Research column

Fig: (left) The mHz QPO dynamic power spectra of 4U 1636-53 from the data collected by XMM-Newton in 2008. (right) The Hilbert spectra from the same data analyzed by HHT whereas the contours are the dynamic power spectra. It is evidently that the Hilbert spectra can provide better precision in time-frequency analysis.

PhD student Hung-En Hsieh: Some of neutron star low mass X-ray binaries (LMXBs) exhibit quasi-periodic oscillation (QPO) of time scale ~100 seconds, called mHz QPO. It is believed caused by the meta-stable thermonuclear reaction on the surface of neutron star. To further understand the nature of this phenomenon. We processed the time-frequency analysis, using Hilbert-Huang transfer (HHT), to the data collected by XMM-Newton for the LMXBs 4U 1636-53. With the precise oscillation phase obtained by HHT, the phase-resolved spectra were constructed. Different from the results of previous studies, among four observations with significant QPO detections, we found that the oscillation is mainly attributed to variations in the area emitting blackbody radiation in three of them. However, the other one shows a variation of temperature with a constant emitting area. Although the cause of the difference is not clear, it might be related to the spectral state of the source that can be observed from a hard color difference in the color–color diagram. This work has been published in Hsieh & Chou Astrophysical Journal (2020), 990, 116

Fig: The pulse phase evolution of accreting millisecond pulsar IGR J17591-234 under the assumption of constant pulsation frequency. The best fitted quadratic curve (the red dashed line) indicates that the spin frequency derivative of the neutron star is (-6±1)x10^-14 Hz/s. According to the accretion torque theory, the surface magnetic field of this neutron star is estimated as about 4x10⁸ Gauss.

Prof. Yi Chou and undergraduate student of department of physics Kaho Tse: Using the data collected by NICER, an X-ray telescope that started to be operated in mid of 2017, for the newly discovered accreting millisecond X-ray pulsar (AMXP) IGR J17591-2342 during its outburst from August to October 2018, we investigated the binary and neutron star properties of the system. After considering the time noise induced by the accretion rate, in addition to obtaining the precise orbital parameters, we found that the neutron star is spun-down in a rate of (-6±1)x10^-14 Hz/s. The magnetic field is then estimated as about 4x10⁸ Gauss by using the accretion torque theory. Besides, by analyzing the energy-dependent pulse profile properties, we found that the pulsed fractional amplitude increases from 1 to 5 keV and then decreases for higher energy bands. Furthermore, the soft phase lag phenomenon that is usually seen in other AMXPs is also observed from ∼4 to 12 keV with a value of 1.44μs. With the supporting evidences form the X-ray spectrum detected by NICER , we concluded that these phenomena can be explained by the two- component model with a relatively strong blackbody component and an additional unpulsed disk blackbody component. These results have been published Tse, Chou & Hsieh, Astrophysical Journal (2020), 899, 120.

Fig: The Asteroid Rotation Period Survey Using the CNEOST

Tingshuo Yeh: With the collaboration between NCUIA and Purple Mountain Observatory (PMO), we collected asteroid lightcurves in early 2017 and early 2018 by the Chinese Near-Earth Object Survey Telescope (CNEOST). In total 4522 lightcurves, we found 506 reliable rotation periods, most of them are main-belt asteroids. Among the reliable rotation periods, we found 16 candidates with a possible rotation period of <~2 hr. This kind of asteroids are special because they should be destroyed by the centrifugal force introduced by their high rotation speed, but they still exist, the reason why they survive is the problem we would like to solve. This result is published in Yeh et al. (including Chan-Kao Chang, Zhong-Yi Lin and Wing-Huen Ip) 2020, AJ, 160, 73.

Prof. Ngeow: Kundu et al (2019) identified 5 extra-tidal RR Lyrae that could be associated with globular cluster NGC5024, shown as red squares in the above picture. However, 4 of them actually belong to a nearby globular cluster NGC5053, where the known RR Lyrae in this globular cluster is marked with green crosses. Using Gaia and ZTF data, we further searched for possible extra-tidal RR Lyrae that could be associated with both of the globular clusters, and we found none. This work is published in Ngeow et al (2020) AJ 160:31.

