Companion Wind Research Articles

The Intrabinary Shock and Companion Star of Redback Pulsar J2215+5135

PSR J2215+5135 (J2215) is a “redback” spider pulsar, where the intrabinary shock (IBS) wraps around the pulsar rather than the stellar-mass companion. Spider orbital light curves are modulated, dominated by their binary companion thermal emission in the optical bands and by IBS synchrotron emission in the X-rays. We report on new XMM-Newton X-ray and U-band observations of J2215. We produce orbital light curves and use them to model the system properties. Our best-fit optical light model gives a neutron star mass M NS = 1.98 ± 0.08 M ⊙, lower than previously reported. However, uncertainty in the stellar atmosphere metallicity, a parameter to which J2215 is unusually sensitive, requires us to consider an acceptable systematic plus statistical range of M NS ∼ 1.85–2.3 M ⊙. From the X-ray analysis, we find that the IBS wraps around the pulsar but with a pulsar-wind-to-companion-wind-momentum ratio unusually close to unity, implying a flatter IBS geometry than seen in other spiders. Estimating the companion wind momentum and speed from the X-ray light curve, we find a companion mass-loss rate of Ṁc≳10−10M⊙ yr−1 so that J2215 may become an isolated millisecond pulsar in ∼1 Gyr. Our X-ray analyses place constraints on the magnetization and particle density of the pulsar wind and support models of magnetic reconnection and particle acceleration in the highly magnetized relativistic IBS.

Spindown of Pulsars Interacting with Companion Winds: The Impact of Magnetospheric Compression

Pulsars in binary systems with strong companion winds can have the magnetopause separating their magnetosphere from the wind located well within their light cylinder. This bow-like enclosure effectively creates a waveguide that confines the pulsar’s electromagnetic fields and can significantly alter its spindown. In this paper, we study the spindown of compressed pulsar magnetospheres in such systems. We parameterize the confinement as the ratio between the equatorial position of the magnetopause (or standoff distance) R m and the pulsar’s light cylinder R LC. Using particle-in-cell simulations, we quantify the pulsar spindown for a range of compressions, R m/R LC = 1/3–1, and inclination angles, χ = 0°…90°, between magnetic and rotation axes. Our strongly confined models (R m/R LC = 1/3) show two distinct limits. For χ = 0°, the spindown of a compressed pulsar magnetosphere is enhanced by approximately a factor of three compared to an isolated pulsar due to the increased number of open magnetic field lines. Conversely, for χ = 90°, the compressed pulsar spins down at less than 40% of the rate of an isolated reference pulsar due to the mismatch between the pulsar wind stripe wavelength and the waveguide size. We apply our analysis to the 2.77 s oblique rotator (χ = 60°) in the double-pulsar system PSR J0737-3039. With the numerically derived spindown estimate, we constrain its surface magnetic field to B * ≈ (7.3 ± 0.2) × 1011 G. We discuss the time modulation of its period derivative, the effects of compression on its braking index, and implications for the radio eclipse in PSR J0737-3039.

Hydrodynamical simulations of wind interaction in spider systems. A step toward understanding transitional millisecond pulsars

The detected population of "spiders", referring to millisecond pulsar binaries, has significantly grown in the past decade thanks to multiwavelength follow-up investigations of unidentified Fermi sources. These systems consist of low-mass stellar companions orbiting rotation-powered millisecond pulsars in short periods of a few hours up to day. Among them, a subset of intriguing objects called transitional millisecond pulsars (tMSPs) has been shown to exhibit a remarkable behavior, transitioning between pulsar-binary and faint low-mass X-ray binary states over a span of a few years. Our objective is to study the interaction of stellar winds in tMSPs in order to understand their observational properties. To this end we focus on the parameter range that places the system near Roche-lobe overflow. Employing the adaptative mesh refinement (AMR) AMRVAC 2.0 code, we performed 2D hydrodynamical (HD) simulations of the interaction between the flows from both stars, accounting for the effects of gravity and orbital motion. By studying the mass loss and launch speed of the winds, we successfully recreated two phenomenologically distinct regimes: the accretion stream and the pulsar radio state. We also identified the tipping point that marks the sharp transition between these two states. In the accretion stream state, we discover a very strong variability induced by the pulsar wind. In the pulsar state, we reconstructed the corresponding X-ray light curves of the system that produces the characteristic double-peak pattern of these systems. The position of the peaks is shifted due to orbital motion and the leading peak is weaker due to eclipsing by the companion. This study highlights the importance of gravity and orbital motion in the interaction between the companion and pulsar winds. Our setup allows the study of the complex interaction between the pulsar wind and an accretion stream during mass transfer. We suggest that a smaller leading peak in X-rays is indicative of a nearly edge-on system.

