Detectability of accretion-induced bosenovae in the Milky Way

We estimate collapse rates of axion stars in our galaxy based on the axion minicluster mass function of the Milky Way dark matter halo. We consider axionlike particles (ALP) with different temperature evolution of the axion mass, including the QCD axion with <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>m</a:mi><a:mi>a</a:mi></a:msub><a:mo>=</a:mo><a:mn>50</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi mathvariant="normal">μ</a:mi><a:mi>eV</a:mi></a:math>. Combining estimates for the present-day axion star mass function from our previous work with the axion star accretion model predicted by self-similar growth, we can infer the expected number of bosenovae occurring within the Milky Way. Our estimates suggest that for an observation time of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msub><d:mi>t</d:mi><d:mi>obs</d:mi></d:msub><d:mo>=</d:mo><d:mn>1</d:mn><d:mtext> </d:mtext><d:mtext> </d:mtext><d:mi>yr</d:mi></d:math>, the majority of the up to <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mo>∼</f:mo><f:msup><f:mn>10</f:mn><f:mn>13</f:mn></f:msup></f:math> bosenovae per galaxy occur in the densest miniclusters with initial overdensity parameter <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mi mathvariant="normal">Φ</h:mi><h:mo>≲</h:mo><h:msup><h:mn>10</h:mn><h:mn>4</h:mn></h:msup></h:math>. We discuss the detectability of such recurring axion bursts within our galactic vicinity and find that, for models with derivative couplings including axion-fermion interactions, potential broadband axion dark matter experiments can probe a large range of ALP masses <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>6</k:mn></k:mrow></k:msup><k:mtext> </k:mtext><k:mtext> </k:mtext><k:mi>eV</k:mi></k:math> and with moderate improvements even the quantum chromodynamics axion case. For axions with nonderivative-type interactions like the axion-photon coupling, our analysis suggests that optimistic predictions with order-one dark matter abundance of axion stars <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msub><m:mi>f</m:mi><m:mo>⋆</m:mo></m:msub><m:mo>∼</m:mo><m:mn>1</m:mn></m:math> can be probed by dedicated burst searches.