30,437 research outputs found
ROSAT PSPC observations of Cygnus-A : X-ray spectra of the cooling flow and hot spots
We present a {\it ROSAT} Position Sensitive Proportional Counter (PSPC)
observation of the powerful radio galaxy Cygnus-A. The X-ray emission in the
{\it ROSAT} band is dominated by thermal emission from the hot intracluster
medium of the associated cluster. Image deprojection confirms the existence of
a significant cluster cooling flow with total mass deposition rate of and a (Hubble time) cooling radius of \,kpc. Spectral data show the gradient in the emission-weighted mean
temperature with the temperature decreasing towards the centre of the cluster.
We also find signatures of the radio source: in particular, we detect the X-ray
emission from the western radio hot spot previously found by the {\it ROSAT}
High Resolution Imager (HRI). We find the emission from the hot spot to be hard
and discuss the physical implications of this result.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Special relativistic effects on the strength of the fluorescent K-alpha iron line from black hole accretion disks
The broad iron K emission line, commonly seen in the X-ray spectrum
of Seyfert nuclei, is thought to originate when the inner accretion disk is
illuminated by an active disk-corona. We show that relative motion between the
disk and the X-ray emitting material can have an important influence on the
observed equivalent width (EW) of this line via special relativistic aberration
and Doppler effects. We suggest this may be relevant to understanding why the
observed EW often exceeds the prediction of the standard X-ray reflection
model. Several observational tests are suggested that could disentangle these
special relativistic effects from iron abundance effects.Comment: 5 pages, 3 postscript figures. Accepted for publication in MNRAS
(pink pages). Also available at
http://rocinante.Colorado.EDU/~chris/html_papers/special/special.htm
An RXTE study of M87 and the core of the Virgo cluster
We present hard X-ray observations of the nearby radio galaxy M87 and the
core of the Virgo cluster using the Rossi X-ray Timing Explorer. These are the
first hard X-ray observations of M87 not affected by contamination from the
nearby Seyfert 2 galaxy NGC4388. Thermal emission from Virgo's intracluster
medium is clearly detected and has a spectrum indicative of kT=2.5keV plasma
with approximately 25% cosmic abundances. No non-thermal (power-law) emission
from M87 is detected in the hard X-ray band, with fluctuations in the Cosmic
X-ray Background being the limiting factor. Combining with ROSAT data, we infer
that the X-ray spectrum of the M87 core and jet must be steep (Gamma_core>1.90$
and Gamma_jet>1.75), and we discuss the implications of this result. In
particular, these results are consistent with M87 being a mis-aligned BL-Lac
object.Comment: 8 pages, 2 postscript figures. Accepted for publication in Ap
Are the Nuclei of Seyfert 2 Galaxies Viewed Face-On?
We show from modeling the Fe Kalpha line in the ASCA spectra of four X-ray
bright narrow emission line galaxies (Seyfert types 1.9 and 2) that two equally
viable physical models can describe the observed line profile. The first is
discussed by Turner et al. (1998) and consists of emission from a nearly
pole-on accretion disk. The second, which is statistically preferred, is a
superposition of emission from an accretion disk viewed at an intermediate
inclination of about 48 degrees and a distinct, unresolved feature that
presumably originates some distance from the galaxy nucleus. The intermediate
inclination is entirely consistent with unified schemes and our findings
challenge recent assertions that Seyfert 2 galaxies are preferentially viewed
with their inner regions face-on. We derive mean equivalent widths for the
narrow and disk lines of =60 eV and = 213 eV, respectively. The
X-ray data are well described by a geometry in which our view of the active
nucleus intersects and is blocked by the outer edges of the obscuring torus,
and therefore do not require severe misalignments between the accretion disk
and the torus.Comment: 19 pages, 3 postscript figures. Accepted for publication in ApJ
An extended multi-zone model for the MCG-6-30-15 warm absorber
The variable warm absorber seen with {\em ASCA} in the X-ray spectrum of
MCG6-30-15 shows complex time behaviour in which the optical depth of OVIII
anticorrelates with the flux whereas that of OVII is unchanging. The
explanation in terms of a two zone absorber has since been challenged by {\em
BeppoSAX} observations. These present a more complicated behaviour for the
OVIII edge. We demonstrate here that the presence of a third, intermediate,
zone can explain all the observations. In practice, warm absorbers are likely
to be extended, multi-zone regions of which only part causes directly
observable absorption edges at any given time.Comment: 5 pages, 6 figures, accepted for publication in MNRA
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