Spectral Evolution Magnetar Outburst

Since the launch of the Imaging X-ray Polarimetry Explorer (IXPE), X-ray polarization studies have become practical. Magnetars exhibit strongest magnetic fields in the Universe, making them promising targets for polarization measurements. In 2024, the bright magnetar 1E 1841-045 went into outburst, and we carried out the first IXPE observation of a magnetar during an outburst.

Our energy-resolved polarization analysis shows the broadband polarization degree rising from ~20% at 2 keV to ~70% at 8 keV, with a nearly constant polarization angle aligned. Component‐resolved fits give ~30% polarization for the soft power-law component and ~40-60% for the hard tail. We observe only marginal phase variations, peaking at the hard-X pulse maximum. Comparisons with theoretical models indicate that a synchrotron cascade caused by resonant inverse Compton photons naturally reproduces both the hard spectrum and high polarization, while the soft component matches expectations for Comptonized coronal emission in the inner magnetosphere. These findings demonstrate that X-ray polarimetry uniquely probes magnetar outbursts, directly revealing magnetic field geometry and high energy particle populations beyond the spectroscopy or timing alone.

Paper: https://ui.adsabs.harvard.edu/abs/2025ApJ...985L..35S/abstract 

NASA News: https://www.nasa.gov/missions/ixpe/nasas-ixpe-obtains-first-x-ray-polarization-measurement-of-magnetar-outburst/ 

Sci.News: https://www.sci.news/astronomy/ixpe-x-ray-polarization-magnetar-outburst-13968.html 

phys.org: https://phys.org/news/2025-06-ixpe-ray-polarization-magnetar-outburst.html 

X-ray Polarization of a Magnetar Outburst

Since the launch of the Imaging X-ray Polarimetry Explorer (IXPE), X-ray polarization studies have become practical. Magnetars exhibit strongest magnetic fields in the Universe, making them promising targets for polarization measurements. In 2024, the bright magnetar 1E 1841-045 went into outburst, and we carried out the first IXPE observation of a magnetar during an outburst.

Our energy-resolved polarization analysis shows the broadband polarization degree rising from ~20% at 2 keV to ~70% at 8 keV, with a nearly constant polarization angle aligned. Component‐resolved fits give ~30% polarization for the soft power-law component and ~40-60% for the hard tail. We observe only marginal phase variations, peaking at the hard-X pulse maximum. Comparisons with theoretical models indicate that a synchrotron cascade caused by resonant inverse Compton photons naturally reproduces both the hard spectrum and high polarization, while the soft component matches expectations for Comptonized coronal emission in the inner magnetosphere. These findings demonstrate that X-ray polarimetry uniquely probes magnetar outbursts, directly revealing magnetic field geometry and high energy particle populations beyond the spectroscopy or timing alone.

Paper: https://ui.adsabs.harvard.edu/abs/2025ApJ...985L..35S/abstract 

NASA News: https://www.nasa.gov/missions/ixpe/nasas-ixpe-obtains-first-x-ray-polarization-measurement-of-magnetar-outburst/ 

Sci.News: https://www.sci.news/astronomy/ixpe-x-ray-polarization-magnetar-outburst-13968.html 

phys.org: https://phys.org/news/2025-06-ixpe-ray-polarization-magnetar-outburst.html 

Double Glitches Bracketing a Fast Radio Burst

Understanding the origins and applications of fast radio bursts (FRBs) has become one of the most rapidly evolving fields in astrophysics over the past decade. FRBs are short-duration (sub-second), broadband, and highly dispersed radio pulses, primarily emitted from distant galaxies. Neutron stars and magnetars are considered among the most promising candidates for FRB origins.

The first direct link between FRBs and magnetars was established in 2020 when an FRB was detected from the outbursting magnetar SGR 1935+2154. Broadband analysis indicates that the X-ray burst associated with this FRB had a significantly different spectrum compared to other bursts detected half a day earlier during a burst storm. The process by which FRBs are generated remains unclear.

In our study, we observed two spin-up glitches occurring within a 9-hour interval, bracketing the FRB emitted from SGR 1935+2154 during its 2022 outburst. This observation suggests the possible energy sources for the observed X-ray activities and the FRB itself. Additionally, we witnessed a burst storm, a mini outburst, and a substantial flare approximately two hours before the FRB, indicating that significant amounts of energy and angular momentum were released from the magnetar. These phenomena could explain the rapid spin-down observed between the two glitches. Notably, a spectral softening during these events hints at a change in the magnetospheric environment, suggesting that these X-ray activities might clear the surroundings, and hence enables coherent radio emissions to escape from the magnetar’s magnetosphere.

