The Extreme-ultraviolet Variability Experiment, or EVE, was never designed to measure Doppler velocities. Its mission is to track how the Sun’s extreme ultraviolet output varies over time, information that matters because that radiation is the primary energy input to Earth’s ionosphere and thermosphere. EVE observes the full solar disk from 6 to 106 nanometers with a spectral resolution of just 0.1 nanometers, using its Multiple EUV Grating Spectrographs, known as MEGS. At that modest resolution, detecting the tiny wavelength shifts produced by moving plasma would seem impossible. Yet two fortunate circumstances changed the calculus: the SDO spacecraft holds its solar pointing with exceptional stability, and its geosynchronous orbit provides an unusually stable thermal environment, keeping the instrument’s wavelength scale steady enough to capture genuine solar motions.

The new data product fits Gaussian profiles to 70 carefully selected emission features, each originating from a distinct layer of the solar atmosphere. Roughly half of the lines come from the transition region, a thin boundary layer where temperatures climb from about 25,000 kelvin to 0.6 million kelvin, and half come from the corona above, where temperatures exceed 0.6 million kelvin and can surpass 6 million kelvin during flares. The fitting algorithm models each feature of interest with a Gaussian plus two additional Gaussians for blended neighboring lines and a linear background, yielding fitted values for intensity, wavelength center, and line width. When the GOES X-Ray Sensor detects a flare, the algorithm automatically subtracts a pre-flare spectrum to isolate the flare’s own spectral signature, flagging those events in the data product.

Converting wavelength shifts to velocities is straightforward in principle: the shift between the flare spectrum and the pre-flare spectrum, multiplied by the speed of light and divided by the reference wavelength, gives the line-of-sight velocity. Positive values are red shifts, indicating plasma receding from the observer, which for flares near disk center means downflow. Negative values are blue shifts, indicating approaching plasma, or upflow. The team emphasizes that measured pre-flare wavelengths, rather than theoretical values from the CHIANTI spectral database, must serve as the reference, because small systematic offsets in the instrument’s wavelength scale would otherwise contaminate the result.

Not every spectral line is trustworthy for this work. Many EUV features are blends of multiple emissions at MEGS resolution, and different lines dominate at different plasma temperatures, so a blended feature can shift spuriously as solar activity changes. The team used CHIANTI model spectra for quiet Sun, active region, and flare conditions to estimate this blend uncertainty for each of the 70 features, expressing it as a velocity error. Forty-two lines, nine from MEGS-A and thirty-three from MEGS-B, came in below the 30 kilometers-per-second threshold and are flagged as the best choices for studying flare dynamics. The rest carry blend errors large enough to swamp any genuine solar signal.

Perhaps the most technically demanding part of the study involved disentangling an optical artifact from real solar physics. The MEGS-B instrument uses two Rowland-circle spectrographs in tandem, and because its CCD sensor is flat rather than curved along the Rowland circle, most wavelengths are slightly out of focus. That defocus makes the measured wavelength scale sensitive to where an active region sits on the solar disk. Raytrace modeling of the original optical design confirmed that active regions near the east limb produce wavelength shifts in one direction, west-limb regions in the opposite direction, with the sign flipping between central and outer wavelengths. Earlier reports of surprisingly fast prograde-rotation velocities of about 50 kilometers per second in coronal lines, published by Hudson and colleagues in 2022, turn out to be an artifact of this optical behavior rather than a genuine solar flow.

To validate and calibrate the correction, the team exploited a rare window in April 2019 near the minimum of solar cycle 24, when a single active region, NOAA 12738, crossed the disk alone over six days. By subtracting a spectrum from a spotless day to remove the full-disk contribution, and then fitting the remaining active-region spectrum, the researchers measured wavelength shifts that matched a tuned raytrace model remarkably well. The tuning revealed that the Sun’s center sits about 2.5 arc-minutes east of MEGS-B’s optical center, a misalignment inherited from the compromise positioning of EVE’s three channels and the jolts of launch. The resulting correction equations, expressed as parabolic functions of wavelength scaled by flare position, are now available for users of the data product, though the corrections are not applied automatically because the processing pipeline does not know each flare’s location.

The payoff comes in the flare statistics. For the X2.2 flare of 15 February 2011, the first X-class event of the SDO mission, all chromospheric, transition region, and cool coronal features below 1 million kelvin showed maximum red shifts averaging 75 plus or minus 24 kilometers per second, while hotter coronal lines showed blue shifts averaging minus 114 plus or minus 75 kilometers per second, with the Fe XIV line at 21.14 nanometers reaching an extraordinary minus 213 kilometers per second. The X9.0 flare of 3 October 2024 told a similar story, with downflows averaging 37 plus or minus 16 kilometers per second and upflows averaging minus 94 plus or minus 58 kilometers per second. In both cases, and across a broader sample of 15 disk-center X-class flares, the velocity maxima almost always occurred during the impulsive phase, the brief interval when magnetic reconnection releases energy most furiously.

