The team’s solution was to abandon the traditional measurement strategies altogether and instead interrogate the nucleus through its own electrons, stripped down to a bare handful. In a study published in Nature Communications, the researchers performed high-precision extreme ultraviolet spectroscopy of highly charged ions, atoms from which dozens of electrons have been removed, using the Tokyo-EBIT facility at the University of Electro-Communications. By measuring the emission line energies of lutetium ions carrying charges of 59+ and 60+ and ytterbium ions carrying charges of 58+ and 59+, and comparing those energies against state-of-the-art atomic structure calculations, they extracted the tiny influence of nuclear size on atomic energy levels. The verdict: the lutetium nucleus is actually slightly smaller than that of ytterbium, exactly as the expected odd-even staggering predicts.

To understand why this measurement required such an exotic approach, it helps to consider what a nuclear charge radius actually is and how it normally gets measured. The nucleus is astonishingly small compared with the cloud of electrons that surrounds it, typically by a factor of tens of thousands. When an electron orbits far from the nucleus, as it does in a neutral atom or a singly charged ion, it spends almost no time inside the nuclear volume, so the finite size of the nucleus barely perturbs its energy. The resulting energy shift, known as the volume shift, is real but minuscule, and teasing it out of optical spectra has historically required exquisite laser spectroscopy and careful comparisons, usually within a single element across a chain of isotopes. Comparing radii between different elements was notoriously difficult, because the atomic physics differences between neighboring elements swamp the nuclear signal.

Highly charged ions change the calculus dramatically. When an atom is ionized many times, the remaining electrons are pulled into tight orbits that lie close to the nucleus, in some cases penetrating deep into the nuclear interior. There they directly sample the distribution of nuclear charge, and the volume shift is amplified enormously relative to what a neutral atom would show. A nucleus that is a few percent larger or smaller than its neighbor leaves a measurable fingerprint on the extreme ultraviolet emission lines of these stripped ions. In effect, the highly charged ion acts as an exquisitely sensitive probe that has been threaded directly into the nucleus, converting a nuclear property into an atomic wavelength that can be measured with spectroscopic precision.

The instrument that made this possible is Tokyo-EBIT, an Electron Beam Ion Trap developed at the University of Electro-Communications in the 1990s and refined over decades of operation. Inside the device, a monoenergetic electron beam is compressed by a strong magnetic field to high current density, creating a laboratory plasma unlike any natural one. When atoms of a chosen element are injected into this beam, they collide repeatedly with the fast electrons and are stripped of their electrons in successive steps, climbing the charge-state ladder until they reach the desired ionization stage. Tokyo-EBIT boasts world-class performance in both electron beam energy and current, which allowed the team to generate and observe lutetium and ytterbium ions in the extremely high charge states around 58+ to 60+, where only a handful of bound electrons remain.

The experimental protocol demanded unusual care. The team alternately recorded high-resolution extreme ultraviolet emission spectra of the lutetium and ytterbium ions, switching between elements so that instrumental drifts and plasma conditions would affect both measurements equally. The emission line energies of Lu59+, Lu60+, Yb58+, and Yb59+ were then compared line by line. Because the ions in each pair share nearly identical electronic configurations, the differences in their emission energies could be attributed almost entirely to the nuclear volume shift rather than to ordinary atomic structure effects. The latest high-precision atomic structure calculations served as the theoretical bridge, converting the observed energy differences into differences in nuclear charge radii with a rigor that earlier techniques could not match.

The result settles a puzzle that has lingered through generations of nuclear data compilations. Earlier evaluations, built on electron scattering experiments, muonic atom spectroscopy, and laser spectroscopy of neutral atoms and singly charged ions, had implied that lutetium’s nucleus was larger than ytterbium’s, an apparent reversal of the odd-even staggering that governs nuclear sizes across the rare-earth region. The new measurement shows that no reversal exists. The nucleus of lutetium, with its odd number of protons, is slightly smaller than the even-proton nucleus of ytterbium, in keeping with the zigzag pattern that reflects the pairing tendency of protons and neutrons inside the nucleus. The long-debated reversal problem, as it came to be known, has been resolved.

