arXiv subjects

Hirofumi Noda

Publications and source records attributed to Hirofumi Noda.

At least 73 records · Page 4Linked to original sources

arXiv2021

Suzaku Detection of Solar Wind Charge Exchange Emission from a Variety of Highly-ionized Ions in an Interplanetary Coronal Mass Ejection

X-ray emission generated through solar-wind charge exchange (SWCX) is known to contaminate X-ray observation data, the amount of which is often significant or even dominant, particularly in the soft X-ray band, when the main target is comparatively weak diffuse sources, depending on the space weather during the observation. In particular, SWCX events caused by interplanetary coronal mass ejections (ICMEs) tend to be spectrally rich and to provide critical information about the metal abundance in the ICME plasma. We analyzed the SN1006 background data observed with Suzaku on 2005 September 11 shortly after an X6-class solar flare, signatures of which were separately detected together with an associated ICME. We found that the data include emission lines from a variety of highly ionized ions generated through SWCX. The relative abundances of the detected ions were found to be consistent with those in past ICME-driven SWCX events. Thus, we conclude that this event was ICME-driven. In addition, we detected a sulfur XVI line for the first time as one from the SWCX emission, the fact of which suggests that it is the most spectrally-rich SWCX event ever observed. We suggest that observations of ICME-driven SWCX events can provide a unique probe to study the population of highly-ionized ions in the plasma, which is difficult to measure in currently-available in-situ observations.

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arXiv2020

Origin of the in-orbit instrumental background of the Hard X-ray Imager onboard Hitomi

Understanding and reducing the in-orbit instrumental backgrounds are essential to achieving high sensitivity in hard X-ray astronomical observations. The observational data of the Hard X-ray Imager (HXI) on board the Hitomi satellite provides useful information on the background components, owing to its multi-layer configuration with different atomic numbers: the HXI consists of a stack of four layers of Si (Z = 14) detectors and one layer of CdTe (Z = 48, 52) detector surrounded by well-type BGO (Bi4Ge3O12) active shields. Based on the observational data, the backgrounds of top Si layer, the three underlying Si layers, and the CdTe layer are inferred to be dominated by different components, namely, low-energy electrons, albedo neutrons, and proton-induced radioactivation, respectively. Monte Carlo simulations of the in-orbit background of the HXI reproduce the observed background spectrum of each layer well, thereby verifying the above hypothesis quantitatively. In addition, we suggest the inclusion of an electron shield to reduce the background.

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arXiv2020

NuSTAR Non-detection of a Faint Active Galactic Nucleus in an Ultraluminous IR Galaxy with Kpc-scale Fast Wind

Large-scale outflows are generally considered as a possible evidence that active galactic nuclei (AGNs) can severely affect their host galaxies. Recently an ultraluminous IR galaxy (ULIRG) at $z=0.49$, AKARI J0916248+073034, was found to have a galaxy-scale [OIII] $λ$5007 outflow with one of the highest energy-ejection rates at $z<1.6$. However, the central AGN activity estimated from its torus mid-IR (MIR) radiation is weak relative to the luminous [OIII] emission. In this work we report the first NuSTAR hard X-ray follow-up of this ULIRG to constrain its current AGN luminosity. The intrinsic 2-10 keV luminosity shows a 90% upper-limit of $3.0\times10^{43}$ erg s$^{-1}$ assuming Compton-thick obscuration ($N_{\rm H}=1.5\times10^{24}$ cm$^{-2}$), which is only 3.6% of the luminosity expected from the extinction corrected [OIII] luminosity. With the NuSTAR observation, we succeed to identify that this ULIRG has a most extreme case of X-ray deficit among local ULIRGs. A possible scenario to explain the drastic declining in both of the corona (X-ray) and torus (MIR) is that the primary radiation from the AGN accretion disk is currently in a fading status, as a consequence of a powerful nuclear wind suggested by its powerful ionized outflow in the galaxy scale.

