arXiv subjects

Wataru Iwakiri

Publications and source records attributed to Wataru Iwakiri.

At least 19 recordsLinked to original sources

arXiv2026

X-ray and Hα superflare on an RS CVn-type star, UX Arietis: Constraint on the flare location from radial velocity change during the flare

We report on a giant stellar flare from the RS CVn-type binary UX Arietis, detected with the Monitor of All-sky X-ray Image (MAXI) and followed by a 12-day optical spectroscopic campaign using the 3.8~m Seimei Telescope. The flare released $5 \times 10^{37}$~erg in X-rays (0.1--100~keV) and $(2$--$6) \times 10^{36}$~erg in the H$α$ line, placing it among the most energetic events of its kind. The H$α$ light curve showed sinusoidal modulation atop an exponential decay, consistent with reappearance of the flaring region due to binary rotation. At orbital phase 0, when the primary star is farthest from the observer, 40\% of the H$α$ flux was obscured, while at phase 0.5 the full emission was visible. This suggests the H$α$ emitting region is located at a relatively low latitude and is comparable in size to the stellar disk. Radial velocity modulation implies that the region lies at $\sim19\,R_{\odot}$ from the system's rotation axis, farther out than the stellar limb at $14.4\,R_{\odot}$. Photometric monitoring with the Chuo-university Astronomical Telescope revealed a large low-latitude starspot covering $\sim25\%$ of the surface. These findings are consistent with a scenario in which the flare occurred above the starspot, and the H$α$-emitting plasma was magnetically confined in a loop extending at least $5\,R_{\odot}$ above the stellar surface. From the MAXI data and assuming a radiatively cooling plasma, the electron density and volume are estimated to be $10^{10}$~cm$^{-3}$ and $1 \times 10^{35}$~cm$^3$, respectively. If cubic in shape, this corresponds to $7\,R_{\odot}$, consistent with the H$α$ region height. These results provide direct constraints on the geometry of the plasma and its spatial relationship with the starspot in one of the most energetic stellar flares ever observed.

astro-ph.SR↗

arXiv2026

Magnetic-field Order in the Southwestern Rim of RCW 86 Constrained Using X-Ray Polarimetry

RCW 86 is a supernova remnant whose origin has recently been linked to an off-center explosion within a cavity created by its progenitor star. In the southwestern region, the forward shock is thought to have reached the cavity wall, encountering diverse environmental conditions. We report on the spatially resolved X-ray polarimetric observation of RCW 86 with the Imaging X-ray Polarimetry Explorer (IXPE). In the 2--4.5 keV energy band we find no significant detection of polarization. Employing a dedicated background subtraction procedure and Bayesian spectropolarimetric fitting, we derive 99% upper limits on the polarization degree of the synchrotron component: 15% in higher-statistics regions and 30%--40% in lower-statistics regions. These upper limits on the polarization degree in several regions exclude the possibility of a strongly coherent magnetic field down to the subparsec scale, and that of a moderately coherent one on the scale of the synchrotron features as resolved by IXPE. The results indicate that the shocks in the southwestern rim of RCW 86 propagate more slowly than the unshocked ejecta at their locations, yet exceed the measured proper motion speeds. This behavior is consistent with reflected shocks occurring in tenuous regions of the shocked ejecta, distinct from regions that are radio-bright.

astro-ph.HE↗

arXiv2026

In-orbit Demonstration of X-ray Pulsar Navigation with NinjaSat

This study demonstrated the pulsar navigation capability of the CubeSat X-ray observatory NinjaSat, which is equipped with two Gas Multiplier Counters (GMCs). The GMCs are sensitive to the 2-50 keV energy band and have an effective area of 16 cm^2 per module at 6 keV. We verified the timing accuracy by observing the Crab Pulsar and confirmed stable timing performance within 100 microseconds. To demonstrate pulsar navigation, we applied a method that optimizes orbital parameters to maximize the significance of the pulsar X-ray pulse profile, known as the Significance Enhancement of Pulse-profile with Orbit-dynamics (SEPO) method. We observed the Crab Pulsar with a total exposure of approximately 100 ks at different epochs and analyzed the data transmitted to the ground. By comparing the optimized orbit with the satellite position derived from Global Positioning System data, we quantitatively evaluated the navigation performance. The results show that the position component along the Crab line of sight was consistently constrained within approximately 40 km, and the three-dimensional position error ranged from 27 to 370 km depending on the observation epoch. These results demonstrate the feasibility of applying a CubeSat-class X-ray observatory to pulsar navigation and provide the first experimental verification that the accuracy of the SEPO method depends on the seasonal geometry between the orbital plane and the pulsar direction.

