arXiv subjects

Kohei Ichikawa

Publications and source records attributed to Kohei Ichikawa.

At least 91 records · Page 5Linked to original sources

arXiv2019

NuSTAR Discovery of Dead Quasar Engine in Arp 187

Recent active galactic nucleus (AGN) and quasar surveys have revealed a population showing rapid AGN luminosity variability by a factor of $\sim10$. Here we present the most drastic AGN luminosity decline by a factor of $\gtrsim 10^{3}$ constrained by a NuSTAR X-ray observation of the nearby galaxy Arp 187, which is a promising "dead" quasar whose current activity seems quiet but whose past activity of $L_\mathrm{bol} \sim 10^{46}$ erg s$^{-1}$ is still observable at a large scale by its light echo. The obtained upper bound of the X-ray luminosity is $\log (L_{\rm 2-10 keV}/{\rm erg} {\rm s}^{-1}) < 41.2$, corresponding to $\log (L_\mathrm{bol}/{\rm erg} {\rm s}^{-1}) < 42.5$, indicating an inactive central engine. Even if a putative torus model with $N_\mathrm{H} \sim 1.5 \times 10^{24}$ cm$^{-2}$ is assumed, the strong upper-bound still holds with $\log (L_{\rm 2-10 keV}/{\rm erg} {\rm s}^{-1}) < 41.8$ or $\log (L_\mathrm{bol}/{\rm erg} {\rm s}^{-1}) < 43.1$. Given the expected size of the narrow line region, this luminosity decrease by a factor of $\gtrsim 10^3$ must have occurred within $\lesssim 10^4$ yr. This extremely rapid luminosity/accretion shutdown is puzzling and it requires one burst-like accretion mechanism producing a clear outer boundary for an accretion disk. We raise two possible scenarios realizing such an accretion mechanism: a mass accretion 1) by the tidal disruption of a molecular cloud and/or 2) by the gas depletion as a result of vigorous nuclear starformation after rapid mass inflow to the central engine.

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arXiv2019

BAT AGN Spectroscopic Survey -- XIII. The nature of the most luminous obscured AGN in the low-redshift universe

We present a multi wavelength analysis of 28 of the most luminous low-redshift narrow-line, ultra-hard X-ray selected active galactic nuclei (AGN) drawn from the 70 month Swift/BAT all-sky survey, with bolometric luminosities of log(L_bol/erg/s) > 45.25. The broad goal of our study is to determine whether these objects have any distinctive properties, potentially setting them aside from lower-luminosity obscured AGN in the local Universe. Our analysis relies on the first data release of the BAT AGN Spectroscopic Survey (BASS/DR1) and on dedicated observations with the VLT, Palomar, and Keck observatories. We find that the vast majority of our sources agree with commonly used AGN selection criteria which are based on emission line ratios and on mid-infrared colours. Our AGN are predominantly hosted in massive galaxies (9.8 < log(M_*/M_sun) < 11.7); based on visual inspection of archival optical images, they appear to be mostly ellipticals. Otherwise, they do not have distinctive properties. Their radio luminosities, determined from publicly available survey data, show a large spread of almost 4 orders of magnitude - much broader than what is found for lower X-ray luminosity obscured AGN in BASS. Moreover, our sample shows no preferred combination of black hole masses (M_BH) and/or Eddington ratio (lambda_Edd), covering 7.5 < log(M_BH/M_sun) < 10.3 and 0.01 < lambda_Edd < 1. Based on the distribution of our sources in the lambda_Edd-N_H plane, we conclude that our sample is consistent with a scenario where the amount of obscuring material along the line of sight is determined by radiation pressure exerted by the AGN on the dusty circumnuclear gas.

