arXiv subjects

Katsuji Koyama

Publications and source records attributed to Katsuji Koyama.

At least 127 records · Page 7Linked to original sources

arXiv2004

X-ray Study of the Intermediate-Mass Young Stars Herbig Ae/Be Stars

We present the ASCA results of intermediate-mass pre-main-sequence stars (PMSs), or Herbig Ae/Be stars (HAeBes). Among the 35 ASCA pointed-sources, we detect 11 plausible X-ray counterparts. X-ray luminosities of the detected sources in the 0.5-10 keV band are in the range of log LX ~30-32 ergs s-1, which is systematically higher than those of low-mass PMSs. This fact suggests that the contribution of a possible low-mass companion is not large. Most of the bright sources show significant time variation, particularly, two HAeBes - MWC 297 and TY CrA - exhibit flare-like events with long decay timescales (e-folding time ~ 10-60 ksec). These flare shapes are similar to those of low-mass PMSs. The X-ray spectra are successfully reproduced by an absorbed one or two-temperature thin-thermal plasma model. The temperatures are in the range of kT ~1-5 keV, which are significantly higher than those of main-sequence OB stars (kT < 1 keV). These X-ray properties are not explained by wind driven shocks, but are more likely due to magnetic activity. On the other hand, the plasma temperature rises as absorption column density increases, or as HAeBes ascend to earlier phases. The X-ray luminosity reduces after stellar age of a few x10^6 years. X-ray activity may be related to stellar evolution. The age of the activity decay is apparently near the termination of jet or outflow activity. We thus hypothesize that magnetic activity originates from the interaction of the large scale magnetic fields coupled to the circumstellar disk. We also discuss differences in X-ray properties between HAeBes and main-sequence OB stars.

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arXiv2003

A Discovery of a Peculiar Pulsar in the Small Magellanic Cloud

We report on a peculiar X-ray binary pulsar IKT1 = RXJ0047.3-7312 observed with XMM-Newton in Oct. 2000. The X-ray spectrum is described by a two-component spectrum. The hard component has a broken power-law with respective photon indices of 0.2 and 1.8, below and above the break energy at 5.8 keV. The soft component can be modeled by a blackbody of kT = 0.6 keV. The X-ray flux shows a gradual decrease and periodic variations of about 4000 s. The averaged flux in 0.7-10.0 keV is 2.9x10^-12 ergs/cm^2/s, which is ~10 times brighter than that in a ROSAT observation in Nov. 1999. In addition to the 4000-s variation, we found coherent pulsations of 263 +/- 1 s. These discoveries strengthen the Be/X-ray binary scenario proposed by the ROSAT and ASCA observations on this source, and confirm that most of the hard sources in the Small Magellanic Cloud are X-ray binary pulsars. A peculiar property of this XBP is that the coherent pulsations are found only in the soft component, and the folded light curve shows a flat top shape with a sharp dip. We discuss the nature of this XBP focusing on the peculiar soft component.

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arXiv2003

Constraints On the Diffusive Shock Acceleration From the Nonthermal X-ray Thin Shells In SN1006 NE Rim

Characteristic scale lengths of nonthermal X-rays from the SN1006 NE rim, which are observed by Chandra, are interpreted in the context of the diffusive shock acceleration on the assumption that the observed spatial profile of nonthermal X-rays corresponds to that of accelerated electrons with energies of a few tens of TeV. To explain the observed scale lengths, we construct two simple models with a test particle approximation, where the maximum energy of accelerated electrons is determined by the age of SN1006 (age-limited model) or the energy loss (energy loss-limited model), and constrain the magnetic field configuration and the diffusion coefficients of accelerated electrons. When the magnetic field is nearly parallel to the shock normal, the magnetic field should be in the range of 20-85 micro Gauss and highly turbulent both in upstream and downstream, which means that the mean free path of accelerated electrons is on the order of their gyro-radius (Bohm limit). This situation can be realized both in the age-limited and energy loss-limited model. On the other hand, when the magnetic field is nearly perpendicular to the shock normal, which can exist only in the age-limited case, the magnetic field is several micro Gauss in the upstream and 14-20 micro Gauss in the downstream, and the upstream magnetic field is less turbulent than the downstream.

