arXiv subjects

A. W. Strong

Publications and source records attributed to A. W. Strong.

At least 73 records · Page 4Linked to original sources

arXiv2005

A new estimate of the Galactic interstellar radiation field between 0.1 microns and 1000 microns

Cosmic-ray electrons and positrons propagating in the Galaxy produce diffuse gamma-rays via the inverse Compton (IC) process. The low energy target photon populations with which the cosmic-rays interact during propagation are produced by stars, this stellar light being reprocessed by Galactic dust. Detailed modelling of the Galactic stellar distribution, dust distribution, and treatment of the absorption and scattering of light is therefore required to obtain accurate models for the low energy Galactic photon distribution and spectrum. Using a realistic Galactic stellar distribution model, and dust distribution, we calculate the diffuse radiation field from stars in the Galaxy (the `optical' radiation field), including absorption and scattering. Using a dust heating code, we self-consistently calculate the infra-red radiation field for the same dust model used for the optical calculation; both transient and equilibrium heating are included. We present the calculated radiation field spectra and distributions, and will use these to calculate the expected Galactic diffuse IC gamma-ray spectrum.

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arXiv2004

Propagation of cosmic rays: nuclear physics in cosmic-ray studies

The nuclei fraction in cosmic rays (CR) far exceeds the fraction of other CR species, such as antiprotons, electrons, and positrons. Thus the majority of information obtained from CR studies is based on interpretation of isotopic abundances using CR propagation models where the nuclear data and isotopic production cross sections in p- and alpha-induced reactions are the key elements. This paper presents an introduction to the astrophysics of CR and diffuse gamma rays and discusses some of the puzzles that have emerged recently due to more precise data and improved propagation models. Merging with cosmology and particle physics, astrophysics of CR has become a very dynamic field with a large potential of breakthrough and discoveries in the near future. Exploiting the data collected by the CR experiments to the fullest requires accurate nuclear cross sections.

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arXiv2004

Diffuse Galactic continuum gamma rays. A model compatible with EGRET data and cosmic-ray measurements

We present a study of the compatibility of some current models of the diffuse Galactic continuum gamma rays with EGRET data. A set of regions sampling the whole sky is chosen to provide a comprehensive range of tests. The range of EGRET data used is extended to 100 GeV. The models are computed with our GALPROP cosmic-ray propagation and gamma-ray production code. We confirm that the "conventional model" based on the locally observed electron and nucleon spectra is inadequate, for all sky regions. A conventional model plus hard sources in the inner Galaxy is also inadequate, since this cannot explain the GeV excess away from the Galactic plane. Models with a hard electron injection spectrum are inconsistent with the local spectrum even considering the expected fluctuations; they are also inconsistent with the EGRET data above 10 GeV. We present a new model which fits the spectrum in all sky regions adequately. Secondary antiproton data were used to fix the Galactic average proton spectrum, while the electron spectrum is adjusted using the spectrum of diffuse emission itself. The derived electron and proton spectra are compatible with those measured locally considering fluctuations due to energy losses, propagation, or possibly details of Galactic structure. This model requires a much less dramatic variation in the electron spectrum than models with a hard electron injection spectrum, and moreover it fits the gamma-ray spectrum better and to the highest EGRET energies. It gives a good representation of the latitude distribution of the gamma-ray emission from the plane to the poles, and of the longitude distribution. We show that secondary positrons and electrons make an essential contribution to Galactic diffuse gamma-ray emission.

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arXiv2004

A new determination of the extragalactic diffuse gamma-ray background from EGRET data

We use the GALPROP model for cosmic-ray propagation to obtain a new estimate of the Galactic component of gamma rays, and show that away from the Galactic plane it gives an accurate prediction of the observed EGRET intensities in the energy range 30 MeV - 50 GeV. On this basis we re-evaluate the extragalactic gamma-ray background. We find that for some energies previous work underestimated the Galactic contribution at high latitudes and hence overestimated the background. Our new background spectrum shows a positive curvature similar to that expected for models of the extragalactic emission based on the blazar population.

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arXiv2004

The distribution of cosmic-ray sources in the Galaxy, gamma-rays, and the gradient in the CO-to-H2 relation

We present a solution to the apparent discrepancy between the radial gradient in the diffuse Galactic gamma-ray emissivity and the distribution of supernova remnants, believed to be the sources of cosmic rays. Recent determinations of the pulsar distribution have made the discrepancy even more apparent. The problem is shown to be plausibly solved by a variation in the CO-to-N(H2) scaling factor. If this factor increases by a factor of 5-10 from the inner to the outer Galaxy, as expected from the Galactic metallicity gradient, we show that the source distribution required to match the radial gradient of gamma-ray can be reconciled with the distribution of supernova remnants as traced by current studies of pulsars. The resulting model fits the EGRET gamma-ray profiles extremely well in longitude, and reproduces the mid-latitude inner Galaxy intensities better than previous models.

