V. Tatischeff

Vinča Institute of Nuclear Sciences, Beograd, Central Serbia, Serbia

Are you V. Tatischeff?

Claim your profile

Publications (110)233.95 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a European Space Agency hard X-ray/γ-ray observatory for astrophysics, covering photon energies from 15 keV to 10 MeV. It was launched in 2002, and since then the Bismuth Germanate (BGO) detectors of the Anti-Coincidence Shield (ACS) of the Spectrometer on INTEGRAL (SPI) have detected many hard X-ray (HXR) bursts from the Sun, producing light curves at photon energies above ≈ 100 keV. The spacecraft has a highly elliptical orbit, providing long uninterrupted observing (about 90 % of the orbital period) with nearly constant background due to the shorter time needed to cross Earth’s radiation belts. However, because of technical constraints, INTEGRAL cannot be pointed at the Sun, and high-energy solar photons are always detected in nonstandard observation conditions. To make the data useable for solar studies, we have undertaken a major effort to specify the observing conditions through Monte Carlo simulations of the response of ACS for several selected flares. We checked the performance of the model employed for the Monte Carlo simulations using the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations for the same sample of solar flares. We conclude that although INTEGRAL was not designed to perform solar observations, ACS is a useful instrument for solar-flare research. In particular, its relatively large effective area allows determining good-quality HXR/γ-ray light curves for X- and M-class solar flares and, in some cases, probably also for C-class flares.
    Solar Physics 05/2014; 289(5). · 3.26 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: The observed primordial 7Li abundance in metal-poor halo stars is found to be lower than its Big-Bang nucleosynthesis (BBN) calculated value by a factor of approximately three. Some recent works suggested the possibility that this discrepancy originates from missing resonant reactions which would destroy the 7Be, parent of 7Li. The most promising candidate resonances which were found include a possibly missed 1- or 2- narrow state around 15 MeV in the compound nucleus 10C formed by 7Be+3He and a state close to 7.8 MeV in the compound nucleus 11C formed by 7Be+4He. In this work, we studied the high excitation energy region of 10C and the low excitation energy region in 11C via the reactions 10B(3He,t)10C and 11B(3He,t)11C, respectively, at the incident energy of 35 MeV. Our results for 10C do not support 7Be+3He as a possible solution for the 7Li problem. Concerning 11C results, the data show no new resonances in the excitation energy region of interest and this excludes 7Be+4He reaction channel as an explanation for the 7Li deficit.
    12/2013; Physical Review C(88).
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: INTEGRAL is a hard X-ray/gamma-ray observatory for astrophysics (ESA) covering photon energies from 15 keV to 10 MeV. It was launched in 2002 and since then the BGO detectors of the Anti-Coincidence shield (ACS) of the SPI spectrometer have detected many hard X-ray (HXR) bursts from the Sun, producing lightcurves at photon energies above ~ 100 keV. The spacecraft has a highly elliptical orbit, providing a long uninterrupted observing time (about 90% of the orbital period) with nearly constant background due to the reduction of the crossing time of the Earth's radiation belts. However, due to technical constraints, INTEGRAL cannot point to the Sun and high-energy solar photons are always detected in non-standard observation conditions. To make the data useful for solar studies, we have undertaken a major effort to specify the observing conditions through Monte-Carlo simulations of the response of ACS for several selected flares. We check the performance of the model employed for the Monte-Carlo simulations using RHESSI observations for the same sample of solar flares. We conclude that, despite the fact that INTEGRAL was not designed to perform solar observations, ACS is a useful instrument in solar flare research. In particular, its relatively large effective area allows the determination of good-quality HXR/gamma-ray lightcurves for X- and M-class solar flares and, in some cases, probably also for C-class flares.
    08/2013;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics.