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arXiv2003

Thermal and Non-thermal X-Rays from the LMC Super Bubble 30 Dor C

We report on the discovery of thermal and non-thermal X-rays from the shells of the super bubble (SB) 30 Dor C in the Large Magellanic Cloud (LMC). The X-ray morphology is a nearly circular shell with a radius of about 40 pc, which is bright on the northern and western sides. The spectra of the shells are different from region to region. The southern shell shows clear emission lines, and is well fitted with a model of a thin-thermal plasma (kT = 0.21keV) in non-equilibrium ionization (NEI) plus a power-law component. This thermal plasma is located inside of the H alpha emission, which is the outer edge of the shell of the SB. The northern and western sides of the SB are dim in H alpha emission, but are bright in non-thermal (power-law) X-rays with a photon index of 2.1-2.9. The non-thermal X-ray shell traces the outer boundary of the radio shell. These features of thin-thermal and non-thermal X-rays are similar to those of SN 1006, a prototype of synchrotron X-ray shell, but the non-thermal component of 30 Dor C is about ten-times brighter than that of SN 1006. 30 Dor C is the first candidate of an extragalactic SB, in which energetic electrons are accelerating in the shell. The age is much older than that of SN 1006, and hence the particle acceleration time in this SB may be longer than those in normal shell-like SNRs. We found point-like sources associated with some of tight star clusters. The X-ray luminosity and spectrum are consistent with those of young clusters of massive stars. Point-like sources with non-thermal spectra are also found in the SB. These may be background objects (AGNs) or stellar remnants (neutron stars or black holes).

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arXiv2003

X-Ray Observation on the Monoceros R2 Star-Forming Region with the Chandra ACIS-I Array

We report on the results of the Chandra observation on the central region of the Monoceros R2 cloud (Mon R2), a high-mass star-forming region (SFR). With a deep exposure of 100 ks, we detected 368 X-ray sources, 80% of which were identified with the NIR counterparts. We systematically analyzed the spectra and time variability of most of the X-ray emitting sources and provided a comprehensive X-ray source catalog for the first time. Using the J-, H-, and K-band magnitudes of the NIR counterparts, we estimated the evolutionary phase (classical T Tauri stars and weak-lined T Tauri stars) and the mass of the X-ray emitting sources, and analyzed the X-ray properties as a function of the age and mass. We found a marginal hint that classical T Tauri stars have a slightly higher temperature (2.4 keV) than that of weak-lined T Tauri stars (2.0 keV). A significant fraction of the high- and intermediate-mass sources have a time variability and high plasma temperatures (2.7 keV). We performed the same analysis for other SFRs, the Orion Nebula Cluster and Orion Molecular Cloud-2/3, and obtained similar results to Mon R2. This supports the earlier results of this observation obtained by Kohno et al. (2002, ApJ, 567, 423) and Preibisch et al. (2002, A&A, 392, 945) that high- and intermediate- mass young stellar objects emit X-rays via magnetic activity. We also found a significant difference in the spatial distribution between X-ray and NIR sources.

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arXiv2003

Fine Structures of Shock of SN 1006 with the Chandra Observation

The north east shell of SN 1006 is the most probable acceleration site of high energy electrons (up to ~ 100 TeV) with the Fermi acceleration mechanism at the shock front. We resolved non-thermal filaments from thermal emission in the shell with the excellent spatial resolution of Chandra. The thermal component is extended widely over about ~ 100 arcsec (about 1 pc at 1.8 kpc distance) in width, consistent with the shock width derived from the Sedov solution. The spectrum is fitted with a thin thermal plasma of kT = 0.24 keV in non-equilibrium ionization (NEI), typical for a young SNR. The non-thermal filaments are likely thin sheets with the scale widths of ~ 4 arcsec (0.04 pc) and ~ 20 arcsec (0.2 pc) at upstream and downstream, respectively. The spectra of the filaments are fitted with a power-law function of index 2.1--2.3, with no significant variation from position to position. In a standard diffusive shock acceleration (DSA) model, the extremely small scale length in upstream requires the magnetic field nearly perpendicular to the shock normal. The injection efficiency (eta) from thermal to non-thermal electrons around the shock front is estimated to be ~ 1e-3 under the assumption that the magnetic field in upstream is 10 micro G. In the filaments, the energy densities of the magnetic field and non-thermal electrons are similar to each other, and both are slightly smaller than that of thermal electrons. in the same order for each other. These results suggest that the acceleration occur in more compact region with larger efficiency than previous studies.

