Searches for transient astrophysical sources often reveal unexpected classes of objects that are useful physical laboratories. In a recent survey for pulsars and fast transients, we have uncovered four millisecond-duration radio transients all more than 40° from the Galactic plane. The bursts' properties indicate that they are of celestial rather than terrestrial origin. Host galaxy and intergalactic medium models suggest that they have cosmological redshifts of 0.5 to 1 and distances of up to 3 gigaparsecs. No temporally coincident x- or gamma-ray signature was identified in association with the bursts. Characterization of the source population and identification of host galaxies offers an opportunity to determine the baryonic content of the universe.

The merger 1 of close binary systems containing two neutron stars should produce a burst of gravitational waves, as predicted by the theory of general relativity 2 . A reliable estimate of the double-neutron-star merger rate in the Galaxy is crucial in order to predict whether current gravity wave detectors will be successful in detecting such bursts. Present estimates of this rate are rather low 3−7 , because we know of only a few doubleneutron-star binaries with merger times less than the age of the Universe. Here we report the discovery of a 22-ms pulsar, PSR J0737−3039, which is a member of a highly relativistic double-neutron-star binary with an orbital period of 2.4 hours. This system will merge in about 85 Myr, a time much shorter than for any other known neutron-star binary. Together with the relatively low radio luminosity of PSR J0737−3039, this timescale implies an order-of-magnitude increase in the predicted merger rate for double-neutron-star systems in our Galaxy (and in the rest of the Universe). PSR J0737−3039 was discovered during a pulsar search carried out using a multibeam receiver 8 on the Parkes 64-m radio telescope in New South Whales, Australia. The original detection showed a large change in apparent pulsar period during the 4-min observation time, suggesting that the pulsar is a member of a tight binary system. Follow-up observations undertaken at Parkes consisting of continuous ∼ 5-hour observations showed that the orbit has a very short period (2.4 hrs) and a significant eccentricity (0.088). The derived orbital parameters implied that the system is relatively massive, probably consisting of two neutron stars, and predicted a huge rate of periastron advanceω due to effects of general relativity. Indeed, after only a few days of pulse-timing observations we were able to detect a significant value ofω.Interferometric observations made using the Australia Telescope Compact Array (ATCA) in the 20-cm band gave an improved position and flux density for the pulsar. Knowledge of the pulsar position with subarcsecond precision allowed determination of the rotational period derivative,Ṗ , and other parameters from the available data span. Table 1 reports results derived from a coherent phase fit to data taken over about five months. The measured value ofω = 16.88 • yr −1 is about four times that of PSR B1913+16 (ref. 9), previously the highestknown. If the observedω is entirely due to general relativity, it indicates a total system mass M = 2.58 ± 0.02 M , where M is the mass of the Sun. Figure 1 shows the constraints on the masses of the pulsar and its companion resulting from the observations so far and the mean pulse profile as an inset. The shaded region indicates values that are ruled out by the mass function M f and the observeḋ ω constrains the system to lie between the two diagonal lines. Together, these constraints imply that the pulsar mass m p is less than 1.35 M and that the companion mass m c is greater than 1.24 M . The derived upper limit on m p is consistent with the

We present the discovery and follow‐up observations of 142 pulsars found in the Parkes 20‐cm multibeam pulsar survey of the Galactic plane. These new discoveries bring the total number of pulsars found by the survey to 742. In addition to tabulating spin and astrometric parameters, along with pulse width and flux density information, we present orbital characteristics for 13 binary pulsars which form part of the new sample. Combining these results from another recent Parkes multibeam survey at high Galactic latitudes, we have a sample of 1008 normal pulsars which we use to carry out a determination of their Galactic distribution and birth rate. We infer a total Galactic population of 30 000 ± 1100 potentially detectable pulsars (i.e. those beaming towards us) having 1.4‐GHz luminosities above 0.1 mJy kpc2. Adopting the Tauris & Manchester beaming model, this translates to a total of 155 000 ± 6000 active radio pulsars in the Galaxy above this luminosity limit. Using a pulsar current analysis, we derive the birth rate of this population to be 1.4 ± 0.2 pulsars per century. An important conclusion from our work is that the inferred radial density function of pulsars depends strongly on the assumed distribution of free electrons in the Galaxy. As a result, any analyses using the most recent electron model of Cordes & Lazio predict a dearth of pulsars in the inner Galaxy. We show that this model can also bias the inferred pulsar scaleheight with respect to the Galactic plane. Combining our results with other Parkes multibeam surveys we find that the population is best described by an exponential distribution with a scaleheight of 330 pc. Surveys underway at Parkes and Arecibo are expected to improve the knowledge of the radial distribution outside the solar circle, and to discover several hundred new pulsars in the inner Galaxy.

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The Parkes multi-beam pulsar survey is a sensitive survey of a strip along the Galactic plane with \b\ < 5<degrees> and l = 260 degrees to l = 50 degrees. It uses a 13-beam receiver on the 64-m Parkes radio telescope. receiving two polarizations per beam over a 288-MHz bandwidth centred on 1374 MHz. The receiver and data acquisition systems are described in some detail. For pulsar periods in the range 0.1-2 s and dispersion measures of less than 300 cm(-3) pc, the nominal limiting flux density of the survey is about 0.2 mJy. At shorter or longer periods or higher dispersions, the sensitivity is reduced. Timing observations are carried out for pulsars discovered in the survey for 12-18 months after confirmation to obtain accurate positions, spin parameters, dispersion measures, pulse shapes and mean flux densities. The survey is proving to be extremely successful, with more than 600 pulsars discovered so far. We expect that, when complete, this one survey will come close to finding as many pulsars as all previous pulsar surveys put together. The newly discovered pulsars tend to be young, distant and of high radio luminosity. They will form a valuable sample for studies of pulsar emission properties, the Galactic distribution and evolution of pulsars, and as probes of interstellar medium properties. This paper reports the timing and pulse shape parameters for the first 100 pulsars timed at Parkes, including three pulsars with periods of less than 100 ms which are members of binary systems. These results are briefly compared with the parameters of the previously known population

We report the detection of ionized intracluster gas in the globular cluster 47 Tucanae. Pulsars in this cluster with a negative period derivative, which must lie in the distant half of the cluster, have significantly higher measured integrated electron column densities than the pulsars with a positive period derivative. We derive the plasma density within the central few pc of the cluster using two different methods which yield consistent values. Our best estimate of n e = 0.067 ± 0.015 cm −3 is about 100 times the free electron density of the ISM in the vicinity of 47 Tucanae, and the ionized gas is probably the dominant component of the intracluster medium.

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