86 research outputs found
Astrometry of HO Masers in Nearby Star-Forming Regions with VERA --- IV. L1448C
We have carried out multi-epoch VLBI observations with VERA (VLBI Exploration
of Radio Astrometry) of the 22~GHz HO masers associated with a Class 0
protostar L1448C in the Perseus molecular cloud. The maser features trace the
base of collimated bipolar jet driven by one of the infrared counter parts of
L1448C named as L1448C(N) or L1448-mm A. We detected possible evidences for
apparent acceleration and precession of the jet according to the
three-dimensional velocity structure. Based on the phase-referencing VLBI
astrometry, we have successfully detected an annual parallax of the HO
maser in L1448C to be 4.310.33~milliarcseconds (mas) which corresponds to
a distance of 23218~pc from the Sun. The present result is in good
agreement with that of another HO maser source NGC~1333 SVS13A in the
Perseus molecular cloud, 235~pc. It is also consistent with the photometric
distance, 220~pc. Thus, the distance to the western part of the Perseus
molecular cloud complex would be constrained to be about 235~pc rather than the
larger value, 300~pc, previously reported.Comment: 15 pages, 5 figures, accepted for publication in PAS
The MAXI Mission on the ISS: Science and Instruments for Monitoring All Sky X-Ray Images
The MAXI (Monitor of All-sky X-ray Image) mission is the first astronomical
payload to be installed on the Japanese Experiment Module-Exposed Facility
(JEM-EF) on the ISS. It is scheduled for launch in the middle of 2009 to
monitor all-sky X-ray objects on every ISS orbit. MAXI will be more powerful
than any previous X-ray All Sky Monitor (ASM) payloads, being able to monitor
hundreds of AGN. MAXI will provide all sky images of X-ray sources of about 20
mCrab in the energy band of 2-30 keV from observation on one ISS orbit (90
min), about 4.5 mCrab for one day, and about 1 mCrab for one month. A final
detectability of MAXI could be 0.2 mCrab for 2 year observations.Comment: 12 pages, 11 figures, accepted for publication in Publications of the
Astronomical Society of Japa
Bright X-ray flares from the BL Lac object Mrk 421, detected with MAXI in 2010 January and February
Strong X-ray flares from the blazar Mrk 421 were detected in 2010 January and
February through the 7 month monitoring with the MAXI GSC. The maximum 2 -- 10
keV flux in the January and February flares was measured as 120 +- 10 mCrab and
164 +- 17 mCrab respectively; the latter is the highest among those reported
from the object. A comparison of the MAXI and Swift BAT data suggests a convex
X-ray spectrum with an approximated photon index of about 2. This spectrum is
consistent with a picture that MAXI is observing near the synchrotron peak
frequency. The source exhibited a spectral variation during these flares,
slightly different from those in the previous observations, in which the
positive correlation between the flux and hardness was widely reported. By
equating the halving decay timescale in the January flare, s, to the synchrotron cooling time, the magnetic field was
evaluated as B = 0.045 G , where is the jet
beaming factor. Assuming that the light crossing time of the emission region is
shorter than the doubling rise time, s,
the region size was roughly estimated as cm
. These are consistent with the values previously reported. For
the February flare, the rise time, s, gives a
loose upper limit on the size as cm ,
although the longer decay time s, indicates
B = 0.015 G , which is weaker than the previous results.