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arXiv2003

Review of Discrete X-Ray Sources in the Small Magellanic Cloud: Summary of the ASCA Results and Implication on the Recent Star Forming Activity

We made 22 observations on the Small Magellanic Cloud (SMC) and covered full regions by the end of the ASCA mission. We detected 106 discrete sources with a criterion of S/N > 5 and performed systematic analyses on all of the sources. We determined the source positions with an ~40'' error radius (90% confidence) for sources detected in the central 20' radius of the GIS. We detected coherent pulsations from 17 sources. Among them, eight were newly discovered during this study. We classified most of these pulsars as X-ray binary pulsars (XBPs) based on their properties, such as the flux variability and the existence of an optical counterpart. We detected X-ray emission from eight supernova remnants (SNRs). Among them, five SNRs showed emission lines in their spectra, hence we regarded the five as thermal SNRs. We found that XBPs and thermal SNRs in the SMC can be clearly separated by their spectral hardness ratio. Applying this empirical law to faint (thus unclassified) sources, we found 19 XBP candidates and four thermal SNR candidates. We also found several tens of candidates for active galactic nuclei, both from the hardness ratio and the logN--logS relation of extragalactic sources. Based on these ASCA results and further information from other sattelites, we compiled comprehensive catalogues of discrete X-ray sources in the Small Magellanic Cloud. Using the catalogues, we derived the spatial distributions of XBPs and SNRs. XBPs and SNRs were found to be concentrated in the main body and eastern wing, which resembles the distribution of young stars with ages of ~2e7yr. By comparing the source populations in the SMC and our Galaxy, we suggest that the star-forming rate (per unit mass) in the SMC was much higher than the Galaxy 1e7yr ago. We also discuss the recent change of the star-forming rate in the SMC.

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arXiv2003

Chandra Observations of a Non-Thermal Supernova Remnant Candidate AX J1843.8-0352 and its Surroundings

We present the Chandra results of AX J1843.8-0352, a supernova remnant (SNR) recently identified with ASCA. Chandra spatially resolved two components from this SNR: non-thermal and thermal ones. The morphology of the non-thermal component is clumpy and elliptical, elongated from the north to the south with a mean diameter of about 9 arcmin. The spectrum is fitted with a power-law model of photon index 2.1 and the east rim is associated with the non-thermal radio sources C and F (Helfand et al. 1989). Therefore the non-thermal component is probably synchrotron X-rays by energetic electrons accelerated at the shell of the SNR. The thermal component is the brightest clump located within the non-thermal component and shows a spectrum of a thin plasma of about 0.7 keV temperature. Notable discovery is its peculiar morphology; a head of 50 arcsec x 30 arcsec size near the south-east rim of the SNR and a 30 arcsec-long ``jet'' pointing to the southwest. Although this emission is associated with the west part of the radio source F, the absorption is twice larger than that of the non-thermal X-rays, or the bulk of the SNR emission. Therefore, it is unclear whether this peculiar plasma is a thermal component associated with AX J1843.8-0352, a Galactic source located in the far side of our Galaxy, or an extragalactic source.

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arXiv2003

Diffuse Hard X-ray Sources Discovered with the ASCA Galactic Plane Survey

We found diffuse hard X-ray sources, G11.0+0.0, G25.5+0.0, and G26.6-0.1 in the ASCA Galactic plane survey data. The X-ray spectra are featureless with no emission line, and are fitted with both models of a thin thermal plasma in non-equilibrium ionization and a power-law function. The source distances are estimated to be 1-8 kpc, using the best-fit NH values on the assumption that the mean density in the line of sight is 1 H cm^-3. The source sizes and luminosities are then 4.5-27 pc and (0.8-23)x10^33 ergs/s. Although the source sizes are typical to supernova remnants (SNR) with young to intermediate ages, the X-ray luminosity, plasma temperature, and weak emission lines in the spectra are all unusual. This suggests that these objects are either shell-like SNRs dominated by X-ray synchrotron emission, like SN 1006, or, alternatively, plerionic SNRs. The total number of these classes of SNRs in our Galaxy is also estimated.

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arXiv2002

Errata: The X-Ray Emission Measure (EM) of the Young Stellar Objects (YSOs) in the Rho Ophiuchi (Rho Oph) and Monoceros R2 Dark Clouds (Mon R2)

