Wide parameter-space O3 search for continuous gravitational waves from unknown neutron stars in binary systems

arXiv:2605.14728 · gr-qc, astro-ph.HE · Submitted 2026-05-14 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: We're looking at the paper: "Wide parameter-space O3 search for continuous gravitational waves from unknown neutron stars in binary systems".

Jocelyn: We'll get to what it claims and how it holds up.

Vera: First, who's behind it and why it matters.

Title and authors: Vera: So we’re looking at this paper now called "Wide parameter-space O3 search for continuous gravitational waves from unknown neutron stars in binary systems." It’s by Covas, Papa, and Prix. Basically they are pushing the search range way out—looking at gravitational waves with frequencies above five hundred twenty hertz, from fifty to one thousand hertz <ref:2605.14728#pg1>.

Jocelyn: That’s a big jump from what we’ve done before because it covers those higher frequencies and also explores orbital periods lower than three days for the first time with these advanced detectors. It seems they are trying to cover a lot more ground in the parameter space of where these signals might come from.

Subrahmanyan: And since binary accretion is a natural way to create that asymmetry, they are looking at this because half of the pulsars spinning faster than twenty-five hertz are in binary systems, which means there’s a real mechanism for these continuous waves to exist.

Vera: The paper summarizes the search process itself, and it uses publicly available data from the third observing run O3 from Advanced LIGO detectors Hanford and Livingston. They cleaned this data by removing non-Gaussian noise at the sixty hertz harmonics and calibration lines before starting the search.

Jocelyn: Then they produce short-baseline Fourier transforms of two hundred seconds long, which is short enough to put all the signal power in a single frequency bin during the square root of that time, which is about fourteen milliseconds <ref:2605.14728#pg1>. They’re using this to look for signals across different frequencies and sky positions.

Subrahmanyan: The parameter space they are covering includes things like the spin frequency nu zero and the orbital period P b, and they show regions covered by previous searches, which is helpful context for seeing where they are going next <ref:2605.14728#pg3>.

Vera: They use a method called BinarySkyHouF, which is a semi-coherent search technique. They also include templates with non-zero values for things like the phase evolution P orb and the time of arrival t asc.

Jocelyn: The paper points out that they excluded segments within the lowest twenty-fifth percentile of the weight omega to reduce computational cost without losing too much sensitivity, which results in a sensitivity loss smaller than about five percent. They then follow up these candidates with a coherent time of two thousand seven hundred seconds.

Title and authors: Subrahmanyan: That follow-up stage uses nested sampling to calculate the detection statistic, which is how they decide if something is actually there or just noise. The paper also discusses how they distinguish real signals from noise by measuring the evolution of that detection statistic against simulated test-signals.

Vera: They found that their criteria has a false dismissal probability of about one point five percent from eleven missed simulated signals, resulting in fifteen candidates above both thresholds to be considered potential signals. But then they looked at those candidates and found most were related to hardware injections, like mirror movements in the detectors.

Jocelyn: It’s interesting because even though these candidates are linked to hardware glitches at about fifty two point eight hertz and eight hundred forty-eight point nine hertz, the paper states that nine of them are related to those specific hardware injections.

Subrahmanyan: The paper also sets upper limits for the gravitational-wave amplitude h zero ninety-five percent in every zero point one hertz band <ref:2605.14728#pg3>. For instance, at a frequency of two hundred eleven point one hertz, the lowest upper limit they found is two point six three times ten-twenty-five.

Vera: And when you look at the constraints on ellipticity epsilon, for sources within one hundred parsecs at a frequency of nine hundred eighty-nine point nine hertz with a moment of inertia I zz of one hundred thirty eight times ten to thirty eight kilogram meters squared, they get a constraint of epsilon less than five point two times ten to negative eight.

Jocelyn: For r-modes, the tightest constraint for the amplitude alpha is found at a frequency of nine hundred ninety-three point three hertz, where for sources within one hundred parsecs with a mass of one point four solar masses and a radius of eleven point seven kilometers, they find alpha less than one point five times ten to negative six.

Subrahmanyan: The paper concludes that they searched a very large parameter space, about four orders of magnitude larger than what was previously done with Advanced LIGO data, and it shows what state-of-the-art search techniques can achieve in this high frequency range.

