In order to explore any change caused by the G2 cloud approaching, we have monitored the flux density of Sgr A* at 22 GHz from Feb. 2013 to Aug. 2014 with a sub-array of Japanese VLBI Network . The observation period included the expected periastron
dates. The number of observation epochs was 283 days. We have observed no significant microwave enhancement of Sgr A* in the whole observation period. The average flux density in the period is $S=1.23+/-0.33$ Jy. The average is consistent with the usually observed flux density range of Sgr A* at 22 GHz.
The 22 GHz H2O maser in Orion KL has shown extraordinary burst events in 1979-1985 and 1998-1999, sometimes called supermaser. We have conducted monitoring observations of the supermaser in Orion KL using VERA, VLBI Exploration of Radio Astrometry, i
n the current third burst since 2011 March. Three flux maxima are detected in 2011 and 2012 with rising and falling timescales of 2-7 months. Time variations of the supermaser seem symmetric for all of the active phases. The maximum total flux density of 135000 Jy is observed in 2012 June while it is still one order of magnitude lower than those in previous bursts. The supermaser consists of two spatially different components at different velocities. They are elongated along a northwest-southeast direction perpendicular to the low-velocity outflow driven by Source I. Proper motions of the supermaser features with respect to Source I are measured toward west and southwest directions, almost parallel to the low-velocity outflow. The flux density and linewidth show an anti-correlation as expected for an unsaturated maser emission. The supermaser is located close to the methylformate (HCOOCH3) line and continuum emission peaks in the Orion Compact Ridge detected by ALMA. The broader velocity range of the weak HCOOCH3 emission at the supermaser position would be an evidence of a shock front. On the other hand, the 321 GHz H2O line is not detected at the position of the supermaser. It can be explained qualitatively by one of the theoretical H2O excitation models without extraordinary conditions. Our results support a scenario that the supermaser is excited in the dense gas interacting with the low-velocity outflow in the Compact Ridge. The extremely high flux density and its symmetric time variation for rising and falling phases could be explained by a beaming effect during the amplification process rather than changes in physical conditions.
We performed the observation of the flux densities of SgrA* at 90 and 102GHz in order to detect the time lag between these frequencies using the Nobeyama Millimeter Array, which was previously reported at lower frequencies. We detected a radio flare
during the observation period on 6 April 2005 and calculated the z-transformed discrete correlation function between the light curves. The time lag between these frequencies was not detected. If the expanding plasma model which explains the time lag at lower frequencies is valid, the light curve at 90GHz would be delayed with respect to the one at 102GHz. This result suggests that the plasma blobs ejected near the Galactic Center black hole may be widely diverse especially in optical thickness. Another possibility is that the major portion of the flux above 100GHz does not originate from the blobs.
We performed the observation of the flux densities of Sgr A* at 90 and 102 GHz on 6 April 2005 using the Nobeyama Millimeter Array in order to detect the time lag between these frequencies. We constructed light curves covering a few hour with 1 min b
in, and the Intra-Day Variability, which had a rising phase and intensity peak, of Sgr A* is clearly seen at both frequencies. We calculated the z-transformed discrete correlation function between the light curves of Sgr A* at 90 and 102 GHz. The derived time lag of the flares at these frequencies was approximately zero, contrary to our expectations based on the previously reported time lag at lower frequencies. If the radio flares of Sgr A* are explained by the expanding plasma model, the light curve at 90 GHz would be delayed with respect to the one at 102 GHz. However, we could not find such a delay with statistical significance in our data.
Large deployable antennas with a mesh surface woven by fine metal wires are an important technology for communications satellites and space radio telescopes. However, it is difficult to make metal mesh surfaces with sufficient radio-frequency (RF) pe
rformance for frequencies higher than millimeter waves. In this paper, we present the RF performance of metal mesh surfaces at 43 GHz. For this purpose, we developed an apparatus to measure the reflection coefficient, transmission coefficient, and radiative coefficient of the mesh surface. The reflection coefficient increases as a function of metal mesh surface tension, whereas the radiative coefficient decreases. The anisotropic aspects of the reflection coefficient and the radiative coefficient are also clearly seen. They depend on the front and back sides of the metal mesh surface and the rotation angle. The transmission coefficient was measured to be almost constant. The measured radiative coefficients and transmission coefficients would cause significant degradation of the system noise temperature. In addition, we carried out an astronomical observation of a well-known SiO maser source, R Cas, by using a metal mesh mirror on the NRO 45-m radio telescope Coude system. The metal mesh mirror considerably increases the system noise temperature and slightly decreases the peak antenna temperature. These results are consistent with laboratory measurements.
