Constraints on the injection energy of positrons in the Galactic centre region

Recent observations of the 511 keV positron-electron annihilation line in the Galactic centre region by the INTEGRAL/SPI spectrometer have stirred up new speculations about the origin of the large corresponding positron injection rate. Beyond astrophysical candidates, new mechanisms have been put forward. We focus on the annihilation of light dark matter particles and review the various gamma-ray radiation components related to such a source of mono-energetic positrons in addition to the 511 keV line itself. We study the influence of the degree of ionisation of the bulge on this radiation, and its possible effects on the observational constraints on the mass of the hypothetical light dark matter particle or the injection energy of a mono-energetic source of positrons in general.Comment: 4 pages, 7 figures, 1 table. Accepted for publication in the proceedings of the 6th INTEGRAL Workshop on the Obscured Universe (ESA SP-622). 2-8 July 2006, Moscow, Russi

Evidence for 1809 keV Gamma-Ray Emission from 26Al Decays in the Vela Region with INTEGRAL/SPI

The Vela region is a promising target for the detection of 1.8 MeV gamma-rays emitted by the decays of radioactive 26Al isotopes produced in hydrostatic or explosive stellar nucleosynthesis processes. COMPTEL has claimed 1.8 MeV gamma-ray detection from Vela at a 3sigma level with a flux of 3.6 10^-5 ph/cm^2/s. In this paper, we present first results of our search for 1.8 MeV gamma-rays from Vela with the spectrometer SPI aboard INTEGRAL. Using the data set acquired during 1.7 Ms at the end of 2005 in the frame of our AO-3 open-time observation, we determine a flux of (6.5 \pm 1.9(stat) \pm 2.4(syst)) 10^-5 ph/cm^2/s from 26Al decays in the Vela region.Comment: 4 pages, 8 figures. Accepted for publication in ESA SP-622 (Proceedings of the 6th INTEGRAL Workshop, Moscow, 2006 07 03-07

HARPO: a TPC as a gamma-ray telescope and polarimeter

A gas Time Projection Chamber can be used for gamma-ray astronomy with excellent angular-precision and sensitivity to faint sources, and for polarimetry, through the measurement of photon conversion to $e^+e^-$ pairs. We present the expected performance in simulations and the recent development of a demonstrator for tests in a polarized photon beam.Comment: SPIE Astronomical Telescopes + Instrumentation, Ultraviolet to gamma ray, Montr\'eal, Canada 2014. v2: note added in proof. Copyright 2014 SPIE. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibite

Soft gamma-ray background and light Dark Matter annihilation

The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the (S- or P-wave) annihilation of light Dark Matter particles into e+e-. In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard X-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. We find that any extra contribution to this unresolved background at 511 keV should be lower than about 4 keV/cm^2/s/sr. We also estimate analytically the extragalactic background due to Dark Matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be one order of magnitude lower, so that the hypothesis of a light Dark Matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross-sections.Comment: 10 pages, 8 figures, 1 table, accepted for publication in Physical Review D, improved with 4 new figures, 1 new table and 1 new par

Influence of the Earth on the background and the sensitivity of the GRM and ECLAIRs instruments aboard the Chinese-French mission SVOM

SVOM (Space-based multi-band astronomical Variable Object Monitor) is a future Chinese-French satellite mission which is dedicated to Gamma-Ray Burst (GRB) studies. Its anti-solar pointing strategy makes the Earth cross the field of view of its payload every orbit. In this paper, we present the variations of the gamma-ray background of the two high energy instruments aboard SVOM, the Gamma-Ray Monitor (GRM) and ECLAIRs, as a function of the Earth position. We conclude with an estimate of the Earth influence on their sensitivity and their GRB detection capability.Comment: 24 pages, 15 figures, accepted for publication in Experimental Astronom

Étude de l'annihilation électron-positon dans la région du centre Galactique avec le spectromètre INTEGRAL/SPI

