Millisecond Pulsars In Globular Clusters Research Articles

Likely Detection of γ-Ray Pulsations of PSR J1717+4308A in NGC 6341 and Implication of the γ-Ray Millisecond Pulsars in Globular Clusters

We report our analysis results for the globular cluster (GC) NGC 6341 (M92), as a millisecond pulsar (MSP) J1717+4308A has recently been reported found in this GC. The data used are from the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope (Fermi). We detect γ-ray pulsations of the MSP at a 4.4σ confidence level (the corresponding weighted H-test value is ∼28.4). This MSP, the fourth γ-ray pulsar found in a GC, does not have significant off-pulse emission and has γ-ray luminosity and efficiency 1.3 × 1034 erg s−1 and 1.7%, respectively. In order to have a clear view on the properties of the known GC γ-ray MSPs, we reanalyze the Fermi-LAT data for the other three ones. These four MSPs share the properties of either having high (∼1036 erg s−1) or being in the GCs that contain only limited numbers of known MSPs. In addition, we find that PSRs J1823−3021A and B1821−24, in NGC 6624 and NGC 6626, respectively, have detectable off-pulse γ-ray emission and PSR J1835−3259B in NGC 6652 does not. Using the obtained off-pulse spectra or spectral upper limits, we constrain the numbers of other MSPs in the four GCs. The results are consistent with the numbers of the radio pulsars reported in them. While at least in NGC 6624 and NGC 6626, the contribution of other MSPs to their observed γ-ray emission can not be ignored, our study indicates that the presence of a bright MSP could be the dominant factor for whether a GC is detectable at γ-rays or not.

Formation of Low-mass Black Holes and Single Millisecond Pulsars in Globular Clusters

Close encounters between neutron stars and main-sequence stars occur in globular clusters and may lead to various outcomes. Here we study encounters resulting in the tidal disruption of the star. Using N-body models, we predict the typical stellar masses in these disruptions and the dependence of the event rate on the host cluster properties. We find that tidal disruption events occur most frequently in core-collapsed globular clusters and that roughly 25% of the disrupted stars are merger products (i.e., blue straggler stars). Using hydrodynamic simulations, we model the tidal disruptions themselves (over timescales of days) to determine the mass bound to the neutron star and the properties of the accretion disks formed. In general, we find roughly 80%–90% of the initial stellar mass becomes bound to the neutron star following disruption. Additionally, we find that neutron stars receive impulsive kicks of up to about 20 km s−1 as a result of the asymmetry of unbound ejecta; these kicks place these neutron stars on elongated orbits within their host cluster, with apocenter distances well outside the cluster core. Finally, we model the evolution of the (hypercritical) accretion disks on longer timescales (days to years after disruption) to estimate the accretion rate onto the neutron stars and accompanying spin-up. As long as ≳1% of the bound mass accretes onto the neutron star, millisecond spin periods can be attained. We argue the growing numbers of isolated millisecond pulsars observed in globular clusters may have formed, at least in part, through this mechanism. In the case of significant mass growth, some of these neutron stars may collapse to form low-mass (≲3 M ⊙) black holes.

A census of X-ray millisecond pulsars in globular clusters

ABSTRACT We present a comprehensive census of X-ray millisecond pulsars (MSPs) in 29 Galactic globular clusters (GCs), including 68 MSPs with confirmed X-ray luminosities and 107 MSPs with X-ray upper limits. We compile previous X-ray studies of GC MSPs, and add new analyses of six MSPs (PSRs J1326–4728A, J1326–4728B, J1518+0204C, J1717+4308A, J1737–0314A, and J1807–2459A) discovered in five GCs. Their X-ray spectra are well described by a single blackbody model, a single power-law model, or a combination of them, with X-ray luminosities ranging from 1.9 × 1030to 8.3 × 1031 erg s−1. We find that most detected X-ray MSPs have luminosities between ∼1030 and 3 × 1031 erg s−1. Redback pulsars are a relatively bright MSP population with X-ray luminosities of ∼2 × 1031–3 × 1032 erg s−1. Black widows show a bimodal distribution in X-ray luminosities, with eclipsing black widows between ∼7 × 1030 and 2 × 1031 erg s−1, while the two confirmed non-eclipsing black widows are much fainter, with LX of 1.5–3 × 1030 erg s−1, suggesting an intrinsic difference in the populations. We estimate the total number of MSPs in 36 GCs by considering the correlation between the number of MSPs and stellar encounter rate in GCs, and suggest that between 600 and 1500 MSPs exist in these 36 GCs. Finally, we estimate the number of X-ray-detectable MSPs in the Galactic bulge, finding that 1–86 MSPs with LX > 1033 erg s−1, and 20–900 MSPs with LX > 1032 erg s−1 should be detectable there.

