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Tatehiro Mihara

Publications and source records attributed to Tatehiro Mihara.

At least 19 recordsLinked to original sources
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↗
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

Eclipsing Stellar Flare on the Demon Star Algol Binary System Observed during the MAXI-NICER Follow-up Campaign in 2018

Algol is a well-known eclipsing binary hosting an active and variable star that exhibits frequent stellar flares. Here, we report our pre-planned and coordinated rapid X-ray follow-up observations of an eclipsing flare on Algol. The Monitor of All-sky X-ray Image (MAXI) detected a flare on Algol at 05:52 UT on 2018 July 4. Subsequently, we carried out a prompt X-ray monitoring with the Neutron star Interior Composition Explorer (NICER) starting at 19:45 UT on the same day, and the observation ended at 06:02 UT on 2018 July 6. During the decaying phase of the flare, we successfully detected a 5.8-hour-long eclipse, corresponding to the secondary eclipse in which Algol A blocks the line of sight to Algol B. During the eclipse, the 2-10 keV X-ray flux is decreased to 20% level from $1.9\times10^{-10}~ \mathrm{erg~cm^{-2}~s^{-1} }$ to $4.5\times10^{-11}~ \mathrm{erg~cm^{-2}~s^{-1} }$. We found a configuration of the flare size and location to explain the X-ray observations; e.g., the flare occurred at the latitude 45{\deg}S of the Algol B surface with a flare height of $1.9\times10^{11}~\mathrm{cm}$, corresponding to 0.8 times the stellar radius of Algol B, giving 80% obscuration of the flare loop by Algol A. The apparent absorption increase before the eclipse might originate from coronal mass ejection (CME) in the line of sight ejected during the flare.

astro-ph.SR↗
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 ($1\sigma$ 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

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

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$\sigma$ 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$^{-2}$.

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arXiv2024

NinjaSat: Astronomical X-ray CubeSat Observatory

NinjaSat is an X-ray CubeSat designed for agile, long-term continuous observations of bright X-ray sources, with the size of 6U ($100\times200\times300$ mm$^3$) and a mass of 8 kg. NinjaSat is capable of pointing at X-ray sources with an accuracy of less than $0^{\circ}\hspace{-1.0mm}.1$ (2$\sigma$ confidence level) with 3-axis attitude control. The satellite bus is a commercially available NanoAvionics M6P, equipped with two non-imaging gas X-ray detectors covering an energy range of 2-50 keV. A total effective area of 32 cm$^2$ at 6 keV is capable of observing X-ray sources with a flux of approximately 10$^{-10}$ erg cm$^{-2}$ s$^{-1}$. The arrival time of each photon can be tagged with a time resolution of 61 $\mu$s. The two radiation belt monitors continuously measure the fluxes of protons above 5 MeV and electrons above 200 keV trapped in the geomagnetic field, alerting the X-ray detectors when the flux exceeds a threshold. The NinjaSat project started in 2020. Fabrication of the scientific payloads was completed in August 2022, and satellite integration and tests were completed in July 2023. NinjaSat was launched into a Sun-synchronous polar orbit at an altitude of about 530 km on 2023 November 11 by the SpaceX Transporter-9 mission. After about three months of satellite commissioning and payload verification, we observed the Crab Nebula on February 9, 2024, and successfully detected the 33.8262 ms pulsation from the neutron star. With this observation, NinjaSat met the minimum success criterion and stepped forward to scientific observations as initially planned. By the end of November 2024, we successfully observed 21 X-ray sources using NinjaSat. This achievement demonstrates that, with careful target selection, we can conduct scientific observations effectively using CubeSats, contributing to time-domain astronomy.

astro-ph.IM↗
arXiv2024

Evidence of non-Solar elemental composition in the clocked X-ray burster SRGA J144459.2$-$604207