    06/2013;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: The study of both supernova remnants and the hot and cold phases of the interstellar medium are essential for understanding the final stages of stellar evolution and their feedback on the evolution of galaxies through injection of energy and heavy elements. These studies are also crucial for understanding the physics of supernovae, their cosmological implication, and the origin of galactic cosmic rays. The unique capabilities of Athena+ will allow us to explore a new parameter space. Spatially-resolved high-resolution spectroscopy using Athena+ X-IFU of young remnants will allow to characterize individual parcels of ejected material in the line of sight in terms of kinematics, ionization and composition, providing access to the three dimensional geometry of the explosion. Athena+ will also allow studying shock physics and particle acceleration in supernova remnants, as well as their interaction with their environment. Athena+ X-IFU will also characterize the ionization mechanisms competing in forming the complex structures of the hot interstellar medium, likely to keep the echo of past star formation activity, both in our Galaxy and nearby ones. For the first time the dust and gas of the densest cold medium, like in the Galactic Centre environment, will be studied. Athena+ X-IFU will observe, along with the Mg K and Si K edges, which are the main tracers of the silicates content of the ISM, the Fe K edge with unprecedented sensitivity and energy-resolution. This will allow us to study for the first time the nature of Fe-bearing dust in such regions.
    06/2013;
  • [show abstract] [hide abstract]
    ABSTRACT: We investigate the origin of the diffuse 6.4 keV line emission recently detected by Suzaku and the source of H_2ionization in the diffuse molecular gas of the Galactic Center (GC) region. We show that Fe atoms and H_2 molecules in the diffuse interstellar medium of the GC are not ionized by the same particles. The Fe atoms are most likely ionized by X-ray photons emitted by Sgr A* during a previous period of flaring activity of the supermassive black hole. The measured longitudinal intensity distribution of the diffuse 6.4 keV line emission is best explained if the past activity of Sgr A$* lasted at least several hundred years and released a mean 2-100 keV luminosity > 10^38} erg s^{-1}. The H_2 molecules of the diffuse gas can not be ionized by photons from Sgr A*, because soft photons are strongly absorbed in the interstellar gas around the central black hole. The molecular hydrogen in the GC region is most likely ionized by low-energy cosmic rays, probably protons rather than electrons, whose contribution into the diffuse 6.4 keV line emission is negligible.
    The Astrophysical Journal Letters 06/2013; 771(2). · 6.35 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: The decay of ^{19}O(β^{-}) and ^{19}Ne(β^{+}) implanted in niobium in its superconducting and metallic phases was measured using purified radioactive beams produced by the SPIRAL GANIL facility. Half-lives and branching ratios measured in the two phases are consistent within a 1σ error bar. This measurement casts strong doubts on the predicted strong electron screening in a superconductor, the so-called superscreening. The measured difference in screening potential energy is 110(90) eV for ^{19}Ne and 400(320) eV for ^{19}O. Precise determinations of the half-lives were obtained for ^{19}O, 26.476(9) s, and for ^{19}Ne, 17.254(5) s.
    Physical Review Letters 01/2013; 110(3):032501. · 7.94 Impact Factor
  • V. Tatischeff, A. Decourchelle, G. Maurin
    [show abstract] [hide abstract]
    ABSTRACT: We have studied in detail the production of nonthermal line and continuum X-rays by interaction of accelerated electrons and ions with a neutral ambient gas, and have applied the resulting models to XMM-Newton observations of the X-ray emission emanating from the Arches cluster region near the Galactic center. The diffuse X-ray emission prominent in the 6.4keV Fe Kα line surrounding the very massive cluster is likely excited by low-energy cosmic ray ions produced in the ongoing supersonic collision between the star cluster and an adjacent molecular cloud. The nonthermal emission from this region probably offers at present the best available signature for a source of low-energy hadronic cosmic rays in the Galaxy.