The Exotic Zoo of Millisecond Pulsars in Globular Clusters: a multi-wavelength study

This work is focused on the study of millisecond pulsars in globular cluster by using multi-wavelength observations. Radio observations have been used to search for and timing the pulsars. An alternative method to search for very faint pulsars is first presented and then successfully applied to observations of stellar system Terzan 5, leading to the discovery of three new pulsars. The update of the timing solutions of 9 pulsars in M28 is then presented. The timing solutions now cover a data span of 10 years and provide spin, astrometric and orbital properties for all the systems. For the case of the eccentric binaries M28C and M28D, post-Keplerian corrections to the orbit have been measured in order to derive the masses of the binary components. The measurement of the pulsar proper motions allowed to constrain the cluster motion as a whole and its orbit around the Galaxy. Finally, the pulsar spin and orbital period derivatives have been used to measure their accelerations induced by the cluster potential field. Optical observations have been used to search for millisecond pulsar optical counterparts. Six new companion stars have been discovered. In particular, four companions turned out to be He white dwarfs. One companion turned out to be a faint and non-degenerate object, strongly affected by heating of the stellar side exposed to the pulsar flux. Finally, one companion is a main-sequence star which shows Hα emission likely due to a low-level mass transfer. Furthermore, we identified the companion star to a transient low-mass X-ray binary. This companion turned out to be a sub-giant branch star and therefore this system is likely in a very early phase of the mass accretion stage. Conclusion are drawn in the final chapter, where the evolution of millisecond pulsars is discussed and possible future developments are suggested.

Population Syntheses of Millisecond Pulsars from the Galactic Disk and Bulge

Abstract We present the results of a population synthesis of radio and γ-ray millisecond pulsars (MSPs) from the galactic disk (GD). Using 92 radio MSPs detected in 13 radio surveys and 54 Fermi MSPs detected as point sources in the first point source catalog, we establish six free parameters corresponding to the overall factor and the exponents of the period and period derivative dependence for each of the radio and γ-ray empirical luminosity models. We test three high-energy emission models described by the two-pole caustic slot-gap, outer-gap, and pair-starved polar-cap geometries. The simulated distributions of pulsar properties adequately describe the distributions of detected MSPs from the GD. We explore the γ-ray emission from groups of MSPs in globular clusters and in the galactic bulge. The simulation predicts reasonable numbers of Fermi MSPs detected in the other point source catalogs and anticipates a bright future for Fermi observations of MSPs, expecting a total of ≈170 MSP detections from the GD within 10 years. Our numbers of simulated MSPs in globular clusters are in agreement with those derived from Fermi detections. The simulation predicts that about 11,000 MSPs in the galactic bulge are required to explain the γ-ray galactic center excess.

Discovery of Three New Millisecond Pulsars in Terzan 5

Abstract We report on the discovery of three new millisecond pulsars (MSPs; namely J1748−2446aj, J1748−2446ak, and J1748−2446al) in the inner regions of the dense stellar system Terzan 5. These pulsars have been discovered thanks to a method, alternative to the classical search routines, that exploited the large set of archival observations of Terzan 5 acquired with the Green Bank Telescope over five years (from 2010 to 2015). This technique allowed the analysis of stacked power spectra obtained by combining ∼206 hr of observation. J1748−2446aj has a spin period of ∼2.96 ms, J1748−2446ak of ∼1.89 ms (thus it is the fourth fastest pulsar in the cluster) and J1748−2446al of ∼5.95 ms. All three MSPs are isolated, and currently we have timing solutions only for J1748−2446aj and J1748−2446ak. For these two systems, we evaluated the contribution to the measured spin-down rate of the acceleration due to the cluster potential field, thus estimating the intrinsic spin-down rates, which are in agreement with those typically measured for MSPs in globular clusters (GCs). Our results increase the number of pulsars known in Terzan 5 to 37, which now hosts 25% of the entire pulsar population identified, so far, in GCs.