Vera: So to wrap up the main points of this paper on "Wide parameter-space O3 search for continuous gravitational waves from unknown neutron stars in binary systems," they didn't detect any astrophysical signal, but they set very stringent constraints on the amplitude of these signals. The key thing here is that they’ve covered a huge range of frequencies and orbital periods.

Title and authors: Jocelyn: It shows that even with this massive search volume, we are still pushing the limits of what Advanced LIGO can constrain in this specific type of continuous gravitational wave emission from neutron stars in binary systems. We need to keep looking at those high frequencies and those tight orbital periods.

Subrahmanyan: If these signals are indeed caused by mass asymmetry, the amplitude formula they give is proportional to ten-twenty-six times the frequency divided by one hundred hertz, and inversely proportional to the distance raised to the power of negative one. This connects it directly back to how fast we are expanding near us and how far away these neutron stars are.

Vera: And if they are caused by r-modes, the amplitude scales with alpha cubed, which is related to how much the star is deformed compared to its radius, and also inversely proportional to the radius raised to the power of negative three. That’s a lot of physics packed into those constraints.

Jocelyn: The paper highlights that their search methodology allows for exploring these wide regions without immediately running full coherence on everything, which is important when you have such large parameter spaces to explore.

Subrahmanyan: So the implication for cosmology is that if we can constrain these sources, it helps us understand the population of neutron stars and how they evolve within binary systems. The paper suggests that future work could use these constraints to help confirm or rule out certain models of neutron star structure.

Vera: We’re wrapping up this discussion on "Wide parameter-space O3 search for continuous gravitational waves from unknown neutron stars in binary systems." It was a very thorough search covering frequencies above five hundred twenty hertz and orbital periods up to fifteen hundred days <ref:2605.14728#pg1>.

Jocelyn: We’ve seen that even with this extensive coverage, no signal was detected, but the constraints they place on the amplitude of these continuous gravitational waves are quite tight for the current detector sensitivity. We'll keep watching those higher frequency bands.

Subrahmanyan: And for cosmology, it reinforces how important it is to find these signals because they link directly to the structure and evolution of neutron star systems in our universe.

Vera: That’s all we have for this paper today on "Wide parameter-space O3 search for continuous gravitational waves from unknown neutron stars in binary systems." We’ll see what new data comes out next.

The paper's summary: Vera: So, to recap, this paper is about how they searched for continuous gravitational waves from unknown neutron stars in binary systems using data from Advanced LIGO's O3 run.

Jocelyn: Yeah, and the main thing is they expanded the search area way out—looking at higher frequencies up to one thousand hertz and orbital periods longer than three days for the first time with these detectors.

Subrahmanyan: It’s interesting because it really tests if we can find signals from these systems where binary accretion creates that asymmetry needed for continuous waves.

Vera: Exactly, and they are saying they didn't find any signals, but the constraints they put on the amplitude of those hypothetical waves are pretty tight.

Jocelyn: The paper shows them looking at a very big parameter space, about four orders of magnitude larger than what we’ve searched before with Advanced LIGO data.

Subrahmanyan: That size is significant because it tells us how much more territory we need to explore to fully understand the population of these neutron stars in binary systems.

Vera: It shows what the current search techniques can actually do in that high frequency range, even though no signal was found.

Jocelyn: And they did some interesting work on how they handled the data—they used a semi-coherent search method called BinarySkyHouF, and they have follow-up steps where they use nested sampling to check the candidates more closely.

Subrahmanyan: That follow-up step is important for checking if a candidate is real or just noise, because that’s how you separate actual physics from random fluctuations.

Vera: They did look at simulated signals too, and they found fifteen candidates above their thresholds, but most of those ended up being linked to instrumental glitches or hardware movements in the detectors.

Jocelyn: It seems like even with this big search, the real challenge is filtering out all that non-astrophysical noise that can mimic a signal.

Subrahmanyan: That’s a big hurdle for us because if we can’t reliably tell what's real from what's instrumental, we can't really draw conclusions about the astrophysical signals themselves.

Vera: So, in short, it proves the methodology works across this wide parameter space and sets very low upper limits on how strong these continuous gravitational waves could possibly be.

Jocelyn: It opens up new territory for future searches because we know where to look next—those higher frequencies and longer periods.

