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Hiroyuki Maehara

Publications and source records attributed to Hiroyuki Maehara.

At least 19 recordsLinked to original sources
arXiv2026

Systematic and Statistical Properties of Prominence Eruptions on the M-dwarf YZ CMi

M-dwarfs produce frequent flares, and their associated mass ejections are expected to significantly affect the habitability of close-in exoplanets. Recent spectroscopic observations have revealed several prominence eruptions-indicative of stellar mass ejections-on M-dwarfs through Doppler shifts of the H$\alpha$ line. However, systematic and statistical studies, particularly regarding their association with white-light flares, have been limited due to the lack of intensive and continuous simultaneous photometric and spectroscopic monitoring of the same target star. We conducted one month of continuous spectroscopic observations of the active M-dwarf YZ CMi using the 3.8-m Seimei telescope with an unprecedentedly high time cadence of $\sim$1 min, simultaneously with TESS. We detected four prominence eruptions, among which two events showed rapid, short-duration eruptions with velocities of $\sim$300-500 km s$^{-1}$ and durations of $\sim$5 min. Such short-duration events may have been missed in previous observations due to insufficient time cadence. We further performed a systematic analysis using 35 H$\alpha$ flares on YZ CMi observed simultaneously with TESS. Notably, most prominence eruptions (6 out of 7) were not associated with detectable white-light flares. This result suggests that most observed prominence eruptions on M-dwarfs may have occurred near the stellar limb, where white-light flares are difficult to detect, which may imply a potential observational bias in their detectability due to low contrast with the background emission. These first statistical constraints, together with the discovery of rapid, short-duration prominence eruptions, indicate that previous observations may have underestimated both the frequency and velocities of mass ejections on M-dwarfs due to observational biases and highlight the necessity of reassessing their impact on close-in exoplanets.

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arXiv2026

New assignments of the CaH $A^{2}Π-X^{2}Σ^{+}$ transitions in the sunspot umbral spectrum and effective temperature estimation using molecular lines

High-resolution spectroscopy observations of sunspots offer a unique natural laboratory for detailed molecular spectroscopy. Calcium monohydride (CaH) is a vital spectroscopic tracer in cool stellar and solar environments, where its electronic transitions are used for line identification and temperature diagnostics, yet its high-excitation transitions remain poorly characterized. Using high-resolution sunspot umbral spectra obtained with the Brault Fourier-transform spectrometer at the McMath-Pierce telescope (Wallace et al. 1999), we investigated the $A^2Π\;-\;X^2Σ^+$ electronic transition of CaH in the $\rm{14400-14900\;cm^{-1}}$ region. We report the assignment of 224 spectral lines spanning vibrational bands with $v^{\prime}, v^{\prime\prime} = 0\text{-}3$; notably, 75 lines in the (3-3) band are reported here for the first time, and the identification of the remaining bands was extended to significantly higher rotational quantum numbers ($J\rm{ _{max}\approx 49.5}$) compared to previous studies. Incorporating these new identifications into spectral simulations that account for overlapping TiO features, we aimed to reproduce the observed umbral spectrum and found modest but measurable improvement in the agreement between the simulated and observed spectra. Minimizing the residuals between observed and simulated spectra, we estimated an effective umbral temperature range of around 4000 K. We compare this effective temperature against those inferred from independent methods to show that molecular features primarily form in the cool umbral cores of the sunspots. This study demonstrates the utility of solar observations in studying the high-energy transitions that are challenging to reproduce in laboratory settings and the potential of molecular lines as sensitive "thermometers" for cool astronomical objects.