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arXiv2019

WISE Discovery of Mid-infrared Variability in Massive Young Stellar Objects

We systematically investigate the mid-infrared (MIR; $λ>3 ~μ$m) time variability of uniformly selected $\sim800$ massive young stellar objects (MYSOs) from the Red MSX Source (RMS) survey. Out of the 806 sources, we obtain reliable 9-year-long MIR magnitude variability data of 331 sources at the 3.4~$μ$m (W1) and 4.6~$μ$m (W2) bands by cross-matching the MYSO positions with ALLWISE and NEOWISE catalogs. After applying the variability selections using ALLWISE data, we identify 5 MIR-variable candidates. The light curves show various classes, with the periodic, plateau-like, and dipper features. Out of the obtained two color-magnitude diagram of W1 and W1$-$W2, one shows "bluer when brighter and redder when fainter" trends in variability, suggesting change in extinction or accretion rate. Finally, our results show that G335.9960$-$00.8532 (hereafter, G335) has a periodic light curve, with a $\approx 690$-day cycle. Spectral energy density model fitting results indicate that G335 is a relatively evolved MYSO; thus, we may be witnessing the very early stages of a hyper- or ultra-compact HII region, a key source for understanding MYSO evolution.

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arXiv2019

A NuSTAR and XMM-Newton Study of the Two Most Actively Star-forming Green Pea Galaxies (SDSS J0749+3337 and SDSS J0822+2241)

We explore X-ray evidence for the presence of active galactic nuclei (AGNs) in the two most actively star-forming Green Pea galaxies (GPs), SDSS J0749+3337 and SDSS J0822+2241, which have star-formation rates (SFRs) of $123~M_\odot$ yr$^{-1}$ and $78~M_\odot$ yr$^{-1}$, respectively. The GPs have red mid-infrared (MIR) spectral energy distributions and higher 22 $μ$m luminosities than expected from a proxy of the SFR (H$α$ luminosity), consistent with hosting AGNs with 2-10 keV luminosities of $\sim10^{44}$ erg s$^{-1}$. We thus obtain and analyze the first hard ($>$ 10 keV) X-ray data observed with NuSTAR and archival XMM-Newton data below 10 keV. From the NuSTAR $\approx$20 ksec data, however, we find no significant hard X-ray emission. By contrast, soft X-ray emission with 0.5--8 keV luminosities of $\approx10^{42}$ erg s$^{-1}$ is significantly detected in both targets, which can be explained only by star formation (SF). A possible reason for the lack of clear evidence is that a putative AGN torus absorbs most of the X-ray emission. Applying a smooth-density AGN torus model, we determine minimum hydrogen column densities along the equatorial plane ($N_{\rm H}^{\rm eq}$) consistent with the non-detection. The results indicate $N_{\rm H}^{\rm eq} \gtrsim 2\times10^{24}$ cm$^{-2}$ for SDSS J0749+3337 and $N_{\rm H}^{\rm eq} \gtrsim 5\times10^{24}$ cm$^{-2}$ for SDSS J0822+2241. Therefore, the GPs may host such heavily obscured AGNs. Otherwise, no AGN exists and the MIR emission is ascribed to SF. Active SF in low-mass galaxies is indeed suggested to reproduce red MIR colors. This would imply that diagnostics based on MIR photometry data alone may misidentify such galaxies as AGNs.

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arXiv2019

Transition of BH feeding from the quiescent regime into star-forming cold disk regime

We study the properties of rotating accretion flows onto supermassive black holes (SMBHs) using axisymmetric two-dimensional hydrodynamical simulations with radiative cooling and BH feedback. The simulations resolve the accretion dynamics of gas outside from the BH influence radius through an inner accretion disk. For lower Bondi accretion rates in units of the Eddington rate ($\dot{M}_{\rm B}\ll 10^{-3}~\dot{M}_{\rm Edd}$), the BH feeding is suppressed due to turbulent motion by several orders of magnitudes from the Bondi rate. Thus, the radiative luminosity results in as low as $\sim 10^{-10}-10^{-7}~L_{\rm Edd}$, where $L_{\rm Edd}$ is the Eddington luminosity. For higher rates of $\dot{M}_{\rm B}> 10^{-3}~\dot{M}_{\rm Edd}$, the optically-thin accreting gas cools via free-free emission and forms a geometrically-thin disk, which feeds the BH efficiently and increases the radiative luminosity to $> 10^{-3}~L_{\rm Edd}$. The transitional behavior of accreting BHs in galactic nuclei from radiatively inefficient phases to cold disk accretion naturally explains (1) the reason for the offset between the observed luminosities and theoretical predictions for nearby quiescent SMBHs, and (2) the conditions to fuel gas into the nuclear SMBH. In addition, the cold disk formed in galactic nuclei tends to be gravitationally unstable and leads to star formation when the Bondi rate is as high as $ \dot{M}_{\rm B} > 10^{-2}~M_\odot~{\rm yr}^{-1}$. This is a plausible explanation of the correlation observed between star formation rates and BH feeding rates in Seyfert galaxies.