Probing the emission mechanism and nature of the pulsating compact object in the X-ray binary SAX J1324.4−6200

Recently, there has been renewed interest in the Be X-ray binary (Be/XRB) SAX J1324.4−6200 because of its spatial coincidence with a variable γ-ray source detected by Fermi/LAT. To explore more thoroughly its properties, new X-ray observations were carried out in 2023 by NuSTAR, XMM-Newton, and Swift satellites, jointly covering the energy range from 0.2 − 79 keV. SAX J1324.4−6200 was caught at an X-ray flux of ∼10−11 erg cm−2 s−1. The X-ray spectrum fits well with an absorbed power law with a high energy cutoff. Other acceptable fits require an additional blackbody component (kTbb ≈ 1.1 keV) or a Gaussian in absorption (Egabs ≈ 6.9 keV). We measured a NuSTAR spin period of 175.8127 ± 0.0036 s and an XMM-Newton spin period of 175.862 ± 0.025 s. We show that all the available spin period measurements of SAX J1324.4−6200, spanning 29 yr, are highly correlated with time, resulting in a remarkably stable spin-down of Ṗ = 6.09 ± 0.06 × 10−9 s s−1. We find that if SAX J1324.4−6200 hosts an accretion-powered pulsar, accretion torque models indicate a surface magnetic field of ∼1012 − 13 G. The X-ray properties emerging from our analysis strenghten the hypothesis that SAX J1324.4−6200 belongs to the small group of persistent Be/XRBs. We also performed radio observations with the Parkes Murriyang telescope, to search for radio pulsations. However, no radio pulsations compatible with the rotational ephemeris of SAX J1324.4−6200 were detected. We rule out the hypothesis that SAX J1324.4−6200 is a γ-ray binary where the emission is produced by interactions between the pulsar and the companion winds. Other models commonly used to account for the production of γ-rays in accreting pulsars cannot reproduce the bright emission from SAX J1324.4−6200. We examined other possible mechanisms behind the γ-ray emission and note that there is a ∼0.5% chance probability that an unknown extragalactic active galactic nucleus (AGN) observed through the Galactic plane may coincidentally fall within the Fermi/LAT error circle of the source and be responsible for the γ-ray emission.

Influence of tangential and normal components of wind speed on the flight distance of the projectile

Based on the mathematical model proposed by the authors for determining the force of frontal air resistance motion of the projectile, the influence of the accompanying (oncoming) wind on its flight range is investigated.
 Because the nature of the behavior of the frontal air resistance depends significantly on the speed of the projectile supersonic, subsonic or subsonic, then the functional dependence of the frontal air resistance force on
 speeds are described separately for each in particular. Values of characteristic functional coefficients dependencies are determined using the results of polygon studies, which are given in the tables shooting
 In the works of other authors, the magnitude of displacements of the flight range of the projectile due to the action of the companion (oncoming) wind was determined using certain dependencies. The values of correction coefficients and resistance coefficient was determined experimentally with a certain accuracy. In addition, pressure was not taken into account of wind on the side surface of the projectile and the influence of wind speed on the speed of sound in air, the latter depends on the direction of movement of the projectile in space.
 The work considers the influence of the tangential and normal components of the accompanying (oncoming) wind, in relation to the projectile's trajectory, to its dynamics. The effect of wind speed is also taken into account at the speed of sound in air.
 The obtained theoretical results allow us to state that the normal component of the wind is significant affects the flight range of the projectile if its trajectory is convex.
 The effect of the normal component of the speed of the accompanying (oncoming) wind on the movement of the projectile is not significant, if the trajectory of the projectile is flat. However, in the case of projectile movement along a convex trajectory, its influence becomes significant Discrepancies between the tabular displacements of projectile flight distances due to the action of the companion of (oncoming) wind given in the firing tables, and displacements of projectile flight range determined at taking into account the tangential and normal components of the wind and the effect of the wind on the speed of sound in the air is essential. The obtained theoretical studies allow to automate the process of solving the straight line or inverse problems of external ballistics with arbitrary deterministic and non-deterministic values factors.

