Hypernovae/GRB in the Galactic Center as possible sources of Galactic Positrons

TL;DR

The paper addresses the origin of the Galaxy's central 511 keV positron annihilation emission by evaluating potential sources and highlighting the insufficiency of SN Ia alone to account for the observed flux. It proposes a novel posited contributor: bipolar Wolf-Rayet explosions leading to SN Ic hypernovae (exemplified by SN2003dh) that can eject large amounts of $^{56}$Ni and enable substantial positron escape through anisotropic ejecta. Semi-analytic estimates show each SN2003dh-like event could emit $\sim10^{54}$ positrons, and a bulge-rate of $\sim0.2$ per millennium would suffice to explain the INTEGRAL-SPI measurement, in contrast to SN Ia rates required by other models. The work underscores the potential significance of SN Ic hypernovae as Galactic positron sources and calls for 3D hydrodynamic modeling and late-time SN observations to quantify their contributions and distinguish them from SN Ia inputs.

Abstract

The observation of a strong and extended positron-electron line annihilation emission in the central regions of the Galaxy by INTEGRAL-SPI, consistent with the Galactic bulge geometry, without any counterpart in the gamma-ray range, neither at high energy nor in the 1809 keV $^{26}$Al decay line, is challenging. Leaving aside the geometrical question, we address the problem of the adequate positron sources, showing the potentiality of a new category of SN Ic, exemplified by SN2003dh, which is associated to a gamma-ray burst. This kind of supernova/hypernova/GRB event is interpreted as the result of a bipolar Wolf-Rayet explosion, which produces a large amount of $^{56}$Ni and ejects it at high velocity along the rotation axis. The bulk of positrons resulting from $^{56}$Co decay escapes in the surrounding medium due to the rapid thinning of the ejecta in the polar direction. We show that a rate of about 0.02 SN2003dh-like events per century in the central region of the Galaxy is sufficient to explain the positron flux detected by INTEGRAL-SPI. In order to explain this flux by SN Ia events alone, a rate of 0.5 per century is necessary, much higher than indicated by Galactic evolutionary models applied to the bulge. Further observations of late light curves of SNe Ia and SNe Ic in the bulge of spiral galaxies, together with 3D hydrodynamic calculations of anisotropic ejections of $^{56}$Ni in SN Ic/GRB events, will allow to estimate the separate contributions of SNe Ia and SNe Ic to positron injection.