Radio and millimeter monitoring of Sgr A: Spectrum, variability, and constraints on the G2 encounter

Geoffrey C. Bower, Sera Markoff, Jason Dexter, Mark A. Gurwell, James M. Moran, Andreas Brunthaler, Heino Falcke, P. Chris Fragile, Dipankar Maitra, Dan Marrone, Alison Peck, Anthony Rushton, Melvyn C.H. Wright

Research output: Contribution to journalArticlepeer-review

85 Scopus citations


We report new observations with the Very Large Array, Atacama Large Millimeter Array, and Submillimeter Array at frequencies from 1.0 to 355 GHz of the Galactic Center black hole, Sagittarius A∗. These observations were conducted between 2012 October and 2014 November. While we see variability over the whole spectrum with an amplitude as large as a factor of 2 at millimeter wavelengths, we find no evidence for a change in the mean flux density or spectrum of Sgr A∗ that can be attributed to interaction with the G2 source. The absence of a bow shock at low frequencies is consistent with a cross-sectional area for G2 that is less than 2 × 1029 cm2. This result fits with several model predictions including a magnetically arrested cloud, a pressure-confined stellar wind, and a stellar photosphere of a binary merger. There is no evidence for enhanced accretion onto the black hole driving greater jet and/or accretion flow emission. Finally, we measure the millimeter wavelength spectral index of Sgr A∗ to be flat; combined with previous measurements, this suggests that there is no spectral break between 230 and 690 GHz. The emission region is thus likely in a transition between optically thick and thin at these frequencies and requires a mix of lepton distributions with varying temperatures consistent with stratification.

Original languageEnglish (US)
Article number69
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 20 2015


  • Galaxy: center Supporting material: machine-readable tables
  • accretion, accretion disks
  • black hole physics
  • galaxies: active
  • galaxies: jets

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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