Fast Radio Bursts from Axion Star Fa Peng Huang fapeng.huang@wustl.edu Department of Physics and McDonnell Center for the Space Sciences, Washington University in St. Louis based on the work with James H. Buckley, P. S. Bhupal Dev, Francesc Ferrer, arXiv:2004.06486 The 2020 Phenomenology Symposium@University of Pittsburgh May 4th, 2020
Outline ➢ Research motivation, Fast Radio Bursts and axion star ➢ Fast radio bursts from axion stars moving through pulsar magnetospheres ➢ Summary and outlook
Motivation: FRBs In recent ten years, Fast Radio Bursts (FRBs) become the most mysterious phenomenon in astrophysics and cosmology, especially from 2013 (D. Thornton, et al., (2013) Science, 341, 53) . They are intense, transient radio signals with large dispersion measure, light years away. However, their origin and physical nature are still obscure. ost, a µ Jy radio signal O (0.1) to O (100) Jy means that the total ene � O (10 38 ) to O (10 40 ) erg, Duration: millisecond s ft 0 . 1 . z . 2 . 2. We focus on FRBs events with frequency range 800 MHz to 1.4GHz, mainly observed by Parkes, ASKAP, and UTMOST. We do not include other non- repeating FRBs with frequencies lower than 800 MHz, like the events from CHIME and Pushchino, which may be better explained by a lighter axion or other sources. From Universe Today
<latexit sha1_base64="qPrgjaojC0ulxNavxFhOevF23MQ=">AB+nicdVDLSsNAFJ3UV62vVpduBovgxpCktam7gi7cCBXsA9pYJtNpO3SCTMTpcR8ihsXirj1S9z5N04fgoeuHA4517uvcePGJXKsj6MzNLyupadj23sbm1vZMv7DYljwUmDcwZF20fScJoSBqKkbakSAo8Blp+eOzqd+6JUJSHl6rSUS8A1DOqAYKS318gXbukmO7VJ62Uu6vM9V2sXLdOulNyBS3TcUqVqOJe3LqulVom9YMRbBAvZd/7/Y5jgMSKsyQlB3bipSXIKEoZiTNdWNJIoTHaEg6moYoINJLZqen8FArfTjgQleo4Ez9PpGgQMpJ4OvOAKmR/O1Nxb+8TqwGVS+hYRQrEuL5okHMoOJwmgPsU0GwYhNEBZU3wrxCAmElU4rp0P4+hT+T5qOaZdM56pcrJ0v4siCfXAjoANXFADF6AOGgCDO/AnsCzcW8Gi/G67w1Yyxm9sAPG+fte6TsA=</latexit> <latexit sha1_base64="ykNG8kLK/57w7IUlkcgOdsN9E8=">AB63icdVDLSsNAFJ3UV62vqks3g0VwFZK0NnVX0IXLCvYBbSiT6aQdOjMJMxOhP6CGxeKuPWH3Pk3TtoKnrgwuGce7n3njBhVGnH+bAKa+sbm1vF7dLO7t7+QfnwqKPiVGLSxjGLZS9EijAqSFtTzUgvkQTxkJFuOL3K/e49kYrG4k7PEhJwNBY0ohjpXBrwtDQsVxzbrVf9mgMd2/Oq9YZniH9x6fsN6NrOAhWwQmtYfh+MYpxyIjRmSKm+6yQ6yJDUFDMyLw1SRKEp2hM+oYKxIkKsWtc3hmlBGMYmlKaLhQv09kiCs146Hp5EhP1G8vF/y+qmOGkFGRZJqIvByUZQyqGOYPw5HVBKs2cwQhCU1t0I8QRJhbeLJQ/j6FP5POp7tVm3vtlZpXq/iKITcArOgQt80AQ3oAXaAIMJeABP4Nni1qP1Yr0uWwvWauY/ID19gkRu45H</latexit> <latexit sha1_base64="qPrgjaojC0ulxNavxFhOevF23MQ=">AB+nicdVDLSsNAFJ3UV62vVpduBovgxpCktam7gi7cCBXsA9pYJtNpO3SCTMTpcR8ihsXirj1S9z5N04fgoeuHA4517uvcePGJXKsj6MzNLyupadj23sbm1vZMv7DYljwUmDcwZF20fScJoSBqKkbakSAo8Blp+eOzqd+6JUJSHl6rSUS8A1DOqAYKS318gXbukmO7VJ62Uu6vM9V2sXLdOulNyBS3TcUqVqOJe3LqulVom9YMRbBAvZd/7/Y5jgMSKsyQlB3bipSXIKEoZiTNdWNJIoTHaEg6moYoINJLZqen8FArfTjgQleo4Ez9PpGgQMpJ4OvOAKmR/O1Nxb+8TqwGVS+hYRQrEuL5okHMoOJwmgPsU0GwYhNEBZU3wrxCAmElU4rp0P4+hT+T5qOaZdM56pcrJ0v4siCfXAjoANXFADF6AOGgCDO/AnsCzcW8Gi/G67w1Yyxm9sAPG+fte6TsA=</latexit> FRB-Axion star correlation Axion or axion-like particle motivated from strong CP problem or string theory is still one of the most attractive and promising DM candidate. A collection of axions can condense into a bound Bose- Einstein condensate called an axion star. The typical axion star mass is 10 � 13 M � The fact that the energy released by FRBs is close to , which is the typical axion star mass, and 10 � 13 M � that their frequency (several hundred MHz to several GHz) coincides with that expected from eV axion µ particles, motivates us to further explore whether the axion-FRB connection can be made viable in a pulsar magnetosphere and tested with the future data.
