Masahiro Suganuma

Inner Size of a Dust Torus in the Seyfert 1 Galaxy NGC 4151

The most intense monitoring observations yet made were carried out on the Seyfert 1 galaxy NGC 4151 in the optical and near-infrared wave bands. A lag from the optical light curve to the near-infrared light curve was measured. The lag time between the V and K light curves at the flux minimum in 2001 was precisely 48 days, as determined by a cross-correlation analysis. The correlation between the optical luminosity of an active galactic nucleus (AGN) and the lag time between the UV/optical and the near-infrared light curves is presented for NGC 4151 in combination with previous lag-time measurements of NGC 4151 and other AGNs in the literature. This correlation is interpreted as thermal dust reverberation in an AGN, where the near-infrared emission from an AGN is expected to be the thermal reradiation from hot dust surrounding the central engine at a radius where the temperature equals that of the dust sublimation temperature. We find that the inner radius of the dust torus in NGC 4151 is ~0.04 pc corresponding to the measured lag time, well outside the broad-line region determined by other reverberation studies of the emission lines.

LONG-TERM OPTICAL CONTINUUM COLOR VARIABILITY OF NEARBY ACTIVE GALACTIC NUCLEI

We examine whether the spectral energy distribution of optical continuum emission of active galactic nuclei (AGNs) changes during flux variation, based on accurate and frequent monitoring observations of 11 nearby Seyfert galaxies and QSOs carried out in the B, V, and I bands for seven years by the MAGNUM telescope. The multi-epoch flux data in any two different bands obtained on the same night show a very tight linear flux-to-flux relationship for all target AGNs. The flux of the host galaxy within the photometric aperture is carefully estimated by surface brightness fitting to available high-resolution Hubble Space Telescope images and MAGNUM images. The flux of narrow emission lines in the photometric bands is also estimated from available spectroscopic data. We find that the non-variable component of the host galaxy plus narrow emission lines for all target AGNs is located on the fainter extension of the linear regression line of multi-epoch flux data in the flux-to-flux diagram. This result strongly indicates that the spectral shape of AGN continuum emission in the optical region (~4400-7900 A) does not systematically change during flux variation. The trend of spectral hardening that optical continuum emission becomes bluer as it becomes brighter, which has been reported by many studies, is therefore interpreted as the domination of the variable component of the nearly constant spectral shape of an AGN as it brightens over the non-variable component of the host galaxy plus narrow lines, which is usually redder than AGN continuum emission.

VARIATION OF INNER RADIUS OF DUST TORUS IN NGC4151

Long-term optical and near-infrared monitoring observations for a type 1 active galactic nucleus (AGN) NGC 4151 were carried out for six years from 2001 to 2006 by using the MAGNUM telescope, and delayed response of flux variations in the K(2.2 μm) band to those in the V(0.55 μm) band was clearly detected. Based on cross-correlation analysis, we precisely measured a lag time Δt for eight separate periods and we found that Δt is not constant, changing between 30 and 70 d during the monitoring period. Since Δt is the light travel time from the central energy source out to the surrounding dust torus, this is the first convincing evidence that the inner radius of the dust torus did change in an individual AGN. In order to relate such a change of Δt with a change of AGN luminosity L, we present a method of taking an average of the observed V-band fluxes that corresponds to the measured value of Δt, and we find that the time-changing track of NGC 4151 in the Δt versus L diagram during the monitoring period deviates from the relation Δt ∝ L 0.5 expected from dust reverberation. This result, combined with the elapsed time from period to period for which Δt was measured, indicates that the timescale of dust formation is about one year, which should be taken into account as a new constraint in future studies of dust evolution in AGNs.

Multicolor imaging photometer for the MAGNUM project

We present the optical, mechanical and electronic design of MAGNUM-MIP. The MAGNUM project plans to carry out multi color monitoring observations for hundreds of AGNs over several years under remote and automated operation. MAGNUM- MIP has two channels that offer optical and IR broad-band imaging observations at the same time. The IR channel has a SBRC InSb 256 by 256 array which covers a wavelength range from 1 to 4 microns, and the optical channel uses a 1024 by 1024 SITE CCD which covers 0.35 micron to 1 micron. The two channels use the same optics and a beam splitter. We adopted a reflecting optical system in order to get good imaging quality over the wide wavelength range. Because the monitoring is expected to be carried out remotely for several years with minimum manual support and maintenance, the camera is designed to work with only semi-annual maintenance. It has a mechanical cooler, a low outgas design, and an automated vacuum system.

Multiple Regression Analysis of the Variable Component in the Near-Infrared Region for Type 1 AGN MCG +08-11-011

We propose a new method of analyzing a variable component for type 1 active galactic nuclei (AGNs) in the near-infrared wavelength region. This analysis uses a multiple regression technique and divides the variable component into two components originating in the accretion disk at the center of an AGN and from the dust torus that far surrounds the disk. Applying this analysis to the long-term VHK monitoring data of MCG +08-11-011 that were obtained by the MAGNUM project, we found that the (H - K) color temperature of the dust component is T = 1635 ± 20 K, which agrees with the sublimation temperature of dust grains, and that the time delay of K to H variations is Δt ≈ 6 days, which indicates the existence of a radial temperature gradient in the dust torus. As for the disk component, we found that the power-law spectrum of fν ∝ να in the V to near-infrared HK bands varies with a fixed index of α ≈ -0.1 to +0.4, which is broadly consistent with the irradiated standard disk model. The outer part of the disk therefore extends out to a radial distance where the temperature decreases to radiate the light in the near-infrared.

