arXiv subjects

V. Tatischeff

Publications and source records attributed to V. Tatischeff.

At least 73 records · Page 4Linked to original sources

arXiv2005

Physical implications of INTEGRAL/SPI gamma-ray line measurements of the 2003 October 28 solar flare

The very powerful X-class solar flare of 2003 October 28 was detected with the INTEGRAL spectrometer as an intense gamma-ray flash of about 15 minutes. Despite the non-standard incidence of the solar gamma-rays, time-resolved spectra including several nuclear gamma-ray lines were obtained. Such a measurement with a high-energy-resolution instrument can provide valuable information of the isotopic abundances of the ambient solar material, as well as the composition, directionality and energy spectra of the accelerated nuclei. First results on the measured gamma-ray line ratios and time history of the neutron-capture line are presented.

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arXiv2004

Superbubbles and Energetic Particles in the Galaxy. I: Collective effects of particle acceleration

Observations indicate that most massive stars in the Galaxy appear in groups, called OB associations, where their strong wind activity generates large structures known as superbubbles, inside which the subsequent supernovae (SNe) explode, in tight space and time correlation. Acknowledging this fact, we investigate four main questions: 1) does the clustering of massive stars and SN explosions influence the particle acceleration process usually associated with SNe, and induce collective effects which would not manifest around isolated supernova remnants?; 2) does it make a difference for the general phenomenology of Galactic Cosmic Rays (GCRs), notably for their energy spectrum and composition?; 3) can this help alleviate some of the problems encountered within the standard GCR source model?; and 4) Is the link between superbubbles and energetic particles supported by observational data, and can it be further tested and constrained? We argue for a positive answer to all these questions. Theoretical, phenomenological and observational aspects are treated in separate papers. Here, we discuss the interaction of massive stellar winds and SN shocks inside superbubbles and indicate how this leads to specific acceleration effects. We also show that due to the high SN explosion rate and low diffusion coefficient, low-energy particles experience repeated shock acceleration inside superbubbles.

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arXiv2004

High-energy emission from the stellar wind collisions in gamma-2 Velorum

The binary system gamma-2 Velorum (WC8+O7.5) contains the nearest known Wolf-Rayet star to the Sun, at a distance of 258$_{-31}^{+41}$ pc. Its strong radio emission shows evidence for a partially absorbed nonthermal component, which has been interpreted as synchrotron emission from electrons accelerated in the colliding wind region. Inverse Compton cooling of these electrons in the intense UV radiation field from the O-type companion star could produce a significant hard X-ray and gamma-ray emission, whose flux depends on the ratio of the energy densities of magnetic to seed photon fields. The Vela region was observed with the INTEGRAL satellite in 2003, as part of the Core Programme. No signals from gamma-2 Vel are detected in the images obtained with the IBIS/ISGRI coded aperture instrument in the energy ranges 20-40 and 40-80 keV. From the derived 3$σ$ upper limits, we show that the average magnetic field near the region of stellar wind collision should be relatively high, greater than about 1 G. The high-energy emission of gamma-2 Vel might be detected with the forthcoming GLAST experiment.

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arXiv2003

Gamma-ray lines from cosmic-ray interactions with interstellar dust grains

As pointed out by Lingenfelter and Ramaty (1977), the shapes of some gamma-ray lines produced by cosmic-ray interactions with the interstellar medium potentially contain valuable information on the physical properties of dust grains, including their compositions and size distributions. The most promising of such lines are at 847, 1369, 1779 and 6129 keV, from 56-Fe*, 24-Mg*, 28-Si* and 16-O*, respectively. We performed detailed calculations of their profiles using, in particular, available laboratory measurements combined with optical model calculations to evaluate the energy distributions of the recoiling excited nuclei. We show that the line shapes are mainly sensitive to relatively large interstellar grains, with radii greater than 0.25 microns. Line fluxes from the inner Galaxy are then predicted.

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arXiv2003

Calibration of the Spectrometer aboard the INTEGRAL satellite

SPI, the Spectrometer on board the ESA INTEGRAL satellite, to be launched in October 2002, will study the gamma-ray sky in the 20 keV to 8 MeV energy band with a spectral resolution of 2 keV for photons of 1 MeV, thanks to its 19 germanium detectors spanning an active area of 500 cm2. A coded mask imaging technique provides a 2 deg angular resolution. The 16 deg field of view is defined by an active BGO veto shield, furthermore used for background rejection. In April 2001 the flight model of SPI underwent a one-month calibration campaign at CEA in Bruyères le Châtel using low intensity radioactive sources and the CEA accelerator for homogeneity measurements and high intensity radioactive sources for imaging performance measurements. After integration of all scientific payloads (the spectrometer SPI, the imager IBIS and the monitors JEM-X and OMC) on the INTEGRAL satellite, a cross-calibration campaign has been performed at the ESA center in Noordwijk. A set of sources has been placed in the field of view of the different instruments in order to compare their performances and determine their mutual influence. Some of those sources had already been used in Bruyères during the SPI standalone test. For the lowest energy band calibration an X-ray generator has been used. We report on the scientific goals of this calibration activity, and present the measurements performed as well as some preliminary results.