A spectral feature was detected in 1970 in the gamma-ray emission from the central regions of the Milky Way, during balloon flight observations. Located near 511 keV, this feature was soon attributed to the gamma-ray line tracing the annihilation of electrons with their anti-particules, positrons. However, none of the multiple astrophysical scenarii contemplated to explain the production of positrons in the Galactic bulge has been able to reproduce the high injection rate deduced from the flux of the 511 keV line, close to ten to the fourty-third positrons per second.Launched in 2002, the European gamma-ray satellite INTEGRAL was provided with a spectrometer, SPI, whose unprecedented imaging and spectral capabilities in this energy range enable us to further study the source of the 511 keV line detected in the Galactic centre region.Indeed, a better determination of the spatial extent of the source, the intrinsic width of the line and the fraction of positrons annihilating in-flight, directly or via the formation of ortho-Positronium atoms, would improve our knowledge of both the annihilation medium and the initial source of positrons, and could allow us to discriminate between the various explanatory scenarii.The first part of this thesis deals with a key ingredient in the extraction of the annihilation spectrum : the optimization of the instrumental background model. New data screening and tracer selection procedures are presented. Classical multi-linear models are compared to neural and bayesian networks. Finally, three years of observation are used to constrain the width of the source and derive its spectrum.The second part of the thesis focuses on one of the possible scenarii explaining the high positron injection rate deduced from the flux of the 511 keV line : the annihilation of light dark matter particles into electron-positron pairs. The various radiation mechanisms involved are modeled and confronted to observations in order to set an upper limit on the injection energy of positrons -- e.g. on the mass of the light dark matter particle -- found to vary between 3 and 7.5 MeV, depending on the degree of ionization of the interstellar medium.En 1970, des vols ballons ont mis en évidence une structure dans le spectre d'émission gamma de la région du centre Galactique. Située vers 511 keV, cette signature spectrale a rapidement été associée à la raie traçant l'annihilation d'électrons avec leurs anti-particules, les positons.Cependant, les multiples scénarios astrophysiques envisagés pour expliquer l'injection de positons dans le bulbe Galactique n'ont pas permis de reproduire le taux élevé déduit du fux de la raie à 511 keV, de l'ordre de dix puissance quarante-trois par seconde.Lancé fin 2002, le satellite gamma européen INTEGRAL est doté d'un spectromètre, SPI, présentant des capacités d'imagerie et une résolution spectrale à 511 keV sans précédent, qui en font l'instrument adéquat pour affiner notre connaissance de la morphologie spatiale et du spectre de la source de cette émission d'annihilation, afin de distinguer le scénario le plus probable. En effet, la détermination de l'étendue de la source, de la largeur de la raie et de la fraction des positons s'annihilant en vol ou après thermalisation, directement ou via la formation d'atomes de Positronium nous renseignent à la fois sur le milieu dans lequel a lieu l'annihilation finale et sur la source initiale des positons.La première partie de cette thèse a permis de déterminer les méthodes optimales de modélisation du bruit de fond instrumental du spectromètre Spi indispensables à l'analyse des observations de la raie à 511 keV. Des procédures de nettoyage des données et de sélection des indicateurs du bruit de fond ont été mises en ÷uvre. Des modèles utilisant des techniques neuronales ont été élaborés et comparés à des méthodes statistiques plus classiques afin d'évaluerla possibilité de prendre en compte les non-linéarités du bruit et les effets liés à l'activité solaire. Au terme de cette étude, un modèle spatial a été ajusté aux données correspondant à trois années d'observation par INTEGRAL de la raie à 511 keV et un spectre d'annihilation a été extrait.La seconde partie de cette thèse est consacrée à l'un des scénarios qui pourraient expliquer le taux d'injection de positons observé dans le bulbe Galactique : l'annihilation de particules de matière noire légères en paires électron-positon. Les diverses composantes radiatives liées à ce phénomène ont été modélisées et confrontées aux observations gamma passées, afin d'en déduire une limite supérieure sur l'énergie d'injection des positons - et donc sur la masse de la particule de matière noire légère - qui varie entre 3 et 7,5 MeV selon l'ionisation du milieu interstellaire