ALMA and VLA measurements of frequency-dependent time lags in
  Sagittarius A*: evidence for a relativistic outflow
  Radio and mm-wavelength observations of Sagittarius A* (Sgr A*), the radio
source associated with the supermassive black hole at the center of our Galaxy,
show that it behaves as a partially self-absorbed synchrotron-emitting source.
The measured size of Sgr A* shows that the mm-wavelength emission comes from a
small region and consists of the inner accretion flow and a possible collimated
outflow. Existing observations of Sgr A* have revealed a time lag between light
curves at 43 GHz and 22 GHz, which is consistent with a rapidly expanding
plasma flow and supports the presence of a collimated outflow from the
environment of an accreting black hole.
  Here we wish to measure simultaneous frequency-dependent time lags in the
light curves of Sgr A* across a broad frequency range to constrain direction
and speed of the radio-emitting plasma in the vicinity of the black hole. Light
curves of Sgr A* were taken in May 2012 using ALMA at 100 GHz using the VLA at
48, 39, 37, 27, 25.5, and 19 GHz. As a result of elevation limits and the
longitude difference between the stations, the usable overlap in the light
curves is approximately four hours. Although Sgr A* was in a relatively quiet
phase, the high sensitivity of ALMA and the VLA allowed us to detect and fit
maxima of an observed minor flare where flux density varied by ~10%.
  The fitted times of flux density maxima at frequencies from 100 GHz to 19
GHz, as well as a cross-correlation analysis, reveal a simple
frequency-dependent time lag relation where maxima at higher frequencies lead
those at lower frequencies. Taking the observed size-frequency relation of Sgr
A* into account, these time lags suggest a moderately relativistic (lower
estimates: 0.5c for two-sided, 0.77c for one-sided) collimated outflow.
