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A&A 496, 77-83 (2009)
DOI: 10.1051/0004-6361/20078984

Jet-lag in Sagittarius A*: what size and timing measurements tell us about the central black hole in the Milky Way

H. Falcke^{1, 2}, S. Markoff³, and G. C. Bower<sup>4

1</sup>  Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
    e-mail: H.Falcke@astro.ru.nl
²  ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
³  Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098SJ Amsterdam, The Netherlands
⁴  UC Berkeley, 601 Campbell Hall, Astronomy Department & Radio Astronomy Lab, Berkeley, CA 94720, USA

Received 3 November 2007 / Accepted 28 December 2008

Abstract
Context. The black hole at the Galactic Center, Sgr A*, is the prototype of a galactic nucleus at a very low level of activity. Its radio through submm-wave emission is known to come from a region close to the event horizon, however, the source of the emission is still under debate. A successful theory explaining the emission is based on a relativistic jet model scaled down from powerful quasars.
Aims. We want to test the predictive power of this established jet model against newly available measurements of wavelength-dependent time lags and the size-wavelength structure in Sgr A*.
Methods. Using all available closure amplitude VLBI data from different groups, we again derived the intrinsic wavelength-dependent size of Sgr A*. This allowed us to calculate the expected frequency-dependent time lags of radio flares, assuming a range of in- and outflow velocities. Moreover, we calculated the time lags expected in the previously published pressure-driven jet model. The predicted lags are then compared to radio monitoring observations at 22, 43, and 350 GHz.
Results. The combination of time lags and size measurements imply a mildly relativistic outflow with bulk outflow speeds of . The newly measured time lags are reproduced well by the jet model without any major fine tuning.
Conclusions. The results further strengthen the case for the cm-to-mm wave radio emission in Sgr A* as coming from a mildly relativistic jet-like outflow. The combination of radio time lag and VLBI closure amplitude measurements is a powerful new tool for assessing the flow speed and direction in Sgr A*. Future VLBI and time lag measurements over a range of wavelengths will reveal more information about Sgr A*, such as the existence of a jet nozzle, and measure the detailed velocity structure of a relativistic jet near its launching point for the first time.

Key words: black hole physics -- galaxies: active -- galaxies: jets -- galaxies: nuclei -- Galaxy: center -- radio continuum: general

© ESO 2009

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