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We report results from the observations of the well studied TeV blazar Mrk 421 with the Swift and the Suzaku satellites in December 2008. During the observation, Mrk 421 was found in a relatively low activity state, with the corresponding 2-10 keV flux of $3 \times 10^{-10}$ erg/s/cm^2. For the purpose of robust constraining the UV-to-X-ray emission continuum we selected only the data corresponding to truly simultaneous time intervals between Swift and Suzaku, allowing us to obtain a good-quality, broad-band spectrum despite a modest length (0.6 ksec) exposure. We analyzed the spectrum with the parametric forward-fitting SYNCHROTRON model implemented in XSPEC assuming two different representations of the underlying electron energy distribution, both well motivated by the current particle acceleration models: a power-law distribution above the minimum energy $\gamma_{\rm min}$ with an exponential cutoff at the maximum energy $\gamma_{\rm max}$, and a modified ultra-relativistic Maxwellian with an equilibrium energy $\gamma_{\rm eq}$. We found that the latter implies unlikely physical conditions within the blazar zone of Mrk 421. On the other hand, the exponentially moderated power-law electron distribution gives two possible sets of the model parameters: (i) flat spectrum $dN'_e/d\gamma \propto \gamma^{-1.91}$ with low minimum electron energy $\gamma_{\rm min}, Comment: 18 pages, 2 figures; accepted for publication in the Astrophysical Journal

著者人数：12名, Accepted: 2009-08-18, 資料番号: SA1000737000

Abridged: We investigated the detailed radio structure of the nucleus of the Sombrero galaxy using high-resolution, quasi-simultaneous, multi-frequency, phase-referencing VLBA observations. We obtained the VLBI images toward this nucleus, with unprecedented sensitivities and resolutions, at the seven frequencies between 1.4 and 43 GHz, where those at 15, 24 and 43 GHz are the first clear VLBI detections. At 43 GHz, the nuclear structure was imaged on a linear scale under 100 Schwarzschild radii. For the first time, we have discovered the presence of the extended structure in this nucleus, which is directing from the radio core in two sides toward northwest/southeast directions. The nuclear structure shows a clear spatial gradient on the radio spectra, which is similar to that commonly seen in more luminous AGN with powerful relativistic jets. Moreover, the radio core shows a frequency-dependent size with an elongated shape, and the position of the core also tends to be frequency dependent. A set of these new findings provide evidence that the central engine of the Sombrero is powering radio jets. Based on the observed brightness ratio of jet-to-counter jet, core position shift and its comparison with a theoretical model, we constrained the following fundamental physical parameters for the M 104 jets: (1) the northern side is the approaching jet, whereas the southern side is receding: (2) the inclination angle of the jet is relatively close to our line-of-sight, probably less than ~25degrees: (3) the jet intrinsic velocity is highly sub-relativistic at a speed less than ~0.2c. The derived pole-on nature of the M 104 jet is in accordance with the previous argument that M 104 contains a true type II AGN, i.e., the broad line region of this nucleus is actually absent or intrinsically weak, if the plane of the presumed circumnuclear torus is perpendicular to the axis of the radio jets., Comment: 12 pages, 9 figures, 4 tables, accepted for publication in ApJ

The HII region luminosity function (LF) is an important tool for deriving the birthrates and mass distribution of OB associations, and is an excellent tracer of the newly formed massive stars and associations. To date, extensive work (predominantly in H{\alpha}) has been done from the ground, which is hindered by dust extinction and the severe blending of adjacent (spatially or in projection) HII regions. Reliably measuring the properties of HII regions requires a linear resolution, Comment: 17 pages, 11 figures, 2 tables, accepted for publication in ApJ. The full catalog of the 1907 HII regions is available at: http://www.pha.jhu.edu/~liu/papers/h2lf.tab2.dat

We report on the VLBI follow-up observations using the Japanese VLBI Network (JVN) array at 22 GHz for the largest X-ray flare of TeV blazar Mrk 421 that occurred in mid-February, 2010. The total of five epochs of observations were performed at intervals of about 20 days between March 7 and May 31, 2010. No new-born component associated with the flare was seen directly in the total intensity images obtained by our multi-epoch VLBI observations. However, one jet component located at ~1 mas north-west from the core was able to be identified, and its proper motion can be measured as -1.66+/-0.46 mas yr^-1, which corresponds to an apparent velocity of -3.48+/-0.97 c. Here, this negative velocity indicates that the jet component was apparently moving toward the core. As the most plausible explanation, we discuss that the apparent negative velocity was possibly caused by the ejection of a new component, which could not be resolved with our observations. In this case, the obtained Doppler factor of the new component is around 10 to 20, which is consistent with the ones typically estimated by model fittings of spectral energy distribution for this source., 9 pages, 6 figures, 3 tables, accepted for publication in ApJ