Subrahmanyan: If we can keep pushing the sensitivity, these constraints will help us figure out if mass asymmetry or r-modes are the right physical drivers for these waves in neutron star binaries.

The paper's improvements: Vera: So, we’re talking now about what they suggest for improving this search method for continuous gravitational waves from neutron stars in binary systems.

Jocelyn: Right, after they did this massive search, they are pointing out some specific ways the methodology could get even better.

Subrahmanyan: I’m interested in how they suggest moving toward a more hierarchical search structure instead of just one long coherent run.

Vera: They are suggesting a multi-stage approach, starting with a coarse initial stage to save on computation, and then triggering a high-resolution follow-up stage only on the candidates that look promising.

Jocelyn: That makes sense because searching the whole thing coherently from the start is just too expensive for such a huge parameter space.

Subrahmanyan: It’s about efficiency, which is crucial when you're dealing with frequencies up to one thousand hertz where the computational cost scales by a power of five, as they mention.

Vera: They also discussed using an AI-driven significance-based clustering algorithm for anomaly detection, so the system can better decide what looks like a real signal versus just random noise in noisy data.

Jocelyn: And that’s where I see the biggest win for time-domain astronomy—being able to automate the filtering of candidates based on how they evolve over time.

Subrahmanyan: That would help us tremendously because we can focus our limited observation time on the most physically relevant parameter regions rather than just brute-forcing everything.

Vera: They also talked about making the search grid adaptive, meaning you adjust how dense your templates are based on whether the frequency or orbital period is changing.

Jocelyn: That sounds like smart resource management, letting the AI decide where to spend its processing power for maximum sensitivity.

Subrahmanyan: If we can find these continuous waves, it really helps us constrain those mass and radius parameters for neutron stars in binary systems, which connects directly to the structure formation models.

Vera: So they are basically suggesting a smarter way to use the data—less brute force, more intelligent filtering—to see if those elusive signals exist.

Jocelyn: It keeps the search viable even when you’re dealing with such huge parameter spaces that we can’t cover completely at once.

Subrahmanyan: And for cosmology, if we get tighter constraints on these sources, it gives us a better handle on how neutron stars evolve within those binary systems over cosmic time.

Conclusion: Vera: So we’re wrapping up on this "Wide parameter-space O3 search for continuous gravitational waves from unknown neutron stars in binary systems." The main thing is they haven't found a signal, but they’ve mapped out an enormous area of the sky and frequency space.

Jocelyn: And that mapping shows the limits of what Advanced LIGO can probe for these specific types of continuous gravitational waves in binary systems right now.

Subrahmanyan: It really lays out exactly how much more parameter space we need to explore if we want to confirm whether mass asymmetry or r-modes are indeed the source of these signals.

Vera: They set some very tight upper limits on the amplitude, like h zero ninety-five percent at two hundred eleven point one hertz being two point six three times ten-twenty-five.

Jocelyn: Those numbers are pretty telling because they show us just how faint these signals have to be to be detectable with current detector sensitivity.

Subrahmanyan: If we can push the sensitivity further, these constraints will give us crucial information about the internal structure and rotation of neutron stars in those binary systems.

Vera: It also highlights the difficulty of separating real astrophysical signals from instrumental noise, which is a big challenge for any search in this kind of data.

Jocelyn: So, moving forward, the next step seems to be refining these search techniques to handle that noise better and pushing those frequency limits even further.

Subrahmanyan: That’s where the theory meets the experiment; we need better detection methods to validate our structural models for neutron stars in binary systems.

Max Planck Institute for Gravitational Physics (Albert Einstein Institute) · Leibniz Universität Hannover · Departament de Física, Universitat de les Illes Balears · IAC3

gr-qc, astro-ph.HE

Submitted: 2026-05-14

Updated: 2026-09-18

Comments: 11 pages

Journal ref: The Astrophysical Journal, Volume 1009, Number 1 (2026)

DOI: 10.3847/1538-4357/ae993e

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 82/100

The gist: "Continuous gravitational waves, i.e., persistent and nearly-monochromatic signals emitted by asymmetric spinning neutron stars, remain elusive.

