arXiv subjects

Hirokazu Odaka

Publications and source records attributed to Hirokazu Odaka.

At least 37 records · Page 2Linked to original sources

arXiv2025

XRISM constraints on unidentified X-ray emission lines, including the 3.5 keV line, in the stacked spectrum of ten galaxy clusters

We stack 3.75 Megaseconds of early XRISM Resolve observations of ten galaxy clusters to search for unidentified spectral lines in the $E=$ 2.5-15 keV band (rest frame), including the $E=3.5$ keV line reported in earlier, low spectral resolution studies of cluster samples. Such an emission line may originate from the decay of the sterile neutrino, a warm dark matter (DM) candidate. No unidentified lines are detected in our stacked cluster spectrum, with the $3σ$ upper limit on the $m_{\rm s}\sim$ 7.1 keV DM particle decay rate (which corresponds to a $E=3.55$ keV emission line) of $Γ\sim 1.0 \times 10^{-27}$ s$^{-1}$. This upper limit is 3-4 times lower than the one derived by Hitomi Collaboration et al. (2017) from the Perseus observation, but still 5 times higher than the XMM-Newton detection reported by Bulbul et al. (2014) in the stacked cluster sample. XRISM Resolve, with its high spectral resolution but a small field of view, may reach the sensitivity needed to test the XMM-Newton cluster sample detection by combining several years worth of future cluster observations.

astro-ph.HE↗

arXiv2025

Emission and Absorption Features of Magnetically Driven Disk Winds in Black Hole X-Ray Binaries

We investigate accretion disk winds commonly observed in galactic black hole (BH) X-ray binaries (XRB), which manifest as blueshifted absorption features in X-ray spectra. We model these winds as ideal magnetohydrodynamic outflows of hot plasma driven by global magnetic fields threading the accretion disk around the BH. Using Monte Carlo simulations with MONACO, we solve three-dimensional radiative transfer equations to determine the large-scale ionization structure that produces the observed ionic column densities. Focusing on the high/soft state of the BH XRB, where disk emission provides the dominant source of ionizing X-rays, we calculated synthetic spectra showing resonance absorption and scattered emission from ions in various charge states. Our results demonstrate that systems viewed at high polar angles exhibit prominent multi-ion absorption lines with asymmetric profiles, accompanied by P-Cygni-like emission features that partially reproduce the characteristics seen in the observed spectra. This further implies that even a dense disk wind with a high polar angle is unlikely to be saturated due to effective scattering.

astro-ph.HE↗

arXiv2025

Comparing XRISM cluster velocity dispersions with predictions from cosmological simulations: are feedback models too ejective?

The dynamics of the intra-cluster medium (ICM), the hot plasma that fills galaxy clusters, are shaped by gravity-driven cluster mergers and feedback from supermassive black holes (SMBH) in the cluster cores. XRISM measurements of ICM velocities in several clusters offer insights into these processes. We compare XRISM measurements for nine galaxy clusters (Virgo, Perseus, Centaurus, Hydra A, PKS\,0745--19, A2029, Coma, A2319, Ophiuchus) with predictions from three state-of-the-art cosmological simulation suites, TNG-Cluster, The Three Hundred Project GADGET-X, and GIZMO-SIMBA, that employ different models of feedback. In cool cores, XRISM reveals systematically lower velocity dispersions than the simulations predict, with all ten measurements below the median simulated values by a factor $1.5-1.7$ on average and all falling within the bottom $10\%$ of the predicted distributions. The observed kinetic-to-total pressure ratio is also lower, with a median value of $2.2\%$, compared to the predicted $5.0-6.5\%$ for the three simulations. Outside the cool cores and in non-cool-core clusters, simulations show better agreement with XRISM measurements, except for the outskirts of the relaxed, cool-core cluster A2029, which exhibits an exceptionally low kinetic pressure support ($<1\%$), with none of the simulated systems in either of the three suites reaching such low levels. The non-cool-core Coma and A2319 exhibit dispersions at the lower end but within the simulated spread. Our comparison suggests that the three numerical models may overestimate the kinetic effects of SMBH feedback in cluster cores. Additional XRISM observations of non-cool-core clusters will clarify if there is a systematic tension in the gravity-dominated regime as well.

