arXiv subjects

Kazunori Ishibashi

Publications and source records attributed to Kazunori Ishibashi.

23 records · Page 2Linked to original sources

arXiv2006

X-ray Variability in the Young Massive Triple theta2 Ori A

Massive stars rarely show intrinsic X-ray variability. The only O-stars credited to be intrinsically variable are theta1 Ori C due to effects from magnetic confinement of its wind, and theta2 Ori A suspected of similar activity. Early Chandra observations have shown that the most massive star system in the Orion Trapezium Cluster, theta2 Ori A, shows rapid variability on time scales of hours. We determine X-ray fluxes and find that the star shows very strong variability over the last 5 years. We observed a second large X-ray outburst in November 2004 with the high resolution transmission grating spectrometer on-board Chandra. In the low state X-ray emissivities indicate temperatures well above 25 MK. In the high state we find an extended emissivity distribution with high emissivities in the range from 3 MK to over 100 MK. The outburst event in stellar terms is one of the most powerful ever observed and the most energetic one in the ONC with a lower total energy limit of 1.5x10^37 ergs. The line diagnostics show that under the assumption that the line emitting regions in the low states are as close as within 1 -- 2 stellar radii from the O-star's photosphere, whereas the hard states suggest a distance of 3 -- 5 stellar radii. The two outbursts are very close to the periastron passage of the stars. We argue that the high X-ray states are possibly the result of reconnection events from magnetic interactions of the primary and secondary stars of the spectroscopic binary. Effects from wind collisions seem unlikely for this system. The low state emissivity and R-ratios strengthen the predicament that the X-ray emission is enhanced by magnetic confinement of the primary wind. We also detect Fe fluorescence indicative of the existence of substantial amounts of neutral Fe in the vicinity of the X-ray emission.

astro-ph↗

arXiv2006

Chandra/HETGS Observations of the Capella System: the Primary as a Dominating X-ray Source

Using the Chandra/High Energy Transmission Grating Spectrometer (hereafter HETGS) we have detected Doppler motion of Capella's X-ray emission lines in the 6 -- 25AA wave-band. The observed motion follows the expected orbital motion of Capella's primary. This finding implies that the primary G8 III star, not the secondary G1 III star in the Hertzsprung gap, has been the dominant source of hot 10^{6.8} -- 10^{7}K plasma at least in the last six years. In addition, the results demonstrate the long-term stability of the HETGS and demonstrate small uncertainties of 25 and 33 km/s in the velocity determination with the HEG and MEG, respectively.

astro-ph↗

arXiv2005

The Chandra High Energy Transmission Grating: Design, Fabrication, Ground Calibration and Five Years in Flight

Details of the design, fabrication, ground and flight calibration of the High Energy Transmission Grating, HETG, on the Chandra X-ray Observatory are presented after five years of flight experience. Specifics include the theory of phased transmission gratings as applied to the HETG, the Rowland design of the spectrometer, details of the grating fabrication techniques, and the results of ground testing and calibration of the HETG. For nearly six years the HETG has operated essentially as designed, although it has presented some subtle flight calibration effects.

astro-ph↗

arXiv2003

In-Flight Calibration of the Chandra High Energy Transmission Grating Spectrometer

We present results from in-flight calibration of the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-ray Observatory. Basic grating assembly parameters such as orientation and average grating period were measured using emission line sources. These sources were also used to determine the locations of individual CCDs within the flight detector. The line response function (LRF) was modeled in detail using an instrument simulator based on pre-flight measurements of the grating alignments and periods. These LRF predictions agree very well with in-flight observations of sources with narrow emission lines. Using bright continuum sources, we test the consistency of the detector quantum efficiencies by comparing positive orders to negative orders.

astro-ph↗

arXiv2003

Latitude-dependent effects in the stellar wind of Eta Carinae

The Homunculus reflection nebula around eta Carinae provides a rare opportunity to observe the spectrum of a star from multiple latitudes. We present STIS spectra of several positions in the Homunculus, showing directly that eta Car has an aspherical stellar wind. P Cygni absorption in Balmer lines depends on latitude, with high velocities and strong absorption near the poles. Stronger absorption at high latitudes is surprising, and it suggests higher mass flux toward the poles, perhaps resulting from radiative driving with equatorial gravity darkening on a rotating star. Reflected profiles of He I lines are more puzzling, offering clues to the wind geometry and ionization structure. During eta Car's high-excitation state in March 2000, the wind was fast and dense at the poles, with higher ionization at low latitudes. Older STIS data obtained since 1998 reveal that this global stellar-wind geometry changes during eta Car's 5.5 year cycle, and may suggest that this star's spectroscopic events are shell ejections. Whether or not a companion star triggers these outbursts remains ambiguous. The most dramatic changes in the wind occur at low latitudes, while the dense polar wind remains relatively undisturbed during an event. The apparent stability of the polar wind also supports the inferred bipolar geometry. The wind geometry and its variability have critical implications for understanding the 5.5 year cycle and long-term variability, but do not provide a clear alternative to the binary hypothesis for generating eta Car's X-rays.

astro-ph↗