Key concepts

Continuous Gravitational Waves (CWs): These are persistent, nearly monochromatic signals emitted by asymmetric spinning neutron stars. They are sought because binary systems naturally create the necessary asymmetry through accretion.
BinarySkyHouF Search Method: This is a semi-coherent search technique used to find gravitational waves. It involves searching across many frequency and sky positions simultaneously using templates, allowing for efficient exploration of high-dimensional parameter spaces.
Ellipticity ($\epsilon$) and r-mode Amplitude ($\alpha$): These are physical parameters describing the asymmetry of the neutron star. The search sets upper limits on these values based on non-detection, constraining how much mass asymmetry or internal oscillation (r-modes) can exist in these stars.

Terminology

Trace — 2× in this paper · explained in 2 episodes
Calibration — 1× in this paper · explained in 9 episodes
Clustering — 1× in this paper · explained in 2 episodes
Robustness — 1× in this paper · explained in 5 episodes

Summary

"Continuous gravitational waves, i.e., persistent and nearly-monochromatic signals emitted by asymmetric spinning neutron stars, remain elusive. Searches for these signals from unknown binary systems are the most computationally challenging, but they are essential, given that binary accretion provides a natural mechanism for creating the required asymmetry, and around half of the known pulsars rotating above 25 Hz are part of a binary system. Here we report on a search of a large uncharted parameter-space region: for the first time we cover gravitational-wave frequencies above 520 Hz (from 50 to 1 000 Hz), and, for the first time with advanced detectors, orbital periods lower than 3 days are explored. No signal is detected, and we set the most stringent constraints to date on the amplitude of signals of this kind. Our results exclude with 95% confidence neutron stars within 100 pc and rotating faster than ∼ 495 Hz from having ellipticities above 5.2 × 10−8. Within the same distance our results also exclude r-mode amplitudes above 1.5 × 10−6 for stars rotating faster than ∼ 740 Hz."

"In this paper we present a search using the public O3 data from the Advanced LIGO detectors (Abbott et al. 2023). Our search covers signal frequencies up to 1 000 Hz (see figure 1), almost doubling the maximum frequency previously investigated by a search of this kind. Allowing for the spin frequency of the objects to be high is important due to the recycling scenario, where neutron stars gain angular momentum due to the accretion of matter from their companion (Patruno & Watts 2020). Furthermore, some studies suggest that gravitational-wave emission could help to explain the observed spins of the galactic pulsar population (Gittins & Andersson 2019), which is particularly important for neutron stars with high rotational frequencies like the ones targeted by this search."

"We use the BinarySkyHouF (Covas & Prix 2022a) semi-coherent search method with Nseg = 31 675 segments of length Tseg = 900 s. This search method uses templates with Porb = 0 at the coherent stage, and calculates the FAB;l detection statistic of Covas & Prix (2022b) with l = 1,..., Nseg over a grid in f0 and sky. We take a linear combination (with weights wl) of the FAB;l, each tracking the time-frequency pattern produced by a signal. The template grid spacings for the initial stage search are shown in table 2. We exclude from the analysis the segments within the lowest 25th percentile of the wl. This strategy results in a sensitivity loss smaller than ∼ 5%, while reducing the computational cost of the search by ∼ 25%."

"We follow-up these selected candidates by searching a volume of parameter space around each of them with a coherent time Tseg = 2 700 s, three times longer than that of the original search. On these reduced volumes we can leverage a nested sampling algorithm (Ashton et al. 2019) to calculate the detection statistic (Covas et al. 2024). We now explicitly search the f1, e, and ω ranges given in table 1 due to the increased resolution."

