Constraints on positron annihilation kinematics in the inner Galaxy

Abstract

Context. The annihilation of cosmic positrons with electrons in the interstellar medium results in the strongest persistent γ-ray line signal in the sky. For the past 50 years, this 511 keV emission -predominantly from the galactic bulge region and from a low surfacebrightness disk -has puzzled observers and theoreticians. A key issue for understanding positron astrophysics is found in cosmic-ray propagation, especially at low kinetic energies ( 10 MeV). Aims. We want to shed light on how positrons propagate and the resulting morphology of the annihilation emission. We approach this "positron puzzle" by inferring kinematic information of the 511 keV line in the inner radian of the Galaxy. This constrains propagation scenarios and positron source populations in the Milky Way. Methods. By dissecting the positron annihilation emission as measured with INTEGRAL/SPI, we derive spectra for individual and independent regions in the sky. The centroid energies of these spectra around the 511 keV line are converted into Doppler-shifts, representing the line-of-sight velocity along different galactic longitudes. This results in a longitude-velocity diagram of positron annihilation. From high-resolution spectra, we also determine Doppler-broadening from γ-ray line shape parameters to study annihilation conditions as they vary with galactic longitude. Results. We find line-of-sight velocities in the 511 keV line that are consistent with zero, as well as with galactic rotation from CO measurements (2-3 km s −1 deg −1 ), and measurements of radioactive 26 Al (7.5-9.5 km s −1 deg −1 ). The velocity gradient in the inner ±30 • is determined to be 4 ± 6 km s −1 deg −1 . The width of the 511 keV line is constant as a function of longitude at 2.43 ± 0.14 keV, with possibly different values towards the disk. The positronium fraction is found to be 1.0 along the galactic plane. Conclusions. The weak signals in the disk leave open the question whether positron annihilation is associated with the high velocities seen in 26 Al or rather with ordinarily rotating components of the Milky Way's interstellar medium. We confirm previous results that positrons are slowed down to the 10 eV energy scale before annihilation, and constrain bulk Doppler-broadening contributions to 1.25 keV in the inner radian. Consequently, the true annihilation conditions remain unclear.

Primordial Black Holes as a Dark Matter Candidate Are Severely Constrained by the Galactic Center 511 keVγ-Ray Line

“…The Galactic Center 511 keV gamma-ray line has been observed for a few decades and its origin has remained unknown throughout. Recent attempts at measuring the Doppler shifts have also not led to the identification of the source [10] (note that a similar search technique has also been proposed for the 3.5 keV line [69,70]). The observed flux of this gamma-ray line indicates that the rate of positron annihilation at the Galactic Center is ∼ 10 50 per year [62].…”

“…In this work, we take one step towards answering this question. We show that combining the observation that light PBHs can produce e ± pairs via evaporation [7] with the fact that an intense 511 keV gamma-ray line has been observed in the Galactic Center [8][9][10][11][12][13][14][15] can efficiently constrain PBHs in a mass range which cannot yet be constrained by any other technique. Our technique is robust because the morphology of the 511 keV gamma-ray line does not permit a dark matter explanation [16,17].…”

Primordial Black Hole Dark Matter in the Context of Extra Dimensions

“…5a of Ref. [56] (black crosses). These correspond to the total 511 keV flux within galactic latitudes −10.5 • < b < 10.5 • , in 5 equally-spaced longitude bins within −30 • < < 30 • .…”

Understanding the origin of the positron annihilation line and the physics of supernova explosions

“…[59]), all the decay positrons have in common that they first have to slow down to explain the measured signal: β + -decay typically emits MeV positrons, but the line shape suggests positrons at eV energies (e.g. [20,44,69,71]). This slowing down is connected to the propagation of cosmic-ray positrons, and the interplay of the environmental conditions near the initial source and the properties of the interstellar medium between source and region of annihilation.…”