Paper:  https://www.nature.com/articles/s41586-023-07012-5?fbclid=IwAR1u9uDjWiY32Myeg7u5YjGTUC0Fa3eG-fH-0YXUup7yDLZkK8kfQfWoCmE 

彰師大校園頭條: https://www.ncue.edu.tw/p/406-1000-25018,r93.php?Lang=zh-tw&fbclid=IwAR05K5oUT4K0d6oWZt8UtARS9WOe5OVoQhsK7xIOso9V-kRjEPbYGcj3_1A 

台北天文館: https://tam.gov.taipei/News_Content.aspx?n=EF86D8AF23B9A85B&sms=F32C4FF0AC5C2801&s=8DB31ADE02DFA03E 

科技新報: https://technews.tw/2024/02/21/the-correlation-between-magnetars-and-frbs/ 

NASA/JPL News: https://www.jpl.nasa.gov/news/nasa-telescopes-find-new-clues-about-mysterious-deep-space-signals 

Kyoto U News: https://www.kyoto-u.ac.jp/ja/research-news/2024-02-15?fbclid=IwAR1r8TuPEPdgSGsezXlHUUrk-Kc_TTFRUw0FCb0Cznn5rCuNHgALEYXoga8 

Sky & Telescope: https://skyandtelescope.org/astronomy-news/neutron-star-glitches-key-mysterious-radio-bursts/ 

Universe Today: https://www.universetoday.com/165744/another-clue-into-the-true-nature-of-fast-radio-bursts/ 

Space.com: https://www.space.com/magnetic-dead-star-glitches-fast-radio-bursts-neutron-star 

phys.org https://phys.org/news/2024-02-clue-true-nature-fast-radio.html 

朝日新聞: https://www.asahi.com/articles/ASS2G339FS2FPLBJ001.html 

日本經濟新聞: https://www.nikkei.com/article/DGXZQOUC111K10R10C24A2000000/ 

High Energy Astrophysics Picture of the Week: https://heasarc.gsfc.nasa.gov/docs/objects/heapow/archive/compact_objects/frb_nustar.html 

SMC X-1: An Unexpected Excursion on Superorbital Period accompanies with Spin-Up Acceleration

The superorbital modulation period of SMC X-1 has been found to be unstable, though previous observations suggesting a timescale of approximately 3200 days for the occurrence of "excursion" events. With continuous MAXI and Swift BAT observations, a new excursion event was detected to have occurred during 2020-2021, approximately 1600 days after the previous excursion in 2015-2016. Furthermore, the spin-up rate increased significantly one year prior to this event, indicating that the triggering mechanism of the superorbital excursion may be an inside-out process. However, this connection was not observed in the previous excursion, and it remains unclear whether this is simply a coincidence or if there exists a threshold for showing a connection between these two phenomena.

Paper: https://ui.adsabs.harvard.edu/abs/2023MNRAS.520.3436H/abstract 

Development of Stacking HHT Algorithm and Its Application to Gravitational Wave Data

The detection of the gravitational wave signal denotes the new era of multi-messenger astronomy. Precisely detecting the time-frequency property is important. Based on the Hilbert-Huang transform (HHT), we developed a stacking algorithm and systematically apply it to known gravitational wave events and simulated core-collapse supernovae (CCSNe) events, which will be one of the most important targets in the next generation of gravitational wave observatories. In known binary black hole coalescence cases, the time-frequency maps show much better details compared to those wavelet spectra. Moreover, the oscillation in the instantaneous frequency caused by mode-mixing could be reduced. In CCSNe events, the initial stage of different modes of oscillations can be clearly separated. 

Paper: https://ui.adsabs.harvard.edu/abs/2022arXiv220706714H/abstract 

A Comprehensive Study of Pulsar Wind Nebulae

A pulsar wind nebulae (PWN) is formed owing to the pulsar wind from a young pulsar interacting with surrounding materials. PWNe are natural laboratories to test the particle acceleration in the Universe to the ultra-relativistic regime.  The standard model of particle transfer was based on the best-studied PWN, the Crab nebula. However, recent studies of a few PWNe like 3C 58 and G21.5-0.9, suggest that diffusion plays a role more important than the advection. We carried out a comprehensive study of a large sample of young PWNe and confirm that diffusion could explain the photon-index profile and the surface brightness profile of most PWNe. A few PWNe show discrepancies between the photon-index-fit and the surface-brightness-fit, indicating that further modeling is needed. 

