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A low-frequency, stochastic gravitational radiation background can be detected through the irregularities it induces in pulsar arrival times. In this poster we re-examine pulsar timing data presented in Kaspi, Taylor and Ryba (1994) [Ap.J.,428, p. 713] and present an optimal statistical framework for using timing data from a single pulsar to constrain the energy density in a gravitational wave background. Observations of PSRB1855+09 yield an upper limit (95% confidence) 1.0 × 10−8or (90% confidence) 4.8 × 10−9of the closure density at frequency 4.4 × 10−9Hz. This result probably rules out cosmological models that use cosmic strings as seeds for galaxy formation. Using combined observations of the orbital decay of four binary pulsars we also derive weaker limits at frequencies as low as 10−12Hz.

We have observed 1.7 × 106 individual pulses from the millisecond pulsar PSR B1937+21. About one pulse in 10,000 has more than 20 times the mean "pulse-on" flux density, and individual pulses as large as 300 times the average were observed. Comparable behavior has previously been observed only for the Crab pulsar. Giant pulses from PSR B1937+21 are seen in both the main pulse and interpulse components, and their amplitude distribution has roughly a power-law shape. Strong pulses differ greatly from the average emission: they are narrower, systematically delayed by some 40-50 μs, and many are nearly 100% circularly polarized. In addition to their potential importance for elucidating the physics of the emission region, the giant pulses may be useful for high precision timing measurements.

The Fallbrook Low-Frequency Immediate Response Telescope (FLIRT) is an experiment designed to detect prompt radio counterparts to gamma ray bursts (GRBs) at 74 MHz. FLIRT is notified of GRBs by the BACODINE and radio observations begin as early as one minute after the detection of a GRB. The goal of FLIRT is to detect radio counterparts to GRBs and measure their distances by measuring the dispersion of the radio bursts. The dispersion of a radio signal is a function of the source's distance from earth and its galactic coordinates. Radio emission at 74 MHz which was emitted coincident with the GRB at the source would be observed to follow a GRB by 15 s < or approx. {delta}t < or approx. 30 min with galactic GRBs producing shorter delays and cosmological GRBs producing longer delays. Our 1.6 MHz bandwidth is divided into four frequency channels to allow direct measurement of the dispersion across our band. The dispersion measure calculated across the radio bandwidth can then be compared to the dispersion measure implied by the time delay between the GRB and the radio burst. By requiring that these two independent measures of the dispersion agree, we can rule out transients causedmore » by radio interference. This paper discusses the current status of FLIRT.« less

Andromeda is a wide-field, imaging, hard x-ray/soft gamma-ray instrument capable of detecting gamma-ray bursts (GRBs) a factor approximately 20 times fainter than GRO/BATSE. During one-year of a two-year mission, it could determine whether GRBs are Galactic or cosmological in origin by searching for an excess of bursts towards the nearby Andromeda galaxy (M31). As a pointed, imaging instrument with sensitivity in the 10 - 200 keV band significantly better than previous coded-aperture instruments, Andromeda is capable of carrying out important secondary science objectives: for example, studying the soft-gamma- repeater and x-ray transient populations of M31 and the Galactic bulge. Andromeda is a coded aperture gamma-ray telescope consisting of a hexagonal coded mask coupled to an alkali- halide imaging scintillation detector, a flight-proven technology adapted from the balloon- borne Caltech gamma-ray imaging payload (GRIP). The new instrument is optimized for the 10 - 200 keV band and has 1.5 degree angular resolution over a 17 degree FWHM field of view. Andromeda is designed to be a small, low-cost mission, and draws its design largely from existing instrumentation. Andromeda was submitted to the STEDI program, and will also be proposed as a NASA Small Explorer.

