TY - JOUR
T1 - Sagittarius A∗ as an origin of the Galactic PeV cosmic rays?
AU - Fujita, Yutaka
AU - Murase, Kohta
AU - Kimura, Shigeo S.
N1 - Funding Information:
This work was supported by MEXT KAKENHI No. 15K05080 (YF). The work of K.M. is supported by NSF Grant No. PHY-1620777 (KM). This work is partly supported by NASA NNX13AH50G and by IGC post-doctoral fellowship program (S.S.K).
Publisher Copyright:
© 2017 IOP Publishing Ltd and Sissa Medialab srl .
PY - 2017/4/20
Y1 - 2017/4/20
N2 - Supernova remnants (SNRs) have commonly been considered as a source of the observed PeV cosmic rays (CRs) or a Galactic PeV particle accelerator (»Pevatron»). In this work, we study Sagittarius A∗ (Sgr A∗), which is the low-luminosity active galactic nucleus of the Milky Way Galaxy, as another possible canditate of the Pevatron, because it sometimes became very active in the past. We assume that a large number of PeV CRs were injected by Sgr A∗ at the outburst about 107 yr ago when the Fermi bubbles were created. We constrain the diffusion coefficient for the CRs in the Galactic halo on the condition that the CRs have arrived on the Earth by now, while a fairly large fraction of them have escaped from the halo. Based on a diffusion-halo model, we solve a diffusion equation for the CRs and compare the results with the CR spectrum on the Earth. The observed small anisotropy of the arrival directions of CRs may be explained if the diffusion coefficient in the Galactic disk is smaller than that in the halo. Our model predicts that a boron-to-carbon ratio should be energy-independent around the knee, where the CRs from Sgr A∗ become dominant. It is unlikely that the spectrum of the CRs accelerated at the outburst is represented by a power-law similar to the one for those responsible for the gamma-ray emission from the central molecular zone (CMZ) around the Galactic center.
AB - Supernova remnants (SNRs) have commonly been considered as a source of the observed PeV cosmic rays (CRs) or a Galactic PeV particle accelerator (»Pevatron»). In this work, we study Sagittarius A∗ (Sgr A∗), which is the low-luminosity active galactic nucleus of the Milky Way Galaxy, as another possible canditate of the Pevatron, because it sometimes became very active in the past. We assume that a large number of PeV CRs were injected by Sgr A∗ at the outburst about 107 yr ago when the Fermi bubbles were created. We constrain the diffusion coefficient for the CRs in the Galactic halo on the condition that the CRs have arrived on the Earth by now, while a fairly large fraction of them have escaped from the halo. Based on a diffusion-halo model, we solve a diffusion equation for the CRs and compare the results with the CR spectrum on the Earth. The observed small anisotropy of the arrival directions of CRs may be explained if the diffusion coefficient in the Galactic disk is smaller than that in the halo. Our model predicts that a boron-to-carbon ratio should be energy-independent around the knee, where the CRs from Sgr A∗ become dominant. It is unlikely that the spectrum of the CRs accelerated at the outburst is represented by a power-law similar to the one for those responsible for the gamma-ray emission from the central molecular zone (CMZ) around the Galactic center.
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U2 - 10.1088/1475-7516/2017/04/037
DO - 10.1088/1475-7516/2017/04/037
M3 - Article
AN - SCOPUS:85019480846
VL - 2017
JO - Journal of Cosmology and Astroparticle Physics
JF - Journal of Cosmology and Astroparticle Physics
SN - 1475-7516
IS - 4
M1 - 037
ER -