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arXiv2026

Discovery of a compact hierarchical triple main-sequence star system while searching for binary stars with compact objects

We have discovered a compact hierarchical triple main-sequence star system, which is cataloged as Gaia DR3 1010268155897156864 or TIC 21502513. Hereafter, we call it ``G1010''. G1010 consists of a primary (the most massive) star and inner binary that orbit each other. The primary star is a $0.85_{-0.03}^{+0.03}\;{\rm M}_\odot$ main-sequence (MS) star, and the inner binary components are $0.63_{-0.02}^{+0.02}$ and $0.61_{-0.02}^{+0.02}\;{\rm M}_\odot$ MS stars. The outer and inner orbital periods are $277.2_{-1.3}^{+1.6}$ and $\sim 18.26$ days, respectively. G1010 is categorized as a single-lined spectroscopic binary, and its orbital solution indicates that G1010 possibly accompanies a massive compact object, such as a neutron star or massive white dwarf. In order to confirm the presence of a massive compact object, we have performed several-times low signal-to-ratio (SNR) and one-time high SNR spectroscopic observations, and determined the outer orbital parameters. Moreover, we have deeply analyzed the high SNR spectroscopic data, and found that G1010 accompanies not a massive compact object, but an inner binary. We have investigated G1010's light curve in Transiting Exoplanet Survey Satellite (TESS), and concluded that the inner binary is actually an eclipsing binary, not included in TESS Eclipsing Binary Stars. We have obtained the inner orbital parameters from the TESS light curve. G1010 is similar to compact hierarchical triple star systems previously discovered by eclipse timing variation analysis. Our discovery has shown that such triple star systems can be discovered by combination of low- and high-SNR spectroscopic observations with the help of Gaia DR3 and the upcoming Gaia DR4/DR5.

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arXiv2026

The Preliminary Mauve Science Programme: Science themes identified for the first year of operations

Mauve is a low-cost small satellite developed and operated by Blue Skies Space Ltd. The payload features a 13 cm telescope connected with a fibre that feeds into a UV-Vis spectrometer. The detector covers the 200-700 nm range in a single shot, obtaining low resolution spectra at R~20-65. Mauve has launched on 28th November 2025, reaching a 510 km Low-Earth Sun-synchronous orbit. The satellite will enable UV and visible observations of a variety of stellar objects in our Galaxy, filling the gaps in the ultraviolet space-based data. The researchers that have already joined the mission have defined the science themes, observational strategy and targets that Mauve will observe in the first year of operations. To date 10 science themes have been developed by the Mauve science collaboration for year 1, with observational strategies that include both long duration monitoring and short cadence snapshots. Here, we describe these themes and the science that Mauve will undertake in its first year of operations.

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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.

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arXiv2026

Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. III. Comparison between Starspot Mapping, Zeeman Doppler Imaging, and Multiwavelength Variability

Recent simultaneous multiwavelength observations of a nearby young solar-type star EK Dra in the optical, H$α$ spectrum, and X-ray, have provided evidence for stellar prominence eruptions associated with superflares. The large prominence eruption is suggested to have been caused by a large mid-latitude spot on the polarity inversion lines near the stellar limb from the concurrent Zeeman Doppler Imaging (ZDI) and optical photometry by the TESS. In this study, we perform starspot mapping for the TESS data of EK Dra to investigate the relation of starspots and magnetic fields from the photometry and ZDI. We also explore the multiwavelength rotational variability ascribed to starspots and active regions for the TESS, B-band, H$α$, and X-ray light curves. As a result, we find that (i) spot locations deduced from the TESS light curve are mostly consistent with the intensity map from the ZDI except for a polar spot, and (ii) the H$α$ light curve exhibits clear periodicity with respect to the TESS light curve because the H$α$ line is radiated around spots in the chromosphere. The X-ray light curve does not show such association probably because of multiple spots on high activity level and extended spatial structure of coronal active regions. The results provide clues to explore their association with stellar flares at different heights of active regions in chromospheric and coronal lines. Our study also enables us to quantify the stellar XUV radiation from the magnetic fields of active stars toward understanding atmospheric evolution of exoplanets.