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arXiv2019

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). II. Physical Properties derived from the SED Fitting with Optical, Infrared, and Radio Data

We present physical properties of radio galaxies (RGs) with $f_{\rm 1.4 GHz} >$ 1 mJy discovered by Subaru Hyper Supreme-Cam (HSC) and VLA Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. For 1056 FIRST RGs at $0 < z \leq 1.7$ with HSC counterparts in about 100 deg$^2$, we compiled multi-wavelength data of optical, near-infrared (IR), mid-IR, far-IR, and radio (150 MHz). We derived their color excess ($E (B-V)_{*}$), stellar mass, star formation rate (SFR), IR luminosity, the ratio of IR and radio luminosity ($q_{\rm IR}$), and radio spectral index ($α_{\rm radio}$) that are derived from the SED fitting with CIGALE. We also estimated Eddington ratio based on stellar mass and integration of the best-fit SEDs of AGN component. We found that $E (B-V)_{*}$, SFR, and IR luminosity clearly depend on redshift while stellar mass, $q_{\rm IR}$, and $α_{\rm radio}$ do not significantly depend on redshift. Since optically-faint ($i_{\rm AB} \geq 21.3$) RGs that are newly discovered by our RG survey tend to be high redshift, they tend to not only have a large dust extinction and low stellar mass but also have high SFR and AGN luminosity, high IR luminosity, and high Eddington ratio compared to optically-bright ones. The physical properties of a fraction of RGs in our sample seem to differ from a classical view of RGs with massive stellar mass, low SFR, and low Eddington ratio, demonstrating that our RG survey with HSC and FIRST provides us curious RGs among entire RG population.

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arXiv2019

Discovery of a strong ionized-gas outflow in an AKARI-selected Ultra-luminous Infrared Galaxy at z = 0.5

In order to construct a sample of ultra-luminous infrared galaxies (ULIRGs, with infrared luminosity, $L_{\rm IR} > 10^{12}$ L$_{\odot}$) at 0.5 < z < 1, we are conducting an optical follow-up program for bright 90-$μ$m FIR sources with a faint optical (i < 20) counterpart selected in the AKARI Far-Infrared Surveyor (FIS) Bright Source catalog (Ver.2). AKARI-FIS-V2 J0916248+073034, identified as a ULIRG at z = 0.49 in the spectroscopic follow-up observation, indicates signatures of an extremely strong outflow in its emission line profiles. Its [OIII] 5007Å emission line shows FWHM of 1830 km s$^{-1}$ and velocity shift of -770 km s$^{-1}$ in relative to the stellar absorption lines. Furthermore, low-ionization [OII] 3726Å 3729Å doublet also shows large FWHM of 910 km s$^{-1}$ and velocity shift of -380 km s$^{-1}$. After the removal of an unresolved nuclear component, the long-slit spectroscopy 2D image possibly shows that the outflow extends to radius of 4 kpc. The mass outflow and energy ejection rates are estimated to be 500 M$_{\odot}$ yr$^{-1}$ and $4\times10^{44}$ erg s$^{-1}$, respectively, which imply that the outflow is among the most powerful ones observed in ULIRGs and QSOs at 0.3 < z < 1.6. The co-existence of the strong outflow and intense star formation (star formation rate of 990 M$_{\odot}$ yr$^{-1}$) indicates that the feedback of the strong outflow has not severely affect the star-forming region of the galaxy.