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arXiv2020

Experimental studies on the charge transfer inefficiency of CCD developed for the soft X-ray imaging telescope Xtend aboard the XRISM satellite

We present experimental studies on the charge transfer inefficiency (CTI) of charge-coupled device (CCD) developed for the soft X-ray imaging telescope, Xtend, aboard the XRISM satellite. The CCD is equipped with a charge injection (CI) capability, in which sacrificial charge is periodically injected to fill the charge traps. By evaluating the re-emission of the trapped charge observed behind the CI rows, we find that there are at least three trap populations with different time constants. The traps with the shortest time constant, which is equivalent to a transfer time of approximately one pixel, are mainly responsible for the trailing charge of an X-ray event seen in the following pixel. A comparison of the trailing charge in two clocking modes reveals that the CTI depends not only on the transfer time but also on the area, namely the imaging or storage area. We construct a new CTI model with taking into account with both transfer-time and area dependence. This model reproduces the data obtained in both clocking modes consistently. We also examine apparent flux dependence of the CTI observed without the CI technique. The higher incident X-ray flux is, the lower the CTI value becomes. It is due to a sacrificial charge effect by another X-ray photon. This effect is found to be negligible when the CI technique is used.

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arXiv2020

Optical Blocking Performance of CCDs Developed for the X-ray Astronomy Satellite XRISM

We have been developing P-channel Charge-Coupled Devices (CCDs) for the upcoming X-ray Astronomy Satellite XRISM, planned to be launched in 2021. While the basic design of the CCD camera (Soft X-ray Imager: SXI) is almost the same as that of the lost Hitomi (ASTRO-H) observatory, we are planning to reduce the "light leakages" that is one of the largest problems recognized in Hitomi data. We adopted a double-layer optical blocking layer on the XRISM CCDs and also added an extra aluminum layer on the backside of them. We develop a newly designed test sample CCD and irradiate it with optical light to evaluate the optical blocking performance. As a result, light leakages are effectively reduced compared with that of the Hitomi CCDs. We thus conclude that the issue is solved by the new design and that the XRISM CCDs satisfy the mission requirement for the SXI.

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arXiv2020

Tracing the co-evolution path of super massive black holes and spheroids with AKARI-selected ultra-luminous IR galaxies at intermediate redshifts

We present the stellar population and ionized-gas outflow properties of ultra-luminous IR galaxies (ULIRGs) at $z=$ 0.1-1.0, which are selected from AKARI FIR all-sky survey. We construct a catalog of 1077 ULIRGs to examine feedback effect after major mergers. 202 out of the 1077 ULIRGs are spectroscopically identified by SDSS and Subaru/FOCAS observations. Thanks to deeper depth and higher resolution of AKARI compared to the previous IRAS survey, and reliable identification from WISE MIR pointing, the sample is unique in identifying optically-faint (i$\sim$20) IR-bright galaxies, which could be missed in previous surveys. A self-consistent spectrum-SED decomposition method, which constrains stellar population properties in SED modeling based on spectral fitting results, has been employed for 149 ULIRGs whose optical continua are dominated by host galaxies. They are massive galaxies ($M_{\rm star}\sim10^{11}$-$10^{12}$ M$_{\odot}$), associated with intense star formation activities (SFR $\sim$ 200-2000 M$_{\odot}$ yr$^{-1}$). The sample covers a range of AGN bolometric luminosity of $10^{10}$-$10^{13}$ L$_{\odot}$, and the outflow velocity measured from [OIII] 5007A line shows a correlation with AGN luminosity. Eight galaxies show extremely fast outflows with velocity up to 1500-2000 km s$^{-1}$. However, the co-existence of vigorous starbursts and strong outflows suggests the star formation has not been quenched during the ULIRG phase. By deriving stellar mass and mass fraction of young stellar population, we find no significant discrepancies between stellar properties of ULIRGs with weak and powerful AGNs. The results are not consistent with the merger-induced evolutionary scenario, which predicts that SF-dominated ULIRGs show smaller stellar mass and younger stellar populations compared to AGN-dominated ULIRGs.