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arXiv2025

Discovery of multi-temperature coronal mass ejection signatures from a young solar analogue

Coronal mass ejections (CMEs) on the early Sun may have profoundly influenced the planetary atmospheres of early Solar System planets. Flaring young solar analogues serve as excellent proxies for probing the plasma environment of the young Sun, yet their CMEs remain poorly understood. Here we report the detection of multi-wavelength Doppler shifts in Far-Ultraviolet (FUV) and optical lines during a flare on the young solar analog EK Draconis. During and before a Carrington-class ($\sim$10$^{32}$ erg) flare, warm FUV lines ($\sim$10$^5$ K) exhibit blueshifted emission at 300-550 km s$^{-1}$, indicative of a warm eruption. 10 minutes later, the H$α$ line shows slow (70 km s$^{-1}$), long-lasting ($\gtrsim$2 hrs) blueshifted absorptions, suggesting a cool ($\sim$10$^4$ K) filament eruption. This provides evidence of multi-temperature and multi-component nature of a stellar CME. If Carrington-class flares/CMEs occurred frequently on the young Sun, they may have cumulatively impacted the early Earth's magnetosphere and atmosphere.

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arXiv2025

Do Young Suns Produce Frequent, Massive CMEs? Results from Five-year Dedicated Optical Observations of EK Draconis and V889 Hercules

We report results from a five-year (132-night) dedicated observational campaign targeting two nearby young solar-type stars, EK Draconis ($\sim$50-125 Myr age) and V889 Hercules ($\sim$30 Myr age), using the 3.8m Seimei Telescope and Transiting Exoplanet Survey Satellite. The aim is to observationally constrain statistical properties of flaring radiation/heating as well as coronal mass ejections (CMEs), through high time-cadence H$α$ spectroscopy. We obtained an unprecedented sample of 15 H$α$ superflares, including two blueshifted absorption, two blueshifted emission, one redshifted emission, and nine line broadening events. We obtain the following results: (1) Larger flares exhibit broader H$α$ line widths, up to 14.1$_{\pm 2.4}$ Å, indicating higher chromospheric heating than solar flares. (2) The long-lasting redshifted event at $\sim$100 km s$^{-1}$ may indicate dense post-flare loops. (3) H$α$ blueshifted absorptions/emissions provide evidence of massive filament/prominence eruptions, the core structures of CMEs. One newly identified event showed an unexpected rapid decrease in velocity. (4) The lower limit of the CME/eruption association rate with superflares is 27$_{-16}^{+25}$%, yielding occurrence rates of 0.21$_{\pm0.12}$ and $<$0.32$^{+0.46}_{-0.32}$ events per day for EK Draconis and V889 Hercules, respectively. (5) We derived the first direct estimate of the lower limit of the mass-loss rate driven by super-CMEs ($\gtrsim10^{33}$ erg) for EK Dra as $4 \times (10^{-13}$-$10^{-12})$ $M_{\odot}$ yr$^{-1}$, comparable to the stellar wind mass loss at a similar age. This study provides critical observational constraints on the radiation and plasma environment around young solar-type stars and the early Sun, which can drive planetary space weather and stellar mass/angular momentum loss.

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arXiv2025

Time evolution of white-light flare accompanied by probable postflare loops on M-type dwarf EV Lacertae

White-light flares are explosive phenomena accompanied by brightening of continuum from near-ultraviolet to optical, which occur on the Sun and stars. In order to investigate the mechanism of white-light flares, we carried out simultaneous optical photometry (TESS : 6000-10000 Å) and spectroscopy (Seimei Telescope : 4100-8900 Å) of a M-dwarf EV Lac on 2019 September 14. We detected a flare with high-time-cadence ($\sim 50$ sec) spectroscopic observation. At the peak, the continuum of the flare component is well fitted by a blackbody spectrum with temperature of $T = 8122 \pm 273$ K, which is comparable with the results of previous studies that reported the spectral energy distribution of near-ultraviolet to optical during the flare could be approximated by single-temperature blackbody radiation at $T \sim 10^{4}$ K. We also estimated the time evolution of the flare temperature during the decay phase. The radiative energy of this flare within the optical range is $4.4 \times 10^{32}$ erg, taking into account the time-dependent variation in the decreasing flare temperature and expanding flare area. Furthermore, we detected a delayed increase in the flux of H$α$ after the photometric flare peak, secondary increase, and gradual increase even after the white-light flare ended. Comparison of our results with light curves obtained by the Sun-as-a-star analysis of solar flares indicates that these signals may be due to postflare loops near the stellar limb. Our result about time evolution of white-light continuum will help to gain more insight into the mechanism of white-light flares both on the Sun and stars. Additionally, since extreme ultraviolet radiation from flare loops plays a key role in planetary atmospheric escape, the existence of postflare loops on stellar flares and its time evolution will help future studies about habitability of close-in planets.