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arXiv2003

New determinations of gamma-ray line intensities of the Ep = 550 keV and Ep = 1747 keV resonances of the 13-C(p,gamma)14-N reaction

Gamma-ray angular distributions for the resonances at Ep = 550 keV and 1747 keV of the radiative capture reaction 13-C(p,g)14-N have been measured, using intense proton beams on isotopically pure 13-C targets. Relative intensities for the strongest transitions were extracted with an accuracy of typically five per cent, making these resonances new useful gamma-ray standards for efficiency calibration in the energy range Egamma = 1.6 to 9 MeV.

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arXiv2003

INTEGRAL/SPI ground calibration

Three calibration campaigns of the spectrometer SPI have been performed before launch in order to determine the instrument characteristics, such as the effective detection area, the spectral resolution and the angular resolution. Absolute determination of the effective area has been obtained from simulations and measurements. At 1 MeV, the effective area is 65 cm^2 for a point source on the optical axis, the spectral resolution ~2.3 keV. The angular resolution is better than 2.5 deg and the source separation capability about 1 deg. Some temperature dependant parameters will require permanent in-flight calibration.

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arXiv2003

Cross sections relevant to gamma-ray line emission in solar flares: $^3$He-induced reactions on $^{16}$O nuclei

Gamma-ray production cross sections have been measured for gamma-ray lines copiously emitted in the $^3$He bombardment of $^{16}$O nuclei: the 937, 1042 and 1081 keV lines of $^{18}$F and the 1887 keV line of $^{18}$Ne. Four Ge detectors with BGO shielding for Compton suppression were used to measure the angular distributions of the gamma-rays. The excitation functions have been obtained for $^3$He bombarding energies from 3.7 to 36 MeV. Total cross sections are tabulated for calculations relevant to gamma-ray astronomy. The importance of these lines as diagnosis for the presence and properties of accelerated $^3$He in solar flares is discussed in light of the measured cross sections.

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arXiv2003

D(18F,pa)15N reaction applied to nova gamma-ray emission

The 18F(p,alpha)15O reaction is recognized to be one of the most important reactions for nova gamma-ray astronomy as it governs the early E <= 511keV gamma emission. However in the nova temperature regime, its rate remains largely uncertain due to unknown low-energy resonance strengths. We report here the measurement of the D(18F,p)19F(alpha)15N one-nucleon transfer reaction, induced by a 14 MeV 18F radioactive beam impinging on a CD2 target; outgoing protons and 15N (or alpha-particles) were detected in coincidence in two silicon strip detectors. A DWBA analysis of the data resulted in new limits to the contribution of low-energy resonances to the rate of the 18F(p,alpha)15O reaction.

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arXiv2002

Study of the ^18F(p, alpha)^15O reaction for application to nova gamma-ray emission

The ^18F(p, alpha)^15O reaction is recognized as one of the most important reaction for nova gamma-ray astronomy as it governs the early =< 511 keV emission. However, its rate remains largely uncertain at nova temperatures due to unknown low-energy resonance strengths. In order to better constrain this reaction rate, we have studied the one-nucleon transfer reaction, D(^18F,p alpha)^15N, at the CRC-RIB facility at Louvain La Neuve.

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arXiv2002

A new experiment for the determination of the 18F(p,alpha) reaction rate at nova temperatures

The 18F(p,alpha) reaction was recognized as one of the most important for gamma ray astronomy in novae as it governs the early 511 keV emission. However, its rate remains largely uncertain at nova temperatures. A direct measurement of the cross section over the full range of nova energies is impossible because of its vanishing value at low energy and of the short 18F lifetime. Therefore, in order to better constrain this reaction rate, we have performed an indirect experiment taking advantage of the availability of a high purity and intense radioactive 18F beam at the Louvain La Neuve RIB facility. We present here the first results of the data analysis and discuss the consequences.

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arXiv2001

Shape of the 4.438 MeV gamma-ray line of ^12C from proton and alpha-particle induced reactions on ^12C and ^16O

We calculated in detail the angular distribution of gamma-rays and the resulting shape of the gamma-ray line produced by the nuclear deexcitation of the 4.439 MeV state of ^12C following proton and alpha-particle interactions with ^12C and ^16O in the energy range from threshold to 100 MeV per nucleon, making use of available experimental data. In the proton energy range from 8.6 to 20 MeV, the extensive data set of a recent accelerator experiment on gamma-ray line shapes and angular distributions was used to deduce parameterizations for the gamma-ray emission of the 2^+, 4.439 MeV state of ^12C following inelastic proton scattering off ^12C and proton induced spallation of ^16O. At higher proton energies and for alpha-particle induced reactions, optical model calculations were the main source to obtain the needed reaction parameters for the calculation of gamma-ray line shapes and angular distributions. Line shapes are predicted for various interaction scenarios of accelerated protons and alpha-particles in solar flares.

