N. Bartel

Science with the Australian Square Kilometre Array Pathfinder

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline the ASKAP project and summarise its headline science goals as defined by the community at large.

Giant pulses from PSR B1937+21 with widths ≤15 nanoseconds and Tb ≥ 5 × 1039 K, the highest brightness temperature observed in the universe

Giant radio pulses of the millisecond pulsar B1937+21 were recorded with the S2 VLBI system at 1.65 GHz with NASA/JPLs 70 m radio telescope at Tidbinbilla, Australia. These pulses have been observed as strong as 65,000 Jy with widths ≤15 ns, corresponding to a brightness temperature of Tb ≥ 5 × 1039 K, the highest observed in the universe. The vast majority of these pulses occur in 5.8 and 8.2 μs windows at the very trailing edges of the regular main pulse and interpulse profiles, respectively. Giant pulses occur, in general, with a single spike. Only in one case of 309 was the structure clearly more complex. The cumulative distribution is fitted by a power law with index -1.40 ± 0.01 with a low-energy but no high-energy cutoff. We estimate that giant pulses occur frequently but are only rarely detected. When corrected for the directivity factor, 25 giant pulses are estimated to be generated in one neutron star revolution alone. The intensities of the giant pulses of the main pulses and interpulses are not correlated with each other nor with the intensities or energies of the main pulses and interpulses themselves. Their radiation energy density can exceed 300 times the plasma energy density at the surface of the neutron star and can even exceed the magnetic field energy density at that surface. We therefore do not think that the generation of giant pulses is linked to the plasma mechanisms in the magnetosphere. Instead we suggest that it is directly related to discharges in the polar cap region of the pulsar.

SN 1993J VLBI. II. Related Changes of the Deceleration, Flux Density Decay, and Spectrum

Phase-referenced multifrequency VLBI observations of supernova 1993J in the nearby galaxy M81 at 34 epochs from t ¼ 30 to � 3000 days after shock breakout show the detailed characteristics of the non–self-similar expansion. For the first year, the supernova expanded almost freely, with the outer radius of the radio shell ro / t mðtÞ and mðt Þ¼ 0:919 � 0:019. During the following 4 years the deceleration grew to mðt Þ¼ 0:781 � 0:009, the expansion velocity slowed down by about half from originally v0 � 17; 200 km s � 1 to just � 8900 km s � 1 , and the supernova swept up a mass of � 0.3 M� . Momentum conservation suggests that this mass is about equal to the mass of the shocked ejecta, likely composed of all the low-mass envelope of the progenitor left over after mass transfer to a purported binary companion, therefore supporting the binary scenario. Subsequently the deceleration changed again, this time decreasing to mðt Þ¼ 0:860 � 0:011 from t ¼ 5 to 8 yr, consistent with hydrodynamic simulations. This late upturn of mðtÞ may be caused by the massive inner ejecta, which have reached the increasingly decelerated shocked ejecta, passing now through the reverse shock and pushing the low-mass envelope. This upturn of mðtÞ is mirrored by changes in the slope of the radio light curves and a flattening of the radio spectra, all consistent with power laws, with spectral indices changing from � ¼� 0:85 � 0:03 to � 0:64 � 0:03. The magnetic field in the radio shell region is inferred to decrease from originally / r � 0:99 o to / r � 1:45 o toward the end of our observations. After 7 years the radio light curves start to drop rapidly, suggesting that the supernova is now expanding into a zone of the circumstellar medium with a steep density profile. Subject headings: supernovae: individual (SN 1993J) — techniques: interferometric

Angular broadening of pulsars and the distribution of interstellar plasma fluctuations

Comparison of angular and temporal broadening for 10 pulsars suggests a uniform distribution of scattering material in the interstellar plasma with localized, strong scattering near young supernova remnants. Angular broadening and temporal broadening both measure the integrated strength of scattering along the line of sight, but with different weighting functions. Comparison of the two thus yields information on the distribution of scattering material along the line of sight. We report new measurements of angular diameter for the scattering disks of eight pulsars, and discuss two measurements from the literature. We compare these results with published measurements of temporal broadening

