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Katsuji Koyama

Publications and source records attributed to Katsuji Koyama.

At least 145 records · Page 8Linked to original sources
arXiv2001

X-Ray Observations on the Galactic Center Region

This paper reports on the early Chandra view of the Galactic center (GC) activities. The massive black hole Sgr A* is extremely faint, while more bright diffuse X-ray emission is prevailing in the circumnuclear disk. Another high temperature plasma is found in the Sgr A East shell. This may indicate that Sgr A East is a supernova remnant, although no clear X-ray shell is found. A hint of non-thermal X-ray filaments is found, suggesting the presence of an acceleration site of extremely high-energy cosmic rays. The giant molecular cloud Sgr B2 is established to be an X-ray reflection nebula, possibly arising from the past Sgr A activities. Chandra further discovered high temperature shells, suggesting multiple supernova explosions near the GC region.

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arXiv2001

X-ray Detection from Bona-fide and Candidate Brown Dwarfs in the Rho Ophiuchi Cloud with Chandra

We present results of an X-ray search from bona-fide and candidate brown dwarfs in the Rho Ophiuchi cloud cores with the Chandra X-ray Observatory. The selected areas are two fields near the cloud center and are observed with the ACIS-I array of a 17'x17' size and a ~100 ks exposure. Among 18 bona-fide and candidate brown dwarfs listed by the infrared spectroscopy, we find X-ray emission from 7 sources above 99.9% confidence level. Therefore ~40% of the infrared-selected brown dwarfs in this cloud emit X-rays. For the brightest 4 sources, the X-ray spectra are made and are fitted with a thin-thermal plasma model of a temperature 1-2.5 keV. The X-rays are also time variable with rapid flares from 2 of the brown dwarfs. Assuming 2 keV temperature and using the empirical relation of Av vs. NH, we estimate the X-ray luminosity or its upper limit of the other faint or non-X-ray sources. The X-ray luminosity (Lx) of the X-ray-detected sources is in the range of 0.3-90x10^28 ergs s^-1, while the luminosity ratio of X-ray to bolometric (Lx/Lbol) is 10^-3 - 10^-5, similar to those of low-mass pre-main-sequence and dMe stars. All these results suggest that the X-ray origin of brown dwarfs is the same as low-mass stars; strong magnetic activity at the stellar surface.

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arXiv2001

X-ray source population of the Galactic center region obtained with ASCA

From the ASCA X-ray point-source list in the Galactic center 5x5 degree^2 region, we found the clear correlation between the position of the sources and the absorption. This fact implies that the major part of the absorption is due to the cold interstellar matter (ISM) in the line of sight. Using the correlation, we estimate the distribution of the cold ISM. We also found that the ratio of high mass binaries to low mass binaries is significantly smaller than that in the whole Galaxy or SMC, which implies that the past starburst activity in the Galactic center region was rather quiet.

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arXiv2001

A Diffuse X-Ray Source, AX J1843.8-0352: Association with the Radio Complex G28.6-0.1 and Identification of a New Supernova Remnant

ASCA discovered an extended source in the Scutum constellation. The X-ray morphology is an elliptical shape elongated from north to south with a mean diameter of about 10'. The image center is located at RA = 18h43m53s, DEC = 03D52'55'' (hereafter, AX J1843.8-0352). The north and south rims of AX J1843.8-0352 are associated with non-thermal radio sources C and F of the G28.6-0.1 complex (Helfand et al. 1989). The X-ray spectrum was fitted with a model of either a thin thermal plasma in non-equilibrium ionization of a temperature 5.4 keV or a power-law of photon index 2.1. The absorption column is (2.4-4.0)E22 /cm2, which probably places this source in the Scutum arm. With a reasonable distance assumption of about 7 kpc, we estimate the mean diameter and X-ray luminosity to be ~20 pc and ~3E34 erg/s, respectively. Although a Sedov solution for a thin thermal plasma model gives parameters of a young shell-like SNR, no strong emission lines are found with the metal abundances being ~solar. Thus, a more likely scenario for both the radio and X-ray spectra and the morphologies is that AX J1843.8-0352 is a shell-like SNR which predominantly emits synchrotron X-rays.

