Long-term monitoring of Sgr A* at 7 mm with VERA and KaVA
Kazunori Akiyama, Motoki Kino, Bong Won Sohn, Sang Sung Lee, Sascha Trippe, Mareki Honma
aa r X i v : . [ a s t r o - ph . GA ] N ov The Galactic Center: Feeding and Feedback in a Normal GalacticNucleusProceedings IAU Symposium No. 303, 2013A.C. Editor, B.D. Editor & C.E. Editor, eds. c (cid:13) Long-term monitoring of Sgr A* at 7 mmwith VERA and KaVA
K. Akiyama , , M. Kino , B. Sohn , S. Lee , S. Trippe , M. Honma , ,KaVA AGN WG et al. Department of Astronomy, Graduate School of Science, The University of Tokyoemail: [email protected] Mizusawa VLBI Observatory, National Astronomical Observatory of Japan Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency Graduate University for Advanced Studies Korea Astronomy & Space Science Institute Department of Physics and Astronomy, Seoul National University
Abstract.
We present the results of radio monitoring observations of Sgr A* at 7 mm (i.e.43 GHz) with VLBI Exploration of Radio Astrometry (VERA), which is a VLBI array inJapan. VERA provides angular resolutions on millisecond scales, resolving structure within 100Schwarzschild radii of Sgr A* similar to Very Large Baseline Array (VLBA). We performedmulti-epoch observations of Sgr A* in 2005 - 2008, and started monitoring it again with VERAfrom January 2013 for tracing the current G2 encounter event. Our preliminary results in 2013show that Sgr A* on mas scales has been in ordinary state as of August 2013, although somefraction of the G2 cloud already passed pericenter of Sgr A* in April 2013. We will continue onmonitoring Sgr A* with VERA and newly developed KaVA (KVN and VERA Array).
1. Introduction
There are plenty of evidences for the presence of a super-massive black hole (SMBH)with a mass of 4 × M ⊙ (e.g., Gillessen et al. 2009) associated with the Galactic centerSagittarius A* (Sgr A*). The angular size of its Schwarzschild radius (1 R Sch ∼ µ as)is the largest among known black hole candidates due to its proximity ( D ∼ sch . Though detailed imaging of its structure has suffered from the effect of interstel-lar scattering (e.g., Lo et al. 1998), its intrinsic structure start appearing at wavelengthshorter than ∼ Year
Figure 1.
The time variations of the flux and structure of Sgr A* measured with VERA in2005-2008 (Akiyama et al. 2013). From top to bottom, vertical axes correspond to flux, size ofthe major axis, and intrinsic size, respectively. The squares are our results, while the circles andthe crosses are VLBA results.
Monitoring of Sgr A* with VLBI is of great importance to investigate relation betweenvariations in the radio flux and size at the vicinity of its SMBH, and probe a possiblemechanism of its variability. To do this, We performed multi-epoch observations of SgrA* with VLBI Exploration of Radio Astrometry (VERA).
2. VERA Observations in 2005 - 2008
We performed multi-epoch observations of Sgr A* at 7 mm (i.e. 43 GHz) from 2004to 2008 (Akiyama et al. 2013). We detected quiescent flux levels and no radio flares (see,Figure 1). However, comparisons with previous Very Long Baseline Array (VLBA) resultsshowed that Sgr A* underwent a flaring event for at least 10 days in 2007 May reportedin Lu et al. (2011). Our data show that the intrinsic size of Sgr A* remained (withinerrors) unchanged compared to the size before and after the flaring event, indicatingthat the brightness temperature of Sgr A* was increased. The duration of this flaringevent is less than 31 days, which is shorter than the refractive time scale of 3 months(Bower et al. 2004). Moreover, it is difficult to explain the increase in the spectral indexat this flaring event reported in Lu et al. (2011) by a simple interstellar scattering model(Rickett 1990). Hence, the flaring event is most likely associated with changes in theintrinsic properties of Sgr A*.Considering observed structure, it is unlikely that the flaring event is associated withan ejection of relativistic component or a temporal one-shot plasma heating such asan expanding plasma blob (e.g. Yusef-Zadeh et al. 2006) or a hot spot orbiting aroundthe central black hole (e.g. Broderick & Loeb 2006). Thus, the flaring event is likelyto be associated with a brightness increase of the photosphere. Since the synchrotroncooling time-scale at mm wavelengths is much shorter than than the duration of theflare, understanding the flare requires a mechanism that heats electrons continuously ontimescales much longer than the orbital timescales of the accretion disk such as a standingshock in an accretion flow. In the future, simultaneous multi-frequency flux monitoringfrom cm to sub-mm wavelengths would be helpful to constrain the properties of electrondistribution at the flaring event. gr A* observations with VERA and KaVA M a i n t enan c e S ea s on M a i n t enan c e S ea s on Figure 2.
Preliminary results of VERA observations at 7 mm (i.e. 43 GHz) in the first half of2013. Left: The upper panel shows a time variation in the total flux, while the lower panel showsa time variation in the major/minor axis size of the best-fit elliptical Gaussian model. Right:The image at 7 mm on 7th May 2013, with the best-fit elliptical Gaussian model (shown in agreen line). The image is restored using a circular Gaussian beam with a FWHM of 0.4 mas.
3. New VERA Observations from January 2013
Recently, the gas cloud G2 with three earth masses has been detected on its way to SgrA* (Gillessen et al. 2012). G2 will approach pericenter at a distance of only 1500-2200R sch (15-22 mas) in early-2014 (Phifer et al. 2013; Gillessen et al. 2013). The dynamicevolution around Sgr A* in the next few years will be an unique probe for understandingthe properties of the accretion flow/jet and the feeding processes of the SMBH.Related with the G2 encounter with Sgr A*, two flaring phenomena are expected inradio regime as of November 2013. First, G2 is expected to trigger flaring of Sgr A*,since the radio flux is strongly related to the mass accretion rate ˙ M to the centralblack hole, scaling with ˙ M (e.g. Mahadevan 1997) for the accretion flow and ˙ M . (e.g.Falcke & Biermann 1995) for the jet. A possible increase in the mass accretion rate wouldcause a flux increase as well as structural changes in the visible structure detectable withVLBI, like an increase in the size of the radio emitting region (e.g. Mo´scibrodzka et al.2012) or appearance of the visible jet. Second, Narayan et al. (2012) predict that theinteraction between G2 and the accretion disk will produce a bow-shock luminous inradio regime around pericenter. VLBI observations would be important for constrainingthe size and the compact feature of the shock region.In order to trace this exciting event, we performed monitoring observations with VERAat 7 mm with an interval of 3 weeks. Here, we show preliminary results by August 2013in Figure 3. The radio flux and the major/minor axis size show a variation within 10 %level, consistent with previous activities. Our preliminary results show that Sgr A* onmas scales has been quiescent as of August 2013, although the head part of G2 alreadypassed pericenter in April 2013 (Gillessen et al. 2013). Considering a free-fall time scaleof few months from pericenter to the central SMBH, our results indicate that the feedingrate of the G2 head part is comparable or less than the current accretion rate of Sgr A*,or most of the accreted gas is lost on the way to the black hole via a disk outflow in theaccretion disk.
4. Future observations with VERA and KaVA
We are going to continue monitoring with newly developed KVN and VERA Array(KaVA). KaVA consists of 7 stations in Korean VLBI Network (KVN, Lee et al. 2011) K. Akiyama et al.
IshigakijimaIrikiTamnaYonsei Ulsan OgasawaraMizusawa
Figure 3.