71 research outputs found
Excess Hard X-ray Emission from the Obscured Low Luminosity AGN In the Nearby Galaxy M 51 (NGC 5194)
We observed the nearby galaxy M~51 (NGC 5194) with BeppoSAX. The X-ray
properties of the nucleus below 10 keV are almost the same as the ASCA results
regarding the hard component and the neutral Fe K line, but the
intensity is about half of the ASCA 1993 data. Beyond this, in the BeppoSAX PDS
data, we detected a bright hard X-ray emission component which dominates above
10 keV. The 10 -- 100 keV flux and luminosity of this component are
respectively erg s cm and erg
s. These are about 10 times higher than the extrapolation from the soft
X-ray band, and similar to the flux observed with Ginga, which found a bright
power law component in 2 -- 20 keV band. Considering other wavelength
properties and the X-ray luminosity, together with strong neutral Fe K line,
the hard X-ray emission most likely arises from a low luminosity active
nucleus, which is obscured with a column density of cm.
This suggests that hidden low luminosity AGNs may well be present in other
nearby galaxies. We interpret the discrepancy between Ginga and other X-ray
satellites to be due to a large variability of absorption column density toward
the line of sight over several years, suggesting that the Compton thick
absorption material may be present on a spatial scale of a parsec. Apart from
the nucleus, several ultra-luminous off-nuclear X-ray sources detected in M~51
exhibit long-term time variability, suggesting the state transition similar to
that observed in Galactic black hole candidates.Comment: 18 pages, 8 figures, accepted for A&
The Declined Activity in the Nucleus of NGC 1316
NGC 1316 (Fornax A) is a radio galaxy with prototypical double lobes, where
the magnetic field intensity is accurately measured via the inverse-Compton
technique. The radio-emitting electrons in the lobes are inferred to have a
synchrotron life time of 0.1 Gyr. Considering the lobe energetics, we estimate
the past nuclear X-ray luminosity of NGC 1316 to be at least 4 times 10^{34} W
(4 times 10^{41} erg s^{-1}). Thus, the nucleus was rather active at least 0.1
Gyr ago. In contrast, we confirmed with ASCA and ROSAT that the nucleus of NGC
1316 is very faint in X-rays at present, with the 2--10 keV luminosity of any
AGN-like hard component being < 2 times 10^{33} W (2 times 10^{40} erg s^{-1})
even assuming a nuclear obscuration up to 10^{28} m^{-2} (10^{24} cm^{-2}).
This is at least an order of magnitude lower than the estimated past activity,
indicating that the nucleus is presently very inactive. From these two results,
we conclude that the nucleus of NGC 1316 has become dormant during the last 0.1
Gyr. This suggests the possible abundance of ``dormant'' quasars in nearby
galaxies.Comment: 7 pages, 2 figures, to be published in the Astrophysical Journal
Letter
Extreme X-ray spectral variability in the Seyfert 2 Galaxy NGC 1365
We present multiple Chandra and XMM-Newton observations of the type 1.8
Seyfert Galaxy NGC 1365, which shows the most dramatic X-ray spectral changes
observed so far in an AGN: the source switched from reflection dominated to
transmission dominated and back in just 6 weeks. During this time the soft
thermal component, arising from a ~1 kpc region around the center, remained
constant. The reflection component is constant at all timescales, and its high
flux relative to the primary component implies the presence of thick gas
covering a large fraction of the solid angle. The presence of this gas, and the
fast variability time scale, suggest that the Compton-thick to Compton thin
change is due to variation in the line-of-sight absorber, rather than to
extreme intrinsic emission variability. We discuss a structure of the
circumnuclear absorber/reflector which can explain the observed X-ray spectral
and temporal properties.Comment: 12 pages, 2 figures, accepted for publication in ApJ Letter
Detection of an iron K Emission Line from the LINER NGC 4579
We present the results of an ASCA observation of the LINER NGC 4579. A
point-like X-ray source is detected at the nucleus with a 2-10 keV luminosity
of 1.5x10^41 ergs/s assuming a distance of 16.8 Mpc. The X-ray spectrum is
represented by a combination of a power-law with a photon index of ~1.7 and
soft thermal component with kT~0.9 keV. An iron K emission line is detected at
6.73+/-0.13 keV (rest frame) with an equivalent width of 490 +180/-190 eV and