astro-ph.GA↗

arXiv2025

Stratified wind from a super-Eddington X-ray binary is slower than expected

Accretion discs in strong gravity ubiquitously produce winds, seen as blueshifted absorption lines in the X-ray band of both stellar mass X-ray binaries (black holes and neutron stars), and supermassive black holes. Some of the most powerful winds (termed Eddington winds) are expected to arise from systems where radiation pressure is sufficient to unbind material from the inner disc ($L\gtrsim L_{\rm Edd}$). These winds should be extremely fast and carry a large amount of kinetic power, which, when associated with supermassive black holes, would make them a prime contender for the feedback mechanism linking the growth of those black holes with their host galaxies. Here we show the XRISM Resolve spectrum of the Galactic neutron star X-ray binary, GX 13+1, which reveals one of the densest winds ever seen in absorption lines. This Compton-thick wind significantly attenuates the flux, making it appear faint, although it is intrinsically more luminous than usual ($L\gtrsim L_{\rm Edd}$). However, the wind is extremely slow, more consistent with the predictions of thermal-radiative winds launched by X-ray irradiation of the outer disc, than with the expected Eddington wind driven by radiation pressure from the inner disc. This puts new constraints on the origin of winds from bright accretion flows in binaries, but also highlights the very different origin required for the ultrafast ($v\sim 0.3c$) winds seen in recent Resolve observations of a supermassive black hole at similarly high Eddington ratio.

astro-ph.HE↗

arXiv2025

Development of Solar Flare X-ray Polarimeter with Micro-Pixel CMOS Sensors

We are developing an X-ray polarimeter using micro-pixel CMOS sensors for solar flare X-ray polarimetry. The system consists of a 2.5-$μ$m pixel CMOS image sensor with a 12.8$\times$12.8 mm$^2$ imaging area and a readout system based on a Zynq System-on-Chip. While previous studies have validated this concept, no realistic feasibility studies have been conducted for the solar flare X-ray polarization observation. In this work, we performed polarization sensitivity measurements at synchrotron facilities. The results show that our polarimeter is sensitive to the X-ray polarization, exhibiting a modulation factor of 5-15% at an energy range of 6-22 keV. The measurements also determined the thickness of the sensitive layer to be approximately 5 $μ$m, and the thicknesses of the insensitive layers to be 0.8 $μ$m (Si), 2.1 $μ$m (SiO2), and 0.24 $μ$m (Cu). These measured thicknesses lead to a quantum efficiency of 3-4% at 10 keV. Based on these experimental evaluations, we estimated the sensitivity of the micro-pixel CMOS polarimeter system. We found that, when combined with a telescope with an effective area of $\sim$10 cm$^2$, this system can detect X-ray polarization with a polarization degree of a few percent for M-class flares.

astro-ph.IM↗

arXiv2025

XRISM/Resolve View of Abell 2319: Turbulence, Sloshing, and ICM Dynamics

We present results from XRISM/Resolve observations of the core of the galaxy cluster Abell 2319, focusing on its kinematic properties. The intracluster medium (ICM) exhibits temperatures of approximately 8 keV across the core, with a prominent cold front and a high-temperature region ($\sim$11 keV) in the northwest. The average gas velocity in the 3 arcmin $\times$ 4 arcmin region around the brightest cluster galaxy (BCG) covered by two Resolve pointings is consistent with that of the BCG to within 40 km s$^{-1}$ and we found modest average velocity dispersion of 230-250 km s$^{-1}$. On the other hand, spatially-resolved spectroscopy reveals interesting variations. A blueshift of up to $\sim$230 km s$^{-1}$ is observed around the east edge of the cold front, where the gas with the lowest specific entropy is found. The region further south inside the cold front shows only a small velocity difference from the BCG; however, its velocity dispersion is enhanced to 400 km s$^{-1}$, implying the development of turbulence. These characteristics indicate that we are observing sloshing motion with some inclination angle following BCG and that gas phases with different specific entropy participate in sloshing with their own velocities, as expected from simulations. No significant evidence for a high-redshift ICM component associated with the subcluster Abell 2319B was found in the region covered by the current Resolve pointings. These results highlight the importance of sloshing and turbulence in shaping the internal structure of Abell 2319. Further deep observations are necessary to better understand the mixing and turbulent processes within the cluster.