Yet the resolution of one puzzle opens the door to another. The charge radii of rare-earth elements obtained in this study not only call into question the picture built on conventional experimental data but also suggest that the odd-even effects in this region may be significantly more pronounced than current nuclear structure theories can fully explain. Deformed nuclei, those whose shapes deviate from spherical, dominate the rare-earth region, and their radii encode rich information about collective rotation, pairing correlations, and the interplay of protons and neutrons in a strongly interacting quantum system. If the staggering is stronger than theory predicts, then something important about the nuclear force and the pairing of nucleons in deformed systems remains to be understood, and the rare-earth nuclei are pointing the way.

What makes the demonstration especially significant is its generality. Because the method compares emission line energies of highly charged ions of different elements directly, it enables the kind of cross-element comparison that laser spectroscopy of neutral atoms and singly charged ions could never easily provide. The researchers plan to extend the technique beyond lutetium and ytterbium to neighboring rare-earth elements and to radioactive nuclides, mapping out whether the enhanced odd-even staggering is a general feature of the region or specific to particular nuclei. Systematic measurements across a chain of highly charged ions could deliver a new, self-consistent dataset on nuclear charge radii in the heavy-element region, precisely where deformed nuclei make the physics richest and the traditional methods most strained.

The work also represents a striking repurposing of a technique born in a different corner of physics. Highly charged ion spectroscopy was cultivated over decades to advance relativistic atomic physics and to support nuclear fusion science, where the extreme plasmas of fusion devices strip atoms to high charge states and demand accurate atomic data for diagnostics. The present study shows that the same machinery, together with refined methods of high-precision wavelength analysis, rare-earth element injection into the trap, and high-precision atomic structure calculation, can serve as a precision tool for nuclear structure research. The researchers intend to pursue this line vigorously, with applications extending to fusion plasma diagnostics, astrophysics, and the development of short-wavelength light sources. A puzzle about the size of two adjacent nuclei, probed by electrons that orbit within a whisper of the nuclear surface, has thus become a demonstration that the boundary between atomic and nuclear physics is thinner, and more useful, than anyone had fully appreciated.

Subject of Research: Measurement of nuclear charge radii in rare-earth elements using extreme ultraviolet spectroscopy of highly charged ions

Article Title: Exploring nuclear properties by atomic spectroscopy

Article References: Exploring nuclear properties by atomic spectroscopy. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: nuclear charge radii, highly charged ions, atomic spectroscopy, extreme ultraviolet, Tokyo-EBIT, lutetium, ytterbium, odd-even staggering, rare-earth nuclei, nuclear structure, deformed nuclei, volume shift

The appeal of pushing coherent control to short wavelengths is easy to understand. XUV and X-ray photons interact with electrons in ways that optical light cannot, probing inner-shell and site-selective excitations on femtosecond and even attosecond timescales. If researchers could control quantum interference at these energies, they would gain a handle on ultrafast electronic dynamics in atoms and molecules with unprecedented precision. The obstacle has been equally clear: at short wavelengths, intense light tends to rip electrons away entirely through photoionization, and any intermediate state created along the way decays rapidly. Producing two intense short-wavelength pulses with a stable, adjustable relative phase has also been a formidable technical challenge.

The team attacked both problems at the FERMI free-electron laser facility in Trieste, Italy. FERMI is a seeded free-electron laser, meaning its XUV pulses inherit the coherence of an external seed laser rather than emerging from the noisy spontaneous amplification that characterizes most such machines. In its two-colour mode, the seed laser imprints a coherent energy modulation on a high-energy electron beam, and two separate sets of undulators then amplify two different harmonics of the seed frequency. The result is a pair of XUV pulses whose photon energies are exact integer multiples of the seed energy and whose relative phase can be shifted at will using a phase shifter placed between the undulator sets.

The target was helium, chosen for its simplicity and for one particularly useful property. The researchers aimed to drive the atom from its ground state to the long-lived metastable state He(1s2s ¹S), which sits 20.615 electronvolts above the ground state but cannot be reached by a single dipole-allowed photon. Instead, the team used a two-photon stimulated Raman transition: a pump photon lifts the atom into the ionization continuum, and a Stokes photon of slightly lower energy brings it back down into the metastable state. Because the Raman resonance depends only on the difference between the two photon energies, the scheme works even though both photons individually carry far more energy than the excitation itself. By selecting harmonics whose orders differ by four, and tuning the seed laser to 5.154 electronvolts, the team satisfied the resonance condition exactly.

