Hideyo Kunieda

The Variable iron k emission line in MCG-6-30-15

We report on the variability of the iron K emission line in the Seyfert 1 galaxy MCG--6-30-15 during a four-day ASCA observation. The line consists of a narrow core at an energy of about 6.4 keV, and a broad red wing extending to below 5 keV, which are interpreted as line emission arising from the inner parts of an accretion disk. The narrow core correlates well with the continuum flux whereas the broad wing weakly anti-correlates. When the source is brightest, the line is dominated by the narrow core, whilst during a deep minimum, the narrow core is very weak and a huge red tail appears. However, at other times when the continuum shows rather rapid changes, the broad wing is more variable than the narrow core, and shows evidence for correlated changes contrary to its long time scale behaviour. The peculiar line profile during the deep minimum spectrum suggests that the line emitting region is very close to a central spinning (Kerr) black hole where enormous gravitational effects operate.

X-ray evidence for Seyfert activity buried in the infrared galaxy NGC 4945

We have observed the infrared galaxy NGC 4945 using the Japanese X-ray astronomy satellite, Ginga. The X-ray spectrum is found to be composed of three components: hard X-ray emission which is heavily absorbed by cold material with column density of about 10 24.7 cm −2 ; a soft X-ray component seen in the 2-10 keV band; and an iron emission line. The hard component exhibits a power-law spectrum with photon index of about 1.7. The mean X-ray luminosity in the 2-20 keV band, after correction for absorption is 3×10 42 ergs s −1 with significant intensity variation on a time scale of several hours. These results are thought to be evidence for an AGN (active galactic nucleus) in the galaxy

The Variable O VIII Warm Absorber in MCG–6–30–15

We present the results of a 4 day ASCA observation of the Seyfert galaxy MCG-6-30-15, focussing on the nature of the X-ray absorption by the warm absorber, characterizd by the K-edges of the intermediately ionized oxygen, OVII and OVIII. We confirm that the column density of OVIII changes on a timescale of

Supermirror hard-x-ray telescope

\sim 10^4

Suzaku Observations of the Hard X-ray Variability of MCG-6-30-15: the Effects of Strong Gravity Around a Kerr Black Hole

~s when the X-ray continuum flux decreases. The significant anti-correlation of column density with continuum flux gives direct evidence that the warm absorber is photoionized by the X-ray continuum. From the timescale of the variation of the OVIII column density, we estimate that it originates from gas within a radius of about

Discovery of a prominent cyclotron absorption feature from the transient X-ray pulsar X0331+53

10^{17}\cm

The X-Ray Spectral Variability of Markarian 766

of the central engine. In contrast, the depth of the OVII edge shows no response to the continuum flux, which indicates that it originates in gas at larger radii. Our results strongly suggest that there are two warm absorbing regions; one located near or within the Broad Line Region, the other associated with the outer molecular torus, scattering medium or Narrow Line Region.

Revealing the High Energy Emission from the Obscured Seyfert Galaxy MCG -5-23-16 with Suzaku

The practical use of a grazing x-ray telescope is demonstrated for hard-x-ray imaging as hard as 40 keV by means of a depth-graded d-spacing multilayer, a so-called supermirror. Platinum-carbon multilayers of 26 layer pairs in three blocks with a different periodic length d of 3-5 nm were designed to enhance the reflectivity in the energy range from 24 to 36 keV at a grazing angle of 0.3 deg. The multilayers were deposited on thin-replica-foil mirrors by a magnetron dc sputtering system. The reflectivity was measured to be 25%-30% in this energy range; 20 mirror shells thus deposited were assembled into the tightly nested grazing-incidence telescope. The focused hard-x-ray image was observed with a newly developed position-sensitive CdZnTe solid-state detector. The angular resolution of this telescope was found to be 2.4 arc min in the half-power diameter.

Characterization of the Advanced Satellite for Cosmology and Astrophysics x-ray telescope: preflight calibration and ray tracing

Suzaku has, for the first time, enabled the hard X-ray variability of the Seyfert 1 galaxy MCG-6-30-15 to be measured. The variability in the 14-45 keV band, which is dominated by a strong reflection hump, is quenched relative to that at a few keV. This directly demonstrates that the whole reflection spectrum is much less variable than the power-law continuum. The broadband spectral variability can be decomposed into two components - a highly variable power-law and constant reflection - as previously inferred from other observations in the 2-10 keV band. The strong reflection and high iron abundance give rise to a strong broad iron line, which requires the inner disc radius to be at about 2 gravitational radii. Our results are consistent with the predictions of the light bending model which invokes the very strong gravitational effects expected very close to a rapidly spinning black hole.

An ASCA Observation of M51 (NGC 5194): Iron K Emission from an Obscured Active Galactic Nucleus

A remarkable absorption feature at 28.5 keV, attributable to electron cyclotron resonance, has been discovered in the 1.9-60-keV X-ray spectrum of the recurrent transient X-ray pulsar X0331 + 53. The observed resonance energy implies a neutron star surface magnetic field of 2.5(1 + z) x 10 to the 12th G, where z is the gravitational redshift. The detection was made with the Ginga observatory in October 1989, during an outburst of this transient with a flux level of about 0.3 Crab. The feature is very deep and has been resolved with excellent statistics. This is the fourth unambiguous detection of cyclotron resonant scattering features from X-ray pulsars, suggesting that these features are a common phenomenon among these objects. An empirical relation found between the cyclotron resonance energy and the spectral cutoff energy suggests that the magnetic field strengths of the known X-ray pulsars are clustered in a range (1-4) x 10 to the 12th G. 30 refs.