Crab-like Supernova Remnant Research Articles

A dust twin of Cas A: cool dust and 21 μm silicate dust feature in the supernova remnant G54.1+0.3

We present infrared (IR) and submillimeter observations of the Crab-like supernova remnant (SNR) G54.1+0.3 including 350 micron (SHARC-II), 870 micron (LABOCA), 70, 100, 160, 250, 350, 500 micron (Herschel) and 3-40 micron (Spitzer). We detect dust features at 9, 11 and 21 micron and a long wavelength continuum dust component. The 21 micron dust coincides with [Ar II] ejecta emission, and the feature is remarkably similar to that in Cas A. The IRAC 8 micron image including Ar ejecta is distributed in a shell-like morphology which is coincident with dust features, suggesting that dust has formed in the ejecta. We create a cold dust map that shows excess emission in the northwestern shell. We fit the spectral energy distribution of the SNR using the continuous distributions of ellipsoidal (CDE) grain model of pre-solar grain SiO2 that reproduces the 21 and 9 micron dust features and discuss grains of SiC and PAH that may be responsible for the 10-13 micron dust features. To reproduce the long-wavelength continuum, we explore models consisting of different grains including Mg2SiO4, MgSiO3, Al2O3, FeS, carbon, and Fe3O4. We tested a model with a temperature-dependent silicate absorption coefficient. We detect cold dust (27-44 K) in the remnant, making this the fourth such SNR with freshly-formed dust. The total dust mass in the SNR ranges from 0.08-0.9 Msun depending on the grain composition, which is comparable to predicted masses from theoretical models. Our estimated dust masses are consistent with the idea that SNe are a significant source of dust in the early Universe.

Optical identification of the 3C 58 pulsar wind nebula

Context. The Crab-like supernova remnant 3C 58 contains the young pulsar PSR J0295+6449, which powers a radio plerion and a compact torus-like pulsar wind nebula visible in X-rays. Aims. We have performed deep optical imaging of the 3C 58 field to detect the optical counterpart of the pulsar and its wind nebula. Methods. The imaging was carried out with the Nordic Optical Telescope. We also analyzed the archival images of the field obtained with the Chandra/ACIS-S and HRC-S in X-rays and with the Spitzer/IRAC in the mid-infrared. Results. We detect a faint extended elliptical optical object with B = 24. m 06 ± 0.08 and V = 23. 11 ± 0.04, whose center and peak brightness position are consistent at the sub-arcsecond level with the position of the pulsar. The morphology of the object and the orientation of its major axis are in excellent agreement with the torus region of the pulsar wind nebula seen almost edge on in the X-rays, although its extension is only about a half of what is in X-rays. This suggests that in the optical we see only the brightest central part of the torus nebula with the pulsar. The position and morphology of the object are also practically identical to the counterpart of the torus region recently detected in the mid-infrared bands. We do not resolve any point-like source within the nebula that could be identified with the pulsar and estimate that the contribution of the pulsar to the observed optical flux is <10%. Using the archival Chandra/ACIS-S data we analyzed the spectrum of the pulsar+nebula X-ray emission extracted from the spatial region constrained by the optical/infrared source position and extent and find that a single absorbed power law provides an acceptable spectral fit. Combining this fit with the optical and infrared fluxes of the detected candidate torus nebula counterpart, we compile a tentative multi-wavelength spectrum of the central part of the pulsar nebula. Within the uncertainties of the interstellar extinction towards 3C 58, it is reminiscent of either the Crab or PSR B0540-69 pulsar wind nebula spectra. Conclusions. The position, morphology, and spectral properties of the detected source strongly suggest that it is the optical/midinfrared counterpart of the 3C 58 pulsar + its wind nebula system. This makes 3C 58 the third member, together with the Crab and PSR B0540-69, of such a system as identified in the optical and mid-infrared.

