Editing Axion (section)

=== Astrophysical axion searches ===
Axion-like bosons could have a signature in astrophysical settings. In particular, several works have proposed axion-like particles as a solution to the apparent transparency of the Universe to TeV photons ([[Very-high-energy gamma ray|very-high-energy gamma rays]]).<ref>
{{cite journal
 |last1=De Angelis |first1=A.
 |last2=Mansutti   |first2=O.
 |last3=Roncadelli |first3=M.
 |year=2007
 |title=Evidence for a new light spin-zero boson from cosmological gamma-ray propagation?
 |journal=Physical Review D
 |volume=76 |issue=12 |page=121301
 |bibcode=2007PhRvD..76l1301D  |s2cid=119152884
 |doi=10.1103/PhysRevD.76.121301 |arxiv=0707.4312
}}
</ref><ref>
{{cite journal
 |last1=De Angelis  |first1=A.
 |last2=Mansutti    |first2=O.
 |last3=Persic      |first3=M.
 |last4=Roncadelli  |first4=M.
 |year=2009
 |title=Photon propagation and the very high energy gamma-ray spectra of blazars: How transparent is the Universe?
 |journal=Monthly Notices of the Royal Astronomical Society: Letters
 |volume=394  |issue=1  |pages=L21–L25
 |doi=10.1111/j.1745-3933.2008.00602.x  |doi-access=free
 |arxiv=0807.4246
 |bibcode=2009MNRAS.394L..21D  |s2cid=18184567
}}
</ref> It has also been demonstrated that, in the large magnetic fields threading the atmospheres of compact astrophysical objects (e.g., [[magnetar]]s), photons will convert much more efficiently. This would in turn give rise to distinct absorption-like features in the spectra detectable by early 21st century telescopes.<ref>
{{cite journal
 |last1=Chelouche |first1=Doron
 |last2=Rabadan   |first2=Raul
 |last3=Pavlov    |first3=Sergey S.
 |last4 =Castejon |first4=Francisco
 |year=2009
 |title=Spectral signatures of photon–particle oscillations from celestial objects
 |journal=The Astrophysical Journal
 |series=Supplement Series
 |volume=180  |issue=1  |pages=1–29
 |arxiv=0806.0411  |doi=10.1088/0067-0049/180/1/1  |s2cid=5018245
 |bibcode=2009ApJS..180....1C
}}
</ref> A new (2009) promising means is looking for quasi-particle refraction in systems with strong magnetic gradients. In particular, the refraction will lead to beam splitting in the radio light curves of highly magnetized pulsars and allow much greater sensitivities than currently achievable.<ref>
{{cite journal
 |last1=Chelouche |first1=Doron
 |last2=Guendelman |first2=Eduardo I.
 |year=2009
 |title=Cosmic analogs of the Stern–Gerlach experiment and the detection of light bosons
 |journal=The Astrophysical Journal
 |volume=699  |issue=1  |pages=L5–L8
 |doi=10.1088/0004-637X/699/1/L5  |arxiv=0810.3002
 |bibcode=2009ApJ...699L...5C  |s2cid=11868951
}}
</ref> The [[International Axion Observatory]] (IAXO) is a proposed fourth generation [[helioscope]].<ref>
{{cite web
 |title=The International Axion Observatory
 |publisher=[[CERN]]
 |url=http://iaxo.web.cern.ch/content/home-international-axion-observatory
 |access-date=19 March 2016
}}
</ref>

