Millisecond Radio Pulsars Research Articles

A strong upper limit on the pulsed radio luminosity of the compact object 1RXS J141256.0+792204

The ROSAT X-ray source 1RXS J141256.0+792204 has recently been identified as a likely compact object whose properties suggest it could be a very nearby radio millisecond pulsar at d = 80 - 260pc. We investigated this hypothesis by searching for radio pulsations using the Westerbork Synthesis Radio Telescope. We observed 1RXS J141256.0+792204 at 385 and 1380MHz, recording at high time and frequency resolution in order to maintain sensitivity to millisecond pulsations. These data were searched both for dispersed single pulses and using Fourier techniques sensitive to constant and orbitally modulated periodicities. No radio pulsations were detected in these observations, resulting in pulsed radio luminosity limits of L_400 ~ 0.3 (d/250pc)^2 mJy kpc^2 and L_1400 ~ 0.03 (d/250pc)^2 mJy kpc^2 at 400 and 1400MHz respectively. The lack of detectable radio pulsations from 1RXS J141256.0+792204 brings into question its identification as a nearby radio pulsar, though, because the pulsar could be beamed away from us, this hypothesis cannot be strictly ruled out.

Does Submillisecond Pulsar XTE J1739−285 Contain a Weak Magnetic Neutron Star or Quark Star?

ABSTRACTThe possible detection of the fastest spinning nuclear‐powered pulsar XTE J1739−285, of frequency 1122 Hz (0.8913 ms), arouses us to constrain the mass and radius of its central compact object and to infer the stellar matter composition: neutrons or quarks. Spun up by the accreting materials to such a high rotating speed, the compact star should have either a small radius or short innermost stable circular orbit. By the empirical relation between the upper kilohertz quasi‐periodic oscillation frequency and star spin frequency, a strong constraint on mass and radius is obtained of 1.51 M⊙ and 10.9 km, respectively, which excludes most equations of state (EOSs) of normal neutrons and strongly hints the star promisingly to be a strange quark star. Furthermore, the star magnetic field is estimated to be about 4 × 107 G<B<109 G, which reconciles with those of millisecond radio pulsars, revealing the clues of the evolution linkage of two types of astrophysical objects.

The birth properties of Galactic millisecond radio pulsars

We model the population characteristics of the sample of millisecond pulsars (MSPs) within a distance of 1.5 kpc. We find that for a braking index n = 3, the birth magnetic field distribution of the neutron stars as they switch on as radio-emitting MSPs can be represented by a Gaussian in the logarithm with mean log B(G) = 8.1 and σ log B = 0.4 and their birth spin period by a Gaussian with mean P 0 = 4 ms and σ P 0 = 1.3 ms. We assume no field decay during the lifetime of MSPs. Our study, which takes into consideration acceleration effects on the observed spin-down rate, shows that most MSPs are born with periods that are close to the currently observed values and with average characteristic ages that are typically larger by a factor of ∼1.5 compared to the true age. The Galactic birth rate of the MSPs is deduced to be ≥3.2 x 10 -6 yr -1 near the upper end of previous estimates and larger than the semi-empirical birth rate ∼10 -7 yr -1 of the low-mass X-ray binaries (LMXBs), the currently favoured progenitors. The mean birth spin period deduced by us for the radio MSPs is a factor of ∼2 higher than the mean spin period observed for the accretion and nuclear powered X-ray pulsars, although this discrepancy can be resolved if we use a braking index n = 5, the value appropriate to spin-down caused by angular momentum losses by gravitational radiation or magnetic multipolar radiation. We discuss the arguments for and against the hypothesis that accretion-induced collapse (AIC) may constitute the main route to the formation of the MSPs, pointing out that on the AIC scenario the low magnetic fields of the MSPs may simply reflect the field distribution in isolated magnetic white dwarfs which has recently been shown to be bi-modal with a dominant component that is likely to peak at fields below 10 3 G which would scale to neutron star fields below 10 9 G, under magnetic flux conservation.

Monopole-charged Pulsars and Relevant Issues

An aligned pulsar whose rotation axis and magnetic dipole axis are parallel should be positively charged. We consider the electromagnetic field both inside and outside the star under a specific condition and derive the total charge of a pulsar. The statistical relation between the pulsar's rotation energy loss rate (or the period derivative) and the period may imply that the millisecond radio pulsars with small periods could be low-mass bare strange stars.