Time variability of X-ray sources in the M 31 centre field

We present an extension to our XMM-Newton X-ray source catalogue of M 31, containing 39 newly found sources. In order to classify and identify more of the sources we search for X-ray time variability in XMM-Newton archival data of the M 31 centre field. As a source list we used our extended catalogue based on observations covering the time span from June 2000 to July 2004. We then determined the flux or at least an upper limit at the source positions for each observation. Deriving the flux ratios for the different observations and searching for the maximum flux difference we determined variability factors. We also calculated the significance of the flux ratios. Using hardness ratios, X-ray variability and cross correlations with catalogues in the X-ray, optical, infrared and radio regimes, we detected three super soft source candidates, one supernova remnant and six supernova remnant candidates, one globular cluster candidate, three X-ray binaries and four X-ray binary candidates. Additionally we identified one foreground star candidate and classified fifteen sources with hard spectra, which may either be X-ray binaries or Crab-like supernova remnants in M 31 or background active galactic nuclei. The remaining five sources stay unidentified or without classification. Based on the time variability results we suggest six sources, which were formerly classified as hard, to be X-ray binary candidates. The classification of one other source (XMMM31 J004236.7+411349) as a supernova remnant, has to be rejected due to the distinct time variability we found. We now classify this source as a foreground star.

A Massive-Star-forming Infrared Loop around the Crab-like Supernova Remnant G54.1+0.3: Post-Main-Sequence Triggered Star Formation?

We report the discovery of a star-forming loop around the young, Crab-like supernova remnant (SNR) G54.1+0.3 using the AKARI infrared satellite. The loop consists of at least 11 young stellar objects (YSOs) embedded in a ringlike diffuse emission of radius ~1'. The YSOs are bright in the mid-infrared and are also visible in the Spitzer Space Telescope Galactic plane survey images. Their Spitzer colors are similar to those of Class II YSOs in [3.6] – [5.8] but significantly redder in [8] – [24], i.e., 0 < [3.6] – [5.8] < 1.2 and 5 < [8] – [24] < 9. Most of them have near-infrared counterparts in the 2MASS JHKs images, and some of them have an optical counterpart too. Their JHKs colors and magnitudes indicate that the YSOs are massive (10 M☉) pre-main-sequence stars at the same distance to the SNR, i.e., 8 kpc, which supports the association of the star-forming loop with the SNR. The dereddened spectral energy distributions are similar to early Herbig Be stars, which are early B type pre-main-sequence stars with inner disks that have been destroyed. The confinement to a loop structure indicates that the YSOs are young, i.e., 2 Myr. We propose that their formation is triggered by the progenitor star of G54.1+0.3, which has a mass of 15 M☉. The triggering must have occurred near the end of the progenitor's life, possibly after it had evolved off the main sequence.

Hubble Space TelescopeObservations of Oxygen‐rich Supernova Remnants in the Magellanic Clouds. III. WFPC2 Imaging of the Young, Crab‐like Supernova Remnant SNR 0540−69.3

Hubble Space Telescope images with the Wide Field Planetary Camera 2 of the young, oxygen-rich, Crab-like supernova remnant SNR0540-69.3 in the Large Magellanic Cloud (LMC) reveal details of the emission distribution and the relationship between the expanding ejecta and synchrotron nebula. The emission distributions appear very similar to those seen in the Crab nebula, with the ejecta located in a thin envelope surrounding the synchrotron nebula. The [O III] emission is more extended than other tracers, forming a faint skin around the denser filaments and synchrotron nebula, as also observed in the Crab. The [O III] exhibits somewhat different kinematic structure in long-slit spectra, including a more extended high-velocity emission halo not seen in images. Yet even the fastest expansion speeds in SNR 0540 s halo are slow when compared to most other young supernova remnants, though the Crab nebula has similar slow expansion speeds. We show a striking correspondence between the morphology of the synchrotron nebula observed in an optical continuum filter with that recently resolved in Chandra X-ray images. We argue that the multi-component kinematics and filamentary morphology of the optical emission-line features likely result from magnetic Rayleigh-Taylor instabilities that form as the synchrotron nebula expands and sweeps up ejecta, as seen in the Crab nebula. Our images and spectra help to refine our understanding of SNR 0540 in several more detailed respects: they confirm the identification of H(alpha)+[N II] in the red spectrum, they show that the systemic velocity of SNR 0540 is not significantly different from that of the LMC, and they hint at a lower Ne abundance than the Crab (potentially indicating a more massive progenitor star).

