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<!-- <h1>AMEGO: The All-sky Medium Energy Gamma-ray Observatory</h1> -->

<h2>Astro 2020 Decadal Review White Papers</h2>
  <p>Below are the white papers submitted to the Decadal Review that
  are relevant to AMEGO.  These provide a good overview of the science
  that can be done with an observatory like AMEGO.</p>

  <h3>AMEGO Mission paper</h3>
  <ul>
    <li><a href="https://arxiv.org/abs/1907.07558">All-sky Medium Energy Gamma-ray Observatory: Exploring the Extreme Multimessenger Universe</a></li>
  </ul>

  <h3>Multi-Messenger Astronomy and Astrophysics</h3>
  <ul>
    <li><a href="https://arxiv.org/abs/1903.04461">Opportunities for Multimessenger Astronomy in the 2020s</a></li>
    <li><a href="https://arxiv.org/abs/1903.04472">Gamma Rays and Gravitational Waves</a></li>
    <li><a href="https://arxiv.org/abs/1903.03582">A Summary of Multimessenger Science with Neutron Star Mergers</a></li>
    <li><a href="https://arxiv.org/abs/1903.05765">Neutrinos, Cosmic Rays and the MeV Band</a></li>
    <li><a href="https://arxiv.org/abs/1903.05660">Cosmic Rays and Interstellar Medium with Gamma-Ray Observations at MeV Energies</a></li>
    <li><a href="https://arxiv.org/abs/1903.04607">High-Energy Polarimetry - a new window to probe extreme physics in AGN jets</a></li>
    <li><a href="https://arxiv.org/abs/1903.04634">Energetic Particles of Cosmic Accelerators I (Galactic Acclerators)</a></li>
    <li><a href="https://arxiv.org/abs/1903.04639">Energetic Particles of Cosmic Accelerators II (AGNs and GRBs)</a></li>
    <li><a href="https://ui.adsabs.harvard.edu/abs/2019BAAS...51c.100M/abstract">Prompt Emission Polarimetry of Gamma-Ray Bursts</a></li>
    <!-- <li>Gamma-Ray Science in the 2020</li> -->
  </ul>
  <h3>Formation and Evolution of Compact Objects, Cosmology and Fundamental Physics</h3>
  <ul>
    <li><a href="https://arxiv.org/abs/1903.06106">Supermassive black holes at high redshifts</a></li>
    <!-- <li>Prospects for Pulsar Studies at MeV Energies</li> -->
    <li><a href="https://arxiv.org/abs/1903.07553">Prospects for AGN Studies at Hard X-ray through MeV Energies</a></li>
    <li><a href="https://arxiv.org/abs/1903.05648">Magnetars as Astrophysical Laboratories of Extreme Quantum Electrodynamics: The Case for a Compton Telescope</a></li>    
    <li><a href="https://arxiv.org/abs/1903.05699">Prospects for detection of synchrotron halos around middle-age pulsars</a></li>
    <!-- <li>Particle Acceleration in PWNe</li> -->
    <li><a href="https://arxiv.org/abs/1903.05845">Looking Under a Better Lamppost: MeV-scale Dark Matter Candidates</a></li>
    <!-- <li>MeV Background</li> -->
    <!-- <li>GRB Polarimetry</li> -->
  </ul>
  <h3>Stars and Stellar Evolution, Galaxy Evolution</h3>
  <ul>
    <li><a href="https://arxiv.org/abs/1902.02915">Catching Element Formation In The Act</a></li>
    <li><a href="https://arxiv.org/abs/1903.05569">Positron Annihilation in the Galaxy</a></li>
  </ul>

<h2>AMEGO Science</h2>
  
<p>The three groundbreaking capabilities of AMEGO continuum sensitivity, 
nuclear line spectroscopy and polarization combine to address questions 
in the following key science areas. </p>

<div class="figure">
<center><img src="images/SED_science_themes.png" alt="SED_themes"></center>
<p>Characterizing the medium-energy gamma-ray band is essential to distinguish between spectral models and to understand the mechanism that produce emission from jets, compact objects, and shocks.</p>
</div>

