Multifrequency Observations of Giant Radio Pulses from the Millisecond Pulsar B1937+21

Giant pulses are short, intense outbursts of radio emission with a power-law intensity distribution that have been observed from the Crab Pulsar and PSR B1937+21. We have undertaken a systematic study of giant pulses from PSR B1937+21 using the Arecibo telescope at 430, 1420, and 2380 MHz. At 430 MHz, interstellar scattering broadens giant pulses to durations of $\sim50 \mu$secs, but at higher frequencies the pulses are very short, typically lasting only $\sim1$-$2 \mu$secs. At each frequency, giant pulses are emitted only in narrow (\lsim10 \mus) windows of pulse phase located $\sim 55$-$70 \mu$sec after the main and interpulse peaks. Although some pulse-to-pulse jitter in arrival times is observed, the mean arrival phase appears stable; a timing analysis of the giant pulses yields precision competitive with the best average profile timing studies. We have measured the intensity distribution of the giant pulses, confirming a roughly power-law distribution with approximate index of -1.8, contributing \gsim0.1% to the total flux at each frequency. We also find that the intensity of giant pulses falls off with a slightly steeper power of frequency than the ordinary radio emission.Comment: 21 pages, 10 Postscript figures; LaTeX with aaspp4.sty and epsf.tex; submitted to Ap

Phase Structure of Compact Star in Modified Quark-Meson Coupling Model

The K$^-$ condensation and quark deconfinement phase transitions are investigated in the modified quark-meson coupling model. It is shown that K$^-$ condensation is suppressed because of the quark deconfinement when $B^{1/4}<$202.2MeV, where $B$ is the bag constant for unpaired quark matter. With the equation of state (EOS) solved self-consistently, we discuss the properties of compact stars. We find that the EOS of pure hadron matter with condensed K$^-$ phase should be ruled out by the redshift for star EXO0748-676, while EOS containing unpaired quark matter phase with $B^{1/4}$ being about 180MeV could be consistent with this observation and the best measured mass of star PSR 1913+16. We then probe into the change of the phase structures in possible compact stars with deconfinment phase as the central densities increase. But if the recent inferred massive star among Terzan 5 with M$>$1.68M$_{\odot}$ is confirmed, all the present EOSes with condensed phase and deconfined phase would be ruled out and therefore these exotic phases are unlikely to appear within neutron stars.Comment: 11 pages, 5 figure

The Triple Pulsar System PSR B1620-26 in M4

The millisecond pulsar PSR B1620-26, in the globular cluster M4, has a white dwarf companion in a half-year orbit. Anomalously large variations in the pulsar's apparent spin-down rate have suggested the presence of a second companion in a much wider orbit. Using timing observations made on more than seven hundred days spanning eleven years, we confirm this anomalous timing behavior. We explicitly demonstrate, for the first time, that a timing model consisting of the sum of two non-interacting Keplerian orbits can account for the observed signal. Both circular and elliptical orbits are allowed, although highly eccentric orbits require improbable orbital geometries. The motion of the pulsar in the inner orbit is very nearly a Keplerian ellipse, but the tidal effects of the outer companion cause variations in the orbital elements. We have measured the change in the projected semi-major axis of the orbit, which is dominated by precession-driven changes in the orbital inclination. This measurement, along with limits on the rate of change of other orbital elements, can be used to significantly restrict the properties of the outer orbit. We find that the second companion most likely has a mass m~0.01 Msun --- it is almost certainly below the hydrogen burning limit (m<0.036 Msun, 95% confidence) --- and has a current distance from the binary of ~35 AU and orbital period of order one hundred years. Circular (and near-circular) orbits are allowed only if the pulsar magnetic field is ~3x10^9 G, an order of magnitude higher than a typical millisecond pulsar field strength. In this case, the companion has mass m~1.2x10^-3 Msun and orbital period ~62 years.Comment: 12 pages, 6 figures, 3 tables. Very minor clarifications and rewording. Accepted for publication in the Astrophys.

Coherently Dedispersed Polarimetry of Millisecond Pulsars

We present a large sample of high-precision, coherently-dedispersed polarization profiles of millisecond pulsars (MSPs) at frequencies between 410 and 1414 MHz. These data include the first polarimetric observations of several of the pulsars, and the first low-frequency polarization profiles for others. Our observations support previous suggestions that the pulse shapes and polarimetry of MSPs are more complex than those of their slower relatives. An immediate conclusion is that polarimetry-based classification schemes proposed for young pulsars are of only limited use when applied to millisecond pulsars.Comment: 28 pages, 10 figures. Text matches version that appeared in ApJS. Full paper with high-resolution figures available at ftp://ftp.jb.man.ac.uk/pub/psr/papers/msppolpton.ps.g

Modeling the strangeness content of hadronic matter

The strangeness content of hadronic matter is studied in a string-flip model that reproduces various aspects of the QCD-inspired phenomenology, such as quark clustering at low density and color deconfinement at high density, while avoiding long range van der Waals forces. Hadronic matter is modeled in terms of its quark constituents by taking into account its internal flavor (u,d,s) and color (red, blue, green) degrees of freedom. Variational Monte-Carlo simulations in three spatial dimensions are performed for the ground-state energy of the system. The onset of the transition to strange matter is found to be influenced by weak, yet not negligible, clustering correlations. The phase diagram of the system displays an interesting structure containing both continuous and discontinuous phase transitions. Strange matter is found to be absolutely stable in the model.Comment: 14 pages, 1 table, 8 eps figures, revtex. Submitted to Phys. Rev. C, Presented at INPC2001 Berkeley, Ca. july 29-Aug

Measurement of Relativistic Orbital Decay in the PSR B1534+12 Binary System

We have made timing observations of binary pulsar PSR B1534+12 with radio telescopes at Arecibo, Green Bank, and Jodrell Bank. By combining our new observations with data collected up to seven years earlier, we obtain a significantly improved solution for the astrometric, spin, and orbital parameters of the system. For the first time in any binary pulsar system, no fewer than five relativistic or "post-Keplerian" orbital parameters are measurable with useful accuracies in a theory-independent way. We find the orbital period of the system to be decreasing at a rate close to that expected from gravitational radiation damping, according to general relativity, although the precision of this test is limited to about 15% by the otherwise poorly known distance to the pulsar. The remaining post-Keplerian parameters are all consistent with one another and all but one of them have fractional accuracies better than 1%. By assuming that general relativity is the correct theory of gravity, at least to the accuracy demanded by this experiment, we find the masses of the pulsar and companion star each to be 1.339+-0.003 Msun and the system's distance to be d = 1.1+-0.2 kpc, marginally larger than the d ~ 0.7 kpc estimated from the dispersion measure. The increased distance reduces estimates of the projected rate of coalescence of double neutron-star systems in the universe, a quantity of considerable interest for experiments with terrestrial gravitational wave detectors such as LIGO.Comment: 17 pages, 4 figures, submitted to the Ap