astro-ph.IM↗

arXiv2026

Polarization of reflected X-ray emission from the Sgr A molecular complex: multiple flares, multiple sources?

The extended X-ray emission observed in the direction of several molecular clouds in the central molecular zone of our Galaxy exhibits spectral and temporal properties consistent with the X-ray echo scenario. This concept postulates that the observed signal is a light-travel-time delayed reflection of a short ($δt<$1.5 yr) and bright ($L_{\rm X}>10^{39} {\rm erg s^{-1}}$) flare that was most probably produced a few hundred years ago by Sgr A*. This scenario predicts a distinct polarization signature for the reflected X-ray continuum, with the polarization vector being perpendicular to the direction toward the primary source and the polarization degree being determined by the scattering angle. We report the results of two deep observations of the currently brightest (in reflected emission) molecular complex Sgr A taken with the Imaging X-ray Polarimetry Explorer in 2022 and 2023. We confirm the previous polarization measurement for a large region encompassing the Sgr A complex with high significance. We reveal an inconsistent polarization pattern for the brightest reflection region in its center. The X-ray polarization from this region is almost perpendicular to the expected direction in the case of Sgr A* illumination and shows smaller degree of polarization compared to the large region. This could indicate the simultaneous propagation of several illumination fronts throughout the CMZ, with the origin of one of them not being Sgr A*. The primary source could be associated with the Arches stellar cluster or a currently unknown source located closer to the illuminated cloud, potentially lowering the required luminosity of the primary source. Although significantly deeper observations with IXPE would be required to unequivocally distinguish between the scenarios, a combination of high-resolution imaging and micro-calorimetric spectroscopy offers an additional promising path forward.

astro-ph.HE↗

arXiv2025

Super-Eddington Accretion Geometry: a Remarkable Stability of the Hidden Ultraluminous X-Ray Source Cygnus X-3

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of $ 21.2 \pm 0.4 \% $ and polarization angle of $ 92.2 \pm 0.5^\circ $, well compatible with the first hard-state IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, which we attribute to reflection from the optically thick envelope surrounding the central source, our result indicates that both are very stable on year-long timescale. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel.

astro-ph.HE↗

arXiv2025

Unveiling the Nature of Superorbital Modulation of SMC X-1 using NinjaSat

We report a long-term, high-cadence timing and spectral observation of the X-ray pulsar SMC X-1 using NinjaSat, a 6U CubeSat in low-Earth orbit, covering nearly a full superorbital cycle. SMC X-1 is a high-mass X-ray binary exhibiting a 0.7 s X-ray pulsar and a non-stationary superorbital modulation with periods ranging from approximately 40 to 65 days. Its peak luminosity of $1.3\times10^{39}$~\lumcgs\ makes it a local analogue of ultraluminous X-ray pulsars powered by supercritical accretion. We find that the spin-up rate during the high state remains consistent with the long-term average, with no significant correlation between spin-up rate and flux. This result indicates that the modulation is primarily geometric rather than accretion-driven. The hardness ratio and spectral shape are stable throughout the entire superorbital cycle, supporting obscuration by optically thick material or energy-independent scattering. In addition, the 2--20 keV pulse profile varies with superorbital phase, which may be explained either by variable covering fraction due to geometric obscuration, or by free precession of the neutron star. This represents the first complete measurement of spin-up rate and spectral evolution across a single superorbital cycle in SMC X-1, highlighting the scientific capability of CubeSat-based observatories.

astro-ph.HE↗

arXiv2025

Return of the Clocked Burster: Exceptionally Short Recurrence Time in GS 1826-238