<h3>Understand the formation, evolution, and acceleration mechanisms in astrophysical jets</h3>
<p><a href="https://en.wikipedia.org/wiki/Blazar">Blazars</a> are the most powerful subset of Active Galactic Nuclei (AGN). Their relativistic jets 
are oriented close to our line of sight and and emission from these jets 
dominates their broadband emission. In many blazars, known as MeV blazars, 
the peak energy output is in the AMEGO band. MeV 
blazars have larger-than-average jet power and accretion luminosity and 
are often found at <a href="https://www.nasa.gov/feature/goddard/2017/nasas-fermi-discovers-the-most-extreme-blazars-yet">very large redshift</a>. They harbor the most massive black 
holes, with masses exceeding a billion solar masses. MeV blazars are thus <a href="http://adsabs.harvard.edu/abs/2017ApJ...837L...5A">powerful probes</a> of the growth of supermassive black holes across the history 
of the Universe.</p>

<p>AMEGO will offer a unique view into blazars' emission mechanisms. 
Observations of the poorly explored 0.3-100 MeV energy band with AMEGO 
will measure their spectral energy distributions and variability with unprecedented sensitivity and 
link the observed characteristics to crucial physical parameters of these 
systems, such as the maximum energy to which particles are accelerated, the 
strength of the magnetic fields, the content of the jet, and the location 
of the gamma-ray emission site. The polarization measurement capability of 
AMEGO provides a crucial test of the jet content, because hadronic models 
predict a much higher degree of polarization than leptonic models.</p>
<p>The detection and characterization of the MeV blazar population is critical 
to understand black hole formation and growth at high redshift.  Its ground-
breaking sensitivity and all-sky survey capability ensure that AMEGO will 
measure the properties of a large sample of MeV blazars.</p>
	
<h3>Identify the physical processes in the extreme conditions around compact objects</h3>
<p>In the energy range of AMEGO, important open questions about <a href="https://en.wikipedia.org/wiki/Neutron_star">neutron stars </a>
can be resolved. Neutron stars, with their extreme densities, electric potentials, 
and magnetic fields, display a great variety of observational characteristics 
and behavior (see <a href=“http://adsabs.harvard.edu/abs/2013FrPhy...8..679H”>
Harding et al. 2013</a> for a review). The relation between the diverse classes 
of neutron stars, from magnetars to millisecond pulsars, is a decades-old mystery. 
One of the intriguing puzzles is the connection between the rotation-powered pulsar 
(RPP) population and the <a href="https://en.wikipedia.org/wiki/Magnetar">magnetars</a>, magnetically powered neutron stars that undergo 
violent bursting and whose quiescent emission alone far exceeds their spin-down 
luminosities. Many RPPs have surface dipole magnetic field strengths inferred 
from their period and period derivatives that are as high as those of magnetars, 
and some magnetars have dipole fields lower than many RPPs. What determines their 
very different behavior? Magnetar quiescent emission consists of a hot thermal 
component plus a hard non-thermal component with spectra measured by <a href="http://sci.esa.int/integral/">INTEGRAL</a> 
up to 200 keV (<a href="http://adsabs.harvard.edu/abs/2008arXiv0810.4801K">
Kuiper, den Hartog, Hermen 2008</a>) and upper limits from <a href="https://en.wikipedia.org/wiki/Compton_Gamma_Ray_Observatory">COMPTEL</a> and <a href="http://www-glast.stanford.edu/">Fermi</a> 
indicating a cutoff in the 1-10 MeV range. On the other hand, many RPP spectra 
have non-thermal emission that extends to GeV energies. Recently PSR J1119-6127, 
a radio, X-ray and gamma-ray RPP, exhibited a magnetar-like burst 
(<a href=“http://adsabs.harvard.edu/abs/2016ATel.9274....1K”>Kennea et al. 2016</a>; 
<a href=“http://adsabs.harvard.edu/abs/2016ApJ...829L..25G”> G&ouml;&#0287;&uuml;&#0351; et al. 2016</a>) 
that was accompanied by a glitch and a turn-off of the radio 
(<a href=“http://adsabs.harvard.edu/abs/2016ATel.9286....1B “>Burgay et al. 2016</a>) 
and gamma-ray (<a href=“http://adsabs.harvard.edu/abs/2016ATel.9378....1Y”>
Younes et al. 2016</a>) pulsations. Following the burst,the X-ray spectrum 
developed a hotter thermal component and a hard non-thermal component very 
similar to the quiescent spectra of magnetars 
(<a href=“http://adsabs.harvard.edu/abs/2016ApJ...829L..21A”>Archibald et al. 2016</a>). 
This suggests a strong connection between magnetars and high-magnetic field 
pulsars and raises the possibility of a continuum between the two neutron 
star classes, but the nature and cause is still unclear. If the hard X-ray 
emission is due to cyclotron resonant upscattering by accelerated particles 
in the near-surface fields (<a href=“http://adsabs.harvard.edu/abs/2007Ap%26SS.308..109B”>
Baring &amp; Harding 2007</a>; <a href=“http://adsabs.harvard.edu/abs/2007Ap%26SS.308..631B”>
Beloborodov &amp; Thompson 2007</a>) the cutoff energy due to pair 
production and photon splitting is very sensitive to the magnetic field 
direction at the emission location 
(<a href=“http://adsabs.harvard.edu/abs/2013ApJ...777..114B”>Beloborodov 2013</a>). 
Measurement of the phase-resolved quiescent spectra of magnetars in the 300 keV 
to 10 MeV band with AMEGO will determine the high-energy cutoffs, and assess 
how similar they are to the sub-GeV turnover seen in the high-field transitional 
RPP PSR J1513-5908 (<a href=“http://adsabs.harvard.edu/abs/2010ApJ...714..927A”>
Abdo et al. 2010</a>). They will also reveal whether the magnetic field has a 
significant toroidal component, thought to be a critical characteristic 
distinguishing magnetars and RPPs. Observation of high magnetic field RPPs 
following magnetar-like bursts could identify transient toroidal fields and 
answer the question of whether magnetic field structure distinguishes magnetar 
vs. RPP behavior. 
</p>

