Publications of the Korean Astronomical Society pISSN: 1225-1534 30: 129 ∼ 131, 2015 September eISSN: 2287-6936 c 2015. The Korean Astronomical Society. All rights reserved. http://dx.doi.org/10.5303/PKAS.2015.30.2.129 FLUX MONITORING OF 6.7 GHz METHANOL MASER TO SYSTEMATICALLY RESEARCH PERIODIC VARIATIONS USING THE HITACHI 32-m Koichiro Sugiyama 1 , Yoshinori Yonekura 1 , Kazuhito Motogi 2 , Yu Saito 3 , Kenta Fujisawa 2 , Shota Ishii 3 , Munetake Momose 3 , Mareki Honma 4 , Fumie Tazaki 4 , Kei E. I. Tanaka 5 , Takashi Hosokawa 6 , Mizuho Uchiyama 7 , and Kohei Inayoshi 8 1 Center for Astronomy, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan 2 The Research Institute for Time Studies, Yamaguchi University, 1677-1 Yoshida, Yamaguchi, Yamaguchi 753-8511, Japan 3 College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan 4 Mizusawa VLBI Observatory, NAOJ, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 5 Department of Astronomy, University of Florida, 211 Bryant Space Science Center, PO Box 112055, Gainesville, FL 32611, USA 6 Department of Physics and Research Center for the Early Universe, The University of Tokyo, Tokyo 113-0033, Japan 7 Institute of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan 8 Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA E-mail: koichiro@mx.ibaraki.ac.jp, y.yone@mx.ibaraki.ac.jp (Received November 30, 2014; Reviced May 31, 2015; Aaccepted June 30, 2015) ABSTRACT We have initiated single-dish monitoring observations of ∼400 methanol maser sources at 6.7 GHz using the Hitachi 32-m radio telescope from December 2012 to systematically research periodic flux variations, which are observed in some methanol maser sources associated with high-mass (proto-)stars. In our monitoring, we have made daily monitoring, so that each source has been observed every nine days with an integration time of 5 min (typical 3 σ detection sensitivities of 0.9 Jy). The monitoring observations help us statistically understand periodic flux variations with a period longer than 50 days. As an initial result, we present a new detection of periodic flux variations in the 6.7 GHz methanol maser source G 036.70+00.09. The period of the flux variations is ∼53 days (∼0.019 cycles day -1 ), and seems to be stable over 9 cycles, at least until the middle of August 2014. Key words: Masers: methanol — Stars: formation — Stars: massive — Stars: variables 1. INTRODUCTION 6.7 GHz methanol masers show a characteristic “pe- riodic” flux variation, discovered by Goedhart et al. (2003). The periodic variations have been detected in thirteen methanol maser sources so far, for which the observed period ranges between 30 and 668 days (Goed- hart et al. (2004), 2009; Araya et al. (2010); Szymczak et al. (2011); Fujisawa et al. (2014); Maswanganye et al. (2014)). It is suggested that there are some possible mechanisms which could cause periodic varia- tions around high-mass (proto-)stars, such as colliding- wind binaries (van der Walt , 2011) and periodic ac- cretion onto binary systems (Araya et al. , 2010). Of the various mechanisms, we note the stellar pulsation model of high-mass protostars suggested by Inayoshi et al. (2013), because of its occurrence at the pre-main sequence phase, easy explanation of sinusoidal contin- uous variations, and expected periods of the order of 10–100 days. This model also predicts that the period- luminosity (P-L) relation can be described with a power http://pkas.kas.org law. It is predicted that the P-L relation indicates a cor- relation among the periodicity, the luminosity, and the accretion rate during the pulsational instability phase. If the relation can be verified by observations, we can estimate the accretion rate onto the surface of the pro- tostar as well as its mass and radius solely from the periodicity of the pulsation. The estimation of these physical parameters is quite difficult even using Ata- cama Large Millimeter/submillimeter Array. 6.7 GHz methanol masers with periodic flux variations are suit- able targets to observationally verify the P-L relation. The number of such sources detected so far, however, is rather limited. To increase the number of sources, we have initiated daily monitoring observations using Hitachi 32-m telescope, since Dec. 2012. 2. OBSERVATIONS As target sources, we selected ∼400 methanol masers at 6.7 GHz, which are all the ones observable from Hi- tachi station (declination > -30 deg) allowing us to verify the P-L relation systematically. Daily monitor was performed from Dec. 2012 to Jan. 2014 and from 129