Neutron stars in globular clusters: Formation and observational manifestations

Population synthesis is used to model the number of neutron stars in globular clusters that are observed as low-mass X-ray sources and millisecond radio pulsars. The dynamical interactions between binary and single stars in a cluster are assumed to take place only with a continuously replenished “background” of single stars whose properties keep track of the variations in parameters of the cluster as a whole and the evolution of single stars. We use the hypothesis that the neutron stars forming in binary systems from components with initial masses of ∼8–12 M⊙ during the collapse of degenerate O-Ne-Mg cores through electron captures do not acquire a high space velocity. The remaining neutron stars (from single stars with masses >8 M⊙ or from binary components with masses >12 M⊙) are assumed to be born with high space velocities. According to this hypothesis, a sizeable fraction of the forming neutron stars remain in globular clusters (about 1000 stars in a cluster with a mass of 5 × 105M⊙). The number of millisecond radio pulsars forming in such a cluster in the case of accretion-driven spinup in binary systems is found to be ∼10, in agreement with observations. Our modeling also reproduces the observed shape of the X-ray luminosity function for accreting neutron stars in binary systems with normal and degenerate components and the distribution of spin periods for millisecond pulsars.

Binary and Millisecond Pulsars.

We review the main properties, demographics and applications of binary and millisecond radio pulsars. Our knowledge of these exciting objects has greatly increased in recent years, mainly due to successful surveys which have brought the known pulsar population to over 1700. There are now 80 binary and millisecond pulsars associated with the disk of our Galaxy, and a further 103 pulsars in 24 of the Galactic globular clusters. Recent highlights have been the discovery of the first ever double pulsar system and a recent flurry of discoveries in globular clusters, in particular Terzan 5.Electronic Supplementary MaterialSupplementary material is available for this article at 10.12942/lrr-2005-7.

An X‐Ray Variable Millisecond Pulsar in the Globular Cluster 47 Tucanae: Closing the Link to Low‐Mass X‐Ray Binaries

We report the discovery of peculiar X-ray spectral variability in the binary radio millisecond pulsar PSR J0024--7204W in the globular cluster 47 Tucanae. The observed emission consists of a dominant non-thermal component, which is eclipsed for a portion of the orbit, and a thermal component, which appears to be persistent. We propose that the non-thermal X-rays originate in a relativistic intrabinary shock, formed due to interaction between the relativistic particle wind from the pulsar and matter from the main-sequence companion star, while the thermal photons are from the heated magnetic polar caps of the millisecond pulsar. At optical wavelengths, the emission exhibits large-amplitude variations at the orbital period, which can be attributed to heating of one side of the tidally-locked secondary star by the pulsar wind. The observed X-ray and optical properties of PSR J0024--7204W are remarkably similar to those of the low mass X-ray binary and X-ray millisecond pulsar SAX J1808.4--3658 in quiescence. This supports the conjecture that the non-thermal X-ray emission and optical modulations seen in the SAX J1808.4--3658 system in a quiescent state are due to interaction between the wind from a reactivated rotation-powered pulsar and matter from the companion star. The striking similarities between the two systems provide support for the long-sought connection between millisecond radio pulsars and accreting neutron star systems.

Modelling the spin equilibrium of neutron stars in low-mass X-ray binaries without gravitational radiation

In this paper we discuss the spin equilibrium of accreting neutron stars in low-mass X-ray binaries (LMXBs). We demonstrate that, when combined with a naive spin-up torque, the observed data lead to inferred magnetic fields which are at variance with those of Galactic millisecond radio pulsars. This indicates the need for either additional spin-down torques (e.g. gravitational radiation) or an improved accretion model. We show that a simple consistent accretion model can be arrived at by accounting for radiation pressure in rapidly accreting systems (above a few per cent of the Eddington accretion rate). In our model the inner disc region is thick and significantly sub-Keplerian and the estimated equilibrium periods are such that the LMXB neutron stars have properties that accord well with the Galactic millisecond radio pulsar sample. The implications for future gravitational-wave observations are also discussed briefly.

