706 research outputs found
A powerful and highly variable off-nuclear X-ray source in the composite starburst/Seyfert 2 galaxy NGC 4945
We report on a powerful and variable off-nuclear X-ray source in the nearby
spiral galaxy NGC 4945. Two ROSAT PSPC observations show the source to brighten
in 0.5--2.0 keV flux by a factor of about 9 on a time-scale of 11 months or
less. It is seen by ASCA about one month after the second PSPC pointing, and is
seen to have dimmed by a factor of > 7 in a ROSAT HRI pointing about one year
after the second PSPC pointing. Its maximum observed 0.8--2.5 keV luminosity is
about 8E38 erg/s, making it brighter than any known persistent X-ray binary in
the Milky Way. Its total X-ray luminosity is probably larger than 1.2E39 erg/s.
The observed variability argues against a superbubble interpretation, and the
off-nuclear position argues against a low-luminosity active galactic nucleus.
The source is therefore probably either an ultra-powerful X-ray binary or an
ultra-powerful supernova remnant. Optical monitoring has not identified any
supernovae in NGC 4945 during the time of the X-ray observations, and any
supernova would have had to have been either very highly absorbed or
intrinsically optically faint.Comment: 5 pages, uuencoded compressed tar file, MNRAS in pres
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
ROSAT PSPC and Hri Observations of the Composite Starburst/Seyfert 2 Galaxy NGC 1672
The nearby barred spiral galaxy NGC 1672 is thought to have a weak Seyfert
nucleus in addition to its strong starburst activity. Observations with the
PSPC and HRI instruments on board the ROSAT X-ray satellite show that three
X-ray sources with luminosities (1--2)\times 10^{40} erg/s are clearly
identified with NGC 1672. The strongest X-ray source lies at the nucleus, and
the other two lie near the ends of the prominent bar, locations that are also
bright in H-alpha and near-infrared images. The nuclear source is resolved by
the HRI on about the scale of the recently identified nuclear ring, and one of
the sources at the ends of the bar is also probably resolved. The X-ray
spectrum of the nuclear source is quite soft, having a Raymond--Smith plasma
temperature of about 0.7 keV and little evidence for intrinsic absorption. The
ROSAT band X-ray flux of the nuclear source appears to be dominated not by
X-ray binary emission but rather by diffuse gas emission. While the properties
of the nuclear source are generally supportive of a superbubble interpretation,
its large density and emission measure stretch the limits that can be
comfortably accommodated by such models. We do not detect direct emission from
the putative Seyfert nucleus, although an alternative model for the nuclear
source is thermal emission from gas that is photoionized by a hidden Seyfert
nucleus. The spectra of the other two X-ray sources are harder than that of the
nuclear source, and superbubble models for them have the same strengths and
weaknesses.Comment: 11 pages, uuencoded compressed postscript, MNRAS in pres
AXJ1749+684: a narrow emission-line galaxy with a flat X-ray spectrum
We report the serendipitous detection of an X-ray source, AXJ1749+684, with
the ASCA Gas Imaging Spectrometer. AXJ1749+684 is identified with a
LINER/starburst-type spiral galaxy KUG 1750+683A at a redshift z = 0.05. It has
a hard X-ray spectrum, consistent with that of the X-ray background (XRB) in
the 1-10 keV band. Despite the optical classification, the X-ray luminosity
cannot be explained by starburst activity. Combined with spatial variations in
the optical emission line ratios, this suggests the presence of an obscured
Seyfert nucleus embedded within a starforming galaxy. Similar behaviour could
explain the ambiguous properties of the faint narrow-line X-ray galaxies
(NLXGs) emerging from deep X-ray surveys.Comment: MNRAS Letters in press, 6 pages, 7 figures in MNRAS LaTex styl
Iron Fluorescent Line Emission from Black Hole Accretion Disks with Magnetic Reconnection-Heated Corona
We investigate the iron K fluorescent line produced by hard X-ray
photons from magnetic reconnection-heated corona. The hot corona with
temperature being about K can irradiate the underlying disk with a
continuum X-ray spectrum produced via thermal Comptonization. Then the iron
atoms in the disk photoelectrically absorb X-ray photons and radiate K
line photons. Therefore, the activity of corona is responsible to the iron line
emission from the underlying disk. In previous studies, oversimplified X-ray
photon sources are often assumed above the disk in order to compute the iron
line profile or power-law line emissivity profiles are assumed with an index
being a free parameter. We adopt the more realistic corona model constructed by
Liu et al. in which the corona is heated by magnetic energy released through
the reconnection of magnetic flux loops and which has no free parameter. Then
the accretion energy is dominantly dissipated in the corona, in which X-ray
photons are efficiently produced and irradiate the underlying disk. We find the
local emmisivity of iron line on the disk is approximated as . The iron line profiles derived from this model
give excellent fits to the observational data of MCG-6-30-15 with the profiles
derived theoretically for for energy band 4-7keV. Possible
origins of line variability are briefly discussed.Comment: 17 pages, 7 figures, accepted for publication in Ap
An RXTE Observation of the Seyfert 1 Galaxy MCG-6-30-15 : X-ray Reflection and the Iron Abundance
We report on a 50 ks observation of the bright Seyfert 1 galaxy MCG--6-30-15
with the Rossi X-ray Timing Explorer. The data clearly show the broad
fluorescent iron line (equivalent width ~ 250 eV), and the Compton reflection
continuum at higher energies. A comparison of the iron line and the reflection
continuum has enabled us to constrain reflective fraction and the elemental
abundances in the accretion disk. Temporal studies provide evidence that
spectral variability is due to changes in both the amount of reflection seen
and the properties of the primary X-ray source itself.Comment: 6 pages, Late
Diffraction-limited Subaru imaging of M82: sharp mid-infrared view of the starburst core
We present new imaging at 12.81 and 11.7 microns of the central ~40"x30"
(~0.7x0.5 kpc) of the starburst galaxy M82. The observations were carried out
with the COMICS mid-infrared (mid-IR) imager on the 8.2m Subaru telescope, and
are diffraction-limited at an angular resolution of <0".4. The images show
extensive diffuse structures, including a 7"-long linear chimney-like feature
and another resembling the edges of a ruptured bubble. This is the clearest
view to date of the base of the kpc-scale dusty wind known in this galaxy.
