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H. Awaki

Publications and source records attributed to H. Awaki.

27 records · Page 2Linked to original sources
arXiv2003

Discovery of Bright Variable X-ray Sources in NGC 1569 with Chandra

From the analysis of a ~100 ks Chandra observation of the dwarf starburst galaxy NGC 1569, we have found that the X-ray point sources, CXOU 043048.1+645050 and CXOU 043048.6+645058, showed significant time variability. During this observation, the X-ray flux of CXOU 043048.1+645050 increased by 10 times in only 2 x 10^4 s. Since the spectrum in its bright phase was fitted with a disk blackbody model with kT_in ~0.43 keV and the bolometric luminosity is L_bol ~10^38 ergs s^-1, this source is an X-ray binary with a stellar mass black-hole. Since the spectrum in its faint phase was also fitted with a disk blackbody model, the time variability can be explained by a change of the accretion rate onto the black hole. The other variable source, CXOU 043048.6+645058, had a flat spectrum with a photon index of ~1.6. This source may be an X-ray binary with an X-ray luminosity of several x 10^37 ergs s^-1. In addition, three other weak sources showed possible time variability. Taking all of the variability into account may suggest an abundant population of compact X-ray sources in NGC 1569.

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arXiv2002

Extreme X-ray Iron Lines in Active Galactic Nuclei

We analyze X-ray spectra of heavily obscured (N_H > 10^{24} cm^{-2}) active galaxies obtained with Chandra, concentrating on the iron K alpha fluorescence line. We measure very large equivalent widths in most cases, up to 5 keV in the most extreme example. The geometry of an obscuring torus of material near the active galactic nucleus (AGN) determines the Fe emission, which we model as a function of torus opening angle, viewing angle, and optical depth. The starburst/AGN composite galaxies in this sample require small opening angles. Starburst/AGN composite galaxies in general therefore present few direct lines of sight to their central engines. These composite galaxies are common, and their large covering fractions and heavy obscuration effectively hide their intrinsically bright X-ray continua. While few distant obscured AGNs have been identified, we propose to exploit their signature large Fe K alpha equivalent widths to find more examples in X-ray surveys.

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arXiv2000

Discovery of a Luminous, Variable, Off-Center Source in the Nucleus of M82 with the Chandra HRC

We present results from observations of the most famous starburst galaxy M82 with the High-Resolution Camera onboard the Chandra X-Ray Observatory. We found nine sources in the central 1' x 1' region, but no source was detected at the galactic center. Comparing the observations on 1999 October 28 and on 2000 January 20, we found four of the nine sources showed significant time variability. In particular, CXOM82J095550.2+694047, which is 9" away from the galactic center, showed extremely large time variability. We conclude that this source is the origin of the hard X-ray time variability of M82 detected with ASCA. Assuming a spectral shape obtained by the ASCA observation, its luminosity in the 0.5 -- 10 keV band changed from 1.2E40 erg/s on 1999 October 28 to 8.7E40 erg/s on 2000 January 20.

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arXiv1999

A search for broad infrared recombination lines in NGC 1068

We report infrared spectroscopy of the prototypical Seyfert 2 galaxy NGC 1068, aiming at detection of broad components of hydrogen recombination lines that originate in the obscured broad-line region. Using the Short Wavelength Spectrometer on board the Infrared Space Observatory, we have observed for the first time the regions of Brackett beta 2.626um and Pfund alpha 7.460um, and present improved data for Brackett alpha 4.052um. No significant broad components are detected, implying an equivalent visual extinction to the broad-line region of at least 50 magnitudes and an obscuring column density of at least 10^23 cm^-2. While consistent with a highly obscured broad-line region, as required by the classical unified scenario, these limits are not yet significant enough to discriminate strongly between different torus models or to constrain properties of the gas causing the very large X-ray obscuration. We discuss the systematic limitations of infrared broad-line region searches and suggest that Brackett alpha may often be the most favorable transition for future searches.

