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Fast optical spectroscopic observations of PSR J1023+0038 over one orbital period

This paper presents the first minute-timescale, full-orbital-cycle optical spectroscopic observations of the transitional millisecond pulsar PSR J1023+0038, revealing rapid variability in emission line properties and asymmetric structures that support a scenario of coexisting accretion and matter ejection in its sub-luminous disc state.

Original authors: M. M. Messa, M. C. Baglio, P. D'Avanzo, G. Illiano, F. Coti Zelati, K. Alabarta, D. de Martino, Y. D. Hu, A. Miraval Zanon, A. Reguitti, S. Campana

Published 2026-07-29

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CC BY 4.0

Original authors: M. M. Messa, M. C. Baglio, P. D'Avanzo, G. Illiano, F. Coti Zelati, K. Alabarta, D. de Martino, Y. D. Hu, A. Miraval Zanon, A. Reguitti, S. Campana

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a cosmic dance floor where stars sometimes pair up, spinning and swirling in a gravitational waltz. In some of these duets, one partner is a dead star called a neutron star, so dense that a teaspoon of its material would weigh a billion tons on Earth. These neutron stars can be "millisecond pulsars," which are like cosmic lighthouses spinning hundreds of times a second, beaming radio waves out into space. Usually, they are powered by their own rapid spin, but sometimes, they get a second wind. If their partner star is close enough, the neutron star can start stealing gas from it, forming a swirling disk of super-hot material. This is the "accretion" phase. The fascinating mystery scientists are trying to solve is how these systems switch back and forth between being powered by their spin and being powered by the stolen gas. It's like a car that can suddenly switch from running on its own battery to running on fuel, and then back again, without anyone touching the ignition. Understanding this helps us figure out how neutron stars grow up and how they interact with their neighbors.

Now, let's zoom in on a specific celebrity of this cosmic dance floor: a star system named PSR J1023+0038 (or J1023 for short). This system is a "transitional millisecond pulsar," meaning it's the perfect example of a neutron star that has been caught switching between its two power modes. For a long time, astronomers have watched J1023, but they mostly looked at it with X-ray eyes or took snapshots of its light. In this new study, a team of astronomers decided to take a high-speed video of J1023 using a giant telescope called the Gran Telescopio Canarias. They didn't just take a few pictures; they grabbed a spectrum (a detailed breakdown of the light's colors) every single minute for over four hours, covering a full orbit of the two stars around each other. It's like watching a movie of a spinning top, frame by frame, to see exactly how the wobble changes.

What they found was a lot of drama happening in the light. The team looked at specific "fingerprints" in the light, called emission lines, which are like glowing signatures left by hydrogen and helium gas swirling in the disk. They measured two things: how bright these lines were (the "equivalent width") and how fast the gas was moving (the "full width at half maximum," or FWHM). They discovered that these properties were changing wildly on timescales of just minutes. Sometimes, the gas seemed to slow down, and at the exact same time, the brightness of the line dropped. The authors suggest this might be a sign that the inner part of the gas disk is getting kicked out or "ejected" from the system, perhaps when the neutron star switches to its "low mode" of activity. It's as if the gas disk is hiccuping, spitting out little blobs of material every few minutes.

The team also tried to map where this gas was coming from using a technique called "Doppler tomography," which is like creating a speed-map of the swirling gas. The map showed that the gas wasn't swirling in a perfect, symmetrical circle. Instead, it looked lopsided, with more gas on one side than the other. This asymmetry hints that some of the material is being thrown out of the system, possibly by a "propeller" effect where the spinning neutron star flings the gas away. However, the authors are careful to say this isn't a proven fact. The data is tricky, and while the "propeller" idea fits the shape of the map, other factors—like the fact that the neutron star isn't spinning down as fast as a propeller should make it—make the picture complicated. They also looked for signs of strong winds blowing off the star, like a "P Cygni" signature, but their telescope wasn't sharp enough to see those specific features clearly.

Finally, they looked at the overall glow of the system (the "continuum"). They saw a gentle, rhythmic brightening and dimming that matched the orbital period of the stars. This is likely because the companion star is being heated up by the neutron star's intense light, acting like a cosmic spotlight that shines brighter when the heated side faces us. Interestingly, when the system was at its brightest, the gas lines looked "redder," suggesting the inner parts of the disk might be clearing out or changing shape.

In short, this paper gives us the first minute-by-minute movie of J1023's light. It suggests that the gas disk is a chaotic, restless place, constantly shifting and occasionally spitting out material. While the authors can't say for sure exactly what is causing these changes without more data (especially X-ray data taken at the same time), their findings support the idea that accretion and outflows are happening at the same time. It's a reminder that even in the quiet, "sub-luminous" state of a neutron star, the cosmic dance is far from still.

