Natal Kick Velocity Research Articles

Double and single recycled pulsars: an evolutionary puzzle?

We investigate the statistics of isolated recycled pulsars and double neutron star binaries in the Galactic disk. Since recycled pulsars are believed to form through accretion and spinup in close binaries, the isolated objects presumably originate from disrupted progenitors of double neutron stars. There are a comparable number of double neutron star systems compared to isolated recycled pulsars. We find that standard evolutionary models cannot explain this fact, predicting several times the number of isolated recycled pulsars than those in double neutron star systems. We demonstrate, through population synthesis calculations, that the velocity distribution of isolated recycled pulsars is broader than for binary systems. When this is accounted for in a model for radio pulsar survey selection effects, which include the effects of Doppler smearing for the double neutron star binaries, we find that there is a small (25%) bias towards the detection of double neutron star systems. This bias, however, is not significant enough to explain the observational discrepancy if standard (sigma = 265 km/s) neutron star natal kick velocities are invoked in binary population syntheses. Population syntheses in which the 1D Maxwellian velocity dispersion of the natal kick is sigma=170 km/s are consistent with the observations. These conclusions further support earlier findings the neutron stars formed in close interacting binaries receive significantly smaller natal kicks than the velocities of Galactic single pulsars would seem to indicate.

UNDERSTANDING COMPACT OBJECT FORMATION AND NATAL KICKS. II. THE CASE OF XTE J1118 + 480

In recent years, an increasing number of proper motions have been measured for Galactic X-ray binaries. When supplemented with accurate determinations of the component masses, orbital period, and donor effective temperature, these kinematical constraints harbor a wealth of information on the system's past evolution. Here, we consider all this available information to reconstruct the full evolutionary history of the black hole X-ray binary XTE J1118+480, assuming that the system originated in the Galactic disk and the donor has solar metallicity. This analysis accounts for four evolutionary phases: mass transfer through the ongoing X-ray phase, tidal evolution before the onset of Roche-lobe overflow, motion through the Galactic potential after the formation of the black hole, and binary orbital dynamics due to explosive mass loss and possibly a black hole natal kick at the time of core collapse. We find that right after black hole formation, the system consists of a ~6.0-10.0 solar masses black hole and a ~1.0-1.6 solar masses main-sequence star. We also find that that an asymmetric natal kick is not only plausible but required for the formation of this system, and derive a lower and upper limit on the black hole natal kick velocity magnitude of 80 km/s and 310 km/s, respectively.

Pulsar spin-velocity alignment from single and binary neutron star progenitors

The role of binary progenitors of neutron stars (NSs) in the apparent distribution of space velocities and spin-velocity alignment observed in young pulsars is studied. We performed a Monte Carlo synthesis of pulsar populations originated from single and binary stars with different assumptions about the NS natal kick (kick―spin alignment, kick amplitude and kick reduction in electron-capture supernovae in binary progenitors with initial main-sequence masses from the range 8-11 M ⊙ which experienced mass exchange due to Roche lobe overflow). The calculated spin-velocity alignment in pulsars is compared with data inferred from radio polarization measurements. The observed space velocity of pulsars is found to be mostly affected by the natal kick velocity form and its amplitude; the fraction of binaries is not important here for reasonably large kicks. The natal kick―spin alignment is found to strongly affect the spin-velocity correlation of pulsars. Comparison with the observed pulsar spin―velocity angles favours a sizeable fraction of binary progenitors and kick-spin angles ∼5°―20°.

The role of primordial kicks on black hole merger rates

Primordial stars are likely to be very massive $\geq30\Msun$, form in isolation, and will likely leave black holes as remnants in the centers of their host dark matter halos in the mass range $10^{6}-10^{10}\Ms$. Such early black holes, at redshifts z$\gtsim10$, could be the seed black holes for the many supermassive black holes found in galaxies in the local universe. If they exist, their mergers with nearby supermassive black holes may be a prime signal for long wavelength gravitational wave detectors. We simulate formation of black holes in the center of high redshift dark matter halos and explore implications of initial natal kick velocities conjectured by some formation models. The central concentration of early black holes in present day galaxies is reduced if they are born even with moderate kicks of tens of km/s. The modest kicks allow the black holes to leave their parent halo, which consequently leads to dynamical friction being less effective on the lower mass black holes as compared to those still embedded in their parent halos. Therefore, merger rates may be reduced by more than an order of magnitude. Using analytical and illustrative cosmological N--body simulations we quantify the role of natal kicks of black holes formed from massive metal free stars on their merger rates with supermassive black holes in present day galaxies. Our results also apply to black holes ejected by the gravitational slingshot mechanism.