Paper: https://ui.adsabs.harvard.edu/abs/2022ApJ...927...87H/abstract 

Outburst of SGR 1830-0645 and Its Peak Migration

The soft gamma-ray repeater SGR 1830-0645 was discovered on 2020 October. We use NICER to monitor it for eight months and derive an accurate ephemeris of freq = 0.096 Hz and a frequency derivative of  -6.2E-14 Hz s^{-1}. During the monitoring, SGR 1830-0648 emerged 84 short bursts that have a strong preference for occurring close to the surface-emission pulse maximum. This likely implies a very low altitude for the burst emission region and a triggering mechanism connected to the surface-active zone. Moreover, we detected a significant migration of pulse peaks during the outburst tail. Two possible scenarios, the tectonic motion of the crust and the untwisting of the magnetosphere were proposed. 

Paper1: https://ui.adsabs.harvard.edu/abs/2022ApJ...924..136Y/ 

Paper2: https://ui.adsabs.harvard.edu/abs/2022ApJ...924L..27Y/  

NICER Monitoring of Swift J1555.2-5402

The soft gamma-ray repeater Swift J1555.2-5402 was discovered with a short burst detected with the Swift BAT on 2021 June 3. We use NICER to monitor it for a month and detected its 3.86-s periodicity, and the period derivative is measured to be 3.05E-11 ss^{-1}. The equatorial surface magnetic field, characteristic age, and spin-down luminosity are derived under the dipole field approximation to be 3.5E14 G, 2.0 kyr, and 2.1E34 erg s^{-1},  respectively. Swift J1555.2-5402 is classified as a new magnetar and no radio emission is observed. A short burst is simultaneously detected with Swift BAT and NICER, but the spectral property of the burst is barely constrained.

Paper: https://ui.adsabs.harvard.edu/abs/2021ApJ...920L...4E/ 

Crab Pulsar: Discovery of the X-ray enhancement coincide with giant radio pulses

The Crab pulsar is one of the most famous pulsars. It was born as a supernova explosion in 1054. Excepted for the normal, periodic pulses, the Crab pulsar occasionally emits giant radio pulses (GRPs), which are extremely short, millisecond-duration pulses in the radio band. The mechanism of GRPs remains controversial but they could provide insights into the mysterious phenomenon of fast radio bursts (FRBs). The enhancement of the optical pulses during the GRPs has been found in 2003. However, the X-ray enhancement has not yet been observed for ~20 years. We performed a series of simultaneous multi-wavelength observations with the NICER X-ray telescope and Usuda, Kashima radio telescopes, and found an X-ray enhancement of 3.8 % with 5 sigma detection significance. This discovery implies that these giant pulses are hundreds of times more energetic than previously thought.  This could be interpreted by the plasmoid model although future theoretical modeling would be needed. Moreover, this observation provides important hints for future multi-wavelength observations to reveal the relationship between GRPs and FRBs.

Paper: https://science.sciencemag.org/content/372/6538/187 

NASA Press release: https://www.nasa.gov/feature/goddard/2021/nasa-s-nicer-finds-x-ray-boosts-in-the-crab-pulsar-s-radio-bursts 

RIKEN press release: https://www.riken.jp/press/2021/20210409_1/ 

NCUE press: https://www.ncue.edu.tw/p/406-1000-4235,r93.php?Lang=zh-tw

M51 ULX-7: A Possible Dipping Pulsating ULX

M51 ULX-7 has been confirmed to exhibit a neutron star with a spin period of 2.8 s and an orbital period of 2 days. Utilizing Chandra light curve observed in 2012, we observed three dips that possibly occurred with a period of 2 days. If this phenomenon, which is the first time observed in a PULX system, is caused by the vertical structure at the outer accretion disk, it could provide a constrain of the inclination angle of ~60 degree. Unfortunately, we could not observe the change in the hardness ratio, possibly due to limited statistic. Future observations would be needed to test the stability and the spectral behavior of the dips and explore the physical mechanisms.

Paper: https://ui.adsabs.harvard.edu/abs/2021ApJ...909....5H/ 

Burst Storm of SGR 1935+2154

SGR 1935+2154 is a young magnetar that outbursts repeatedly.  During the NICER observation on 2020 April 27, it emitted hundreds of X-ray bursts within a few hours. Later on the same day, SGR 1935+2154 emitted another X-ray burst that coincide with the first Galactic fast radio burst (FRB). We found that all 24 NICER and GBM bursts are very similar temporally to the FRB-related burst, but have strikingly different spectral behavior. The FRB-relatd burst is perhaps indicative of an uncommon locale for its origin, which is possibly in quasi-polar open or closed magnetic field lines that extend to high altitudes.

