田代 信

The Hard X-ray Detector (HXD-II) is one of the three instruments onboard the Astro-E2 satellite scheduled for launch in 2005. The HXD-II consists of 16 main counters (Well units), surrounded by 20 active shield counters (Anti units). The Anti units have a large geometrical area of similar to 800 cm(2) with an uncollimated field of view covering similar to 2 pi steradian. Utilizing 2.6 cm thick BGO crystals: they realize a large effective area of 400 cm(2) for 1 MeV photons. In the energy range of 300-5000 keV, the expected effective area is significantly larger than those of other gamma-ray burst instruments, such as CGRO/BATSE, HETE-2/FREGATE, and GLAST/GBM. Therefore. the Anti units act as a Wideband All-sky, Monitor (WAM) for gamma-ray bursts in the energy range of 50-5000 keV.

WIDGET is a robotic telescope for rnonitoring the HETE-2 field-of-view to detect Gamma-Ray Burst optical flashes or possible optical precursors. The system has 62 degrees x 62 degrees wide field-of-view which covers about 80% of HETE-2 one with a, 2kx2k Apogee U10 CCD camera and a Canon EF 24 mm f/1.4 wide-angle lens without a bandpass filter. WIDGET has been in operation since June 2004 at Akeno observing site where is about 200 kin apart from Tokyo. Typical limiting magnitude with S/N=3 at the site is V = 10(mag) for 5 seconds exposure and V = 11(mag) for 30 seconds exposure. We had already six coincident observations with HETE-2 position alerts. It was, however, cloudy for all cases due to rainy season in Japan. The expected number of coincident observations under clear sky is about 5 events per year. We will extend the system in early 2005 for Swift era to monitor optical transients in wider field-of-view, multi-color or polarization modes.

The Hard X-ray Detector (HXD-II) is one of the three instruments onboard the Astro-E2 satellite scheduled for launch in 2005. The HXD-II consists of 16 main counters (Well units), surrounded by 20 active shield counters (Anti units). The Anti units have a large geometrical area of similar to 800 cm(2) with an uncollimated field of view covering similar to 2 pi steradian. Utilizing 2.6 cm thick BGO crystals: they realize a large effective area of 400 cm(2) for 1 MeV photons. In the energy range of 300-5000 keV, the expected effective area is significantly larger than those of other gamma-ray burst instruments, such as CGRO/BATSE, HETE-2/FREGATE, and GLAST/GBM. Therefore. the Anti units act as a Wideband All-sky, Monitor (WAM) for gamma-ray bursts in the energy range of 50-5000 keV.

The properties of 32K CdZnTe (4 x 4 mm(2) large, 2 rum thick) detectors have been studied in the pre-flight calibration of the Burst Alert Telescope (BAT) on-board the Swift Gamma-ray Burst Explorer (scheduled for launch in November 2004). In order to understand the energy response of the BAT CdZnTe array, we first quantify the mobility-lifetime (mu tau) products of carriers in individual CdZnTe detectors, which produce a position dependency in the charge induction efficiency and results in a low-energy tail in the energy spectrum. Based on a new method utilizing Co-57 spectra obtained at different bias voltages, the mu tau for electrons ranges from 5.0 x 10(-4) to 1.0 x 10(-2) cm(2) V-1 while the mu tau for holes ranges from 1.3 x 10(-5) to 1.8 x 10(-4) cm(2) V-1. We find that this wide distribution of mu tau products explains the large diversity in spectral shapes between CdZnTe detectors well. We also find that the variation of mu tau products can be attributed to the difference of crystal ingots or manufacturing harness. We utilize the 32K sets of extracted mu tau products to develop a spectral model of the detector. In combination with Monte Carlo simulations, we can construct a spectral model for any photon energy or any incident angle. (c) 2005 Elsevier B.V. All rights reserved.

