22 GHz Monitoring of SGR A* During the G2 Peri-Center Passing with the Japanese VLBI Network

We have been performing daily VLBI monitoring of the ux density of Sagittarius (Sgr) A* at 22GHz from February 2013 to August 2014 using a sub-array of the Japanese VLBI Network (JVN). Thepurpose of this monitoring is to explore the ux density variability at daily time resolution for a periodlonger than one year with the G2 cloud approaching. The ux density of Sgr A* is basically stable duringthe observational period, though there are some small variations. The average and scattering range areconsistent with the previously observed values. We have observed no strong are of Sgr A* although it isnear the expected peri-center passing.Key words: journals: individual: Sagittarius A*, VLBI1. INTRODUCTIONSgr A* is a compact radio source at the center of theMilky Way Galaxy, which contains a super massiveblack hole of 4 million solar masses (Ghez et al. (2008),Gillessen et al. (2009)). Gillessen et al. (2012) found asmall cloud, G2, near the black hole. The G2 cloud waspredicted to come closer to the peri-center distance of200 AU in the spring of 2014 (Gillessen et al. (2013)).There were some simulations which predicted that theemission of SgrA* might increase due to this event (e.g.Narayan et al. (2012), Saitoh et al. (2014)). Moni-toring observations of this event are very important todetermine what happens at the Galactic center. Wehave conducted daily monitoring of the ux density ofSgr A* at 22 GHz. The purpose of this monitoring is toexplore the ux density variability on the time resolu-tion of a day for the period long than one year with theG2 cloud approaches.2. OBSERVATIONSWe used a sub-array of the Japanese VLBI Network(JVN) for daily monitoring of the ux density of SgrA* at 22 GHz from February 25, 2013 to August 12,2014. The sub-array consists of the Mizusawa 10-m ra-http://pkas.kas.orgdio telescope, Takahagi /Hitachi 32-m radio telescopes,and Gifu 11-m radio telescope. The Tsukuba 32-m ra-dio telescope and Kashima 34-m radio telescope havejoined it on occasion. Because the sub-array has pro-jected baselines of 90-140 km on the line of site from SgrA*, we can measure the ux density of Sgr A* avoidingcontamination from the extended structure surround-ing Sgr A* and the ux density decrease from partiallyresolved-out ux arising from using the longer projectedbaselines. We used the K5/VSSP32 VLBI data acquisi-tion (DAQ) system (Kondo et al. (2006)), which storedthe VLBI data with 2 bit sampled 32 MHz bandwidth(128 M bps data rate). The observed data was trans-ported to JAXA over the internet, and was correlated bya software correlator in JAXA, Sagamihara after eachdaily observation. Three quasars, NRAO530, J1626-2951, and J1924-2914, were observed each day as uxcalibrators, and Sgr A* was observed twice each day.The on-source observational time of each source was 10minutes per day. The system temperature of each ra-dio telescope was between 100-200K except during thesummer season. The uncertainty of the correlated uxdensity over the whole period was about 0.3 Jy.649