GRAIL, an omni-directional gravitational wave detector

A cryogenic spherical and omni-directional resonant-mass detector proposed by the GRAIL collaboration is described.Comment: 5 pages, 4 figs., contribution to proceedings GW Data Analysis Workshop, Paris, nov. 199

Identification of the Mass Donor Star's Spectrum in SS 433

We present spectroscopy of the microquasar SS 433 obtained near primary eclipse and disk precessional phase Psi = 0.0, when the accretion disk is expected to be most ``face-on''. The likelihood of observing the spectrum of the mass donor is maximized at this combination of orbital and precessional phases since the donor is in the foreground and above the extended disk believed to be present in the system. The spectra were obtained over four different runs centered on these special phases. The blue spectra show clear evidence of absorption features consistent with a classification of A3-7 I. The behavior of the observed lines indicates an origin in the mass donor. The observed radial velocity variations are in anti-phase to the disk, the absorption lines strengthen at mid-eclipse when the donor star is expected to contribute its maximum percentage of the total flux, and the line widths are consistent with lines created in an A supergiant photosphere. We discuss and cast doubt on the possibility that these lines represent a shell spectrum rather than the mass donor itself. We re-evaluate the mass ratio of the system and derive masses of 10.9 +/- 3.1 Msun and 2.9 +/- 0.7 Msun for the mass donor and compact object plus disk, respectively. We suggest that the compact object is a low mass black hole. In addition, we review the behavior of the observed emission lines from both the disk/wind and high velocity jets.Comment: submitted to ApJ, 24 pages, 7 figure

Magnetically-driven explosions of rapidly-rotating white dwarfs following Accretion-Induced Collapse

We present 2D multi-group flux-limited diffusion magnetohydrodynamics (MHD) simulations of the Accretion-Induced Collapse (AIC) of a rapidly-rotating white dwarf. We focus on the dynamical role of MHD processes after the formation of a millisecond-period protoneutron star. We find that including magnetic fields and stresses can lead to a powerful explosion with an energy of a few Bethe, rather than a weak one of at most 0.1 Bethe, with an associated ejecta mass of ~0.1Msun, instead of a few 0.001Msun. The core is spun down by ~30% within 500ms after bounce, and the rotational energy extracted from the core is channeled into magnetic energy that generates a strong magnetically-driven wind, rather than a weak neutrino-driven wind. Baryon loading of the ejecta, while this wind prevails, precludes it from becoming relativistic. This suggests that a GRB is not expected to emerge from such AICs during the early protoneutron star phase, except in the unlikely event that the massive white dwarf has sufficient mass to lead to black hole formation. In addition, we predict both negligible 56Ni-production (that should result in an optically-dark, adiabatically-cooled explosion) and the ejection of 0.1Msun of material with an electron fraction of 0.1-0.2. Such pollution by neutron-rich nuclei puts strong constraints on the possible rate of such AICs. Moreover, being free from ``fallback,'' such highly-magnetized millisecond-period protoneutron stars may later become magnetars, and the magnetically-driven winds may later transition to Poynting-flux-dominated, relativistic winds, eventually detectable as GRBs at cosmological distances. However, the low expected event rate of AICs will constrain them to be, at best, a small subset of GRB and/or magnetar progenitors.Comment: 16 pages, 8 figures, paper accepted to ApJ; High resolution version available at http://hermes.as.arizona.edu/~luc/aic_mhd/aic_mhd.htm

Magnetic fileds of coalescing neutron stars and the luminosity function of short gamma-ray burst

Coalescing neutron star binaries are believed to be the most reliable sources for ground-based detectors of gravitational waves and likely progenitors of short gamma-ray bursts. In the process of coalescence, magnetic fields of neutron stars can induce interesting observational manifestations and affect the form of gravitational wave signal. In this papaer we use the population synthesis method to model the expected distribution of neutron star magnetic fields during the coalescence under different assumptions on the initial parameters of neutron stars and their magnetic field evolution. We discuss possible elecotrmagnetic phenomena preceding the coalescence of magnetized neutron star binaries and the effect of magnetic field on the gravitational wave signal. We find that a log-normal (Gaussian in logarithms) distribution of the initial magnetic fields of neutron stars, which agrees with observed properties of radio pulsars, produces the distribution of the magnetic field energy during the coalescence that adequately describes the observed luminosity function of short gamma-ray bursts under different assumptions on the field evolution and initial parameters of neutron stars. This agreement lends further support to the model of coalescing neutron star binaries as progenitors of gamma-ray bursts.Comment: v.2, LATEX, 25 pages, inc. 7 ps figures, Astron. Lett., in press. Typos corrected, reference adde

The Role of Helium Stars in the Formation of Double Neutron Stars

We have calculated the evolution of 60 model binary systems consisting of helium stars in the mass range of M_He= 2.5-6Msun with a 1.4Msun neutron star companion to investigate the formation of double neutron star systems.Orbital periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe overflow starting from the helium main sequence to after the ignition of carbon burning in the core. We have also examined the evolution into a common envelope phase via secular instability, delayed dynamical instability, and the consequence of matter filling the neutron star's Roche lobe. The survival of some close He-star neutron-star binaries through the last mass transfer episode (either dynamically stable or unstable mass transfer phase) leads to the formation of extremely short-period double neutron star systems (with P<~0.1days). In addition, we find that systems throughout the entire calculated mass range can evolve into a common envelope phase, depending on the orbital period at the onset of mass transfer. The critical orbital period below which common envelope evolution occurs generally increases with M_He. In addition, a common envelope phase may occur during a short time for systems characterized by orbital periods of 0.1-0.5 days at low He-star masses (~ 2.6-3.3Msun). The existence of a short-period population of double neutron stars increases the predicted detection rate of inspiral events by ground-based gravitational-wave detectors and impacts their merger location in host galaxies and their possible role as gamma-ray burst progenitors. We use a set of population synthesis calculations and investigate the implications of the mass-transfer results for the orbital properties of DNS populations.Comment: 30 pages, Latex (AASTeX), 1 table, 8 figures. To appear in ApJ, v592 n1 July 20, 200