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and gamma-ray detection. The high-atomic number of the materials (Z(Cd) = 48, Z(Te) = 52) gives a high quantum efficiency in comparison with Si. The large band-gap energy (E-g = 1.5 eV) allows to operate the detector at room temperature. Based on recent achievements in high-resolution CdTe detectors, in the technology of ASICs and in bump-bonding, we have proposed the novel hard X-ray and gamma-ray detectors for the NeXT mission in Japan. The high-energy response of the super mirror onboard NeXT will enable us to perform the first sensitive imaging observations up to 80keV. The focal plane detector, which combines a fully depleted X-ray CCD and a pixellated CdTe detector, will provide spectra and images in the wide energy range from 0.5 to 80keV. In the soft gamma-ray band up to similar to 1 MeV, a narrow field-of-view Compton gamma-ray telescope utilizing several tens of layers of thin Si or CdTe detector will provide precise spectra with much higher sensitivity than present instruments. The continuum sensitivity will reach several x 10(-8) photons(-1) keV(-1) cm(-1) in the hard X-ray region and a few X 10(-7) photons(-1) keV(-1) cm(-2) in the soft gamma-ray region. (c) 2005 Elsevier B.V. All rights reserved.

The properties of 32K CdZnTe (4 x 4 mm(2) large, 2 rum thick) detectors have been studied in the pre-flight calibration of the Burst Alert Telescope (BAT) on-board the Swift Gamma-ray Burst Explorer (scheduled for launch in November 2004). In order to understand the energy response of the BAT CdZnTe array, we first quantify the mobility-lifetime (mu tau) products of carriers in individual CdZnTe detectors, which produce a position dependency in the charge induction efficiency and results in a low-energy tail in the energy spectrum. Based on a new method utilizing Co-57 spectra obtained at different bias voltages, the mu tau for electrons ranges from 5.0 x 10(-4) to 1.0 x 10(-2) cm(2) V-1 while the mu tau for holes ranges from 1.3 x 10(-5) to 1.8 x 10(-4) cm(2) V-1. We find that this wide distribution of mu tau products explains the large diversity in spectral shapes between CdZnTe detectors well. We also find that the variation of mu tau products can be attributed to the difference of crystal ingots or manufacturing harness. We utilize the 32K sets of extracted mu tau products to develop a spectral model of the detector. In combination with Monte Carlo simulations, we can construct a spectral model for any photon energy or any incident angle. (c) 2005 Elsevier B.V. All rights reserved.

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and gamma-ray detection. The high-atomic number of the materials (Z(Cd) = 48, Z(Te) = 52) gives a high quantum efficiency in comparison with Si. The large band-gap energy (E-g = 1.5 eV) allows to operate the detector at room temperature. Based on recent achievements in high-resolution CdTe detectors, in the technology of ASICs and in bump-bonding, we have proposed the novel hard X-ray and gamma-ray detectors for the NeXT mission in Japan. The high-energy response of the super mirror onboard NeXT will enable us to perform the first sensitive imaging observations up to 80keV. The focal plane detector, which combines a fully depleted X-ray CCD and a pixellated CdTe detector, will provide spectra and images in the wide energy range from 0.5 to 80keV. In the soft gamma-ray band up to similar to 1 MeV, a narrow field-of-view Compton gamma-ray telescope utilizing several tens of layers of thin Si or CdTe detector will provide precise spectra with much higher sensitivity than present instruments. The continuum sensitivity will reach several x 10(-8) photons(-1) keV(-1) cm(-1) in the hard X-ray region and a few X 10(-7) photons(-1) keV(-1) cm(-2) in the soft gamma-ray region. (c) 2005 Elsevier B.V. All rights reserved.

