The Neutron Star Mass Distribution

In recent years, the number of pulsars with secure mass measurements has increased to a level that allows us to probe the underlying neutron star (NS) mass distribution in detail. We critically review the radio pulsar mass measurements. For the first time, we are able to analyze a sizable population of NSs with a flexible modeling approach that can effectively accommodate a skewed underlying distribution and asymmetric measurement errors. We find that NSs that have evolved through different evolutionary paths reflect distinctive signatures through dissimilar distribution peak and mass cutoff values. NSs in double neutron star and neutron star-white dwarf systems show consistent respective peaks at 1.33 Msun and 1.55 Msun suggesting significant mass accretion (delta m~0.22 Msun) has occurred during the spin-up phase. The width of the mass distribution implied by double NS systems is indicative of a tight initial mass function while the inferred mass range is significantly wider for NSs that have gone through recycling. We find a mass cutoff at ~2.1 Msun for NSs with white dwarf companions which establishes a firm lower bound for the maximum NS mass. This rules out the majority of strange quark and soft equation of state models as viable configurations for NS matter. The lack of truncation close to the maximum mass cutoff along with the skewed nature of the inferred mass distribution both enforce the suggestion that the 2.1 Msun limit is set by evolutionary constraints rather than nuclear physics or general relativity, and the existence of rare super-massive NSs is possible.Comment: 13 pages, 4 figures, 2 tables. ApJ in press. A completely new and more flexible statistical model applied. Astrophysical results remained same as arXiv:1011.429

Gamma-Ray Bursts and the Cosmic Star Formation Rate

We have tested several models of GRB luminosity and redshift distribution functions for compatibility with the BATSE 4B number versus peak flux relation. Our results disagree with recent claims that current GRB observations can be used to strongly constrain the cosmic star formation history. Instead, we find that relaxing the assumption that GRBs are standard candles renders a very broad range of models consistent with the BATSE number-flux relation. We explicitly construct two sample distributions, one tracing the star formation history and one with a constant comoving density. We show that both distributions are compatible with the observed fluxes and redshifts of the bursts GRB970508, GRB971214, and GRB980703, and we discuss the measurements required to distinguish the two models.Comment: 12 pages, 2 postscript figures, uses AAS LaTex macros v4.0. To be published in Astrophysical Journal Letters, accepted August 20, 1998. Revised for publicatio

The distance and radius of the neutron star PSR B0656+14

We present the result of astrometric observations of the radio pulsar PSR B0656+14, made using the Very Long Baseline Array. The parallax of the pulsar is pi = 3.47 +- 0.36 mas, yielding a distance of 288 +33 -27 pc. This independent distance estimate has been used to constrain existing models of thermal x-ray emission from the neutron star's photosphere. Simple blackbody fits to the x-ray data formally yield a neutron star radius R_inf ~ 7-8.5 km. With more realistic fits to a magnetized hydrogen atmosphere, any radius between ~13 and ~20 km is allowed.Comment: 7 pages including 1 figure. Submitted to ApJL. AASte

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