The X-ray source populations in M31 and M33

First, the X-ray source populations of M31 and M33 as known from Einstein and ROSAT observations are presented. Then, chandra results on the galaxies are shortly summarized which not only spatially resolved the centre areas but also supernova remnants (SNRs) in both galaxies, and led to source catalogues of restricted areas with high astrometric accuracy. Also luminosity function studies and studies of individual sources based on chandra and XMM-Newton observations led to a better knowledge of the X-ray source populations. After that I will concentrate on XMM-Newton surveys, in which more than 850 and 400 X-ray sources were detected in M31 and M33, respectively, down to a 0.2–4.5 keV luminosity of less than 1035 erg s−1. EPIC hardness ratios as well as informations from earlier X-ray, optical, and radio catalogues were used to distinguish between different source classes (SNRs, supersoft sources (SSSs), X-ray binaries (XRBs), globular cluster sources within the galaxies, and foreground stars, and objects in the background). However, many sources could only be classified as hard.They may either be XRBs or Crab-like SNRs in the galaxies or background sources. Within M31, two globular cluster sources could be identified as low mass XRBs as they showed type I X-ray bursts. Many of the SSSs in both galaxies were identified as optical novae. Due to the high frequency of outbursts in the bulge area of M31 many novae can be monitored at the same time which makes the investigation of class properties much easier compared to novae in the Milky Way or the Magellanic Clouds.

An XMM-Newton survey of M 31

In an analysis of XMM-Newton archival observations of the bright Local Group spiral galaxy M 31 we study the population of X-ray sources (X-ray binaries, supernova remnants) down to a 0.2–4.5 keV luminosity of 4.4 . EPIC hardness ratios and optical and radio information are used to distinguish between different source classes. The survey detects 856 sources in an area of 1.24 square degrees. We correlate our sources with earlier M 31 X-ray catalogues and use information from optical, infra-red and radio wavelengths. As sources within M 31 we detect 21 supernova remnants (SNR) and 23 SNR candidates, 18 supersoft source (SSS) candidates, 7 X-ray binaries (XRBs) and 9 XRB candidates, as well as 27 globular cluster sources (GlC) and 10 GlC candidates, which most likely are low mass XRBs within the GlC. Comparison to earlier X-ray surveys reveal transients not detected with XMM-Newton, which add to the number of M 31 XRBs. There are 567 sources classified as hard, which may either be XRBs or Crab-like SNRs in M 31 or background AGN. The number of 44 SNRs and candidates more than doubles the X-ray-detected SNRs. 22 sources are new SNR candidates in M 31 based on X-ray selection criteria. Another SNR candidate may be the first plerion detected outside the Galaxy and the Magellanic Clouds. On the other hand, six sources are foreground stars and 90 foreground star candidates, one is a BL Lac-type active galactic nucleus (AGN) and 36 are AGN candidates, one source coincides with the Local Group galaxy M 32, one with a background galaxy cluster (GCl) and another is a GCl candidate, all sources not connected to M 31.

XMM-Newton survey of the Local Group galaxy M 33

In an XMM-Newton raster observation of the bright Local Group spiral galaxy M 33 we study the population of X-ray sources (X-ray binaries, supernova remnants) down to a 0.2-4.5 keV luminosity of 10 35 erg s −1 - more than a factor of 10 deeper than earlier ROSAT observations. EPIC hardness ratios and optical and radio information are used to distinguish between different source classes. The survey detects 408 sources in an area of 0.80 square degree. We correlate these newly detected sources with earlier M 33 X-ray catalogues and information from optical, infra-red and radio wavelengths. As M 33 sources we detect 21 supernova remnants (SNR) and 23 SNR candidates, 5 super-soft sources and 2 X-ray binaries (XRBs). There are 267 sources classified as hard, which may either be XRBs or Crab-like SNRs in M 33 or background AGN. The 44 confirmed and candidate SNRs more than double the number of X-ray detected SNRs in M 33. 16 of these are proposed as SNR candidates from the X-ray data for the first time. On the other hand, there are several sources not connected to M 33: five foreground stars, 30 foreground star candidates, 12 active galactic nucleus candidates, one background galaxy and one background galaxy candidate. Extrapolating from deep field observations we would expect 175 to 210 background sources in this field. This indicates that about half of the sources detected are within M 33.