Detecting dark matter with imploding pulsars in the galactic center.

The paucity of old millisecond pulsars observed at the galactic center of the Milky Way could be the result of dark matter accumulating in and destroying neutron stars. In regions of high dark matter density, dark matter clumped in a pulsar can exceed the Schwarzschild limit and collapse into a natal black hole which destroys the pulsar. We examine what dark matter models are consistent with this hypothesis and find regions of parameter space where dark matter accumulation can significantly degrade the neutron star population within the galactic center while remaining consistent with observations of old millisecond pulsars in globular clusters and near the solar position. We identify what dark matter couplings and masses might cause a young pulsar at the galactic center to unexpectedly extinguish. Finally, we find that pulsar collapse age scales inversely with the dark matter density and linearly with the dark matter velocity dispersion. This implies that maximum pulsar age is spatially dependent on position within the dark matter halo of the Milky Way. In turn, this pulsar age spatial dependence will be dark matter model dependent.

Gamma-rays from millisecond pulsar population within the central stellar cluster in the Galactic Centre

Abstract It was proposed that the central dense stellar cluster in the Galactic Centre, containing the mass of ∼4 times larger than that of the central black hole, had been formed as a result of a merging of several massive globular clusters. These globular clusters are expected to provide a large number of millisecond pulsars (MSPs) within the central parsec of the Galactic Centre. We propose that the GeV γ-ray emission observed from the Galactic Centre is in fact a cumulative effect of the emission from several globular clusters captured by the Galactic Centre black hole. Moreover, the MSPs in globular clusters accelerate leptons in their wind zones to energies of the order of a few tens of TeV injecting them into the dense infrared and optical radiation region present within the central parsec. We calculate the expected TeV γ-ray emission produced by these leptons by the inverse Compton scattering process in the soft radiation field. It is shown that this emission can be responsible for the multi-TeV γ-rays observed by the Cherenkov telescopes from the Galactic Centre for reasonable densities of the soft radiation, diffusion models for the propagation of leptons, their injection parameters (fluxes and spectral proprieties). If the energy conversion efficiency from the pulsars to the relativistic leptons is of the order of 10 per cent, then about a thousand of MSPs have to be present in the central cluster in the Galactic Centre.

Numerical modelling of -ray emission produced by electrons originating from the magnetospheres of millisecond pulsars in globular clusters

Globular clusters are source of gamma-ray radiation. At GeV energies, their emission is attributed to magnetospheric activity of millisecond pulsars residing in the clusters. Inverse Compton scattering of ambient photon fields on relativistic particles diffusing through cluster environment is thought to be the source of GeV-TeV emission of globular clusters. Using pair starved polar cap model gamma-ray emission from synthetic millisecond pulsar was modelled. In addition to pulsar emission characteristics, the synthetic pulsar model yielded spectra of electrons escaping pulsar magnetosphere. To simulate gamma-ray emission of globular cluster, both products of synthetic millisecond pulsar modelling were used. Gamma-ray spectra of synthetic millisecond pulsars residing in the cluster were summed to produce the magnetospheric component of cluster emission. Electrons ejected by these pulsars were injected into synthetic globular cluster environment. Their diffusion and interaction, both, with cluster magnetic field and ambient photon fields, were performed with Bednarek & Sitarek (2007) model yielding ICS component of cluster emission. The sum of the magnetospheric and ICS components gives the synthetic gamma-ray spectrum of globular cluster. The synthetic cluster spectrum stretches from GeV to TeV energies. Detailed modelling was preformed for two globular clusters: Ter 5 and 47 Tuc. Simulations are able to reproduce (within errors) the shape and the flux level of the GeV part of the spectrum observed for both clusters with the Fermi/LAT instrument. The synthetic flux level obtained in the TeV part of the clusters' spectrum is in agreement with a H.E.S.S. upper limit determined for 47 Tuc, and with emission level recently detected for Ter 5 with H.E.S.S. telescope. The synthetic globular cluster model, however, is not able to reproduce the exact shape of the TeV spectrum observed for Ter 5.

Can the known millisecond pulsars help in the detection of intermediate-mass black holes at the centers of globular clusters?

