a starlike source of radiation by interstellar dust in a clumpy medium, and by calculating the resulting equilibrium dust temperatures and infrared emission spectrum of both the clumps and the interclump medium. In particular, we find that the temperature of dust in clumps is lower than in the interclump medium if clumps are optically thick. Comparison with Monte Carlo simulations of radiative transfer in the same environment shows that the analytic model yields a good approximation of dust temperatures and the emerging UV to FIR spectrum of radiation for all three types of source distributions mentioned above. Our analytical model provides a numerically expedient way to estimate radiative transfer in a variety of interstellar conditions and can be applied to a wide range of astrophysical environments, from star forming regions to starburst galaxies.

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arXiv1999

Radiative Transfer in Clumpy and Fractal Media

A Monte Carlo model of radiative transfer in multi-phase dusty media is applied to the situation of stars and clumpy dust in a sphere or a disk. The distribution of escaping and absorbed photons are shown for various filling factors and densities. Analytical methods of approximating the escaping fraction of radiation, based on the Mega-Grains approach, are discussed. Comparison with the Monte Carlo results shows that the escape probability formulae provide a reasonable approximation of the escaping/absorbed fractions, for a wide range of parameters characterizing a clumpy dusty medium. A possibly more realistic model of the interstellar medium is one in which clouds have a self-similar hierarchical structure of denser and denser clumps within clumps, resulting in a fractal distribution of gas and dust. Monte Carlo simulations of radiative transfer in such multi-phase fractal media are compared with the two-phase clumpy case.

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arXiv1998

The Submillimeter Frontier: A Space Science Imperative

A major goal of modern astrophysics is to understand the processes by which the universe evolved from its initial simplicity, as seen in measurements of the Cosmic Microwave Background, to the universe we see today, with complexity on all scales. While the diffuse background measurements of COBE reveal the importance of the far infrared and submillimeter in early galaxy and star formation, the understanding of the development of complex structure requires high resolution imaging and spectroscopy. We present a concept for a space mission called SPECS, the Submillimeter Probe of the Evolution of Cosmic Structure, which provides these capabilities. SPECS is a cold, spatial and spectral Michelson interferometer with adjustable baselines ranging up to 1 km. It has Hubble sensitivity and angular resolution in the far IR and submillimeter, spectral resolution up to 10,000, and a 14' field of view. SPECS will be able to image thermal dust continuum and infrared cooling and diagnostic line emission over a wide range of redshifts, providing extinction-free astrophysical probes of young galaxies and early cosmic structures, and measures of the luminosity and heavy element formation history of the universe. SPECS would also have the potential to improve vastly our knowledge of protostars, protoplanetary systems, Active Galactic Nuclei and other objects in the local universe. We recommend that a concerted effort be made during the next decade to develop and test certain critical technologies (photon-counting far IR detectors, formation flying spacecraft, cold, lightweight mirrors, and active coolers), so that SPECS can be deployed in about 2015.

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arXiv1997

Soft X-ray Scattering and Halos from Dust

Small angle scatterings of X-rays by interstellar dust particles create halos around x-ray sources. By observing these halos with sufficient angular resolution it is possible to derive information about both the spatial distribution of the dust along the line of sight to the source and some of the physical properties of the scattering dust particles. Previous analyses of halos have been used to infer that the scattering dust particles must be fluffy. These fluffy dust particles have also been suggested as a possible solution to the interstellar carbon "crisis." This crisis results from the discrepancy between the available ISM carbon abundance and the amount required to be in dust to account for the average ISM extinction. Dwek (1997), however, pointed out that fluffy dust particles may not solve the carbon crisis. In light of this, we re-examine the evidence for fluffy dust in the ISM from x-ray scattering data. We find that previous studies overestimated the intensity of the halos by using the RG approximation, which fails for energies below 1 keV. Using the exact Mie solution we find that below 1 keV the scattering becomes much less efficient. Applying the Mie calculations to the x-ray halo observed around Nova Cygni 1992, we conclude that the data can be explained by scattering from regular dust, and does not require interstellar dust particles to be fluffy.

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arXiv1997

The Evolution of the Elemental Abundances in the Gas and Dust Phases of the Galaxy

We present models for the evolution of the elemental abundances in the gas and dust phases of the interstellar medium (ISM) of our Galaxy by generalizing standard models for its dynamical and chemical evolution. In these models, the stellar birthrate history is determined by the infall rate of primordial gas, and by its functional dependence on the mass surface density of the stars and gas. We adopt a two component model for the Galaxy, consisting of a central bulge and an exponential disk with different infall rates and stellar birthrate histories. Condensation in stellar winds, Type Ia and Type II supernovae, and the accretion of refractory elements onto preexisting grains in dense molecular clouds are the dominant contributors to the abundance of elements locked up in the dust. Grain destruction by sputtering and evaporative grain-grain collisions in supernova remnants are the most important mechanisms that return these elements back to the gas phase. We calculate the dust production rate by the various dust sources, analyze the origin of the elemental depletion pattern, and study the relation between dust abundance and ISM metallicity, and the evolution of the the dust abundance and composition at each Galactocentric radius as a function of time. The derived relation of dust mass with metallicity is compared to the observed Galactic dust abundance gradient, and to the M$_{dust}$ versus log(O/H) relation that is observed in external Dwarf galaxies. The dependence of dust composition on the mass of the progenitor star, and the delayed recycling of newly synthesized dust by low mass stars back to the ISM give rise to variations in the dust composition as a function of time. Our models provide a framework for the self-consistent inclusion of dust in

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