arXiv subjects

Shinji Horiuchi

Publications and source records attributed to Shinji Horiuchi.

29 records · Page 2Linked to original sources

arXiv2018

Searches for H$_2$O Masers toward Narrow-Line Seyfert 1 galaxies

We present searches for 22 GHz H$_2$O masers toward 36 narrow-line Seyfert 1 galaxies (NLS1s), selected from known NLS1s with $v_{sys}$ $\lesssim$ 41000 km/s. Out of the 36 NLS1s in our sample, 11 NLS1s have been first surveyed in our observations, while the observations of other NLS1s were previously reported in literature. In our survey, no new water maser source from NLS1s was detected at 3 $σ$ rms level of 8.4 mJy to 144 mJy, which depends on different observing conditions or inhomogeneous sensitivities of each observation using three different telescopes. It is likely that non-detection of new masers in our NLS1 sample is primarily due to insufficient sensitivities of our observations. Including the five known NLS1 masers, the total detection rate of the H$_2$O maser in NLS1s is not remarkably different from that of type 2 Seyfert galaxies or LINERs. However, more extensive and systematic searches of NLS1 would be required for statistical discussion of the detection rate of the NLS1 maser, compared with that of type 2 Seyferts or LINERs.

astro-ph.GA↗

arXiv2017

Characterizing the transition from diffuse atomic to dense molecular clouds in the Magellanic clouds with [CII], [CI], and CO

We present and analyze deep Herschel/HIFI observations of the [CII] 158um, [CI] 609um, and [CI] 370um lines towards 54 lines-of-sight (LOS) in the Large and Small Magellanic clouds. These observations are used to determine the physical conditions of the line--emitting gas, which we use to study the transition from atomic to molecular gas and from C^+ to C^0 to CO in their low metallicity environments. We trace gas with molecular fractions in the range 0.1 0.45 in both the LMC and SMC. Ionized carbon is the dominant gas-phase form of this element that is associated with molecular gas, with C^0 and CO representing a small fraction, implying that most (89% in the LMC and 77% in the SMC) of the molecular gas in our sample is CO-dark H2. The mean X_CO conversion factors in our LMC and SMC sample are larger than the value typically found in the Milky Way. When applying a correction based on the filling factor of the CO emission, we find that the values of X_CO in the LMC and SMC are closer to that in the Milky Way. The observed [CII] intensity in our sample represents about 1% of the total far-infrared intensity from the LOSs observed in both Magellanic Clouds.

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arXiv2016

Post-Outburst Radio Observations of the High Magnetic Field Pulsar PSR J1119-6127

We have carried out high frequency radio observations of the high magnetic field pulsar PSR J1119-6127 following its recent X-ray outburst. While initial observations showed no evidence of significant radio emission, subsequent observations detected pulsed emission across a large frequency band. In this letter, we report on the initial disappearance of the pulsed emission and its prompt reactivation and dramatic evolution over several months of observation. The periodic pulse profile at S-band (2.3 GHz) after reactivation exhibits a multi-component emission structure, while the simultaneous X-band (8.4 GHz) profile shows a single emission peak. Single pulses were also detected at S-band near the main emission peaks. We present measurements of the spectral index across a wide frequency bandwidth, which captures the underlying changes in the radio emission profile of the neutron star. The high frequency radio detection, unusual emission profile, and observed variability suggest similarities with magnetars, which may independently link the high energy outbursts to magnetar-like behavior.

astro-ph.HE↗

arXiv2016

Search for sub-millimeter H2O masers in active galaxies - the detection of a 321 GHz H2O maser in NGC4945

We present further results of a search for extragalactic submillimeter H2O masers using the Atacama Large Millimeter/Submillimeter Array (ALMA). The detection of a 321 GHz H2O maser in the nearby Type 2 Seyfert galaxy, the Circinus galaxy, has previously been reported, and here the spectral analysis of four other galaxies is described. We have discovered H2O maser emission at 321 GHz toward the center of NGC 4945, a nearby Type 2 Seyfert. The maser emission shows Doppler-shifted velocity features with velocity ranges similar to those of the previously reported 22 GHz H2O masers, however the non-contemporaneous observations also show differences in velocity offsets. The sub-parsec-scale distribution of the 22 GHz H2O masers revealed by earlier VLBI (Very Long Baseline Interferometry) observations suggests that the submillimeter masers could arise in an edge-on rotating disk. The maser features remain unresolved by the synthesized beam of ~ 0.54 (~30 pc) and are located toward the 321 GHz continuum peak within errors. A marginally detected (3 sigma) high-velocity feature is redshifted by 579 km/s with respect to the systemic velocity of the galaxy. Assuming that this feature is real and arises from a Keplerian rotating disk in this galaxy, it is located at a radius of ~0.020 pc (~1.5 x 10^5 Schwarzschild radii), which would enable molecular material closer to the central engine to be probed than the 22 GHz H2O masers. This detection confirms that submillimeter H2O masers are a potential tracer of the circumnuclear regions of active galaxies, which will benefit from higher angular resolution studies with ALMA.

