🔭 astrophysics

A FAST search for radio pulsations during the dormant state of the AMSPs IGR J00291+5934 and MAXI J1957+032

Using the Five-hundred-meter Aperture Spherical Telescope (FAST), researchers conducted a deep L-band radio search for pulsations from the accreting millisecond pulsars IGR J00291+5934 and MAXI J1957+032 during their quiescent states but detected no signals, establishing the most stringent upper limits on pulsed radio flux density for persistent AMSPs to date.

Original authors: A. Marino, E. Parent, F. Coti Zelati, M. C. Baglio, A. Papitto, A. Sanna, A. Anitra, C. Kazantsev, N. Rea, A. Borghese, L. Burderi, T. Di Salvo, C. Espinoza, X. Hou, R. Iaria, G. Illiano, D. M. Russel

Published 2026-06-19

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CC BY 4.0

Original authors: A. Marino, E. Parent, F. Coti Zelati, M. C. Baglio, A. Papitto, A. Sanna, A. Anitra, C. Kazantsev, N. Rea, A. Borghese, L. Burderi, T. Di Salvo, C. Espinoza, X. Hou, R. Iaria, G. Illiano, D. M. Russell, R. Sathyaprakash

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 is a giant, noisy dance floor. In the center of this floor, there are two very specific types of dancers: Accreting Millisecond Pulsars (AMSPs) and Transitional Millisecond Pulsars (tMSPs).

Both are essentially dead stars (neutron stars) that used to be spinning slowly. But they found a partner (a smaller star) and started stealing its "dinner" (gas and dust). As they ate this food, they spun up incredibly fast—hundreds of times per second—like a figure skater pulling in their arms.

Here is the mystery the paper investigates:

The tMSPs are like versatile dancers. When they are eating (accreting matter), they shine in X-rays. But when they finish eating and go into a "quiet" state, they suddenly switch on a radio beacon, flashing a signal like a lighthouse that we can see from Earth.
The AMSPs are the stubborn cousins. They also eat and spin fast, and they shine in X-rays. But when they finish eating and go quiet, they should theoretically switch on that same radio lighthouse. Yet, despite years of looking, we have never seen them turn on. They seem to just go silent.

The Goal of the Study
The authors wanted to solve this mystery by using the FAST telescope, which is like the world's largest, most sensitive ear (a 500-meter dish in China). They decided to listen very carefully to two specific AMSPs, named IGR J00291+5934 and MAXI J1957+032, during their "quiet" times.

Think of it like trying to hear a whisper in a library. The authors wanted to see if these two "stubborn" stars finally decided to whisper their radio signals, or if they were truly mute.

How They Did It

The Setup: They pointed the giant FAST ear at these two stars for about 20 to 50 minutes each.
The Safety Check: Before listening for radio whispers, they used X-ray telescopes (Swift) and optical cameras (Las Cumbres Observatory) to make sure the stars were actually "quiet." They wanted to confirm the stars had stopped eating. If the stars were still eating, the radio signal might be drowned out by the noise of the meal.
- Result: The checks confirmed the stars were indeed quiet. The "dinner" had stopped.
The Search: They used powerful computers to sift through the radio data, looking for a repeating pattern (a pulse) that matches the known spinning speed of these stars. They looked for signals across a wide range of frequencies, just in case the signal was hidden or shifted.

The Findings
The result was a bit disappointing for the "radio lighthouse" theory, but very important for science:

No Radio Signals Found: Even with the world's best ear, they heard nothing. No radio pulses.
The Silence is Deep: They calculated that if these stars were sending radio signals, those signals would have to be incredibly faint—fainter than any other known AMSP we've tried to find before. They set a "silence limit" (upper limit) of about 3 to 6 micro-Janskys. To put that in perspective, it's like trying to hear a cricket chirp from a mile away, and even the most sensitive microphone couldn't pick it up.

Why Didn't They Hear Anything?
The paper suggests a few reasons why these stars might be "mute" even when they are quiet:

They are just too faint: Maybe these specific stars are naturally "ultra-faint" radio stars, weaker than their cousins.
The Beam Missed Us: Imagine a lighthouse. If the light beam is spinning but pointing away from your house, you won't see it. Maybe the radio beam from these stars is just not pointing at Earth.
The "Fog" of Gas: This is the most interesting theory. The authors suggest that even though the stars stopped eating, there might still be a thin, invisible "fog" of gas left over from the meal surrounding the star. This fog could be absorbing or scattering the radio waves, acting like a thick blanket that muffles the sound before it reaches us. This is more likely to happen in systems where the stars are very close together (short orbital periods).

The Bottom Line
This paper is a very thorough "search for the missing radio signal." The authors used the best tools available to listen to two quiet stars. They found nothing. This adds to a growing list of evidence that AMSPs might be fundamentally different from tMSPs. While tMSPs happily switch on their radio beacons when they stop eating, AMSPs might stay silent, perhaps because they are too faint, pointing the wrong way, or because they are still surrounded by a "fog" that blocks the signal.

The study doesn't claim to have solved the whole mystery, but it has set the strictest rules yet on how quiet these stars can be, helping future astronomers know exactly how sensitive their next "ears" need to be to finally hear them.

