1,227 research outputs found
Einstein observations of active galaxies and quasars
The radio galaxies Centaurus A and Signus B are discussed. In both these sources, a comparison of the radio and imaged X-ray flux is allowed for the measurement of the magnetic fields. Einstein observations of quasars are discussed. The number of known X-ray emitting QSO's was increased from 3 to 22 and the distances where these QSO's were seen to correspond to an age of 15 billion years. It was shown that these quasars contributed significantly to the X-ray background
The Elusive Active Nucleus of NGC 4945
We present new HST NICMOS observations of NGC 4945, a starburst galaxy
hosting a highly obscured active nucleus that is one of the brightest
extragalactic sources at 100 keV. The HST data are complemented with ground
based [FeII] line and mid--IR observations. A 100pc-scale starburst ring is
detected in Pa alpha, while H_2 traces the walls of a super bubble opened by
supernova-driven winds. The conically shaped cavity is particularly prominent
in Pa alpha equivalent width and in the Pa alpha/H_2 ratio. Continuum images
are heavily affected by dust extinction and the nucleus of the galaxy is
located in a highly reddened region with an elongated, disk-like morphology. No
manifestation of the active nucleus is found, neither a strong point source nor
dilution in CO stellar features, which are expected tracers of AGN activity.
Even if no AGN traces are detected in the near-IR, with the currently available
data it is still not possible to establish whether the bolometric luminosity of
the object is powered by the AGN or by the starburst: we demonstrate that the
two scenarios constitute equally viable alternatives. However, the absence of
any signature other than in the hard X-rays implies that, in both scenarios,
the AGN is non-standard: if it dominates, it must be obscured in all
directions, conversely, if the starburst dominates, the AGN must lack UV
photons with respect to X-rays. An important conclusion is that powerful AGNs
can be hidden even at mid-infrared wavelengths and, therefore, the nature of
luminous dusty galaxies cannot be always characterized by long-wavelength data
alone but must be complemented with sensitive hard X-ray observations.Comment: Accepted for publication in A&A, high quality color pictures
available at http://www.arcetri.astro.it/~marconi/colpic.htm
Prevalence of X-ray variability in the Chandra Deep Field South
We studied the X-ray variability of sources detected in the Chandra Deep
Field South (Giacconi et al. 2002), nearly all of which are low to moderate z
AGN (Tozzi et al. 2001). We find that 45% of the sources with >100 counts
exhibit significant variability on timescales ranging from a day up to a year.
The fraction of sources found to be variable increases with observed flux,
suggesting that >90% of all AGNs possess intrinsic variability. We also find
that the fraction of variable sources appears to decrease with increasing
intrinsic absorption; a lack of variability in hard, absorbed AGNs could be due
to an increased contribution of reflected X-rays to the total flux. We do not
detect significant spectral variability in the majority (~70%) of our sources.
In half of the remaining 30%, the hardness ratio is anti-correlated with flux,
mimicking the high/soft-low/hard states of galactic sources. The X-ray
variability appears anti-correlated with the luminosity of the sources, in
agreement with previous studies. High redshift sources, however, have larger
variability amplitudes than expected from extrapolations of their low-z
counterparts, suggesting a possible evolution in the accretion rate and/or size
of the X-ray emitting region. Finally, we discuss some effects that may produce
the observed decrease in the fraction of variable sources from z=0.5 out to
z=2.Comment: 24 pages, including 15 figures and 1 table. In press on Ap
ROSAT HRI observations of Centaurus A
We present results from a sensitive high-resolution X-ray observation of the
nearby active galaxy Centaurus A (NGC 5128) with the ROSAT HRI. The 65~ksec
X-ray image clearly distinguishes different components of the X-ray emission
from Cen A: the nucleus and the jet, the diffuse galaxy halo, and a number of
individual sources associated with the galaxy. The luminosity of the nucleus
increased by a factor of two compared to an earlier ROSAT observation in 1990.