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arXiv1999

An RXTE Observation of NGC 6300: a new bright Compton reflection Dominated Seyfert 2 Galaxy

Scanning and pointed RXTE observations of the nearby Seyfert 2 galaxy NGC 6300 reveal that it is a source of hard X-ray continuum and large equivalent width Fe K$α$ emission. These properties are characteristic of Compton-reflection dominated Seyfert 2 galaxies. The continuum can be modeled as Compton-reflection; subsolar iron abundance is required and a high inclination preferred. However, the poor energy resolution of RXTE means that this description is not unique, and the continuum can also be modeled using a ``dual absorber'', i.e. a sum of absorbed power laws. Observations with higher energy resolution detectors will cleanly discriminate between these two models. Optical observations support the Compton-reflection dominated interpretation as $L_X/L_{[OIII]}$ is low. NGC 6300 is notable because with $F_{2-10}\approx 6.4 \times 10^{-12} ergs cm^{-2} s^{-1}$, it is the second brightest such object known.

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arXiv1999

Broad Band X-Ray Observations of the Narrow Line X-Ray Galaxy NGC 5506

We present a detailed analysis of broad band X-ray data of the Seyfert 2 galaxy NGC5506. 2-10 keV band are detected during a 1-day ASCA observation, while no significant change in the 2-10 keV continuum shape is found. The ASCA spectrum consists of an absorbed power-law, a 'soft excess' below 2 keV, and an Fe K$α$ emission line at 6.4 keV. The 'soft excess' can be well described by either thermal emission from very low abundance material at a temperature kT$\simeq$0.8 keV, or scattered/leaking flux from the primary power-law plus a small amount of thermal emission. Analysis of ROSAT HRI data reveals that the soft X-ray emission is extended on kpc scales in this object, and the extended component may account for most of the soft X-ray excess observed by the ASCA. The result suggests that in this type 2 AGN, the 'soft excess' at least partly comes from an extended region, imposing serious problem for the model in which the source is partially covered. Fe K$α$ profile is complex and can not be satisfactorily modeled by a single gaussian. Models of either double gaussians, or a narrow gaussian plus a line from a relativistic accretion disk viewed at an inclination of about 40$\pm10^\circ$ provide good fits to the data. However, the inclination of the disk can be substantially larger if there is a small amount of excessive Fe K edge absorption. The intermediate inclinations for NLXGs are consistent with the ideas that the inner accretion disk is aligned with the outer obscuring torus.

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arXiv1997

On the nature of the X-ray absorption in the Seyfert 2 galaxy NGC 4507

We present results of the ASCA observation of the Seyfert 2 galaxy NGC 4507. The 0.5-10 keV spectrum is rather complex and consists of several components: (1) a hard X-ray power law heavily absorbed by a column density of about 3 10^23 cm^-2, (2) a narrow Fe Kalpha line at 6.4 keV, (3) soft continuum emission well above the extrapolation of the absorbed hard power law, (4) a narrow emission line at about 0.9 keV. The line energy, consistent with highly ionized Neon (NeIX), may indicate that the soft X-ray emission derives from a combination of resonant scattering and fluorescence in a photoionized gas. Some contribution to the soft X-ray spectrum from thermal emission, as a blend of Fe L lines, by a starburst component in the host galaxy cannot be ruled out with the present data.

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arXiv1996

ASCA PV observations of the Seyfert 2 galaxy NGC 4388: the obscured nucleus and its X-ray emission

We present results on the Seyfert 2 galaxy NGC4388 in the Virgo cluster observed with ASCA during its PV phase. The 0.5-10 keV X-ray spectrum consists of multiple components; (1) a continuum component heavily absorbed by a column density NH = 4E23 cm-2 above 3 keV; (2) a strong 6.4 keV line (EW = 500 eV); (3) a weak flat continuum between 1 and 3 keV; and (4) excess soft X-ray emission below 1 keV. The detection of strong absorption for the hard X-ray component is firm evidence for an obscured active nucleus in this Seyfert 2 galaxy. The absorption corrected X-ray luminosity is about 2E42 erg/s. This is the first time that the fluorescent iron-K line has been detected in this object. The flat spectrum in the intermediate energy range may be a scattered continuum from the central source. The soft X-ray emission below 1 keV can be thermal emission from a temperature kT = 0.5 keV, consistent with the spatially extended emission observed by ROSAT HRI. However, the low abundance (0.05 Zs) and high mass flow rate required for the thermal model and an iron-K line stronger than expected from the obscuring torus model are puzzling. An alternative consistent solution can be obtained if the central source was a hundred times more luminous over than a thousand years ago. All the X-ray emission below 3 keV is then scattered radiation.

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