The burst alert telescope (BAT) is one of three instruments on the Swift MIDEX spacecraft to study gamma-ray bursts (GRBs). The BAT first detects the GRB and localizes the burst direction to an accuracy of 1-4 arcmin within 20s after the start of the event. The GRB trigger initiates an autonomous spacecraft slew to point the two narrow field-of-view (FOV) instruments at the burst location within 20-70 s so to make follow-up X-ray and optical observations. The BAT is a wide-FOV, coded-aperture instrument with a CdZnTe detector plane. The detector plane is composed of 32,768 pieces of CdZnTe (4x4x2 mm), and the coded-aperture mask is composed of similar to 52,000 pieces of lead (5x5x1 mm) with a 1-m separation between mask and detector plane. The BAT operates over the 15-150 keV energy range with similar to 7 keV resolution, a sensitivity of similar to 10(-8) ergg s(-1) cm(-2), and a 1.4 sr (half-coded) FOV. We expect to detect >100 GRBs/year for a 2-year mission. The BAT also performs an all-sky hard X-ray survey with a sensitivity of similar to 2 m Crab (systematic limit) and it serves as a hard X-ray transient monitor.

The burst alert telescope (BAT) is one of three instruments on the Swift MIDEX spacecraft to study gamma-ray bursts (GRBs). The BAT first detects the GRB and localizes the burst direction to an accuracy of 1-4 arcmin within 20s after the start of the event. The GRB trigger initiates an autonomous spacecraft slew to point the two narrow field-of-view (FOV) instruments at the burst location within 20-70 s so to make follow-up X-ray and optical observations. The BAT is a wide-FOV, coded-aperture instrument with a CdZnTe detector plane. The detector plane is composed of 32,768 pieces of CdZnTe (4x4x2 mm), and the coded-aperture mask is composed of similar to 52,000 pieces of lead (5x5x1 mm) with a 1-m separation between mask and detector plane. The BAT operates over the 15-150 keV energy range with similar to 7 keV resolution, a sensitivity of similar to 10(-8) ergg s(-1) cm(-2), and a 1.4 sr (half-coded) FOV. We expect to detect >100 GRBs/year for a 2-year mission. The BAT also performs an all-sky hard X-ray survey with a sensitivity of similar to 2 m Crab (systematic limit) and it serves as a hard X-ray transient monitor.

The Hard X-ray Detector (HXD-II), one of instruments onboard the Astro-E2 satellite to be launched in February 2005, is in the final stage of its development. The HXD-II probes the universe in the energy range of 10-600 keV with a sensitivity by an order of magnitude better than those of previous missions. The assembly of the HXD-II completed in January 2004, followed by a series of pre-launch qualification tests. As a result, the design goals of the HXD-II have been met. These include; a background level of 5 × 10 -6 counts/s/keV/cm 2 at 200 keV for GSO and 1 × 10 -5 counts/s/keV/cm 2 at 30 keV for PIN; energy resolutions of 2.9 keV (PIN diode, at 59.5 keV) and 10 % (GSO scintillator, at 662 keV); and low energy thresholds of 10 keV for PIN diodes and 30 keV for GSO scintillators. The measured background predicts a continuum sensitivity of a few ×10 -6 photons/s/keV/cm 2 . Anti-Counter units surrounding the HXD-II provide 50 keV-5 MeV information on gamma-ray bursts and bright X-ray transients.

The Soft Gamma-ray Detector (SGD) on board NeXT (Japanese future high energy astrophysics mission) is a Compton telescope with narrow field of view (FOV), which utilizes Compton kinematics to enhance its background rejection capabilities. It is realized as a hybrid semiconductor gamma-ray detector which consists of silicon and CdTe (Cadmium Telluride) detectors. It can detect photons in a wide energy band (0.05-1 MeV) at a background level of 5 × 10-7counts/s/cm2/keV; the silicon layers are required to improve the performance at a lower energy band (<0.3 MeV). Excellent energy resolution is the key feature of the SGD, allowing to achieve both high angular resolution and good background rejection capability. An additional capability of the SGD, its ability to measure gamma-ray polarization opens up a new window to study properties of astronomical objects. We will present the development of key technologies to realize the SGD; high quality CdTe, low noise front-end ASIC and bump bonding tecnology. Energy resolutions of 1.7 keV (FWHM) for CdTe pixel detectors and 1.1 keV for Si strip detectors have been measured. We also present the validation of MC simulation used to evaluate the performance of the SGD. © 2004 IEEE.