<latexit sha1_base64="qPrgjaojC0ulxNavxFhOevF23MQ=">AB+nicdVDLSsNAFJ3UV62vVpduBovgxpCktam7gi7cCBXsA9pYJtNpO3SCTMTpcR8ihsXirj1S9z5N04fgoeuHA4517uvcePGJXKsj6MzNLyupadj23sbm1vZMv7DYljwUmDcwZF20fScJoSBqKkbakSAo8Blp+eOzqd+6JUJSHl6rSUS8A1DOqAYKS318gXbukmO7VJ62Uu6vM9V2sXLdOulNyBS3TcUqVqOJe3LqulVom9YMRbBAvZd/7/Y5jgMSKsyQlB3bipSXIKEoZiTNdWNJIoTHaEg6moYoINJLZqen8FArfTjgQleo4Ez9PpGgQMpJ4OvOAKmR/O1Nxb+8TqwGVS+hYRQrEuL5okHMoOJwmgPsU0GwYhNEBZU3wrxCAmElU4rp0P4+hT+T5qOaZdM56pcrJ0v4siCfXAjoANXFADF6AOGgCDO/AnsCzcW8Gi/G67w1Yyxm9sAPG+fte6TsA=</latexit> Axion star-Neutron star encounter Dilute axion star is balanced by kinetic pressure and self-gravity, with the following radius ◆ 2 ✓ 10 � 12 M � ✓ 10 µ eV ◆ 1 R dilute ∼ = 270 km ∼ a G N M a m 2 m a M a a In this work, we assume that dense axion stars with a mass around can survive to the present, and 10 � 13 M � have a chance to encounter a neutron star. The radius of a dense axion star is q g a γγ × 10 13 GeV R dense ∼ 0 . 47 a ◆ 0 . 3 r ✓ 10 µ eV M a m , × m a 10 � 13 M �
<latexit sha1_base64="roRvdci61iEDlZhgLtUfTscAtnM=">AB7HicdVBNSwMxEJ2tX7V+VT16CRbBU8lurdVbQ8eK7it0K4lm6ZtaDa7JFmhlP4GLx4U8eoP8ua/MdtWUNEHA4/3ZpiZFyaCa4Pxh5NbWl5ZXcuvFzY2t7Z3irt7TR2nijKfxiJWtyHRTHDJfMONYLeJYiQKBWuFo4vMb90zpXksb8w4YUFEBpL3OSXGSr6L704L3WIJlzGuVvA5ykit6uGMeBhXPORaJUMJFmh0i+dXkzTiElDBdG67eLEBOiDKeCTQudVLOE0BEZsLalkRMB5PZsVN0ZJUe6sfKljRopn6fmJBI63EU2s6ImKH+7WXiX147Nf2zYMJlkhom6XxRPxXIxCj7HPW4YtSIsSWEKm5vRXRIFKHG5pOF8PUp+p80vbJbKXvXJ6X65SKOPBzAIRyDCzWowxU0wAcKHB7gCZ4d6Tw6L87rvDXnLGb24Qect0+B+I3X</latexit> Tidal effects A gravitationally bound object approaching a star closer than Roche limit will be disrupted by tidal effects. The Roche limit is ◆ 1 / 3 ✓ 2 M NS r t = R a M a Tidal disruption may quickly rip apart the dilute axion star, producing a stream of axion debris, long before a dilute axion star enters the magnetosphere of neutron star. For100 km dilute axion, the Roche limit is about km. 10 6 For a dense axion star, the radius is smaller e tidal deformation ratio: than 1m and the Roche limit is below 10 km. � R a 9 M NS Thus, a dense axion star can reach the resonant = 8 ⇡⇢ AS r 3 R a conversion region without being tidally ripped.
