Constraints on Axionlike Particles from a Hard X-Ray Observation of Betelgeuse

We use the first observation of Betelgeuse in hard x rays to perform a novel search for axionlike particles (ALPs). Betelgeuse is not expected to be a standard source of x rays, but light ALPs produced in the stellar core could be converted back into photons in the Galactic magnetic field, producing a detectable flux that peaks in the hard x-ray band ($E_{\gamma }>10\mathrm{keV}$). Using a 50 ks observation of Betelgeuse by the NuSTAR satellite telescope, we find no significant excess of events above the expected background. Using models of the regular Galactic magnetic field in the direction of Betelgeuse, we set a 95% C.L. upper limit on the ALP-photon coupling of $g_{a\gamma }<(0.5–1.8)\times {10}^{-11}{\mathrm{GeV}}^{-1}$ (depending on magnetic field model) for ALP masses $m_a<(5.5–3.5)\times {10}^{-11}\mathrm{eV}$.

Figure 1

Astrophysical constraints on the ALP-photon coupling in relation to the Betelgeuse bound from the present work: SN 1987A [11], Fermi-LAT extragalactic SNe [12], M87 [16], and Chandra NGC 1275 [14].

Figure 2

Top: x-ray spectra from FPMA (left) and FPMB (right) for the Betelgeuse source (red) and background (gray and blue for before and after normalization) regions. The error bars overlaid are the statistic uncertainties ($\sqrt{N}$). Bottom: Source spectra after subtracting the normalized background. The error bars are calculated by Sumw2 with ROOT software [51]. The predicted ALP-produced x-ray spectra assuming transverse magnetic field $B_T=1.4\mu \mathrm{G}$, mass $m_a={10}^{-11}\mathrm{eV}$, and coupling $g_{a\gamma }=1.5\times {10}^{-11}{\mathrm{GeV}}^{-1}$, that would be detected by the NuSTAR instrument are overlaid. The stellar model parameters are described in Table S1 [25]. The spectra are binned to a width of 1 keV, though analysis is performed on unbinned data.

Figure 3

Comparison of 95% upper limits of $g_{a\gamma }$ from NuSTAR Betelgeuse observation is shown in the red band: upper solid line for the most conservative stellar model and $B_T$, lower dashed line for the most optimistic stellar model and $B_T$, and the interval for the other combinations. Overlaid are the region of interest for the QCD axion [55], the current limits from CAST [10], ADMX [56], ABRACADABRA-10 cm [57], and SN 1987A [11], the projected sensitivity of ALPS-II [22] and Baby-IAXO [23, 24]. Also labeled are the ALP regions suggested by hints of excess gamma-ray transparency or cooling of horizontal-branch stars [58, 59].

Figure 4

Evolution on the derived 95% C.L. upper limit of $g_{a\gamma }$ with remaining time until the core collapse for Betelgeuse. Results are shown for $m_a=1.0\times {10}^{-11}$ and $m_a=1.0\times {10}^{-10}\mathrm{eV}$ assuming $B_T=1.4\mu \mathrm{G}$ (more masses are shown in the Supplemental Material [25]). The solid black line shows the fitting. The width of the violet band indicates the uncertainty due to choice of $B_T$.