INTEGRAL spectroscopy of the accreting millisecond pulsar XTE J1807-294 in outburst

the date of receipt and acceptance should be inserted later Abstract. The fourth transient accreting millisecond pulsar XTE J1807-294 was observed during its February/March 2003 outburst by INTEGRAL, partly simultaneously with the XMM-Newton and RXTE satel- lites. We present here the first study of the 0.5-200 keV broad-band spectra of the source. On February 28, the source spectrum was consistent with thermal Comptonization by electrons of temperature � 40 keV, considerably larger than the value (� 10 keV) previously derived from the low energy XMM-Newton data alone. The source is detected by INTEGRAL up to 200 keV with a luminosity in the energy band (0.1-200) keV of 1.3 × 10 37 erg s −1 (assuming a distance of 8 kpc). 22 days later the luminosity dropped to 3.6 × 10 36 erg s −1 . A re-analysis of XMM-Newton data yields the orbital Doppler variations of the pulse period and refines the previous ephemeris. For this source having shortest orbital period of any known binary radio or X-ray millisecond pulsar, we constrain the companion mass Mc < 0.022M⊙, assuming minimum mass transfer driven by gravitational radiation. Only evolved dwarfs with a C/O composition are consistent with the Roche lobe and gravitational radiation constraints, while He dwarfs require an unlikely low inclination.

Formation of massive skyrmion stars

We discuss different channels of formation of massive rapidly rotating neutron stars. We use a population synthesis code to estimate numbers of massive neutron stars in different evolutionary stages. For stiff equations of state the existence of neutron stars with masses > 2 Mis possible. An especially interesting situation occurs if the equation of state based on the Skyrme theory is realized in nature. A neutron star increases its mass by accretion from a secondary companion. Significant growth of a neutron star mass due to accretion is possible only for certain values of initial parameters of the binary. In this paper we show that a significant part of massive neutron stars with M > 2 Mcan be observed as millisecond radio pulsars, as X-ray sources in pairs with white dwarfs, and as accreting neutron stars with very low magnetic fields.

Gravitational Radiation from an Accreting Millisecond Pulsar with a Magnetically Confined Mountain

The amplitude of the gravitational radiation from an accreting neutron star undergoing polar magnetic burial is calculated. During accretion, the magnetic field of a neutron star is compressed into a narrow belt at the magnetic equator by material spreading equatorward from the polar cap. In turn, the compressed field confines the accreted material in a polar mountain which is misaligned with the rotation axis in general, producing gravitational waves. The equilibrium hydromagnetic structure of the polar mountain, and its associated mass quadrupole moment, are computed as functions of the accreted mass, M_a, by solving a Grad-Shafranov boundary value problem. The orientation- and polarization-averaged gravitational wave strain, h_c ~ 6e-24 for a 0.6 kHz source at 1 kpc with M_a > 10^-5 M_Sun, exceeds previous estimates that failed to treat equatorward spreading and flux freezing self-consistently. It is concluded that an accreting millisecond pulsar emits a persistent, sinusoidal gravitational wave signal at levels detectable, in principle, by long baseline interferometers after phase-coherent integration, provided that the polar mountain is hydromagnetically stable. Magnetic burial also reduces the magnetic dipole moment, mu, monotonically, implying a novel, observationally testable scaling h_c(mu). The implications for the rotational evolution of (accreting) X-ray and (isolated) radio millisecond pulsars are explored.

The role of general relativity in the evolution of low-mass X-ray binaries

We study the evolution of low-mass X-ray binaries hosting a neutron star and of millisecond binary radio pulsars using numerical simulations that take into account the detailed evolution of the companion star, of the binary system, and of the neutron star. According to general relativity, when energy is released during accretion or due to magnetodipole radiation during the pulsar phase, the system loses gravitational mass. Moreover, the neutron star can collapse to a black hole if its mass exceeds a critical limit, which depends on the equation of state of ultradense matter and is typically ∼ 2M � . These facts have some interesting consequences. (i) In a millisecond radio pulsar the mass-energy is lost with a specific angular momentum that is smaller than the specific angular momentum of the system, resulting in a positive contribution to the orbital period derivative. If this contribution is dominant and can be measured, we can extract information about the moment of inertia of the neutron star, since the energy loss rate depends on it. Such a measurement can help to put constraints on the equation of state of ultradense matter. (ii) In low-mass X-ray binaries below the bifurcation period (∼18 h), the neutron star survives the ‘period gap’ only if its mass is smaller than the maximum non-rotating mass when the companion becomes fully convective and accretion pauses. Since

Indirect Evidence of an Active Radio Pulsar in the Quiescent State of the Transient Millisecond Pulsar SAX J1808.4-3658

Millisecond radio pulsars are neutron stars that have been spun up by the transfer of angular momentum during the low-mass X-ray binary phase. The transition from an accretion-powered pulsar to a rotation-powered pulsar takes place on evolutionary timescales at the end of the accretion process; however, it may also occur sporadically in systems undergoing transient X-ray activity. We have obtained the first optical spectrum of the low-mass transient X-ray pulsar SAX J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar B1957+20, this X-ray pulsar shows a large optical modulation at the orbital period due to an irradiated companion star. Using the brightness of the companion star as a bolometer, we conclude that a very high irradiating luminosity, a factor of ~100 larger than directly observed, must be present in the system. This most likely derives from a rotation-powered neutron star that resumes activity during quiescence.