We consider the possibility of detecting intermediate-mass (103–104M⊙) black holes, whose existence at the centers of globular clusters is expected from optical and infrared observations, using precise pulse arrival timing for the millisecond pulsars in globular clusters known to date. For some of these pulsars closest to the cluster centers, we have calculated the expected delay times of pulses as they pass in the gravitational field of the central black hole. The detection of such a time delay by currently available instruments for the known pulsars is shown to be impossible at a black hole mass of 103M⊙ and very problematic at a black hole mass of 104M⊙. In addition, the signal delay will have a negligible effect on the pulsar periods and their first derivatives compared to the current accuracy of their measurements.

ChandraX‐Ray Observations of 19 Millisecond Pulsars in the Globular Cluster 47 Tucanae

We present spectral and long-timescale variability analyses of Chandra X-Ray Observatory ACIS-S observations of the 19 millisecond pulsars (MSPs) with precisely known positions in the globular cluster 47 Tucanae. The X-ray emission of the majority of these MSPs is well described by a thermal (blackbody or neutron star hydrogen atmosphere) spectrum with a temperature Teff ~ (1-3) × 106 K, emission radius Reff ~ 0.1-3 km, and luminosity LX ~ 1030-1031 ergs s-1. For several MSPs, there are indications that a second thermal component is required, similar to what is seen in some nearby field MSPs. The observed radiation most likely originates from the heated magnetic polar caps of the MSPs. The small apparent scatter in LX is consistent with thermal emission from the polar caps of a global dipole field, although the small emission areas may imply either a more complex small-scale magnetic field configuration near the neutron star surface or nonuniform polar cap heating. The radio eclipsing binary MSPs 47 Tuc J, O, and W show a significant nonthermal (power-law) component, with spectral photon index Γ ~ 1-1.5, which most likely originates in an intrabinary shock formed due to interaction between the relativistic pulsar wind and matter from the stellar companion. We reexamine the X-ray-spin-down luminosity relation (LX-Ė relation) and find that for the MSPs with thermal spectra LX ∝ Ėβ, where β ~ 0.2 ± 1.1. Due to the large uncertainties in both parameters, the result is consistent with both the linear LX-Ė relation and the flatter LX ∝ Ė0.5 predicted by polar cap heating models. In terms of X-ray properties, we find no clear systematic differences between MSPs in globular clusters and in the field of the Galaxy. We discuss the implications of these results on the present understanding of the X-ray emission properties of MSPs.

High‐Energy Emission from Millisecond Pulsars

The X-ray and gamma-ray spectrum of rotation-powered millisecond pulsars is investigated in a model for acceleration and pair cascades on open field lines above the polar caps. Although these pulsars have low surface magnetic fields, their short periods allow them to have large magnetospheric potential drops, but the majority do not produce sufficient pairs to completely screen the accelerating electric field. The accelerating particles maintain high Lorentz factors and undergo cyclotron resonant absorption of radio emission, that produces and maintains a large pitch angle, resulting in a strong synchrotron component. The resulting spectra consist of several distinct components: curvature radiation from primary electrons dominating from 1 - 100 GeV, synchrotron radiation from primary and secondary electrons dominating up to about 100 MeV, and much weaker inverse-Compton radiation from primary electrons at 0.1 - 1 TeV. We find that the relative size of these components depends on pulsar period, period derivative, and neutron star mass and radius with the level of the synchrotron component also depending sensitively on the radio emission properties. This model is successful in describing the observed X-ray and gamma-ray spectrum of PSR J0218+4232 as synchrotron radiation, peaking around 100 MeV and extending up to a turnover around several GeV. The predicted curvature radiation components from a number of millisecond pulsars, as well as the collective emission from the millisecond pulsars in globular clusters, should be detectable with AGILE and GLAST. We also discuss a hidden population of X-ray-quiet and radio-quiet millisecond pulsars which have evolved below the pair death line, some of which may be detectable by telescopes sensitive above 1 GeV.