Paper: https://www.nature.com/articles/s41550-020-01292-x 

NASA JPL Press release: https://www.jpl.nasa.gov/news/nasa-missions-help-pinpoint-the-source-of-a-unique-x-ray-radio-burst 

NICER Science Nugget: https://heasarc.gsfc.nasa.gov/docs/nicer/science_nuggets/20200507.html 

Swift J1818.0-1607: A Missing Link Between High-B-Field Rotation-Powered Pulsar and Magnetars

On March 12, 2020, the Neil Gehrels Swift observatory detected a new soft Gamma-ray repeater, Swift J1818.0-1607. We performed a prompt observation and detected a 1.36 s coherent pulsating signal from Swift J1818.0-1607. The follow-up observations reveal a long-term spin-down rate of  nu_dot=-2.48E-11 s^-2, corresponding to a strong magnetic field of  2.5E14 G and a young characteristic age of 470 years. Two glitches and a highly variable spin-down rate supports the young nature of this object. This source is close to the magnetar group on the P-P_dot diagram, but the presence of the radio emission and  the large spin-down luminosity fit the characteristic of high-B field rotation-powered pulsars. We suggest that Swift J1818.0-1607 is a link between these two extreme neutron star populations. 

Related Press Release: NICER Science Nugget, RIKEN News. 

Spin and Superorbital Evolution of SMC X-1

SMC X-1 is an accreting X-ray pulsar with a peak luminosity close to 10^39 erg/s. Its X-ray flux has a high-low state transition with a timescale of 40 - 60 days. This variability is named the superorbital modulation because its period is longer than the orbital period of this system (3.9 days). In this research, we find that the "superorbital excursion event," of which the superorbital modulation period evolves to 40 days, is recurrently and likely occurs every ~3150 days. Moreover, we track the spin period evolution with the help of MAXI. We also find that the spin frequency residual possibly anti-correlates with the superorbital frequency during the third excursion event (~MJD 57000).  The spin period evolution of SMC X-1 is archived here.

4U 0114+650: An X-ray Pulsar Switching between Disk-Wind and Direct-Wind Accretion

The evolution of the spin period in an accreting pulsar is strongly affected by the accretion form and the strength of the pulsar's magnetic field. I investigated the spin period evolution of 4U 0114+650 and found that this system likely switched between the direct-wind accretion and the disk-wind accretion phases. I also found that the orbital profile and the superorbital modulation amplitude varied with time. These changes have some connection to the spin period evolution, implying the formation of the accretion disk during specific epochs. 

Magnetars' Surface Temperature Anisotropy as the Effect of Toroidal Fields

Magnetars are extraordinary pulsars with long spin periods, high thermal luminosities, strong magnetic fields, and bursting behaviors. Recently, the boundary between magnetars and rotation-powered pulsars (RPPs) is blurred after the discovery of low magnetic field magnetars and magnetar-like activities of two RPPs. The evolutionary link between magnetars, RPPs, and other subtypes of neutron stars is still under development.

The magneto-thermal evolution model suggests that the magnetic energy is transferred to heat through the dissipation process in the crust. This model explains the systematically high surface temperature of magnetars, and unify the evolutionary pattern of all NS populations. To test if all the magnetars have strong toroidal fields that will destroy the symmetry of the surface temperature profile, I perform a systematic analysis on the soft X-ray pulse profiles of magnetars and compare them with the theoretical ones with different size of hotspots, viewing geometries, and beaming functions. 

White Dwarf Pulsar Observed in X-ray Band

Similar to X-ray binaries, white dwarfs can drain materials from their companions and emit X-rays. AR Sco is such a system in which the accretion disk is truncated by the magnetic field of the white dwarf. It is an unique intermediate polar system showing non-thermal radio pulsation. Through the collaboration with Prof. Jumpri Takata, we found it X-ray pulsation. Utilizing the dynamic power spectrum, I found that the signal is intermittently determined and the pulse shape varies with respect to the orbital phase. We suggest the relativistic electrons accelerated by the magnetic dissipation process on the companion star surface are trapped in the WD’s closed magnetic field lines. The observed pulsation is the emission from the first magnetic mirror point.