110 research outputs found
The Radial Structure of the Cygnus Loop Supernova Remnant --- Possible evidence of a cavity explosion ---
We observed the North-East (NE) Limb toward the center region of the Cygnus
Loop with the ASCA Observatory. We found a radial variation of electron
temperature (kTe) and ionization timescale (log(\tau)) whereas no variation
could be found for the abundances of heavy elements. In this paper, we
re-analyzed the same data set and new observations with the latest calibration
files. Then we constructed the precise spatial variations of kTe, log(\tau),
and abundances of O, Ne, Mg, Si, and Fe over the field of view (FOV). We found
a spatial variation not only in kTe and in log(\tau) but also in most of heavy
elements. As described in Miyata et al. (1994), values of kTe increase and
those of log(\tau) decrease toward the inner region. We found that the
abundance of heavy elements increases toward the inner region. The radial
profiles of O, Ne, and Fe show clear jump structures at a radius of 0.9 Rs,
where Rs is the shock radius. Outside of 0.9 Rs, abundances of all elements are
constant. On the contrary, inside of 0.9 Rs, abundances of these elements are
20--30 % larger than those obtained outside of 0.9 Rs. The radial profile of
kTe also shows the jump structure at 0.9 Rs. This means that the hot and metal
rich plasma fills the volume inside of 0.9 Rs. We concluded that this jump
structure was the possible evidence for the pre-existing cavity produced by the
precursor. If the ejecta fills inside of 0.9 Rs, the total mass of the ejecta
was roughly 4\Msun. We then estimated the main-sequence mass to be roughly
15\Msun, which supports the massive star in origin of the Cygnus Loop supernova
remnant and the existence of a pre-existing cavity.Comment: 37 pages, 14 figures. Accepted for publication of Ap
ASCA Observations of the Starburst-Driven Superwind Galaxy NGC 2146: Broad Band (0.6 - 9 keV) Spectral Properties
We report ASCA GIS and SIS observations of the nearby (D = 11.6 Mpc), nearly
edge-on, starburst galaxy NGC 2146. These X-ray spectral data complement ROSAT
PSPC and HRI imaging discussed by Armus et al., 1995. The broad band (0.6-9
keV) X-ray spectrum of NGC 2146 is best described by a two component model: the
soft X-ray emission with a Raymond-Smith thermal plasma model having a
temperature of kT keV; the hard X-ray emission with a thermal plasma
model having kT keV or a power-law model having a photon index of
. We do not find compelling evidence of substantial excess absorption
above the Galactic value. The soft (hard) thermal component provides about 30%
(70%) of the total luminosity in the 0.5 - 2.0 keV energy band, while in the
2-10 keV energy range only the hard component plays a major role. The spectral
results allow us to set tighter constraints on the starburst-driven superwind
model, which we show can satisfactorily account for the luminosity, mass, and
energy content represented by the soft X-ray spectral component. We estimate
that the mass outflow rate ( 9 M per year) is about an order of
magnitude greater than the predicted rate at which supernovae and stellar winds
return mass into the interstellar medium and, therefore, argue that the flow is
strongly "mass-loaded" with material in and around the starburst. The estimated
outflow velocity of the hot gas is close to the escape velocity from the
galaxy, so the fate of the gas is not clear. We suggest that the hard X-ray
spectral component is due to the combined emission of X-ray binaries and/or
young supernovae remnants associated with the starburst.Comment: 26 pages plus 4 figures, LaTex manuscript, Accepted for publication
in the Astrophysical Journa
ASCA and contemporaneous ground-based observations of the BL Lacertae objects 1749+096 and 2200+420 (BL Lac)
We present ASCA observations of the radio-selected BL Lacertae objects
1749+096 (z=0.32) and 2200+420 (BL Lac, z=0.069) performed in 1995 Sept and
Nov, respectively. The ASCA spectra of both sources can be described as a first
approximation by a power law with photon index Gamma ~ 2. This is flatter than
for most X-ray-selected BL Lacs observed with ASCA, in agreement with the
predictions of current blazar unification models. While 1749+096 exhibits
tentative evidence for spectral flattening at low energies, a concave continuum
is detected for 2200+420: the steep low-energy component is consistent the
high-energy tail of the synchrotron emission responsible for the longer
wavelengths, while the harder tail at higher energies is the onset of the
Compton component. The spectral energy distributions from radio to gamma-rays
are consistent with synchrotron-self Compton emission from a single homogeneous
region shortward of the IR/optical wavelengths, with a second component in the