Studies of orbital parameters and pulse profile of the accreting millisecond pulsar XTE J1807–294

The accreting millisecond pulsar XTE J1807-294 was observed as a Target of Opportunity (ToO) by XMM-Newton on March 22, 2003 after its discovery on February 21, 2003 by RXTE . The source was detected in its bright phase with an observed average count rate of 33.3 cts s-1 in the EPIC-pn camera in the 0.5–10 keV energy band (3.7 mCrab). Using the earlier established best-fit orbital period of 40.0741 ± 0.0005 min from the RXTE observations and considering a circular binary orbit as first approximation, we derived a value of 4.8 ± 0.1 lt-ms for the projected orbital radius of the binary system and an epoch of the orbital phase of MJD 52 720.67415(16). The barycentric mean spin period of the pulsar was derived as 5.2459427 ± 0.0000004 ms. The pulsar's spin-pulse profile showed a prominent (1.5 ms FWHM) pulse, with energy and orbital phase dependence in the amplitude and shape. The measured pulsed fraction in four energy bands was found to be 3.1 ± 0.2% (0.5–3.0 keV), 5.4 ± 0.4% (3.0–6.0 keV), 5.1 ± 0.7% (6.0–10.0 keV) and 3.7 ± 0.2% (0.5–10.0 keV), respectively. Studies of spin-profiles with orbital phase and energy showed significant increase in its pulsed fraction during the second observed orbit of the neutron star, gradually declining in the subsequent two orbits, which was associated with sudden but marginal increase in mass accretion. From our investigations of orbital parameters and estimation of other properties of this compact binary system, we conclude that XTE J1807-294 is very likely a candidate for a millisecond radio pulsar.

Ultracompact Binaries as Bright X-Ray Sources in Elliptical Galaxies

Chandra observations of distant (D ~ 10 Mpc) elliptical galaxies have revealed large numbers of low-mass X-ray binaries (LMXBs) accreting at > 10-8 M☉ yr-1. The majority of these LMXBs reside in globular clusters (GCs), and it has been suggested that many of the field LMXBs also originated in GCs. We show here that ultracompact (UC) binaries with orbital periods of 8-10 minutes and He or C/O donors of 0.06-0.08 M☉ naturally provide the observed from gravitational radiation losses alone. Such systems are predicted to be formed in the dense GC environment, a hypothesis supported by the 11.4 minute binary 4U 1820-30, the brightest persistent LMXB in a Galactic GC. These binaries have short enough lifetimes ( 1038 ergs s-1) that we calculate their luminosity function under a steady state approximation. This yields a luminosity function slope in agreement with that observed for 6 × 1037 ergs s-1 < L < 5 × 1038 ergs s-1, encouraging us to use the observed numbers of LMXBs per GC mass to calculate the accumulated number of UC binaries. For a constant birthrate over 8 Gyr, the number of UC binaries that have evolved through this bright phase is ~4000 in a 107 M☉ GC, consistent with dynamical interaction calculations. Perhaps most importantly, if all UC binaries become millisecond radio pulsars, then the observed normalization agrees with the inferred number of millisecond radio pulsars in 47 Tuc and Galactic GCs in general.

The formation of millisecond radio pulsars in close binary systems

An analysis of the basic parameters of a sample of radio and X-ray pulsars that are members of close binary systems is used to separate them into several families according to the nature of the pulsar companions and the previous evolution of the systems. To quantitatively describe the main parameters of close binaries containing neutron stars, we have performed numerical modeling of their evolution. The main driving forces of the evolution of these systems are the nuclear evolution of the donor, the magnetically coupled and radiation-induced stellar winds of the donor, and gravitational-wave radiation. We have considered donors that are low-mass stars in various stages of their evolution, nondegenerate helium stars, and degenerate stars. The systems studied are either the products of the normal evolution of close binaries with large initial component-mass ratios or result from inelastic collisions of old neutron stars with single and binary low-mass, main-sequence stars in the dense cores of globular clusters. The formation of single millisecond pulsars requires either the dynamical disruption of a low-mass (≲0.1M⊙) donor or its complete evaporation under the action of the X-ray radiation of the millisecond pulsar. The observed properties of binary radio pulsars with eccentric orbits combined with the bimodal spatial-velocity distribution of single radio pulsars suggest that it may be possible to explain the observed rotational and spatial motions of all radio pulsars as a result of their formation in close binaries. In this case, neutron stars formed from massive single stars or the components of massive wide binaries probably cannot acquire the high spatial velocities or rapid rotation rates that are required for the birth of a radio pulsar.

