Detection of the 511 keV Galactic position annihilation line with COSI. (arXiv:1912.00110v1 [astro-ph.HE])

Detection of the 511 keV Galactic position annihilation line with COSI. (arXiv:1912.00110v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kierans_C/0/1/0/all/0/1">Carolyn A. Kierans</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Boggs_S/0/1/0/all/0/1">Steven E. Boggs</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zoglauer_A/0/1/0/all/0/1">Andreas Zoglauer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lowell_A/0/1/0/all/0/1">Alex W. Lowell</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sleator_C/0/1/0/all/0/1">Clio C. Sleator</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Beechert_J/0/1/0/all/0/1">Jacqueline Beechert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brandt_T/0/1/0/all/0/1">Terri J. Brandt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jean_P/0/1/0/all/0/1">Pierre Jean</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lazar_H/0/1/0/all/0/1">Hadar Lazar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Roberts_J/0/1/0/all/0/1">Jarred M. Roberts</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Siegert_T/0/1/0/all/0/1">Thomas Siegert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tomsick_J/0/1/0/all/0/1">John A. Tomsick</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ballmoos_P/0/1/0/all/0/1">Peter von Ballmoos</a>

The signature of positron annihilation, namely the 511 keV $gamma$-ray line,
was first detected coming from the direction of the Galactic center in the
1970’s, but the source of Galactic positrons still remains a puzzle. The
measured flux of the annihilation corresponds to an intense, probably steady,
source of positron production, with an annihilation rate on the order of
$sim10^{43}$~e$^{+}$/s. The 511 keV emission is the strongest diffuse
$gamma$-ray line signal and it shows a concentration towards the Galactic
center region. An additional low-surface brightness component is aligned with
the Galactic disk; however, the morphology of the latter is not well
constrained. The Compton Spectrometer and Imager (COSI) is a balloon-borne soft
$gamma$-ray (0.2–5 MeV) telescope designed to perform wide-field imaging and
high-resolution spectroscopy. One of its major goals is to further our
understanding of Galactic positrons. COSI had a 46-day balloon flight in
May-July 2016 from Wanaka, New Zealand, and here we report on the detection and
spectral analyses of the 511 keV emission from those observations. To isolate
the Galactic positron annihilation emission from instrumental background, we
have developed a technique to separate celestial signals utilizing the COMPTEL
Data Space. With this method, we find a 7.2$sigma$ detection of the 511 keV
line. We find that the spatial distribution is not consistent with a single
point source, and it appears to be broader than what has been previously
reported.

The signature of positron annihilation, namely the 511 keV $gamma$-ray line,
was first detected coming from the direction of the Galactic center in the
1970’s, but the source of Galactic positrons still remains a puzzle. The
measured flux of the annihilation corresponds to an intense, probably steady,
source of positron production, with an annihilation rate on the order of
$sim10^{43}$~e$^{+}$/s. The 511 keV emission is the strongest diffuse
$gamma$-ray line signal and it shows a concentration towards the Galactic
center region. An additional low-surface brightness component is aligned with
the Galactic disk; however, the morphology of the latter is not well
constrained. The Compton Spectrometer and Imager (COSI) is a balloon-borne soft
$gamma$-ray (0.2–5 MeV) telescope designed to perform wide-field imaging and
high-resolution spectroscopy. One of its major goals is to further our
understanding of Galactic positrons. COSI had a 46-day balloon flight in
May-July 2016 from Wanaka, New Zealand, and here we report on the detection and
spectral analyses of the 511 keV emission from those observations. To isolate
the Galactic positron annihilation emission from instrumental background, we
have developed a technique to separate celestial signals utilizing the COMPTEL
Data Space. With this method, we find a 7.2$sigma$ detection of the 511 keV
line. We find that the spatial distribution is not consistent with a single
point source, and it appears to be broader than what has been previously
reported.

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