On the Origin of X‐Ray Emission from Millisecond Pulsars in 47 Tucanae

The observed spectra and X-ray luminosities of millisecond pulsars in 47 Tuc can be interpreted in the context of theoretical models based on strong, small-scale, multipole fields on the neutron star surface. For multipole fields that are relatively strong compared to the large-scale dipole field, the emitted X-rays are thermal and likely result from polar cap heating associated with the return current from the polar gap. On the other hand, for weak multipole fields, the emission is nonthermal and results from synchrotron radiation of e? pairs created by curvature radiation. The X-ray luminosity LX is related to the spin-down power Lsd, expressed in the form LX L with ? ~ 0.5 and ~1 for strong and weak multipole fields, respectively. If the polar cap size is of the order of the length scale of the multipole field s, the polar cap temperature is ~3 ? 106 K(Lsd/1034 ergs s-1)1/8(s/3 ? 104 cm)-1/2. A comparison of the X-ray properties of millisecond pulsars in globular clusters and in the Galactic field suggests that the emergence of relatively strong small-scale multipole fields from the neutron star interior may be correlated with the age and evolutionary history of the underlying neutron star.

Formation of Short-Period Binary Pulsars in Globular Clusters.

We present a new dynamical scenario for the formation of short-period binary millisecond pulsars in globular clusters. Our work is motivated by the recent observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc, most neutron stars acquire binary companions through exchange interactions with primordial binaries. The resulting systems have semimajor axes in the range approximately 0.1-1 AU and neutron star companion masses approximately 1-3 M middle dot in circle. For many of these systems, we find that when the companion evolves off the main sequence and fills its Roche lobe, the subsequent mass transfer is dynamically unstable. This leads to a common envelope phase and the formation of short-period neutron star-white dwarf binaries. For a significant fraction of these binaries, the decay of the orbit due to gravitational radiation will be followed by a period of stable mass transfer driven by a combination of gravitational radiation and tidal heating of the companion. The properties of the resulting short-period binaries match well those of observed binary pulsars in 47 Tuc.

Discovery of Kilohertz Quasi-periodic Oscillations from 4U 1820−303 with [ITAL]Rossi X-Ray Timing Explorer[/ITAL

We have detected high-frequency (HF) quasi-periodic oscillations (QPOs) from the low-mass X-ray binary 4U 1820-303 during observations performed in 1996 October using the Rossi X-ray Timing Explorer. The QPOs are visible when the source occupies the low-state luminosity range LX = 2.4-3.1 × 1037 ergs s-1 (2-20 keV, at 6.4 kpc); the centroid frequency of the main QPO peak varies between 546 ± 2 Hz and 796 ± 6 Hz and is tightly correlated with the source count rate. The measured QPO widths are typically ~20 Hz, with mean rms amplitude 4.1% ± 0.3%. At the upper end of this luminosity range a second significant QPO peak appears with frequency 1065 ± 7 Hz, width 40 ± 20 Hz, and rms amplitude 3.2% ± 0.8%. When both QPOs are visible simultaneously, the difference between their frequencies is 275 ± 8 Hz. When the source brightens beyond LX = 3.1 × 1037 ergs s-1 (~10% of the Eddington limit for a helium-rich envelope), neither QPO is detected. Neither the magnetospheric beat frequency model nor the sonic point model of HF QPOs provides a perfect explanation of the phenomenology we observe. These results represent the first detection of kilohertz QPO activity in a globular cluster X-ray binary, and provide a new method of directly comparing the properties of cluster and noncluster neutron star binaries. If the highest QPO frequency we observe is identified with the marginally stable orbit in the accretion disk, the neutron star mass may be ~2 M☉, 35%-50% more massive than usually assumed. This may have consequences for the current evolutionary scenarios for this source and also for the debate about the evolution of millisecond pulsars in globular clusters.

The effect of encounters on the eccentricity of binaries in clusters

We derive analytical expressions for the change in the orbital eccentricity of a binary following a distant encounter with a third star on a hyperbolic or parabolic orbit. To establish the accuracy of these expressions, we present detailed comparisons with the results of direct numerical integrations of the equations of motion for the three bodies. We treat with particular care the difficult case of a binary with zero initial eccentricity. In this case, we show that the eccentricity $\delta e$ induced by the encounter declines in general as a power-law, $\delta e\propto (a/\rp)^{5/2}$, where $a$ is the binary semi-major axis and $\rp$ is the periastron distance of the encounter. This power-law arises from the octupole-level secular perturbation of the binary. In contrast, non-secular quadrupole-level perturbations induce an eccentricity change that declines exponentially with $\rp$. These non-secular effects can become dominant at sufficiently small $\rp$, for a sufficiently high relative velocity, or for a sufficiently massive perturber. We also derive cross sections for eccentricity change and compare our results with those of previous studies based on numerical scattering experiments. Our results have important implications for a number of astrophysical problems including, in particular, the evolution of binary millisecond pulsars in globular clusters.