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arXiv2004

SPI Measurements of the Diffuse Galactic Hard X-ray Continuum

INTEGRAL Spectrometer SPI data from the first year of the Galactic Centre Deep Exposure has been analysed for the diffuse continuum from the Galactic ridge. A new catalogue of sources from the INTEGRAL Imager IBIS has been used to account for their contribution to the celestial signal. Apparently diffuse emission is detected at a level ~10% of the total source flux. A comparison of the spectrum of diffuse emission with that from an analysis of IBIS data alone shows that they are consistent. The question of the contribution of unresolved sources to this ridge emission is still open.

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arXiv2004

Al(26) Studies with INTEGRAL's Spectrometer SPI

Al(26) radioactivity traces recent nucleosynthesis throughout the Galaxy, and is known to be produced in massive stars and novae. The map from its decay gamma-ray line suggests massive stars to dominate, but high-resolution line spectroscopy is expected to supplement imaging of Al(26) source regions and thus to help decide about the Al(26) injection process and interstellar environment, hence about the specific massive-star subgroup and phase which produces interstellar Al(26). The INTEGRAL Spectrometer SPI has observed Galactic Al(26) radioactivity in its 1809 keV gamma-ray line during its first inner-Galaxy survey. Instrumental background lines make analysis difficult; yet, a clear signal from the inner Galaxy agrees with expectations. In particular, SPI has constrained the line width to exclude previously-reported line broadenings corresponding to velocities >500 km/s. The signal-to-background ratio of percent implies that detector response and background modeling need to be fine-tuned to eventually enable line shape deconvolution in order to extract source location information along the line of sight.

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arXiv2004

Diffuse Gamma Rays: Galactic and Extragalactic Diffuse Emission

"Diffuse" gamma rays consist of several components: truly diffuse emission from the interstellar medium, the extragalactic background, whose origin is not firmly established yet, and the contribution from unresolved and faint Galactic point sources. One approach to unravel these components is to study the diffuse emission from the interstellar medium, which traces the interactions of high energy particles with interstellar gas and radiation fields. Because of its origin such emission is potentially able to reveal much about the sources and propagation of cosmic rays. The extragalactic background, if reliably determined, can be used in cosmological and blazar studies. Studying the derived "average" spectrum of faint Galactic sources may be able to give a clue to the nature of the emitting objects.

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arXiv2003

Early SPI/INTEGRAL measurements of galactic 511 keV line emission from positron annihilation

We report the first measurements of the 511 keV line emission from the Galactic Centre (GC) region performed with the spectrometer SPI on the space observatory INTEGRAL (International Gamma-Ray Astrophysics Laboratory). Taking into account the range of spatial distribution models which are consistent with the data, we derive a flux of $9.9^{+4.7}_{-2.1} \times 10^{-4}$ ph cm$^{-2}$ s$^{-1}$ and an intrinsic line width of $2.95^{+0.45}_{-0.51}$ keV (FWHM). The results are consistent with other high-spectroscopy measurements, though the width is found to be at the upper bound of previously reported values.

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arXiv2003

The INTEGRAL Science Data Centre (ISDC)

The INTEGRAL Science Data Centre (ISDC) provides the INTEGRAL data and means to analyse them to the scientific community. The ISDC runs a gamma ray burst alert system that provides the position of gamma ray bursts on the sky within seconds to the community. It operates a quick-look analysis of the data within few hours that detects new and unexpected sources as well as it monitors the instruments. The ISDC processes the data through a standard analysis the results of which are provided to the observers together with their data.

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arXiv2003

First results from the INTEGRAL Galactic plane scans

Scans of the Galactic plane performed at regular intervals constitute a key element of the guaranteed time observations of the INTEGRAL observing programme. These scans are done for two reasons: frequent monitoring of the Galactic plane in order to detect transient sources, and time resolved mapping of the Galactic plane in continuum and diffuse line emission. This paper describes first results obtained from the Galactic plane scans executed so far during the early phase (Dec 2002 - May 2003) of the nominal mission.

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arXiv2003

Antiprotons in CR: What Do They Tell Us?

Recent measurements of the CR antiproton flux have been shown to pose a problem for conventional propagation models. In particular, models consistent with secondary/primary nuclei ratio in CR produce too few antiprotons, while matching the ratio and the antiproton flux requires ad hoc assumptions. This may indicate an additional local CR component or new phenomena in CR propagation in the Galaxy. We discuss several possibilities which may cause this problem.

