205 research outputs found
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.Comment: 15 pages including 4 Postscript figures; accepted for publication in
ApJ
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.Comment: 8 pages, LateX, 5 figures (included). Uses mn.sty and epsfig.sty. To
appear in MNRA
Spectral Statistics and Luminosity Function of a Hard X-ray Complete Sample of Brightest AGNs
We investigated the statistics of the X-ray spectral properties of a complete
flux-limited sample of bright AGNs from HEAO-1 all-sky catalogs to provide the
bright end constraint of the evolution of AGN hard X-ray luminosity function
(HXLF) and the AGN population synthesis model of the X-ray background. Spectral
studies have been made using ASCA and XMM-Newton observation data for almost
all AGNs in this sample.Comment: PTPTex v0.88, 2 pages with 4 figures, Proceedings of the
"Stellar-Mass, Intermediate -Masss, and Supermassive Black Holes" in Kyoto,
Japa
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.Comment: 9 pages, 5 Postscript figures, to be published in MNRA
Effects of tidal interactions on the gas flows of elliptical galaxies
During a Hubble time, cluster galaxies may undergo several mutual encounters
close enough to gravitationally perturb their hot, X-ray emitting gas flows. We
ran several 2D, time dependent hydrodynamical models to investigate the effects
of such perturbations on the gas flow inside elliptical galaxies. In
particular, we studied in detail the modifications occurring in the scenario
proposed by D'Ercole et al. (1989), in which the galactic interstellar medium
produced by the aging galactic stellar population, is heated by SNIa at a
decreasing rate. We find that, although the tidal interaction in our models
lasts less than 1 Gyr, its effect extends over several Gyrs. The tidally
induced turbulent flows create dense filaments which cool quickly and accrete
onto the galactic center, producing large spikes in the global Lx. Once this
mechanism starts, it is fed by gravity and amplified by SNIa. In cooling flow
models without supernovae the amplitude of the Lx fluctuations due to the tidal
interaction is substantially reduced. We conclude that, if SNIa significantly
contribute to the energetics of the gas flows in ellipticals, then the observed
spread in the Lx-Lb diagram may be caused, at least in part, by this mechanism.
On the contrary, tidal interactions cannot be responsible for the observed
spread if the pure cooling flow scenario applies (abridged).Comment: 21 pages, 8 figures, to be published in ApJ (main journal
A variability study of the Seyfert 2 galaxy NGC 6300 with XMM-Newton
We present the results of timing analysis of the XMM-Newton observation of
the Seyfert 2 galaxy NGC 6300. The hard X-ray spectrum above 2 keV consists of
a Compton-thin-absorbed power law, as is often seen in Seyfert 2 galaxies. We
clearly detected rapid time variability on a time scale of about 1000 s from
the light curve above 2 keV. The excess variance of the time variability
(sigma2_RMS) is calculated to be ~0.12, and the periodogram of the light curve
is well represented by a power law function with a slope of 1.75. In contrast
with previous results from Seyfert 2 nuclei, these variability characteristics
are consistent with those of Seyfert 1 galaxies. This consistency suggests that
NGC 6300 has a similar black hole mass and accretion properties as Seyfert 1
galaxies. Using the relation between time variability and central black hole
mass by Hayashida et al. (1998), the black hole mass of NGC 6300 is estimated
to be ~2.8x10^5 Mo. Taking uncertainty of this method into account, the black
hole mass is less than 10^7 Mo. Taking the bolometric luminosity of 3.3x10^43
erg/s into consideration, this yields an accretion rate of > 0.03 of the
Eddington value, and comparable with estimates from Seyfert 1 galaxies using
this method. The time variability analysis suggests that NGC 6300 actually has
a Seyfert 1 nucleus obscured by a thick matter, and more generally provides a
new pillar of support for the unified model of Seyfert galaxies based on
obscuration.Comment: 11 pages, 6 figures, accepted for publication in Ap
XMM-Newton Observations of NGC 507: Super-solar Metal Abundances in the Hot ISM
We present the results of the X-ray XMM-Newton observations of NGC 507, a
dominant elliptical galaxy in a small group of galaxies, and report
'super-solar' metal abundances of both Fe and a-elements in the hot ISM of this
galaxy. We find Z_Fe = 2-3 times solar inside the D25 ellipse of NGC 507. This
is the highest Z_Fe reported so far for the hot halo of an elliptical galaxy;
this high Iron abundance is fully consistent with the predictions of stellar
evolution models, which include the yield of both type II and Ia supernovae.
The spatially resolved, high quality XMM spectra provide enough statistics to
formally require at least three emission components: two soft thermal
components indicating a range of temperatures in the hot ISM, plus a harder
component, consistent with the integrated output of low mass X-ray binaries
(LMXBs). The abundance of a-elements (most accurately determined by Si) is also
found to be super-solar. The a-elements to Fe abundance ratio is close to the
solar ratio, suggesting that ~70% of the Iron mass in the hot ISM was
originated from SNe Type Ia. The a-element to Fe abundance ratio remains
constant out to at least 100 kpc, indicating that SNe Type II and Ia ejecta are
well mixed in a scale much larger than the extent of the stellar body.Comment: 29 pages, 6 figures, Accepted in ApJ (v613, Oct. 1, 2004); Minor
revisions after referee's comments; A high-resolution pdf file available at
http://hea-www.harvard.edu/~kim/pap/N507_XMM.pd
Decoupled and inhomogeneous gas flows in S0 galaxies
A recent analysis of the "Einstein" sample of early-type galaxies has
revealed that at any fixed optical luminosity Lb S0 galaxies have lower mean
X-ray luminosity Lx per unit Lb than ellipticals. Following a previous
analytical investigation of this problem (Ciotti & Pellegrini 1996), we have
performed 2D numerical simulations of the gas flows inside S0 galaxies in order
to ascertain the effectiveness of rotation and/or galaxy flattening in reducing
the Lx/Lb ratio. The flow in models without SNIa heating is considerably
ordered, and essentially all the gas lost by the stars is cooled and
accumulated in the galaxy center. If rotation is present, the cold material
settles in a disk on the galactic equatorial plane. Models with a time
decreasing SNIa heating host gas flows that can be much more complex. After an
initial wind phase, gas flows in energetically strongly bound galaxies tend to
reverse to inflows. This occurs in the polar regions, while the disk is still
in the outflow phase. In this phase of strong decoupling, cold filaments are
created at the interface between inflowing and outflowing gas. Models with more
realistic values of the dynamical quantities are preferentially found in the
wind phase with respect to their spherical counterparts of equal Lb. The
resulting Lx of this class of models is lower than in spherical models with the
same Lb and SNIa heating. At variance with cooling flow models, rotation is
shown to have only a marginal effect in this reduction, while the flattening is
one of the driving parameters for such underluminosity, in accordance with the
analytical investigation.Comment: 32 pages LaTex file, plus 5 .ps figures and macro aasms4.sty --
Accepted on Ap
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