arXiv subjects

R. Diehl

Publications and source records attributed to R. Diehl.

85 records · Page 5Linked to original sources

arXiv2000

Modelling Radioactivities and other Observables from OB Associations

Recent observations of the diffuse Galactic gamma-ray glow at 1.809 MeV, attributed to the radioactive decay of 26Al, point towards a massive star origin of this radioactive isotope. Wolf Rayet stars and core-collapse supernovae appear to dominate the production of this isotope. Massive stars are commonly located in clusters and OB associations, regions of recent star formation. We thus discuss the temporal evolution of 26Al, and 60Fe within evolving OB associations. The goal of this study is to utilize the associated gamma-ray lines as a diagnostic tool for the study of correlated star formation, and also to provide more detailed models for the interpretation of data obtained with COMPTEL on the Compton Observatory. We investigate the effects of possible aluminum yield enhancements, predicted for some massive close binary systems. In addition to the ejection of chemically processed matter, massive stars also drive strong stellar winds and emit large fluxes of ionizing radiation. This energy and radiation input into the interstellar medium (ISM) is crucial for the dynamical evolution of the gas and subsequent star forming activity in the galactic disk. We discuss population synthesis models for a variety of star formation histories, and compare the predicted gamma-ray line lightcurves to COMPTEL measurements in the Cygnus region. Radioactive tracers such as 26Al and 60Fe provide a unique gamma-ray tracer of Galactic star formation activity, complementary to other methods using spectral information in the radio, IR or optical bands.

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arXiv1999

COMPTEL limits on 26Al 1.809 MeV line emission from gamma^2 Velorum

The Wolf-Rayet binary system gamma^2 Vel (WR 11} is the closest known Wolf-Rayet (WR) star. Recently, its distance has been redetermined by parallax measurements with the HIPPARCOS astrometric satellite yielding 258^{+41}_{-31} pc, significantly lower than previous estimates (300 -- 450 pc). Wolf-Rayet stars have been proposed as a major source of the Galactic 26Al observed at 1.809 MeV. The gamma-ray telescope COMPTEL has previously reported 1.8 MeV emission from the Vela region, yet located closer to the Galactic plane than the position of gamma^2 Vel. We derive an upper 1.8 MeV flux limit of 1.1 10^{-5} ph cm^{-2} s^{-1} (2 σ) for the WR star. With the new distance estimate, COMPTEL measurements place a limit of (6.3^{+2.1}_{-1.4}) 10^{-5} M_\odot the 26Al yield of gamma^2 Vel, thus constrains theories of nucleosynthesis in Wolf-Rayet stars. We discuss the implications in the context of the binary nature of gamma^2 Vel and present a new interpretation of the IRAS Vela shell.

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arXiv1999

Titanium-44: Its Effective Decay Rate in Young Supernova Remnants, and its Abundance in Cas A

Radioactive isotopes such as 44Ti offer probably the most direct probe into nucleosynthesis environments in the interior of exploding stars, when the associated gamma-ray activities in the explosion remnant are detected and translated back to the isotopic abundances at the time of the explosion. In this paper, we assert that the procedure may not necessarily be straightforward, at least in the case of 44Ti, an orbital-electron capture decay isotope. Using the analytic model of McKee and Truelove (1995) for young supernova remnants, and assuming the existence of overdense 56Fe-dominated clumps that contain also 44Ti, we show that a high degree of ionization may be caused by the reverse shock so that the electron-capture rate of 44Ti could be significantly reduced from its laboratory value. When applied to Cas A, this increases under certain conditions the current 44Ti-activity by a factor 1.5 - 2.5, which yields a better compatibility between the COMPTEL observation of the 1.16 MeV line activity associated with the 44Ti decay and the SN model predictions of the initial 44Ti abundance. This possibility is, however, subject to various uncertainties, and in particular to the unknown properties and radial distribution of the the clumps in the ejecta.