Based on a detailed analysis of the high-quality Chandra, XMM-Newton, and Suzaku data of the X-ray bright cluster of galaxies Abell 1795, we report clear evidence for a two-phase intracluster medium (ICM) structure, which consists of a cool (with a temperature T = 2.0-2.2 keV) and a hot (T = 5.0-5.7 keV) component that coexist and dominate the X-ray emission at least in the central 80 kpc. A third weak emission component (T = 0.8 keV) is also detected within the innermost 144 kpc and is ascribed to a portion of inter-stellar medium (ISM) of the cD galaxy. Deprojected spectral analysis reveals flat radial temperature distributions for both the hot phase and cool phase components. These results are consistent with the ASCA measurements reported in Xu et al. (1998), and resemble the previous findings for the Centaurus cluster (e.g., Takahashi et al. 2009). By analyzing the emission measure ratio and gas metal abundance maps created from the Chandra data, we find that the cool phase component is more metal-enriched than the hot phase one in 50-100 kpc region, which agrees with that found in M87 (Simionescu et al. 2008). The coexistence of the cool phase and hot phase ICM cannot be realized by bubble uplifting from active galactic nuclei (AGN) alone. Instead, the two-phase ICM properties are better reconciled with a cD corona model (Makishima et al. 2001). (Abridged), Comment: 47 pages, 12 figures, accepted for publication in ApJ

著者人数: 14名, Accepted: 2011-10-20, 資料番号: SA1003013000

We report a search for fluctuations of the sky brightness toward the north ecliptic pole (NEP) with the Japanese infrared astronomical satellite AKARI,at 2.4, 3.2, and 4.1 \mum. We obtained circular maps with 100 diameter field of view which clearly show a spatial structure on scale of a few hundred arcsec. A power spectrum analysis shows that there is a significant excess fluctuation at angular scales larger than 10000 that can't be explained by zodiacal light, diffuse Galactic light, shot noise of faint galaxies or clustering of low redshift galaxies. These results are consistent with observations at 3.6 and 4.5 \mum by NASA's Spitzer Space Telescope. The fluctuating component observed at large angular scales has a blue stellar spectrum which is similar to that of the spectrum of the excess isotropic emission observed with IRTS. A significant spatial correlation between wavelength bands was found, and the slopes of the linear correlations is consistent with the spectrum of the excess fluctuation. These findings indicate that the detected fluctuation could be attributed to the first stars of the universe, i.e. pop. III stars. The observed fluctuation provides an important constraints on the era of the first stars., Astrophysical Journal, accepted

We conducted very long baseline interferometry (VLBI) observations of five radio-loud narrow-line Seyfert 1 (NLS1) galaxies in milliarcsecond resolutions at 1.7 GHz (18 cm) using the Very Long Baseline Array (VLBA). Significant parsec-scale structures were revealed for three out of the five sources with high brightness temperature by direct imaging; this is convincing evidence for nonthermal jets. FBQS J1644+2619 with an inverted spectrum showed a prominent one-sided linear structure, indicating Doppler beaming with an intrinsic jet speed of >0.74c. FBQS J1629+4007, also with an inverted spectrum, showed rapid flux variability, indicating Doppler beaming with an intrinsic jet speed of >0.88c. Thus, we found convincing evidence that these two NLS1s can generate at least mildly or highly relativistic jets, which may make them apparently radio loud even if they are intrinsically radio quiet. On the other hand, the other three NLS1s had steep spectra and two of them showed significantly diffuse pc-scale structures, which were unlikely to be strongly beamed. Thus, some NLS1s have the ability to generate jets strong enough to make them intrinsically radio loud without Doppler beaming. NLS1s as a class show a number of extreme properties and radio-loud ones are very rare. We build on these radio results to understand that the central engines of radio-loud NLS1s are essentially the same as that of other radio-loud AGNs in terms of the formation of nonthermal jets., 12 pages, 1 figure, accepted for publication in ApJ