The high spatial resolution of the ROSAT HRI shows that most of the knots in
the jet are extended both along and perpendicular to the jet axis. We report
the detection of a new X-ray feature, at the opposite side of the X-ray jet
which is probably due to compression of hot interstellar gas by the expanding
southwestern inner radio lobe.Comment: To be published in Astrophys. Journal Letters. 4 pages, 3 plate
Hubble Space Telescope Imaging in the Chandra Deep Field South: III. Quantitative Morphology of the 1Ms Chandra Counterparts and Comparison with the Field Population
We present quantitative morphological analyses of 37 HST/WFPC2 counterparts
of X-ray sources in the 1 Ms Chandra Deep Field-South (CDFS). We investigate:
1) 1-D surface brightness profiles via isophotal ellipse fitting; 2) 2-D, PSF-
convolved, bulge+disk+nucleus profile-fitting; 3) asymmetry and concentration
indices compared with all ~3000 sources in our three WFPC2 fields; and 4) near-
neighbor analyses comparing local environments of X-ray sources versus the
field control sample. Significant nuclear point-source optical components
appear in roughly half of the resolved HST/WFPC2 counterparts, showing a narrow
range of F_X/F_{opt,nuc} consistent with the several HST-unresolved X-ray
sources (putative type-1 AGN) in our fields. We infer roughly half of the
HST/WFPC2 counterparts host unobscured AGN, which suggests no steep decline in
the type-1/type-2 ratio out to the redshifts z~0.5-1 typical of our sources.
The concentration indices of the CDFS counterparts are clearly larger on
average than those of the field distribution, at 5-sigma, suggesting that the
strong correlation between central black hole mass and host galaxy properties
(including concentration index) observed in nearby galaxies is already evident
by z~0.5-1. By contrast, the asymmetry index distribution of the 21 resolved
CDFS sources at I<23 is indistinguishable from the I<23 field. Moreover, the
frequency of I<23 near neighbors around the CDFS counterparts is not
significantly different from the field sample. These results, combined with
previous similar findings for local samples, suggest that recent merger/
interaction history is not a good indicator of AGN activity over a substantial
range of look-back time.Comment: 30 pages, incl. 8 figures; accepted for publication in the
Astrophysical Journa
Conformal Symmetry and the Three Point Function for the Gravitational Axial Anomaly
This work presents a first study of a radiative calculation for the
gravitational axial anomaly in the massless Abelian Higgs model. The two loop
contribution to the anomalous correlation function of one axial current and two
energy-momentum tensors, , is computed
at an order that involves only internal matter fields. Conformal properties of
massless field theories are used in order to perform the Feynman diagram
calculations in the coordinate space representation. The two loop contribution
is found not to vanish, due to the presence of two independent tensor
structures in the anomalous correlator.Comment: 34 pages, 5 figures, RevTex, Minor changes, Final version for Phys.
Rev.
Jet driven motions in the Narrow Line region of NGC1068
We have obtained HST FOC f/48 long-slit spectroscopy of the inner 4" of the
Narrow Line Region of NGC 1068 between 3500-5400A with a spectral resolution of
1.78A/pixel. At a spatial scale of 0.0287" per pixel these data provide an
order of magnitude improvement in resolution over previous ground based spectra
and allow us to trace the interaction between the radio jet and the gas in the
NLR. Our results show that, within +/-0.5" of the radio-jet the emission lines
are kinematically disturbed and split into two components whose velocity
separation is 1500 km/sec. The filaments associated with the radio lobe also
show a redshifted kinematic disturbance of the order of 300 km/sec which
probably is a consequence of the expansion of the radio plasma.
Furthermore, the material enveloping the radio-jet is in a much higher
ionization state than that of the surrounding NLR gas. The highest excitation
is coincident with the jet axis where emission in the coronal line of [FeVII]
3769A is detected and the HeII 4686A is strong but where [OII] 3727A is
depressed. This large localized increase in ionization on the jet axis is
accompanied by the presence of an excess continuum. Because the electron
density is substantially larger in the jet compared to the surrounding NLR,
these results can only be explained if there is a more intense ionizing
continuum associated with the jet. This can be accomplished in a variety of
ways which include an intrinsically anisotropic nuclear radiation field, a
reduced gas covering factor or the presence of a local ionization source.
The morphology, kinematics and, possibly, the ionization structure of the NLR
in the vicinity of the jet of NGC 1068 are a direct consequence of the
interaction with the radio outflow.Comment: 11 pages, 5 figures, ApJ Letters in pres
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