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arXiv1999

Image Reconstruction of COMPTEL 1.8 MeV 26Al Line Data

We present a new algorithm, called Multiresolution Regularized Expectation Maximization (MREM), for the reconstruction of gamma-ray intensity maps from COMPTEL data. The algorithm is based on the iterative Richardson-Lucy scheme to which we added a wavelet thresholding step in order to eliminate image-noise in the reconstruction. The wavelet thresholding explicitly accounts for spatial correlations in the data, and adapts the angular resolution locally, depending on the significance of the signal in the data. We compare the performance of MREM to that of the maximum entropy and the Richardson-Lucy algorithms by means of Monte-Carlo simulations of COMPTEL 1.809 MeV gamma-ray line observations. The simulations demonstrate that the maximum entropy and Richardson-Lucy algorithms provide virtually identical reconstructions which are heavily disturbed by image noise. MREM largely suppresses this noise in the reconstructions, showing only the significant structures that are present in the data. Application of MREM to COMPTEL 1.8 MeV gamma-ray line data results in a 1.809 MeV sky map that is much smoother than the maximum entropy or Richardson-Lucy reconstructions presented previously. The essential features of this map are (1) an asymmetric galactic ridge emission reaching from l=45 deg to l=240 deg, (2) a bright localised emission feature in the Cygnus region around (l,b)=(80 deg,0 deg), (3) two emission spots at l=317 deg and l=332 deg situated in the galactic plane, and (4) an extended emission region around (l,b)=(160 deg, 0 deg). Comparison of the MREM map to the simulated reconstructions demonstrates that the 1.809 MeV emission is confined to the galactic plane.

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arXiv1999

Assessment of Tracers of 1.8 MeV Emission

We examined the question of a possible correlation between 1.8 MeV emission and intensity distributions observed at other wavelengths by comparing the 1.8 MeV data to an extended set of all-sky maps, covering the entire explored wavelength range from the radio band up to high-energy gamma-rays. This analysis revealed that tracers of the old stellar population or the local interstellar medium provide only a poor description of the data. Tracers of the young stellar population considerably improve the fit. Residuals are minimized for the free-free emission map obtained by the DMR at microwave wavelengths and the 158 um fine-structure line map of C+ observed by FIRAS. Within the statistics of the present data, both maps provide an overall satisfactory fit. We therefore conclude that adequate tracers of 26Al sources have now been identified. Implications for 26Al source types are discussed (see also Knodlseder, these proceedings).

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arXiv1999

A New Look at Image Reconstruction of COMPTEL 1.8 MeV Data

Deconvolving COMPTEL gamma-ray data into images presents a major methodological challenge. We developed a new algorithm called Multiresolution Regularized Expectation Maximization (MREM), which explicitly accounts for spatial correlations in the image by using wavelets (Dixon et al. 1998). We demonstrate that MREM largely suppresses image noise in the reconstruction, showing only significant structures that are present in the data. Application to 1.8 MeV data results in a sky map that is much smoother than the maximum entropy reconstructions presented previously, but it shows the same characteristic emission features which have been established earlier.

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arXiv1998

COMPTEL skymapping: a new approach using parallel computing

Large-scale skymapping with COMPTEL using the full survey database presents challenging problems on account of the complex response and time-variable background. A new approach which attempts to address some of these problems is described, in which the information about each observation is preserved throughout the analysis. In this method, a maximum-entropy algorithm is used to determine image and background simultaneously. Because of the extreme computing requirements, the method has been implemented on a parallel computer, which brings large gains since the response computation is fully parallelizable. The zero level is left undetermined in this method. Results using data from 7 years of COMPTEL data are presented.

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arXiv1998

Evidence for a Galactic gamma ray halo

We present quantitative statistical evidence for a $γ$-ray emission halo surrounding the Galaxy. Maps of the emission are derived. EGRET data were analyzed in a wavelet-based non-parametric hypothesis testing framework, using a model of expected diffuse (Galactic + isotropic) emission as a null hypothesis. The results show a statistically significant large scale halo surrounding the center of the Milky Way as seen from Earth. The halo flux at high latitudes is somewhat smaller than the isotropic gamma-ray flux at the same energy, though of the same order (O(10^(-7)--10^(-6)) ph/cm^2/s/sr above 1 GeV).