Detecting the outcome posed its own challenge. The two-photon Raman cross-section is tiny compared with the single-photon ionization cross-section, so the signal of interest is buried under a flood of photoelectrons and photoions. The researchers exploited a clever separation scheme: metastable helium atoms, carrying roughly 20 electronvolts of internal energy, liberate electrons when they strike a detector and are therefore registered by a position-sensitive multichannel plate, while ground-state atoms pass unnoticed. A small static electric field across the interaction region sweeps charged particles away from the atomic beam, so only neutral metastable atoms travel the roughly 500 microseconds to the detector. The measurement is thus essentially immune to the overwhelming ionization background.

The first results confirmed that a genuine stimulated Raman process was at work. Using the seventh and eleventh harmonics, circularly polarized with the same handedness as required by the dipole selection rules for a two-photon transition, the team measured a strong metastable yield on resonance. Flipping the helicity of one pulse, which renders the two-photon transition dipole-forbidden, collapsed the yield to a few percent of its former value, with the residue attributable to imperfect circular polarization. Sweeping the seed laser photon energy across the resonance for four different harmonic pairs produced clean, Gaussian-shaped Raman lines centred precisely at the predicted metastable excitation energy, with no measurable shift even at the highest pulse intensities.

Theoretical calculations underpinned the interpretation. The team solved the time-dependent Schrödinger equation for a single active electron in a model helium potential, capturing all nonlinearities associated with the strong fields. The calculations reproduced the measured lineshapes and revealed that the Stark shift of the resonance remains negligible up to an intensity product of about 10⁵ terawatts squared per square centimetre. They also uncovered a striking feature: at high intensities, where the ground state is almost completely depleted by ionization, the metastable population saturates just below one percent yet survives, because it is produced early in the pulse while ground-state atoms are still abundant and is far less susceptible to subsequent ionization than the ground state itself.

The centrepiece of the study, however, was the demonstration of interference between two Raman pathways. In a single two-colour Raman transition, all the phases of the fields and dipole moments cancel in the cross-section and are unobservable. But when two coherent pathways connect the same initial and final states through different continuum states, the pathways become indistinguishable and their relative phase enters the cross-section as a cosine modulation. The team created such a pair by exploiting a weak, unamplified harmonic that accompanies the two strong ones: the intense Stokes photon of the main pathway doubles as the pump photon of a second pathway, completed by the weak lower harmonic as its Stokes photon. Although the weak harmonic carries only about 0.1 percent of the pulse energy, the much larger Raman cross-section at lower photon energies compensates, making the second pathway competitive.

Scanning the phase of the upper harmonic with the FERMI phase shifter, the researchers watched the metastable yield oscillate periodically, a direct signature of quantum interference deep in the XUV. For the harmonic pair involving the fundamental seed laser as the weak Stokes photon, the fringe visibility reached about 50 percent, and time-dependent Schrödinger equation calculations using the experimental field strengths matched the measured modulation closely. The decisive control experiment followed: inserting an aluminium filter to block the fundamental radiation removed the second pathway entirely, and with it the modulation vanished. The interference, and the phase control it enables, was unambiguously real.

The achievement opens a route to phase-sensitive coherent control of atomic and molecular processes at short wavelengths, including site-selective core excitations and ultrafast electronic dynamics that optical control schemes cannot address. The authors point to particularly intriguing prospects near autoionizing resonances, where theory predicts dramatic Stark shifts and strong lineshape alterations that their experimental approach is ideally suited to explore. As free-electron laser technology matures toward two independently tunable, fully coherent pulses, the dream of writing and rewriting quantum pathways with XUV and X-ray light moves from theoretical proposal to laboratory routine.