The observation by the BATSE instrument on the Compton Gamma Ray Observatory that gamma-ray bursts (GRBs) are distributed isotropically around the Earth but nonuniformly in distance has led to the widespread conclusion that GRBs are most likely to be at cosmological distances, making them the most luminous sources known in the Universe. If bursts arise from events that occur in normal galaxies, such as neutron star binary inspirals, then they will also occur in our Galaxy about every hundred thousand to million years. The gamma-ray flux at the Earth due to a Galactic GRB would far exceed that from even the largest solar flares. The absorption of this radiation in the atmosphere would substantially increase the stratospheric nitric oxide concentration through photodissociation of N$_2$, greatly reducing the ozone concentration for several years through NO$_x$ catalysis, with important biospheric effects due to increased solar ultraviolet flux. A nearby GRB may also leave traces in anomalous radionuclide abundances., Comment: uuencoded, gzip-ed postscript; 6 pages; submitted to ApJ Letters

The discovery last year of a planetary system orbiting a millisecond pulsar raises important questions in pulsar evolution, planet formation, and planetary dynamics. We review the literature concerning pulsar-planetary systems, emphasizing particularly the contributions to the meeting Planets around Pulsars held at Caltech in 1992.

The measurement or constraint of the masses of neutron stars and their binary companions tests theories of neutron star structure and of pulsar formation and evolution. We have measured the rate of the general relativistic advance of the longitude of periastron for the pulsar PSR B1802$-$07: $\dot\omega=0\fdg060\pm0\fdg009\,\mbox{yr}^{-1}$, which implies a total system mass, pulsar plus companion star, of $M=1.7\pm0.4\,\Msun$. We also present a much improved measurement of the rate of periastron advance for PSR B2303+46: $\dot\omega = 0\fdg0099\pm0\fdg0002\,\mbox{yr}^{-1}$, implying $M=2.53\pm0.08\,\Msun$ for this system. We discuss the available constraints on distribution of mass between the pulsars and their companions, and we compare the pulsar masses with other determinations of neutron star masses., Comment: 10 pages plus 3 figures, latex, accepted ApJ Letters, sets-001

We present the results of recent Arecibo observations of the relativistic double-neutron-star binary PSR B1534+12. The timing solution includes measurements of five post-Keplerian orbital parameters, whose values agree well with the predictions of general relativity. The observations show that the pulse profile is evolving secularly at both 1400 MHz and 430 MHz. This effect is similar to that seen in PSR B1913+16, and is almost certainly due to general relativistic precession of the pulsar's spin axis. We also present high-quality polarimetric profiles at both observing frequencies.

We have built a new radio astronomical receiving system designed specifically for very high precision timing and polarimetry of fast pulsars. Unlike most detectors currently used to study pulsars, this intrument does not square the received signal at the time of observation. Instead, voltages proportional to the instantaneous electric vectors of incoming signals are digitized, time-tagged, and recorded on high speed magnetic media. We have tested the system using a 5 MHz bandwidth signal with 2-bit digitization at the Greenbank 140 foot telescope. Full polarization information was obtained with a 0.2μs time resolution. We have used this system to study the giant pulses emitted by PSR B0531+21 and PSR B1937+21, to determine high precision dispersion measures, and to perform high precision timing and polarimetry.

Eclipsing binary pulsars systems highlight an important stage in the evolution of isolated millisecond pulsars. In these systems, the pulsar’s companion is losing mass due to Roche lobe overflow and/or a stellar wind induced by intercepted energy from the pulsar flux. Eventual evaporation of the companions could yield isolated millisecond puslars. PSR B1744-24A was the second eclipsing millisecond pulsar to be discovered (Lyneet al. 1990). It is in a 1.8 hr orbit with a ∼ 0.1 M⊙companion. Its eclipses show some variability, and the pulsar is undetectable at about 25% of observing epochs, presumably because it is completely enveloped by the companion’s outflow (Nice & Thorsett 1992).