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arXiv2025

High-Time-Cadence Spectroscopy and Photometry of Stellar Flares on M-dwarf YZ Canis Minoris with Seimei Telescope and TESS. II. Statistical Properties of Blue/Red Asymmetries in the H$α$ Line

M-dwarfs frequently produce flares, and their associated coronal mass ejections (CMEs) may threaten the habitability of close-in exoplanets. M-dwarf flares sometimes show prominence eruption signatures, observed as blue/red asymmetries in the H$α$ line. In Paper I, we reported four candidates of prominence eruptions, which shows large diversity in their durations and velocities. In this study, we statistically investigate how blue/red asymmetries are related with their flare and starspot properties, using the dataset from 27 H$α$ flares in Paper I and previously reported 8 H$α$ flares on an M-dwarf YZ Canis Minoris. We found that these asymmetry events tend to show larger H$α$ flare energies compared to non-asymmetry events. In particular, 5 out of 6 blue asymmetry events are not associated with white-light flares, whereas all 7 red asymmetry events are associated with white-light flares. Furthermore, their starspot distributions estimated from the TESS light curve show that all prominence eruption candidates occurred when starspots were located on the stellar disk center as well as on the stellar limb. These results suggest that flares with lower heating rates may have a higher association rate with prominence eruptions and/or the possibility that prominence eruptions are more detectable on the limb than on the disk center on M-dwarfs. These results provide significant insights into CMEs that can affect the habitable world around M-dwarfs.

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arXiv2025

Temporal Variation of Flare Occurrence Rates via the Spot Evolution on the Sun and Solar-type Stars

The spot evolution on the Sun and solar-type stars is important for understanding the nature of consequential flaring activity. This study statistically investigates the variance of flare occurrence rate through the time evolution of spots on the Sun and solar-type stars. We have compiled the 28-year catalogs of solar flares and their source sunspots obtained from solar surface observations by NOAA and GOES for the Sun. Also, we combined the cataloged stellar flares with the time evolution of starspots estimated by light curves obtained by the 4-year Kepler mission for solar-type stars. For the obtained 24124 solar flares and 180 stellar flares, we calculate the flare occurrence distribution with respect to $t_\mathrm{flare}-t_\mathrm{max}$, which represents the timing of flare through the spot evolution, where $t_\mathrm{flare}$ is the flare occurrence time, and $t_\mathrm{max}$ is the time when the source spot takes its maximum area. When normalized by the spot lifetime, we found that the flare occurrence distribution for $t_\mathrm{flare}-t_\mathrm{max}$ shows a similar distribution regardless of spot size or flare energy, suggesting that the Sun and the solar-type star share the same physical process in the spot-to-flare activity. On this basis, we propose a formula for the time variation of the flare occurrence rate per spot. Also, the correlation between the temporal variation of flare occurrence rate and the time evolution of spot area and the lack of difference in flare occurrence rate between the emergence and decaying phases provide a milestone for the nature of flare-productive spots.

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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.