astro-ph.HE↗

arXiv2025

Unraveling the structure of the stratified ultra-fast outflows in PDS 456 with XRISM

Multiple clumpy wind components ($v_{out}\sim0.2-0.3c$) in the luminous quasar PDS 456 have recently been resolved by XRISM in the Fe-K band for the first time. In this paper, we investigate the structure of ultra-fast outflows (UFOs) using coordinated observations from XRISM, XMM-Newton, and NuSTAR, along with the self-consistently calculated photoionization model \texttt{PION}. Our results reveal a stratified ionization structure, characterized by a relation between wind velocity and ionization parameter $v_{out}\proptoξ^{(0.14\pm0.04)}$. To evaluate the impact of the screening effect, we tested all possible order permutations of six \texttt{PION} components. We find that highly ionized UFOs ($\logξ>4.5$) are insensitive to their relative positions, whereas the soft X-ray UFO ($\logξ\sim3$ and $v_{out}\sim0.27c$) and the lowest-ionized hard X-ray UFO ($\logξ\sim4.1$ and $v_ {out}\sim0.23c$) are statistically favored -- based on the evidence from both the C-statistic and Bayesian analysis -- to occupy the middle and innermost layers, respectively. This suggests a possible trend where slower UFOs are launched from regions closer to the supermassive black hole (SMBH). The soft X-ray UFO is found to be thermally unstable, regardless of its relative position. However, its location remains unclear. Our sequence analysis and its similarity to hard X-ray UFOs suggest that they may be co-spatial, while variability constraints support its location within the broad-line region at sub-parsec scales. Simulations with the gate-valve opened XRISM show that high-resolution soft X-ray data can enhance the reliability of our results. Furthermore, simulations with the future X-ray mission NewAthena demonstrate its capability to resolve the absorber sequence and spatial distributions, enabling the determination of UFO structures and their roles in AGN feedback.

astro-ph.HE↗

arXiv2025

XRISM spectroscopy on orbital modulation of Fe Ly$α$ lines in Cygnus X-3

To understand physical processes such as mass transfer and binary evolution in X-ray binaries, the orbital parameters of the system are fundamental and crucial information. Cygnus X-3 is a high-mass X-ray binary composed of a compact object of unknown nature and a Wolf-Rayet star, which is of great interest in the context of wind-fed mass accretion and binary evolution. Here we present XRISM/Resolve high-resolution spectroscopy focusing on the Fe Ly$α$ lines in its hypersoft state. We perform an orbital phase-resolved spectral analysis of the lines to study the orbital modulation of the emission and absorption lines. It is found that the emission lines reflect the orbital motion of the compact object whose estimated velocity amplitude is $430^{~~+150}_{~~-140}~~\mathrm{km\,s^{~-1}}$, while the absorption lines show a variation that can be interpreted as originating from the stellar wind. We discuss possible mass ranges for the binary components using the mass function with the estimated value of the velocity amplitude in this work, combined with the relation between the mass loss rate and the orbital period derivative and the empirical mass and mass loss rate relation for Galactic Wolf-Rayet stars. They are constrained to be $(1.3\text{-}5.1)\,M_\odot$ and $(9.3\text{-}12)\,M_\odot$ for the assumed inclination angle of $i = 25$ deg, which becomes more relaxed to $(1.3\text{-}24)\,M_\odot$ and $(9.3\text{-}16)\,M_\odot$ for $i = 35$ deg, respectively. Thus, it remains unclear whether the system harbors a black hole or a neutron star.