CHIME Discovery of a Binary Pulsar with a Massive Nondegenerate Companion

Of the more than 3000 radio pulsars currently known, only ∼300 are in binary systems, and only five of these consist of young pulsars with massive nondegenerate companions. We present the discovery and initial timing, accomplished using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope, of the sixth such binary pulsar, PSR J2108+4516, a 0.577 s radio pulsar in a 269 day orbit of eccentricity 0.09 with a companion of minimum mass 11 M ⊙. Notably, the pulsar undergoes periods of substantial eclipse, disappearing from the CHIME 400–800 MHz observing band for a large fraction of its orbit, and displays significant dispersion measure and scattering variations throughout its orbit, pointing to the possibility of a circumstellar disk or very dense stellar wind associated with the companion star. Subarcsecond resolution imaging with the Karl G. Jansky Very Large Array unambiguously demonstrates that the companion is a bright, V ≃ 11 OBe star, EM* UHA 138, located at a distance of 3.26(14) kpc. Archival optical observations of EM* UHA 138 approximately suggest a companion mass ranging from 17.5 M ⊙ < M c < 23 M ⊙, in turn constraining the orbital inclination angle to 50.°3 ≲ i ≲ 58.°3. With further multiwavelength follow-up, PSR J2108+4516 promises to serve as another rare laboratory for the exploration of companion winds, circumstellar disks, and short-term evolution through extended-body orbital dynamics.

UV spectropolarimetry with Polstar: massive star binary colliding winds

The winds of massive stars are important for their direct impact on the interstellar medium, and for their influence on the final state of a star prior to it exploding as a supernova. However, the dynamics of these winds is understood primarily via their illumination from a single central source. The Doppler shift seen in resonance lines is a useful tool for inferring these dynamics, but the mapping from that Doppler shift to the radial distance from the source is ambiguous. Binary systems can reduce this ambiguity by providing a second light source at a known radius in the wind, seen from orbitally modulated directions. From the nature of the collision between the winds, a massive companion also provides unique additional information about wind momentum fluxes. Since massive stars are strong ultraviolet (UV) sources, and UV resonance line opacity in the wind is strong, UV instruments with a high resolution spectroscopic capability are essential for extracting this dynamical information. Polarimetric capability also helps to further resolve ambiguities in aspects of the wind geometry that are not axisymmetric about the line of sight, because of its unique access to scattering direction information. We review how the proposed MIDEX-scale mission Polstar can use UV spectropolarimetric observations to critically constrain the physics of colliding winds, and hence radiatively-driven winds in general. We propose a sample of 20 binary targets, capitalizing on this unique combination of illumination by companion starlight, and collision with a companion wind, to probe wind attributes over a range in wind strengths. Of particular interest is the hypothesis that the radial distribution of the wind acceleration is altered significantly, when the radiative transfer within the winds becomes optically thick to resonance scattering in multiple overlapping UV lines.

Fitting generalized Pareto distribution based on the expected order statistics and its application for ice accretion hazard mapping

The extreme value analysis of the ice accretion thickness can be carried out based on the peaks-over-threshold (POT) approach to take advantage of the available rare samples in assessing the ice accretion thickness hazard. For the POT approach, the generalized Pareto distribution (GPD) with location, scale, and shape parameters has been frequently employed, although the estimators of parameters of GPD by using existing fitting methods could be associated with significant bias or variability. In the present study, we propose a new nonlinear least-squares fitting method by considering the expected order statistics. We compare its performance with some existing methods through simulation analysis, showing that the proposed method outperforms some well-known existing methods for a range of values of the shape parameter.We then apply the POT approach with GPD as well as the block maximum approach with Gumbel distribution to estimate T-year return period value of the annual maximum ice accretion thickness, xT, for Canadian sites. The analysis results indicate that xT is sensitive to whether the POT or the annual maximum approach with Gumbel distribution is employed. For the considered cases, the use of GPD fitted to POT values is preferred. Also, based on the wind-on-ice analysis results, it is suggested a companion wind load factor of about 0.5 to 0.55 could be adopted.