These are boom times for the study of recycled radio pulsars, and not only because of the 1993 Nobel Prize in Physics awarded to Russell Hulse and Joe Taylor for their discovery of the first binary pulsar. A new generation of sensitive surveys aimed at high galactic latitudes and globular clusters is turning up these pulsars at a remarkable rate: the most recent pulsar catalogue, already outdated, lists 29 binary pulsars and a dozen more that were probably spun-up by mass transfer from a companion that has since been destroyed or lost. In aggregate, these pulsars have become a sample large enough for meaningful statistical investigations, while individually they show a rich variety of fascinating astrophysics.

The discovery of the first millisecond pulsar 11 years ago revitalized pulsar astronomy and led to a number of large-scale, continuing surveys that have so far discovered at least 16 other fast pulsars in the galactic disk. On page 603 of this issue, Bell, Bailes and Bessell now report the discovery of the probable companion star to the pulsar J0437-4715. At a distance of about 140 parsecs, PSR J0437-4715 is not only our nearest millisecond pulsar but possibly even our nearest radio pulsar, and is, like most millisecond pulsars, in a binary system. The discovery of the companion star provides an independent estimate of the age of the pulsar, which can also be deduced from the rate of slowing of the pulsar.

On 7 December 185, astronomers at the imperial observatory of Lo-Yang, in central China, reported a 'guest star' in the southern sky. Their records of its appearance, gradual fading and disappearance constitute the oldest compelling historical account of a supernova. It is widely believed that the remnant of this explosion is MSH14–63, but the identification is problematic, in particular because the position of MSH14–63with respect to the Sun and the horizon would have made it invisible for some of the time when the guest star was said to be visible. I argue here that the Chinese astronomers actually witnessed the birth of the pulsar PSR1509–58 in the supernova remnant MSH15–52. This is then only the second pulsar, after that in the Crab nebula (PSR0531 + 21), to have a known age. Timing measurements of the age of PSR1509–58 are consistent with its birth ~1,800 years ago, and future measurements of the distance to MSH15–52 and its rate of expansion should conclusively establish or disprove this identification.

We discuss observations of giant pulses from the millisecond pulsar PSR B1937+21 at 430 MHz, with a baseband sampling system at Arecibo (Shrauner et al., these proceedings) and coherent dedispersion techniques. About one pulse or interpulse per hundred thousand is stronger than 100 times the mean flux density, and the giant pulse strengths follow a power law distribution somewhat shallower than that of the Crab giant pulses. The individual giant pulses appear consistent with impulses shorter than a few microseconds, convolved with an exponential due to interstellar scattering. They are systematically delayed with respect to the average emission, and many are nearly 100% circularly polarized.

We report surveyed regions and flux limits for several large-area surveys at 430 MHz for radio pulsars using the 305 m reflector at Arecibo, Puerto Rico. The surveys used integration times between 8 and 67 s and covered a total area of ~515 square degrees. No previously undiscovered pulsars were detected in these regions, although all previously known pulsars were detected. The nondetection of new pulsars places constraints on the population of low-luminosity pulsars and must be taken into account in estimates of the local density of millisecond pulsars.

We present ROSAT observations toward six known millisecond radio pulsars. These observations yielded upper limits to the X-ray flux in the ROSAT band (0.1-2.4 keV) for five pulsars and a possible association of an X-ray source with PSR B1821-24, in the globular cluster M28. At the 99.9% confidence level, the source is pulsed at the expected radio pulsar frequency. We compare our results with predicted X-ray luminosities by Seward & Wang Oegelman. The X-ray luminosities of PSRs B1257+12 and J0437-4715, millisecond pulsars with similar periods and spin-down rates, are found to differ by more than a factor of 25. X-ray emission from radio pulsars has been ascribed to a thermal component arising from a surface hot spot and a power-law magnetospheric component (Halpern & Ruderman). In the context of this model and these observations, we argue that the orientation of the magnetic and rotation axes with respect to the line of sight is very different for PSR J0437-4715 compared to PSR B1257+12. Finally, we suggest that the beaming factor for X-ray emission is independent of the pulsar period, unlike that for radio emission; if so, most millisecond pulsars are visible in the radio but no at X-ray energies.