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arXiv2018

SOFIA/FORCAST resolves 30 - 40 um extended dust emission in nearby active galactic nuclei

We present arcsecond-scale observations of the active galactic nuclei (AGNs) of seven nearby Seyfert galaxies observed from the Stratospheric Observatory For Infrared Astronomy (SOFIA) using the 31.5 and 37.1 um filters of the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST). We isolate unresolved emission from the torus and find extended diffuse emission in six 37.1 um residual images in our sample. Using Spitzer/IRS spectra, we determine the dominant mid-infrared (MIR) extended emission source and attribute it to dust in the narrow line region (NLR) or star formation. We compare the optical NLR and radio jet axes to the extended 37.1 um emission and find coincident axes for three sources. For those AGNs with extended emission coincident with the optical axis, we find that spatial scales of the residual images are consistent with 0.1 - 1 kpc scale distances to which dust can be heated by the AGN. Using previously published subarcsecond 1 - 20 um imaging and spectroscopic data along with our new observations, we construct broadband spectral energy distributions (SEDs) of the AGNs at wavelengths 1 - 40 um. We find that three AGNs in our sample tentatively show a turnover in the SED between 30 - 40 um. Using results from Clumpy torus models and the Bayesian inference tool BayesClumpy, we find that the posterior outputs for AGNs with MIR turnover revealed by SOFIA/FORCAST have smaller uncertainties than AGNs that do not show a turnover.

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arXiv2018

Discovery of Dying Active Galactic Nucleus in Arp 187: Experience of Drastic Luminosity Decline within $10^4$ years

Arp 187 is one of the fading active galactic nuclei (AGN), whose AGN activity is currently decreasing in luminosity. We investigate the observational signatures of AGN in Arp 187, which trace various physical scales from less than 0.1 pc to the nearly 10 kpc, to estimate the longterm luminosity change over $10^{4}$ years. The VLA 5 GHz, 8 GHz, and the ALMA 133 GHz images reveal bimodal jet lobes with $\sim$5 kpc size and the absence of the central radio-core. The 6dF optical spectrum shows that Arp 187 hosts narrow line region with the estimated size of $\sim$1 kpc, and the line strengths give the AGN luminosity of $L_{\rm bol}=1.5 \times 10^{46}$ erg s$^{-1}$. On the other hand, the current AGN activity estimated from the AGN torus emission gives the upper bound of $L_{\rm bol} < 2.2 \times 10^{43}$ erg s$^{-1}$. The absence of the radio-core gives the more strict upper bound of the current AGN luminosity of $L_{\rm bol} < 8.0 \times 10^{40}$ erg s$^{-1}$, suggesting that the central engine is already quenched. These multi-wavelength signatures indicate that Arp 187 hosts a "dying" AGN: the central engine is already dead, but the large scale AGN indicators are still observable as the remnant of the past AGN activity. The central engine has experienced the drastic luminosity decline by a factor of $\sim10^{3-5}$ fainter within $\sim10^{4}$ years, which is roughly consistent with the viscous timescale of the inner part of the accretion disk within $\sim$500 years.

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arXiv2018

BAT AGN Spectroscopic Survey - XI. The Covering Factor of Dust and Gas in Swift/BAT Active Galactic Nuclei

We quantify the luminosity contribution of active galactic nuclei (AGN) to the 12 $μ$m, mid-infrared (MIR; 5-38 $μ$m), and the total IR (5-1000 $μ$m) emission in the local AGN detected in the all-sky 70-month Swift/Burst Alert Telescope (BAT) ultra hard X-ray survey. We decompose the IR spectral energy distributions (SEDs) of 587 objects into AGN and starburst components using AGN torus and star-forming galaxy templates. This enables us to recover the AGN torus emission also for low-luminosity end, down to $\log (L_{14-150}/{\rm erg}~{\rm s}^{-1}) \simeq 41$, which typically have significant host galaxy contamination. We find that the luminosity contribution of the AGN to the 12 $μ$m, the MIR, and the total IR band is an increasing function of the 14-150 keV luminosity. We also find that for the most extreme cases, the IR pure-AGN emission from the torus can extend up to 90 $μ$m. The obtained total IR AGN luminosity through the IR SED decomposition enables us to estimate the fraction of the sky obscured by dust, i.e., the dust covering factor. We demonstrate that the median of the dust covering factor is always smaller than that of the X-ray obscuration fraction above the AGN bolometric luminosity of $\log (L_{\rm bol}/{\rm erg}~{\rm s}^{-1}) \simeq 42.5$. Considering that X-ray obscuration fraction is equivalent to the covering factor coming from both the dust and gas, it indicates that an additional neutral gas component, along with the dusty torus, is responsible for the absorption of X-ray emission.

