Kensuke Imanishi

Quasi-periodic X-Ray Flares from the Protostar YLW 15

With ASCA, we have detected three X-ray flares from the class I protostar YLW 15. The flares occurred every ~20 hr and showed an exponential decay with time constant 30-60 ks. The X-ray spectra are explained by a thin thermal plasma emission. The plasma temperature shows a fast rise and slow decay for each flare with kTpeak ~ 4-6 keV. The emission measure of the plasma shows this time profile only for the first flare, and remains almost constant during the second and third flares, at the level of the tail of the first flare. The peak flare luminosities, LX,peak, were ~5-20 × 1031 ergs s-1, which are among the brightest X-ray luminosities observed to date for class I protostars. The total energy released in each flare was 3-6 × 1036 ergs. The first flare is well reproduced by the quasi-static cooling model, which is based on solar flares, and it suggests that the plasma cools mainly radiatively, confined by a semicircular magnetic loop of length ~14 R☉, with diameter-to-length ratio ~0.07. The two subsequent flares were consistent with the reheating of the same magnetic structure as in the first flare. The large-scale magnetic structure and the periodicity of the flares imply that the reheating events of the same magnetic loop originate in an interaction between the star and the disk due to the differential rotation.

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.

Chandra and ASCA Observations of the X-Ray-brightest T Tauri Stars in the ρ Ophiuchi Cloud

We present the Chandra Advanced CCD Imaging Spectrometer and ASCA Gas Imaging Spectrometer results for a series of four long-term observations on DoAr 21, ROXs 21, and ROXs 31, the X-ray-brightest T Tauri stars in the ? Ophiuchi cloud. In the four observations with a net exposure of ~600 ks, 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 (~105 yr) than ROXs 21 and ROXs 31 (~106 yr), these results may indicate that the soft component gradually increases as T Tauri stars age. The abundances are generally subsolar and vary from element to element. Both high first ionization potential (FIP) 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 ~1033 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.

A Systematic Study of X-Ray Flares from Low-Mass Young Stellar Objects in the

We report on the results of a systematic study of X-ray flares from low-mass young stellar objects, using two deep exposure Chandra observations of the main region of the ρ Ophiuchi star-forming cloud. From 195 X-ray sources, including class I-III sources and some young brown dwarfs, we detected a total of 71 X-ray flares. Most of the flares have the typical profile of solar and stellar flares, fast rise and slow decay, while some bright flares show unusually long rise timescales. We derived the time-averaged temperature (� kT � ), luminosity (� LX� ), rise and decay timescales (τr and τd) of the flares, finding that (1) class I-II sources tend to have a highkT � , which sometimes exceeds 5keV, (2) the distribution ofLXduring flares is nearly the same for all classes from ∼10 29.5 to ∼10 31.5 ergs −1 , although there is a marginal hint of a higherLXdistribution for class I than class II-III, and (3) positive and negative log-linear correlations are found between τr and τd ,a ndkTand τr. In order to explain these relations, we used the framework of magnetic reconnection model with heat conduction and chromospheric evaporation to formulate the observational parameters (τr, τd ,a ndkT� ) as a function of the pre-flare (coronal) electronic density (nc), the half-length of the reconnected magnetic loop (L), and magnetic field strength (B). The observed correlations are well reproduced if loop lengths are nearly the same for all classes, regardless of the existence of an accretion disk. The estimated loop length is almost comparable to the typical stellar radius of these objects (10 10 -10 11 cm), which indicates that the observed flares are triggered by solar-type loops, rather than larger ones (∼ 10 12 cm) connecting the star with its inner accretion disk. The higherkTobserved for class I sources may be explained by a slightly higher magnetic field strength (≈ 500G) than for class II-III sources (200-300G).

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We report the ASCA results of the Great Annihilator 1E 1740.7-2942 obtained with five pointing observations in a time span of 3.5 yr. The X-ray spectrum for each period is well fitted with a single power law absorbed by a high column of gas. The X-ray flux changes by a factor of 2 from period to period, but the other spectral parameters show no significant change. The photon index is flat with ? = 0.9-1.3. The column density of hydrogen NH is ~ 1.0 ? 1023 H cm-2 and that of iron NFe is ~ 1019 Fe cm-2. These large column densities indicate that 1E 1740.7-2942 is near the Galactic center. The column density ratio leads the iron abundance to be 2 times larger than the other elements in a unit of the solar ratio. The equivalent width of the K? line from a neutral iron is less than 15 eV in 90% confidence. This indicates that the iron column density within several parsecs from 1E 1740.7-2942 is less than 5 ? 1017 Fe cm-2. In addition, the derived hydrogen column density is about one-sixth of that of giant molecular clouds in the line of sight. All these facts support the fact that 1E 1740.7-2942 is not in a molecular cloud, but possibly in front of it; the X-rays are not powered by accretion from a molecular cloud, but from a companion star like ordinary X-ray binaries.

Ophiuchi Star-Forming Region with Chandra

We report on the results of the Chandra ACIS-I observation on the central region of the Monoceros R2 cloud (Mon R2), a high-mass star-forming region (SFR) at a distance of 830pc. With a deep exposure of ∼ 100ks, we detected 368 X-ray sources, ∼80% of which were identified with the near-infrared (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 (2.4keV) than that of weak-lined T Tauri stars (2.0keV). A significant fraction of the high- and intermediate-mass sources have a time variability and high plasma temperatures (2.7keV) similar to those of low-mass sources (2.0keV). 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.

Further Studies of 1E 1740.7–2942 with ASCA

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.

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

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 ~ 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.

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

We present the detailed study of the response function of X- ray CCD cameras (XIS; X-ray Imaging Spectrometer). A pulse height distribution for monochromatic X-rays show a low- energy tail component and several weak lines in addition to main peak corresponding to incident X-ray energy. We divided the response function into six components; main peak, sub peak, triangle component, constant component, Si escape, and Si line. Each of them represents different physical processes in the CCD. We did the data fitting, numerical calculations, and Monte Carlo simulations to study energy dependence of the shape and intensity of these components, and made the response function as a function of X-ray energy.

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

We report on an 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 +1.0 −0.7 and a column density of 1.3 +2.9 −1.3 × 10 22 cm −2 . An unabsorbed flux at 0.7–10.0 keV was estimated to be 8 × 10 −13 erg cm −2 s −1 corresponding to an X-ray luminosity of 4 × 10 35 erg s −1 at an SMC distance of 62 kpc.