We present results from observations of 104 pulsars made between 1989 August and 1993 April, including timing solutions for 96 of them. Pulse profiles were recorded at four frequencies in the range 0.4-1.64 GHz, yielding topocentric pulse arrival times with uncertainties of order 10(exp -3) periods. Models fitted to the timing data yield accurate positions, periods, period derivatives, and dispersion measures for each pulsar. Nine of the measured period derivatives are new, and most of the parameters represent improvements upon previous measurements. In a few cases we correct some erroneous parameter values from the published literature. A glitch was observed in the PSR B1800-21 pulse arrival times, and we fit a simple exponential model to the post-glitch recovery. We present graphs of the observed pulse shapes and their evolution with frequency, a table of measured pulase widths, and quantitative estimates of the long-term timing stability of each pulsar.

Accurate determination of the spin-down rates of millisecond pulsars requires consideration of the apparent acceleration of the pulsars due to their high transverse velocities. We show that for several nearby pulsars the neglect of this effect leads to substantial errors in inferred pulsar ages and magnetic fields. Two important ramifications follow. (1) The intrinsic magnetic field strengths of all millisecond pulsars lie below 5 x 10(exp 8) G, strengthening an earlier suggestion of a 'gap' between the magnetic field strengths of millisecond pulsars and of high-mass binary pulsars such as PSR B1913+16, which are thought to have been formed by mass transfer in low-mass and high-mass X-ray binaries, respectively. This result suggests that the magnetic field strengths of recycled pulsars are related to their formation and evolution in binary systems. (2) The corrected characteristic ages of several millisecond pulsars appear to be greater than the age of the Galactic disk. We reconcile this apparent paradox by suggesting that some millisecond pulsars were born with periods close to their current periods. This conclusion has important implications for the interpretation of the cooling ages of white dwarf companions, the birthrate discrepancy between millisecond pulsars and their X-ray binary progenitors, and the possible existence of a class of weakly magnetized (B much less than 10(exp 8)G), rapidly rotating neutron stars.

When a massive star collapses to a neutron star, rapidly losing over half its mass in a symmetric supernova explosiosn, any planets orbiting the star will be unbound. However, to explain the observed space velocity and binary fraction of radio pulsars, an asymmetric kick must be given to the neutron star of birth.

Because ordinary pulsars are distributed in the Galaxy like their massive progenitor stars, sensitive searches for them have been concentrated in the Galactic plane. The greater age and scale height of millisecond pulsars, in contrast, suggest that their local population should be fairly isotropic. We have used the Arecibo radio telescope to search for pulsars in 180 square degrees of sky at high Galactic latitude. We have found two new slow pulsars, in addition to detecting the one previously known pulsar in our survey region, but no new millisecond pulsars. The implications for millisecond pulsar population modeling are discussed

PSR B1620-26, in the globular cluster M4, is a millisecond pulsar with a ∼0.3 M ○. companion in a ∼0.7 AU orbit. It was recently realized (Backer 1993) that timing observations of this pulsar show, in addition to a linear spin-down, a large-period second derivative naturally interpreted as evidence for a varying acceleration, or jerk, of the pulsar binary. We describe timing observations of PSR B 1620-26 taken over a 5 year period. The measured acceleration and jerk are inconsistent with acceleration by the mean gravitational field of M4, and an encounter with another star close enough to cause the observed acceleration is very unlikely

This paper presents an analysis of 1.7 yrs of high-precision timing measurements of the millisecond pulsar PSR 1744-24A. The timing position of the pulsar shows it to be 40 arcsec from the core of the globular cluster Terzan 5, coincident with a steep-spectrum continuum source. The observed negative pulsar period derivative of -1.9 +/- 0.2 x 10^(-20) is biased by acceleration of the system in the gravitational fields of the cluster and possibly the Galactic center. A strong attenuation of the pulsed signal occurs near the eclipse edges, and excess time delays of the pulsed signal are observed during eclipses. Eclipse characteristics vary as a function of frequency and, even at a single frequency, orbital phases of eclipse edges are somewhat variable. The upper limit on changes in the orbital period puts a lower limit of 40 Myr on the time scale of orbital evolution. The implications of these observations are discussed.