The Orbital Eccentricities of Binary Millisecond Pulsars in Globular Clusters

Low-mass binary millisecond pulsars are born with very small orbital eccentricities, typically of order ei ∼ 10−6−10−3. In globular clusters, however, higher eccentricities ef ≫ ei can be induced by dynamical interactions with passing stars. Using both analytical perturbation calculations and numerical integrations, we have shown (Heggie & Rasio 1996) that the cross section for this process is much larger than previously estimated. This is because, even for initially circular binaries, the induced eccentricity ef for an encounter with pericentre separation rp beyond a few times the binary semi-major axis a declines only as a power-law, ef ∝ (rp/a)−5/2, and not as an exponential. We find that all currently known low-mass binary millisecond pulsars in globular clusters must have been affected by interactions, with their current eccentricities being at least an order of magnitude larger than at birth (Rasio & Heggie 1995).

Does the coalescence of white dwarfs produce millisecond pulsars in globular clusters?

view Abstract Citations (20) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Does the Coalescence of White Dwarfs Produce Millisecond Pulsars in Globular Clusters? Chen, Kaiyou ; Leonard, Peter J. T. Abstract Isolated main-sequence stars with initial masses in the range from about 1 to 8 or 9 solar masses will evolve into C + O white dwarfs with masses up to about 1.1 solar mass, and those main-sequence stars with initial masses in the range from about 8 or 9 to 10 or 11 solar masses may produce O + Ne(+Mg) white dwarfs with masses up to about 1.4 solar mass or less. Many such white dwarfs should exist in globular clusters, and pairs of them should coalesce frequently via collisional processes, especially if primordial binaries are present. It is quite possible that most of the short-period millisecond pulsars in the precore-collapse globular clusters have been produced via mergers of white dwarf pairs with total masses exceeding the Chandrasekhar limit. Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/186916 Bibcode: 1993ApJ...411L..75C Keywords: Coalescing; Globular Clusters; Pulsars; Stellar Evolution; White Dwarf Stars; Neutron Stars; Supernovae; Astrophysics; GALAXY: GLOBULAR CLUSTERS: GENERAL; STARS: SUPERNOVAE: GENERAL; STARS: NEUTRON; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (7)

Low-mass X-ray binaries and millisecond pulsars in globular clusters

Globular clusters harbor many millisecond pulsars and 10 bright low-mass X-ray binary systems. It has been generally believed that almost all millisecond pulsars are spun up by accretion from a companion star during a (progenitor) low-mass X-ray binary (LMXB) phase. However, most of the short-period millisecond pulsars are found in the apparently younger globular clusters, which have much lower core densities than those characteristic of the apparently older globulars containing the majority of the LMXBs. The vast majority of the globular LMXBs are X-ray burst sources, and among them there are no bright sources similar to those in the Galactic disk, such as Sco X-1, Cyg X-2, and GX 5-1. Furthermore, the radio pulse properties of the very short period millisecond pulsars in globular clusters differ from those of pulsars spun up in the Galactic disk, such as PSR 1937 + 21 and PSR 1957 + 20. Only the properties of the latter class are consistent with what is expected for the spin-up scenario. We argue that, in most globular cluster LMXBs, the neutron stars were formed spinning rapidly, and are likely to be the descendants, rather than the progenitors, of weakly magnetized millisecond pulsars.

Evacuation of gas from globular clusters by winds from millisecond pulsars

WHY is so little gas observed within globular clusters? A typical globular cluster contains ∼103 post-turn off stars, each of which will lose ∼0.2M⊙ before its asymptotic giant branch phase of evolution, and ∼0.1M⊙ during this phase1, and so should accumu-late 102∼103M⊙ of gas in the 108-year interval between passages of the globular cluster through the galactic disk. (At each disk passage, gas ram pressure will remove the accumulated material2,3.) But observational searches show that there is scant intracluster gas; in many clusters, there is less than 1M⊙ of gas, orders of magnitude less than theoretical predictions3. The recent discovery4,5 of multiple millisecond pulsars in globular clusters may resolve this long-standing problem: the relativistic wind from these pulsars is enough to drive gas from stellar mass loss out of the globular cluster.