Axions can resonantly convert into photons in the [[magnetosphere]]s of [[neutron star]]s.<ref>
{{cite journal
 |last1=Pshirkov |first1=Maxim S.
 |last2=Popov    |first2=Sergei B.
 |year=2009
 |title=Conversion of Dark matter axions to photons in magnetospheres of neutron stars
 |journal=Journal of Experimental and Theoretical Physics
 |volume= 108 |issue=3 |pages=384–388
 |doi=10.1134/S1063776109030030
 |arxiv=0711.1264
 |bibcode= 2009JETP..108..384P
 |s2cid=119269835
}}
</ref> The emerging photons lie in the GHz frequency range and can be potentially picked up in radio detectors, leading to a sensitive probe of the axion parameter space. This strategy has been used to constrain the axion–photon coupling in the mass range {{val|5|–|11|u=μeV/''c''<sup>2</sup>}}, by re-analyzing existing data from the [[Green Bank Telescope]] and the [[Effelsberg 100-m Radio Telescope|Effelsberg 100&nbsp;m Radio Telescope]].<ref>
{{cite journal
 |last1=Foster     |first1=Joshua W.
 |last2=Kahn       |first2=Yonatan
 |last3=Macias     |first3=Oscar
 |last4=Sun        |first4=Zhiquan
 |last5=Eatough    |first5=Ralph P.
 |last6=Kondratiev |first6=Vladislav I.
 |last7=Peters     |first7=Wendy M.
 |last8=Weniger    |first8=Christoph
 |last9=Safdi      |first9=Benjamin R.
 |year=2020
 |title=Green Bank and Effelsberg Radio Telescope Searches for Axion Dark Matter Conversion in Neutron Star Magnetospheres
 |journal=Physical Review Letters
 |volume=125 |number=17 |pages= 171301
 |doi=10.1103/PhysRevLett.125.171301
 |pmid=33156637
 |arxiv=2004.00011
 |bibcode= 2020PhRvL.125q1301F
 |s2cid=214743261
}}
</ref> A novel, alternative strategy consists in detecting the transient signal from the encounter between a neutron star and an axion minicluster in the [[Milky Way]].<ref>
{{cite journal
 |last1=Edwards   |first1=Thomas D. P. 
 |last2=Kavanagh  |first2=Bradley J.
 |last3=Visinelli |first3=Luca
 |last4=Weniger   |first4=Christoph
 |year=2021
 |title=Transient Radio Signatures from Neutron Star Encounters with QCD Axion Miniclusters
 |journal=Physical Review Letters
 |volume=127 |number=13 |pages= 131103
 |doi=10.1103/PhysRevLett.127.131103
 |pmid=34623827 
 |arxiv=2011.05378
 |bibcode=2021PhRvL.127m1103E 
 |s2cid=226300099 
}}
</ref>

Axions can be produced in the Sun's core when X-rays scatter in strong electric fields. The [[CERN Axion Solar Telescope|CAST]] solar telescope is underway, and has set limits on coupling to photons and electrons. Axions may also be produced within neutron stars by nucleon–nucleon [[bremsstrahlung]]. The subsequent decay of axions to gamma rays allows constraints on the axion mass to be placed from observations of neutron stars in gamma-rays using the [[Fermi Gamma-ray Space Telescope]]. From an analysis of four neutron stars, Berenji et al. (2016) obtained a 95% [[confidence interval]] upper limit on the axion mass of {{val|0.079|u=eV/c2}}.<ref>
{{cite journal
 |last1=Berenji |first1=B. 
 |last2=Gaskins |first2=J.
 |last3=Meyer   |first3=M.
 |year=2016
 |title=Constraints on axions and axionlike particles from Fermi Large Area Telescope observations of neutron stars
 |journal=Physical Review D
 |volume=93  |issue=14  |page=045019
 |arxiv=1602.00091  |doi=10.1103/PhysRevD.93.045019
 |bibcode=2016PhRvD..93d5019B  |s2cid=118723146
}}
</ref> In 2021 it has been also suggested<ref>{{cite journal |last1=Buschmann |first1=Malte |last2=Co |first2=Raymond T. |last3=Dessert |first3=Christopher |last4=Safdi |first4=Benjamin R. |title=Axion Emission Can Explain a New Hard X-Ray Excess from Nearby Isolated Neutron Stars |journal=Physical Review Letters |date=12 January 2021 |volume=126 |issue=2 |page=021102 |doi=10.1103/PhysRevLett.126.021102 |pmid=33512228 |arxiv=1910.04164 |bibcode=2021PhRvL.126b1102B |s2cid=231764983 }}</ref><ref>{{cite web |last=O'Callaghan |first=Jonathan |date=2021-10-19 |title=A Hint of Dark Matter Sends Physicists Looking to the Skies |url=https://www.quantamagazine.org/a-hint-of-dark-matter-sends-physicists-looking-to-the-skies-20211019/ |access-date=2021-10-25 |website=Quanta Magazine |language=en }}</ref> that a reported<ref>{{cite journal |last1=Dessert |first1=Christopher |last2=Foster |first2=Joshua W. |last3=Safdi |first3=Benjamin R. |title=Hard X-Ray Excess from the Magnificent Seven Neutron Stars |journal=The Astrophysical Journal |date=November 2020 |volume=904 |issue=1 |pages=42 |doi=10.3847/1538-4357/abb4ea |arxiv=1910.02956 |bibcode=2020ApJ...904...42D |s2cid=203902766 |doi-access=free }}</ref> excess of hard X-ray emission from a system of neutron stars known as the [[The Magnificent Seven (neutron stars)|magnificent seven]] could be explained as axion emission.

