Low frequency monitoring of SgrA* with the GMRT

ATel #5323; Nirupam Roy (MPIfR), Nissim Kanekar (NCRA-TIFR)
on 26 Aug 2013; 10:55 UT
Credential Certification: Nirupam Roy (roy.nirupam@gmail.com)

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The Giant Metrewave Radio Telescope (GMRT) was used in its Cycle 24 (proposal 24_053) to monitor the flux density of the Galactic Centre black hole SgrA* at low radio frequencies between April and August 2013, to search for flux density changes in SgrA* due to the close approach of the gas cloud G2 (Gillessen et al. 2012, Nature, 481, 51). Images of the SgrA* field were made at 610 MHz at five epochs between April and August 2013, and we also analysed archival GMRT 610 MHz data from March 22, 2013. Only the longer GMRT baselines were used for the imaging, so as to resolve out most of the strong extended emission from the field; the angular resolution of the different images is ~ 6"x4". The 610 MHz flux density of SgrA* was measured to be 392 mJy (March 22), 437 mJy (April 24), 380 mJy (May 23), 385 mJy (June 4), 418 mJy (July 5) and 374 mJy (August 2); the error on each measurement is ~10%, dominated by systematic errors in the GMRT flux density scale. All flux density estimates are consistent with each other (within 10%), with the flux density 0.5 +/- 0.2 on February 8 (reported by Kassim et al. 2013, ATel #4922), and the quiescent state flux density of (0.45 +/- 0.10) Jy reported by An et al. (2005, ApJ, 634, L49) from earlier GMRT 610 MHz observations. We thus find no significant increase of the flux density of SgrA* over the period March - August 2013, during the pericentre passage of the cloud G2. Specifically, there is no evidence for a change in the flux density of a few Jy below 1 GHz, as predicted to arise in various models due to non-thermal emission from a bow shock (e.g. Sadowski et al. 2013, MNRAS, 433, 2165).

We thank the staff of the GMRT who made these observations possible. The GMRT is operated by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research (NCRA-TIFR).