Discovery of a 136 Millisecond Radio and X-Ray Pulsar in Supernova Remnant G54.1+0.3

We report the discovery of a pulsar with period P = 136 ms and dispersion measure 308 cm(-3) pc in a deep observation of the supernova remnant (SNR) G54.1+0.3 with the Arecibo radio telescope. Timing measurements of the new pulsar, J1930+1852, reveal a characteristic age of P/2 (P) over dot = 2900 yr and a spin-down luminosity of (E) over dot = 1.2 x 10(37) ergs s(-1). We have subsequently searched archival ASCA X-ray data of this SNR and detect pulsations with a consistent period. These findings ensure that PSR J1930+1852 is the pulsar that powers the Crab-like SNR G54.1+0.3. Together with existing Chandra observations of the SNR, we derive an X-ray pulsed fraction (2-10 keV) of approximate to27%. We also find that the cooling efficiency of the pulsar wind nebula (PWN) is intermediate between those of the Vela and Crab PWNe: L-x(2-10 keV) similar to0.002 (E) over dot. PSR J1930+1852 is a very weak radio source, with a period-averaged flux density at 1180 MHz of 60 muJy. For a distance of 5 kpc, its luminosity, similar to1 mJy kpc(2), is among the lowest for known young pulsars.

New Constraints on Neutron Star Cooling from [ITAL]Chandra[/ITAL] Observations of 3C 58

3C58 is a young Crab-like supernova remnant. Historical evidence strongly suggests an association of the remnant with supernova SN 1181, which would make 3C58 younger than the Crab Nebula. Recent Chandra observations have identified the young 65 ms pulsar J0205+6449 at its center, embedded in a compact nebula which, we show here, appears to be confined by the pulsar wind termination shock. We present new Chandra observations of this compact nebula and embedded pulsar which set strong upper limits on thermal emission originating from the neutron star surface. These limits fall far below predictions of standard neutron star cooling, requiring the presence of exotic cooling processes in the neutron star core.

X‐Ray Observations of the Supernova Remnant G21.5−0.9

We present the analysis of archival X-ray observations of the supernova remnant (SNR) G21.5-0.9. Based on its morphology and spectral properties, G21.5-0.9 has been classified as a Crab-like SNR. For that reason, it was chosen as a Chandra calibration target. In their early analysis of part of these calibration data, Slane and coworkers discovered a low surface brightness, extended emission. They interpreted this component as the blast wave formed in the supernova explosion. XMM-Newton observations by Warwick and coworkers revealed the nonthermal nature of this emission, suggesting that it is instead an extension of the synchrotron nebula. In this paper, we revisit the Chandra analysis using new calibration data, improving the statistics by a factor of 2. We also include ROSAT and ASCA observations. Our analysis confirms the nonthermal nature of the extended emission. Advanced CCD Imaging Spectrometer images indicate that this component is not limb-brightened and that it shows knotty structures and a bright filament 2' north of the center. We find no evidence of line emission from any part of the remnant. We can reject a collisional equilibrium ionization thermal model at solar abundances and nonequilibrium ionization (NEI) models (such as a plane-parallel shock model with different ionization ages and constant temperature or an NEI model with a single ionization age and a constant temperature). The entire remnant is best fitted with a power-law model with a photon index steepening away from the center. The total unabsorbed flux FX(0.5-10 keV) is 1.1 × 10-10 ergs cm-2 s-1 with an 85% contribution from the 40'' radius core. Timing analysis of the High-Resolution Camera data failed to detect any pulsations. We put a 16% upper limit on the pulsed fraction. We derive the physical parameters of the putative pulsar and compare them with those of other plerions (such as the Crab nebula and 3C 58).

The Crab-like Supernova Remnant G21.5−0.9 at Millimeter Wavelengths

We present a BIMA array image of the supernova remnant G21.5-0.9 at 94 GHz with angular resolution 86 × 45. On scales larger than our synthesized beam, our results do not indicate any radial or local variations in the acceleration or synchrotron loss processes for the relativistic particles emitting at radio wavelengths. However, the FWHM size of the radio remnant is significantly greater than that of the X-ray remnant. Either any low-frequency spectral break is distributed across the remnant or, more probably, the break frequency is in fact higher than ~100 GHz.