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arXiv2018

Gravitational waves from supermassive black hole binaries in ultra-luminous infrared galaxies

Gravitational waves (GWs) in the nano-hertz band are great tools for understanding the cosmological evolution of supermassive black holes (SMBHs) in galactic nuclei. We consider SMBH binaries in high-$z$ ultra-luminous infrared galaxies (ULIRGs) as sources of a stochastic GW background (GWB). ULIRGs are likely associated with gas-rich galaxy mergers containing SMBHs that possibly occur at most once in the life of galaxies, unlike multiple dry mergers at low redshift. Adopting a well-established sample of ULIRGs, we study the properties of the GWB due to coalescing binary SMBHs in these galaxies. Since the ULIRG population peaks at $z>1.5$, the amplitude of the GWB is not affected even if BH mergers are delayed by as long as $\sim $ 10 Gyrs. Despite the rarity of the high-$z$ ULIRGs, we find a tension with the upper limits from Pulsar Timing Array (PTA) experiments. This result suggests that if a fraction $f_{\rm m,gal}$ of ULIRGs are associated with SMBH binaries, then no more than $20 f_{\rm m,gal}(λ_{\rm Edd}/0.3)^{5/3}(t_{\rm life}/30~{\rm Myr})~\%$ of the binary SMBHs in ULIRGs can merge within a Hubble time, for plausible values of the Eddington ratio of ULIRGs ($λ_{\rm Edd}$) and their lifetime ($t_{\rm life}$).

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arXiv2018

The emission and distribution of dust of the torus of NGC 1068

We present observations of NGC 1068 covering the $19.7-53.0$ $μ$m wavelength range using FORCAST and HAWC+ onboard SOFIA. Using these observations, high-angular resolution infrared (IR) and sub-mm observations, we find an observational turn-over of the torus emission in the $30-40$ $μ$m wavelength range with a characteristic temperature of $70-100$ K. This component is clearly different from the diffuse extended emission in the narrow line and star formation regions at 10-100 $μ$m within the central 700 pc. We compute $2.2-432$ $μ$m 2D images using the best inferred \textsc{clumpy} torus model based on several nuclear spectral energy distribution (SED) coverages. We find that when $1-20$ $μ$m SED is used, the inferred result gives a small torus size ($<4$ pc radius) and a steep radial dust distribution. The computed torus using the $1-432$ $μ$m SED provides comparable torus sizes, $5.1^{+0.4}_{-0.4}$ pc radius, and morphology to the recently resolved 432 $μ$m ALMA observations. This result indicates that the $1-20$ $μ$m wavelength range is not able to probe the full extent of the torus. The characterization of the turn-over emission of the torus using the $30-60$ $μ$m wavelength range is sensitive to the detection of cold dust in the torus. The morphology of the dust emission in our 2D image at 432 $μ$m is spatially coincident with the cloud distribution, while the morphology of the emission in the $1-20$ $μ$m wavelength range shows an elongated morphology perpendicular to the cloud distribution. We find that our 2D \textsc{clumpy} torus image at 12 $μ$m can produce comparable results to those observed using IR interferometry.

