An asymmetric distribution of positrons in the Galactic disk revealed by γ-rays

Abstract

Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk ( approximately 10-50 degrees from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10(41) positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

A leptonic/hadronic jet model for the low-mass microquasar XTE J1118+480

“…Our model also predicts that electron-positron pairs are produced at a significant rate. This is interesting in connection with the results of Weidenspointner et al (2008), who found that the 0.511 MeV annihilation line emission detected outside the galactic plane traces the asymmetric spatial distribution of low-mass X-ray binaries in the Galaxy (see, however, Bouchet et al 2010).…”

Cosmic Gamma-Ray Spectroscopy

“…The scientific surprise of this emission had already been apparent in earlier results from the Compton Observatory [103], and was consolidated by SPI measurements: The annihilation emission predominantly arises in the inner Galaxy in an extended region of size 10 • . By comparison, the disk of the Galaxy is much fainter, wit a bulge-to-disk intensity ratio of 1.4 from a total luminosity of 2 10 −3 ph cm −2 s −1 [70,144].…”

“…SPI images (figure 20) include hints for deviations from the ideal symmetry expected for dark-matter origins of the bulge emission. Asymmetric disk emission was interpreted as possibly related to X-ray binaries [144]. But in fact, each of the candidate sources may have its own deviations from perfect Galactic symmetry, and conclusions are model dependent and rely on the quality of the imaging information.…”

“…left: Significance map of a pixelized maximum-likelihood imaging deconvolution [8]. right: Intensity map of a multi-resolution expectation maximization imaging deconvolution [144]. The centroid of the bulge emission may hold clues to the nature of the sources in the bulge, relating a small offset or an asymmetric inner-disk component (depending on analysis approach; see text) to spatial distributions as expected from stellar or, alternatively, dark-matter related sources.…”

String loop moduli stabilisation and cosmology in IIB flux compactifications

“…Hence these scenarios are not plagued by any CMP. 17 However, recently it has been discovered with the INTEGRAL spectrometer SPI [234] that the 511 KeV line emission appears to be asymmetric. This distribution of positron annihilation resembles that of low mass X-ray binaries, suggesting that these systems may be the dominant origin of the positrons and so reducing the need for more exotic explanations, such as the one presented in this paper.…”