The apparent discovery of a planet around the neutron star PSR 1829- 10 raises the question, are planets around pulsars rare or merely difficult to detect? To address that issue we have begun a survey for low-mass companions in archival timing data of ≃ 185 pulsars. In this Letter we demonstrate the sensitivity of our algorithm for two timing-quiet pulsars: PSR 1919+21 and PSR 1937+21

The eclipsing msec binary pulsar PSR 1744 - 24A was observed at radio frequencies between 800 MHz and 1.66 GHz. It is found that the pulsar is eclipsed behind its 0.1-solar-mass companion for up to half of each 109-min orbit, indicating that the companion's eclipsing region is much larger than its Roche lobe. Eclipse duration varies with observing frequency as nu exp beta, with beta = -0.63 + or - 0.18. The pulsed signal is weaker near eclipse boundaries, but no other changes in the pulses are observed. The signal sometimes disappears entirely for intervals as long as several hours. Delayed pulse arrival times after some such anomalous disappearances may suggest that they are caused by excess material in the vicinity of the binary system. 13 refs.

The mean polarization profiles of several fast pulsars were measured. Observations of pulsar PSR 1937 + 21 are similar to previously published results, except that a shallower position angle was found to swing across the pulse at low frequency; for the first time, the occurrence of orthogonal mode switching at all frequencies is explicitly demonstrated. Contrary to previous reports, it is shown that PSR 1953 + 29 is substantially linearly polarized in its first and third components. In addition, the first polarization measurements of the recently discovered millisecond pulsar PSR 1957 + 20 are presented. No evidence is found that the polarization properties of these recycled pulsars differ from those of their slower counterparts. Available data on the spectral development of millisecond pulsar profiles is also reviewed. It is emphasized that millisecond pulsars severely test phenomenological models such as the one proposed by Rankin (1983, 1990). 27 refs.

Information obtained over the past year on the eclipsing millisecond pulsar PSR 1957 + 20 and its orbiting companion is discussed. The pulsar is found to be similar in many ways to other millisecond pulsars: its spin parameters are extremely stable, its period derivative is very small, its profile has a strong interpulse, and its radio spectrum has a steep power-law index. The orbit is nearly circular, and the mass function implies a companion mass not much greater than 0.022 solar. Eclipses last for approximately 56 and 50 min at 318 and 430 MHz, respectively, corresponding to a nu exp - 0.41 + or - 0.09 dependence of eclipse duration on frequency. The available evidence points strongly toward a system in which the radiation from the pulsar heats the companion to the point of ablation, thereby driving a stellar wind that trails outward and behind the companion. 28 refs.

Polarization data for PSR 1957 + 20, collected at 321, 430, and 606 MHz are presented. The integrated profile of the pulsar showed that the main pulse is narrower than that of any known pulsar, with the slight broadening at 321 MHz, which is ascribed to scattering in the interstellar medium. In addition, circular polarization observations of three eclipses at 430 MHz were made in an attempt to investigate the time delay noted previously by Fruchter (1988, 1989). In plots of magnetic induction vs orbital phase, average parallel magnetic fields of about -2.7 and 0.7 gauss were found before and after the eclipse, respectively.