We report the discovery of an exceptionally short burst recurrence time in the well-known clocked burster GS 1826$-$238, observed with the CubeSat X-ray observatory NinjaSat. In 2025 May, GS 1826$-$238 underwent a soft-to-hard state transition for the first time in 10 years. On June 23, NinjaSat began monitoring GS 1826$-$238 in the hard state and continued until it returned to a steady soft state. During this period, we detected 19 X-ray bursts: 14 during the hard state, 4 in the transitional state, and 1 in the soft state. In the hard state, we identified a new clocked bursting epoch, during which the burst recurrence time remained highly stable and unprecedentedly short among the clocked bursting phases of GS 1826$-$238, with $t_{\rm rec} = 1.603 \pm 0.040$ hr ($1σ$ error). Previous observations showed that the burst recurrence time in GS 1826$-$238 decreased with increasing mass accretion rate, reached its minimum value of $t_{\rm rec} \sim 3$ hr, and then increased again. The observed 1.6 hr recurrence time is therefore exceptionally short, indicating anomalous ignition conditions. We propose that this phenomenon reflects fuel accumulation over a smaller fraction of the neutron star surface, resulting in a higher local accretion rate compared to earlier epochs. This scenario naturally accounts for the exceptionally short recurrence time, together with the observed reductions during bursts in blackbody normalization (proportional to the emitting area) and fluence. We also discuss possible contributions from residual heat in the neutron star crust or the presence of an additional soft spectral component.

astro-ph.HE↗

arXiv2025

Multiband Optical Photometric and Spectroscopic Monitoring of the 2024 Flare Event in Transition Blazar OP313

Blazars are active galactic nuclei known for their extreme variability, offering unique opportunities to study jet physics and high-energy emission mechanisms. In 2024, the Flat Spectrum Radio Quasar (FSRQ) OP313 underwent a remarkable flare event, during which the gamma-ray flux observed by the Fermi Large Area Telescope (Fermi/LAT) increased by a factor of 60 over its average value. The flare peak lasted less than two days. Using optical telescopes, we conducted 100-day time-scale observations. Multi-wavelength data revealed that OP313 entered an active state 50 days prior to the flare and remained active for at least 50 days afterward. We propose that this prolonged activity results from variations in electron density within the shock front due to changes in the accretion rate. Concurrently, OP313's spectrum transitioned from an FSRQ-like state to a BL Lac-like state, characterized by a significant increase in the synchrotron peak frequency and the disappearance of broad-line region emission lines. In the post-flare phase, we observed a decoupling between synchrotron radiation and inverse Compton scattering, along with a possible decrease in the magnetic field strength within the shock front.

astro-ph.HE↗

arXiv2025

The 2020 Superburst of 4U 1608-522 and its impact on the accretion disk

Superbursts are rare events observed from bursting neutron star low mass X-ray binaries. They are thought to originate from unstable burning of the thick layer of Carbon on the surface of the neutron star, causing the observed X-ray flashes to last several hours. Given their fluence it has long been thought that superbursts may have significant effects on the accretion flow around the neutron star. In this paper, we first present evidence for a new superburst observed from 4U 1608-522 by MAXI during the 2020 outburst, around 00:45 UTC on 16 July 2020. We compare some of the properties of this superburst and the underlying outburst with the events recorded on May 5 2005 by RXTE and most recently in 2025 by MAXI. We then present our spectral analysis of NICER and Insight-HXMT data obtained before and after the 2020 superburst event. Our results indicate that the inner disk temperature and the radius show a systematic evolution in the following few days, which may be related to the superburst. We show that the timescale of the observed evolution can not be governed by viscous timescales unless the viscosity parameter is unrealistically low.

astro-ph.HE↗

arXiv2025

Quasi-periodic variation during a fast X-ray outburst of a high-mass X-ray binary MAXI J0709-159 / LY CMa observed by NICER