Dr. Chien-De Lee: HO Pup is an new discovered dwarf nova by ZTF and NCU. Dwarf nova is a mass transfer binary including a mass donor star and a white dwarf surrounded by the accretion disk. In 2019, we observed the variation of Hα line from 3/15-3/22 with CFHT/EsPADOnS spectroscopy. The brightening part of the outburst was fully observed at these eight nights. When the outer disk accretes enough mass, here coms the photometric brightening. Before the luminosity maximum, the absorption part of Hα line was obviously shown in the spectra of HO Pup. Mass transfer rate of accretion play a major role behind this photometric and spectroscopic variability. (Including Prof. C.-C. Ngeow and Ph.D student J.-Y. Ou)

Figure. The flare frequency distribution of different energy release levels of M-type stars.

PhD student Li-Ching Huang: The longevity and the abundance of M-type stars in our Galaxy, provide a good condition of life development. The space weather is also important to the habitability of the exoplanets. Stars with strong chromospheric activity is a hazard to the possible life. Therefore, we studied the flare activities on late- type stars. We use the photometric data from Kepler and the spectral data from LAMOST and YNAO 2.4-m telescope to study the flare activities of the 12 M-type eclipsing binaries. The lightcurve data show that the flare frequency of the binary systems (red line in the figure) is similar with the stars with hyperflares (brown line), but the high energy flare events are less frequent. Compare with the single stars (violet line), the binary systems show a 10 times flare frequency. This result is published in Huang, et al., 2020, ApJ, 892, 1 (including Prof. Wing-Huen Ip, and Chia-Lung Lin).

陳文屏教授：利用 Gaia 太空望遠鏡的精確恆星距離與運動數據，研究距離我們約750光年，年齡約1億年的年輕星團 Blanco 1，一共指認出644顆成員星，包括488顆位於星團核心，表示此星團目前仍處於動力穩定狀態，但已經顯示質量分層效應，也就是大質量成員星集中在中央區域。我們首度發現 Blanco 1 有潮汐尾，分別在星團核心（33光年）兩端，延伸長達180光年，亦顯示星團正開始瓦解。右圖展示星團成員的質量函數，在0.2～0.3太陽質量達到最高。垂直線標示數據的完整極限，目前尚未涵蓋棕矮星 （< 0.1太陽質量）。這項研究成果發表在 Zhang et al. Astrophysical Journal (2020), 889, 99

Fig: Radial Acceleration Relation in CLASH Galaxy Clusters

Dr. Yong Tian: For the first time, we reveal a tight radial acceleration relation (RAR) in 20 CLASH galaxy clusters. In this figure, the axes are from two independent measurements: the total acceleration, g_tot, by weak-lensing and strong-lensing; the baryonic acceleration, g_bar, by X-ray gas, stellar mass, and the brightest cluster galaxy (BCG). The resulting RAR is described by g_tot=(g_bar g_‡)^1/2 with g_‡=(2.02±0.11)×10⁻⁹ ms⁻². The slope is consistent with the low acceleration limit of the RAR in galaxies, whereas the intercept implies a much larger acceleration scale. Tian et al. (Including Chung-Ming Ko), 2020, ApJ, 896, 70

Fig. Asteroid Discovery and Light Curve Extraction Using the Hough Transform: A Rotation Period Study for Subkilometer Main-belt Asteroids

Dr. Chan-Kao Chang: Using the Hough transform, a line detecting algorithm, more than 3000 new asteroids were discovered from a high-cadence observation of the Pan-STARRS 1 (PS1). This observation was originally planned to collected asteroid lightcurves for rotation period measurement and conducted in October 2016. Because of the high cadence observation (i.e., images were continuously taken one after one for the entire night), the detections of an asteroid would line up as a straight line and, therefore, these line-up detections can be used to identify asteroids.

With the collected lightcurves of these new asteroids, 122 reliable rotation periods were obtained and 13 of them are super-fast rotators (i.e., asteroids in the size range of rubble-pile structure, a weak aggregate of large and small components held together by gravity rather than material strength, have rotation periods < 2 hr). This kind of asteroids is not expected because they suppose to be destroyed due to the exceedingly centrifugal force introduced by a rotation period of < 2hr. How these super-fast rotators can survive is still a mystery. (Lo, Kai-Jie, et al. (including Chan-Kao Chang; Wing-Huen Ip; Wen-Ping Chen) 2020, AJ, 159, 25L)