Observations of the supernova remnant G54.1+0.3: X-ray spectrum and evidence for an X-ray jet

We present in this paper analyses of the $ROSAT$ PSPC and $ASCA$ SIS and GIS observations of the Crab-like supernova remnant (SNR) G54.1+0.3. Its spectrum obtained by $ROSAT$ PSPC favors a power law model with a photon index of -0.8$^{+0.8}_{-2.0}$, absorbed energy flux in 0.1-2.4 keV of 1.0$\times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$, and absorption column density of 12.3$^{+8.0}_{-3.2}\times 10^{21}$ cm$^{-2}$. $ASCA$ SIS observation shows that its spectrum can also be best fitted with power law model. The fitted parameters are, photon index -1.9$^{+0.1}_{-0.2}$, absorbed energy flux in 0.7-2.1 keV 6.5$\times10^{-13}$ erg cm$^{-2}$ s$^{-1}$, and column density 17.9$^{+2.8}_{-2.5}\times 10^{21}$ cm$^{-2}$. The high absorption column density indicates a distance similar to the radius of the galaxy. The 0.1-2.4 keV X-ray luminosity of G54.1+0.3 is 3.2$\times10^{33}$$d_{10}^2$ erg s$^{-1}$, where $d_{10}$ is the distance in 10 kpc. With an image restoration method we have obtained high spatial resolution X-ray image of the remnant, which clearly shows an X-ray jet pointing to the northeast with a length about 40$\arcsec$ from the center of the nebula. Its X-ray luminosity in 0.1-2.4 keV is about 5.1$\times10^{32}$$d_{10}^2$ erg s$^{-1}$. The X-ray jet is consistent with the radio extension to the northeast in both direction and position. We propose that the X-ray jet is connected with the pulsar assumed to exist in the remnant.

Discovery of X‐Ray Emission from G328.4+0.2, a Crab‐like Supernova Remnant

G328.4+0.2 is a moderately small (5 arcmin x 5arcmin) Galactic radio supernova remnant (SNR) at a distance of at least 17 kpc that has been long suggested to be Crab-like. Here we report on the detection with ASCA of the X-ray emission from the SNR. The X-ray source is faint with an observed flux of (6.0 +/- 0.8)E-13 erg/s/cm^2 over the 2-10 keV band. The emission is heavily cut-off at low energies and no flux is detected below 2 keV. Spectral analysis confirms that the column density to the source is indeed large, N_H ~ 1E23 atoms/cm^2, and consistent with the total column density of hydrogen through the Galaxy at this position. Good fits to the spectrum can be obtained for either thermal plasma or nonthermal power-law models, although the lack of detected line emission as well as other evidence argues against the former interpretation. The power-law index we find, \alpha_P = 2.9 (+0.9,-0.8), is consistent with other Crab-like SNRs. In the radio band G328.4+0.2 is nearly as luminous as the Crab Nebula, yet in the X-ray band luminosity it is some 70 times fainter. Nevertheless its inferred soft X-ray band luminosity is greater than all but the brightest pulsar-powered synchrotron nebulae and implies that G328.4+0.2 contains a rapidly spinning, as yet undetected, pulsar that is losing energy at a rate of approximately 1E38 erg/s.

G328.4+0.2: A Large and Luminous Crab‐like Supernova Remnant

We report on radio continuum and H I observations of the radio source G328.4+0.2 using the Australia Telescope Compact Array. Our results confirm G328.4+0.2 to be a filled-center nebula with no surrounding shell, showing significant linear polarization and an almost flat spectral index. These results lead us to conclude that G328.4+0.2 is a Crab-like, or plerionic, supernova remnant (SNR), presumably powered by an unseen central pulsar. H I absorption toward G328.4+0.2 puts a lower limit on its distance of 17.4 ± 0.9 kpc, making it the largest (D = 25 pc) and most luminous (LR = 3 × 1035 ergs s-1) Crab-like SNR in the Galaxy. We infer G328.4+0.2 to be significantly older than the Crab Nebula, but powered by a pulsar which is fast spinning (P < 20 ms) and which has a comparatively low magnetic field (B < 1012 G). We propose G328.4+0.2, G74.9+1.2, and N157B as a distinct group of large-diameter, high-luminosity Crab-like SNRs, all powered by fast-spinning low-field pulsars.

