Limits on Low Frequency Radio Flux Density Changes for Sgr A*

ATel #5159; Andreas Brunthaler (MPIfR), Heino Falcke (Nijmegen/ASTRON/MPIfR)
on 24 Jun 2013; 18:53 UT
Credential Certification: Geoffrey C Bower (gbower@astro.berkeley.edu)

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We report new limits on changes low frequency emission for Sgr A* based on Jansky VLA observations obtained on 2013 June 09 (ATEL #5153) . Data were obtained through the NRAO Service Observing program (https://science.nrao.edu/science/service-observing/service-observing). The JVLA is currently in the compact C-configuration, providing low resolution observations in which Sgr A* is heavily confused by diffuse structure associated with Sgr A West and East. Resolution at 1 GHz is ~>20 arcsec. We use a visibility differencing technique with recent D-configuration observations that provides improved limits on any point source structures in the field. This uses the subset of visibilities that fall in the same (u,v) cells between observations in two separate epochs.

We measure flux density differences relative to the 2013 April 26 observations:
1.5 GHz -0.04 +/- 0.73 Jy
3.1 GHz -0.08 +/- 0.33 Jy
5.3 GHz 0.02 +/- 0.07 Jy
8.9 GHz -0.22 +/- 0.12 Jy
13.9 GHz -0.00 +/- 0.03 Jy
21.1 GHz -0.23 +/- 0.03 Jy
32.0 GHz -0.19 +/- 0.04 Jy
40.9 GHz -0.40 +/- 0.06 Jy
Frequencies are the main for each of the spectral windows associated with the VLA observing bands. A spectrum is available at http://astro.berkeley.edu/~gbower/fluxdifference_20130609-20130426.png

These differences are similar to those found in comparison between previous epochs with compact configurations (ATEL #5025). The decline in the 40 GHz flux density is consistent that found with traditional imaging analysis (ATEL #5153). These results indicate no detectable radio emission in or near to Sgr A* associated with an SGR (ATeL #5006) or with shocked emission driven by the G2 cloud.