These patterns match the standard picture of flare physics, often called the CSHKP model, in which reconnection drives chromospheric evaporation upward into hot coronal loops while cooler material drains downward. The transition from red shifts to blue shifts between 1 and 2 million kelvin, first noted by Milligan and Dennis in 2009 using Hinode data, appears clearly in the EVE results. The findings also align with earlier EVE-based analyses, including Hudson and colleagues’ 2011 report of a 50 kilometers-per-second red shift in He II and a 100 kilometers-per-second blue shift in Fe XXIV, and Otsu and Asai’s 2024 detection of a dramatic minus 400 kilometers-per-second blue shift during a filament eruption. For limb flares, the team found that even after location corrections, coronal lines still blue-shift by about 150 kilometers per second relative to the gradual phase, hinting at non-radial outflows that may accompany coronal mass ejections.

The practical lesson for solar physicists is that the EVE Level 4 Lines product, despite the instrument’s modest resolution and full-disk field of view, opens a routine path to measuring flare plasma dynamics across an entire solar cycle. Users must apply the optical wavelength-shift correction based on flare location for anything away from disk center, or risk mistaking instrumental artifacts for solar flows of 50 to 200 kilometers per second. But with 42 X-class flares already analyzed for MEGS-A and 103 for MEGS-B, and countless smaller events awaiting study, the new data product promises to deepen understanding of the magnetic explosions that drive space weather, disturb satellite orbits, and occasionally paint auroras across skies far from the poles.

Subject of Research: Measurement of solar flare Doppler velocities using the SDO EVE Level 4 Lines data product

Article Title: Solar Doppler Velocity Results from the SDO EVE Level 4 Lines Data Product

Article References: Solar Doppler Velocity Results from the SDO EVE Level 4 Lines Data Product. (n.d.). https://doi.org/10.1007/s11207-026-02727-w

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02727-w

Keywords: solar flares, Doppler velocity, SDO, EVE, extreme ultraviolet, corona, transition region, space weather, magnetic reconnection, spectroscopy, Solar Dynamics Observatory, MEGS

X-ray binaries are among the brightest non-explosive beacons in the universe. In these systems, a compact object, either a black hole, a neutron star or a white dwarf, strips gas from a companion star and heats it to millions of degrees as it spirals inward, outshining the Sun by factors of millions. Typical X-ray binaries radiate most of their energy above 0.3 kiloelectronvolts, an energy range where missions such as ROSAT, XMM-Newton and Chandra have been highly sensitive. Cooler accreting sources, whose emission peaks in the extreme ultraviolet between roughly 912 and 100 angstroms, present a fundamentally harder problem: interstellar hydrogen absorbs extreme-ultraviolet photons efficiently across most of the sky, and no dedicated extreme-ultraviolet survey mission has operated since the 1990s. As a result, sources whose output is concentrated in this band have been systematically undercounted, despite theoretical predictions that they should exist in substantial numbers.

The new study took a direct approach to this gap. Rather than relying on standard survey catalogs, which typically classify sources using hardness ratios computed over conventional X-ray bands, the team reprocessed Chandra observations of nearby galaxies and specifically searched for point-like, non-nuclear sources detected primarily or exclusively in the 0.15 to 0.3 kiloelectronvolt band. This required careful accounting for a subtle instrumental effect: Chandra’s sensitivity at the softest energies has gradually declined over the decades since its launch, particularly in the lowest energy channels. By quantifying this evolution using the galaxy cluster Abell 1795 as a calibration reference, and fitting the decline in soft-band count rates as a function of observing epoch, the researchers ensured that a source appearing faint in soft X-rays in an early observation was not simply a casualty of changing detector response.

What emerged from the search was a population of sources unlike anything in the standard X-ray binary zoo. The most luminous examples radiate close to the canonical 10^38 erg per second Eddington-level output associated with accretion onto a stellar-mass compact object, but they do so in a band so narrow that standard surveys, tuned to harder photons, often register nothing at all. Spectral modeling indicates that the observed X-ray emission is only the tip of the iceberg. For blackbody temperatures in the range implied by the observed colors, the bolometric correction is large, meaning that the total energy output, most of which emerges in the extreme ultraviolet, is likely several times higher than what the X-ray band alone reveals. For accretion-disk models the correction is estimated at roughly three to four times larger than for a pure blackbody at comparable temperatures. By this measure, hypersoft sources rank among the most energetic steady objects in their host galaxies.