The crab Nebula's wisps in radio and optical

We present four new, high-resolution VLA radio images of the Crab Nebula, taken between 2001 February 25 and April 17. The radio images show systematic variability in the Crabs radio emission throughout the region near the pulsar. The principal geometry of the variable features is that of elliptical ripples very similar to the optical wisps. The radio wisps are seen to move systematically outward with projected speeds of up to 0.3c. Comparing the new radio images with our earlier ones from 1998 and 2000, we show that there are also more slowly moving features somewhat farther away from the pulsar. In particular, there is a prominent moving feature to the northwest of the pulsar that has a projected speed of the order of 104 km s-1. Striation is seen throughout the nebula, suggesting the presence of wavelike disturbances propagating through the synchrotron bubble. The radio images were taken simultaneously with HST optical observations as part of a unique observing campaign to obtain simultaneous, time-resolved, high-resolution images of the Crab in different wavebands. Comparing the radio to the optical images, we find that the radio wisps are sometimes displaced from the optical ones or have no optical counterparts. We also find that some optical wisps in particular, the brightest optical wisps near the pulsar, do not seem to have radio counterparts. In the exterior of the nebula, by contrast, there is generally a good correspondence between the radio and optical features.

VLBI limits on the proper motion of the ‘core’ of the superluminal quasar 3C345

VLBI (very-long-baseline interferometry) observations between 1971 and 1983 have been used to determine the positions of the ‘core’ of the quasar 3C345 relative to the more distant compact quasar NRAO512 with a fractional uncertainty as small as 2 parts in 108. The core of 3C345 appears stationary in right ascension to within 20 arc μs yr−1, a subluminal bound corresponding to 0.7c. The apparent velocities of the jets are superluminal, up to 14c in magnitude.

SN 1993J VLBI. III. The Evolution of the Radio Shell

A sequence of images of supernova 1993J at 31 epochs, from 50 days to ~9 yr after shock breakout, shows the evolution of the expanding radio shell of an exploded star in detail. The images were obtained from 24 observing sessions at 8.4 GHz and 19 at 5.0 GHz and from our last session at 1.7 GHz. The images are all phase-referenced to the stable reference point of the core of the host galaxy M81. This allows us to display them relative to the supernova explosion center. The earliest image shows an almost unresolved source with a radius of 520 AU. The shell structure becomes discernible 175 days after shock breakout. The brightness of the ridge of the projected shell is not uniform, but rather varies by a factor of 2, having a distinct peak or maximum to the southeast and a gap or minimum to the west. Over the next ~350 days, this pattern rotates counterclockwise, with the gap rotating from west to north-northeast. After 2 years, the structure becomes more complex with hot spots developing in the east, south, and west. The pattern of modulation continues to change, and after 5 years the hot spots are located to the north-northwest, south, and south-southeast. After 9 years, the radio shell has expanded to a radius of 19,000 AU. The brightness in the center of the images is lower than expected for an optically thin, spherical shell. Absorption in the center is favored over a thinner shell in the back and/or front. Allowing for absorption, we find that the thickness of the shell is 25% ? 3% of its outer radius. We place a 3 ? upper limit of 4.4% on the mean polarization of the bright part of the shell, consistent with internal Faraday depolarization. We find no compact source in the central region above a brightness limit of 0.05 mJy beam-1 at 8.4 GHz, corresponding to 30% of the current spectral luminosity of the Crab Nebula. We conclude either that any pulsar nebula in the center of SN 1993J is much fainter than the Crab or that there is still significant internal radio absorption.