    01/2013;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: Recent observations of high ionization rates of molecular hydrogen in diffuse interstellar clouds point to a distinct low-energy cosmic-ray component. Supposing that this component is made of nuclei, two models for the origin of such particles are explored and low-energy cosmic-ray spectra are calculated which, added to the standard cosmic ray spectra, produce the observed ionization rates. The clearest evidence of the presence of such low-energy nuclei between a few MeV per nucleon and several hundred MeV per nucleon in the interstellar medium would be a detection of nuclear \gamma-ray line emission in the range E_ 0.1 - 10 MeV, which is strongly produced in their collisions with the interstellar gas and dust. Using a recent \gamma-ray cross section compilation for nuclear collisions, \gamma-ray line emission spectra are calculated alongside with the high-energy \gamma-ray emission due to {\pi} 0 decay, the latter providing normalization of the absolute fluxes by comparison with Fermi-LAT observations of the diffuse emission above E \gamma = 0.1 GeV. Our predicted fluxes of strong nuclear \gamma-ray lines from the inner Galaxy are well below the detection sensitivies of INTEGRAL, but a detection, especially of the 4.4-MeV line, seems possible with new-generation \gamma-ray telescopes based on available technology. We predict also strong \gamma-ray continuum emission in the 1-8 MeV range, which in a large part of our model space for low-energy cosmic rays exceeds considerably estimated instrument sensitivities of future telescopes.
    The Astrophysical Journal 12/2012; 763(2). · 6.73 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: The high energy activity in the inner few degrees of the Galactic center is traced by diffuse radio, X-ray and gamma-ray emission. The physical relationship between different components of diffuse gas emitting at multiple wavelengths is a focus of this work. We first present radio continuum observations using Green Bank Telescope and model the nonthermal spectrum in terms of a broken power-law distribution of GeV electrons emitting synchrotron radiation. We show that the emission detected by Fermi is primarily due to nonthermal bremsstrahlung produced by the population of synchrotron emitting electrons in the GeV energy range interacting with neutral gas. The extrapolation of the electron population measured from radio data to low and high energies can also explain the origin of FeI 6.4 keV line and diffuse TeV emission, as observed with Suzaku, XMM-Newton, Chandra and the H.E.S.S. observatories. The inferred physical quantities from modeling multi-wavelength emission in the context of bremsstrahlung emission from the inner 300x120 parsecs of the Galactic center are constrained to have the cosmic ray ionization rate 1-10x10^{-15} s^-1, molecular gas heating rate elevating the gas temperature to 75-200K, fractional ionization of molecular gas 10^{-6} to 10^{-5}, large scale magnetic field 10-20 micro Gauss, the density of diffuse and dense molecular gas 100 and 10^3 cm^{-3} over 300pc and 50pc pathlengths, and the variability of FeI Kalpha 6.4 keV line emission on yearly time scales. Important implications of our study are that GeV electrons emitting in radio can explain the GeV gamma-rays detected by Fermi and that the cosmic ray irradiation model, like the model of the X-ray irradiation triggered by past activity of Sgr A*, can also explain the origin of the variable 6.4 keV emission from Galactic center molecular clouds.
    The Astrophysical Journal 06/2012; 762(1). · 6.73 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: The γ-ray spectra ol the strongest solar flares often show a broad and complex structure in the 0.1-10 MeV region sitting on a bremsstrahlung continuum. This structure is composed of several outstanding narrow lines and of thousands of unresolved narrow and broad lines forming a quasi-continuum. The major part of this emission is due to prompt deexcitation lines following nuclear interactions of accelerated light and heavy ions with the atomic nuclei composing the solar atmosphere. A similar emission is expected from interactions of galactic cosmic rays with the interstellar gas and dust. Experimental nuclear reaction studies coupled with extensive calculations have been done in the last one and a half decade at Orsay for the modelisation of this γ-ray emission. After a description of the nuclear reaction studies the analysis of one solar flare spectrum and predictions for the emission from the inner Galaxy will be presented.
    Journal of Physics Conference Series 05/2012; 366(1):2026-.
