Vincent Tatischeff

Low-energy measurement of the Be-7(p,gamma) B-8 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. Comment: Accepted for publication in Phys. Rev. Lett

Properties of the energetic particle distributions during the October 28, 2003 solar flare from INTEGRAL/SPI observations

Analysis of spectra obtained with the gamma-ray spectrometer SPI onboard INTEGRAL of the GOES X17-class flare on October 28, 2003 is presented. In the energy range 600 keV - 8 MeV three prominent narrow lines at 2.223, 4.4 and 6.1 MeV, resulting from nuclear interactions of accelerated ions within the solar atmosphere could be observed. Time profiles of the three lines and the underlying continuum indicate distinct phases with several emission peaks and varying continuum-to-line ratio for several minutes before a smoother decay phase sets in. Due to the high-resolution Ge detectors of SPI and the exceptional intensity of the flare, detailed studies of the 4.4 and 6.1 MeV line shapes was possible for the first time. Comparison with calculated line shapes using a thick target interaction model and several energetic particle angular distributions indicates that the nuclear interactions were induced by downward-directed particle beams with alpha-to-proton ratios of the order of 0.1. There are also indications that the 4.4 MeV to 6.1 MeV line fluence ratio changed between the beginning and the decay phase of the flare, possibly due to a temporal evolution of the energetic particle alpha-to-proton ratio.

First identification and modelling of SPI background lines

On Oct. 17, 2002, the ESA INTEGRAL observatory was launched into a highly elliptical orbit. SPI, a high resolution Ge spectrometer covering an energy range of 20-8000 keV, is one of its two main instruments. We use data recorded early in the mission (i.e. in March 2003) to characterize the instrumental background, in particular the many gamma-ray lines produced by cosmic-ray interactions in the instrument and spacecraft materials. More than 300 lines and spectral features are observed, for about 220 of which we provide identifications. An electronic version of this list, which will be updated continuously, is available for download at CESR. We also report first results from our efforts to model these lines by ab initio Monte Carlo simulation.

Evidence for nonlinear diffusive shock acceleration of cosmic rays in the 2006 outburst of the recurrent nova RS ophiuchi

Spectroscopic observations of the 2006 outburst of the recurrent nova RS Ophiuchi at both infrared (IR) and X-ray wavelengths have shown that the blast wave has decelerated at a higher rate than predicted by the standard test-particle adiabatic shock wave model. Here we show that the observed evolution of the nova remnant can be explained by the diffusive shock acceleration of particles at the blast wave and the subsequent escape of the highest energy ions from the shock region. Nonlinear particle acceleration can also account for the difference of shock velocities deduced from the IR and X-ray data. The maximum energy that accelerated electrons and protons can have achieved in few days after outburst is found to be as high as a few TeV. Using the semianalytic model of nonlinear diffusive shock acceleration developed by Berezhko & Ellison, we show that the postshock temperature of the shocked gas measured with RXTE PCA and Swift XRT imply a relatively moderate acceleration efficiency characterized by a proton injection rate ηinj 10-4.

Energetic Constraints on In Situ Production of Short-Lived Radionuclei in the Early Solar System

We calculate upper limits on the amount of short-lived radionuclei that can be produced by nonthermal nucleosynthesis in the early solar system. Using energetic constraints obtained from X-ray observations of young stellar objects, we show that irradiation of bare solids can produce 10Be and 41Ca at levels compatible with a homogeneous distribution over the entire protoplanetary disk up to the comet-forming region. 53Mn and 36Cl cannot be produced at canonical levels together with 10Be and 41Ca, unless we posit a heterogeneous spatial distribution. The high level of 7Be suggested recently is barely compatible with a coproduction of 10Be up to the cometary reservoir and may indicate the irradiation of a gas phase. Finally, we show that the maximum amount of irradiation-induced 26Al can only account for a homogeneous distribution of this radionuclide over a rocky reservoir of 2-3 M⊕ and that the well-defined canonical 26Al/27Al ratio observed in Ca-Al-rich inclusions is probably not compatible with an in situ production in the embedded phase of the Sun. If extinct 26Al is detected in the cometary material from the Stardust mission, the nucleosynthetic process that produced this preeminent high-resolution chronometer should be searched for in stellar events contemporary with the birth of the Sun.