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arXiv2001

Gamma-Ray Line Emission from 7-Li and 7-Be Production by Cosmic-Rays

We calculate the total gamma-ray line emission at ~450 keV that accompanies 7-Li and 7-Be production by cosmic-ray interactions, including the delayed line emission at 0.478 MeV from 7-Be radioactive decay. We present a new gamma-ray spectroscopic test which has the potential to give direct information on the nature of the interstellar regions into which 7-Be ions propagate and decay. Finally, we evaluate the intensity of the predicted diffuse emission from the central radian of the Galaxy.

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arXiv2001

Low energy measurement of the 7Be(p,gamma)8B cross section

We have measured the cross section of the 7Be(p,gamma)8B reaction for E_cm = 185.8 keV, 134.7 keV and 111.7 keV using a radioactive 7Be target (132 mCi). Single and coincidence spectra of beta^+ and alpha particles from 8B and 8Be^* decay, respectively, were measured using a large acceptance spectrometer. The zero energy S factor inferred from these data is 18.5 +/- 2.4 eV b and a weighted mean value of 18.8 +/- 1.7 eV b (theoretical uncertainty included) is deduced when combining this value with our previous results at higher energies.

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arXiv2000

Lithium-6 from Solar Flares

By introducing a hitherto ignored Li-6 producing process, due to accelerated He-3 reactions with He-4, we show that accelerated particle interactions in solar flares produce much more Li-6 than Li-7. By normalizing our calculations to gamma-ray data we demonstrate that the Li-6 produced in solar flares, combined with photospheric Li-7, can account for the recently determined solar wind lithium isotopic ratio, obtained from measurements in lunar soil, provided that the bulk of the flare produced lithium is evacuated by the solar wind. Further research in this area could provide unique information on a variety of problems, including solar atmospheric transport and mixing, solar convection and the lithium depletion issue, and solar wind and solar particle acceleration.

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arXiv1999

Nonthermal X-Rays from the Galactic Ridge: a Tracer of Low Energy Cosmic Rays ?

A distinct low energy cosmic-ray component has been proposed to explain the essentially constant Be/Fe ratio at low metallicities. Atomic collisions of such low energy ions produce characteristic nonthermal X-ray emission. In this paper, we study the possible contribution of such X-rays to the Galactic ridge emission. We show that they would account for < 10% of the 10-60 keV luminosity of the thin Galactic disk component detected with RXTE. They could make a more significant contribution in the 0.5-10 keV energy range, provided that the nonthermal ion population extends down to about 1 MeV/nucleon and delivers about 10$^{42}$ erg s$^{-1}$ to the interstellar medium, comparable to the total power suplied by the Galactic supernovae. But since the nonthermal X-rays in this energy range are essentially produced below the thresholds of the Be-producing cross sections, their detection does not necessarily imply a low energy cosmic-ray origin for the spallogenic light elements. A significant contribution of nonthermal X-rays could alleviate the problem of the origin of the hard component observed with ASCA in the Scutum arm region.

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arXiv1998

Diffuse 0.5-1 keV X-Rays and Nuclear Gamma-Rays from Fast Particles in the Local Hot Bubble

We show that interactions of fast particles with the boundary shell of the local hot bubble could make an important contribution to the 0.5-1 keV diffuse X-ray background observed with ROSAT. The bulk of these nonthermal X-rays are due to line emission from fast O ions of energies around 1 MeV/nucleon. This is the typical energy per particle in the ejecta of the supernova which is thought to have energized the bubble. We find that there is sufficient total energy in the ejecta to produce X-rays of the required intensity, subject to the details of the evolution of the fast particle population since the supernova explosion (about 3 10$^5$ years ago based on the age of the Geminga pulsar). The unequivocal signature of lines from deexcitations in fast ions is their large width ($δE/E$~0.1 for O lines), which will clearly distinguishes them from X-ray lines produced in a hot plasma. If a small fraction of the total ejecta energy is converted into accelerated particle kinetic energy (>~30 MeV/nucleon), the gamma-ray line emission produced in the boundary shell of the local hot bubble could account for the recently reported COMPTEL observations of nuclear gamma-ray lines from a broad region towards the Galactic center.

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arXiv1998

X-Rays from Accelerated Ion Interactions

We have developed in detail the theory of X-ray line and continuum production due to atomic interactions of accelerated ions, incorporating in our calculations information from a broad range of laboratory measurements. We applied our calculations to the Orion region from which nuclear gamma-ray lines were observed with the COMPTEL instrument on CGRO. The accelerated particles which produce this gamma-ray emission via nuclear reactions also produce X-ray lines via atomic interactions. We predict strong line emission in the range from 0.5 to 1 keV, mainly due to de-excitations in fast O ions. While much of the diffuse X-ray emission observed with ROSAT from Orion could be due to accelerated ions, the current X-ray data do not provide unambiguous signatures for such an origin. If future observations with high spectral resolution would confirm the predicted X-rays, the combined analysis of the X-ray and gamma-ray data will set important constraints on the origin of the accelerated particles and their interaction model.

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