In February and March 2024, a series of many Type I X-ray bursts from the accreting neutron star SRGA J144459.2$-$604207, which has been identified by multiple X-ray satellites, with the first reports coming from INTEGRAL and NinjaSat. These observations reveal that after exhibiting very regular behavior as a ``clocked'' burster, the peak luminosity of the SRGA J144459.2$-$604207 X-ray bursts shows a gradual decline. The observed light curves exhibit a short plateau feature, potentially with a double peak, followed by a rapid decay in the tail-features unlike those seen in previously observed clocked bursters. In this study, we calculate a series of multizone X-ray burst models with various compositions of accreted matter, specifically varying the mass fractions of hydrogen ($X$), helium ($Y$), and heavier CNO elements or metallicity ($Z_{\rm CNO}$). We demonstrate that a model with higher $Z_{\rm CNO}$ and/or lower $X/Y$ compared to the solar values can reproduce the observed behavior of SRGA J144459.2$-$604207. Therefore, we propose that this new X-ray burster is likely the first clocked burster with non-solar elemental compositions. Moreover, based on the X-ray burst light curve morphology in the decline phase observed by NinjaSat, a He-enhanced model with $X/Y \approx 1.5$ seems preferred over high-metallicity cases. We also give a brief discussion on the implications for the neutron star mass, binary star evolution, inclination angle, and the potential for a high-metallicity scenario, the last of which is closely related to the properties of the hot CNO cycle.

astro-ph.HE↗
arXiv2024

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$20 s, shorter than other clocked burster systems. As the persistent X-ray emission declined by a factor of five, X-ray bursts showed a notable change in its morphology: the rise time became shorter from 4.4(7) s to 0.3(3) s (1$\sigma$ errors), and the peak amplitude increased by 44%. The burst recurrence time $\Delta t_{\rm rec}$ also became longer from 2 hr to 10 hr, following the relation of $\Delta t_{\rm rec} \propto F_{\rm per}^{-0.84}$, 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 $\Delta t_{\textrm{rec}}$ 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↗
arXiv2024

High-speed Readout System of X-ray CMOS Image Sensor for Time Domain Astronomy

We developed an FPGA-based high-speed readout system for a complementary metal-oxide-semiconductor (CMOS) image sensor to observe soft X-ray transients in future satellite missions, such as HiZ-GUNDAM. Our previous research revealed that the CMOS image sensor has low-energy X-ray detection capability (0.4-4 keV) and strong radiation tolerance, which satisfies the requirements of the HiZ-GUNDAM mission. However, CMOS sensors typically have small pixel sizes (e.g., $\sim$10 ${\rm \mu m}$), resulting in large volumes of image data. GSENSE400BSI has 2048$\times$2048 pixels, producing 6 Mbyte per frame. These large volumes of observed raw image data cannot be stored in a satellite bus system with a limited storage size. Therefore, only X-ray photon events must be extracted from the raw image data. Furthermore, the readout time of CMOS image sensors is approximately ten times faster than that of typical X-ray CCDs, requiring faster event extraction on a timescale of $\sim$0.1 s. To address these issues, we have developed an FPGA-based image signal processing system capable of high-speed X-ray event extraction onboard without storing raw image data. The developed compact system enabled mounting on a CubeSat mission, facilitating early in-orbit operation demonstration. Here, we present the design and results of the performance evaluation tests of the proposed FPGA-based readout system. Utilizing X-ray irradiation experiments, the results of the X-ray event extraction with the onboard and offline processing methods were consistent, validating the functionality of the proposed system.

astro-ph.IM↗
arXiv2024

X-ray Iron absorption line in Swift J1858.6-0814

We present the spectral analysis of bright steady states in an outburst of the transient neutron star low-mass X-ray binary (NS-LMXB) Swift J1858.6-0814 observed with NICER. We detected an ionized iron K absorption line (H-like Fe) at 6.97keV in the spectrum. We estimated the photoionization parameter using the ratio of the equivalent widths (EWs) of the FeXXVI (H-like) (17+\-5eV) and FeXXV (He-like) (<3eV) and discuss the origin of the iron absorption line. The irradiated gas producing the absorption line would locate within (3-6)*1E9cm from the X-ray source. We suggest that the observed H-like Fe absorption line originates from the highly-ionized gas in the inner accretion disk in Swift J1858.6-0814.

astro-ph.HE↗
arXiv2023

Accretion spin-up and a strong magnetic field in the slow-spinning Be X-ray binary MAXI J0655-013