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arXiv2003

A New Estimate of the Extragalactic Gamma-Ray Background from EGRET Data

We use the GALPROP model for cosmic-ray (CR) propagation to obtain our best prediction of the Galactic component of gamma rays, and show that away from the Galactic plane it gives an accurate prediction of the observed EGRET intensities. On this basis we re-evaluate the isotropic extragalactic gamma-ray background (EGRB). We find that for some energies previous work underestimated the Galactic contribution and hence overestimated the background. Our new EGRB spectrum shows a positive curvature similar to that expected for models of the extragalactic emission based on the blazar population.

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arXiv2003

Evaluation of Models for Diffuse Continuum Gamma Rays in EGRET Range

The GALPROP model for cosmic-ray (CR) propagation produces explicit predictions for the angular distribution of Galactic diffuse gamma rays. We compare our current models with EGRET spectra for various regions of the sky. This allows a critical test of alternative hypotheses for the observed GeV excess. We show that a population of hard-spectrum gamma-ray sources cannot be solely responsible for the excess since it also appears at high latitudes; on the other hand a hard CR electron spectrum model cannot explain the gamma-ray excess in the inner Galaxy. Hence some combination of these explanations is suggested.

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arXiv2003

Propagation of Light Elements in the Galaxy

The origin and evolution of isotopes of the lightest elements H2, He3, Li, Be, B in the universe is a key problem in such fields as astrophysics of CR, Galactic evolution, non-thermal nucleosynthesis, and cosmological studies. One of the major sources of these species is spallation by CR nuclei in the interstellar medium. On the other hand, it is the B/C ratio in CR and Be10 abundance which are used to fix the propagation parameters and thus the spallation rate. We study the production and Galactic propagation of isotopes of elements Z<6 using the numerical propagation code GALPROP and updated production cross sections.

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arXiv2003

Propagation of secondary antiprotons and cosmic rays in the Galaxy

Recent measurements of the cosmic ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary to primary nuclei ratios (e.g., B/C) produce too few antiprotons. In the present paper we discuss one possibility to overcome these difficulties. Using the measured antiproton flux AND B/C ratio to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh local "unprocessed" component at low energies perhaps associated with the Local Bubble, thus decreasing the measured secondary to primary nuclei ratio. The independent evidence for SN activity in the solar vicinity in the last few Myr supports this idea. The model reproduces antiprotons, B/C ratio, and elemental abundances up to Ni (Z<=28). Calculated isotopic distributions of Be and B are in perfect agreement with CR data. The abundances of three "radioactive clock" isotopes in CR, 10Be, 26Al, 36Cl, are all consistent and indicate a halo size z_h~4 kpc based on the most accurate data taken by the ACE spacecraft.

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arXiv2003

Propagation model for cosmic ray species in the Galaxy

In a recent paper (Moskalenko et al., 2002), it has been shown that the flux of secondary cosmic ray (CR) antiprotons appears to be contradictory to measurements of secondary to primary nuclei ratios in cosmic rays when calculated in the same Galactic propagation model. The contradiction appears as a value of the diffusion coefficient necessary to match the secondary ratios antiproton/proton and B/C. In particlular, it was shown that the reacceleration models designed to match secondary to primary nuclei ratios produce too few antiprotons. It is, however, clear that some reacceleration is unavoidable in the turbulent interstellar medium. Here we discuss an idea of how to improve reacceleration model by allowing for the damping of interstellar turbulences on the small scale by cosmic rays, mostly protons. This would lead to increase in the mean free path lenghts at low energies, the well-known phenomena empirically discovered in the Leaky Box models, thus producing less secondary nuclei. Antiprotons will remain almost non-affected due to their high energy threshold of production cross section.

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arXiv2002

Challenging cosmic ray propagation with antiprotons. Evidence for a "fresh" nuclei component?

Recent measurements of the cosmic ray (CR) antiproton flux have been shown to challenge existing CR propagation models. It was shown that the reacceleration models designed to match secondary to primary nuclei ratios (e.g., boron/carbon) produce too few antiprotons. Matching both the secondary to primary nuclei ratio and the antiproton flux requires artificial breaks in the diffusion coefficient and the primary injection spectrum suggesting the need for other approaches. In the present paper we discuss one possibility to overcome these difficulties. Using the measured antiproton flux AND B/C ratio to fix the diffusion coefficient, we show that the spectra of primary nuclei as measured in the heliosphere may contain a fresh local "unprocessed" component at low energies perhaps associated with the Local Bubble, thus decreasing the measured secondary to primary nuclei ratio. The independent evidence for SN activity in the solar vicinity in the last few Myr supports this idea. The model reproduces antiprotons, B/C ratio, and elemental abundances up to Ni (Z<=28). Calculated isotopic distributions of Be and B are in perfect agreement with CR data. The abundances of three "radioactive clock" isotopes in CR, 10Be, 26Al, 36Cl, are all consistent and indicate a halo size z_h~4 kpc based on the most accurate data taken by the ACE spacecraft.

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