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The discovery of the Galactic center pulsar SGR J1745–29 has provided an important new window into plasma processes in the Galactic center (GC) interstellar medium, the population of compact objects in the GC, and the prospects for probing general relativistic effects through timing of a Sgr A* pulsar companion. We discuss here radio observations of the pulsar and how they are providing fresh insights. In particular, our results show that recent pulsar surveys had the sensitivity to detect many pulsars in the GC region without significant losses due to interstellar scattering. This raise the question of why only this pulsar close to Sgr A* has been detected.
The Australian Square Kilometre Array Pathfinder (ASKAP) will give us an unprecedented opportunity to investigate the transient sky at radio wavelengths. In this paper we present VAST, an ASKAP survey for Variables and Slow Transients. VAST will exploit the wide-field survey capabilities of ASKAP to enable the discovery and investigation of variable and transient phenomena from the local to the cosmological, including flare stars, intermittent pulsars, X-ray binaries, magnetars, extreme scattering events, interstellar scintillation, radio supernovae, and orphan afterglows of gamma-ray bursts. In addition, it will allow us to probe unexplored regions of parameter space where new classes of transient sources may be detected. In this paper we review the known radio transient and variable populations and the current results from blind radio surveys. We outline a comprehensive program based on a multi-tiered survey strategy to characterise the radio transient sky through detection and monitoring of transient and variable sources on the ASKAP imaging timescales of 5 s and greater. We also present an analysis of the expected source populations that we will be able to detect with VAST.
Of all possible black hole sources, the event horizon of the Galactic Center black hole, Sgr A*, subtends the largest angular scale on the sky. It is therefore a prime candidate to study and image plasma processes in strong gravity and it even allows imaging of the shadow cast by the event horizon. Recent mm-wave VLBI and radio timing observations as well as numerical GRMHD simulations now have provided several breakthroughs that put Sgr A* back into the focus. Firstly, VLBI observations have now measured the intrinsic size of Sgr A* at multiple frequencies, where the highest frequency measurements have approached the scale of the black hole shadow. Moreover, measurements of the radio variability show a clear time lag between 22 GHz and 43 GHz. The combination of size and timing measurements, allows one to actually measure the flow speed and direction of magnetized plasma at some tens of Schwarzschild radii. This data strongly support a moderately relativistic outflow, consistent with an accelerating jet model. This is compared to recent GRMHD simulation that show the presence of a moderately relativistic outflow coupled to an accretion flow Sgr A*. Further VLBI and timing observations coupled to simulations have the potential to map out the velocity profile from 5-40 Schwarzschild radii and to provide a first glimpse at the appearance of a jet-disk system near the event horizon. Future submm-VLBI experiments would even be able to directly image those processes in strong gravity and directly confirm the presence of an event horizon.
We present here 3 epochs of 3 and 7 millimeter wavelength VLBI observations and 2 epochs of lower frequency VLBA imaging of the gamma-ray blazar NRAO 530. These observations document the evolution of the parsec scale jet in this source during the brightest flare in 3 decades. New jet components were created during the flare and are probably related to an increase in gamma-ray activity. The components travel at superluminal velocities, further confirming the connection between superluminal sources and gamma-ray blazars. The rapid evolution of the source makes tracking of components difficult. It appears that either components significantly decelerate or that there is rapid cooling and acceleration of elections. We may be identifying structure due to standing shocks. The jet is bent on all scales between 100 μarcsec to 10 arcsec. The results indicate the ability of 3 mm wavelength imaging to probe the nuclei of blazars rapidly and reliably.
3 mm λ VLBI observations of the extremely flat spectrum QSO NRAO 530 at the peak of its brightest recorded flare show a resolved core only slightly changed from its pre-outburst state. Observations with Haystack, Kitt Peak and Owens Valley observatories in April 1994, prior to the onset of the flare found a single component with Tb = 8 × 1010 K. One year later, within one week of the peak of flare at 3 mm λ, observations with Haystack, Kitt Peak and Hat Creek observatories revealed a component comparable in size and flux density with Tb = 7 × 1010 K. Table 1 details the source models for the two experiments.
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