Unraveling the Eclipse Mechanism of a Binary Millisecond Pulsar Using Broadband Radio Spectra

Abstract The frequency dependent eclipses of the radio emission from millisecond pulsars in compact binary systems provide an opportunity to understand the eclipse mechanism and to determine the nature of the eclipsing medium. We combine multifrequency observations from the upgraded Giant Metrewave Radio Telescope and model the broadband radio spectrum in the optically thick to thin transition regime to constrain the eclipse mechanism. The best-fit model to the eclipse phase spectra favors synchrotron absorption by relativistic electrons. We are able to strongly constrain the frequency of onset of the eclipse to 345 ± 5 MHz, which is an order of magnitude more precise than previous estimates. The dependence on the magnetic field strength of synchrotron absorption allowed us to estimate the magnetic field strength of the eclipse medium to be ∼13 G, which is very similar to the values obtained by considering a pressure balance between the incident pulsar wind and the stellar wind of the companion. Applying this method to other millisecond binary pulsars will enable us to determine if the eclipse mechanisms are all the same and also estimate the wind and magnetic field properties of the companion stars. The method could also be applied to other systems where pulsars interact with companion winds in binary systems and in all cases it will lead to a better understanding of the evolutionary processes.

XMM-Newton Observes the Intrabinary Shock of PSR J1959+2048

Abstract In a multi-orbit (100 ks) XMM-Newton exposure of the original black widow pulsar, PSR J1959+2048, we measure the strong orbital modulation caused by intrabinary shock (IBS) emission. The IBS light curve peak appears asymmetric, which we attribute to sweep-back effects in the companion wind. We also see evidence for an X-ray eclipse by the companion and its wind. Together with the IBS fit, this supports an edge-on i ∼ 90° view of the system and a modest ∼1.8 M ⊙ mass for the recycled pulsar. Our IBS fit parameters imply a wind flux that, if persistent, would evaporate the companion within a few Gyr.

Chandra and HST studies of six millisecond pulsars in the globular cluster M13

ABSTRACT We analyse 55 ks of Chandra X-ray observations of the Galactic globular cluster (GC) M13. Using the latest radio timing positions of six known millisecond pulsars (MSPs) in M13 from Wang et al. (2020), we detect confident X-ray counterparts to five of the six MSPs at X-ray luminosities of LX(0.3–8 keV)∼3 × 1030–1031 erg s−1, including the newly discovered PSR J1641+3627F. There are limited X-ray counts at the position of PSR J1641+3627A, for which we obtain an upper limit LX &amp;lt; 1.3 × 1030 erg s−1. We analyse X-ray spectra of all six MSPs, which are well described by either a single blackbody (BB) or a single power-law model. We also incorporate optical/UV imaging observations from the Hubble Space Telescope (HST) and find optical counterparts to PSR J1641+3627D and J1641+3627F. Our colour–magnitude diagrams indicate the latter contains a white dwarf, consistent with the properties suggested by radio timing observations. The counterpart to J1641+3627D is only visible in the V band; however, we argue that the companion to J1641+3627D is also a white dwarf, since we see a BB-like X-ray spectrum, while MSPs with non-degenerate companions generally show non-thermal X-rays from shocks between the pulsar and companion winds. Our work increases the sample of known X-ray and optical counterparts of MSPs in GCs.

On the vanishing orbital X-ray variability of the eclipsing binary millisecond pulsar 47 Tuc W

ABSTRACT Redback millisecond pulsars (MSPs) typically show pronounced orbital variability in their X-ray emission due to our changing view of the intrabinary shock (IBS) between the pulsar wind and stellar wind from the companion. Some redbacks (‘transitional’ MSPs) have shown dramatic changes in their multiwavelength properties, indicating a transition from a radio pulsar state to an accretion-powered state. The redback MSP 47 Tuc W showed clear X-ray orbital variability in the Chandra ACIS-S observations in 2002, which were not detectable in the longer Chandra HRC-S observations in 2005–06, suggesting that it might have undergone a state transition. However, the Chandra observations of 47 Tuc in 2014–15 show similar X-ray orbital variability as in 2002. We explain the different X-ray light curves from these epochs in terms of two components of the X-ray spectrum (soft X-rays from the pulsar versus harder X-rays from the IBS), and different sensitivities of the X-ray instruments observing in each epoch. However, when we use our best-fitting spectra with HRC response files to model the HRC light curve, we expect a more significant and shorter dip than that observed in the 2005–06 Chandra data. This suggests an intrinsic change in the IBS of the system. We use the icarus stellar modelling software, including calculations of heating by an IBS, to model the X-ray, optical, and UV light curves of 47 Tuc W. Our best-fitting parameters point towards a high-inclination system ($i \sim 60 \deg$), which is primarily heated by the pulsar radiation, with an IBS dominated by the companion wind momentum.