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arXiv1997

26Al in the local interstellar medium

We estimate the 1.8 MeV luminosity of the Sco-Cen association due to radioactive decay of 26Al to (4-15) 10e-5 ph cm**-2 s**-1. We propose a low surface brightness, limb brightened bubble for the 1.8 MeV intensity distribution. The detectibility of this distribution with existing gamma-ray telescopes is discussed.

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arXiv1997

Constraints from $^{26}$Al Measurements on the Galaxy's Recent Global Star Formation Rate and Core Collapse Supernovae Rate

Gamma-rays from the decay of $^{26}$Al offer a stringent constraint on the Galaxy's global star formation rate over the past million years, supplementing other methods for quantifying the recent Galactic star formation rate, such as equivalent widths of H$α$ emission. Advantages and disadvantages of using $^{26}$Al gamma-ray measurements as a tracer of the massive star formation rate are analyzed. Estimates of the Galactic $^{26}$Al mass derived from COMPTEL measurements are coupled with a simple, analytical model of the $^{26}$Al injection rate from massive stars and restrict the Galaxy's recent star formation rate to \hbox{5 $\pm$ 4 M\sun yr$^{-1}$}. In addition, we show that the derived $^{26}$Al mass implies a present day \hbox{Type II + Ib} supernovae rate of 3.4 $\pm$ 2.8 per century, which seems consistent with other independent estimates of the Galactic core collapse supernova rate. If some independent measure of the massive star initial mass function or star formation rate or \hbox{Type II + Ib} supernovae rate were to become available (perhaps through estimates of the Galactic $^{60}$Fe mass), then a convenient way to restrain, or possibly determine, the other parameters is presented.

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arXiv1996

COMPTEL detection of the high-energy gamma-ray source 2CG 135+01

We present an analysis of COMPTEL observations made between November 1991 and May 1994 of 2CG 135+01, a bright gamma-ray source located near the Galactic plane. At energies above 1 MeV, an excess consistent with the position of 2CG 135+01 is detected in the sum of the observations, at flux levels which are a factor of 10-100 below those published in the past. The detection significance of this excess, when the possible presence of underlying Galactic diffuse emission is neglected, is 6.6 sigma for 3 degrees of freedom. The differential photon spectrum in the 1-30 MeV energy range can be described by a power law with a spectral index of $1.95^{+0.2}_{-0.3}$. Due to the uncertainties involved in modelling the Galactic-disk diffuse emission underneath the source, the absolute flux levels must be considered uncertain by a factor of two. They are consistent with the extrapolation of the time-averaged spectrum of 2CG 135+01 measured with EGRET, thereby strengthening the identification. No significant temporal correlation between the gamma-ray emission and the monitored radio emission of the possible counterpart radio source GT 0236+610 (showing a 26.5 day modulation) is found.

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arXiv1996

Modelling the 1.8 MeV Sky: Tests for Spiral Structure

COMPTEL imaging analysis revealed a patchy, asymmetric distribution of diffuse 1.8 MeV emission along the Galactic plane, which is attributed to the decay of radioactive 26Al in the ISM. If massive stars were the major source of Galactic 26Al, the 1.8 MeV emission should be asymmetric and trace the spiral arms of the Galaxy, presumed site of massive star formation. Using model fits, we indeed find weak evidence in the COMPTEL data that the observed 1.8 MeV emission is at least partly confined to spiral arms. We derive a total Galactic 26Al mass of 2.5 Msol from which at least 0.7 Msol can be attributed to massive stars.

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arXiv1996

1.8 MeV Emission from the Carina Region

Significant 1.8 MeV emission from the Carina region has been detected by COMPTEL. The emission is concentrated within 6 degrees or less near the Carina nebula NGC 3372, one of the brightest HII regions known in our Galaxy. This region contains a wealth of extreme young open clusters whose massive stars possibly contributed to an enrichment of 26Al in the ISM within the last few million years. The relation of these clusters and the peculiar object Eta Carinae with the observed emission is discussed. The 26Al yield of the clusters is estimated using current theoretical nucleosynthesis models.

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