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arXiv2020

Discovery of a transient X-ray source Suzaku J1305-4930 in NGC 4945

We report the serendipitous discovery of a transient X-ray source, Suzaku J1305$-$4930, $\sim$3 kpc southwest of the nucleus of the Seyfert 2 galaxy NGC 4945. Among the seven Suzaku observations of NGC 4945 from 2005 to 2011, Suzaku J1305$-$4930 was detected four times in July and August in 2010. The X-ray spectra are better approximated with a multi-color disk model than a power-law model. At the first detection on 2010 July 4--5, its X-ray luminosity was $(8.9^{+0.2}_{-0.4}) \times 10^{38}$ erg s$^{-1}$ and the temperature at the inner-disk radius ($kT_{\rm in}$) was $1.12\pm0.04$ keV. At the last detection with Suzaku on 2010 August 4--5, the luminosity decreased to $(2.2^{+0.3}_{-0.8}) \times10^{38}$ erg s$^{-1}$ and $kT_{\rm in}$ was $0.62\pm0.07$ keV. The source was not detected on 2011 January 29, about six months after the first detection, with a luminosity upper limit of $2.4\times10^{38}$ erg s$^{-1}$. We also find an absorption feature which is similar to that reported in Cyg X-1. Assuming the standard disk, we suggest that Suzaku J1305$-$4930 consists of a black hole with a mass of $\sim$10 $M_{\odot}$. The relation between the disk luminosity and $kT_{\rm in}$ is not reproduced with the standard model of a constant inner radius but is better approximated with a slim-disk model.

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arXiv2020

Dust Reverberation Mapping of Type 2 AGN NGC 2110 Realized with X-ray and 3-5 $μ$m IR monitoring

The dust reverberation mapping is one of powerful methods to investigate the structure of the dusty tori in AGNs, and it has been performed on more than a hundred type 1 AGNs. However, no clear results have been reported on type 2 AGNs because their strong optical-UV extinction completely hides their accretion disc emission. Here we focus on an X-ray-bright type 2 AGN, NGC 2110, and utilize 2-20 keV X-ray variation monitored by MAXI to trace disc emission, instead of optical-UV variation. Comparing it with light curves in the WISE infrared (IR) W1 band ($λ= 3.4$ $μ$m) and W2 band ($λ= 4.6$ $μ$m) with cross-correlation analyses, we found candidates of the dust reverberation time lag at $\sim60$ days, $\sim130$ days, and $\sim1250$ days between the X-ray flux variation and those of the IR bands. By examining the best-fitting X-ray and IR light curves with the derived time lags, we found that the time lag of $\sim130$ days is most favoured. With this time lag, the relation between the time lag and luminosity of NGC 2110 is consistent with those in type 1 AGNs, suggesting that the dust reverberation in NGC 2110 mainly originates in hot dust in the torus innermost region, the same as in type 1 AGNs. As demonstrated by the present study, X-ray and IR simultaneous monitoring can be a promising tool to perform the dust reverberation mapping on type 2 AGNs.

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arXiv2020

Inverse FIP Effects in Giant Solar Flares Found from Earth X-Ray Albedo with Suzaku/XIS

We report X-ray spectroscopic results for four giant solar flares occurred on 2005 September 7 (X17.0), 2005 September 8 (X5.4), 2005 September 9 (X6.2), and 2006 December 5 (X9.0), obtained from Earth albedo data with the X-ray imaging spectrometer (XIS) onboard Suzaku. The good energy resolution of the XIS (FWHM$\sim$100 eV) enables us to separate a number of line-like features and detect the underlying continuum emission. These features include Si He$α$, Si Ly$α$, S He$α$, S Ly$α$, Ar He$α$, and Ca He$α$ originating from solar flares as well as fluorescent Ar K$α$ and Ar K$β$ from the Earth atmosphere. Absolute elemental abundances (X/H) averaged over the four flares are obtained to be $\sim$2.0 (Ca), $\sim$0.7 (Si), $\sim$0.3 (S), and $\sim$0.9 (Ar) at around flare peaks. This abundance pattern is similar to those of active stars' coronae showing inverse first ionization potential (i-FIP) effects, i.e., elemental abundances decrease with decreasing FIP with a turnover at the low end of FIP. The abundances are almost constant during the flares, with an exception of Si which increases by a factor of $\sim$2 in the decay phase. The evolution of the Si abundance is consistent with a picture that the i-FIP plasma originates from the chromosphere evaporation and then mixes with the surrounding low-FIP biased materials. Flare-to-flare abundance varied by a factor of 2, agreeing with past observations of solar flares. Finally, we emphasize that Earth albedo data acquired by X-ray astronomy satellites like Suzaku and XRISM can significantly contribute to studies of solar physics.