Delayed X- and Gamma-Ray Line Emission from Solar Flare Radioactivity

We have studied the radioactive line emission expected from solar active regions after large flares, following the production of long-lived radioisotopes by nuclear interactions of flare-accelerated ions. This delayed X- and gamma-ray line emission can provide unique information on the accelerated particle composition and energy spectrum, as well as on mixing processes in the solar atmosphere. Total cross sections for the formation of the main radioisotopes by proton, 3He, and α-particle reactions are evaluated from available data combined with nuclear reaction theory. Thick-target radioisotope yields are provided in tabular form, which can be used to predict fluxes of all of the major delayed lines at any time after a gamma-ray flare. The brightest delayed line for days after the flare is found to be the 511 keV positron-electron annihilation line resulting from the decay of several β+ radioisotopes. After ~2 days however, the flux of the e+-e- annihilation line can become lower than that of the 846.8 keV line from the decay of 56Co into 56Fe. Our study has revealed other delayed gamma-ray lines that appear to be promising for detection, e.g., at 1434 keV from the radioactivity of both the isomer 52Mnm (T1/2 = 21.1 minutes) and the ground state 52Mng (T1/2 = 5.59 days), 1332 and 1792 keV from 60Cu (T1/2 = 23.7 minutes), and 931.1 keV from 55Co (T1/2 = 17.5 hr). The strongest delayed X-ray line is found to be the Co Kα at 6.92 keV, which is produced from both the decay of the isomer 58Com (T1/2 = 9.04 hr) by the conversion of a K-shell electron and the decay of 57Ni (T1/2 = 35.6 hr) by orbital electron capture. Prospects for observation of these lines with RHESSI or future space instruments are discussed.

Discovery of K-Shell Emission Lines of Neutral Atoms in the Galactic Center Region

The K-shell emission line of neutral iron from the Galactic center (GC) region is one of the keys concerning the structure and activity of the GC. The origin is still an open question, but possibly due either to X-ray radiation or to electron bombarding to neutral atoms. To address this issue, we analyzed the Suzaku X-ray spectrum from the GC region of intense neutral iron line emission, and report on the discovery of K˛ lines of neutral argon, calcium, chrome, and manganese atoms. The equivalent widths of these K˛ lines indicate that the metal abundances in the GC region should be � 1.6 and � 4 of the solar value, depending on the X-ray and electron origins, respectively. On the other hand, the metal abundances in the hot plasma in the GC region are found to be � 1–2 solar. These results favor that the origin of the neutral K˛ lines is due to X-ray irradiation.

Comparison of low-energy resonances in15N(α,γ)19F and15O(α,γ)19Ne and related uncertainties

A disagreement between two determinations of Gamma_alpha of the astro- physically relevant level at E_x=4.378 MeV in 19F has been stated in two recent papers by Wilmes et al. and de Oliveira et al. In this work the uncertainties of both papers are discussed in detail, and we adopt the value Gamma_alpha=(1.5^{+1.5}_{-0.8})10^-9eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei Gamma_gamma(19F) approx Gamma_gamma(19Ne), theta^2_alpha(19F) approx theta^2_alpha(19Ne) are discussed, together with the resulting uncertainties on the resonance strengths in 19Ne and on the 15O(alpha,gamma)19Ne rate.A disagreement between two determinations of {Gamma}{sub {alpha}} of the astrophysically relevant level at E{sub x}=4.378 MeV in {sup 19}F has been stated in two recent papers by Wilmes {ital et al.} and de Oliveira {ital et al.} In this work the uncertainties of both papers are discussed in detail, and we adopt the value {Gamma}{sub {alpha}} = (1.5{sub {minus}0.8}{sup +1.5}){times}10{sup {minus}9} eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei {Gamma}{sub {gamma}}({sup 19}F){approx}{Gamma}{sub {gamma}}({sup 19}Ne), {theta}{sub {alpha}}{sup 2}({sup 19}F){approx}{theta}{sub {alpha}}{sup 2}({sup 19}Ne) are discussed, together with the resulting uncertainties on the resonance strengths in {sup 19}Ne and on the {sup 15}O({alpha},{gamma}){sup 19}Ne rate. {copyright} {ital 1997} {ital The American Physical Society}

Variation of the X-ray non-thermal emission in the Arches cloud

The origin of the iron fluorescent line at 6.4 keV from an extended region surrounding the Arches cluster is debated and the non-variability of this emission up to 2009 has favoured the low-energy cosmic ray origin over a possible irradiation by hard X-rays. By probing the variability of the Arches cloud non-thermal emission in the most recent years, including a deep observation in 2012, we intend to discriminate between the two competing scenarios. We perform a spectral fit of XMM-Newton observations collected from 2000 to 2013 in order to build the Arches cloud light curve corresponding to both the neutral Fe Kα line and the X-ray continuum emissions. We reveal a 30 per cent flux drop in 2012, detected with more than 4σ significance for both components. This implies that a large fraction of the studied non-thermal emission is due to the reflection of an X-ray transient source.

The 7Be(d, p)2? Cross Section at Big Bang Energies and the Primordial 7Li Abundance

The WMAP satellite, devoted to observations of the anisotropies of the cosmic microwave background radiation, has recently provided a determination of the baryonic density of the universe with unprecedented precision. Using this, big bang nucleosynthesis calculations predict a primordial 7Li abundance that is a factor of 2-3 higher than that observed in Galactic halo dwarf stars. It has been argued that this discrepancy could be resolved if the 7Be(d, p)2? reaction rate were around a factor of 100 larger than has previously been considered. We have now studied this reaction, for the first time at energies appropriate to the big bang environment, at the CYCLONE radioactive-beam facility at Louvain-la-Neuve. The cross section was found to be a factor of 10 smaller than derived from earlier measurements. It is concluded therefore that nuclear uncertainties cannot explain the discrepancy between observed and predicted primordial 7Li abundances, and an alternative astrophysical solution must be investigated.