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arXiv2024

Multiwavelength Observations of the Black Hole X-ray Binary MAXI J1820$+$070 in the Rebrightening Phase

We report the results of quasi-simultaneous multiwavelength (near-infrared, optical, UV, and X-ray) observations of the Galactic X-ray black hole binary MAXI J1820+070 performed in 2019 May 10-13, $\sim 60$ days after the onset of the first rebrightening phase. It showed a much larger optical-to-X-ray luminosity ratio ($\sim 8$) than in the initial outburst epoch. The primary components of the spectral energy distribution (SED) can be best interpreted by radiatively inefficient accretion flow (RIAF) spectrum showing a luminosity peak in the optical band. By comparison with theoretical calculations, we estimate the mass accretion rate to be $\dot{M}/(8 L_{\rm Edd}/c^2) \sim 10^{-3}$, where $c$ is the light speed and $L_{\rm Edd}$ is the Eddington luminosity. In addition to the RIAF emission, a blue power-law component is detected in the optical-UV SED, which is most likely synchrotron radiation from the jet. The optical spectrum taken at the Seimei telescope shows a weak and narrow H$α$ emission line, whose emitting region is constrained to be $\gtrsim 2 \times 10^{4}$ times the gravitational radius. We suggest that the entire disk structure cannot be described by a single RIAF solution but cooler material responsible for the H$α$ emission must exist at the outermost region.

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arXiv2024

Evolution of Accretion Disk Structure of the Black Hole X-ray Binary MAXI J1820$+$070 during the Rebrightening Phase

To understand the evolution of global accretion disk structure in the ``rebrightening'' phase of MAXI J1820$+$070, we perform a comprehensive analysis of its near infrared/optical/UV to X-ray spectral energy distribution (SED) utilizing data obtained by OISTER, Las Cumbres Observatory (LCO), Swift, NICER, and NuSTAR in 2019. Optical spectra observed with Seimei telescope in 2019 and 2020 are also analyzed. On the basis of the optical and X-ray light curves and their flux ratios, we divide the whole phase into 3 periods, Periods I (flux rise), II (decay), and III (dim). In the first 2 periods, the source stayed in the low/hard state (LHS), where the X-ray (0.3--30 keV) and optical/UV SED can be both fitted with power-law models. We interpret that the X-ray emission arises from hot corona via Comptonization, whereas the optical/UV flux is dominated by synchrotron radiation from the jets, with a partial contribution from the irradiated disk. The optical/UV power-law component smoothly connects to a simultaneous radio flux, supporting its jet origin. Balmer line profiles in the optical spectra indicate that the inner radius of an irradiated disk slightly decreased from $\sim 2\times 10^5 r_{\rm g}$ (Period I) to $\sim 1\times 10^5 r_{\rm g}$ (Period II), where $r_{\rm g}$ is the gravitational radius, implying a change of the hot corona geometry. In Period III, the SED can be reproduced by an advection-dominated accretion flow and jet emission. However, the double-peaked H$α$ emission line indicates that a cool disk remained at large radii.

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arXiv2024

Sun-like stars produce superflares roughly once per century

Stellar superflares are energetic outbursts of electromagnetic radiation, similar to solar flares but releasing more energy, up to $10^{36}$ erg on main sequence stars. It is unknown whether the Sun can generate superflares, and if so, how often they might occur. We used photometry from the Kepler space observatory to investigate superflares on other stars with Sun-like fundamental parameters. We identified 2889 superflares on 2527 Sun-like stars, out of 56450 observed. This detection rate indicates that superflares with energies $>10^{34}$ erg occur roughly once per century on stars with Sun-like temperature and variability. The resulting stellar superflare frequency-energy distribution is consistent with an extrapolation of the Sun's flare distribution to higher energies, so we suggest that both are generated by the same physical mechanism.

