フェルミ探査機、近傍の超新星からのガンマ線を観測せず(NASA’s Fermi Mission Sees No Gamma Rays from Nearby Supernova)

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2024-04-16 NASA

2023年に近くの渦巻銀河(M101)で発生した超新星SN 2023ixfは、地球から約2200万光年離れた木星座方向に位置し、Fermiが2008年の打ち上げ以来観測した中で最も明るい近傍超新星です。この超新星からは宇宙線が光速近くまで加速されると予想されていましたが、NASAのFermiガンマ線宇宙望遠鏡は、それに伴う高エネルギーガンマ線を検出していません。研究者は、超新星が全エネルギーの約1%を宇宙線加速に使っていることを新たに示し、これは以前の推定の10%よりも低い値です。この事実は、超新星が宇宙線の源であるという考えを完全に否定するものではないものの、そのメカニズムについてさらに研究が必要であることを示しています。

<関連情報>

フェルミ大望遠鏡によるコア崩壊型超新星2023ixfの宇宙線加速に関するガンマ線による初期制約 Early-time gamma-ray constraints on cosmic-ray acceleration in the core-collapse SN 2023ixf with the Fermi Large Area Telescope

G. Martí-Devesa, C. C. Cheung, N. Di Lalla, M. Renaud, G. Principe, N. Omodei, F. Acero
arXiv  Submitted on:16 Apr 2024
DOI:https://doi.org/10.48550/arXiv.2404.10487

フェルミ探査機、近傍の超新星からのガンマ線を観測せず(NASA’s Fermi Mission Sees No Gamma Rays from Nearby Supernova)

Abstract

While SNRs have been considered the most relevant Galactic CR accelerators for decades, CCSNe could accelerate particles during the earliest stages of their evolution and hence contribute to the CR energy budget in the Galaxy. Some SNRs have indeed been associated with TeV gamma-rays, yet proton acceleration efficiency during the early stages of an SN expansion remains mostly unconstrained. The multi-wavelength observation of SN 2023ixf, a Type II SN in the nearby galaxy M101, opens the possibility to constrain CR acceleration within a few days after the collapse of the RSG stellar progenitor. With this work, we intend to provide a phenomenological, quasi-model-independent constraint on the CR acceleration efficiency during this event at photon energies above 100 MeV. We performed a maximum-likelihood analysis of gamma-ray data from the Fermi Large Area Telescope up to one month after the SN explosion. We searched for high-energy emission from its expanding shock, and estimated the underlying hadronic CR energy reservoir assuming a power-law proton distribution consistent with standard diffusive shock acceleration. We do not find significant gamma-ray emission from SN 2023ixf. Nonetheless, our non-detection provides the first limit on the energy transferred to the population of hadronic CRs during the very early expansion of a CCSN. Under reasonable assumptions, our limits would imply a maximum efficiency on the CR acceleration of as low as 1%, which is inconsistent with the common estimate of 10% in generic SNe. However, this result is highly dependent on the assumed geometry of the circumstellar medium, and could be relaxed back to 10% by challenging spherical symmetry. A more sophisticated, inhomogeneous characterisation of the shock and the progenitor’s environment is required before establishing whether or not Type II SNe are indeed efficient CR accelerators at early times.

1701物理及び化学
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