Koposov et al. (2015) used data from the Dark Energy Survey to search for new dwarf satellite galaxies (DSGs) of the Milky Way (MW). They found nine candidates in the southern sky, which may be related to the magellanic systems. Among their sample, Reticulum II (Ret II) is the closest and was detected with a significance of more than 48 sigma. Looking for a possible low energy gamma-ray emission from these new DSGs, Siegert et al. (2016) found a 3.1 sigma signal at photon energies around 511 keV from the direction of Ret II, with a flux of 1.7E-4 ph/cm2/s. The origins of such a 511 keV signal from electron-positron annihilations are versatile and probably not attributable to one particular source. The connection between this excess and the galactic centre signal remains unexplored. Massive stars and supernovae have been considered as a possible source of 511 keV photons in the MW since they can produce beta+-unstable isotopes. However they are unlikely to be responsible for the signal in Ret II since we do not expect any in DSGs. Dark matter (DM) has also been proposed as a possible source of low-energy positron emitters in the MW and could be a source of positrons in Ret II as it is dominated by DM. However, the cross section needed to explain the 511 keV signal in Ret II would make the MW brighter at these energies. Another source of low-energy positrons could be microquasars. These may produce electron-positron pair-plasma, predominantly in outbursts. In fact, one single microquasar in Ret II might be enough to explain the whole emission. In this proposal, we ask for 1.5 Ms of observation time towards the direction of Ret II, (l/b) = (266.3/-49.7), which would add to the currently available 250 ks. This observation would consolidate or falsify the discovery of a 511 keV line signal in Ret II and understand its origin if it is indeed present. In addition, observing these DSGs with SPI may help to solve the 40 years old galactic positron puzzle.
Publications
An INTEGRAL/SPI view of reticulum II: particle dark matter and primordial black holes limits in the MeV range - Siegert, Thomas, Boehm, Celine,Calore, Francesca,Diehl, Roland,Krause, Martin G. H.,Serpico, Pasquale D.,Vincent, Aaron C. (2022-03-01) http://ui.adsabs.harvard.edu/#abs/2022MNRAS.511..914S
Diffuse Galactic emission spectrum between 0.5 and 8.0 MeV - Siegert, Thomas, Berteaud, Joanna,Calore, Francesca,Serpico, Pasquale D.,Weinberger, Christoph (2022-04-01) http://ui.adsabs.harvard.edu/#abs/2022A&A...660A.130S
Gamma-Ray Emission of 60Fe and 26Al Radioactivity in Our Galaxy - Wang, W., Siegert, T.,Dai, Z. G.,Diehl, R.,Greiner, J.,Heger, A.,Krause, M.,Lang, M.,Pleintinger, M. M. M.,Zhang, X. L. (2020-02-01) http://ui.adsabs.harvard.edu/#abs/2020ApJ...889..169W
26Al gamma rays from the Galaxy with INTEGRAL/SPI - Pleintinger, Moritz M. M., Diehl, Roland,Siegert, Thomas,Greiner, Jochen,Krause, Martin G. H. (2023-04-01) http://ui.adsabs.harvard.edu/#abs/2023A&A...672A..53P
Temporal Coverage
2017-12-27T17:24:30Z / 2018-12-25T11:13:27Z
Version
1.0
Mission Description
The INTEGRAL (International Gamma-Ray Astrophysics Laboratory) mission, launched by the European Space Agency (ESA) on October 17, 2002, was designed to study high-energy phenomena in the universe. INTEGRAL was operating until february 2025 and it was equipped with three high-energy instruments: the Imager on Board the INTEGRAL Satellite (IBIS), the Spectrometer on INTEGRAL (SPI), and the JEM-X (Joint European Monitor for X-rays). Its Optical Monitoring Camera (OMC) provided optical V-band magnitude measurements, complementing the high-energy observations.
European Space Agency, Siegert, 2025, 'Positron Annihilation in the Dwarf Satellite Galaxy Reticulum II', 1.0, European Space Agency, https://doi.org/10.57780/esa-z7vx9km