astro-ph.HE↗

arXiv2025

X-Ray Radiative Transfer Calculation Based on a Physics-based Model of the Sub-parsec-scale Gases around an Active Galactic Nucleus and Its Application to NGC 3783

Although the X-ray spectra of Seyfert 1 galaxies exhibit absorption lines of He-like iron and H-like iron at blue-shifted velocities of approximately $500 \ \mathrm{km} \ \mathrm{s}^{-1}$, the physical origin of these absorption lines remains uncertain. In this study, we performed X-ray radiative transfer based on the sub-parsec-scale thermally driven outflows. The initial step involved calculating the photoionization equilibrium using the Cloudy code, which is based on three-dimensional radiative hydrodynamic simulations. Subsequently, X-ray radiative transfer was performed using the Monte Carlo simulation for astrophysics and cosmology code. Our findings indicate that when the angle of inclination ranges from $55 \ \mathrm{degrees}$ to $65 \ \mathrm{degrees}$, the transmitted component of the X-ray spectrum displays absorption lines of He-like and H-like iron, exhibiting a blue shift of approximately $500 \ \mathrm{km} \ \mathrm{s}^{-1}$. The results suggest that the absorption lines are generated by a photoionized gas within $0.005 \ \mathrm{pc}$. Additionally, the results indicate that the scattered component of the X-ray spectrum exhibits emission lines originating from neutral iron fluorescence, He-like iron, and H-like iron. The emission lines are broadened by approximately $7000 \ \mathrm{km} \ \mathrm{s}^{-1}$ due to the Keplerian rotation. Furthermore, the model reproduced the H-like iron and H-like iron absorption lines in NGC 3783 observed by the Chandra High Energy Transmission Grating.

astro-ph.HE↗

arXiv2025

Constraining gas motion and non-thermal pressure beyond the core of the Abell 2029 galaxy cluster with XRISM

We report a detailed spectroscopic study of the gas dynamics and hydrostatic mass bias of the galaxy cluster Abell 2029, utilizing high-resolution observations from XRISM Resolve. Abell 2029, known for its cool core and relaxed X-ray morphology, provides an excellent opportunity to investigate the influence of gas motions beyond the central region. Expanding upon prior studies that revealed low turbulence and bulk motions within the core, our analysis covers regions out to the scale radius $R_{2500}$ (670~kpc) based on three radial pointings extending from the cluster center toward the northern side. We obtain accurate measurements of bulk and turbulent velocities along the line of sight. The results indicate that non-thermal pressure accounts for no more than 2% of the total pressure at all radii, with a gradual decrease outward. The observed radial trend differs from many numerical simulations, which often predict an increase in non-thermal pressure fraction at larger radii. These findings suggest that deviations from hydrostatic equilibrium are small, leading to a hydrostatic mass bias of around 2% across the observed area.

astro-ph.CO↗

arXiv2025

The Moon as a Cosmic-Ray Spectrometer: Prospects for MeV Gamma-Ray Observations

The Moon is the closest celestial gamma-ray emitting object. Its gamma-ray emission arises from interactions between Galactic cosmic rays (CRs) and the lunar surface. While the lunar GeV gamma-ray spectrum is dominated by a continuum from hadronic decay processes, the MeV emission exhibits both continuum and distinctive spectral lines from nuclear de-excitation and radioactive decay processes. Using Geant4 Monte Carlo particle simulations, we model the lunar gamma-ray spectrum. Our results demonstrate its consistency with Fermi-LAT observations, and predict that next-generation MeV gamma-ray instruments will detect both the lunar MeV continuum and several key spectral line features, notably the $1.779~\mathrm{MeV}$ line from $\mathrm{^{28}Si}$ de-excitation enhanced by the lunar surface composition, the $e^+e^-$ annihilation line, and radioactive decay lines from $\mathrm{^{22}Na}$ ($τ\approx3.75\,\mathrm{yr}$) and long-lived $\mathrm{^{26}Al}$ ($τ\approx1\,\mathrm{Myr}$). These gamma-ray lines are sensitive to CRs with energies $\lesssim1\,\mathrm{GeV\,nuc^{-1}}$, offering unique temporal probes of CR activity over different timescales. Observations of the lunar MeV gamma-ray spectrum will therefore open a new window to study the current irradiation of the solar-terrestrial environment by low-energy CRs and its long-term temporal evolution.