Technical Summary: A FAST Search for Radio Pulsations in Dormant AMSPs

Problem Statement
Accreting Millisecond Pulsars (AMSPs) and Transitional Millisecond Pulsars (tMSPs) represent low-mass X-ray binaries (LMXBs) containing neutron stars that spin at hundreds of Hz. While tMSPs are known to transition between accretion-powered X-ray states and rotation-powered radio pulsar states during quiescence, radio pulsations have never been detected in quiescent AMSPs, with the sole exception of IGR J18245−2452. This discrepancy raises fundamental questions regarding the "pulsar recycling scenario": whether AMSPs and tMSPs constitute distinct evolutionary classes or different phases of a continuous sequence. Specifically, it remains unclear if AMSPs switch off as radio pulsars when accretion ceases, or if they remain radio-quiet due to external factors such as residual accretion discs, eclipses, or free-free scattering.

Methodology
The authors conducted deep radio observations of two AMSPs, IGR J00291+5934 and MAXI J1957+032, using the Five-hundred-meter Aperture Spherical Telescope (FAST) in the L-band (1–1.5 GHz). These targets were selected as they are the only known AMSPs visible from the FAST location (along with Aql X−1).

Observations:
- IGR J00291+5934: Two 50-minute observations on September 22 and October 23, 2022.
- MAXI J1957+032: One 20-minute observation on October 8, 2023.
- Observations were timed to coincide with the pulsars' inferior conjunction to minimize signal eclipses by circumbinary material.
Multi-wavelength Context: To verify the sources were in a quiescent state (low-luminosity X-ray state), the authors performed simultaneous or near-simultaneous observations with Swift (X-ray) and the Las Cumbres Observatory (optical).
Data Analysis:
- Radio: Data were processed using the PRESTO software package. The authors employed a blind search for dispersion measures (DM) ranging from 0 to 400 pc cm⁻³.
- Search Techniques:
  1. Brute-force folding: Using known X-ray ephemerides, allowing for acceleration searches around the predicted spin frequency.
  2. Acceleration/Jerk search: Using the accelsearch routine to account for orbital evolution and higher-order acceleration effects, given the observation duration was ~30% of the orbital period.
  3. Ascending Node Search: Using SPIDER_TWISTER to correct for potential drifts in the time of ascending node ( $T_{asc}$ ).
  4. Single-pulse search: Searching for sporadic bursts using single_pulse_search.py.
- Upper Limits: In the absence of detections, the authors calculated minimum detectable flux densities ( $S_{min}$ ) using the radiometer equation, corrected for search efficiency ( $E(\delta)$ ) based on pulse duty cycle ( $\delta$ ).

Key Results

Non-Detection: No significant coherent periodic radio signals were detected for either source, neither at their known spin frequencies nor at other frequencies. No convincing single-pulse candidates were found.
State Verification:
- X-ray: Swift observations established upper limits on X-ray luminosity ( $L_X$ ) of $<4.2 - 5.5 \times 10^{32}$ erg s⁻¹ (0.5–10 keV). These values are below the typical luminosity of tMSP "disc states" ( $10^{33} - 10^{34}$ erg s⁻¹), suggesting the sources were in quiescence.
- Optical: For IGR J00291+5934, optical upper limits ( $i' > 19.9 - 21.6$ mag) were consistent with previous quiescent measurements and significantly fainter than outburst states.
Flux Density Limits: Assuming a typical millisecond pulsar duty cycle of 10%, the authors derived the most stringent upper limits to date for persistent AMSPs:
- IGR J00291+5934: $S_{min} < 3.3$ µJy.
- MAXI J1957+032: $S_{min} < 5.6$ µJy.

Significance and Claims
The paper claims these results represent the deepest search for radio pulsations in quiescent AMSPs to date. The authors highlight several implications:

Confirmation of Non-Detection Trend: The results reinforce the historical pattern where ~90% of quiescent AMSPs fail to show radio pulsations, contrasting with tMSPs.
Evolutionary Distinction: The persistent lack of radio activation in AMSPs suggests they may not transition to the rotation-powered radio pulsar state in the same manner as tMSPs. This challenges the view that they are simply different phases of the same evolutionary sequence.
Alternative Explanations: The authors discuss potential reasons for non-detection beyond intrinsic radio faintness, including:
- Residual Accretion: A faint, residual accretion disc could suppress radio emission (though X-ray limits make a "disc state" unlikely).
- Geometric Factors: The pulsar beam may not intersect the line of sight (probability ~66%).
- Free-Free Scattering: Material surrounding the pulsar (potentially driven by a "radio-ejection" mechanism) could absorb radio waves. The authors note a tentative trend where AMSPs with shorter orbital periods (more compact systems) are more susceptible to this effect.
Secular Spin-Down: The lack of radio pulsar activation implies that the observed secular spin-down in some AMSPs cannot be attributed to magnetic dipole radiation in quiescence, pointing instead to other mechanisms like residual torques or weak propeller phases.

The authors conclude that while FAST provides unprecedented sensitivity, the non-detection of radio pulsations in these targets suggests that either AMSPs are intrinsically radio-faint, or environmental factors (like intra-binary material) prevent detection. Future monitoring with facilities like the Square Kilometre Array (SKA) is required to definitively resolve whether AMSPs can ever activate as radio pulsars in quiescence.

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