We report on Hubble Space Telescope observations of the optical transient discovered in the error box of GRB970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph and Near-Infrared Camera and Multi-Object Spectrometer. The observations reveal a point-like source with R = 23.1 +- 0.2 and H = 20.6 +- 0.3, in agreement with the power-law temporal decay seen in previous ground-based monitoring. Unlike the case of GRB970228, no nebulosity is detected surrounding the OT of GRB970508, although Mg I absorption and [O II] emission seen in Keck spectra at a redshift of z = 0.835 suggest the presence of a dense, star-forming medium. The HST observations set very conservative upper limits of R ~24.5 and H ~22.2 on the brightness of any underlying extended source. If this subtends a substantial fraction of an arcsecond, then the R band limit is ~25.5. Subsequent photometry suggests a flattening of the light curve at later epochs. Assuming the OT decline follows a pure power-law and ascribing the flattening to the presence of an underlying component of constant flux, we find that this must have R = 25.4, consistent with the upper limits determined by HST. At z = 0.8, this would correspond to an absolute magnitude in the U band of ~-18, similar to that of the Large Magellanic Cloud. We propose a scenario in which the host galaxy of the GRB is of Magellanic type, possibly being a ``satellite'' of one of the bright galaxies located at few arcseconds from the optical transient., Comment: 5 pages, 1 figure, to appear in the Proceedings of the Fourth Gamma Ray Bursts Symposium, eds. C. A. Meegan, R. Preece, and T. Koshut

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made with the Hubble Space Telescope STIS CCD approximately six months after outburst and with the HST/NICMOS and Keck/NIRC approximately one year after outburst. The unresolved counterpart is detected by STIS at V=28.0 +/- 0.25, consistent with a continued power-law decline with exponent -1.14 +/- 0.05. The counterpart is located within, but near the edge of, a faint extended source with diameter ~0."8 and integrated magnitude V=25.8 +/- 0.25. A reanalysis of HST and NTT observations performed shortly after the burst shows no evidence of proper motion of the point source or fading of the extended emission. Only the extended source is visible in the NICMOS images with a magnitude of H=23.3 +/- 0.1. The Keck observations find K = 22.8 +/- 0.3. Several distinct and independent means of deriving the foreground extinction in the direction of GRB 970228 all agree with A_V = 0.75 +/- 0.2. After adjusting for Galactic extinction, we find that the size of the observed extended emission is consistent with that of galaxies of comparable magnitude found in the Hubble Deep Field (HDF) and other deep HST images. Only 2% of the sky is covered by galaxies of similar or greater surface brightness; therefore the extended source is almost certainly the host galaxy. Additionally, we find that the extinction-corrected V - H and V - K colors of the host are as blue as any galaxy of comparable or brighter magnitude in the HDF. Taken in concert with recent observations of GRB 970508, GRB 971214, and GRB 980703 our work suggests that all four GRBs with spectroscopic identification or deep multicolor broad-band imaging of the host lie in rapidly star-forming galaxies.

We report on observations of the field of GRB~970508 made in early August 1998, 454 days after outburst, with the STIS CCD camera onboard the Hubble Space Telescope. The images, taken in open filter (50CCD) mode, clearly reveal the presence of a galaxy which was obscured in earlier (June 1997) HST images by emission from the optical transient (OT). The galaxy is regular in shape: after correcting for the HST/STIS PSF, it is well-fitted by an exponential disk with a scale length of 0."046 +/- 0."006 and an ellipticity of 0.70 +/- 0.07. All observations are marginally consistent with a continuous decline in OT emission as t^{-1.3} beginning two days after outburst; however, we find no direct evidence in the image for emission from the OT, and the surface brightness profile of the galaxy is most regular if we assume that the OT emission is negligible, suggesting that the OT may have faded more rapidly at late times than is predicted by the power-law decay. Due to the wide bandwidth of the STIS clear mode, the estimated magnitude of the galaxy is dependent on the galaxy spectrum that is assumed. Using colors obtained from late-time ground-based observations to constrain the spectrum, we find V = 25.4 +/- 0.15, a few tenths of a magnitude brighter than earlier ground-based estimates that were obtained by observing the total light of the galaxy and the OT and then subtracting the estimated OT brightness assuming it fades as a single power-law. This again suggests that the OT may have faded faster at late time than the power-law predicts. The position of the OT agrees with that of the isophotal center of the galaxy to 0."01. This remarkable agreement raises the possibility that the GRB may have been associated with either an active galactic nucleus or a nuclear starburst.