A 'pulsar timing array' (PTA), in which observations of a large sample of pulsars spread across the celestial sphere are combined, allows investigation of 'global' phenomena such as a background of gravitational waves or instabilities in atomic timescales that produce correlated timing residuals in the pulsars of the array. The Parkes Pulsar Timing Array (PPTA) is an implementation of the PTA concept based on observations with the Parkes 64-m radio telescope. A sample of 20 ms pulsars is being observed at three radio-frequency bands, 50 cm (ß700 MHz), 20 cm (ß1400 MHz), and 10 cm (ß3100 MHz), with observations at intervals of two to three weeks. Regular observations commenced in early 2005. This paper describes the systems used for the PPTA observations and data processing, including calibration and timing analysis. The strategy behind the choice of pulsars, observing parameters, and analysis methods is discussed. Results are presented for PPTA data in the three bands taken between 2005 March and 2011 March. For 10 of the 20 pulsars, rms timing residuals are less than 1 μs for the best band after fitting for pulse frequency and its first time derivative. Significant 'red' timing noise is detected in about half of the sample. We discuss the implications of these results on future projects including the International Pulsar Timing Array and a PTA based on the Square Kilometre Array. We also present an 'extended PPTA' data set that combines PPTA data with earlier Parkes timing data for these pulsars.
We present seven years of radio observations of SN 1987A made with the Australia Telescope Compact Array. At 1. 4, 2.4, 4.8 and 8.6 GHz, the flux density of the radio remnant has increased monotonically since emission was redetected 1200 days after the explosion. On day 3200, the remnant was expanding at 2800 ± 400 km s −1 , which we interpret as indicating significant deceleration of the fastest moving ejecta. Since day 1787 the spectral index has remained constant at α = −0.95 ± 0.04 (S ∝ ν α ). These observations are all consistent with the shock having encountered a denser, shocked, component of the progenitor's stellar wind. At the current rate of expansion, the shock is expected to encounter the inner optical ring in the year 2006±3, in line with predictions made by hydrodynamic simulations.Using super-resolution, we have also obtained 9 GHz images of the remnant (resolution ≈ 0 ′′ .5) at four epochs. The emission is distributed around the rim of a near-circular shell, but has become increasingly asymmetric with time. There are two "hotspots" to the east and west, aligned along the major axis of the optical ring. This morphology is most likely indicative of an axisymmetric circumstellar medium into which the shock is expanding, consistent with present understanding of the progenitor star and its environment. The two hotspots are increasing in flux density at different rates, which may indicate directional anisotropies in the ejecta. We believe that the northern and southern regions of the remnant are encountering a shocked wind which is less dense and also further from the progenitor star than that in the bright regions of emission. As a result, these regions should eventually brighten and/or extend. Subject headings: supernovae: individual (SN 1987A) -supernova remnantscircumstellar matter
We show that in many methanol maser sources the masers are located in lines, with a velocity gradient along them which suggests that the masers are situated in edge-on circumstellar, or protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92+0.48, in the 12.2 GHz transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only linear in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz and the 12.2 GHz transition from a number of star formation regions, and show that the observed spatial and velocity distribution of methanol masers, and the derived Keplerian masses, are consistent with a circumstellar disk rotating around an OB star. We consider this and other hypotheses, and conclude that about half of these methanol masers are probably located in edge-on circumstellar disks around young stars. This is of particular significance for studies of circumstellar disks because of the detailed velocity information available from the masers.Comment: 38 pages, 13 figures accepted by Ap
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