Hypernovae as possible sources of Galactic positrons

INTEGRAL/SPI has recently observed a strong and extended emission resulting from electron-positron annihilation located in the Galactic center region, consistent with the Galactic bulge geometry, without any counterpart at high gamma-ray energies, nor in the 1809 keV $^{26}$Al decay line. In order to explain the rate of positron injection in the Galactic bulge, estimated to more than 10$^{43}$ s$^{-1}$, the most commonly considered positron injection sources are type Ia supernovae. However, SN Ia rate estimations show that those sources fall short to explain the observed positron production rate, raising a challenging question about the nature of the Galactic positron source. In this context, a possible source of Galactic positrons could be supernova events of a new type, as the recently observed SN2003dh/GRB030329, an exploding Wolf-Rayet star (type Ic supernova) associated with a hypernova/gamma-ray burst; the question about the rate of this kind of events remains open, but could be problematically low. In this paper, we explore the possibility of positron production and escape by such an event in the framework of an asymmetric model, in which a huge amount of $^{56}$Ni is ejected in a cone with a very high velocity; the ejected material becomes quickly transparent to positrons, which spread out in the interstellar medium.Comment: 8 pages, 2 figures. To appear in the Proceedings of the 5th INTEGRAL Workshop: "The INTEGRAL Universe", February 16-20, 2004, Munich, German

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

Soft gamma-ray galactic ridge emission as unveiled by SPI aboard INTEGRAL

The origin of the soft gamma-ray (200 keV - 1 MeV) galactic ridge emission is one of the long-standing mysteries in the field of high-energy astrophysics. Population studies at lower energies have shown that emission from accreting compact objects gradually recedes in this domain, leaving place to another source of gamma-ray emission that is characterised by a hard power-law spectrum extending from 100 keV up to 100 MeV The nature of this hard component has remained so far elusive, partly due to the lack of sufficiently sensitive imaging telescopes that would be able to unveil the spatial distribution of the emission. The SPI telescope aboard INTEGRAL allows now for the first time the simultaneous imaging of diffuse and point-like emission in the soft gamma-ray regime. We present here all-sky images of the soft gamma-ray continuum emission that clearly reveal the morphology of the different emission components. We discuss the implications of our results on the nature of underlying emission processes and we put our results in perspective of GLAST studies of diffuse galactic continuum emission

Imaging the Gamma-Ray Sky with SPI aboard INTEGRAL

The spectrometer SPI on INTEGRAL allows for the first time simultaneous imaging of diffuse and point-like emission in the hard X-ray and soft gamma-ray regime. To fully exploit the capabilities of the instrument, we implemented the MREM image deconvolution algorithm, initially developed for COMPTEL data analysis, to SPI data analysis. We present the performances of the algorithm by means of simulations and apply it to data accumulated during the first 2 mission years of INTEGRAL. Skymaps are presented for the 1809 keV gamma-ray line, attributed to the radioactive decay of 26Al, and for continuum energy bands, covering the range 20 keV - 3 MeV. The 1809 keV map indicates that emission is clearly detected by SPI from the inner Galactic radian and from the Cygnus region. The continuum maps reveal the transition between a point-source dominated hard X-ray sky to a diffuse emission dominated soft gamma-ray sky. From the skymaps, we extract a Galactic ridge emission spectrum that matches well SPI results obtained by model fitting. By comparing our spectrum with the cumulative flux measured by IBIS from point sources, we find indications for the existence of an unresolved or diffuse emission component above ~100 keV.Comment: 12 pages, 7 figure

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

The SVOM mission

SVOM (Space-based multiband astronomical Variable Objects Monitor) is a sino-french mission that is dedicated to Gamma-Ray Burst (GRB) science, expected to be launched in mid 2023. The mission includes four space-based and three ground-based instruments that, working together, will discover GRBs and provide rapid multi-wavelength follow-up in order to obtain a complete coverage of the GRB emission over seven decades in energy, from the trigger up to the very late phases of the afterglow. Thanks to its characteristics, SVOM will play a crucial role in time-domain and multi-messenger astronomy

Perspectives on Gamma-Ray Burst Physics and Cosmology with Next Generation Facilities

High-redshift Gamma-Ray Bursts (GRBs) beyond redshift $\sim6$ are potentially powerful tools to probe the distant early Universe. Their detections in large numbers and at truly high redshifts call for the next generation of high-energy wide-field instruments with unprecedented sensitivity at least one order of magnitude higher than the ones currently in orbit. On the other hand, follow-up observations of the afterglows of high-redshift GRBs and identification of their host galaxies, which would be difficult for the currently operating telescopes, require new, extremely large facilities of at multi-wavelengths. This chapter describes future experiments that are expected to advance this exciting field, both being currently built and being proposed. The legacy of Swift will be continued by SVOM, which is equipped with a set of space-based multi-wavelength instruments as well as and a ground segment including a wide angle camera and two follow-up telescopes. The established Lobster-eye X-ray focusing optics provides a promising technology for the detection of faint GRBs at very large distances, based on which the {THESEUS}, {Einstein Probe} and other mission concepts have been proposed. Follow-up observations and exploration of the reionization era will be enabled by large facilities such as {SKA} in the radio, the 30m class telescopes in the optical/near-IR, and the space-borne {WFIRST} and {JWST} in the optical/near-IR/mid-IR. In addition, the X-ray and $\gamma$-ray polarization experiment POLAR is also introduced.Comment: accepted for publication in Space Science Review; reprinted as a chapter in a book of the Space Sciences Series of ISSI for the proceedings of the ISSI-Beijing workshop " Gamma-Ray Bursts: a Tool to Explore the Young Universe

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