This could be reconciled by invoking a scenario that this flare is a
superposition of unresolved events with a shorter timescale.Comment: 14 pages, 4 figures, accepted for PASJ (Vol. 62 No. 6
MAXI GSC observations of a spectral state transition in the black hole candidate XTE J1752-223
We present the first results on the black hole candidate XTE J1752-223 from
the Gas Slit Camera (GSC) on-board the Monitor of All-sky X-ray Image (MAXI) on
the International Space Station. Including the onset of the outburst reported
by the Proportional Counter Array on-board the Rossi X-ray Timing Explorer on
2009 October 23, the MAXI/GSC has been monitoring this source approximately 10
times per day with a high sensitivity in the 2-20 keV band. XTE J1752-223 was
initially in the low/hard state during the first 3 months. An anti-correlated
behavior between the 2-4 keV and 4-20 keV bands were observed around January
20, 2010, indicating that the source exhibited the spectral transition to the
high/soft state. A transient radio jet may have been ejected when the source
was in the intermediate state where the spectrum was roughly explained by a
power-law with a photon index of 2.5-3.0. The unusually long period in the
initial low/hard state implies a slow variation in the mass accretion rate, and
the dramatic soft X-ray increase may be explained by a sudden appearance of the
accretion disk component with a relatively low innermost temperature (0.4-0.7
keV). Such a low temperature might suggest that the maximum accretion rate was
just above the critical gas evaporation rate required for the state transition.Comment: Publication of Astronomical Society of Japan Vol.62, No.5 (2010) [in
print
Fundamental Parameters of the Milky Way Galaxy Based on VLBI astrometry
We present analyses to determine the fundamental parameters of the Galaxy
based on VLBI astrometry of 52 Galactic maser sources obtained with VERA, VLBA
and EVN. We model the Galaxy's structure with a set of parameters including the
Galaxy center distance R_0, the angular rotation velocity at the LSR Omega_0,
mean peculiar motion of the sources with respect to Galactic rotation (U_src,
V_src, W_src), rotation-curve shape index, and the V component of the Solar
peculiar motions V_sun. Based on a Markov chain Monte Carlo method, we find
that the Galaxy center distance is constrained at a 5% level to be R_0 = 8.05
+/- 0.45 kpc, where the error bar includes both statistical and systematic
errors. We also find that the two components of the source peculiar motion
U_src and W_src are fairly small compared to the Galactic rotation velocity,
being U_src = 1.0 +/- 1.5 km/s and W_src = -1.4 +/- 1.2 km/s. Also, the
rotation curve shape is found to be basically flat between Galacto-centric
radii of 4 and 13 kpc. On the other hand, we find a linear relation between
V_src and V_sun as V_src = V_sun -19 (+/- 2) km/s, suggesting that the value of
V_src is fully dependent on the adopted value of V_sun. Regarding the rotation
speed in the vicinity of the Sun, we also find a strong correlation between
Omega_0 and V_sun. We find that the angular velocity of the Sun, Omega_sun,
which is defined as Omega_sun = Omega_0 + V_sun/R_0, can be well constrained
with the best estimate of Omega_sun = 31.09 +/- 0.78 km/s/kpc. This corresponds
to Theta_0 = 238 +/- 14 km/s if one adopts the above value of R_0 and recent
determination of V_sun ~ 12 km/s.Comment: 14 pages, 6 figures, PASJ in pres
A Large X-ray Flare from a Single Weak-lined T Tauri Star TWA-7 Detected with MAXI GSC
We present a large X-ray flare from a nearby weak-lined T Tauri star TWA-7
detected with the Gas Slit Camera (GSC) on the Monitor of All-sky X-ray Image
(MAXI). The GSC captured X-ray flaring from TWA-7 with a flux of
ergs cm s in 2--20 keV band during the scan
transit starting at UT 2010-09-07 18:24:30.The estimated X-ray luminosity at
the scan in the energy band is 3 ergs s,indicating that
the event is among the largest X-ray flares fromT Tauri stars.Since MAXI GSC
monitors a target only during a scan transit of about a minute per 92 min
orbital cycle, the luminosity at the flare peak might have been higher than
that detected. At the scan transit, we observed a high X-ray-to-bolometric
luminosity ratio, log = ; i.e., the
X-ray luminosity is comparable to the bolometric luminosity. Since TWA-7 has
neither an accreting disk nor a binary companion, the observed event implies
that none of those are essential to generate such big flares in T Tauri stars.Comment: 4 pages, 2 figures, 1 table accepted for publication in PAS
A novel graphene barrier against moisture by multiple stacking large-grain graphene
The moisture barrier properties of stacked graphene layers on Cu surfaces were investigated with the goal of improving the moisture barrier efficiency of single-layer graphene (SLG) for Cu metallization. SLG with large grain size were stacked on Cu surfaces coated with CVD-SLG to cover the grain-boundaries and defective areas of the underneath SLG film, which was confirmed to be oxidized by Raman spectroscopy measurements. To evaluate the humidity resistance of the graphene-coated Cu surfaces, temperature humidity storage (THS) testing was conducted under accelerated oxidation conditions (85 °C and 85% relative humidity) for 100 h. The color changes of the Cu surfaces during THS testing were observed by optical microscopy, while the oxidized Cu into Cu2O and CuO was detected by X-ray photoelectron spectroscopy (XPS). The experimental results were accord with the results of first-principle simulation for the energetic barrier against water diffusion through the stacked graphene layers with different overlap. The results demonstrate the efficiency of SLG stacking approach against moisture for Cu metallization
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