Observational Estimates of Magnetic Fields inside Neutron Stars

2023 Fiscal Year Final Research Report

• Project/Area Number: 21K03624
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 16010:Astronomy-related
• Research Institution: The University of Tokyo
• Principal Investigator: Makishima Kazuo 東京大学, 大学院理学系研究科(理学部), 名誉教授 (20126163)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: 中性子星 / マグネター / 宇宙Ｘ線 / 磁場進化 / パルス位相変調 / トロイダル磁場

Outline of Final Research Achievements

Using archival data from Japanese cosmic X-ray satellites, such as ASCA and Suzaku, we performed timing studies of magnetars (a special type of neutron stars). Then, in seven magnetars, the rotational pulses were found to suffer phase modulation, at a period that is 4 orders of magnitude longer than the pulse period. This can be explained as a manifestation of free precession, when the star is axially deformed due to extremely intense toroidal magnetic fields, reaching 10^{16} Gauss, hidden inside it. When the estimated toroidal field strengths of the 7 object are divided by their dipole magnetic field strengths, the ratio was found to increase toward older objects. This means that the toroidal magnetic field, invisible from outside, lasts longer than the dipole field extending outside the stars. The result can naturally explain the presence of some classes of known neutron stars, which are regarded as aged magnetars.

Free Research Field

Ｘ線宇宙物理学

Academic Significance and Societal Importance of the Research Achievements

我々の結果は、宇宙でおそらく最強の磁場を発見したことを意味する。10の16乗ガウスのトロイダル磁場の存在は、いくつかの傍証から示唆されてはいたものの、その観測的検証に挑んだ研究は、我々の知る限り皆無である。我々が得たパルス位相変調という手掛かりは、データの詳細解析と基礎物理学を通じて到達したものであり、何らかの理論予測に導かれたものではなく、外国の後追いでもない。用いたデータがおもに日本の科学衛星が取得したものであることも鑑みると、本研究の成果はは掛け値なしに100パーセント「日本オリジナル」なものであると自負できる。