radio domain related to a more extended emission region. For 2200+420,
comparing the 1995 Nov state with the optical/GeV flare of 1997 July, we find
that models requiring inverse Compton scattering of external photons provide a
viable mechanism for the production of the highest (GeV) energies during the
flare. An increase of the external radiation density and of the power injected
in the jet can reproduce the flat gamma-ray continuum observed in 1997 July. A
directly testable prediction of this model is that the line luminosity in
2200+420 should vary shortly after (~1 month) a non-thermal synchrotron flare.Comment: 28 pages,6 figures, 5 tables; LaTeX document. accepted for
publication in the Astrophysical Journa
The X-ray spectra and spectral variability of intermediate type Seyfert galaxies: ASCA observations of NGC 4388 and ESO 103-G35
The X-ray spectra of two intermediate type Seyfert galaxies are investigated
using ASCA observations separated by more than a year. Both NGC 4388 and ESO
103-G35 exhibit strong, narrow Fe K alpha line emission and absorption by cold
neutral gas with a column density ~ 10^23 cm^-2, characteristic of the X-ray
spectra of type 2 Seyfert galaxies. The power law continuum flux has changed by
a factor of 2 over a time-scale of ~ 2 years for both objects, declining in the
case of NGC 4388 and rising in ESO 103-G35. No variation was observed in the
equivalent width of the Fe K alpha line in the spectra of NGC 4388, implying
that the line flux declined with the continuum. We find that the strength of
the line cannot be accounted for by fluorescence in line-of-sight material with
the measured column density unless a `leaky-absorber' model of the type favored
for IRAS 04575-7537 is employed. The equivalent width of the Fe K alpha
emission line is seen to decrease between the observations of ESO 103-G35 while
the continuum flux increased. The 1996 observation of ESO 103-G35 can also be
fitted with an absorption edge at 7.4 0.2 keV due to partially ionized
iron, and when an ionized absorber model is fitted to the data it is found that
the equivalent column of neutral hydrogen rises to 3.5 x 10^23 cm^-2. The Fe K
alpha line flux can be accounted by fluorescence in this material alone and
this model is also a good representation of the 1988 and 1991 Ginga
observations. There is then no requirement for a reflection component in the
ASCA spectra of ESO 103-G35 or NGC 4388.Comment: 45 pages, 5 tables, 11 figures. Accepted for publication in the
Astrophysical Journa
The aperiodic variability of Cyg X-1 and GRS 1915 +105 at very low frequencies
We have carried out a timing analysis of Cyg X-1 and GRS 1915+105 using more
than 5 years worth of data from the All Sky Monitor (ASM) on board RXTE. We
have obtained for the first time power density spectra, colour-colour and
colour-intensity plots, cross-correlation functions and phase-lag diagrams to
investigate the variability of the sources at frequencies < 10^{-5} Hz. We find
that the PDS are not flat but consistent with a power-law of index ~-1. This is
the same slope as that found in the power spectra of Cyg X-1 during the soft
state. In fact, the power spectrum of Cyg X-1 in the frequency range
10^{-7}-10^{-5} Hz appears as a continuation of the 10^{-3}-10 Hz power
spectrum during the soft state. Significant variability, expressed as the
fractional rms in the frequency range 2 x 10^{-7}-1 x 10^{-5} Hz, is detected
at a level of 27% and 21% for GRS 1915+105 and Cyg X-1, respectively. Our
results confirm previous suggestions that the innermost regions of the
accretion disc are affected by variations occurring at larger radii from the
black hole, presumably due to changes in the mass accretion rate. We also
observe strong spectral changes: in Cyg X--1 the spectrum softens as the flux
increases when it is in the low/hard state, and hardens with increasing flux in
the high/soft state. GRS 1915+105 follows the same trend as Cyg X-1 in the soft
state, but the hardness ratios show a larger amplitude of variation. The
cross-correlation shows that the variations occur simultaneously in all energy
bands. However, the cross-correlation functions are asymmetric toward negative
lags. The phase spectrum also reveals negative lags at periods larger than a
few days, suggesting that the long term variations in the soft energy band are
delayed with respect to the harder bands.Comment: 8 pages, 9 figures, accepted for publication in A&
A Study of the Populations of X-ray Sources in the Small Magellanic Cloud with ASCA
The Advanced Satellite for Cosmology and Astrophysics (ASCA) has made
multiple observations of the Small Magellanic Cloud (SMC). X-ray mosaic images
in the soft (0.7--2.0 keV) and hard (2.0--7.0 keV) bands are separately
constructed, and the latter provides the first hard X-ray view of the SMC. We