First Detection of Near-Infrared Intraday Variations in the Seyfert 1 Nucleus NGC 4395

We carried out a one-night optical V and near-infrared JHK monitoring observation of the least luminous Seyfert 1 galaxy, NGC 4395, on 2004 May 1, and detected for the first time the intraday flux variations in the J and H bands, while such variation was not clearly seen for the K band. The detected J and H variations are synchronized with the flux variation in the V band, which indicates that the intraday-variable component of near-infrared continuum emission of the NGC 4395 nucleus is an extension of power-law continuum emission to the near-infrared and originates in an outer region of the central accretion disk. On the other hand, from our regular program of long-term optical BVI and near-infrared JHK monitoring observation of NGC 4395 from 2004 February 12 until 2005 January 22, we found large flux variations in all the bands on timescales of days to months. The optical BVI variations are almost synchronized with each other but not completely with the near-infrared JHK variations. The color temperature of the near-infrared variable component is estimated to be T = 1320-1710 K, in agreement with thermal emission from hot dust tori in active galactic nuclei (AGNs). We therefore conclude that the near-infrared variation consists of two components having different timescales, so that a small K-flux variation on a timescale of a few hours would possibly be veiled by large variation of thermal dust emission on a timescale of days.

The Reverberation Radius of the Central Dust Hole in NGC 5548

The most intensive multicolor monitoring observations in the optical and near-infrared wave bands were carried out for the Seyfert 1 galaxy NGC 5548. During a monitoring period from 2001 March to 2003 July, the V(0.55 μm) and K(2.2 μm) fluxes separately reached a minimum state twice. A delayed response of light variations in the K band to those in the V band was detected, and the lag time Δt was measured by a cross-correlation analysis based on a new technique of simulating the light curves to fill in the sampling gaps. The measured lag time is Δt = 48 days in the first minimum state and Δt = 47 days in the second minimum state. The lag time is interpreted as the light-travel time from the central energy source to the surrounding dust torus, because the K light is emitted from the hot dust heated by absorption of ultraviolet and optical light from the central energy source. Compared with lag measurements of the broad emission lines in the literature, the lag time for such dust reverberation is found to be longer, indicating that the inner radius of the dust torus corresponds to an outer edge of the broad emission line region in NGC 5548. Furthermore, the V light curve exhibits short-timescale variations superposed on its global variation. These variations on timescales of 10 days or less are found to correlate with X-ray variations and delay behind them by 1 or 2 days, indicating the optical reprocessing of X-ray emission in the central accretion flow in NGC 5548.

Development of the Compact InfraRed Camera (CIRC) for wildfire detection

The Compact InfraRed Camera (CIRC) is a technology-demonstration payload to be carried on the Small Demonstration Satellite type-2 (SDS-2). The SDS program is a JAXA activity to demonstrate a variety of new technologies and new missions. The CIRC is an infrared camera equipped with an uncooled infrared array detector (microbolometer). The mission of the SDS-2/CIRC project is to demonstrate the potential of the microbolometer, especially for wildfire detection but also for other applications. This paper introduces the detailed design and concept of CIRC. We also discuss preliminary results of the feasibility study on wildfire detection using thermal infrared images.

The Infrared Cloud Monitor for the MAGNUM Robotic Telescope at Haleakala

We present the most successful infrared cloud monitor for a robotic telescope. The system was originally developed for the MAGNUM 2 m telescope, which has been achieving unmanned, automated monitoring observations of active galactic nuclei at Haleakala on the Hawaiian island of Maui since 2001. Using a thermal imager and two aspherical mirrors, it sees at once almost the whole sky at a wavelength of mm. Its outdoor l ∼ 10 component is weatherproof and is totally maintenance-free. The images obtained every 1 or 2 minutes are categorized immediately into several ranks of weather conditions, from which our automated observing system not only decides whether to open or close the dome, but also selects what types of observations should be made. The whole-sky data accumulated over 4 years show that 50%-60% of all nights are photometric, and about 75% are observable with respect to cloud conditions at Haleakala. Many copies of this system are now used all over the world, such as those in Mauna Kea in Hawaii, Atacama in Chile, and Okayama and Kiso in Japan. Online material: color figure

Optical testing activities for the SPICA telescope

SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is a Japan-led infrared astronomical satellite project with a 3-m-class telescope in collaboration with Europe. The telescope is cooled down to temperature below 6 K in space by a combination of mechanical coolers with radiative cooling in space. The telescope has requirements for its total weight to be lighter than 700 kg and for the imaging performance to be diffraction-limited at 5 μm at 6 K. The mirrors will be made of silicon carbide (SiC) or its related material, which has large heritages of the AKARI and Herschel telescopes. The design of the telescope system has been studied by the Europe-Japan telescope working group led by ESA with European industries to meet the requirements. As for optical testing, responsibilities will be split between Europe and Japan so that final optical verification at temperatures below 10 K will be executed in Japan. We present our recent optical testing activities in Japan for the SPICA telescope, which include the numerical and experimental studies of stitching interferometry as well as modifications of the 6-m-diameter radiometer space chamber facility at Tsukuba Space Center in JAXA. We also show results of cryogenic optical testing of the 160-mm and 800-mm lightweight mirrors made of a C/SiC material called HBCesic, which is a candidate mirror material for the SPICA telescope.