NuSTAR as an Axion Helioscope

We present a novel approach to investigating axions and axionlike particles by studying their potential conversion into x-rays within the Sun’s atmospheric magnetic field. Utilizing high-sensitivity data from the nuclear spectroscopic telescope array (NuSTAR) collected during the 2020 solar minimum, along with advanced solar atmospheric magnetic field models, we establish a new limit on the axion-photon coupling strength <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mrow><a:mi>g</a:mi></a:mrow><a:mrow><a:mi>a</a:mi><a:mi>γ</a:mi></a:mrow></a:msub><a:mo>≲</a:mo><a:mn>7.3</a:mn><a:mo>×</a:mo><a:msup><a:mrow><a:mn>10</a:mn></a:mrow><a:mrow><a:mo>−</a:mo><a:mn>12</a:mn></a:mrow></a:msup><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mrow><a:msup><a:mrow><a:mi>GeV</a:mi></a:mrow><a:mrow><a:mo>−</a:mo><a:mn>1</a:mn></a:mrow></a:msup></a:mrow></a:mrow></a:math> at 95% CL for axion masses <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>m</c:mi></c:mrow><c:mrow><c:mi>a</c:mi></c:mrow></c:msub><c:mo>≲</c:mo><c:mn>4</c:mn><c:mo>×</c:mo><c:msup><c:mrow><c:mn>10</c:mn></c:mrow><c:mrow><c:mo>−</c:mo><c:mn>7</c:mn></c:mrow></c:msup><c:mtext> </c:mtext><c:mtext> </c:mtext><c:mi>eV</c:mi></c:mrow></c:math>. This constraint surpasses current ground-based experimental limits, studying previously unexplored regions of the axion-photon coupling parameter space up to masses of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>m</e:mi><e:mi>a</e:mi></e:msub><e:mo>≲</e:mo><e:mn>3.4</e:mn><e:mo>×</e:mo><e:msup><e:mn>10</e:mn><e:mrow><e:mo>−</e:mo><e:mn>4</e:mn></e:mrow></e:msup><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>eV</e:mi></e:math>. These findings mark a significant advancement in our ability to probe axion properties and strengthen indirect searches for dark matter candidates.