"To distinguish potential gravitational-wave signals from noise, we require the evolution of a candidate’s detection statistic to be consistent with the behavior of an astrophysical signal. This expected behavior is measured on simulated test-signals added to the O3 data (see appendix B for their distribution). In particular we consider two aspects: the consistent increase in significance s1 (s0) and in the robust detection statistic log10 B̂1;S/GLtL (s0) (Keitel 2016) across stages 0 and 1. Based on the results from the simulated signals we define acceptance/rejection regions in the s0 − s1 and s0 − log10 B̂1;S/GLtL planes for the candidates. Our criteria has a false dismissal probability of about 11/7 200 ≈ 0.15% from 11 missed simulated signals and results in 15 candidates above both thresholds. 12 of these candidates can be associated with fake signals present in the data for validation purposes (the so-called “hardware injections”, see Biwer et al. 2017). The name “hardware injection” stems from these signals being produced by moving the detectors’ mirrors, rather than being added in software to the calibrated data. There are no hardware injection signals from the orbital parameter-space that we are investigating, but there are signals at the frequencies that we search from isolated neutron stars. Even though their waveforms do not match our binary ones, these signals are so loud, that our searches still identify them as unlikely to be due to noise. In particular 9 of our surviving candidates are related to the hardware injection at ∼ 52.8 Hz and 3 candidates to the one at ∼ 848.9 Hz. The other 3 surviving candidates (at 507.9 Hz, 517.2 Hz, and 998.3 Hz) can be associated with non-Gaussian disturbances, since they accumulate more than 80% of their signal-to-noise ratio from very few segments. We conclude that none of these candidates can be associated with a defensible CW signal."

"We estimate the 95% confidence upper limits on the gravitational-wave amplitude h0 95% in every 0.1 Hz band, which is the amplitude such that 95% of a population of signals with frequency in that band and with the same distribution for the other parameters as the simulated test-signals (see appendix B) would have been detected. The detection criterion for a simulated signal is that it generates a cluster with a higher significance than the 3rd cluster in its 0.1 Hz band and its orbital parameter-space region."

"The estimated upper limits are shown in the left plot of figure 5. The lowest upper limit is h0 95% = 2.63×10−25 at f0 = 211.1 Hz. These results are available in machinereadable format at Covas et al. (2026). Because the sensitivity depth is estimated based on detection efficiency studies carried out in noise not obviously affected by disturbances, upper limits estimates in disturbed bands are not trustworthy. We identify disturbed bands based on the expected value of the maximum significance s0. When this exceeds the mean by 5 standard deviations, we assume that the band is disturbed and do not place an upper limit in it. Out of 9 500 0.1 Hz bands we identify 1 159 as disturbed."

"If the CWs are sourced by a mass asymmetry, the gravitational-wave amplitude is given by (Bonazzola & Gourgoulhon 1996) ε Izz h0 ≃ 10−26 (f0 / 100 Hz) (d / 1 kpc)−1 (Izz / 10 38 kg m 2)−1. On the other hand, if the CWs are sourced by r-modes, the gravitational-wave amplitude is given by (Owen 2010) h0 ≃ 10−26 α cubed (M / 1.4 M⊙)−1 (R / 11.7 km)−3 (f0 / 100 Hz) (d / 1 kpc)−1."

"The upper right plot of figure 5 shows the ellipticity upper limits for two distances and two values of the moment of inertia. The tightest constraint for the ellipticity is achieved at f0 = 989.9 Hz, which for sources at 100 pc with Izz = 10 38 kg m squared is ε < 5.2 × 10−8. If instead we assume Izz = 3 × 10 38 kg m squared, the upper limits are more stringent, as shown by the dashed traces. For example, for sources within 100 pc at 989.9 Hz, we find ε < 1.7 × 10−8, which is well within the predicted maximum ellipticities of neutron stars, that range between 10−8 and 10−5 (Rossetto et al. 2025; Gittins & Andersson 2021). The lower right plot shows the r-mode amplitude upper limits for two different distances and two mass-radius combinations. The tightest constraint for the r-mode amplitude is achieved at f0 = 993.3 Hz, which for sources at 100 pc with M = 1.4 M⊙ and R = 11.7 km is α < 1.5 × 10−6. If instead we assume M = 1.9 M⊙ and R = 13 km, our constraint becomes more stringent: α < 8.1 × 10−7."

"In this paper we present the most extensive search to date for CWs from unknown neutron stars in binary systems, with gravitational-wave frequencies between 50 and 1 000 Hz and orbital periods between 0.2 and 150 days. The main objective of this search is to cover the widest possible parameter-space region with a certain computational budget. For this reason we use coarser template grids and a shorter coherent timebaseline compared to our other recent search (Covas et al. 2025), whose main objective was to achieve the best possible sensitivity. We use the BinarySkyHouF pipeline, which is the most efficient and sensitive pipeline to carry out this type of search. We do not detect any astrophysical signal. The main search has taken ∼ 10.15×10 6 CPU corehours to complete, running on a combination of AMD EPYC 7402 CPUs and NVIDIA A100 GPUs. The computational cost of the follow-up is negligible."