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arXiv2001

Chandra Observation of the Rho Oph Cloud

We observed a 17$'\times17'$ region of the $ρ$ Oph molecular cloud, using the ACIS-I array onboard the {\it Chandra X-ray Observatory} (CXO). A 100-ks exposure observation revealed $\sim$ hundred X-ray sources above the detection limit of $\sim$ 10$^{28}$ ergs s$^{-1}$. About 2/3 of the X-ray sources are identified to an optical and/or infrared counterpart, including significant numbers of class I, II and III sources and a few brown dwarfs. The X-ray detection rate of class I sources is surprisingly high of 70 %. Many X-ray flares, possibly due to the magnetic activity, are found from all of the classes. We make systematic spectral studies on the quiescent and flare X-rays for all the X-ray sources. The X-ray temperatures and absorptions of class I sources are generally larger than those of class II and III sources. Also X-ray flares from class I sources show a hint of higher duty ratio, temperature and luminosity, than those from more evolved classes. We further give brief results on brown dwarfs, younger phase than class I, unclassified and unidentified sources. Details for several selected young stellar objects, including the first detection of a neutral iron line (6.4 keV) from a class I source YLW 16A, are separately addressed.

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arXiv2001

Discovery of X rays from Class 0 protostar candidates in OMC-3

We have observed the Orion Molecular Clouds 2 and 3 (OMC-2 and OMC-3) with the Chandra X-ray Observatory (CXO). The northern part of OMC-3 is found to be particularly rich in new X-ray features; four hard X-ray sources are located in and along the filament of cloud cores. Two sources coincide positionally with the sub$mm$-$mm$ dust condensations of MMS 2 and 3 or an outflow radio source VLA 1, which are in a very early phase of star formation. The X-ray spectra of these sources show an absorption column of (1-3) x 10^23 H cm-2. Assuming a moderate temperature plasma, the X-ray luminosity in the 0.5-10 keV band is estimated to be ~10^30 erg s^-1 at a distance of 450 pc. From the large absorption, positional coincidence and moderate luminosity, we infer that the hard X-rays are coming from very young stellar objects embedded in the molecular cloud cores. We found another hard X-ray source near the edge of the dust filament. The extremely high absorption of 3 x 10^23 H cm^-2 indicates that the source must be surrounded by dense gas, suggesting that it is either a YSO in an early accretion phase or a Type II AGN (e.g. a Seyfert 2), although no counterpart is found at any other wavelength. In contrast to the hard X-ray sources, soft X-ray sources are found spread around the dust filaments, most of which are identified with IR sources in the T Tauri phase.

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arXiv2000

Discovery of a 9-s X-Ray Pulsar, AX J0049-732, in the Small Magellanic Cloud

We report the ASCA discovery of a new X-ray pulsar, AX J0049-732, in the Small Magellanic Cloud (SMC). The pulse shape is sinusoidal with a barycentric period of 9.1320 +/- 0.0004 s. The X-ray spectrum was fitted by an absorbed power-law model with a photon index of ~0.6 and a column density of ~1.3e22 cm^-2. An unabsorbed flux in the 0.7-10.0 keV was estimated to be ~8e-13 erg cm^-2 s^-1 corresponding to the X-ray luminosity of ~4e35 erg s^-1 at the SMC distance of 62 kpc.

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arXiv2000

Discovery of the Slowest X-Ray Pulsar in the SMC, AX J0049.5-7323, with ASCA

The discovery of coherent pulsations with ASCA from an X-ray source, AX J0049.5-7323, is reported. The barycentric period was determined to be 755.5 +/- 0.6 s, which is the longest among X-ray pulsators in the SMC. The X-ray spectrum has been found to be unchanged through ASCA observations, with a photon index of ~0.8 and a luminosity of ~5e35 erg s^-1 (0.7-10 keV). Archival data of the Einstein and the ROSAT satellites indicate that the flux has been < 1e-12 erg s^-1 cm^-2 (< 5e35 erg s^-1) for over 20 years and exhibits a variability with a factor of >10. We argue that AX J0049.5-7323 is an X-ray pulsar with a Be star companion.