astro-ph.HE↗

arXiv2025

XRISM forecast for the Coma cluster: stormy, with a steep power spectrum

The XRISM Resolve microcalorimeter array measured the velocities of hot intracluster gas at two positions in the Coma galaxy cluster: 3'x3' squares at the center and at 6' (170 kpc) to the south. We find the line-of-sight velocity dispersions in those regions to be sigma_z=208+-12 km/s and 202+-24 km/s, respectively. The central value corresponds to a 3D Mach number of M=0.24+-0.015 and the ratio of the kinetic pressure of small-scale motions to thermal pressure in the intracluster plasma of only 3.1+-0.4%, at the lower end of predictions from cosmological simulations for merging clusters like Coma, and similar to that observed in the cool core of the relaxed cluster A2029. Meanwhile, the gas in both regions exhibits high line-of-sight velocity differences from the mean velocity of the cluster galaxies, Delta v_z=450+-15 km/s and 730+-30 km/s, respectively. A small contribution from an additional gas velocity component, consistent with the cluster optical mean, is detected along a sightline near the cluster center. The combination of the observed velocity dispersions and bulk velocities is not described by a Kolmogorov velocity power spectrum of steady-state turbulence; instead, the data imply a much steeper effective slope (i.e., relatively more power at larger linear scales). This may indicate either a very large dissipation scale resulting in the suppression of small-scale motions, or a transient dynamic state of the cluster, where large-scale gas flows generated by an ongoing merger have not yet cascaded down to small scales.

astro-ph.HE↗

arXiv2025

The nature of spectral variability of the accreting pulsar Centaurus X-3 unveiled by NuSTAR observation covering two orbital cycles

We conducted a $369\;{\rm ks}$ NuSTAR observation on the X-ray pulsar Centaurus X-3, which covered two consecutive orbital cycles of the source, including two eclipse durations. We investigated the orbital-phase spectral variability over the two orbital cycles. We divided the entire observation data into multiple segments, each covering an orbital interval of $ΔΦ=0.005$. The phenomenological spectral modeling applied to these orbital-phase-resolved spectra reveals that the photon index is the key parameter with the most variability and a strong correlation with the continuum flux. The photon index becomes softer during the high-flux phases and harder in the low-flux phases. The relation between the photon index and continuum flux remains consistent when investigating specific spin phases, suggesting that the spectral variability originates from extrinsic factors apart from the neutron star. Furthermore, the 3-5 keV pulse fraction also exhibits variability, being enhanced in the high-flux phases and suppressed in the low-flux phases, which indicates the presence of multiple emission components with different pulse fractions. These phenomenological analysis results enabled us to estimate the physical origin of the spectral variability. We successfully fitted the orbital-phase-resolved spectra with a physical model that assumes (1) stable emission from the neutron star, (2) attenuation by inhomogeneous, clumpy stellar wind, and (3) an additional non-pulsed emission component arising from thermal emission from the accretion disk. The thermal emission from the accretion disk can be described by a blackbody with a temperature of $kT\sim0.5\;{\rm keV}$ and a luminosity of $\sim10^{37}\;{\rm erg\;s^{-1}}$.

astro-ph.HE↗

arXiv2025

Characterization of more than three years of in-orbit radiation damage of SiPMs on GRBAlpha and VZLUSAT-2 CubeSats