Errors were made in the calculations of EMs using the best fit normalizations in XSPEC. Lists of the errors and corrections are as follows. (1)Tables 1, 3, and 4 in ApJ, 557, 747 and astro-ph/0104190 (YSOs in Rho Oph): All the EMs in these tables should be multiplied by pi; the log(EM) in the first raw in Table 1, for example, is 52.6(52.2--53.2), not 52.1(51.7--52.7). (2)Figures 7 and 9 in ApJ, 557, 747 and astro-ph/0104190 (YSOs in Rho Oph): The EM plots should be systematically up-shifted by the factor of pi. (3)Table 3 in ApJ, 563, 361 and astro-ph/0108078 (young brown dwarfs in Rho Oph): All the EMs in this table should be multiplied by pi; the log(EM) in the first raw, for example, is 52.1(52.0--52.6), not 51.6(51.5--52.1). (4)Table 1 in ApJ, 567, 423 and astro-ph/0110462 (YSOs in Mon R2): All the EMs in this table should be divided by 10; the log(EM) in the first raw, for example, is 52.5(52.1--52.9), not 53.5(53.1--53.9). All the spectral parameters except EM are unchanged. These errors of EM do not affect any of the conclusions of the relevant papers. In addition, Gregorio-Hetem, J., Montmerle, T., Casanova, S., and Feigelson, E. D.1998, A&A, 331, 193 should be included in the reference list of ApJ, 567, 423 (astro-ph/0110462).

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arXiv2002

Chandra and ASCA Observations of the X-ray-brightest T-Tauri Stars in the Rho Ophiuchi Cloud

We present the Chandra ACIS and ASCA GIS results for a series of four long-term observations on DoAr 21, ROXs 21 and ROXs 31; the X-ray brightest T-Tauri stars (TTSs) in the Rho Ophiuchi cloud. In the four observations with a net exposure of ~600 ksec, we found six, three and two flares from DoAr 21, ROXs 21 and ROXs 31, respectively; hence the flare rate is fairly high. The spectra of DoAr 21 are well fitted with a single-temperature plasma model, while those of ROXs 21 and ROXs 31 need an additional soft plasma component. Since DoAr 21 is younger (~10^5 yr) than ROXs 21 and ROXs 31 (~10^6 yr), these results may indicate that the soft component gradually increases as T-Tauri stars age. The abundances are generally sub-solar and vary from element to element. Both high-FIP (first ionization potential) and low-FIP elements show enhancement over the mean abundances. An unusual giant flare is detected from ROXs 31. The peak luminosity and temperature are ~10^33 ergs s^-1 and ~10 keV, respectively. The temperature reaches its peak value before the flux maximum, and is nearly constant (4--5 keV) during the decay phase, indicating successive energy release during the flare. The abundances and absorption show dramatic variability from the quiescent to flare phase.

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arXiv2002

Chandra Observations of a New Supernova Remnant AX J1843.8-0352/G28.6-0.1

AX J1843.8-0352 is a new X-ray SNR identified to the radio complex G28.6-0.1 with the ASCA satellite. Chandra discovered two distinct components from this SNR: non-thermal and thin thermal X-ray emissions. The non-thermal component is fitted with a power-law spectrum of photon index 2.0. The morphology is complicated, but roughly an elliptical shape with a mean diameter of about 7'-10'. The east to south rims of the ellipse are associated with the non-thermal radio sources C, F and G (Helfand et al. 1989). The power-law slope of the radio spectrum can be smoothly connected to that of X-rays with a break at around the optical-IR band, hence would be due to synchrotron X-rays accelerated probably to > 1 TeV at the shell of the SNR. The thermal component near the southeast rim is a thin plasma of about 0.8 keV temperature. It has the appearance of a "Tadpole" figure with a head of 30"x 40"-size and a tail of 30"-long. Although this emission is associated with the west part of the radio source F, the absorption is about two times larger than that of the non-thermal X-rays, the bulk of the SNR emission. Therefore, together with the peculiar morphology, whether the thermal plasma is a part of the SNR or a background object is unclear.

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arXiv2002

Diffuse X-rays in the Galactic center region -- The zoo of iron line clumps, non-thermal filaments and hot plasmas --

This paper reports the diffuse X-ray features around the Galactic center observed with Chandra. We confirm the ASCA and Ginga discoveries of the large-scale thin-thermal plasma with strong lines in the Galactic center region. In addition, many small clumps of emission lines from neutral (6.4 keV line) to He-like (6.7 keV line) irons are discovered. The 6.4 keV line clumps would be reflection nebulae, while those of the 6.7 keV line are likely SNRs. We also find emission lines of intermediate energy between 6.5-6.7 keV, which are attributable to young SNRs in non equilibrium ionization. Non-thermal filaments or belts with X-ray spectra of no emission line are found, suggesting the Fermi acceleration site in a rapidly expanding shell. All these suggest that multiple-supernovae or extremely large explosion had occurred around the Galactic center region in the recent past.