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arXiv2000

Discovery of 101-s Pulsations from AX J0057.4-7325 in the SMC with ASCA

The results from two ASCA observations of AX J0057.4-7325 = RX J0057.3-7325 are presented. Coherent pulsations with a barycentric period of 101.45 +/- 0.07 s were discovered in the second observation. The X-ray spectrum was found to be hard (photon index ~ 0.9) and unchanged through these observations, except for the flux. The ROSAT archival data show that AX J0057.4-7325 exhibits a flux variation with a factor >~ 10. A discussion on a possible optical counterpart is given.

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arXiv2000

ASCA Observations of the Twin Supernova Remnants in the Large Magellanic Cloud, DEM L316

We report results from an ASCA X-ray study of DEM L316, an emission nebula in the Large Magellanic Cloud (LMC) consisting of two closely-spaced supernova remnants (SNRs). The SIS image shows separate X-ray sources located at the positions of the two radio- and optically-emitting SNR shells, 0547$-$69.7A and 0547$-$69.7B (hereafter, shell A and B). The individual X-ray spectrum of each shell is well described by optically-thin thermal emission, although the characteristics of the emission differ in important details between them. Shell A exhibits strong iron L emission, which we attribute to the presence of iron-rich ejecta leading to the suggestion that this SNR originates from a Type Ia SN, an explosion of a moderate mass progenitor. Shell B, on the other hand, has a chemical composition similar to that of the interstellar medium of the LMC and so its X-ray emission is dominated by swept-up matter. The different spectral features strongly disfavor the hypothesis that the two shells are due to one supernova explosion into an interconnected bubble. We could not obtain the evidence which positively supports the collision between two SNRs.

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arXiv2000

Discovery of 172-s Pulsations from a Be/X-ray Binary Candidate AX J0051.6-7311 in the SMC with ASCA

The results from three ASCA observations of AX J0051.6-7311 = RX J0051.9-7311 are reported. Coherent pulsations with a barycentric period of 172.40 +/- 0.03 s were discovered in the third observation, with an exceptionally long exposure time of ~177 ks. The X-ray spectrum was found to remain unchanged through these observations, with a photon index of ~0.9. Energy-resolved pulse profiles in the third observation reveal that the pulsations are mostly due to photons with an energy above ~2 keV. Archival data of ROSAT and Einstein indicate that AX J0051.6-7311 exhibits a flux variation having a factor ~> 20.

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arXiv2000

Discovery of a Slow X-Ray Pulsator, AX J1740.1-2847, in the Galactic Center Region

We report the discovery of an X-ray pulsar AX J1740.1-2847 from the Galactic center region. This source was found as a faint hard X-ray object on 7--8 September 1998 with the ASCA Galactic center survey observation. Then, coherent pulsations of P=729 +/- 14 sec period were detected. The X-ray spectrum is described by a flat power-law of ~= 0.7 photon index. The large absorption column of log NH ~= 22.4 (cm^-2) indicates that AX J1740.1-2847 is a distant source, larger than 2.4 kpc, and possibly near at the Galactic center region. The luminosity in the 2--10 keV band is larger than 2.5 x 10^33 erg/s, or likely to be 3.2 x 10^34 erg/s at the Galactic center distance. Although the slow pulse period does not discriminate whether AX J1740.1-2847 is a white dwarf or neutron star binary, the flat power-law and moderate luminosity strongly favor a neutron star binary.

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arXiv2000

Discovery of Non-Thermal X-Rays from the Shell of RCW86

We report the ASCA (Advanced Satellite for Cosmology and Astrophysics) results of RCW 86, a shell-like supernova remnant (SNR). The bright region in the X-ray band traces the radio clumpy shell, although details of the structure are different. The X-ray spectrum from each part of the shell can not be fitted to a thin thermal plasma model, but requires, at least three components: a low temperature plasma of 0.3 keV, high temperature plasma of > several keV, and a power-law component with a photon index = 3. The abundances of O, Ne, Mg and Si are significantly higher than that of Fe, indicating that RCW 86 is a type II SNR. The absorption column of 3e21 H cm^-2 indicates the distance to the SNR to be several kpc. The power-law component can be interpreted to be synchrotron radiation of high energy electrons. Assuming energy density equipartition between the magnetic field and the electrons, and using the radio and X-ray spectra, we argue that high energy electrons are accelerated up to 20 TeV. The acceleration efficiency is, however, different from shell to shell.