We report on a quasi-periodic variation at $\sim1$ Hz during a fast X-ray outburst of a high-mass X-ray binary MAXI J0709$-$159 / LY CMa observed by the Neutron-star interior composition explorer (NICER). The new X-ray transient MAXI J0709$-$159 was discovered on 2022 January 25. Due to the transient X-ray behavior characterized by the short (a few hours) outburst duration, rapid ($\lesssim$ 1 s) variability with spectral change, and large luminosity swing from $10^{32}$ erg s$^{-1}$ to $10^{37}$ erg s$^{-1}$, the object was considered likely to be a supergiant X-ray binary with a neutron star (NS) categorized as a Supergiant Fast X-ray Transient (SFXT). Follow-up NICER and NuSTAR observations confirmed that the position of the new X-ray object is consistent with a Be star, LY CMa, which has been also identified as a B supergiant. We analyzed the NICER data obtained from 3 hours to 6 days after the discovery. The light curve reveals that the X-ray activity continued for $\sim7$ hours in sparse short flares, each lasting $\lesssim 100$ seconds, and the luminosity instantaneously reached up to $\sim 1\times 10^{38}$ erg s$^{-1}$. The light-curve and spectral features reasonably agree with those expected from accretion of a clumpy stellar-wind onto a magnetized NS. The variability power spectrum during the brightest flare shows a broad peak at $1.1$ Hz resembling a quasi-periodic oscillation (QPO). If the QPO is attributed to the Keplerian orbital frequency at the inner edge of a transient accretion disk truncated by the NS magnetosphere, the NS surface magnetic field is estimated to be $\sim 10^{12}$ G.

astro-ph.HE↗

arXiv2025

X-ray Polarization Detection of the Pulsar Wind Nebula in G21.5-0.9 with IXPE

We present the X-ray polarization observation of G21.5-0.9, a young Galactic supernova remnant (SNR), conducted with the Imaging X-ray Polarimetry Explorer (IXPE) in October 2023, with a total livetime of approximately 837 ks. Using different analysis methods, such as a space-integrated study of the entire region of the PWN and a space-resolved polarization map, we detect significant polarization from the pulsar wind nebula (PWN) at the center of the SNR, with an average polarization degree of ~10% oriented at ~33° (north through east). No significant energy-dependent variation in polarization is observed across the IXPE band (2-8 keV). The polarization map, corrected for the effect of polarization leakage, reveals a consistent pattern in both degree and angle, with little change across the nebula. Our findings indicate the presence of a highly polarized central torus, suggesting low levels of turbulence at particle acceleration sites. Unlike Vela, but similar to the Crab Nebula, we observe substantial differences between radio and X-ray polarization maps. This suggests a clear separation in energy of the emitting particle populations and hints at an important, yet poorly understood, role of instabilities in the turbulence dynamics of PWNe.

astro-ph.HE↗

arXiv2025

Thermonuclear superburst of MAXI J1752$-$457 observed with NinjaSat and MAXI

An uncatalogued bright X-ray transient was detected with MAXI on November 9, 2024, named MAXI J1752$-$457. The NinjaSat X-ray observatory promptly observed the source from November 10 to 18 while the small angular separation from the Sun hampered follow-up campaigns by other X-ray observatories. The MAXI and NinjaSat light curves in the 2-10 keV band showed first and second decaying components at the early and late phases, approximated by exponential functions with e-folding constants of 1.2 $\pm$ 0.2 and 14.9 $\pm$ 0.9 hours (1$σ$ errors), respectively. A single blackbody model reproduces the X-ray spectrum with a softening trend of its temperature decreasing from 1.8 $\pm$ 0.1 keV to 0.59 $\pm$ 0.06 keV. Assuming the unknown source distance at 8 kpc, at which the initial X-ray luminosity roughly corresponds to the Eddington limit, the shrinking blackbody radius was estimated at 5-11 km. This X-ray brightening is interpreted as a superburst in a Galactic low-mass X-ray binary, which is powered by thermonuclear burning triggered presumably by carbon ignition close to the outer crust of the neutron star. The transition between two decaying components occurred at 5.5-7.7 hours, corresponding to the thermal time scale of the burning layer. The ignition column density is estimated to be (2.8-5.1)$\times 10^{12}$ g cm$^{-2}$.