  • [show abstract] [hide abstract]
    ABSTRACT: SEPServer is set out to make the first database of particle and corresponding EM observations of solar energetic particle (SEP) events over roughly three solar cycles. It will also provide users with results from the scientific analysis of multiple datasets using different observational and simulation based methods. Therefore, SEPServer will lead to new perspectives of scientific analysis and will serve as a new asset valuable for SEP and Space Weather research. In this contribution, the event of 13 July 2005 has been used as a case study, which is a proxy for the overall information that the SEPServer will include and at the same time it reveals the capabilities offered to the future users of SEPServer. The analysis of the 13 July 2005 event - focusing on the data driven analysis, i.e., onset and release time determination from SOHO/ERNE, SOHO/EPHIN and ACE/EPAM together with pitch angle distributions from ACE/EPAM, simulations based on WIND/3DP and ACE/EPAM electrons as well as direct comparison of the observed SEP fluxes with the associated electromagnetic emissions - is performed. The physical interpretation and the interconnection of the experimental and the simulation based results are discussed in detail. The 13 July 2005 case study exemplifies the future usage of SEPServer, which will provide a comprehensive and up to date SEP analysis service. Acknowledgements: The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement No 262773 (SEPServer).
    04/2012;
  • [show abstract] [hide abstract]
    ABSTRACT: The 12C(α,γ )16O reaction plays a crucial role in stellar evolution. The rate of this reaction determines directly the 12C-to-16O abundance ratio at the end of the helium burning phase of stars and consequently has a big effect on the subsequent nucleosynthesis and even on the evolution of massive stars. However, despite many experimental studies, the low-energy cross section of 12C(α,γ )16O remains uncertain. The extrapolation of the measured cross sections to stellar energies (E ∼ 300 keV) is made particularly difficult by the presence of the 2+ (Ex = 6.92 MeV) and 1− (Ex = 7.12 MeV) subthreshold states of 16O. To further investigate the contribution of these two subthreshold resonances to the 12C(α,γ )16O cross section, we determine their α-reduced widths via a measurement of the transfer reaction 12C(7Li, t )16O at two incident energies, 28 and 34 MeV. The uncertainties on the determined α-spectroscopic factors and the α-reduced widths were reduced thanks to a detailed distorted-wave Born approximation analysis of the transfer angular distributions measured at the two incident energies. The R-matrix calculations of 12C(α,γ )16O cross section using our obtained α-reduced widths for the 2+ and 1− subthreshold resonances lead to an E1 S factor at 300 keV of 100 ± 28 keV b, which is consistent with values obtained in most of the direct and indirect measurements as well as the NACRE collaboration compilation while the result for the E2 component SE2 (300 keV) = 50 ± 19 keV b disagrees with the NACRE adopted value .
    Physical Review C 03/2012; 85:035804. · 3.72 Impact Factor
  • 01/2012;
  • Source
    V. Tatischeff, J. Kiener
    [show abstract] [hide abstract]
    ABSTRACT: Cosmic rays of kinetic energies below ~1 GeV per nucleon are thought to play a key role in the chemistry and dynamics of the interstellar medium. They are also thought to be responsible for nucleosynthesis of the light elements Li, Be, and B. However, very little is known about the flux and composition of low-energy cosmic rays since the solar modulation effect makes impossible a direct detection of these particles near Earth. We first discuss the information that the light elements have brought to cosmic-ray studies. We then discuss the prospects for detection of nuclear gamma-ray line emission produced by interaction of low-energy cosmic rays with interstellar nuclei.
    09/2011;
  • [show abstract] [hide abstract]
    ABSTRACT: The 4 million solar masses black hole at the Galactic centre (GC) is remarkably quiet today. The molecular cloud Sgr B2, among others, emits strong Fe Kalpha photons as well as hard X-rays up to 100 keV. This has been interpreted as the result of the illumination of the cloud by a past high luminosity period of Sgr A*. This interpretation is still controversial since irradiation by subrelativistic cosmic rays might also account for the observed spectra. We present here the results of 7 years of INTEGRAL monitoring of Sgr B2 and show that its hard X-ray (20-60 keV) flux has decreased by nearly 40% during that period. This cannot be accounted for by cosmic ray interpretations and support the idea that Sgr A* was more active in the past and has been quiescent since about 100 years.