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arXiv2019

X-ray imaging polarimetry with a 2.5-$\mathrmμ$m pixel CMOS sensor for visible light at room temperature

X-ray polarimetry in astronomy has not been exploited well, despite its importance. The recent innovation of instruments is changing this situation. We focus on a complementary MOS (CMOS) pixel detector with small pixel size and employ it as an x-ray photoelectron tracking polarimeter. The CMOS detector we employ is developed by GPixel Inc., and has a pixel size of 2.5 $\mathrmμ$m $\times$ 2.5 $\mathrmμ$m. Although it is designed for visible light, we succeed in detecting x-ray photons with an energy resolution of 176 eV (FWHM) at 5.9 keV at room temperature and the atmospheric condition. We measure the x-ray detection efficiency and polarimetry sensitivity by irradiating polarized monochromatic x-rays at BL20B2 in SPring-8, the synchrotron radiation facility in Japan. We obtain modulation factors of 7.63% $\pm$ 0.07% and 15.5% $\pm$ 0.4% at 12.4 keV and 24.8 keV, respectively. It demonstrates that this sensor can be used as an x-ray imaging spectrometer and polarimeter with the highest spatial resolution ever tested.

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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

Explaining changing-look AGN with state transition triggered by rapid mass accretion rate drop

We model the broadband (optical/UV and X-ray) continuum spectrum of the "changing-look" active galactic nucleus (AGN) Mrk 1018 as it fades from Seyfert 1 to 1.9 in $\sim 8$ years. The brightest spectrum, with Eddington ratio $L/L_{\rm Edd}\sim 0.08$, has a typical type 1 AGN continuum, with a strong "soft X-ray excess" spanning between the UV and soft X-rays. The dimmest spectrum, at $L/L_{\rm Edd} \sim 0.006$, is very different in shape as well as luminosity, with the soft excess dropping by much more than the hard X-rays. The soft X-ray excess produces most of the ionizing photons, so its dramatic drop leads to the disappearance of the broad line region, driving the "changing-look" phenomena. This spectral hardening appears similar to the soft-to-hard state transition in black hole binaries at $L/L_{\rm Edd} \sim 0.02$, where the inner disc evaporates into an advection dominated accretion flow, while the overall drop in luminosity appears consistent with the Hydrogen ionization disc instability. Nonetheless, both processes happen much faster in Mrk 1018 than predicted by disc theory. We critically examine scaling from galactic binary systems, and show that a major difference is that radiation pressure should be much more important in AGNs, so that the sound speed is much faster than expected from the gas temperature. Including magnetic pressure to stabilize the disc shortens the timescales even further. We suggest that all changing-look AGNs are similarly associated with the state transition at $L/L_{\rm Edd} \sim$ a few percent.