Technical Summary: Fast optical spectroscopic observations of PSR J1023+0038 over one orbital period

Problem and Context
Transitional millisecond pulsars (tMSPs), such as PSR J1023+0038 (hereafter J1023), are neutron-star binaries that switch between rotation-powered and accretion-powered states. In their sub-luminous disc state, these systems exhibit complex variability on short timescales, including rapid switches between high and low X-ray modes. While the existence of these modes is well-documented in X-rays, the physical mechanisms driving them and their manifestation in the optical band remain debated. Specifically, the origin of optical variability, the coexistence of accretion and outflow processes, and the nature of the emission structures (e.g., accretion disc vs. propeller-driven ejecta) require further constraint. Previous optical studies have often lacked the temporal resolution or orbital coverage necessary to disentangle these short-timescale phenomena from orbital modulation.

Methodology
The authors present high-time-resolution optical spectroscopic observations of J1023 obtained during its sub-luminous disc state. The observations were conducted using the Gran Telescopio Canarias (GTC) with the OSIRIS instrument (R1000B grism, $\sim$ 1000 resolution, 3630–7500 Å coverage).

Observing Strategy: The campaign ran from March 30–31, 2024, covering approximately 1.3 orbital periods (4.75 hours) with a cadence of roughly 50 seconds (one spectrum per minute), yielding 480 spectra.
Data Reduction: Spectra were reduced using the GTC/OSIRIS Data Reduction System (PypeIt) and the molly package. Wavelength and flux calibrations were performed using arc lamps and the standard star Hiltner 600. Corrections for slit losses were applied based on measured seeing conditions (1.2–4.8 arcsec).
Analysis Techniques:
- Continuum Analysis: Light curves were constructed in $B_e$ (4200 Å), $V_e$ (5500 Å), and $R_e$ (5500–7500 Å) bands to search for orbital modulation.
- Line Properties: Equivalent Width (EW) and Full Width at Half Maximum (FWHM) were measured for prominent emission lines (H $\alpha$ , H $\beta$ , He I, He II).
- Doppler Tomography: Maps of emission brightness in velocity space were reconstructed for H $\alpha$ , H $\beta$ , and He I lines using pydoppler, assuming an inclination of $\sim$ 54 $^\circ$ and a mass ratio of 0.137.
- Correlation Analysis: Pearson and Spearman correlation coefficients were computed between continuum flux, EW, and FWHM.

Key Results

Orbital Modulation: The optical continuum exhibits a statistically significant ( $>8\sigma$ ) sinusoidal modulation peaking at orbital phase $\phi \approx 0.5$ (inferior conjunction of the neutron star, where the irradiated companion face is visible). The amplitude of this modulation is lower than previously reported, potentially due to the dominance of the accretion disc emission or observing conditions.
Spectral Line Variability: Significant variability in the EW and FWHM of emission lines was detected on minute timescales.
- Correlated Behavior: A correlation was found between FWHM minima and decreases in EW for certain phases (e.g., $\phi \approx 0.6$ ). This suggests episodes where the contribution of high-velocity material (inner disc) is suppressed, possibly coinciding with switches to low modes.
- Anti-correlation: The EW of H $\alpha$ shows a pronounced maximum at $\phi \approx 1$ (superior conjunction), anti-correlated with the continuum. This is interpreted as continuum dilution by the irradiated companion star at $\phi \approx 0.5$ .
Doppler Tomography: The maps reveal asymmetric emission structures.
- Asymmetry: Dominant emission is distributed over negative $v_x$ and $v_y$ velocities, extending into the positive $v_x$ , negative $v_y$ quadrant. This morphology resembles that of SW Sex-type cataclysmic variables and previous J1023 observations, suggesting non-axisymmetric structures.
- Comparison with Literature: Unlike some previous maps showing a clear gas stream impact (hot spot), the current maps show an absence of clear gas stream emission and intensified emissivity in specific quadrants. This may indicate a propeller-like mechanism expelling material, though the lack of a clear P Cygni profile limits definitive wind identification.
Companion Contribution: Analysis of the H $\alpha$ profile and Doppler maps indicates a negligible contribution from emission originating directly on the companion star, despite the strong orbital modulation in the continuum.

Significance and Claims
The paper claims to provide the first optical spectroscopic study of a tMSP covering a full orbital period with minute-timescale cadence. The primary significance of the work lies in:

New Constraints: Providing new constraints on the short-timescale behavior of tMSPs in the sub-luminous disc state, specifically the correlation between line width and strength.
Coexistence of Processes: Supporting scenarios where accretion and outflow processes coexist. The observed FWHM/EW correlations and asymmetric Doppler maps are consistent with episodes of matter ejection from the inner disc, potentially linked to mode switches.
Propeller Interpretation: While the Doppler maps suggest a propeller-like scenario (expulsion of inner disc material), the authors note that observational constraints (such as the location of the accreting material relative to the light cylinder and the measured spin-down rate) challenge a dominant propeller regime interpretation. They suggest the pulsar wind may be the driver of mass outflows instead.
Future Requirements: The authors emphasize that the lack of simultaneous X-ray coverage prevents a direct link between the observed optical variability and specific high/low mode switches. They conclude that future multiwavelength campaigns, particularly those with simultaneous X-ray coverage and spanning multiple orbital cycles, are crucial to definitively establish the physical origin of these variability episodes.

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