A Search for Pulsars in Quiescent Soft X‐Ray Transients. I.

We have carried out a deep search at 1.4 GHz for radio pulsed emission from six soft X-ray transient sources observed during their X-ray quiescent phase. The commonly accepted model for the formation of millisecond radio pulsars predicts the presence of a rapidly rotating, weakly magnetized neutron star in the core of these systems. The sudden drop in accretion rate associated with the end of an X-ray outburst causes the Alfven surface to move outside the light cylinder, allowing the pulsar emission process to operate. No pulsed signal was detected from the sources in our sample. We discuss several mechanisms that could hamper the detection and suggest that free-free absorption from material ejected from the system by the pulsar radiation pressure could explain our null result.

Diffusive Nuclear Burning in Neutron Star Envelopes

We calculate the rate of hydrogen burning for neutron stars (NSs) with hydrogen atmospheres and an underlying reservoir of nuclei capable of proton capture. This burning occurs in the exponentially suppressed diffusive tail of H that extends to the hotter depths of the envelope where protons are rapidly captured. This process, which we call diffusive nuclear burning (DNB), can change the H abundance at the NS photosphere on timescales as short as $10^{2-4}$ years. In the absence of diffusion, the hydrogen at the photosphere (where $T\approx 10^6 {\rm K}$ and $\rho\sim 0.1 {\rm g cm^{-2}}$) would last for far longer than a Hubble time. Our work impacts the understanding of the evolution of surface abundances of isolated NSs, which is important to their thermal spectrum and their effective temperature-core temperature relation. In this paper, we calculate the rate of H burning when the overall consumption rate is controlled by the nuclear timescales, rather than diffusion timescales. The immediate application is for H burning on millisecond radio pulsars and in quiescence for the accreting NS Cen X-4. We will apply this work to young radio pulsars and magnetars once we have incorporated the effects of strong $B>10^{12} {\rm G}$ magnetic fields.

Hot White Dwarf Donors in Ultracompact X-Ray Binaries

The discovery of two accreting millisecond X-ray pulsars in binaries with ≈43 minute orbital periods allows for a new probe of the donor's structure. For XTE J1751-305, only a hot white dwarf (WD) can fill the Roche lobe. A cold He WD is a possible solution for XTE J0929-314, although I will show that evolutionary arguments make a hot WD more likely. In addition to being larger than the T = 0 models, these finite entropy, low-mass (Mc < 0.03 M☉) WDs have a minimum mass for a fixed core temperature. If they remain hot as they lose mass and expand, they can evaporate to leave an isolated millisecond radio pulsar. They also adiabatically expand upon mass loss at a rate faster than the growth of the Roche radius if the angular momentum deposited in the disk is not returned to the donor. If the timescale of the resulting runaway mass transfer is shorter than the viscous timescale in the outer disk, then the mass transfer instability of Ruderman & Shaham for He WDs would be realized. However, my estimates of these timescales still make the instability unlikely for adiabatic responses. I close by noting the possible impact of finite temperature WDs on our understanding of AM CVn binaries.

Discovery of a Second Millisecond Accreting Pulsar: XTE J1751−305

We report the discovery by the Rossi X-Ray Timing Explorer Proportional Counter Array of a second transient accreting millisecond pulsar, XTE J1751-305, during regular monitoring observations of the Galactic bulge region. The pulsar has a spin frequency of 435 Hz, making it one of the fastest pulsars. The pulsations contain the signature of orbital Doppler modulation, which implies an orbital period of 42 minutes, the shortest orbital period of any known radio or X-ray millisecond pulsar. The mass function, fX = (1.278 ± 0.003) × 10-6 M☉, yields a minimum mass for the companion of between 0.013 and 0.017 M☉, depending on the mass of the neutron star. No eclipses were detected. A previous X-ray outburst in 1998 June was discovered in archival All-Sky Monitor data. Assuming mass transfer in this binary system is driven by gravitational radiation, we constrain the orbital inclination to be in the range 30°-85° and the companion mass to be 0.013-0.035 M☉. The companion is most likely a heated helium dwarf. We also present results from the Chandra High Resolution Camera-S observations, which provide the best-known position of XTE J1751-305.