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arXiv2018

In-orbit performance and calibration of the Hard X-ray Imager onboard Hitomi (ASTRO-H)

The Hard X-ray Imager (HXI) onboard Hitomi (ASTRO-H) is an imaging spectrometer covering hard X-ray energies of 5-80 keV. Combined with the hard X-ray telescope, it enables imaging spectroscopy with an angular resolution of $1^\prime.7$ half-power diameter, in a field of view of $9^\prime\times9^\prime$. The main imager is composed of 4 layers of Si detectors and 1 layer of CdTe detector, stacked to cover wide energy band up to 80 keV, surrounded by an active shield made of BGO scintillator to reduce the background. The HXI started observations 12 days before the Hitomi loss, and successfully obtained data from G21.5$-$0.9, Crab and blank sky. Utilizing these data, we calibrate the detector response and study properties of in-orbit background. The observed Crab spectra agree well with a powerlaw model convolved with the detector response, within 5% accuracy. We find that albedo electrons in specified orbit strongly affect the background of Si top layer, and establish a screening method to reduce it. The background level over the full field of view after all the processing and screening is as low as the pre-flight requirement of $1$-$3\times10^{-4}$ counts s$^{-1}$ cm$^{-2}$ keV$^{-1}$.

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arXiv2018

Modeling of proton-induced radioactivation background in hard X-ray telescopes: Geant4-based simulation and its demonstration by Hitomi's measurement in a low Earth orbit

Hard X-ray astronomical observatories in orbit suffer from a significant amount of background due to radioactivation induced by cosmic-ray protons and/or geomagnetically trapped protons. Within the framework of a full Monte Carlo simulation, we present modeling of in-orbit instrumental background dominated by radioactivation. To reduce the computation time required by straightforward simulations of delayed emissions from activated isotopes, we insert a semi-analytical calculation that converts production probabilities of radioactive isotopes by interaction of the primary protons into decay rates at measurement time of all secondary isotopes. Therefore, our simulation method is separated into three steps: (1) simulation of isotope production, (2) semi-analytical conversion to decay rates, and (3) simulation of decays of the isotopes at measurement time. This method is verified by a simple setup that has a CdTe semiconductor detector, and shows a 100-fold improvement in efficiency over the straightforward simulation. The simulation framework was tested against data measured with a CdTe sensor in the Hard X-ray Imager onboard the Hitomi X-ray Astronomy Satellite, which was put into a low Earth orbit with an altitude of 570 km and an inclination of 31 degrees, and thus experienced a large amount of irradiation from geomagnetically trapped protons during its passages through the South Atlantic Anomaly. The simulation is able to treat full histories of the proton irradiation and multiple measurement windows. The simulation results agree very well with the measured data, showing that the measured background is well described by the combination of proton-induced radioactivation of the CdTe detector itself and thick Bi4Ge3O12 scintillator shields, leakage of cosmic X-ray background and albedo gamma-ray radiation, and emissions from naturally contaminated isotopes in the detector system.

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arXiv2017

X-Ray bright optically faint active galactic nuclei in the Subaru Hyper Suprime-Cam wide survey

We construct a sample of X-ray bright optically faint active galactic nuclei by combining Subaru Hyper Suprime-Cam, XMM-Newton, and infrared source catalogs. 53 X-ray sources satisfying i band magnitude fainter than 23.5 mag and X-ray counts with EPIC-PN detector larger than 70 are selected from 9.1 deg^2, and their spectral energy distributions (SEDs) and X-ray spectra are analyzed. 44 objects with an X-ray to i-band flux ratio F_X/F_i>10 are classified as extreme X-ray-to-optical flux sources. SEDs of 48 among 53 are represented by templates of type 2 AGNs or starforming galaxies and show signature of stellar emission from host galaxies in the optical in the source rest frame. Infrared/optical SEDs indicate significant contribution of emission from dust to infrared fluxes and that the central AGN is dust obscured. Photometric redshifts determined from the SEDs are in the range of 0.6-2.5. X-ray spectra are fitted by an absorbed power law model, and the intrinsic absorption column densities are modest (best-fit log N_H = 20.5-23.5 cm^-2 in most cases). The absorption corrected X-ray luminosities are in the range of 6x10^42 - 2x10^45 erg s^-1. 20 objects are classified as type 2 quasars based on X-ray luminsosity and N_H. The optical faintness is explained by a combination of redshifts (mostly z>1.0), strong dust extinction, and in part a large ratio of dust/gas.