Quick sketch of the neutron star size Radius of the neutron star is slightly larger than the radius of the LHC circle.
Strong magnetic field in the magnetosphere of Neutron star, Pulsar, Magnetar: the strongest magnetic field in the Universe 1 . Mass: from 1 to 2 solar mass r 0 ∼ 10 − 20km 2. Radius: The typical diameter of neutron star is just half-Marathon. 3. Strongest magnetic field at the surface of the neutron star B 0 ≈ 10 12 − 10 15 G B 0 ∼ 3 . 3 × 10 19 p P ˙ G P P is the period of neutron star 4. Neutron star is surrounded by large region of magnetosphere, where photon becomes massive. r ∼ 100 r 0 Alfven
James H. Buckley , P. S. Bhupal Dev , Francesc Ferrer , FPH, arXiv:2004.06486
Axion-photon conversion in magnetosphere The Lagrangian for axion-photon conversion the magnetosphere L ¼ − 1 4 F μν F μν þ 1 2 ð ∂ μ a ∂ μ a − m 2 a a 2 Þ þ L int þ L QED ; Massive Photon: In the magnetosphere α 2 7 of the neutron star, photon obtains the 4 ð F μν ˜ F μν Þ 2 ; +… L QED ¼ 90 m 4 e effective mass in the magnetized plasma. 45 π ω 2 B 2 Q QED ¼ 7 α mass m 2 ; γ ¼ Q pl − Q QED B 2 crit plasma ¼ 4 πα n e B axion Q plasma ¼ ω 2 ; photon m e � 2 10 12 G Q pl � μ eV 1 sec ∼ 5 × 10 8 : For relativistic axion from neutron Q QED ω B P star, QED mass dominates and there is no resonant conversion. L = − g a γγ aF µ ν ˜ F µ ν = g a γγ a ~ E · ~ B , 4 Axion-photon conversion in external magnetic field G. Raffelt and L. Stodolsky, Phys. Rev. D 37, 1237 (1988)
Axion-photon conversion in magnetosphere Massive Photon: In the magnetosphere of the neutron star, photon obtains the effective mass in the magnetized plasma. s e 2 n e r n e m γ ( r ) = ω p = = 7 . 3 × 10 8 cm � 3 µ eV , m e ⇣ r NS B ð r Þ ⌘ 3 e ð r Þ ¼ 7 × 10 − 2 1 s 1 n e ð r Þ ¼ n GJ B ( r ) = B 0 cm 3 P 1 G r Here, we choose the simplest electron density distribution and magnetic field configuration to clearly see the physics process. Thus, the photon mass is position r dependent, and within some region the photon mass is close to the axion mass.
The Non-adiabatic Resonant Conversion ∼ In the resonant conversion region, the photon e ff ec- tively has almost the same mass as the axion due to plasma e ff ects: ✓ m a � � ◆ 2 10 10 G d ω 2 = 3 ω 2 ◆ 3 � � ✓ r NS P p p � � 1 s . d r r ∼ � � r c µ eV B 0 � � resonant case, and it can be obtained from the well- r = r c r = r c known Landau-Zener probability: P a ! γ = 1 − e � 2 πβ . (10) The non-adiabatic limit corresponds to small β , and we have P a ! γ ≈ 2 πβ with β = ( g a γγ ω B 0 ) 2 / 2¯ � k � . (11) � � � � d ω 2 p / d r � � � r = r c
FRBs Signal: For resonant conversion the radiated power is ✓ ◆ � M a 10 44 GeV · s � 1 � ˙ 10 7 × P a ! γ � � W ∼ 10 � 13 M � egion. Hence, to explain the ty W ∼ 10 44 GeV · s � 1 ˙ o FRBs, ✓ d ˙ W ◆ 2 E FRB F obs ∆ B S = × 10 � 29 (1 + z ) , = 4 π d 2 ∆ B J Jy · ms Hz m S For the benchmark values m a = 10 µ eV, M a = 10 � 13 M � , g a γγ = 10 � 13 GeV � 1 we can naturally explain FRBs. Sensitivity: The smallest detectable flux density of the radio telescope is of order, taking SKA as example ◆ 1 / 2 ✓ 1 ms ◆ 1 / 2 ✓ 10 3 m 2 / K ✓ 1 MHz ◆ S min ≈ 0 . 09 Jy ∆ B t obs A e ff /T sys
James H. Buckley , P. S. Bhupal Dev , Francesc Ferrer , FPH, arXiv:2004.06486
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