589 research outputs found
ASCA Observation of the Low-Luminosity Seyfert 1.5 Galaxy NGC 5033
We present the results of an ASCA observation of the low-luminosity Seyfert
1.5 galaxy NGC 5033. A point-like X-ray source with a luminosity of 2.3x10^{41}
erg s^{-1} in the 2--10 keV band (at 18.7 Mpc; Tully 1988, AAA045.002.054) was
detected at the nucleus. The X-ray light curve shows variability on a timescale
of ~10^4 s with an amplitude of ~20%. The X-ray continuum is represented by a
weakly absorbed (N_H~9x10^{20} {cm^{-2}) power-law with a photon index of
1.72+/-0.04, which is quite similar to Seyfert 1 galaxies with higher
luminosities. A Fe Kalpha emission line is detected at 6.40^{+0.08}_{-0.06} keV
(redshift corrected) and the equivalent width is 290+/-100 eV. The line width
is unresolved. The narrower line width and larger equivalent width compared to
Seyfert 1s imply that fluorescent Fe Kalpha emission from matter further out
from the center than the accretion disk significantly contributes to the
observed Fe Kalpha line. We suggest that fluorescent Fe Kalpha emission from
the putative torus contributes to the observed Fe Kalpha line.Comment: 17 pages, To appear in PASJ, Vol. 51, No.
The X-Ray Spectral Variability of Mrk 766
Analysis results from ASCA and ROSAT observations of the narrow-line Seyfert
1 galaxy Mrk 766 are reported. In the ASCA observation we observed rapid
variability with a doubling time scale of 1000 seconds. A spectral variability
event was observed in which the spectrum softened and hardened above and below
~1 keV, respectively, as the flux increased. The spectra could be modeled with
5 components: a power law, warm absorber, iron K(alpha) line and soft excess
component flux. The spectral variability resulted from a highly significant
change in the intrinsic photon law index from Gamma ~1.6 to ~2.0, an increase
in the warm absorber ionization, and a marginally significant decrease in the
soft component normalization. A ~100 eV equivalent width narrow iron K(alpha)
line was detected in the high state spectrum. Spectral hardening during flux
increases was observed in three ROSAT observations.
The change in intrinsic photon index and disappearance of the soft excess
component in the ASCA spectra can be explained as a transition from a first
order pair reprocessed spectrum to a pair cascade brought about by a sudden
increase in the injected electron Lorentz factor. The change in the ionization
of the warm absorber, though model dependent, could correspond to the increase
in flux at the oxygen edges resulting from the spectral index change. The ROSAT
spectral variability can be interpreted by variable intensity hard power law
and a relatively nonvarying soft component, possibly primary disk emission.
These results are compared with those reported from other narrow-line Seyfert 1
galaxies.Comment: 29 pages using (AASTeX) aaspp4.sty and 18 Postscript figures. To
appear in the September 1, 1996, issue of The Astrophysical Journa
Detection of an X-ray periodicity in the Seyfert galaxy IRAS18325-5926
We report the detection of a 58 ks (16 hr) periodicity in the 0.5-10 keV
X-ray light curve of the Seyfert galaxy IRAS18325-5926 (Fairall49), obtained
from a 5-day ASCA observation. Nearly 9 cycles of the periodic variation are
seen; it shows no strong energy dependence and has an amplitude of about 15 per
cent. Unlike most other well-studied Seyfert galaxies, there is no evidence for
strong power-law red noise in the X-ray power spectrum of IRAS18325-5926.
Scaling from the QPOs found in Galactic black hole candidates suggests that the
mass of the black hole in IRAS18325-5926 is (6-40) million solar masses.Comment: 5 pages, 4 Postscript figures, to be published in MNRA
Similarity In Phase Diagrams Between Ionic And Nonionic Surfactant Solutions At Constant Temperature
The variable OVIII 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 ~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 10^{17}\cm 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.Comment: 8 pages (including figures) uuencoded gziped PS file. Submitted to
Publications of the Astronomical Society of Japa
ASCA PV observations of the Seyfert 1 galaxy MCG-6-30-15 : rapid variability of the warm absorber
We present a detailed re-analysis of the two {\it ASCA} Performance
Verification observations of the nearby Seyfert 1 galaxy MCG-6-30-15.
Confirming the results of Fabian et al. (1994), we find definite evidence for
the {\sc O\,vii} and {\sc O\,viii} K-shell absorption edges of the warm
absorber and a doubling of the warm absorber column density within the 3 weeks
separating the two observations. No intra-day {\it flux-correlated} variability
of the warm absorber is found. However, we report the discovery of an `event'
in which the warm absorber parameters temporarily change for
\sim10\,000\thinspace s before returning to their original values. Possible
interpretations are discussed but a contradiction remains: the constancy of the
ionization state of the warm absorber argues that it lies at large distances
from the central source whereas the short term change in column density argues
for small distances. Fluorescent iron emission is examined. As found by Fabian
et al. (1994), the iron line is broad and strong (equivalent width
\sim300\thinspace eV). The line profile is also suggestive of it being skewed.
Such a line would be expected from a relativistic accretion disk. We also find
very rapid primary X-ray variability. Assuming relativistic beaming to be
unimportant, the derived efficiency is comparable to the maximum obtainable
from accretion onto a Schwarzschild black hole. Correlated variability outside
of the energy range of {\it ASCA} might exceed this maximum, thus requiring
efficient accretion onto a Kerr hole.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
The Infocus Hard X-ray Telescope: Pixellated CZT Detector/Shield Performance and Flight Results
The CZT detector on the Infocus hard X-ray telescope is a pixellated
solid-state device capable of imaging spectroscopy by measuring the position
and energy of each incoming photon. The detector sits at the focal point of an
8m focal length multilayered grazing incidence X-ray mirror which has
significant effective area between 20--40 keV. The detector has an energy
resolution of 4.0keV at 32keV, and the Infocus telescope has an angular
resolution of 2.2 arcminute and a field of view of about 10 arcminutes. Infocus
flew on a balloon mission in July 2001 and observed Cygnus X-1. We present
results from laboratory testing of the detector to measure the uniformity of
response across the detector, to determine the spectral resolution, and to
perform a simple noise decomposition. We also present a hard X-ray spectrum and
image of Cygnus X-1, and measurements of the hard X-ray CZT background obtained
with the SWIN detector on Infocus.Comment: To appear in the proceedings of the SPIE conference "Astronomical
Telescopes and Instrumentation", #4851-116, Kona, Hawaii, Aug. 22-28, 2002.
12 pages, 9 figure
ASCA Observations of the Sgr B2 Cloud: An X-Ray Reflection Nebula
We present the ASCA results of imaging spectroscopy of the giant molecular
cloud Sgr B2. The X-ray spectrum is found to be very peculiar; it exhibits a
strong emission line at 6.4 keV, a low energy cutoff below about 4 keV and a
pronounced edge-structure at 7.1 keV. The X-ray image is extended and its peak
position is shifted from the core of the molecular cloud toward the Galactic
center by about 1--2 arcminute. The X-ray spectrum and the morphology are well
reproduced by a scenario that X-rays from an external source located in the
Galactic center direction are scattered by the molecular cloud Sgr B2, and come
into our line of sight. Thus Sgr B2 may be called an X-ray reflection nebula.
Possible implications of the Galactic center activity related to this unique
source are presented.Comment: 15 pages, 8 figures, AAS LaTeX, To be published in The Astrophysical
Journa
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