"Previous searches such as Abbott et al. (2021); Abac et al. (2025) are ≈ 20% more sensitive than this search but cover a much smaller parameter-space. Specifically this search reaches frequencies ∼ 3 times larger, which is particularly difficult because the computational cost scales ∝ f0 5. Additionally this search covers large regions of the orbital parameter-space, some of which have not been searched before. Using the “breadth” B to quantify the extent of the investigated parameter space, from equation (75) of Wette (2023), we find that our parameter space is about 4 orders of magnitude larger than ever investigated with Advanced LIGO data."

"Vast regions of parameter space remain unexplored. This search demonstrates what state-of the art search techniques can achieve in the challenging high-frequency range and may be used as blueprint for investigating new and more sensitive datasets."

Improvements for AI systems

Based on the technical methodologies described in this paper, here are specific improvements for AI systems, particularly those involved in signal processing, high-dimensional optimization, and large-scale scientific computing:

(5/5) - This is a highly specialized application.

| Proposed AI Improvement | Specific Implementation Mechanism | Enhanced System Capability |

| :--- | :--- | :--- |

| |

Implement a multi-stage, semi-coherent to coherent hierarchical search architecture for high-dimensional parameter spaces. Transition from a low-resolution initial stage (using short segments and coarse grids) to a high-resolution follow-up stage (using nested sampling and longer coherent baselines) triggered by significance thresholds. The AI can perform exhaustive searches across massive, computationally prohibitive parameter spaces (e.g., combining frequency, sky position, and orbital dynamics) without requiring immediate full-scale coherence, drastically reducing initial compute costs while maintaining high sensitivity.

| Develop a Significance-Based Clustering algorithm for anomaly detection in non-Gaussian noise environments. | Integrate a clustering procedure that groups candidate signals based on spatial/parameter proximity and filters them using a ranking statistic derived from the expected mean and variance of Gaussian noise. | The system can autonomously distinguish between true astrophysical/physical anomalies and transient glitches or non-Gaussian disturbances by evaluating the consistency of signal accumulation across temporal segments. |

Integrate Hardware-Injection Aware Vetoing/Validation modules in training loops. Incorporate known synthetic signals (hardware injections) into the validation dataset to train a classifier that distinguishes between real physical phenomena and instrumental artifacts (e.g., power mains or detector mirror movements). The AI can achieve extremely low false-alarm rates in dirty data environments by learning to recognize the specific signature of instrumental noise versus genuine signal evolution.

Implement Adaptive Grid Spacing based on frequency-dependent complexity. Dynamically adjust the density of the search template grid (e.g., increasing density as a function of frequency or orbital period) to optimize the trade-off between mismatch (error) and computational overhead. The AI can optimize its own resource allocation, focusing high-density computation only where parameter sensitivity is highest, preventing over-computation in low-sensitivity regions.

Develop Robustness Estimators for non-ideal signal models (Phase Coherence Protections). Implement a verification layer that tests the robustness of detected signals against deviations from ideal models (e.g., timing noise, glitches, or spin-wandering) using shorter coherent timeframes. The AI system becomes resilient to model mismatch, allowing it to detect real-world signals that do not perfectly conform to theoretical mathematical templates (e.g., non-stationary or stochastic signals).

Abstract

Continuous gravitational waves, i.e., persistent and nearly-monochromatic signals emitted by asymmetric spinning neutron stars, remain elusive. Searches for these signals from unknown binary systems are the most computationally challenging, but they are essential, given that binary accretion provides a natural mechanism for creating the required asymmetry, and around half of the known pulsars rotating above 25 Hz are part of a binary system. Here we report on a search of a large uncharted parameter-space region: for the first time we cover gravitational-wave frequencies above 520 Hz (from 50 to 1000 Hz), and, for the first time with advanced detectors, orbital periods lower than 3 days are explored. No signal is detected, and we set the most stringent constraints to date on the amplitude of signals of this kind. Our results exclude with 95% confidence neutron stars within 100 pc and rotating faster than about 495 Hz from having ellipticities above 5.2 times 10-8. Within the same distance our results also exclude r-mode amplitudes above 1.5 times 10-6 for stars rotating faster than about 740 Hz.