Silicon photomultipliers (SiPMs) are prone to radiation damage which causes an increase of dark count rate. This leads to an increase in low-energy threshold in a gamma-ray detector combining SiPM and a scintillator. Despite this drawback, they are becoming preferred for scintillator-based gamma-ray detectors on CubeSats due to their low operation voltage, small size, linear response to low light intensity and fast response. This increasing popularity of SiPMs among new spaceborne missions makes it important to characterize their long-term performance in the space environment. In this work, we report the change of the dark count rate and low-energy threshold of S13360-3050 PE multi-pixel photon counters (MPPCs) by Hamamatsu, using measurements acquired by the GRBAlpha and VZLUSAT-2 CubeSats at low Earth orbit (LEO) spanning over three years. Such a long measurement of the performance of MPPCs in space has not been published before. GRBAlpha is a 1U CubeSat launched on March 22, 2021, to a 550 km altitude sun-synchronous polar orbit (SSO) carrying on board a gamma-ray detector based on CsI(Tl) scintillator readout by eight MPPCs and regularly detecting gamma-ray transients such as gamma-ray bursts and solar flares in the energy range of ~30-900 keV. VZLUSAT-2 is a 3U CubeSat launched on January 13, 2022 also to a 535 km altitude SSO carrying on board, among other payloads, two gamma-ray detectors similar to the one on GRBAlpha. We have flight-proven the Hamamatsu MPPCs S13360-3050 PE and demonstrated that MPPCs, shielded by 2.5 mm of PbSb alloy, can be used in LEO environment on a scientific mission lasting beyond three years. This manifests the potential of MPPCs being employed in future satellites.

astro-ph.IM↗

arXiv2025

In-orbit Performance of the Soft X-ray Imaging Telescope Xtend aboard XRISM

We present a summary of the in-orbit performance of the soft X-ray imaging telescope Xtend onboard the XRISM mission, based on in-flight observation data, including first-light celestial objects, calibration sources, and results from the cross-calibration campaign with other currently-operating X-ray observatories. XRISM/Xtend has a large field of view of $38.5'\times38.5'$, covering an energy range of 0.4--13 keV, as demonstrated by the first-light observation of the galaxy cluster Abell 2319. It also features an energy resolution of 170--180 eV at 6 keV, which meets the mission requirement and enables to resolve He-like and H-like Fe K$α$ lines. Throughout the observation during the performance verification phase, we confirm that two issues identified in SXI onboard the previous Hitomi mission -- light leakage and crosstalk events -- are addressed and suppressed in the case of Xtend. A joint cross-calibration observation of the bright quasar 3C273 results in an effective area measured to be $\sim420$ cm$^{2}$@1.5 keV and $\sim310$ cm$^{2}$@6.0 keV, which matches values obtained in ground tests. We also continuously monitor the health of Xtend by analyzing overclocking data, calibration source spectra, and day-Earth observations: the readout noise is stable and low, and contamination is negligible even one year after launch. A low background level compared to other major X-ray instruments onboard satellites, combined with the largest grasp ($Ω_{\rm eff}\sim60$ ${\rm cm^2~degree^2}$) of Xtend, will not only support Resolve analysis, but also enable significant scientific results on its own. This includes near future follow-up observations and transient searches in the context of time-domain and multi-messenger astrophysics.

