The formation of the eccentric-orbit millisecond pulsar J1903+0327 and the origin of single millisecond pulsars

The millisecond pulsar J1903+0327 is accompanied by an ordinary G-dwarf star in an unusually wide ($P_{\rm orb} \simeq 95.2$\,days) and eccentric ($e \simeq 0.44$) orbit. The standard model for producing MSPs fails to explain the orbital characteristics of this extraordinary binary, and alternative binary models are unable to explain the observables. We present a triple-star model for producing MSPs in relatively wide eccentric binaries with a normal (main-sequence) stellar companion. We start from a stable triple system consisting of a Low-Mass X-ray Binary (LMXB) with an orbital period of at least 1 day, accompanied by a G-dwarf in a wide and possibly eccentric orbit. Variations in the initial conditions naturally provide a satisfactory explanation for the unexplained triple component in the eclipsing soft X-ray transient 4U~2129+47 or the cataclysmic variable EC 19314-5915. The best explanation for J1903, however, results from the expansion of the orbit of the LMXB, driven by the mass transfer from the evolving donor star to its neutron star companion, which causes the triple eventually to becomes dynamically unstable. Using numerical computations we show that, depending on the precise system configuration at the moment the triple becomes dynamically unstable, the ejection of each of the three components is possible. If the donor star of the LMXB is ejected, a system resembling J1903, will result. If the neutron star is ejected, a single MSP results. This model therefore also provides a straightforward mechanism for forming single MSP in the Galactic disk. We conclude that the Galaxy contains some 30--300 binaries with characteristics similar to J1903, and about an order of magnitude fewer single millisecond pulsars produced with the proposed triple scenario.Comment: ApJ accepted for publicatio

The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of the MIRI Coronagraphs

The imaging channel on the Mid-Infrared Instrument (MIRI) is equipped with four coronagraphs that provide high contrast imaging capabilities for studying faint point sources and extended emission that would otherwise be overwhelmed by a bright point-source in its vicinity. Such bright sources might include stars that are orbited by exoplanets and circumstellar material, mass-loss envelopes around post-main-sequence stars, the near-nuclear environments in active galaxies, and the host galaxies of distant quasars. This paper describes the coronagraphic observing modes of MIRI, as well as performance estimates based on measurements of the MIRI flight model during cryo-vacuum testing. A brief outline of coronagraphic operations is also provided. Finally, simulated MIRI coronagraphic observations of a few astronomical targets are presented for illustration

On the incidence of weak magnetic fields in DA white dwarfs

Context: About 10% of white dwarfs have magnetic fields with strength in the range between about 10^5 and 3x10^8 G. It is not known whether the remaining white dwarfs are not magnetic, or if they have a magnetic field too weak to be detected with the techniques adopted in the large surveys. Aims. We describe the results of the first survey specifically devised to clarify the detection frequency of kG-level magnetic fields in cool DA white dwarfs. Methods: Using the FORS1 instrument of the ESO VLT, we have obtained Balmer line circular spectropolarimetric measurements of a small sample of cool (DA6 - DA8) white dwarfs. Using FORS and UVES archive data, we have also revised numerous white dwarf field measurements previously published in the literature. Results: We have discovered an apparently constant longitudinal magnetic field of \sim9.5 kG in the DA6 white dwarf WD2105-820. This star is the first weak-field white dwarf that has been observed sufficiently to roughly determine the characteristics of its field. The available data are consistent with a simple dipolar morphology with magnetic axis nearly parallel to the rotation axis, and a polar strength of \simeq 56 kG. Our re-evaluation of the FORS archive data for white dwarfs indicates that longitudinal magnetic fields weaker than 10 kG had previously been correctly identified in at least three white dwarfs. Conclusions: We find that the probability of detecting a field of kG strength in a DA white dwarf is of the order of 10% for each of the cool and hot DA stars. If there is a lower cutoff to field strength in white dwarfs, or a field below which all white dwarfs are magnetic, the current precision of measurements is not yet sufficient to reveal it.Comment: Accepted for publication in Astronomy & Astrophysic

Rational optimization of tolC as a powerful dual selectable marker for genome engineering

Selection has been invaluable for genetic manipulation, although counter-selection has historically exhibited limited robustness and convenience. TolC, an outer membrane pore involved in transmembrane transport in E. coli, has been implemented as a selectable/counter-selectable marker, but counter-selection escape frequency using colicin E1 precludes using tolC for inefficient genetic manipulations and/or with large libraries. Here, we leveraged unbiased deep sequencing of 96 independent lineages exhibiting counter-selection escape to identify loss-of-function mutations, which offered mechanistic insight and guided strain engineering to reduce counter-selection escape frequency by ∼40-fold. We fundamentally improved the tolC counter-selection by supplementing a second agent, vancomycin, which reduces counter-selection escape by 425-fold, compared colicin E1 alone. Combining these improvements in a mismatch repair proficient strain reduced counter-selection escape frequency by 1.3E6-fold in total, making tolC counter-selection as effective as most selectable markers, and adding a valuable tool to the genome editing toolbox. These improvements permitted us to perform stable and continuous rounds of selection/counter-selection using tolC, enabling replacement of 10 alleles without requiring genotypic screening for the first time. Finally, we combined these advances to create an optimized E. coli strain for genome engineering that is ∼10-fold more efficient at achieving allelic diversity than previous best practices