is statistically significant at more than 99.9 % confidence. The line center
energy is consistent with Helium-like iron and is significantly higher than 6.4
keV which is expected from fluorescence by "cold" (or a lower ionization state
of) iron. The iron line profile shows no significant red tail in contrast to
Seyfert 1 galaxies although the statistics are limited. The line center energy,
equivalent width, and profile are consistent with an origin in an ionized
accretion disk. However the large mass accretion rate necessary to ionize the
accretion disk is not consistent with the observed luminosity and normal
accretion models.Comment: 15 pages, 5 figures, to appear in The Astrophysical Journa
X-ray Properties of LINERs and Low-luminosity Seyfert Galaxies Observed with ASCA. I. Observations and Results
This paper presents a comprehensive study of the X-ray properties of
low-ionization nuclear emission-line regions (LINERs) and low-luminosity
Seyfert galaxies based on observations obtained with ASCA. We analyzed data of
53 observations of 21 LINERs and 17 low-luminosity Seyferts. X-ray emission has
been detected in all but one object. The X-ray luminosities in the 2-10 keV
band range from 4e39 ergs/s to 5e41 ergs/s, which are 1-3 orders of magnitude
smaller than in classical Seyfert galaxies. The X-ray spectra of most objects
are well described by a canonical model which consists of (1) a soft component
from a thermal plasma with kT<1 keV and (2) a hard component represented by a
power law with a photon index~1.8 or thermal bremsstrahlung emission with kT~10
keV. Several objects do not require the soft thermal component, and their
continua are well fitted by a single power-law model. Some objects show heavy
absorption with column densities in excess of 1e23 cm^-2. We detect in several
objects Fe K line emission with equivalent widths ranging from 50 eV to 2 keV.
Variability on timescales less than a day is uncommon in our sample. By
comparing multiple observations made with ASCA or with published observations
from other satellites, we show that at least eight objects are variable on
timescales of a week to several years. We find that the morphologies of many
objects, both in the soft and hard bands, are consistent with being pointlike
relative to the telescope PSF; a few are clearly extended in either or both
energy bands.Comment: 38 pages, To appear in The Astrophysical Journal Supplements., v13
A Chandra detection of diffuse hard X-ray emission associated with the lobes of the radio galaxy 3C 452
An 80 ksec Chandra ACIS observation of the radio galaxy 3C 452 is reported. A
diffuse X-ray emission associated with the lobes has been detected with high
statistical significance, together with the X-ray nucleus of the host galaxy.
The 0.5--5 keV ACIS spectrum of the diffuse emission is described by a
two-component model, consisting of a soft thermal plasma emission from the host
galaxy halo and a hard non-thermal power-law component. The hard component is
ascribed to the inverse Comptonization of cosmic microwave background photons
by the synchrotron emitting electrons in the lobes, because its spectral energy
index, 0.68+-0.28, is consistent with the radio synchrotron index, 0.78. These
results reveal a significant electron dominance in the lobes. The electrons are
inferred to have a relatively uniform distribution, while the magnetic field is
compressed toward the lobe periphery.Comment: 4 figures, 2 tables, Accepted by ApJL (to appear in the December 1
issue
Interferometric 12CO(J=2-1) image of the Nuclear Region of Seyfert 1 Galaxy NGC 1097
We have mapped the central region of the Seyfert 1 galaxy NGC 1097 in
12CO(J=2-1) with the Submillieter Array (SMA). The 12CO(J=2-1) map shows a
central concentration and a surrounding ring, which coincide respectively with
the Seyfert nucleus and a starburst ring. The line intensity peaks at the
nucleus, whereas in a previously published 12CO(J=1-0) map the intensity peaks
at the starburst ring. The molecular ring has an azimuthally averaged
12CO(J=2-1)/(J=1-0) intensity ratio (R21) of about unity, which is similar to
those in nearby active star forming galaxies, suggesting that most of the
molecular mass in the ring is involved in fueling the starburst. The molecular
gas can last for only about 1.2\times10^8 years without further replenishment
assuming a constant star formation rate and a perfect conversion of gas to
stars. The velocity map shows that the central molecular gas is rotating with