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arXiv2000

Broad-Band X-Ray Study of a Transient Pulsar RX J0059.2-7138

We report on the results of the ASCA and ROSAT observations on RX J0059.2-7138, a transient X-ray pulsar in the Small Magellanic Cloud. The barycentric pulse period has been precisely determined to be 2.763221 +- 0.000004 s. The pulse shape is almost identical in all of the energy bands. The pulse fraction increases with the photon energy below ~2 keV, while it is nearly constant at ~37% above ~2 keV. The X-ray spectrum has been found to consist of two components. One is dominant above 2 keV, and exhibits sinusoidal pulsations. This component is well described by a typical model found in many X-ray binary pulsars, a power-law of photon index 0.4 with an exponential cut-off at 6.5 keV. The other is dominant below 1 keV and shows no significant pulsation. This component is represented by either a broken power-law with photon indices of 2.6 and 5.1 below and above a break energy of 0.9 keV, or a metal-poor thin-thermal plasma with a temperature of 0.37 keV. The phase-averaged luminosity is ~1E38 erg s^-1 (0.1-10.0 keV) for both components. A hint of oxygen over-abundance is found in the absorbing column, possibly due to circumstellar gas ejected from an evolved companion.

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arXiv2000

Deep X-Ray Observations of Supernova Remnants G359.1-0.5 and G359.0-0.9 with ASCA

We present the results of deep ASCA observations of two shell-like radio supernova remnants (SNRs) located in the direction to the Galactic center (GC) region. Unlike the radio morphology, G359.1-0.5 shows center-filled X-rays with prominent K-alpha lines of He-like silicon and H-like sulfur. The plasma requires at least two temperature components: a silicon-dominated cool plasma of 0.6keV temperature and a sulfur-dominated higher temperature plasma of 4.4keV. Because the absorption column is ~ 6x10^22 H cm^-2, this SNR would be near to the GC. The spherical plasma is attributable to supernova ejecta with the total mass of Si and S being about 0.1 solar mass and 0.3 solar mass, respectively. X-rays from G359.0-0.9 trace the partial shell structure of the radio emission. The spectrum is well fitted to a single-temperature plasma of 0.4keV with a non-solar abundance of magnesium or iron. Because the absorption column is not very large, ~ 1.8x10^22 H cm^-2, G359.0-0.9 would be in front of the GC region. The total supernova energy, interstellar density near to the X-ray emitting shell and age of the SNR are estimated to be 1.2x10^51erg, 0.5cm^-3, and 1.8x10^4yr, respectively. We also discuss possible implications on the origin of the large-scale hot plasma surrounding the GC.

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arXiv2000

A Study of the Populations of X-ray Sources in the Small Magellanic Cloud with ASCA

The Advanced Satellite for Cosmology and Astrophysics (ASCA) has made multiple observations of the Small Magellanic Cloud (SMC). X-ray mosaic images in the soft (0.7--2.0 keV) and hard (2.0--7.0 keV) bands are separately constructed, and the latter provides the first hard X-ray view of the SMC. We extract 39 sources from the two-band images with a criterion of S/N>5, and conduct timing and spectral analyses for all of these sources. Coherent pulsations are detected from 12 X-ray sources; five of which are new discoveries. Most of the 12 X-ray pulsars are found to exhibit long-term flux variabilities, hence they are likely to be X-ray binary pulsars (XBPs). On the other hand, we classify four supernova remnants (SNRs) as thermal SNRs, because their spectra exhibit emission lines from highly ionized atoms. We find that XBPs and thermal SNRs in the SMC can be clearly separated by their hardness ratio (the ratio of the count rate between the hard and soft bands). Using this empirical grouping, we find many XBP candidates in the SMC, although no pulsations have yet been detected from these sources. Possible implications on the star-formation history and evolution of the SMC are presented by a comparison of the source populations in the SMC and our Galaxy.

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