Probing the axion-nucleon coupling with supergiant stars

A finite axion-nucleon coupling enables the production of axions in stellar environments via the thermal excitation and subsequent deexcitation of the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mmultiscripts> <a:mrow> <a:mi>Fe</a:mi> </a:mrow> <a:mprescripts/> <a:none/> <a:mrow> <a:mn>57</a:mn> </a:mrow> </a:mmultiscripts> </a:mrow> </a:math> isotope. Given its low-lying excited state at 14.4 keV, <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mmultiscripts> <c:mrow> <c:mi>Fe</c:mi> </c:mrow> <c:mprescripts/> <c:none/> <c:mrow> <c:mn>57</c:mn> </c:mrow> </c:mmultiscripts> </c:mrow> </c:math> can be efficiently excited in the hot cores of supergiant stars, possibly leading to axion emission. The conversion of these axions into photons in the Galactic magnetic field results in a characteristic 14.4 keV line, potentially detectable by hard x-ray telescopes such as NASA’s Nuclear Spectroscopic Telescope Array (u). In this work, we present the first constraints on axion-nucleon couplings derived from u observations of Betelgeuse and discuss the potential insights that could be gained from detecting this line in other nearby supergiants. Our results establish significantly more stringent bounds than those obtained from solar observations, setting a limit of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mo stretchy="false">|</e:mo> <e:msub> <e:mi>g</e:mi> <e:mrow> <e:mi>a</e:mi> <e:mi>γ</e:mi> </e:mrow> </e:msub> <e:msubsup> <e:mi>g</e:mi> <e:mrow> <e:mi>a</e:mi> <e:mi>N</e:mi> </e:mrow> <e:mi>eff</e:mi> </e:msubsup> <e:mo stretchy="false">|</e:mo> <e:mo>&lt;</e:mo> <e:mo stretchy="false">(</e:mo> <e:mn>1.2</e:mn> <e:mi>–</e:mi> <e:mn>2.7</e:mn> <e:mo stretchy="false">)</e:mo> <e:mo>×</e:mo> <e:msup> <e:mn>10</e:mn> <e:mrow> <e:mo>−</e:mo> <e:mn>20</e:mn> </e:mrow> </e:msup> <e:mtext> </e:mtext> <e:mtext> </e:mtext> <e:mrow> <e:msup> <e:mrow> <e:mi>GeV</e:mi> </e:mrow> <e:mrow> <e:mo>−</e:mo> <e:mn>1</e:mn> </e:mrow> </e:msup> </e:mrow> </e:math> for <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:msub> <k:mi>m</k:mi> <k:mi>a</k:mi> </k:msub> <k:mo>≲</k:mo> <k:msup> <k:mn>10</k:mn> <k:mrow> <k:mo>−</k:mo> <k:mn>10</k:mn> </k:mrow> </k:msup> <k:mtext> </k:mtext> <k:mtext> </k:mtext> <k:mi>eV</k:mi> </k:math> .