The galaxies hosting the newly identified sources include well-observed nearby systems such as NGC 3379 and NGC 4472, two elliptical galaxies in the Virgo region with deep Chandra monitoring records, as well as the spiral galaxy M101. In NGC 4472, repeated observations across three different epochs revealed at least two hypersoft sources displaying recurrent or potentially persistent behavior, demonstrating that these are not one-off flares but stable or repeatable emitters. Variability analysis of the sources in NGC 3379 likewise shows that their soft emission persists on timescales long enough to be captured multiple times, ruling out the possibility that they are transient artifacts or background fluctuations. Their point-like morphology and off-nuclear locations distinguish them from diffuse hot gas and from active galactic nuclei, placing them squarely in the category of compact accreting binaries.

The physical nature of these sources remains an open question, but the authors propose that hypersoft sources represent X-ray binaries spanning several classes rather than a single type of object. One leading possibility is that many are accreting white dwarfs, including post-nova systems in which a white dwarf continues to burn hydrogen stably or quasi-stably on its surface after a classical nova eruption. Such systems are of special interest because sufficiently massive accreting white dwarfs are considered strong candidate progenitors of type Ia supernovae, the standardizable candles used to measure cosmic expansion. The classical supersoft sources discovered by ROSAT in the early 1990s in the Large Magellanic Cloud and M31 fit this general picture, with effective temperatures near a few tens of electronvolts, but the new hypersoft population appears to extend to even softer spectra, suggesting either lower temperatures, higher absorbing columns, or different accretion geometries than previously cataloged supersoft sources.

Accreting black holes may also inhabit the hypersoft class. Theoretical work on ultraluminous X-ray sources has long anticipated that super-Eddington or near-Eddington accretion onto stellar-removal black holes could produce cool, disk-dominated spectra peaking at extreme-ultraviolet energies, and at least one ultraluminous ultraviolet source has been directly detected in a nearby galaxy. If a meaningful fraction of hypersoft sources turn out to host black holes, they would provide a new window on the physics of accretion at the softest observable energies and could refine population models of stellar-mass black holes in galaxies. Conversely, if most are white-dwarf systems, they would constrain the rates at which type Ia supernova progenitors evolve in both old and young stellar populations, a longstanding puzzle given that elliptical galaxies like NGC 3379 and NGC 4472 host these sources despite lacking recent star formation.

Beyond their identity, hypersoft sources may matter for galaxy evolution in a more diffuse way. The extreme-ultraviolet photons that dominate their output are capable of ionizing helium and other species in the surrounding interstellar medium, and recent theoretical work has argued that supersoft sources contribute significantly to nebular He II line emission in star-forming galaxies, a spectral feature whose origin has long been debated. If hypersoft sources are as numerous as the new survey technique suggests, their cumulative ionizing output could help explain puzzling emission lines observed in nearby galaxies and might even leave an imprint on the absorption signatures seen in the spectra of very distant, young star-forming galaxies observed by the James Webb Space Telescope. In effect, these dim-seeming objects could be quiet but consequential players in the energy budget of the interstellar gas.

The discovery also carries a practical lesson for observational astronomy. Hypersoft sources evaded detection not because they are rare or faint but because standard survey pipelines, calibrated to the energy bands where most X-ray binaries shine, were effectively blind to them. The researchers point out that Chandra’s declining soft-band sensitivity means that future searches should prioritize the earliest, deepest archival observations, when the observatory’s extreme-ultraviolet-adjacent response was at its best. All of the data and code underlying the new catalog have been released through the Chandra Data Archive and Zenodo, allowing other teams to extend the hunt to additional galaxies. As follow-up observations with optical telescopes, ultraviolet missions and eventually next-generation X-ray observatories home in on individual hypersoft sources, astronomers may find that a significant fraction of the universe’s luminous accreting binaries has been waiting, softly glowing, just below the threshold of notice.

Subject of Research: Discovery of hypersoft X-ray sources, a new low-energy class of luminous accreting cosmic emitters

Article Title: Hypersoft X-ray sources as a low-energy class of luminous cosmic emitter

Article References: Muhibullah, M., Irwin, J. A., & Di Stefano, R. (2026). Hypersoft X-ray sources as a low-energy class of luminous cosmic emitter. Nature Astronomy. https://doi.org/10.1038/s41550-026-02959-7

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02959-7

Keywords: hypersoft X-ray sources, X-ray binaries, extreme ultraviolet, accreting white dwarfs, type Ia supernovae, Chandra X-ray Observatory, black holes, NGC 4472, NGC 3379, M101, supersoft sources, galaxy evolution