1,283 research outputs found
ASCA spectroscopy of the luminous infrared galaxy NGC6240: X-ray emission from a starburst and a buried active nucleus
We present an X-ray spectral study of the prototype far-infrared galaxy
NGC6240 from ASCA. The soft X-ray spectrum (below 2 keV) shows clear signatures
of thermal emission well described with a multi-temperature optically-thin
plasma, which probably originates in a powerful starburst. Strong hard X-ray
emission is also detected with ASCA and its spectrum above 3 keV is extremely
flat with a prominent iron K line complex, very similar to that seen in the
Seyfert 2 galaxy NGC1068 but about an order of magnitude more luminous
[L(3-10keV)=1.4E42 erg/s]. The hard X-ray spectrum indicates that only
reflected X-rays of an active galactic nucleus (AGN) buried in a heavy
obscuration [N(H)>2E24 cm-2] are visible. This is evidence for an AGN in
NGC6240 emitting possibly at a quasar luminosity and suggests its significant
contribution to the far-infrared luminosity.Comment: 9 pages, 6 Postscript figures, to appear in MNRA
Chandra detection of reflected X-ray emission from the type 2 QSO in IRAS 09104+4109
We present X-ray imaging spectroscopy of the extremely luminous infrared
galaxy IRAS 09104+4109 (z=0.442) obtained with the Chandra X-ray Observatory.
With the arcsec resolution of Chandra, an unresolved source at the nucleus is
separated from the surrounding cluster emission. A strong iron K line at 6.4
keV on a very hard continuum is detected from the nuclear source, rendering
IRAS 09104+4109 the most distant reflection-dominated X-ray source known.
Combined with the BeppoSAX detection of the excess hard X-ray emission, it
provides further strong support to the presence of a hidden X-ray source of
quasar luminosity in this infrared galaxy. Also seen is a faint linear
structure to the North, which coincides with the main radio jet. An X-ray
deficit in the corresponding region suggests an interaction between the cluster
medium and the jet driven by the active nucleus.Comment: 5 pages, accepted for publication as a Letter in MNRA
ASCA and ROSAT observations of NGC5548: discrepant spectral indices
We report on simultaneous ASCA and ROSAT observations of the Seyfert galaxy
NGC5548 made during the ASCA Performance Verification phase. Spectral features
due to a warm absorber and reflection are clearly seen in the X-ray spectra. We
find that the continuum spectral shape differs between the ASCA and ROSAT
datasets. The photon-index obtained from the ROSAT PSPC exceeds that from the
ASCA SIS about 0.4. The discrepancy is clear even in the 0.5-2 keV energy band
over which both detectors are sensitive. The spectra cannot be made consistent
by choosing a more complex model. The problem likely lies in the response curve
(estimated effective area) of one, or both, detectors. There may be important
consequences for a wide range of published results.Comment: 9 pages, accepted for publication in MNRA
The iron K line complex in NGC1068: implications for X-ray reflection in the nucleus
We report a new analysis of ASCA data on the iron K line complex in NGC1068.
The line complex basically consists of three components, as previously
reported. A weak red wing of the 6.4 keV fluoresence iron K line is found. A
plausible explanation is Compton scattering in optically thick, cold matter
which can be identified with an obscuring torus or cold gas in the host galaxy.
We also show that this `Compton shoulder' should be observable with ASCA using
a simulated reflection spectrum. In order to explain the two higher energy
lines as well as the cold 6.4 keV line, we fit the ASCA data with a composite
model of cold and warm reflection. This shows that cold reflection dominates
the observed X-ray emission above 4 keV. The two higher energy lines have large
equivalent width with respect to the warm-scattered continuum, suggesting that
efficient resonant scattering operates. The line energies are systematically
lower than those expected from resonant lines for FeXXV and FeXXVI by 100 eV.
The redshifts may be due to either the ionized gas of the warm mirror receding
at a radial velocity of 4000-5000 km/s, or effects of Compton scattering in a
complicated geometry.Comment: 7 pages, 4 figures, to appear in MNRA
Evidence for an intermediate mass black hole and a multi-zone warm absorber in NGC 4395
We report on the results of an analysis in the X-ray band of a recent long
ASCA observation of NGC 4395, the most variable low-luminosity AGN known. A
relativistically-broadened iron line at ~6.4 keV is clearly resolved in the
time-averaged spectrum, with an equivalent width of 310^{+70}_{-90} eV.