The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr(-1) and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z>10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a new-generation wide-field gamma-ray (15-150 keV) detector that will detect bursts, calculate 1'-4' positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5" positions and perform spectroscopy in the 0.2-10 keV band; and a narrow-field UV/optical telescope that will operate in the 170-600 nm band and provide 0".3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of similar to1 mcrab (similar to2x10(-11) ergs cm(-2) s(-1) in the 15-150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. Swift transient data will be rapidly distributed to the astronomical community, and all interested observers are encouraged to participate in follow-up measurements. A Guest Investigator program for the mission will provide funding for community involvement. Innovations from the Swift program applicable to the future include (1) a large-area gamma-ray detector using the new CdZnTe detectors, (2) an autonomous rapid-slewing spacecraft, (3) a multiwavelength payload combining optical, X-ray, and gamma-ray instruments, (4) an observing program coordinated with other ground-based and space-based observatories, and (5) immediate multiwavelength data flow to the community. The mission is currently funded for 2 yr of operations, and the spacecraft will have a lifetime to orbital decay of similar to8 yr.

The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr(-1) and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z>10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a new-generation wide-field gamma-ray (15-150 keV) detector that will detect bursts, calculate 1'-4' positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5" positions and perform spectroscopy in the 0.2-10 keV band; and a narrow-field UV/optical telescope that will operate in the 170-600 nm band and provide 0".3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of similar to1 mcrab (similar to2x10(-11) ergs cm(-2) s(-1) in the 15-150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. Swift transient data will be rapidly distributed to the astronomical community, and all interested observers are encouraged to participate in follow-up measurements. A Guest Investigator program for the mission will provide funding for community involvement. Innovations from the Swift program applicable to the future include (1) a large-area gamma-ray detector using the new CdZnTe detectors, (2) an autonomous rapid-slewing spacecraft, (3) a multiwavelength payload combining optical, X-ray, and gamma-ray instruments, (4) an observing program coordinated with other ground-based and space-based observatories, and (5) immediate multiwavelength data flow to the community. The mission is currently funded for 2 yr of operations, and the spacecraft will have a lifetime to orbital decay of similar to8 yr.

When compared with X-ray astronomy, the gamma-ray astronomy, especially in the energy band from 10 keV to several MeV, is still immature and significant improvements should be done to obtain sensitivity comparable to that achieved in the energy band below 10 keV. In order to fill this "sensitivity gap", the NeXT (New X-ray Telescope) mission has been proposed as a successor of the Astro-E2 mission. The high-energy response of the super mirror will enable us to perform first sensitive imaging observation up to 80 keV. One idea for the focal plane detector is to combine a fully depleted X-ray imaging device (soft X-ray detector) and a pixelated CdTe (cadmium telluride) detector. In the soft gamma-ray band upto similar to1 MeV, a narrow field-of-view Compton gamma-ray telescope utilizing several tens of layers of thin Si or CdTe detector has been proposed to obtain much higher sensitivity than present instruments. (C) 2003 Elsevier B.V. All rights reserved.

When compared with X-ray astronomy, the gamma-ray astronomy, especially in the energy band from 10 keV to several MeV, is still immature and significant improvements should be done to obtain sensitivity comparable to that achieved in the energy band below 10 keV. In order to fill this "sensitivity gap", the NeXT (New X-ray Telescope) mission has been proposed as a successor of the Astro-E2 mission. The high-energy response of the super mirror will enable us to perform first sensitive imaging observation up to 80 keV. One idea for the focal plane detector is to combine a fully depleted X-ray imaging device (soft X-ray detector) and a pixelated CdTe (cadmium telluride) detector. In the soft gamma-ray band upto similar to1 MeV, a narrow field-of-view Compton gamma-ray telescope utilizing several tens of layers of thin Si or CdTe detector has been proposed to obtain much higher sensitivity than present instruments. (C) 2003 Elsevier B.V. All rights reserved.