<p>The high-energy cutoffs in hard spectral components of magnetars are predicted 
to be highly polarized since pair production and photon splitting cutoffs occur 
at different energies and attenuate orthogonal emission modes 
(<a href=“http://adsabs.harvard.edu/abs/2001ApJ...547..929B”>Baring &amp; Harding 2001</a>). 
The AMEGO measurements of this unique signature of near 100% polarization 
near the high-energy cutoff of magnetar quiescent spectra could verify that 
this emission comes from near the neutron star surface and provide an <i>in situ</i> 
measurement of a magnetar magnetic field. </p>


<div class="figure">
<center> <img src="images/magnetar2.png" alt="Magnetar"></center>
<p> AMEGO will have a wide energy band, excellent continuum 
sensitivity, and the necessary energy resolution needed to resolve open 
questions in magnetar physics, demonstrated by the spectral energy 
distribution of magnetar 1RXS J1708-4009 
(<a href=“http://adsabs.harvard.edu/abs/2014ApJ...786L...1H”>Hasco&euml;t et al. 2014</a>). 
The solid blue and dashed blue lines show two different models for the 
shape of the high-energy cutoff.</p></div>
	
<h3>Measure the properties of element formation in dynamic systems</h3>
<p>The creation of "metals" in stars led to the enrichment of gas and
stars in the Galaxy to a mass fraction of about 2%. This process of
Galactic chemical evolution is driven by the star-formation rate, and
the relative rates of various sources of nucleosynthesis occurring in
dynamic environments; predominantly thermonuclear (<a href="https://en.wikipedia.org/wiki/Type_Ia_supernova">Type Ia</a>) and
core-collapse (<a href="https://en.wikipedia.org/wiki/Type_II_supernova">Type II</a>) <a href="https://en.wikipedia.org/wiki/Supernova">Supernovae (SNe)</a>. </p>

<p>The chemical evolution processes are traced with spectroscopic
abundance measurements in stars and the interstellar medium, where
iron (mostly from SN Ia) and oxygen (mostly from SNII) provide a
chemical clock. Gamma-ray line astronomy contributes to this study in
unique ways: for nearby SNe it is possible to directly measure the
freshly synthesized amount of <sup>56</sup>Ni,the decay of which keeps
SNe bright for much longer than the adiabatic cooling of
the eject would otherwise allow. On longer time scales we find
<sup>44</sup>Ti, which deposits energy on a 100-year time scale, and
<sup>26</sup>Al and <sup>60</sup>Fe, which decay on a million-year
time scale. </p>