Subject of Research: Phase-coherent control of stimulated Raman transitions in helium using interfering XUV pathways

Article Title: Phase control of interfering stimulated Raman pathways in the XUV regime

Article References: Eichmann, U., Carlström, S., Patchkovskii, S., Eisebitt, S., Rubensson, J.-E., Söderström, J., Mikosch, J., Allaria, E., Brynes, A., Callegari, C., Di Fraia, M., Manfredda, M., Pal, N., Perosa, G., Plekan, O., Simoncig, A., & Génévriez, M. (2026). Phase control of interfering stimulated Raman pathways in the XUV regime. Nature Photonics. https://doi.org/10.1038/s41566-026-02024-9

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02024-9

Keywords: coherent control, stimulated Raman scattering, extreme ultraviolet, free-electron laser, helium, quantum interference, phase control, nonlinear optics, atomic physics, metastable states, FERMI, Raman excitation

Excitons are bound states formed when an electron, promoted out of its usual place in a crystal, pairs up with the positively charged hole it leaves behind. Like other bosons, excitons can, in principle, condense into a single macroscopic quantum state when cooled sufficiently, producing an exotic phase of matter known as an excitonic insulator. Physicists have made striking progress observing this condensation in engineered low-dimensional systems, such as moiré heterostructures and atomically thin layers, where signatures like perfect Coulomb drag have been reported. But in bulk crystals the story has been far murkier, because the ideal superfluidity of a Bose condensate is nearly impossible when quasiparticles scatter between the bands hosting the electrons and holes.

Instead of long-range quantum coherence, theorists expect a bulk excitonic condensate to manifest as a spatial density order, a superlattice whose period is set by the inverse momentum separation of the bound electrons and holes. The trouble is that such a superlattice looks almost identical to an ordinary charge-density wave, a more mundane ordering of charge driven largely by lattice forces. This ambiguity sits at the heart of the long-running debate over 1T-TiSe2, a quasi-two-dimensional transition metal dichalcogenide that undergoes a phase transition at a critical temperature of roughly 200 kelvin. Earlier momentum-resolved electron energy-loss spectroscopy measurements reported a softening of an electronic collective mode consistent with an exciton condensate’s amplitude mode, but subsequent higher-resolution measurements found no such softening, because Landau damping at finite momentum blurs the signal.

Ultrafast spectroscopy has offered a way around this impasse, since the microscopic interactions underlying excitonic and charge-density-wave orders unfold on distinct timescales. Time-resolved angle-resolved photoemission can track how quickly the spectral gap and folded bands collapse after a laser pulse, and the collapse times depend strongly on the photoexcited carrier density, a hallmark of transient electron–hole dissociation. Yet this technique loses its power above the transition temperature, where long-range order vanishes and only short-range fluctuations remain in the quasi-two-dimensional system. What was needed was a probe sensitive to local electronic structure, one that works even without long-range order.

The Berkeley-led team turned to ultrafast broadband extreme-ultraviolet absorption spectroscopy, a pump–probe technique with element and orbital specificity. The excitonic instability in 1T-TiSe2 originates from the Coulomb attraction between selenium 4p holes and titanium 3d electrons near the Fermi level, so the researchers focused on absorption features involving precisely those states. Guided by first-principles calculations of the orbital-projected density of states, they assigned the observed selenium absorption peaks to transitions from spin–orbit-split selenium 3d core levels into unoccupied selenium 4p states, while the titanium M2,3 edges showed anomalies, including a blueshift of more than two electronvolts and a broad hump spanning 38 to 53 electronvolts, that only time-dependent density functional theory incorporating electron–electron interactions could reproduce.

Among these spectral features, one selenium peak stood out as an ideal reporter. When the team excited the crystal with a 3.4-femtosecond pulse, all the absorption peaks responded within 100 femtoseconds, and coherent oscillations at 6.0 terahertz, corresponding to the A1g phonon in which selenium ions displace out of plane relative to stationary titanium ions, rippled across most of the spectrum. But one selenium peak, labelled peak 2, showed no phonon-induced modulation at all, because its coupling to the A1g mode is negligible. That silence made it a clean window onto the local electronic and excitonic dynamics, free from the interfering rattling of the lattice.

The first surprise came when the researchers lowered the pump fluence nearly tenfold, from 1.7 to 0.2 millijoules per square centimetre, to limit the number of photoexcited carriers. While most transient features scaled down proportionally, the titanium many-body continuum displayed a sign reversal of its transient signal at 200 femtoseconds, a qualitatively different behaviour implying an additional dynamical process beyond simple carrier excitation. Sweeping the sample temperature in roughly 5-kelvin steps across the transition at this low fluence revealed an order-parameter-like onset, showing that this spectral window is sensitive to the establishment of long-range order, likely because the valence charge excitations producing the broad hump are modified by the changing titanium 3d states across the transition.

