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Jun Yokogawa

Publications and source records attributed to Jun Yokogawa.

18 recordsLinked to original sources
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

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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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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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

ASCA Observation of the New Transient X-ray Pulsar XTE J0111.2-7317 in the Small Magellanic Cloud

The new transient X-ray pulsar XTE J0111.2-7317 was observed with Advanced Satellite for Cosmology and Astrophysics (ASCA) on 1998 November 18, a few days after its discovery with the Proportional Counter Array onboard the Rossi X-ray Timing Explorer. The source was detected at a flux level of 3.6x10^-10 erg cm^-2 s^-1 in the 0.7--10.0 keV band, which corresponds to the X-ray luminosity of 1.8x10^38 erg s^-1, if a distance of 65 kpc for this pulsar in the Small Magellanic Cloud is assumed. Nearly sinusoidal pulsations with a period of 30.9497 +/- 0.0004 s were unambiguously detected during the ASCA observation. The pulsed fraction is low and slightly energy dependent with average value of \~27%. The energy spectrum shows a large soft excess below ~2 keV when fitted to a simple power-law type model. The soft excess is eliminated if the spectrum is fitted to an ``inversely broken power-law'' model, in which photon indices below and above a break energy of 1.5 keV are 2.3 and 0.8, respectively. The soft excess can also be described by a blackbody or a thermal bremsstrahlung when the spectrum above ~2 keV is modeled by a power-law. In these models, however, the thermal soft component requires a very large emission zone, and hence it is difficult to explain the observed pulsations at energies below 2 keV. A bright state of the source enables us to identify a weak iron line feature at 6.4 keV with an equivalent width of 50 +/- 14 eV. Pulse phase resolved spectroscopy revealed a slight hardening of the spectrum and marginal indication of an increase in the iron line strength during the pulse maximum.

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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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arXiv1999

Discovery of an X-Ray Pulsar in the SMC: AX J0058-7203

We report on the discovery and analysis of an X-ray pulsar, AX J0058-7203, in the Small Magellanic Cloud. This pulsar exhibits coherent pulsations at P= 280.3 s with a double-peak structure. The X-ray spectrum is well fitted with a simple power-law model of photon index ~ 0.7. No significant change of the pulsation period over the observation was found. A comparison with ROSAT observations in the same field reveals that AX J0058-7203 is highly variable, and is most likely a Be star binary pulsar.

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arXiv1999

ASCA Discovery of a Be X-Ray Pulsar in the SMC: AX J0051-733

ASCA observed the central region of the Small Magellanic Cloud, and found a hard X-ray source, AX J0051-733, at the position of the ROSAT source RX J0050.8-7316, which has an optical counterpart of a Be star. Coherent X-ray pulsations of 323.1 +/- 0.3 s were discovered from AX J0051-733. The pulse profile shows several sub-peaks in the soft (0.7-2.0 keV) X-ray band, but becomes nearly sinusoidal in the harder (2.0-7.0 keV) X-ray band. The X-ray spectrum was found to be hard, and is well fitted by a power-law model with a photon index of 1.0 +/- 0.4. The long-term flux history was examined with the archival data of Einstein observatory and ROSAT; a flux variability with a factor > 10 was found.

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