These structures do not extrapolate to a single central point, implying
multiple ejection sites for the dust. In general, the distribution of dust
probed in the mid-IR anticorrelates with the locations of massive star clusters
that appear in the near-infrared. The 10-21 micron mid-IR emission,
spatially-integrated over the field of view, may be represented by hot dust
with temperature of ~160 K. Most discrete sources are found to have extended
morphologies. Several radio HII regions are identified for the first time in
the mid-IR. The only potential radio supernova remnant to have a mid-IR
counterpart is a source which has previously also been suggested to be a weak
active galactic nucleus. This source has an X-ray counterpart in Chandra data
which appears prominently above 3 keV and is best described as a hot (~2.6 keV)
absorbed thermal plasma with a 6.7 keV Fe K emission line, in addition to a
weaker and cooler thermal component. The mid-IR detection is consistent with
the presence of strong [NeII]12.81um line emission. The broad-band source
properties are complex, but the X-ray spectra do not support the active
galactic nucleus hypothesis. We discuss possible interpretations regarding the
nature of this source.Comment: Accepted for publication in PASJ Subaru special issue. High
resolution version available temporarily at
http://www.astro.isas.jaxa.jp/~pgandhi/pgandhi_m82.pd
Description of Pseudo-Newtonian Potential for the Relativistic Accretion Disk around Kerr Black Holes
We present a pseudo-Newtonian potential for accretion disk modeling around
the rotating black holes. This potential can describe the general relativistic
effects on accretion disk. As the inclusion of rotation in a proper way is very
important at an inner edge of disk the potential is derived from the Kerr
metric. This potential can reproduce all the essential properties of general
relativity within 10% error even for rapidly rotating black holes.Comment: 5 Latex pages including 1 figure. Version to appear in Astrophysical
Journal, V-581, N-1, December 10, 200
The ASCA spectrum of the z=4.72 blazar, GB 1428+4217
The X-ray luminous quasar GB 1428+4217 at redshift 4.72 has been observed
with ASCA. The observed 0.5-10 keV flux is 3.2E-12 erg/s/cm2. We report here on
the intrinsic 4-57 keV X-ray spectrum, which is very flat (photon index of
1.29). We find no evidence for flux variability within the ASCA dataset or
between it and ROSAT data. We show that the overall spectral energy
distribution of GB 1428+4217 is similar to that of lower redshift MeV blazars
and present models which fit the available data. The Doppler beaming factor is
likely to be at least 8. We speculate on the number density of such high
redshift blazars, which must contain rapidly-formed massive black holes.Comment: 5 pages, 3 Postscript figures, to appear in MNRA
First Constraints on Iron Abundance versus Reflection Fraction from the Seyfert~1 Galaxy MCG--6-30-15
We report on a joint ASCA and RXTE observation spanning an 400~ks time
interval of the bright Seyfert~1 galaxy MCG--6-30-15. The data clearly confirm
the presence of a broad skewed iron line ( 266 eV) and
Compton reflection continuum at higher energies reported in our previous paper.
We also investigate whether the gravitational and Doppler effects that affect
the iron line may also be manifest in the reflected continuum. The uniqueness
of this data set is underlined by the extremely good statistics that we obtain
from the approximately four million photons that make up the 2-20 keV RXTE PCA
spectrum alone. This, coupled with the high energy coverage of HEXTE and the
spectral resolution of ASCA in the iron line regime has allowed us to constrain
the relationship between abundance and reflection fraction for the first time
at the 99 per cent confidence level. The reflection fraction is entirely
consistent with a flat disk, i.e. the cold material subtends sr at
the source, to an accuracy of 20 per cent. Monte Carlo simulations show that
the observed strong iron line intensity is explained by an overabundance of
iron by a factor of 2 and an underabundance of the lower-Z elements by a
similar factor. By considering non-standard abundances, a clear and consistent
picture can be made in which both the iron line and reflection continuum come
from the same material such as e.g. an accretion disk.Comment: 8 pages, 8 figures, accepted for publication MNRAS 7/9
- …