Division of Science, NAOJ

Talk by Margherita Grespan
2025-05-14 15:30 - 16:30 (Asia/Tokyo) | 大セミナー室（すばる棟, 国立天文台三鷹キャンパス）
TBD

Masafumi Niwano (NAOJ) : Study of the relationship between circumstellar disc evolution and Be donor activities in BeXBs
2025-05-07 15:30 - 16:30
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May. 7 Wed
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Campus: Mitaka
Seminar: NAOJ Science Colloquium
Date and time: 2025 May 7 (Wed.), 15:30-16:30 JST
Place: The large seminar room / Zoom (hybrid)
Speaker: Masafumi Niwano
Affiliation: NAOJ ADC/DoS
Title: Study of the relationship between circumstellar disc evolution and Be donor activities in BeXBs
Abstract: The mechanism of X-ray outbursts in Be X-ray binaries remains a mystery, and understanding their circumstellar discs is crucial for a solution of the mass-transfer problem. In particular, it is important to identify the Be star activities (e.g. pulsations) that cause mass ejection and, hence, disc formation. Therefore, we investigated the relationship between optical flux oscillations and the infrared (IR) excess in a sample of five Be X-ray binaries. Applying the Lomb-Scargle technique to high-cadence optical light curves from the Transiting Exoplanet Survey Satellite (TESS), we detected several significant oscillation modes in the 3-24 h period range for each source. We also measured the IR excess (a proxy for disc growth) of those five sources, using J-band light curves from Palomar Gattini-IR. In four of the five sources, we found anticorrelations between the IR excess and the amplitude of the main flux oscillation modes. This result is inconsistent with the conventional idea that non-radial pulsations drive mass ejections. We propose an alternative scenario where internal temperature variations in the Be star cause transitions between pulsation-active and mass-ejection-active states.

Is Stephen Hawking's evaporation of black holes wrong? - Black Hole as a new candidate for dark matter -
2025-04-23 15:30 - 16:30
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Apr. 23 Wed
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Campus: Mitaka
Seminar: NAOJ Science Colloquium
Date and time: 2025 April 23 (Wed.), 15:30-16:30 JST
Place: The insei seminar room of Subaru Building / Zoom (hybrid)
Speaker: Kazunori Kohri
Affiliation: DoS, NAOJ
Title: Is Stephen Hawking’s evaporation of black holes wrong? – Black Hole as a new candidate for dark matter –
Abstract: Last year, Prof. Gia Dvali, an expert in quantum gravity theory, criticized Stephen Hawking’s proposal for calculating the evaporation of black holes due to Hawking radiation, insisting that the initial conditions assumed were not appropriate, and if he added the back reaction (the Memory Burden Effect) from the entropy of Hawking radiation emitted by the black hole itself to the calculation, the lifetime of the black hole should be extended even further. Researchers around the world were surprised. My group (Thoss, Burkert and Kohri, MNRAS, 2024, arXiv:2402.17823) was quick to incorporate this correction and pointed out that primordial black holes that have existed since the early Universe may have remained and become dark matter without evaporating. I will explain this topic, which is still the subject of heated debate, in an easy-to-understand way for non-experts in this seminar. https://arxiv.org/abs/2402.17823
