🔭 astrophysics

There and back again: Mysterious optical pulse profile behavior of the transitional millisecond pulsar PSR J1023+0038

Using the 6-m BTA telescope, researchers observed the transitional millisecond pulsar PSR J1023+0038 undergo a rare, 220-second episode where its typically stable double-peaked optical pulse profile abruptly shifted to a single-peaked, sinusoidal shape with a significantly increased pulsed fraction, challenging current models of pulsar wind interactions.

Original authors: S. V. Karpov, Artyom S. Tanashkin, G. M. Beskin, V. L. Plokhotnichenko, Y. A. Shibanov, D. A. Zyuzin

Published 2026-05-22

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Original authors: S. V. Karpov, Artyom S. Tanashkin, G. M. Beskin, V. L. Plokhotnichenko, Y. A. Shibanov, D. A. Zyuzin

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 a cosmic lighthouse, a neutron star named PSR J1023+0038, spinning incredibly fast—about 600 times every second. This star is in a "transitional" phase, meaning it's caught in a tug-of-war between two different lifestyles: one where it eats matter from a companion star (like a hungry monster), and another where it spins freely, powered by its own rotation (like a spinning top).

Usually, when this lighthouse flashes in visible light, it behaves predictably. Think of its light curve as a heartbeat with two distinct bumps per spin. It's like a double-hump camel: a big bump, a dip, a smaller bump, and then a dip again. This pattern has been steady for years, with the light flickering very slightly (less than 1% of its total brightness).

The Mystery Event
However, on two nights in November 2017, astronomers watching this star with a massive 6-meter telescope in Russia saw something bizarre happen.

In the blink of an eye (literally a few seconds), the "double-hump" heartbeat suddenly collapsed into a single, smooth wave. Imagine that double-hump camel suddenly morphing into a single, rolling hill.

Here are the key details of this transformation:

The Shape Change: The light went from two peaks to one smooth, sinusoidal peak.
The Brightness Spike: Not only did the shape change, but the "flicker" became much stronger. The light variation jumped from a tiny 1% to a massive 5%. It was like the lighthouse suddenly shouting instead of whispering.
The Duration: This new, wild behavior lasted for about 220 seconds (roughly 3.5 minutes).
The Flare: During this time, the star was also having a "tantrum," flashing with sudden, bright bursts of light (flares).
The Return: After about 220 seconds, the single peak slowly stretched back out, and the star returned to its normal double-hump shape. The brightness flicker then slowly settled back down to its usual quiet level.

Why is this confusing?
The scientists have a pretty good idea of how the "normal" double-hump works. They believe the star is shooting out a wind of particles. This wind hits a disk of gas swirling around the star, creating a shockwave (like a sonic boom). Because the wind hits the disk from two opposite sides as the star spins, it creates two flashes of light per rotation.

But the single-hump event breaks the rules.

If the wind hits the disk from two sides, you should always see two flashes.
For the two flashes to merge into one, something must have changed the physics of that collision zone. Perhaps the gas disk got thicker or denser in a specific spot, or the magnetic field of the star shifted slightly, causing the two shockwaves to blur together.

The paper suggests a few possibilities, like a temporary buildup of gas in the inner disk or a change in how the star's magnetic field interacts with the gas. However, the authors admit they don't have the full answer yet. It's like seeing a car suddenly drive on two wheels instead of four; you can guess the mechanics might be off, but without seeing under the hood (simultaneous X-ray observations), you can't be sure what broke.

The Bottom Line
This paper is a report on a rare, 3-minute glitch in a cosmic lighthouse. The star briefly changed its rhythm from a "double-beat" to a "single-beat" and got much brighter while doing it. It proves that even in the stable-looking universe, things can change in the blink of an eye, and we still have a lot to learn about how these spinning stars interact with their surroundings.

Problem
Transitional millisecond pulsars (MSPs), such as PSR J1023+0038 (hereafter J1023), exhibit complex behavior, swinging between accretion-powered low-mass X-ray binary (LMXB) states and rotation-powered radio pulsar states. In the subluminous disk state, J1023 displays coherent optical pulsations at the neutron star's spin frequency. These pulsations typically manifest as a smooth, double-peaked profile with a pulsed fraction of $\lesssim 1\%$ , believed to arise from the re-emission of pulsar wind energy by charged particles in the surrounding medium, likely via a shock formed where the wind collides with the accretion disk beyond the light cylinder. While the amplitudes of the peaks vary, the overall double-peaked structure has been observed to remain fixed. The problem addressed by this study is the nature of a rare, dramatic violation of this permanence: a transient transformation of the optical pulse profile into a single-peaked, sinusoidal shape accompanied by a significant increase in pulsed fraction.