Optical Observations of the Binary Pulsar System PSR B1718-19: Implications for Tidal Circularization

We report on Keck and Hubble Space Telescope optical observations of the eclipsing binary pulsar system PSR B1718-19, in the direction of the globular cluster NGC 6342. These reveal a faint star ($m_{\rm F702W}=25.21\pm0.07$; Vega system) within the pulsar's 0\farcs5 radius positional error circle. This may be the companion. If it is a main-sequence star in the cluster, it has radius \rcomp\simeq0.3 \rsun, temperature \teff\simeq3600 K, and mass \mcomp\simeq0.3 \msun. In many formation models, however, the pulsar (spun up by accretion or newly formed) and its companion are initially in an eccentric orbit. If so, for tidal circularization to have produced the present-day highly circular orbit, a large stellar radius is required, i.e., the star must be bloated. Using constraints on the radius and temperature from the Roche and Hayashi limits, we infer from our observations that \rcomp\simlt0.44 \rsun and \teff\simgt3300 K. Even for the largest radii, the required efficiency of tidal dissipation is larger than expected for some prescriptions.Comment: 10 pages, 2 figures, aas4pp2.sty. Accepted for publication in Ap

Does Sub-millisecond Pulsar XTE J1739-285 Contain a Low Magnetic Neutron Star or Quark Star ?

With the possible detection of the fastest spinning nuclear-powered pulsar XTE J1739-285 of frequency 1122 Hz (0.8913 ms), it arouses us to constrain the mass and radius of its central compact object and to imply the stellar matter compositions: 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 kHz quasi-periodic oscillation frequency and star spin frequency, a strong constraint on mass and radius is obtained as 1.51 solar masses and 10.9 km, which excludes most equations of states (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 $4\times10^{7} (G) < B < 10^{9} (G)$, which reconciles with those of millisecond radio pulsars, revealing the clues of the evolution linkage of two types of astrophysical objects.Comment: 10 pages, 2 figures, accepted by PASP 200

Discovery of Five Binary Radio Pulsars

We report on five binary pulsars discovered in the Parkes multibeam Galactic plane survey. All of the pulsars are old, with characteristic ages 1-11 Gyr, and have relatively small inferred magnetic fields, 5-90e8 G. The orbital periods range from 1.3 to 15 days. As a group these objects differ from the usual low-mass binary pulsars (LMBPs): their spin periods of 9-88 ms are relatively long; their companion masses, 0.2-1.1 Msun, are, in at least some cases, suggestive of CO or more massive white dwarfs; and some of the orbital eccentricities, 1e-5 < e < 0.002, are unexpectedly large. We argue that these observed characteristics reflect binary evolution that is significantly different from that of LMBPs. We also note that intermediate-mass binary pulsars apparently have a smaller scale-height than LMBPs.Comment: 5 pages, 4 embedded EPS figs, accepted for publication by ApJ Letter

Merging of Components in Close Binaries: Type Ia Supernovae, Massive White Dwarfs, and Ap stars

The "Scenario Machine" (a computer code designed for studies of the evolution of close binaries) was used to carry out a population synthesis for a wide range of merging astrophysical objects: main-sequence stars with main-sequence stars; white dwarfs with white dwarfs, neutron stars, and black holes; neutron stars with neutron stars and black holes; and black holes with black holes.We calculate the rates of such events, and plot the mass distributions for merging white dwarfs and main-sequence stars. It is shown that Type Ia supernovae can be used as standard candles only after approximately one billion years of evolution of galaxies. In the course of this evolution, the average energy of Type Ia supernovae should decrease by roughly 10%; the maximum and minimum energies of Type Ia supernovae may differ by no less than by a factor of 1.5. This circumstance should be taken into account in estimations of parameters of acceleration of the Universe. According to theoretical estimates, the most massive - as a rule, magnetic - white dwarfs probably originate from mergers of white dwarfs of lower mass. At least some magnetic Ap and Bp stars may form in mergers of low-mass main sequence stars (<1.5 mass of the Sun) with convective envelopes.Comment: 15 pages, 4 figure

New Pulsars from an Arecibo Drift Scan Search

We report the discovery of pulsars J0030+0451, J0711+0931, and J1313+0931 that were found in a search of 470 square degrees at 430 MHz using the 305m Arecibo telescope. The search has an estimated sensitivity for long period, low dispersion measure, low zenith angle, and high Galactic latitude pulsars of ~1 mJy, comparable to previous Arecibo searches. Spin and astrometric parameters for the three pulsars are presented along with polarimetry at 430 MHz. PSR J0030+0451, a nearby pulsar with a period of 4.8 ms, belongs to the less common category of isolated millisecond pulsars. We have measured significant polarization in PSR J0030+0451 over more than 50% of the period, and use these data for a detailed discussion of its magnetospheric geometry. Scintillation observations of PSR J0030+0451 provide an estimate of the plasma turbulence level along the line of sight through the local interstellar medium.Comment: 21 pages, 4 figures, Accepted for Publication in Ap