Cadmium Telluride (CdTe), with its high photon absorption efficiency, has been regarded as a promising semiconductor material for the next generation X/gamma-ray detectors. In order to apply this device to astrophysics, it is essential to investigate the radiation hardness and background properties induced by cosmic-ray protons in orbit. We irradiated Schottky CdTe diodes and a CdTe block with a beam of mono-energetic (150 MeV) protons. The induced activation in CdTe was measured externally with a germanium detector, and internally with the irradiated CdTe diode itself. We successfully identified most of radioactive isotopes induced mainly via (p, xn) reactions, and confirmed that the activation background level of CdTe diode is sufficiently low in orbit. We compared energy resolution and leakage current before and after the irradiation and also monitored the signals from a calibration source during the irradiation. There have been no significant degradation. CdTe diodes are tolerant enough to radioactivity in low earth orbit.

Cadmium Telluride (CdTe), with its high photon absorption efficiency, has been regarded as a promising semiconductor material for the next generation X/gamma-ray detectors. In order to apply this device to astrophysics, it is essential to investigate the radiation hardness and background properties induced by cosmic-ray protons in orbit. We irradiated Schottky CdTe diodes and a CdTe block with a beam of mono-energetic (150 MeV) protons. The induced activation in CdTe was measured externally with a germanium detector, and internally with the irradiated CdTe diode itself. We successfully identified most of radioactive isotopes induced mainly via (p, xn) reactions, and confirmed that the activation background level of CdTe diode is sufficiently low in orbit. We compared energy resolution and leakage current before and after the irradiation and also monitored the signals from a calibration source during the irradiation. There have been no significant degradation. CdTe diodes are tolerant enough to radioactivity in low earth orbit.

With its high stopping power, Cadmium Telluride (CdTe) has been regarded as a promising semiconductor material for the next generation X/γ-ray detectors, and unproved significantly during this decade. In order to apply this device to astrophysics, it is essential to investigate the radiation hardness and background properties induced by cosmic-ray protons. We irradiated Scbottky CdTe diodes and a CdTe block with a beam of mono-energetic (150 MeV) protons. The induced radio-activation in CdTe was measured externally with a germanium detector, and internally with the irradiated CdTe diode itself. We successfully identified most of radioactive isotopes induced mainly via (p, xn) reactions, and confirmed that activation background level of the CdTe diode is sufficiently low in orbit. We compared energy resolution and leakage current before and after the irradiation, and also monitored the signals from a calibration source during the irradiation. There have been no significant degradation. CdTe diodes are enough tolerant to radioactivity in orbit.

An 80 ks Chandra ACIS observation of the radio galaxy 3C 452 is reported. A diffuse X-ray emission associated with the lobes has been detected with high statistical significance, together with the X-ray nucleus of the host galaxy. The 0.5-5 keV ACIS spectrum of the diffuse emission is described by a two-component model, consisting of a soft thermal plasma emission from the host galaxy halo and a hard nonthermal power-law component. The hard component is ascribed to the inverse Comptonization of cosmic microwave background photons by the synchrotron-emitting electrons in the lobes, because its spectral energy index, 0.68 +/- 0.28, is consistent with the radio synchrotron index, 0.78. These results reveal a significant electron dominance in the lobes. The electrons are inferred to have a relatively uniform distribution, while the magnetic field is compressed toward the lobe periphery.