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arXiv2017

BAT AGN Spectroscopic Survey - V. X-ray properties of the Swift/BAT 70-month AGN catalog

Hard X-ray ($\geq 10$ keV) observations of Active Galactic Nuclei (AGN) can shed light on some of the most obscured episodes of accretion onto supermassive black holes. The 70-month Swift/BAT all-sky survey, which probes the 14-195 keV energy range, has currently detected 838 AGN. We report here on the broad-band X-ray (0.3-150 keV) characteristics of these AGN, obtained by combining XMM-Newton, Swift/XRT, ASCA, Chandra, and Suzaku observations in the soft X-ray band ($\leq 10$ keV) with 70-month averaged Swift/BAT data. The non-blazar AGN of our sample are almost equally divided into unobscured ($N_{\rm H}< 10^{22}\rm cm^{-2}$) and obscured ($N_{\rm H}\geq 10^{22}\rm cm^{-2}$) AGN, and their Swift/BAT continuum is systematically steeper than the 0.3-10 keV emission, which suggests that the presence of a high-energy cutoff is almost ubiquitous. We discuss the main X-ray spectral parameters obtained, such as the photon index, the reflection parameter, the energy of the cutoff, neutral and ionized absorbers, and the soft excess for both obscured and unobscured AGN.

astro-ph.HE↗

arXiv2017

Luminous Quasars Do Not Live in the Most Overdense Regions of Galaxies at z~4

We present the cross-correlation between 151 luminous quasars ($M_{ \mathrm{UV}} < -26$) and 179 protocluster candidates at $z \sim 3.8$, extracted from the Wide imaging survey ($ \sim 121~ $deg$^2$) performed with a part of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We find that only two out of 151 quasars reside in regions that are more overdense compared to the average field at $ > 4 σ$. The distributions of the distance between quasars and the nearest protoclusters and the significance of the overdensity at the position of quasars are statistically identical to those found for $g$-dropout galaxies, suggesting that quasars tend to reside in almost the same environment as star-forming galaxies at this redshift. Using stacking analysis, we find that the average density of $g$-dropout galaxies around quasars is slightly higher than that around $g$-dropout galaxies on $1.0 - 2.5$ pMpc scales, while at $ < 0.5$ pMpc that around quasars tends to be lower. We also find that quasars with higher UV-luminosity or with more massive black holes tend to avoid the most overdense regions, and that the quasar near zone sizes are anti-correlated with overdensity. These findings are consistent with a scenario in which the luminous quasar at $z \sim4 $ resides in structures that are less massive than those expected for the progenitors of today's rich clusters of galaxies, and possibly that luminous quasars may be suppressing star formation in their close vicinity.

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arXiv2017

NuSTAR hard X-ray data and Gemini 3D spectra reveal powerful AGN and outflow histories in two low-redshift Lyman-$α$ blobs

We have shown that Lyman-$α$ blobs (LABs) may still exist even at $z\sim0.3$, about 7 billion years later than most other LABs known (Schirmer et al. 2016). Their luminous Ly$α$ and [OIII] emitters at $z\sim0.3$ offer new insights into the ionization mechanism. This paper focuses on the two X-ray brightest LABs at $z\sim0.3$, SDSS J0113$+$0106 (J0113) and SDSS J1155$-$0147 (J1155), comparable in size and luminosity to `B1', one of the best-studied LABs at $z \gtrsim$ 2. Our NuSTAR hard X-ray (3--30 keV) observations reveal powerful active galactic nuclei (AGN) with $L_{2-10{\;\rm keV}}=(0.5$--$3)\times10^{44}$ erg cm$^{-2}$ s$^{-1}$. J0113 also faded by a factor of $\sim 5$ between 2014 and 2016, emphasizing that variable AGN may cause apparent ionization deficits in LABs. Joint spectral analyses including Chandra data constrain column densities of $N_{\rm H}=5.1^{+3.1}_{-3.3}\times10^{23}$ cm$^{-2}$ (J0113) and $N_{\rm H}=6.0^{+1.4}_{-1.1}\times10^{22}$ cm$^{-2}$ (J1155). J0113 is likely buried in a torus with a narrow ionization cone, but ionizing radiation is also leaking in other directions as revealed by our Gemini/GMOS 3D spectroscopy. The latter shows a bipolar outflow over $10$ kpc, with a peculiar velocity profile that is best explained by AGN flickering. X-ray analysis of J1155 reveals a weakly absorbed AGN that may ionize over a wide solid angle, consistent with our 3D spectra. Extinction corrected [OIII] log-luminosities are high, $\sim43.6$. The velocity dispersions are low, $\sim100$--$150$ km s$^{-1}$, even at the AGN positions. We argue that this is a combination of high extinction hiding the turbulent gas, and previous outflows that have cleared the escape paths for their successors.