We present MAXI and NuSTAR observations of the Be X-ray binary, MAXI J0655-013, in outburst. NuSTAR observed the source once early in the outburst, when spectral analysis yields a bolometric (0.1--100 keV), unabsorbed source luminosity of $L_{\mathrm{bol}}=5.6\times10^{36}\mathrm{erg\,s^{-1}}$, and a second time 54 days later, by which time the luminosity dropped to $L_{\mathrm{bol}}=4\times10^{34}\,\mathrm{erg\,s^{-1}}$ after first undergoing a dramatic increase. Timing analysis of the NuSTAR data reveals a neutron star spin period of $1129.09\pm0.04$ s during the first observation, which decreased to $1085\pm1$ s by the time of the second observation, indicating spin-up due to accretion throughout the outburst. Furthermore, during the first NuSTAR observation, we observed quasiperiodic oscillations with centroid frequency $\nu_0=89\pm1$ mHz, which exhibited a second harmonic feature. By combining the MAXI and NuSTAR data with pulse period measurements reported by Fermi/GBM, we are able to show that apparent flaring behavior in the MAXI light-curve is an artifact introduced by uneven sampling of the pulse profile, which has a large pulsed fraction. Finally, we estimate the magnetic field strength at the neutron star surface via three independent methods, invoking a tentative cyclotron resonance scattering feature at $44$ keV, QPO production at the inner edge of the accretion disk, and spin-up via interaction of the neutron star magnetic field with accreting material. Each of these result in a significantly different value. We discuss the strengths and weaknesses of each method and infer that MAXI J0655-013 is likely to have a high surface magnetic field strength, $B_{s}>10^{13}$ G.

astro-ph.HE↗
arXiv2023

GRB 221009A: Discovery of an Exceptionally Rare Nearby and Energetic Gamma-Ray Burst

We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image (MAXI), and Neutron Star Interior Composition Explorer Mission (NICER). This energetic GRB was located relatively nearby (z = 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (b = 4.3 degrees) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (~> 10kpc). We present analysis of the light curves and spectra at X-ray and UV/optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at T0 + 4.5 ks than any previous GRB observed by Swift. In its rest frame GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10^4 long GRBs were as energetic as GRB 221009A; such a large E_gamma,iso implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ~<1 per 1000 yr - making this a truly remarkable opportunity unlikely to be repeated in our lifetime.

astro-ph.HE↗
arXiv2023

Monitoring observations of SMC X-1's excursions (MOOSE)-II: A new excursion accompanies spin-up acceleration

SMC X-1 is a high-mass X-ray binary showing superorbital modulation with an unstable period. Previous monitoring shows three excursion events in 1996--1998, 2005--2007, and 2014--2016. The superorbital period drifts from >60 days to <40 days and then evolves back during an excursion. Here we report a new excursion event of SMC X-1 in 2020--2021, indicating that the superorbital modulation has an unpredictable, chaotic nature. We trace the spin-period evolution and find that the spin-up rate accelerated one year before the onset of this new excursion, which suggests a possible inside-out process connecting the spin-up acceleration and the superorbital excursion. This results in a deviation of the spin period residual, similar to the behaviour of the first excursion in 1996--1998. In further analysis of the pulse profile evolution, we find that the pulsed fraction shows a long-term evolution and may be connected to the superorbital excursion. These discoveries deepen the mystery of SMC X-1 because they cannot be solely interpreted by the warped disc model. Upcoming pointed observations and theoretical studies may improve our understanding of the detailed accretion mechanisms taking place.

astro-ph.HE↗
arXiv2022

Discovery of a new supergiant fast X-ray transient MAXI J0709-159 associated with the Be star LY CMa

