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Radio and Millimeter Monitoring of Sgr A*: Spectrum, Variability, and Constraints on the G2 Encounter

  • Bower, Geoffrey C.
  • Markoff, Sera
  • Dexter, Jason
  • Gurwell, Mark A.
  • Moran, James M.
  • Brunthaler, Andreas
  • Falcke, Heino
  • Fragile, P. Chris
  • Maitra, Dipankar
  • Marrone, Dan
  • Peck, Alison
  • Rushton, Anthony
  • Wright, Melvyn C. H.
Publication Date
Feb 23, 2015
Submission Date
Feb 23, 2015
DOI: 10.1088/0004-637X/802/1/69
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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 October 2012 and November 2014. 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 $2 \times 10^{29}$ cm$^2$. 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.

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