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arXiv2017

BAT AGN Spectroscopic Survey I: Spectral Measurements, Derived Quantities, and AGN Demographics

We present the first catalog and data release of the Swift-BAT AGN Spectroscopic Survey (BASS). We analyze optical spectra of the majority of AGN (77%, 641/836) detected based on their 14-195 keV emission in the 70-month Swift BAT all-sky catalog. This includes redshift determination, absorption and emission line measurements, and black hole mass and accretion rate estimates for the majority of obscured and un-obscured AGN (74%, 473/641) with 340 measured for the first time. With ~90% of sources at z<0.2, the survey represents a significant census of hard-X-ray selected AGN in the local universe. In this first catalog paper, we describe the spectroscopic observations and datasets, and our initial spectral analysis. The FWHM of the emission lines show broad agreement with the X-ray obscuration (~94%), such that Sy 1-1.8 have NH<10^21.9 cm^-2, and Seyfert 2, have NH>10^21.9 cm^-2. Seyfert 1.9 show a range of column densities. Compared to narrow line AGN in the SDSS, the X-ray selected AGN have a larger fraction of dusty host galaxies suggesting these types of AGN are missed in optical surveys. Using the most sensitive [OIII]/Hbeta and [NII]/Halpha emission line diagnostic, about half of the sources are classified as Seyferts, ~15% reside in dusty galaxies that lack an Hbeta detection, but for which the line upper limits imply either a Seyfert or LINER, ~15% are in galaxies with weak or no emission lines despite high quality spectra, and a few percent each are LINERS, composite galaxies, HII regions, or in known beamed AGN.

astro-ph.HE↗

arXiv2017

Cooling timescale of dust tori in dying active galactic nuclei

We estimate the dust torus cooling timescale once the active galactic nucleus (AGN) is quenched. In a clumpy torus system, once the incoming photons are suppressed, the cooling timescale of one clump from $T_{\rm dust}=1000$ K to several $10$ K is less than $10$ years, indicating that the dust torus cooling time is mainly governed by the light crossing time of the torus from the central engine. After considering the light crossing time of the torus, the AGN torus emission at $12~μ$m becomes over two orders of magnitude fainter within $100$ years after the quenching. We also propose that those "dying" AGN could be found using the AGN indicators with different physical scale $R$ such as $12~μ$m band luminosity tracing AGN torus ($R \sim 10$ pc) and the optical [OIII]$\lambda5007$ emission line narrow line regions ($R=10^{2-4}$ pc).

astro-ph.GA↗

arXiv2017

Organized Autotelescopes for Serendipitous Event Survey (OASES): design and performance

Organized Autotelescopes for Serendipitous Event Survey (OASES) is an optical observation project that aims to detect and investigate stellar occultation events by kilometer-sized trans-Neptunian objects (TNOs). In this project, multiple low-cost observation systems for wide-field and high-speed photometry were developed in order to detect rare and short-timescale stellar occultation events. The observation system consists of commercial off-the-shelf $0.28 \ {\rm m}$ aperture $f/1.58$ optics providing a $2.3 \times 1.8$ square-degree field of view. A commercial CMOS camera is coupled to the optics to obtain full-frame imaging with a frame rate greater than $10 \ {\rm Hz}$. As of September 2016, this project exploits two observation systems, which are installed on Miyako Island, Okinawa, Japan. Recent improvements in CMOS technology in terms of high-speed imaging and low readout noise mean that the observation systems are capable of monitoring $\sim 2000$ stars in the Galactic plane simultaneously with magnitudes down to ${\rm V} \sim 13.0$, providing $\sim 20\%$ photometric precision in light curves with a sampling cadence of $15.4 \ {\rm Hz}$. This number of monitored stars is larger than for any other existing instruments for coordinated occultation surveys. In addition, a precise time synchronization method needed for simultaneous occultation detection is developed using faint meteors. The two OASES observation systems are executing coordinated monitoring observations of a dense stellar field in order to detect occultations by kilometer-sized TNOs for the first time.

astro-ph.IM↗