astro-ph.GA↗

arXiv2016

The TANAMI Multiwavelength Program: Dynamic SEDs of Southern Blazars

Simultaneous broadband spectral and temporal studies of blazars are an important tool for investigating active galactic nuclei (AGN) jet physics. We study the spectral evolution between quiescent and flaring periods of 22 radio-loud AGN through multi-epoch, quasi-simultaneous broadband spectra. For many of these sources these are the first broadband studies. We use a Bayesian block analysis of \Fermi/LAT light curves in order to determine time ranges of constant flux for constructing quasi-simultaneous SEDs. The shapes of the resulting 81 SEDs are described by two logarithmic parabolas and a blackbody spectrum where needed. For low states the peak frequencies and luminosities agree well with the blazar sequence, higher luminosity implying lower peak frequencies. This is not true for sources in a high state. The $γ$-ray photon index in Fermi/LAT correlates with the synchrotron peak frequency in low and intermediate states. No correlation is present in high states. The black hole mass cannot be determined from the SEDs. Surprisingly, the thermal excess often found in FSRQs at optical/UV wavelengths can be described by blackbody emission and not an accretion disk spectrum. The "harder-when-brighter" trend, typically seen in X-ray spectra of flaring blazars, is visible in the blazar sequence. Our results for low and intermediate states, as well as the Compton dominance, are in agreement with previous results. Black hole mass estimates using the parameters from Bonchi (2013) are in agreement with some of the more direct measurements. For two sources, estimates disagree by more than four orders of magnitude, possibly due to boosting effects. The shapes of the thermal excess seen predominantly in flat spectrum radio quasars are inconsistent with a direct accretion disk origin.

astro-ph.HE↗

arXiv2014

New detections of HC$_{5}$N toward hot cores associated with 6.7 GHz methanol masers

We present new detections of cyanodiacetylene (HC$_{5}$N) toward hot molecular cores, observed with the Tidbinbilla 34 m radio telescope (DSS-34). In a sample of 79 hot molecular cores, HC$_{5}$N was detected towards 35. These results are counter to the expectation that long chain cyanopolyynes, such as HC$_{5}$N, are not typically found in hot molecular cores, unlike their shorter chain counterpart HC$_{3}$N. However it is consistent with recent models which suggest HC$_{5}$N may exist for a limited period during the evolution of hot molecular cores.

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arXiv2014

Molecular line mapping of the giant molecular cloud associated with RCW 106 - IV. Ammonia towards dust emission

Here we report observations of the two lowest inversion transitions of ammonia with the 70-m Tidbinbilla radio telescope. They were conducted to determine the kinetic temperatures in the dense clumps of the G333 giant molecular cloud associated with RCW 106 and to examine the effect that accurate temperatures have on the calculation of derived quantities such as mass. This project is part of a larger investigation to understand the timescales and evolutionary sequence associated with high-mass star formation, particularly its earliest stages. Assuming that the initial chemical composition of a giant molecular cloud is uniform, any abundance variations within will be due to evolutionary state. We have identified 63 clumps using SIMBA 1.2-mm dust continuum maps and have calculated gas temperatures for most (78 per cent) of these dense clumps. After using Spitzer GLIMPSE 8.0 $μ$m emission to separate the sample into IR-bright and IR-faint clumps, we use statistical tests to examine whether our classification shows different populations in terms of mass and temperature. We find that clump mass and column density show no significant population difference, and that kinetic temperature is the best parameter to distinguish between the gravitationally bound state of each clump. The kinetic temperature was the only parameter found to have a significantly low probability of being drawn from the same population. This suggests that clump radii does not have a large effect on the temperature of a clump, so clumps of similar radii may have different internal heating mechanisms. We also find that while the IR-bright clumps have a higher median virial mass, both samples have a similar range for both virial mass and FWHM. There are 87 per cent (40 of 46) of the clumps with masses larger than the virial mass, suggesting that they will form stars or are already undergoing star formation.