Correlation of Concurrent Extreme Metocean Hazards Considering Seasonality

Simultaneous occurrence of metocean variables can present a multihazard to maritime systems. However, simplified design approaches to assess simultaneous significant wave heights and wind velocities are lacking, especially if seasonality is considered. This is addressed in this study by using extreme significant wave heights and companion wind velocities recorded in the Gulf of Mexico. Time-dependent, generalized extreme value (GEV) models and classical regression are the basis to propose a simplified approach to estimate correlated extreme significant wave heights and wind velocities associated with given return periods, accounting for seasonality and including measures of uncertainty. It is found that the proposed approach is a new but simple method to adequately characterize the concurrent extreme metocean variables and their uncertainty. It is concluded that the method is an effective probabilistic design tool to determine simultaneous extreme significant wave heights and companion wind velocities for desired return periods and seasonality.

On the joint tropical cyclone wind and wave hazard

A simulation-based framework to estimate tropical cyclone (TC) induced wind and wave hazards is proposed by considering the TC tracks, wind field model, and significant wave height model. The proposed approach is applied to several offshore sites located near the coastline of mainland China. Both the block maximum analysis and event-based analysis are carried out. It was concluded that the marginal distribution of the annual maximum wind speed and of annual maximum significant wave height could be modelled using the generalized extreme value distribution. Plots of paired samples in terms of probability of the annual maximum wind speed (significant wave height) and of its companion significant wave height (wind speed) suggested that they could be characterized using Gumbel copula. The ranked correlation coefficient (i.e., Kendall’s tau) varies from about 0.4 to 0.7, depending on the considered paired random variable and site. The implication of the results in assigning the load combination factor for combined TC wind and wave load effect is discussed. The deaggregation results indicate that the identified most likely event causing p-quantile of the annual maximum wind speed differs from that identified from p-quantile of the annual maximum significant wave height. Therefore, for the reliability analysis, risk modelling, and emergency preparedness, it is recommended that two sets of hazard characterizations are be considered: one based on the maximum of wind speed and its companion wave height, and the other based on maximum wave height and its companion wind speed.

T CrB: Radio Observations during the 2016–2017 “Super-active” State

Abstract We obtained radio observations of the symbiotic binary and known recurrent nova T Coronae Borealis following a period of increased activity in the optical and X-ray bands. A comparison of our observations with those made prior to 2015 indicates that the system is in a state of higher emission in the radio as well. The spectral energy distributions are consistent with optically thick thermal bremsstrahlung emission from a photoionized source. Our observations indicate that the system was in a state of increased ionization in the companion wind, possibly driven by an increase in accretion rate, with the radio photosphere located well outside the binary system.

The Synchrotron Emission Pattern of Intrabinary Shocks

Abstract We model millisecond pulsar winds colliding with radiatively driven companion winds in black widow and redback systems. For the redbacks, the geometry of this intrabinary shock (IBS) is quite sensitive to the expected equatorial concentration in the pulsar outflow. We thus analytically extend IBS thin-shock models to pulsar winds. We compute the synchrotron emission from such shocks, including the buildup and cooling of the particle population as it accelerates along the IBS. For reasonable parameters, this IBS flux dominates the binary emission in the X-ray band. The modeling shows subtle variation in spectrum across the IBS peak, accessible to sensitive X-ray studies. As example applications, we fit archival CXO/XMM data from the black widow pulsar J1959+2048 and the redback PSR J2339–0533, finding that the model reproduces well the orbital light-curve profiles and energy spectra. The results show a very hard injected electron spectrum, indicating likely dominance by reconnection. The light-curve fitting is sensitive to the geometric parameters, including the very important orbital inclination i. Coupled with optical fits of the companion star, such IBS X-ray light-curve modeling can strongly constrain the binary geometry and the energetics of the millisecond pulsar wind.