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arXiv2016

X-ray and Optical Correlation of Type I Seyfert NGC 3516 Studied with Suzaku and Japanese Ground-Based Telescopes

From 2013 April to 2014 April, we performed an X-ray and optical simultaneous monitoring of the type 1.5 Seyfert galaxy NGC 3516. It employed Suzaku, and 5 Japanese ground-based telescopes, the Pirka, Kiso Schmidt, Nayuta, MITSuME, and the Kanata telescopes. The Suzaku observations were conducted seven times with various intervals ranging from days, weeks, to months, with an exposure of $\sim50$ ksec each. The optical $B$-band observations not only covered those of Suzaku almost simultaneously, but also followed the source as frequently as possible. As a result, NGC 3516 was found in its faint phase with the 2-10 keV flux of $0.21-2.70 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$. The 2-45 keV X-ray spectra were composed of a dominant variable hard power-law continuum with a photon index of $\sim1.7$, and a non-relativistic reflection component with a prominent Fe-K$α$ emission line. Producing the $B$-band light curve by differential image photometry, we found that the $B$-band flux changed by $\sim2.7 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$, which is comparable to the X-ray variation, and detected a significant flux correlation between the hard power-law component in X-rays and the $B$-band radiation, for the first time in NGC 3516. By examining their correlation, we found that the X-ray flux preceded that of $B$ band by $2.0^{+0.7}_{-0.6}$ days ($1σ$ error). Although this result supports the X-ray reprocessing model, the derived lag is too large to be explained by the standard view which assumes a "lamppost"-type X-ray illuminator located near a standard accretion disk. Our results are better explained by assuming a hot accretion flow and a truncated disk.

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arXiv2016

The New Primary X-ray Component Confirmed in the Seyfert I Galaxy IC4329A

The bright and highly-variable Seyfert I Active Galactic Nucleus, IC4329A, was observed with Suzaku 5 times in 2007 August with intervals of ~ 5 days, and a net exposure of 24-31 ks each. Another longer observation was carried out in 2012 August with a net exposure of 118 ks. In the 6 observations, the source was detected in 2-45 keV with average 2-10 keV fluxes of $(0.67-1.2)\times10^{-10}$ erg cm$^{-2}$ s$^{-1}$. The intensity changed by a factor of 2 among the 5 observations in 2007, and 1.5 within the 2012 observation. Difference spectra among these observations revealed that the variability of IC4329A was carried mainly by a power-law component with a photon index $Γ\sim2.0$. However, in addition to this primary component and the associated reflection, the broad-band Suzaku data required another, harder, and less variable component with $Γ\sim1.4$. The presence of this new continuum was also confirmed by analyzing the same 6 data sets through the spectral decomposition technique developed by Noda et al. 2013a. This $Γ\sim1.4$ continuum is considered to be a new primary component that has not been recognized in the spectra of IC4329A so far, although it was recently identified in those of several other Seyfert I galaxies (Noda et al. 2013a, 2014).

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arXiv2016

Fe-K Line Time variability and Ni abundance of Distant Reflector in Seyfert Galaxies

We have performed systematic studies of narrow Fe-K line (6.4 keV) flux variability and Ni-K line intensity for Seyfert galaxies, using {\em Suzaku} and {\em XMM-Newton} archival data. Significant Fe-K line variability of several tens percents was detected for a pair of observations separated by 1000--2000 days (Cen A, IC 4329 A, NGC 3516, and NGC 4151) and 158 days (NGC 3516). These timescales are larger by a factor of 10--100 than the inner radius of torus, consistent with the view that X-ray reflection by torus is a main origin of a narrow Fe-K line. Ni-K line was detected with $>2σ$ level for the Circinus galaxy, Cen A, MRK 3, NGC 4388, and NGC 4151. A mean and variance of Ni-K$α$ to Fe-K$α$ line intensity ratio are 0.066 and 0.026, respectively. Comparing this with the Monte-Carlo simulation of reflection, Ni to Fe abundance ratio is 1.9$\pm$0.8 solar. We discuss the results and possibility of Ni abundance enhancement.

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