In 2016, a theoretical team from [[Massachusetts Institute of Technology]] devised a possible way of detecting axions using a strong magnetic field that need be no stronger than that produced in an [[Magnetic resonance imaging|MRI]] scanning machine. It would show variation, a slight wavering, that is linked to the mass of the axion. Results from the ensuing experiment published in 2021 reported no evidence of axions in the mass range from 4.1x10<sup>−10</sup> to 8.27x10<sup>−9</sup> eV.<ref>{{cite journal |last1=Salemi |first1=Chiara P. |last2=Foster |first2=Joshua W. |last3=Ouellet |first3=Jonathan L. |last4=Gavin |first4=Andrew |last5=Pappas |first5=Kaliroë M. W. |last6=Cheng |first6=Sabrina |last7=Richardson |first7=Kate A. |last8=Henning |first8=Reyco |last9=Kahn |first9=Yonatan |last10=Nguyen |first10=Rachel |last11=Rodd |first11=Nicholas L. |last12=Safdi |first12=Benjamin R. |last13=Winslow |first13=Lindley |date=2021-08-17 |title=Search for Low-Mass Axion Dark Matter with ABRACADABRA-10 cm |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.081801 |journal=Physical Review Letters |volume=127 |issue=8 |pages=081801 |doi=10.1103/PhysRevLett.127.081801|pmid=34477408 |arxiv=2102.06722 |bibcode=2021PhRvL.127h1801S }}</ref>

In 2022 the polarized light [[Event Horizon Telescope#Messier 87*|measurements]] of [[Messier 87|Messier 87*]] by the [[Event Horizon Telescope]] were used to constrain the mass of the axion assuming that hypothetical clouds of axions could form around a black hole, rejecting the approximate {{val|e=-21|u=eV/c2}} – {{val|e=-20|u=eV/c2}} range of mass values.<ref>{{cite journal |last1=Chen |first1=Yifan |last2=Liu |first2=Yuxin |last3=Lu |first3=Ru-Sen |last4=Mizuno |first4=Yosuke |last5=Shu |first5=Jing |last6=Xue |first6=Xiao |last7=Yuan |first7=Qiang |last8=Zhao |first8=Yue |title=Stringent axion constraints with Event Horizon Telescope polarimetric measurements of M87⋆ |journal=Nature Astronomy |date=17 March 2022 |volume=6 |issue=5 |pages=592–598 |doi=10.1038/s41550-022-01620-3 |arxiv=2105.04572 |bibcode=2022NatAs...6..592C |s2cid=247188135 }}</ref><ref>{{cite news |last1=Kruesi |first1=Liz |title=How light from black holes is narrowing the search for axions |url=https://www.sciencenews.org/article/black-hole-light-axion-particle-search-event-horizon |work=Science News |date=17 March 2022 }}</ref>