ASCA Observations of the Crab-Like Supernova Remnant 3C 58

Abstract We present here X-ray observation of a Crab-like supernova remnant (SNR) 3C 58 with ASCA. We find that the integrated energy spectrum over the nebula is consistent with previous results, showing a power-law spectrum with the photon index of γ = 2.2–2.4, modified by interstellar absorption of about (3–4)' 1021 cm−2. The inclusion of a blackbody component, which is attributable to the central compact source, significantly improves the spectral fit. Stringent upper limits for any line emitting thin hot plasma are established. We find for the first time that the nebular spectrum is harder in the central part of the SNR, becoming softer toward the periphery, while the absorption column is uniform across the nebula. Correspondingly, the nebular size decreases with increasing photon energy, which is a steeper function of the radius than that of the Crab nebula. The results are compared with synchrotron energy-loss models, and the nature of the putative pulsar is discussed. A timing analysis was performed to search for pulsed X-ray emission from the central compact source. No significant pulsations were observed; and we present the upper limit for the pulsed fraction.

Chandra Observations of the Crab-like Supernova Remnant G21.5-0.9.

Chandra observations of the Crab-like supernova remnant G21.5-0.9 reveal a compact central core and spectral variations indicative of synchrotron burn-off of higher energy electrons in the inner nebula. The central core is slightly extended, perhaps indicating the presence of an inner wind-shock nebula surrounding the pulsar. No pulsations are observed from the central region, yielding an upper limit of approximately 40% for the pulsed fraction. A faint outer shell may be the first evidence of the expanding ejecta and blast wave formed in the initial explosion, indicating a composite nature for G21.5-0.9.

Mixed-Morphology Supernova Remnants

While shell-like, Crab-like, and composite supernova remnants (SNRs) represent well-defined classes, there is another group of SNRs with different X-ray and radio morphologies: shell-like in the radio and centrally filled in the X-ray (with little or no limb brightening). We present the results of a systematic X-ray study of 19 such remnants, using ROSAT and ASCA data. We show that they have physical and environmental properties distinct from the other categories, and we propose that they form a unique class that we term supernova remnants. We found two prominent distinctions of this new class: their dominant X-ray emission mechanism is thermal despite their X-ray morphological similarity to Crab-like SNRs, and their emission arises primarily from swept-up interstellar material, not ejecta. The temperature across each remnant is nearly uniform, and the density and pressure are constant or increase toward the remnant center, contrary to the standard Sedov model. Most of these remnants are interacting with molecular or H I clouds, as indicated in some cases by strong infrared line emission or OH masers. The population represents at least 8% of all Galactic SNRs and as much as 25% of all X-ray-detected Galactic SNRs. This suggests that supernovae often explode in an environment that is conducive to the formation of a mixed-morphology remnant. The two most likely mechanisms for forming these remnants—evaporation of many small interior clouds and interior fossil radiation evolved in a denser than normal medium—require a reservoir of material with density higher than the typical intercloud ISM.

Discovery of an Ultrafast X-Ray Pulsar in the Supernova Remnant N157B

We present the serendipitous discovery of 16 ms pulsed X-ray emission from the Crab-like supernova remnant N157B in the Large Magellanic Cloud. This is the fastest spinning pulsar associated with a supernova remnant (SNR). Observations with the Rossi X-Ray Timing Explorer (RXTE), centered on the field containing SN 1987A, reveal an X-ray pulsar with a narrow pulse profile. The archival Advanced Satellite for Cosmology and Astrophysics (ASCA) X-ray data confirm this detection and locate the pulsar within 1' of the supernova remnant N157B, 14' from SN 1987A. The pulsar shows evidence of glitch(es) between the 3.5 yr separating the RXTE and ASCA observations; the mean linear spin-down rate is =5.126×10−14 s s−1. The background-subtracted, pulsed emission is similar to other Crab-like pulsars with a power law of photon index ~1.6. The characteristic spin-down age (~5000 yr) is consistent with the previous age estimate of the SNR. The inferred B-field for a rotationally powered pulsar is ~1×1012 G. Our result confirms the Crab-like nature of N157B ; the pulsar is likely associated with a compact X-ray source revealed by ROSAT HRI observations.

X-Ray Study of Crab-Like and Composite SNRs

We have presented X-ray observations of Crab-like supernova remnants (SNRs) (plerions) and (non-thermal) composite SNRs. We have observed several objects including 3C58, CTA1, and G292.0+1.8. Since space is limited, here we summarize results on G292.0+1.8. For the other objects, please refer to each publication (Slane et al. 1997; Torii et al. 1997).