astro-ph.HE↗

arXiv2025

High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet

Blazars, supermassive black hole systems (SMBHs) with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio to gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10-18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer (IXPE) found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3sigma confidence level). We argue here that this observational result rules out a hadronic origin of the high energy emission, and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic "spring" embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine.

astro-ph.HE↗

arXiv2025

Evolution of the inner accretion flow and the white-dwarf spin pulse during the 2023 outburst in GK Persei

We present our X-ray and optical observations performed by NICER, NuSTAR, and Tomo-e Gozen during the 2023 outburst in the intermediate polar GK Persei. The X-ray spectrum consisted of three components: blackbody emission of several tens of eVs from the irradiated white-dwarf surface, a source possibly including several emission lines around 1 keV, and multi-temperature bremsstrahlung emission from the accretion column. The 351.3-s white-dwarf spin pulse was detected in X-rays, and the observable X-ray flux from the column drastically decreased at the off-pulse phase, which suggests that the absorption of the column by the accreting gas called the curtain was the major cause of the pulse. As the system became brighter in optical, the column became fainter, the pulse amplitude became higher, and the energy dependence of pulses became weaker at $<$8~keV. These phenomena could be explained by the column's more pronounced absorption by the denser curtain as mass accretion rates increased. The blackbody and line fluxes rapidly decreased at the optical decline, which suggests the expansion of the innermost disk edge with decreasing accretion rates. The electron scattering or the column geometry may be associated with almost no energy dependence of high-energy pulses. The irradiated vertically-thick structure at the disk may generate optical QPOs with a period of $\sim$5700 s.

astro-ph.HE↗

arXiv2025

Optical Observations of the High Mass X-ray Binary MAXI J0709$-$159/LY CMa

We report on the optical spectroscopic monitoring of the X-ray transient MAXI J0709$-$159 (identified as the Be star LY CMa) performed for about 1.5 months after the X-ray detection with MAXI. The observed spectrum showed a double-peaked H$α$ line with a peak-to-peak separation of $\sim 230$ km s$^{-1}$, suggestive of the Be disk origin. We also detected a broad wing of the H$α$ line with a line-of-sight velocity of $\gtrsim 900$ km s$^{-1}$, which could be explained by the accretion disk of the compact object or a stellar wind from the Be star. Initially the H$α$ line showed an asymmetric profile with an enhanced blue peak, and then the blue peak decreased in $\sim$ 3 weeks to a similar strength to the red peak. We suggest that the evolution of the blue peak is associated with the X-ray activity and generated by the turbulence of the Be disk due to the passage of the compact object. We also investigated flux variation using the archival TESS data and found quasi-periodic variations with frequencies of $\sim 1$ and $\sim 2$ day$^{-1}$, which were likely caused by the pulsation of the B star. The overall variability properties on timescales of $\sim$ day were similar to those in Be X-ray binaries, rather than supergiant X-ray binaries.

astro-ph.HE↗

arXiv2025

X-ray Polarization of the High-Synchrotron-Peak BL Lacertae Object 1ES 1959+650 during Intermediate and High X-ray Flux States

We report the Imaging X-ray Polarimetry Explorer (IXPE) polarimetric and simultaneous multiwavelength observations of the high-energy-peaked BL Lacertae (HBL) object 1ES 1959+650, performed in 2022 October and 2023 August. In 2022 October IXPE measured an average polarization degree $Π_{\rm X}=9.4\;\!\%\pm 1.6\;\!\%$ and an electric-vector position angle $ψ_{\rm X}=53^{\circ}\pm 5^{\circ}$. The polarized X-ray emission can be decomposed into a constant component, plus a rotating component, with rotation velocity $ω_{\rm EVPA}=(-117\;\!\pm\;\!12)$ ${\rm deg}\;\!{\rm d}^{-1}$. In 2023 August, during a period of pronounced activity of the source, IXPE measured an average $Π_{\rm X}=12.4\;\!\%\pm0.7\;\!\%$ and $ψ_X=20^{\circ}\pm2^{\circ}$, with evidence ($\sim$0.4$\;\!\%$ chance probability) for a rapidly rotating component with $ω_{\rm EVPA}=(1864\;\!\pm\;\!34)$ ${\rm deg}\;\!{\rm d}^{-1}$. These findings suggest the presence of a helical magnetic field in the jet of 1ES 1959+650 or stochastic processes governing the field in turbulent plasma. Our multiwavelength campaigns from radio to X-ray reveal variability in both polarization and flux from optical to X-rays. We interpret the results in terms of a relatively slowly varying component dominating the radio and optical emission, while rapidly variable polarized components dominate the X-ray and provide minor contribution at optical wavelengths. The radio and optical data indicate that on parsec scales the magnetic field is primarily orthogonal to the jet direction. On the contrary, X-ray measurements show a magnetic field almost aligned with the parsec jet direction. Confronting with other IXPE observations, we guess that the magnetic field of HBLs on sub-pc scale should be rather unstable, often changing its direction with respect to the VLBA jet.