    05/2011;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: γ -ray production cross sections have been measured in proton irradiations of N, Ne, and Si and α-particle irradiations of N and Ne. In the same experiment we extracted also line shapes for strong γ -ray lines of 16O produced in proton and α-particle irradiations of O. For the measurements gas targets were used for N, O, and Ne and a thick foil for Si. All targets were of natural isotopic composition. Beams in the energy range up to 26 MeV for protons and 39 MeV for α particles were delivered by the Institut de Physique Nucléaire–Orsay tandem accelerator. The γ rays were detected with four high-purity Ge detectors in the angular range 30◦ to 135◦.We extracted 36 cross-section excitation functions for proton reactions and 14 for α-particle reactions. For the majority of the excitation functions no other data exist to our knowledge. Where comparison with existing data was possible, usually a very good agreement was found. It is shown that these data are very interesting for constraining nuclear reaction models. In particular, the agreement of cross section calculations in the nuclear reaction code TALYS with the measured data could be improved by adjusting the coupling schemes of collective levels in the target nuclei 14N, 20,22Ne, and 28Si. The importance of these results for the modeling of nuclear γ -ray line emission in astrophysical sites is discussed.
    Physical Review C 01/2011; · 3.72 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: The recently claimed observations of non-negligible amounts of 6Li in old halo stars have renewed interest in the Big Bang nucleosynthesis (BBN) of 6Li. One important ingredient in the predicted BBN abundance of 6Li is the low-energy 2H(α,γ)6Li cross section. Up to now, the only available experimental result for this cross section showed an almost constant astrophysical S factor below 400 keV, contrary to theoretical expectations. We report on a new measurement of the 2H(α,γ)6Li reaction using the breakup of 6Li at 150 A  MeV. Even though we cannot separate experimentally the Coulomb contribution from the nuclear one, we find clear evidence for Coulomb-nuclear interference by analyzing the scattering angular distributions. This is in line with our theoretical description, which indicates a drop of the S24 factor at low energies as predicted also by most other models. Consequently, we find even lower upper limits for the calculated primordial 6Li abundance than before.
    Physical Review C 11/2010; · 3.72 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: The 12C(alpha,gamma)16O reaction plays an important role in helium burning in massive stars and their evolution. However, despite many experimental studies, the low-energy cross section of 12C(alpha,gamma)16O still highly uncertain. The extrapolation of the measured cross sections to stellar energies (E~300 keV) is made difficult by the presence of the two sub-threshold states at 6.92 and 7.12 MeV of 16O. In order to further investigate the contribution of these two-subthreshold resonances to the 12C(alpha,gamma)16O cross section, we performed a new determination of the alpha -reduced widths via a measurement of the transfer reaction 12C(7Li,t)16O at two incident energies, 34 and 28 MeV. The measured and calculated differential cross sections will be presented as well as the obtained spectroscopic factors and the alpha -reduced widths
    AIP Conference Proceedings. 10/2010;
  • [show abstract] [hide abstract]
    ABSTRACT: One of the remarkable features of the X‐ray emission from the Galactic center (GC) region is a Kα line at 6.4 keV from an Fe atom. Although some scenarios, such as photo‐ionization by external X‐rays (XRN) or collisional ionization by low energy cosmic‐ray electrons (LECRe) have been proposed, the origin of the 6.4 keV line is still an open question. We analyzed X‐ray data obtained by the X‐ray satellite Suzaku and discovered K‐shell lines of neutral Ar, Ca, Cr, and Mn atoms in addition to Fe and Ni. To explain the observed equivalent widths of the neutral lines, the elemental abundances are required to be ∼1.6 and ∼4 solar in the XRN and LECRe scenarios. On the other hand, the abundances of the various elements in the GC hot plasma were 1–2 solar. Ourresult suggests that the origin of the neutral K‐shell lines is the external X‐rays.
    AIP Conference Proceedings. 08/2010; 1269(1):154-159.