Simulation of a population of gravitational wave-driven neutron stars

I study, via a Monte Carlo simulation, a population of neutron stars evolving through the emission of gravitational waves. A starting population, with ages uniformly distributed back to 100 Myr (or 500 Myr), is evolved in the galactic gravitational potential to the present time. Neutron stars' natal kick velocity and the Gould Belt, with an enhanced formation rate, are taken into account. Neutron stars' ellipticity is assumed to be distributed exponentially with mean value treated as a parameter. The detectability of the emitted gravitational signals, by the first and planned second generation of interferometric detectors, is estimated.

Chemical Abundances in the Secondary Star of the Neutron Star Binary Centaurus X‐4

Using a high-resolution spectrum of the secondary star in the neutron star binary Cen X-4, we have derived the stellar parameters and veiling caused by the accretion disk in a consistent way. We have used a χ2 minimization procedure to explore a grid of 1,500,000 LTE synthetic spectra computed for a plausible range of both stellar and veiling parameters. Adopting the best model parameters found, we have determined atmospheric abundances of Fe, Ca, Ti, Ni, and Al. These element abundances are supersolar ([Fe/H] = 0.23 ± 0.10), but only the abundances of Ti and Ni appear to be moderately enhanced (≥1 σ) as compared with the average values of stars of similar iron content. These element abundances can be explained if the secondary star captured a significant amount of matter ejected from a spherically symmetric supernova (SN) explosion of a 4 M☉ He core progenitor and if we assume solar abundances as primordial abundances in the secondary star. The kinematic properties of the system indicate that the neutron star received a natal kick velocity through an aspherical SN and/or an asymmetric neutrino emission. The former scenario might be ruled out, since our model computations cannot produce acceptable fits to the observed abundances. We have also examined whether this system could have formed in the Galactic halo, and our simulations show that this possibility seems unlikely. We also report a new determination of the Li abundance, consistent with previous studies, that is unusually high and close to the cosmic Li abundance in the Galactic disk.

X‐Ray Counterparts of Runaway Stars

An X-ray search for possible compact companions of runaway OB stars has been conducted using pointed ROSAT observations. Of a list of 71 runaway stars, ROSAT exposures were available for 24, of which 13 are detected. These numbers are nearly 3 times larger than for a previously studied Einstein sample, and spectral information is exploited as well. Luminosities, hardness ratios, and long-term variability are as for normal OB stars and do not suggest the presence of collapsed companions. A result like this is often interpreted as support for dynamical ejection from a dense group rather than a supernova event in a binary as a production process for runaway stars. There are, however, several circumstances that may adversely affect the observability of a compact companion, or after a supernova explosion systems may be disrupted because of the large natal kick velocity imparted to the neutron star as a result of asymmetries in the explosions. It is noted that there is actually evidence for both of these production routes and that they may be expected to occur sequentially in the evolution of OB associations.

Constraints on Natal Pulsar Kicks from Eccentric Binary Pulsars

We investigate the expected space velocities and misalignment angles of the eccentric binary pulsars for a range of possible progenitor systems and natal kick models. In particular, the effect of the orbital separation at the instant of supernova is discussed. The results of our simulations, combined with recent proper-motion and misalignment angle measurements, lead us to conclude that, although the known systems generally provide evidence for asymmetric supernovae, none of the proposed natal kick velocity distributions are adequate. Instead, we find that while kicks are required to explain the space velocities of J0045-7319 and B1259-63, a slow kick velocity distribution with σ = 0 km s -1 and μ = 50 km s-1 predicts space velocities that demonstrate the best agreement with observation. The expected misalignment angle distributions for B1534+12, J0045-7319, and B1259-63 favor larger kicks but do not preclude a slow kick velocity distribution. Such a model is, however, inconsistent with the large kicks usually invoked to explain the velocities of single pulsars. We discuss possible means to reconcile the space velocities of eccentric binary pulsars and the velocity distribution of the pulsar population. These include constraining the progenitor systems of binary pulsars to preclude tight post-common-envelope systems and erroneous pulsar velocity measurements due to an inaccurate distance model.