We present the statistical properties of molecular clumps in the Galactic center 50 km s$^{-1}$ molecular cloud (GCM-0.02-0.07) based on observations of the CS $J=1-0$ emission line with the Nobeyama Millimeter Array. In the cloud, 37 molecular clump
s with local thermal equilibrium (LTE) masses of $2times10^2-6times10^3 M_odot$ were identified by using the {it clumpfind} algorithm. The velocity widths of the molecular clumps are about five-fold those of Galactic disk molecular clouds with the same radius. The virial-theorem masses are three-fold the LTE masses. The mass and size spectra can be described by power laws of $dN/dMpropto M^{-2.6pm0.1}$ ($Mgtrsim 900M_odot$) and $dN/dRpropto R^{-5.9pm0.3}$ ($Rgtrsim 0.35$ pc), respectively. The statistical properties of the region interacting with the Sgr A East shell and those of the non-interacting part of the cloud are significantly different. The interaction probably makes the mass function steeper, from $dN/dMpropto M^{-2.0pm0.1}$ in the non-interacting part to $dN/dMpropto M^{-4.0pm0.2}$ in the interacting region. On the other hand, the interaction presumably truncates the size spectrum on the larger side of $Rsim 0.4$ pc.
Cygnus X-3 (Cyg X-3) is a well-known microquasar with relativistic jets. Cyg X-3 is especially famous for its giant radio outbursts, which have been observed once every few years since their first discovery. Each giant outburst presumably consists of
a series of short-duration flares. The physical parameters of the flares in the giant outbursts are difficult to derive because the successive flares overlap. Here, we report isolated flares in the quiescent phase of Cyg X-3, as observed at 23, 43, and 86 GHz with the 45-m radio telescope at Nobeyama Radio Observatory. The observed flares have small amplitude (0.5--2 Jy) and short duration (1--2 h). The millimeter fluxes rapidly increase and then exponentially decay. The lifetime of the decay is shorter at higher frequency. The radio spectrum of Cyg X-3 during the flares is flat or inverted around the peak flux density. After that, the spectrum gradually becomes steeper. The observed characteristics are consistent with those of adiabatic expanding plasma. The brightness temperature of the plasma at the peak is estimated to be $T_Bgtrsim 1 times 10^{11}$ K. The magnetic field in the plasma is calculated to be $0.2 lesssim H lesssim 30$ G.
In February 2011, a burst event of the H$_{2}$O maser in Orion KL (Kleinmann-Low object) has started after 13-year silence. This is the third time to detect such phenomena in Orion KL, followed by those in 1979-1985 and 1998. We have carried out astr
ometric observations of the bursting H$_{2}$O maser features in Orion KL with VERA (VLBI Exploration of Radio Astrometry), a Japanese VLBI network dedicated for astrometry. The total flux of the bursting feature at the LSR velocity of 7.58 km s$^{-1}$ reaches 4.4$times10^{4}$ Jy in March 2011. The intensity of the bursting feature is three orders of magnitudes larger than that of the same velocity feature in the quiescent phase in 2006. Two months later, another new feature appears at the LSR velocity of 6.95 km s$^{-1}$ in May 2011, separated by 12 mas north of the 7.58 km s$^{-1}$ feature. Thus, the current burst occurs at two spatially different features. The bursting masers are elongated along the northwest-southeast direction as reported in the previous burst in 1998. We determine the absolute positions of the bursting features for the first time ever with a submilli-arcsecond (mas) accuracy. Their positions are coincident with the shocked molecular gas called the Orion Compact Ridge. We tentatively detect the absolute proper motions of the bursting features toward southwest direction. It is most likely that the outflow from the radio source I or another young stellar object interacting with the Compact Ridge is a possible origin of the H$_{2}$O maser burst.
Cyg X-3 is a well-known microquasar with a bipolar relativistic jet. Its famous giant radio outbursts have been repeated once every several years. However, the behavior of the millimeter wave emission has remained unclear because of limitations of ti
me resolution in previous observations. We report here millimeter wave observations of Cyg X-3 experiencing giant outbursts with one of the finest time resolutions. We find a series of short-lived flares with amplitude of 1-2 Jy in the millimeter light curve of the 2008 April-May outburst. They have flat spectra around 100 GHz. We also find abrupt and large amplitude flux density changes with e-folding time of 3.6 minutes or less. The source size of Cyg X-3 is constrained within 0.4 AU and the brightness temperature is estimated to be $T_B gtrsim 1times10^{11}$ K.