Time-resolved spectral analysis of the heavily absorbed soft X-ray band
confirms the existence of a variable, multi-zone warm absorber in this source,
as proposed in a previous analysis of a shorter ASCA observation. The light
curve of the source is wildly variable on timescales of hours or less, and a
factor of nearly 10 change in count-rate was recorded in a period of less than
2000 s. The long observation and variability of the source allowed the power
density spectrum (PDS) to be constructed to an unprecedented level of detail.
There is evidence for a break in the PDS from a slope of \alpha~1 to \alpha~1.8
at a frequency of around 3 \times 10^{-4} Hz. The central black hole mass of
NGC 4395 is estimated to be approximately 10^4-10^5 solar masses using the
break in the PDS, a result consistent with previous analyses using optical and
kinematical techniques.Comment: 8 pages, 6 figures, accepted for publication in MNRA
X-ray absorption in the strong FeII narrow-line Seyfert 1 galaxy Mrk 507
We present results from spectral analysis of ASCA data on the NLS1 Mrk 507.
This galaxy was found to have an exceptionally flat ROSAT spectrum among the
NLS1s. The ASCA spectrum however shows a clear absorption feature in the energy
band below 2 keV, which partly accounts for the flat spectrum observed with the
ROSAT PSPC. The absorption is mainly due to cold gas with a column density of
(2-3)E21 cm-2. A reanalysis of the PSPC data indicates that the absorber is
slightly ionized, covers only part of the central source, or there is extra
soft thermal emission from an extended region. There is also evidence that the
X-ray absorption is complex; an edge feature marginally detected at 0.84 keV
suggests the presence of an additional high ionization absorber which imposes a
strong OVIII edge on the spectrum. After correction for the absorption, the
photon index of the intrinsic continuum, 1.8, obtained from the ASCA data is
quite similar to that of ordinary Seyfert 1 galaxies. Mrk 507 still has one of
the flattest continuum slopes among NLS1, but is no longer exceptional. The
strong optical FeII emission remains unusual in the light of the correlation
between FeII strengths and steepness of soft X-ray slope.Comment: 6 pages, 3 Postscript figures, to be published in MNRA
A Chandra observation of the H2O megamaser IC2560
A short Chandra ACIS-S observation of the Seyfert 2 galaxy IC 2560, which
hosts a luminous nuclear water megamaser, shows: 1) the X-ray emission is
extended; 2) the X-ray spectrum shows emission features in the soft (E<2 keV)
X-ray band; this is the major component of the extended emission; and 3) a very
strong (EW~3.6 keV) iron K line at 6.4 keV on a flat continuum. This last
feature clearly indicates that the X-ray source is hidden behind Compton-thick
obscuration, so that the intrinsic hard X-ray luminosity must be much higher
than observed, probably close to ~3e42 erg/s. We briefly discuss the
implications for powering of the maser emission and the central source.Comment: 5 pages, MNRAS in pres
Do nuclear starbursts obscure the X-ray background?
We propose a model for the source of the X-ray background (XRB) in which low
luminosity active nuclei (L<10^43 erg/s) are obscured by nuclear starbursts
within the inner 100pc. The obscuring material covers most of the sky as seen
from the central source, rather than being distributed in a toroidal structure,
and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is
turbulent with velocity dispersion of a few 100 km/s and cloud-cloud collisions
lead to copious star formation. Although supernovae tend to produce outflows,
most of the gas is trapped in the gravity field of the starforming cluster
itself and the central black hole. A hot (T=10^6-10^7 K) virialised phase of
this gas, comprising a few per cent of the total obscuring material, feeds the
central engine of 10^7 solar masses through Bondi accretion, at a sub-Eddington
rate appropriate for the luminosity of these objects. If starburst-obscured
objects give rise to the residual XRB, then only 10 per cent of the accretion
in active galaxies occurs close to the Eddington limit in unabsorbed objects.Comment: 5 pages, 2 PS figures included in the text, MNRAS in the press. Also
at http://www.ifca.unican.es/~barcons/preprints.htm
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