Methodology
The authors conducted high-temporal-resolution optical observations of J1023 over two consecutive nights (November 14–15, 2017) using the 6-m BTA telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences. The instrument used was the Multi-channel panoramic photopolarimeter (MPPP), operating in a dual-channel regime to simultaneously collect photons in "red" (effective wavelength 5670 Å) and "blue" (4320 Å) bands with $\sim 1 \mu$ s temporal resolution.

Data reduction involved converting photon times of arrival (ToAs) to the Solar System barycenter and correcting for the binary orbital motion. Due to known stochastic variations in the orbital phase of J1023, the authors performed a fine-tuning of the orbital phase using the $Z^2_K$ -statistic (with $K=2$ harmonics) to maximize the detection of the periodic signal. They calculated background-corrected pulsed fractions using two metrics: $p_{area}$ (ratio of the area between the profile and the minimum bin level to the total area) and $p_{rms}$ (root mean square amplitude). The analysis included folding the entire dataset to establish the baseline profile and analyzing individual time chunks (tens to hundreds of seconds) to investigate temporal variability.

Key Contributions and Results
The primary contribution of this work is the detection and characterization of a 220-second episode where the optical pulse profile of J1023 underwent a rapid transformation from its standard double-peaked shape to a single-peaked, nearly sinusoidal shape.

Profile Transformation: During this event, the pulse profile changed from two distinct peaks to a single broad peak. The maximum of this new peak did not align with either of the peaks in the normal state.
Amplitude Increase: The pulsed fraction increased dramatically. In the normal double-peaked state, the combined red and blue channels showed $p_{area} \approx 0.65\%$ and $p_{rms} \approx 0.43\%$ . During the single-peaked interval, these values rose to $p_{area} \approx 3.3\%$ and $p_{rms} \approx 1.9\%$ , representing a four-to-five-fold increase. The peak pulsed fraction reached $\approx 5\%$ within the first few seconds of the event.
Temporal Evolution: The transition to the single-peaked state occurred within seconds. The high-amplitude state persisted for approximately 220 seconds, accompanied by flaring activity in the light curve. The return to the normal state was gradual: the profile first reverted to a double-peaked shape, and only after a delay of several tens of seconds did the pulsed fraction drop back to the normal $\lesssim 1\%$ level.
Spectral Analysis: By comparing the observed pulsed fractions with existing spectral data, the authors estimated the spectral index ( $\alpha$ ) of the pulsed component. The results suggest a hardening of the energy distribution (a decrease in $\alpha$ ) during the single-peaked state, although uncertainties remain large.
Modeling: The authors propose a "toy model" to explain the merging of the two peaks. They suggest that the transformation could result from the broadening of the second peak (D2) due to changes in the synchrotron cooling timescale, driven by moderate variations in the magnetic field ( $B$ ) and Lorentz factor ( $\gamma$ ) of the emitting plasma, potentially caused by an accumulation of matter in the inner disk.

Significance and Claims
The paper claims that this observation reveals a "mysterious" and "enigmatic" feature of J1023, demonstrating that the pulse profile structure is not as permanent as previously thought. The authors emphasize that while the standard double-peaked profile is well-understood as a result of the pulsar wind interacting with the accretion disk, the mechanism behind the transient single-peaked state remains unclear.

The authors explicitly state that a complete picture of this behavior is "yet to be revealed." They discuss several possibilities, including:

Changes in physical conditions within the emitting volume (e.g., magnetic field strength, particle density) leading to the merging of the two shock emission spots.
A global reconfiguration of the neutron star's magnetosphere (though they note this is difficult to conceive for an old MSP with a relatively low magnetic field).
Interactions between pulsar magnetospheric particles and infalling plasma penetrating the closed magnetosphere.

The study concludes that the lack of simultaneous X-ray observations during this specific event limits the ability to definitively identify the X-ray mode or the exact physical processes involved. Consequently, the authors stress the importance of future coordinated X-ray and optical timing and spectral observations to further investigate such transient phenomena in transitional MSPs.

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