An 80 ks Chandra ACIS observation of the radio galaxy 3C 452 is reported. A diffuse X-ray emission associated with the lobes has been detected with high statistical significance, together with the X-ray nucleus of the host galaxy. The 0.5-5 keV ACIS spectrum of the diffuse emission is described by a two-component model, consisting of a soft thermal plasma emission from the host galaxy halo and a hard nonthermal power-law component. The hard component is ascribed to the inverse Comptonization of cosmic microwave background photons by the synchrotron-emitting electrons in the lobes, because its spectral energy index, 0.68 +/- 0.28, is consistent with the radio synchrotron index, 0.78. These results reveal a significant electron dominance in the lobes. The electrons are inferred to have a relatively uniform distribution, while the magnetic field is compressed toward the lobe periphery.

PKS 1413+135 (z=0.24671) is one of very few radio-loud active galactic nuclei (AGNs) with an apparent spiral host galaxy. Previous authors have attributed its nearly exponential infrared cutoff to heavy absorption but have been unable to place tight limits on the absorber or its location in the optical galaxy. In addition, doubts remain about the relationship of the AGN to the optical galaxy given the observed lack of reemitted radiation. We present new Hubble Space Telescope (HST), ASCA, and Very Long Baseline Array observations, which throw significant new light on these issues. The HST observations reveal that the active nucleus of PKS 1413+135 has an extremely red color, (V-H)=6.9 mag, requiring both a spectral turnover at a few microns because of synchrotron aging and an absorbing region the size of a giant molecular cloud. Combining constraints from the HST and ASCA data, we derive an intrinsic column NH=4.6(-1.6)(+2.1) x 10(22) cm(-2) and covering fraction f = 0.12(-0.05)(+0.07). The spin temperature of the molecular absorption lines found by previous authors suggests that the cloud is located in the disk of the optical galaxy, making our sight line rather unlikely (Psimilar to2x10(-4)). The properties of this region appear typical of large giant molecular clouds in our own Galaxy. The H I absorber appears centered 25 mas away from the nucleus, while the X-ray and nearly all of the molecular absorbers must cover the nucleus, implying a rather complicated geometry and cloud structure, in particular requiring a molecular core along our line of sight to the nucleus. Interestingly, the HST/NICMOS data require the AGN to be decentered relative to the optical galaxy by 13+/-4 mas. This could be interpreted as suggestive of an AGN location far in the background compared with the optical galaxy, but it can also be explained by obscuration and/or nuclear structure, which is more consistent with the observed lack of multiple images.

PKS 1413+135 (z=0.24671) is one of very few radio-loud active galactic nuclei (AGNs) with an apparent spiral host galaxy. Previous authors have attributed its nearly exponential infrared cutoff to heavy absorption but have been unable to place tight limits on the absorber or its location in the optical galaxy. In addition, doubts remain about the relationship of the AGN to the optical galaxy given the observed lack of reemitted radiation. We present new Hubble Space Telescope (HST), ASCA, and Very Long Baseline Array observations, which throw significant new light on these issues. The HST observations reveal that the active nucleus of PKS 1413+135 has an extremely red color, (V-H)=6.9 mag, requiring both a spectral turnover at a few microns because of synchrotron aging and an absorbing region the size of a giant molecular cloud. Combining constraints from the HST and ASCA data, we derive an intrinsic column NH=4.6(-1.6)(+2.1) x 10(22) cm(-2) and covering fraction f = 0.12(-0.05)(+0.07). The spin temperature of the molecular absorption lines found by previous authors suggests that the cloud is located in the disk of the optical galaxy, making our sight line rather unlikely (Psimilar to2x10(-4)). The properties of this region appear typical of large giant molecular clouds in our own Galaxy. The H I absorber appears centered 25 mas away from the nucleus, while the X-ray and nearly all of the molecular absorbers must cover the nucleus, implying a rather complicated geometry and cloud structure, in particular requiring a molecular core along our line of sight to the nucleus. Interestingly, the HST/NICMOS data require the AGN to be decentered relative to the optical galaxy by 13+/-4 mas. This could be interpreted as suggestive of an AGN location far in the background compared with the optical galaxy, but it can also be explained by obscuration and/or nuclear structure, which is more consistent with the observed lack of multiple images.