Astronomy and Astrophysics, volume 576A, 41-41 (2015/4-1)
ALMA and VLA measurements of frequency-dependent time lags in Sagittarius A*: evidence for a relativistic outflow.
BRINKERINK C.D., FALCKE H., LAW C.J., BARKATS D., BOWER G.C., BRUNTHALER A., GAMMIE C., IMPELLIZZERI C.M.V., MARKOFF S., MENTEN K.M., MOSCIBRODZKA M., PECK A., RUSHTON A.P., SCHAAF R. and WRIGHT M.
Abstract (from CDS):
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 43GHz and 22GHz, 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 100GHz using the VLA at 48, 39, 37, 27, 25.5, and 19GHz. 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 100GHz to 19GHz, 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.
accretion, accretion disks - black hole physics - radiation mechanisms: non-thermal - Galaxy: center - galaxies: jets
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