the molecular ring in the same direction, while its velocity gradient is much
steeper than that of the ring. This velocity gradient of the central gas is
similar to what is usually observed in some Seyfert 2 galaxies. To view the
active nucleus directly in the optical, the central molecular gas structure can
either be a low-inclined disk or torus but not too low to be less massive than
the mass of the host galaxy itself, be a highly-inclined thin disk or clumpy
and thick torus, or be an inner part of the galactic disk. The R21 value of
~1.9 of the central molecular gas component, which is significantly higher than
the value found at the molecular gas ring, indicates that the activity of the
Seyfert nucleus may have a significant influence on the conditions of the
molecular gas in the central component.Comment: 22 pages, 4 figures, accepted by Ap
Poststarburst Models of LINERs
Since the discovery of low-ionization nuclear emission-line regions in many
galaxies (LINERs), it has been recognized that they constitute a class of
active galactic nuclei (AGNs) which are thought to be powered by gas accretion
onto a central, supermassive black hole. LINERs are observed in approximately
one third of galaxies in the local universe and it has been often thought that
they harbor an AGN-like central engine with moderate activity. However, some
LINERs show no direct evidence for AGNs such as broad emission lines, radio
jets, hard X-ray emission, spectral energy distributions which are inconsistent
with starlight, and so on. For such LINERs (a subset of type 2 LINERs), we
present new poststarburst models which explain some of their most important
optical narrow emission-line ratios. In these models, the ionization sources
are planetary nebula nuclei (PNNs) with temperature of ~ 10^5 K which appear in
the late-phase evolution of intermediate-mass stars with mass between ~ 3
M_solar and ~ 6 M_solar. Such PNNs left in a typical starburst nucleus can
produce an Halpha luminosity of L(Halpha) ~ 10^38 ergs s^-1 for typical
poststarburst LINERs and ~ 10^39 ergs s^-1 only in exceptionally bright cases.
The PNN phase lasts until the death of the lowest-mass stars formed in the
starburst, which is ~ 5 x 10^8 yr for an assumed lower limit of the initial
mass function of 3 M_solar. This long duration appears consistent with the
observed higher frequency of occurrence of LINERs if every galaxy could
experience the starburst activity several times in its life. We therefore
propose that some LINERs which show no direct evidence for AGNs may be
poststarburst nuclei powered by a cluster of PNNs.Comment: 8 pages, 6 figures, emulateapj.sty; To appear in the September 2000
issue of the Astronomical Journa
Heavy X-ray Absorption in Soft X-ray Weak Active Galactic Nuclei
Recent ROSAT studies have identified a significant population of Active
Galactic Nuclei (AGN) that are notably faint in soft X-rays relative to their
optical fluxes. Are these AGN intrinsically X-ray weak or are they just highly
absorbed? Brandt, Laor & Wills have systematically examined the optical and UV
spectral properties of a well-defined sample of these soft X-ray weak (SXW) AGN
drawn from the Boroson & Green sample of all the Palomar Green AGN with z<0.5.
We present ASCA observations of three of these SXW AGN: PG 1011-040, PG
1535+547 (Mrk 486), and PG 2112+059. In general, our ASCA observations support
the intrinsic absorption scenario for explaining soft X-ray weakness; both PG
1535+547 and PG 2112+059 show significant column densities (N_H~10^22-10^23
cm^-2) of absorbing gas. Interestingly, PG 1011-040 shows no spectral evidence
for X-ray absorption. The weak X-ray emission may result from very strong
absorption of a partially covered source, or this AGN may be intrinsically
X-ray weak. PG 2112+059 is a Broad Absorption Line (BAL) QSO, and we find it to
have the highest X-ray flux known of this class. It shows a typical power-law
X-ray continuum above 3 keV; this is the first direct evidence that BAL QSOs
indeed have normal X-ray continua underlying their intrinsic absorption.
Finally, marked variability between the ROSAT and ASCA observations of PG
1535+547 and PG 2112+059 suggests that the soft X-ray weak designation may be
transient, and multi-epoch 0.1-10.0 keV X-ray observations are required to
constrain variability of the absorber and continuum.Comment: 30 pages, 10 figures, LaTeX, uses aaspp4.sty; submitted to Ap
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