astro-ph.IM↗

arXiv2025

Initial operations of the Soft X-ray Imager onboard XRISM

XRISM (X-Ray Imaging and Spectroscopy Mission) is an astronomical satellite with the capability of high-resolution spectroscopy with the X-ray microcalorimeter, Resolve, and wide field-of-view imaging with the CCD camera, Xtend. Xtend consists of the mirror assembly (XMA: X-ray Mirror Assembly) and detector (SXI: Soft X-ray Imager). The SXI is composed of CCDs, analog and digital electronics, and a mechanical cooler. After the successful launch on September 6th, 2023 (UT) and subsequent critical operations, the mission instruments were turned on and set up. The CCDs have been kept at the designed operating temperature of $-110^\circ$C after the electronics and cooling system were successfully set up. During the initial operation phase, which continued for more than a month after the critical operations, we verified the observation procedure, stability of the cooling system, all the observation options with different imaging areas and/or timing resolutions, and time-tagged and automated operations including those for South Atlantic Anomaly passages. We optimized the operation procedure and observation parameters including the cooler settings, imaging areas for the small window modes, and event selection algorithm. We summarize our policy and procedure of the initial operations for the SXI. We also report on a couple of issues we faced during the initial operations and lessons learned from them.

astro-ph.IM↗

arXiv2025

Soft X-ray Imager of the Xtend system onboard XRISM

The Soft X-ray Imager (SXI) is the X-ray charge-coupled device (CCD) camera for the soft X-ray imaging telescope Xtend installed on the X-ray Imaging and Spectroscopy Mission (XRISM), which was adopted as a recovery mission for the Hitomi X-ray satellite and was successfully launched on 2023 September 7 (JST). In order to maximize the science output of XRISM, we set the requirements for Xtend and find that the CCD set employed in the Hitomi/SXI or similar, i.e., a $2 \times 2$ array of back-illuminated CCDs with a $200~μ$m-thick depletion layer, would be practically best among available choices, when used in combination with the X-ray mirror assembly. We design the XRISM/SXI, based on the Hitomi/SXI, to have a wide field of view of $38' \times 38'$ in the $0.4-13$ keV energy range. We incorporated several significant improvements from the Hitomi/SXI into the CCD chip design to enhance the optical-light blocking capability and to increase the cosmic-ray tolerance, reducing the degradation of charge-transfer efficiency in orbit. By the time of the launch of XRISM, the imaging and spectroscopic capabilities of the SXI has been extensively studied in on-ground experiments with the full flight-model configuration or equivalent setups and confirmed to meet the requirements. The optical blocking capability, the cooling and temperature control performance, and the transmissivity and quantum efficiency to incident X-rays of the CCDs are also all confirmed to meet the requirements. Thus, we successfully complete the pre-flight development of the SXI for XRISM.

astro-ph.IM↗

arXiv2025

NuSTAR detection of a hot stellar superflare with a temperature of 95 MK in hard X-rays

A search of the hard X-ray archive data of NuSTAR found a transient source, NuSTAR J230059+5857.4, during an observation of 1E 2259+586 on 2013 April 25. A multi-wavelength analysis using X-ray, optical, and IR data, mostly taken in its quiescent phase, was conducted to identify the origin of NuSTAR J230059+5857.4 and elucidate the phenomena associated with the flare activity. The results indicated that NuSTAR J230059+5857.4 was a stellar flare that occurred on a single M-dwarf, M-dwarf binary, or pre-main-sequence star. NuSTAR J230059+5857.4 exhibited the higher emission measure and higher temperature, 8.60+2.15/-1.73x10^54 cm^-3 and 8.21+2.71/-1.86 keV, respectively, on average than the nominal values of stellar flares reported in the past. The flare loop size estimated on the basis of the model to balance the plasma and magnetic pressures was larger than the stellar radius by a factor of several. Since based on solar flare loops, this flare loop scale is excessively large, we conjecture that the observed large emission measure is possible to be attributed to the observation of mutually-associated multiple flares simultaneously occurring on the stellar surface, known as sympathetic flares. Thanks to the large effective area of NuSTAR in the hard X-ray band, we can conduct detailed discussion about a temperature variation associated with the flare. Investigation of the temperature variation during the flare revealed that the temperature remained significantly higher than during the quiescent phase even after the count rate dropped to around 5% of the peak. The sustained high temperature over the long duration is consistent with the idea of sympathetic flares. We found that it is essential to use theoretical models to evaluate loops and assess temporal changes in temperature as done in this study to determine whether there are multiple flares or not when analyzing flare observation data.

astro-ph.SR↗