The selenium peak told an equally compelling story. From a state-filling perspective, its transient transmission decrease reflects an increased number of selenium 4p holes below the Fermi level, holes that could otherwise bind titanium 3d electrons into excitons. At 25 kelvin the decrease was larger than at 310 kelvin, pointing to additional holes released by the break-up of condensed excitons in the ordered state, and temperature-dependent measurements showed a clear onset of this suppression at the critical temperature. Crucially, because this peak is dominated by local dipole transitions, its time-domain signature is agnostic about whether long-range phase coherence exists among the excitons, meaning the team could hunt for excitonic correlations even above the transition.

What they found there is the study’s headline result. The pump fluence controls the transient mobile carrier density, and hence the plasmon frequency, which governs how quickly screening breaks the bound electron–hole pairs; the initial response time should therefore scale as one over the square root of the fluence. Below the transition temperature, the initial suppression indeed became markedly faster as fluence increased, consistent with earlier photoemission measurements. But remarkably, at 300 kelvin, far above the transition, the same trend held: the response visibly slowed at lower fluence, following the same inverse-square-root scaling with a constant offset attributable to the time needed to create mobile holes in the absence of excitonic interactions. The team considered alternative microscopic processes that depend on carrier density, but none could account for the observed fluence- and temperature-dependent trends, leaving short-range excitonic correlations persisting in the normal state as the natural interpretation.

Then came a second surprise at the transition itself. At 190 kelvin, close to the critical temperature, the initial response was consistently fast and roughly independent of pump fluence, breaking the scaling seen everywhere else. Measuring across temperatures from 23 to 350 kelvin at both low and high fluences showed that at low fluence the response time decreases gradually from both sides, reaching a minimum near the transition, while at high fluence it stays short and temperature-independent. The researchers formalized this with a simple model in which the carrier density decomposes into a temperature-independent susceptibility plus a diverging excitonic susceptibility that grows as a power of the inverse distance from the transition. The accelerated hole production near the transition reflects an enhanced excitonic susceptibility, where local excitonic correlations are so fragile that even a weak laser pulse breaks them, analogous to how enhanced phonon fluctuations in diffuse scattering expose a lattice instability. The authors also ruled out a dominant role for charge-density-wave phonons, noting that transient phonon renormalization of about 20 percent could only account for a 20 percent change in response time, dwarfed by the threefold reduction they measured. Because the material is quasi-two-dimensional, the excitonic correlations above the transition are likely confined in-plane, contributing to short-range fluctuations that lack three-dimensional phase coherence, while previous work suggests excitons also maintain interplane coherence in the ordered state. Together, the findings offer a unified picture in which excitonic correlations shape both the amplitude and the phase of the ordered state, and they provide a template, built on element- and orbital-resolved femtosecond spectroscopy, for hunting excitonic instabilities in other bulk candidates such as Ta2NiSe5 and Ta2Pd3Te5.

Subject of Research: Short-range excitonic correlations and enhanced excitonic susceptibility in the excitonic insulator candidate 1T-TiSe2 probed by few-femtosecond extreme-ultraviolet absorption spectroscopy

Article Title: Short-range excitonic correlations and enhanced excitonic susceptibility in 1T-TiSe2

Article References: Zong, A., Lin, S.-C., Sato, S. A., Berger, E., Nebgen, B. R., Hui, M., Lv, B. Q., Cheng, Y., Xia, W., Guo, Y., Xiang, D., & Zuerch, M. W. (2026). Short-range excitonic correlations and enhanced excitonic susceptibility in 1T-TiSe2. Nature Physics. https://doi.org/10.1038/s41567-026-03423-z

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03423-z

Keywords: excitons, excitonic insulator, 1T-TiSe2, charge-density wave, ultrafast spectroscopy, extreme ultraviolet, phase transition, quantum materials, transition metal dichalcogenide, excitonic susceptibility, femtosecond pump-probe, condensed matter physics

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: Woods, T. N., Chamberlin, P. C., Crotser, D., Jones, A., Borelli, R., Woodraska, D., & Gonzalez, G. (2026). Solar Doppler Velocity Results from the SDO EVE Level 4 Lines Data Product. Solar Physics, 301(9), Article 134. 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