astro-ph.HE↗

arXiv2025

IXPE Observation of the Low-Synchrotron Peaked Blazar S4 0954+65 During An Optical-X-ray Flare

The X-ray polarization observations made possible with the Imaging X-ray Polarimetry Explorer (IXPE) offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here we report on the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength campaign on the source, we detected an optical flare whose peak coincided with the peak of an X-ray flare. This optical-X-ray flare most likely took place in a feature moving along the parsec-scale jet, imaged at 43 GHz by the Very Long Baseline Array. The 43 GHz polarization angle of the moving component underwent a rotation near the time of the flare. In the optical band, prior to the IXPE observation, we measured the polarization angle to be aligned with the jet axis. In contrast, during the optical flare the optical polarization angle was perpendicular to the jet axis; after the flare, it reverted to being parallel to the jet axis. Due to the smooth behavior of the optical polarization angle during the flare, we favor shocks as the main acceleration mechanism. We also infer that the ambient magnetic field lines in the jet were parallel to the jet position angle. The average degree of optical polarization during the IXPE observation was (14.3$\pm$4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3$σ$ level) on the X-ray polarization degree; although a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% ($3σ$). We model the spectral energy distribution (SED) and spectral polarization distribution (SPD) of S4 0954+65 with leptonic (synchrotron self-Compton) and hadronic (proton and pair synchrotron) models. The constraints we obtain with our combined multi-wavelength polarization observations and SED modeling tentatively disfavor hadronic models for the X-ray emission in S4 0954+65.

astro-ph.HE↗

arXiv2025

NinjaSat monitoring of Type-I X-ray bursts from the clocked burster SRGA J144459.2$-$604207

The CubeSat X-ray observatory NinjaSat was launched on 2023 November 11 and has provided opportunities for agile and flexible monitoring of bright X-ray sources. On 2024 February 23, the NinjaSat team started long-term observation of the new X-ray source SRGA J144459.2$-$604207 as the first scientific target, which was discovered on 2024 February 21 and recognized as the sixth clocked X-ray burster. Our 25-day observation covered almost the entire decay of this outburst from two days after the peak at $\sim$100 mCrab on February 23 until March 18 at a few mCrab level. The Gas Multiplier Counter onboard NinjaSat successfully detected 12 Type-I X-ray bursts with a typical burst duration of $\sim$20 s, shorter than other clocked burster systems. As the persistent X-ray emission declined by a factor of five, X-ray bursts showed a notable change in its morphology: the rise time became shorter from 4.4(7) s to 0.3(3) s (1$σ$ errors), and the peak amplitude increased by 44%. The burst recurrence time $Δt_{\rm rec}$ also became longer from 2 hr to 10 hr, following the relation of $Δt_{\rm rec} \propto F_{\rm per}^{-0.84}$, where $F_{\rm per}$ is the persistent X-ray flux, by applying a Markov chain Monte Carlo method. The short duration of bursts is explained by the He-enhanced composition of accretion matter and the relation between $Δt_{\textrm{rec}}$ and $F_{\rm per}$ by a massive neutron star. This study demonstrated that CubeSat pointing observations can provide valuable astronomical X-ray data.

astro-ph.HE↗