Search for positively charged strangelets and other related results with E864 at the AGS

We report on the latest results in the search for positively charged strangelets from E864's 96/97 run at the AGS with sensitivity of about $8\times 10^{-9}$ per central collision. This contribution also contains new results of a search for highly charged strangelets with $Z=+3$. Production of light nuclei, such as $^6He$ and $^6Li$, is presented as well. Measurements of yields of these rarely produced isotopes near midrapidity will help constrain the production levels of strangelets via coalescence. E864 also measures antiproton production which includes decays from antihyperons. Comparisons with antiproton yields measured by E878 as a function of centrality indicate a large antihyperon-to-antiproton ratio in central collisions.Comment: 8 pages, 4 figures; Talk at SQM'98, Padova, Italy (July 20-24th, 1998

Antideuteron yield at the AGS and coalescence implications

We present Experiment 864's measurement of invariant antideuteron yields in 11.5A GeV/c Au + Pt collisions. The analysis includes 250 million triggers representing 14 billion 10% central interactions sampled for events with high mass candidates. We find (1/2 pi pt) d^(2)N/dydpt = 3.5 +/- 1.5 (stat.) +0.9,-0.5 (sys.) x 10^(-8) GeV^(-2)c^(2) for 1.8=0.35 GeV/c (y(cm)=1.6) and 3.7 +/- 2.7 (stat.) +1.4,-1.5 (sys.) x 10^(-8) GeV^(-2)c^(2) for 1.4=0.26 GeV/c, and a coalescence parameter B2-bar of 4.1 +/- 2.9 (stat.) +2.3,-2.4 (sys.) x 10^(-3) GeV^(2)c^(-3). Implications for the coalescence model and antimatter annihilation are discussed.Comment: 8 pages, 4 figures, Latex, submitted to Phys. Rev. Let

Mass dependence of light nucleus production in ultrarelativistic heavy ion collisions

Light nuclei can be produced in the central reaction zone via coalescence in relativistic heavy ion collisions. E864 at BNL has measured the production of ten light nuclei with nuclear number of A=1 to A=7 at rapidity $y\simeq1.9$ and $p_{T}/A\leq300MeV/c$. Data were taken with a Au beam of momentum of 11.5 A $GeV/c$ on a Pb or Pt target with different experimental settings. The invariant yields show a striking exponential dependence on nuclear number with a penalty factor of about 50 per additional nucleon. Detailed analysis reveals that the production may depend on the spin factor of the nucleus and the nuclear binding energy as well.Comment: (6 pages, 3 figures), some changes on text, references and figures' lettering. To be published in PRL (13Dec1999

Antiproton Production in 11.5 A GeV/c Au+Pb Nucleus-Nucleus Collisions

We present the first results from the E864 collaboration on the production of antiprotons in 10% central 11.5 A GeV/c Au+Pb nucleus collisions at the Brookhaven AGS. We report invariant multiplicities for antiproton production in the kinematic region 1.4<y<2.2 and 50<p_T<300 MeV/c, and compare our data with a first collision scaling model and previously published results from the E878 collaboration. The differences between the E864 and E878 antiproton measurements and the implications for antihyperon production are discussed.Comment: 4 pages, 4 figures; accepted for publication in Physical Review Letter

The origin of variability of the intermediate-mass black-hole ULX system HLX-1 in ESO 243-49

The ultra-luminous intermediate-mass black-hole system HLX-1 in the ESO 243-49 galaxy exhibits variability with a possible recurrence time of a few hundred days. Finding the origin of this variability would constrain the still largely unknown properties of this extraordinary object. Since it exhibits an intensity-hardness behavior characteristic of black-hole X-ray transients, we have analyzed the variability of HLX-1 in the framework of the disk instability model that explains outbursts of such systems. We find that the long-term variability of HLX-1 is unlikely to be explained by a model in which outbursts are triggered by thermal-viscous instabilities in an accretion disc. Possible alternatives include the instability in a radiation-pressure dominated disk but we argue that a more likely explanation is a modulated mass-transfer due to tidal stripping of a star on an eccentric orbit around the intermediate-mass black hole. We consider an evolutionary scenario leading to the creation of such a system and estimate the probability of its observation. We conclude, using a simplified dynamical model of the post-collapse cluster, that no more than 1/100 to 1/10 of Mbh < 10^4 Msun IMBHs - formed by run-away stellar mergers in the dense collapsed cores of young clusters - could have a few times 1 Msun Main-Sequence star evolve to an AGB on an orbit eccentric enough for mass transfer at periapse, while avoiding collisional destruction or being scattered into the IMBH by 2-body encounters. The finite but low probability of this configuration is consistent with the uniqueness of HLX-1. We note, however, that the actual response of a standard accretion disk to bursts of mass transfer may be too slow to explain the observations unless the orbit is close to parabolic (and hence even rarer) and/or additional heating, presumably linked to the highly time-dependent gravitational potential, are invoked.Comment: 8 pages, 2 figures. Additional figure, extended discussion. To be published in ApJ, June 10, 2011, v734 -