MULTI-WAVELENGTH OBSERVATIONS OF SUPERNOVA 2011ei: TIME-DEPENDENT CLASSIFICATION OF TYPE IIb AND Ib SUPERNOVAE AND IMPLICATIONS FOR THEIR PROGENITORS

We present X-ray, UV/optical, and radio observations of the stripped-envelope, core-collapse supernova (SN) 2011ei, one of the least luminous SNe IIb or Ib observed to date. Our observations begin with a discovery within � 1 day of explosion and span several months afterward. Early optical spectra exhibit broad, Type II-like hydrogen Balmer profiles that subside rapidly and are replaced by Type Ib-like He-rich features on the timescale of one week. High-cadence monitoring of this transition suggests that absorption attributable to a high velocity (& 12,000 km s −1 ) H-rich shell is not rare in Type Ib events. Radio observations imply a shock velocity of v � 0.13c and a progenitor star mass-loss rate of u M � 1.4 × 10 −5 M⊙ yr −1 (assuming wind velocity vw = 10 3 km s −1 ). This is consistent with independent constraints from deep X-ray observations with Swift-XRT and Chandra. Overall, the multi-wavelength properties of SN2011ei are consistent with the explosion of a lower-mass (3 4 M⊙), compact (R∗ . 1 × 10 11 cm), He core star. The star retained a thin hydrogen envelope at the time of explosion, and was embedded in an inhomogeneous circumstellar wind suggestive of modest episodic mass-loss. We conclude that SN2011ei’s rapid spectral metamorphosis is indicative of time-dependent classifications that bias estimates of explosion rates for Type IIb and Ib objects, and that important information about a progenitor star’s evolutionary state and mass-loss immediately prior to SN explosion can be inferred from timely multi-wavelength observations. Subject headings: supernovae: general — supernova: individual (SN2011ei)

THE LOCATION OF THE CORE IN M81

We report on VLBI observations of M81*, the northeast-southwest oriented nuclear core-jet source of the spiral galaxy M81, at five different frequencies between 1.7 and 14.8 GHz. By phase referencing to supernova 1993J we can accurately locate the emission region of M81* in the galaxys reference frame. Although the emission regions size decreases with increasing frequency while the brightness peak moves to the southwest, the emission region seems sharply bounded to the southwest at all frequencies. We argue that the core must be located between the brightness peak at our highest frequency (14.8 GHz) and the sharp bound to the southwest. This narrowly constrains the location of the core, or the purported black hole in the center of the galaxy, to be within a region of ±0.2 mas or ±800 AU (at a distance of ~4 Mpc). This range includes the core position that we determined earlier by finding the most stationary point in the brightness distribution of M81* at only a single frequency. This independent constraint therefore strongly confirms our earlier core position. Our observations also confirm that M81* is a core-jet source, with a one-sided jet that extends to the northeast from the core, on average curved somewhat to the east, with a radio spectrum that is flat or inverted near the core and steep at the distant end. The brightness peak is unambiguously identified with the variable jet rather than the core, which indicates limitations in determining the proper motion of nearby galaxies and in refining the extragalactic reference frame.

The expansion and radio spectral index of G21.5−0.9: is PSR J1833−1034 the youngest pulsar?

We report on new 5-GHz Very Large Array (VLA) radio observations of the pulsar-powered supernova remnant G21.5-0.9. These observations have allowed us to make a high-quality radio image of this remnant with a resolution of ∼0.7 arcsec. It has a filamentary structure similar to that seen in the Crab Nebula. Radio structure suggestive of the torus seen around the Crab pulsar is tentatively identified. We also compared the new image with one taken ∼15 yr earlier at 1.5 GHz, both to find the expansion speed of the remnant and to make a spectral index image. Between 1991 and 2006, we find that the average expansion rate of the remnant is 0.11 ± 0.02 per cent yr -1 , corresponding, for a distance of 5 kpc, to a speed of 910 ± 160 km s -1 with respect to the centre of the nebula. Assuming undecelerated expansion, this expansion speed implies that the age of G21.5-0.9 is 870 +200 -150 yr, which makes PSR J1833-1034 one of the youngest, if not the youngest, known pulsars in the Galaxy.