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arXiv2002

Centrally Peaked X-Ray Supernova Remnants in the Small Magellanic Cloud Studied with ASCA and ROSAT

This paper presents ASCA/SIS and ROSAT/HRI results of three supernova remnants (SNRs) in the Small Magellanic Cloud: 0103-726, 0045-734, and 0057-7226. The ROSAT/HRI images of these SNRs indicate that the most of the X-ray emissions are concentrated in the center region. Only from 0103-726 are faint X-rays along the radio shell also detected. The ASCA/SIS spectra of 0103-726 and 0045-734 exhibit strong emission lines from highly ionized metals. The spectra were well-fitted with non-equilibrium ionization (NEI) plasma models. The metal abundances are found to be larger than the mean chemical compositions in the interstellar medium (ISM) of the SMC. Thus, X-rays from these two SNRs are attributable to the ejecta gas, although the ages estimated from the ionization timescale are significantly large, ~> 10^4 yr. The chemical compositions are roughly consistent with the type-II supernova origin of a progenitor mass ~< 20 M_solar. The SIS spectrum of 0057-7226 was also fitted with an NEI model of an estimated age ~> 6 x 10^3 yr. Although no constraint on the metal abundances was obtained, the rather weak emission lines are consistent with the low metal abundances in the ISM of the SMC. A possible scenario for the evolution of the morphologies and spectra of SNRs is proposed.

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arXiv2001

Discovery of a Transient X-Ray Pulsar, AX J1841.0-0536, in the Scutum Arm Region with ASCA

We report on the discovery of a transient X-ray pulsar, AX J1841.0-0536, serendipitously found in the Scutum arm region with the ASCA in two separate observations. The X-ray flux is very faint at the beginning, but exhibits two flares in the second observation. The flare flux increases by a factor 10 within only about 1 hr. Coherent pulsations with a period of 4.7394 plus/minus 0.0008 s were detected in the brightest flare phase. The X-ray spectra in the quiescent and flare phases were fitted with an absorbed power-law model with a photon index about 1 plus a narrow Gaussian line at the center energy of 6.4 keV. The interstellar column density of about 3E22 /cm2 may indicate that AX J1841.0-0536 is located at a tangential point of the Scutum arm at about 10 kpc distance. The coherent pulsations, large flux variability and the spectral shape suggest that AX J1841.0-0536 is a Be/X-ray binary pulsar.

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arXiv2001

Unusual diffuse X-ray source in the Galactic center region

We report the ASCA and Chandra discovery of a diffuse X-ray source in the Galactic center region. The X-ray spectrum is fitted with a non-equilibrium ionization (NEI) plasma model of about 6-keV temperature. The model requires higher than solar metal abundances, a young plasma age of $\simeq$ 100 years and a large N_H value of about 10^{23} cm^{-2}. The N_H value constrains the source position to be in the Galactic center region at about 8.5 kpc distance. The high resolution X-ray image with the Chandra ACIS shows a ring of 10'' radius which corresponds to 0.4 pc at the Galactic center, and a tail-like structure. Although the morphology is peculiar, the other X-ray features are likely to be a very young supernova remnant, possibly in a free expansion phase.

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arXiv2001

Chandra Observations of Embedded Young Stellar Objects

This paper reviews the Chandra deep exposure observations of star forming regions, rho-Ophiuchi, Orion Molecular Clouds 2 and 3, Sagittarius B2 and Monoceros R2. The results are; (1) class I protostars are found to exhibit higher temperature plasma than those of T Tauri stars, (2) heavily absorbed X-rays are discovered from the candidates of class 0 protostars, (3) hard and highly variable X-rays are observed from high-mass young stars, and (4) young brown dwarfs emit X-rays similar to those of low-mass young stars.

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arXiv2001

ASCA X-ray source catalogue in the Galactic Center region

The ASCA satellite made 107 pointing observations on a 5 x 5 deg^2 region around the center of our Milky Way Galaxy (the Galactic Center) from 1993 to 1999. In the X-ray images of the 0.7--3 keV or 3--10 keV bands, we found 52 point sources and a dozen diffuse sources. All the point sources are uniformly fitted with an absorbed power-law model. For selected bright sources, Sgr A*, AX J1745.6-2901, A 1742-294, SLX 1744-300, GRO J1744-28, SLX 1737-282, GRS 1734-292, AX J1749.2-2725, KS 1741-293, GRS 1741.9-2853, and an unusual flare source XTE J1739-302, we present further detailed spectral and timing analyses, and discuss their nature. The dozen extended X-ray sources comprise radio supernova remnants, giant molecular clouds, and some new discoveries. Most show emission lines from either highly ionized atoms or low-ionized irons. The X-ray spectra were fitted with either a thin thermal or power-law model. This paper summarizes the results and provides the ASCA X-ray source catalogue in the Galactic Center region.

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