We report on the discovery of a new supergiant fast X-ray transient (SFXT), MAXI J0709$-$159, and its identification with LY CMa (also known as HD 54786). On 2022 January 25, a new flaring X-ray object named MAXI J0709$-$159, was detected by Monitor of All-sky X-ray Image (MAXI). Two flaring activities were observed in the two scans of $\sim 3$ hours apart, where the 2-10 keV flux reached $5\times 10^{-9}$ erg cm$^{-2}$ s$^{-1}$. During the period, the source exhibited a large spectral change suggesting that the absorption column density $N_\mathrm{H}$ increased from $10^{22}$ cm$^{-2}$ to $10^{23}$ cm$^{-2}$. NuSTAR follow-up observation on January 29 identified a new X-ray source with a flux of $6\times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$ at the position consistent with LY CMa, which has been identified as B supergiant as well as Be star, located at the 3 kpc distance. The observed X-ray activity characterized by the short ($\lesssim$ several hours) duration, the rapid ($\lesssim$ a few seconds) variabilities accompanied with spectral changes, and the large luminosity swing ($10^{32}$-$10^{37}$ erg s$^{-1}$) agree with those of SFXT. On the other hand, optical spectroscopic observations of LY CMa revealed a broad $Hα$ emission line, which may indicate the existence of a Be circumstellar disk. These obtained results suggest that the optical companion, LY CMa, certainly has a complex circumstellar medium including dense clumps.

astro-ph.HE↗
arXiv2022

Decades' long-term variations in NS-LMXBs observed with MAXI/GSC, RXTE/ASM and Ginga/ASM

We investigated the decades' long-term X-ray variations in bright low-mass X-ray binaries containing a neutron star (NS-LMXB). The light curves of MAXI/GSC and RXTE/ASM covers $\sim$ 26 yr, and high-quality X-ray light curves are obtained from 33 NS-LMXBs. Among them, together with Ginga/ASM, two sources (GX 3$+$1 and GX 9$+$1) showed an apparent sinusoidal variation with the period of $\sim 5$ yr and $\sim 10$ yr in the 34 yr light curve. Their X-ray luminosities were $(1-4)\times10^{37}$ erg s$^{-1}$ in the middle of the luminosity distribution of the NS-LMXB. Other seven sources (Ser X-1, 4U 1735--444, GX 9$+$9, 4U 1746$-$37, 4U 1708$-$40, 4U 1822$-$000, and 1A 1246$-$588) have also similar sinusoidal variation, although the profiles (amplitude, period, and phase) are variable. Compering the 21 sources with known orbital periods, a possible cause of the long-term sinusoidal variation might be the mass transfer cycles induced by the irradiation to the donor star.

astro-ph.HE↗
arXiv2022

MAXI and NuSTAR observations of the faint X-ray transient MAXI J1848-015 in the GLIMPSE-C01 Cluster

We present the results of MAXI monitoring and two NuSTAR observations of the recently discovered faint X-ray transient MAXI J1848-015. Analysis of the MAXI light-curve shows that the source underwent a rapid flux increase beginning on 2020 December 20, followed by a rapid decrease in flux after only $\sim5$ days. NuSTAR observations reveal that the source transitioned from a bright soft state with unabsorbed, bolometric ($0.1$-$100$ keV) flux $F=6.9 \pm 0.1 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$, to a low hard state with flux $F=2.85 \pm 0.04 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. Given a distance of $3.3$ kpc, inferred via association of the source with the GLIMPSE-C01 cluster, these fluxes correspond to an Eddington fraction of order $10^{-3}$ for an accreting neutron star of mass $M=1.4M_\odot$, or even lower for a more massive accretor. However, the source spectra exhibit strong relativistic reflection features, indicating the presence of an accretion disk which extends close to the accretor, for which we measure a high spin, $a=0.967\pm0.013$. In addition to a change in flux and spectral shape, we find evidence for other changes between the soft and hard states, including moderate disk truncation with the inner disk radius increasing from $R_\mathrm{in}\approx3\,R_\mathrm{g}$ to $R_\mathrm{in}\approx8\,R_\mathrm{g}$, narrow Fe emission whose centroid decreases from $6.8\pm0.1$ keV to $6.3 \pm 0.1$ keV, and an increase in low-frequency ($10^{-3}$-$10^{-1}$ Hz) variability. Due to the high spin we conclude that the source is likely to be a black hole rather than a neutron star, and we discuss physical interpretations of the low apparent luminosity as well as the narrow Fe emission.

astro-ph.HE↗