<p>All these unique tracers of radioactivity produced in SNe reveal their
presence via gamma-ray lines in the MeV band probed by AMEGO. The
typical yields are of order 10<sup>-4</sup> solar masses, so with an event
rate of order one per century a steady state mass of radioactive
aluminum or iron of order one solar mass is present in the disk of our
galaxy. The resulting diffuse gamma-ray line glow of these isotopes
has been detected, and in fact allows one of the best Galaxy-wide
measurements of the Galactic star-formation rate (averaged over a few
million years). </p>

<p> Gamma-ray line observations in the MeV band offer unique opportunities
to reveal the multi-source origins of the metal content of our
galaxy. It furthermore offers an opportunity to probe the central
regions of these explosive sites through the glow of supernova
remnants that occurred in the past millennium, or the detection of SNe
in the local universe. Particularly important is the potential to
detect gamma-ray lines from nearby Type Ia supernova explosions 
(<a href="http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1006.5751">
Horiuchi and Beacom 2010</a>). <b> AMEGO will detect
&#126;6 SN Ia per year, out to a distance of at  least 30 Mpc </b>. The
847 keV line from <sup>56</sup>Co decay directly
yields the <sup>56</sup>Ni mass, which AMEGO will measure precisely for the
nearest SNIa events. </p>

<p>This mass is a critical factor determining the SNIa light curves and
their use as standardizable candles.  Thus AMEGO will open a new
window for testing SNIa calibration as distance indicators, and hence
as probes of cosmic acceleration.  Limited sensitivity in the MeV band
prevented significant progress in this arena. With the huge boost from
AMEGO's sensitivity relative to COMPTEL and INTEGRAL, the study of the
radioactive galaxy will experience a renaissance. We expect to detect
more remnants, better map the diffuse lines (including the puzzling
511 keV line), detect explosions in nearby galaxies, and finally
detect the long-predicted nuclear lines of CNO atoms excited by
Galactic cosmic rays. AMEGO will study isotope production in ways
possible only with gamma rays, where nuclear physics reigns and atomic
sensitivities to the thermal state of the emitting plasma are
minimized. AMEGO will advance all aspects of Astronomy with
Radioactivities through its greatly boosted
sensitivity. </p>
<h3>Test models that predict dark matter signals in the MeV band</h3>

<p>The nature of dark matter is crucial to our understanding of the 
structure and evolution of the Universe after the big bang.  AMEGO 
will be the most sensitive probe  in the MeV energy range to gamma-
ray signatures to two leading dark matter candidates:  Weakly 
	Interacting Massive Particles (WIMPs) and <a href="https://en.wikipedia.org/wiki/Axion">axion</a> like-particles (ALPs).  
This makes it an ideal instrument for dark matter searches.</p>

<p>WIMP dark matter naturally arises in extensions of the 
<a href="https://en.wikipedia.org/wiki/Standard_Model">Standard Model</a> 
of particle physics. Typically WIMPs are thought to have masses &gt; 1 GeV, 
which produce gamma rays with energies starting in the &#126;100 MeV range.  
AMEGO is ideal for searching for low-mass WIMP dark matter with the necessary 
precision in the MeV range for detection.</p>

<div class="figure">
<center><img src="images/ALP_w_inset.png" alt="ALP absorption features"></center>
<p>AMEGO will detect the absorption features in the gamma-ray spectra from interactions between gamma rays and <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.161101">axions in large magnetic fields</a>. Gamma rays are produced in sources such as AGN (shown) or galaxy clusters, and will interact in the surrounding large magnetic fields. This interaction could cause gamma rays to convert into axions or ALPs and back again to gamma rays, yielding absorption features.</p>
</div>

<p>ALPs interacting with photons in the presence of external magnetic fields 
lead to energy-dependent mixing similar to 
<a href="http://journals.aps.org/prd/abstract/10.1103/PhysRevD.37.1237">
neutrino oscillations</a>.  These oscillations yield distinctive features in 
the energy spectra of the target observed. Ideal targets have known large 
magnetic fields, such as magnetars, blazars and galaxy clusters.  Gamma rays 
from these sources often times traverse multiple magnetic-field  environments 
in which gamma rays could convert into an ALP and vice versa.  AMEGO is, with 
its low energy threshold, large field of view, and excellent angular resolution, 
the perfect instrument to search for such a signal and will extend these searches 
to ALP masses by orders of magnitude. The sensitivity surpasses that of current 
and planned dedicated ALP laboratory searches.</p>
	
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