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arXiv2024

Smart Kanata: A Framework for Autonomous Decision Making in Rapid Follow-up Observations of Cataclysmic Variables

Studying the early stages of transient events provides crucial information about the fundamental physical processes in cataclysmic variables (CVs). However, determining an appropriate observation mode immediately after the discovery of a new transient presents challenges due to significant uncertainties regarding its nature. We developed a framework designed for autonomous decision making in prompt follow-up observations of CVs using the Kanata 1.5-m telescope. The system, named Smart Kanata, first estimates the class probabilities of variable star types using a generative model. It then selects the optimal observation mode from three possible options based on the mutual information calculated from the class probabilities. We have operated the system for ~300 days and obtained 21 samples, among which automated observations were successfully performed for a nova and a microlensing event. In the time-series spectra of the nova V4370 Oph, we detected a rapid deepening of the absorption component of the H_alpha line. These initial results demonstrate the capability of Smart Kanata in facilitating rapid observations and improving our understanding of outbursts and eruptions of CVs and other galactic transients.

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arXiv2024

High-Time-Cadence Spectroscopy and Photometry of Stellar Flares on M-dwarf YZ Canis Minoris with Seimei Telescope and TESS. I. Discovery of Rapid and Short-Duration Prominence Eruptions

M-dwarfs show frequent flares and associated coronal mass ejections (CMEs) may significantly impact close-in habitable planets. M-dwarf flares sometimes show red/blue asymmetries in the H$α$ line profile, suggesting prominence eruptions as an early stage of CMEs. However, their high-time-cadence observations are limited. We conducted spectroscopic monitoring observations of the active M-dwarf YZ Canis Minoris with $\sim$1 minute time cadence using the Seimei telescope, simultaneously with the optical photometric observations by Transiting Exoplanet Survey Satellite. We detected 27 H$α$ flares with H$α$ energies ranging from 1.7 $\times$ 10$^{29}$ to 3.8 $\times$ 10$^{32}$ erg and durations from 8 to 319 minutes. Among them, we identified 3 blue asymmetry and 5 red asymmetry events based on criteria using the Bayesian Information Criterion. The maximum velocity of the blue- and red-shifted components ranges from 250 to 450 km s$^{-1}$ and 190 to 400 km s$^{-1}$, respectively. The duration and time evolution show variety, and in particular, we discovered rapid, short-duration blue/red asymmetry events with the duration of 6--8 minutes. Among the 8 blue/red asymmetry events, two blue and one red asymmetry events are interpreted as prominence eruptions because of their fast velocity and time evolution. Based on this interpretation, the lower limit of occurrence frequency of prominence eruptions can be estimated to be $\sim$1.1 events per day. Our discovery of short-duration events suggests that previous studies with low time cadence may have missed these events, potentially leading to an underestimation of the occurrence frequency of prominence eruptions/CMEs.

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arXiv2024

Rising Near-Ultraviolet Spectra in Stellar Megaflares

Flares from M-dwarf stars can attain energies up to $10^4$ times larger than solar flares but are generally thought to result from similar processes of magnetic energy release and particle acceleration. Larger heating rates in the low atmosphere are needed to reproduce the shape and strength of the observed continua in stellar flares, which are often simplified to a blackbody model from the optical to the far-ultraviolet (FUV). The near-ultraviolet (NUV) has been woefully undersampled in spectral observations despite this being where the blackbody radiation should peak. We present Hubble Space Telescope NUV spectra in the impulsive phase of a flare with $E_{\rm{TESS}} \approx 7.5 \times 10^{33}$ erg and a flare with $E_{\rm{TESS}} \approx 10^{35}$ erg and the largest NUV flare luminosity observed to date from an M star. The composite NUV spectra are not well represented by a single blackbody that is commonly assumed in the literature. Rather, continuum flux rises toward shorter wavelengths into the FUV, and we calculate that an optical $T=10^4$ K blackbody underestimates the short wavelength NUV flux by a factor of $\approx 6$. We show that rising NUV continuum spectra can be reproduced by collisionally heating the lower atmosphere with beams of $E \gtrsim 10$ MeV protons or $E \gtrsim 500$ keV electrons and flux densities of $10^{13}$ erg cm$^{-2}$ s$^{-1}$. These are much larger than canonical values describing accelerated particles in solar flares.

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