Constraining magnetic fields through plasma lensing: application to the Black Widow pulsar

In regions with strongly varying electron density, radio emission can be magnified significantly by plasma lensing. In the presence of magnetic fields, magnification in time and frequency will be different for two circular polarizations. We show how these effects can be used to measure or constrain the magnetic field parallel to the line of sight, $B_\parallel$, as well as its spatial structure, $\sigma_{B_\parallel}$, in the lensing region. In addition, we discuss how generalized Faraday rotation can constrain the strength of the perpendicular field, $B_\perp$. We attempt to make such measurements for the Black Widow pulsar, PSR~B1957+20, in which plasma lensing was recently discovered. For this system, pressure equilibrium suggests $B\gtrsim 20\,$G at the interface between the pulsar and companion winds, where the radio eclipse starts and ends, and where most lensing occurs. We find no evidence for large-scale magnetic fields, with, on average, $B_\parallel=0.02\pm0.09\,$G over the egress lensing region. From individual lensing events, we strongly constrain small scale magnetic structure to $\sigma_B<10\,$mG, thus excluding scenarios with a strong but rapidly varying field. Finally, from the lack of reduction of average circular polarization in the same region, we rule out a strong, quasi-transverse field. We cannot identify any plausible scenario in which a large magnetic field in this system is concealed, leaving the nature of the interface between the pulsar and companion winds an enigma. Our method can be applied to other sources showing plasma lensing, including other eclipsing pulsars and fast radio bursts, to study the local properties of the magnetic field.

Pressure Balance and Intrabinary Shock Stability in Rotation-powered-state Redback and Transitional Millisecond Pulsar Binary Systems

Abstract A number of low-mass millisecond pulsar (MSP) binaries in their rotation-powered state exhibit double-peaked X-ray orbital modulation centered at inferior pulsar conjunction. This state, which has been known to persist for years, has recently been interpreted as emission from a shock that enshrouds the pulsar. However, the pressure balance for such a configuration is a crucial unresolved issue. We consider two scenarios for pressure balance: a companion magnetosphere and stellar mass loss with gas dominance. It is found that the magnetospheric scenario requires several kilogauss poloidal fields for isobaric surfaces to enshroud the MSP, as well as for the magnetosphere to remain stable if there is significant mass loss. For the gas-dominated scenario, it is necessary that the companion wind loses angular momentum prolifically as an advection- or heating-dominated flow. Thermal bremsstrahlung cooling in the flow may be observable as a UV to soft X-ray component independent of orbital phase if the mass rate is high. We formulate the general requirements for shock stability against gravitational influences in the pulsar rotation-powered state for the gas-dominated scenario. We explore stabilizing mechanisms, principally irradiation feedback, which anticipates correlated shock emission and companion variability and predicts F γ /F X ≲ 14 for the ratio of pulsar magnetospheric γ-ray to total shock soft-to-hard X-ray fluxes. This stability criterion implies an unbroken extension of X-ray power-law emission to hundreds of keV for some systems. We explore observational discriminants between the gas-dominated and magnetospheric scenarios, motivating contemporaneous radio through γ-ray monitoring of these systems.

The young Be-star binary Circinus X-1

AbstractCir X-1 is a young X-ray binary exhibiting X-ray flux changes of four orders of magnitude over several decades. It has been observed many times since the launch of the Chandra X-ray Observatory with high energy transmission grating spectrometer and each time the source gave us a vastly different look. At its very lowest X-ray flux we found a single 1.7 keV blackbody spectrum with an emission radius of 0.5 km. Since the neutron star in Cir X-1 is only few thousand years old we identify this as emission from an accretion column since at this youth the neutron star is assumed to be highly magnetized. At an X-ray flux of 1.8×10−11 erg cm−2 s−1 this implies a moderate magnetic field of a few times of 1011 G. The photoionized X-ray emission line properties at this low flux are consistent with B5-type companion wind. We suggest that Cir X-1 is a very young Be-star binary.

Computational and experimental investigation of free vibration and flutter of bridge decks

A modified rigid-object formulation is developed, and employed as part of the fluid–object interaction modeling framework from Akkerman et al. (J Appl Mech 79(1):010905, 2012. https://doi.org/10.1115/1.4005072 ) to simulate free vibration and flutter of long-span bridges subjected to strong winds. To validate the numerical methodology, companion wind tunnel experiments have been conducted. The results show that the computational framework captures very precisely the aeroelastic behavior in terms of aerodynamic stiffness, damping and flutter characteristics. Considering its relative simplicity and accuracy, we conclude from our study that the proposed free-vibration simulation technique is a valuable tool in engineering design of long-span bridges.