astro-ph.GA↗

arXiv2013

Submillimeter H2O Maser in Circinus Galaxy - A New Probe for the Circumnuclear Region of Active Galactic Nuclei

We present the first detection of extragalactic submillimeter H2O maser in the 321 GHz transition toward the center of Circinus galaxy, the nearby Type 2 Seyfert using the Atacama Large Millimeter/Submillimeter Array. We find that Doppler features of the detected 321 GHz H2O maser straddle the systemic velocity of the galaxy as seen in the spectrum of the known 22 GHz H2O maser in the galaxy. By comparing the velocities of the maser features in both transitions, it can be deduced that the 321 GHz maser occurs in a region similar to that of the 22 GHz maser, where the sub-parsec-scale distribution of the 22 GHz maser was revealed by earlier very long baseline interferometry observations. The detected maser features remain unresolved at the synthesized beam of ~0".66 (~15 pc) and coincide with the 321 GHz continuum peak within small uncertainties. We also present a tentative detection of the highest velocity feature (redshifts up to ~635 km/s) in the galaxy. If this high-velocity feature arises from a Keplerian rotating disk well established in this galaxy, it is located at a radius of ~0.018 pc (~1.2 x 10^5 Schwarzschild radii), which might probe molecular material closest to the central engine.

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arXiv2008

An Approach Detecting the Event Horizon of Sgr A*

Imaging the vicinity of a black hole is one of the ultimate goals of VLBI astronomy. The closest massive black hole, Sgr A*, located at the Galactic center, is the leading candidate for such observations. Combined with recent VLBI recording technique and submillimeter radio engineering, we now have sufficient sensitivity for the observations. Here we show performance simulations of submillimeter VLBI arrays for imaging Sgr A*. Good images are obtained from submillimeter VLBI arrays in the southern hemisphere composed of more than 10 stations. We also note that even with a small array, we can estimate the shadow size and then the mass of the black hole from visibility analysis. Now, all we need is to construct a submillimeter VLBI array in the southern hemisphere if we wish to unveil the black hole environment of Sgr A*. Key words: black hole, event horizon, Sgr A*, submillimeter

astro-ph↗

arXiv2005

Ten milliparsec-scale structure of the nucleus region in Centaurus A

We present the results of a VLBI Space Observatory Programme (VSOP) observation of the subparsec structure in Centaurus A at 4.9 GHz. Owing to its proximity, our Centaurus A space-VLBI image is one of the highest spatial resolution images of an AGN ever made -- 0.01 pc per beam. The elongated core region is resolved into several components over 10 milli-arcseconds long (0.2 pc) including a compact component of brightness temperature 2.2x10^10K. We analyze the jet geometry in terms of collimation. Assuming the strongest component to be the core, the jet opening angle at ~ 5,000 r_s (Schwarzchild radii) from the core is estimated to be ~ 12 degree, with collimation of the jet to ~ 3 degree continuing out to ~ 20,000 r_s. This result is consistent with previous studies of the jet in M87, which favor MHD disk outflow models. Future space VLBI observations at higher frequencies will probably be able to image the collimation region, within 1,000 r_s of the center of Centaurus A, together with the accretion disk itself.

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arXiv2003

Linear Polarization Observations of Water Masers in W3 IRS5

We present a magnetic field mapping of water maser clouds in the star-forming region W3 IRS5, which has been made on the basis of the linear polarization VLBI observation. Using the Very Long Baseline Array (VLBA) at 22.2 GHz, 16 of 61 detected water masers were found to be linearly polarized with polarization degrees up to 13%. Although 10 polarized features were widely distributed in the whole W3 IRS5 water maser region, they had similar position angles of the magnetic field vectors (~75 deg from the north). The magnetic field vectors are roughly perpendicular to the spatial alignments of the maser features. They are consistent with the hourglass model of the magnetic field, which was previously proposed to explain the magnetic field in the whole W3 Main region (r~0.1 pc). They are, on the other hand, not aligned to the directions of maser feature proper motions observed previously. This implies that the W3 IRS5 magnetic field was controlled by a collapse of the W3 Main molecular cloud rather than the outflow originated from W3 IRS5.

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