extract 39 sources from the two-band images with a criterion of S/N>5, and
conduct timing and spectral analyses for all of these sources. Coherent
pulsations are detected from 12 X-ray sources; five of which are new
discoveries. Most of the 12 X-ray pulsars are found to exhibit long-term flux
variabilities, hence they are likely to be X-ray binary pulsars (XBPs). On the
other hand, we classify four supernova remnants (SNRs) as thermal SNRs, because
their spectra exhibit emission lines from highly ionized atoms. We find that
XBPs and thermal SNRs in the SMC can be clearly separated by their hardness
ratio (the ratio of the count rate between the hard and soft bands). Using this
empirical grouping, we find many XBP candidates in the SMC, although no
pulsations have yet been detected from these sources. Possible implications on
the star-formation history and evolution of the SMC are presented by a
comparison of the source populations in the SMC and our Galaxy.Comment: 11 pages, 39 Figures, to be published in ApJ Supplement. Tables (body
and figures also) are available at
http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job
ASCA X-ray source catalogue in the Galactic Center region
The ASCA satellite made 107 pointing observations on a 5 x 5 deg^2 region
around the center of our Milky Way Galaxy (the Galactic Center) from 1993 to
1999. In the X-ray images of the 0.7--3 keV or 3--10 keV bands, we found 52
point sources and a dozen diffuse sources. All the point sources are uniformly
fitted with an absorbed power-law model. For selected bright sources, Sgr A*,
AX J1745.6-2901, A 1742-294, SLX 1744-300, GRO J1744-28, SLX 1737-282, GRS
1734-292, AX J1749.2-2725, KS 1741-293, GRS 1741.9-2853, and an unusual flare
source XTE J1739-302, we present further detailed spectral and timing analyses,
and discuss their nature.
The dozen extended X-ray sources comprise radio supernova remnants, giant
molecular clouds, and some new discoveries. Most show emission lines from
either highly ionized atoms or low-ionized irons.
The X-ray spectra were fitted with either a thin thermal or power-law model.
This paper summarizes the results and provides the ASCA X-ray source catalogue
in the Galactic Center region.Comment: 33 pages, 8 figures, Accepted for publication in ApJS, also found in
http://www.star.le.ac.uk/~mas/research/paper/#Sakano2001apj
The Nature of Ultra-Luminous Compact X-Ray Sources in Nearby Spiral Galaxies
Studies were made of ASCA spectra of seven ultra-luminous compact X-ray
sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. 1994), M81 X-6
(Fabbiano 1988b; Kohmura et al. 1994; Uno 1997), IC 342 Source 1 (Okada et al.
1998), Dwingeloo 1 X-1 (Reynolds et al. 1997), NGC 1313 Source B (Fabbiano &
Trinchieri 1987; Petre et al. 1994), and two sources in NGC 4565 (Mizuno et al.
1999). With the 0.5--10 keV luminosities in the range 10^{39-40} ergs/s, they
are thought to represent a class of enigmatic X-ray sources often found in
spiral galaxies. For some of them, the ASCA data are newly processed, or the
published spectra are reanalyzed. For others, the published results are quoted.
The ASCA spectra of all these seven sources have been described successfully
with so called multi-color disk blackbody (MCD) emission arising from
optically-thick standard accretion disks around black holes. Except the case of
M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not
to exceed the Eddington limits, the black holes are inferred to have rather
high masses, up to ~100 solar masses. However, the observed innermost disk
temperatures of these objects, Tin = 1.1--1.8 keV, are too high to be
compatible with the required high black-hole masses, as long as the standard
accretion disks around Schwarzschild black holes are assumed. Similarly high
disk temperatures are also observed from two Galactic transients with
superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high
disk temperature may be explained by the black hole rotation, which makes the
disk get closer to the black hole, and hence hotter.Comment: submitted to ApJ, December 199
Erratum: ASCA Observations of the Absorption-Line Features from the Super-Luminal Jet Source GRS 1915+105
We have discovered errors in the calculation of some of the Einstein
coefficients in Table 5 and some plots in Figure 3. Due to the errors, the
square-root section of the curves of growth of Fe xxvi Ka and Kb, and Ni xxviii
Ka in Figure 3 were a few times underestimated. We correct Table 5 and re-plot
Figure 3 together with the unaffected curves. The program to calculate the
corrected curves of growth is available at
http://www.hp.phys.titech.ac.jp/kotani/cog/index.htmlComment: Erratum with 1 table and 1 figure submitted to ApJ. Original paper
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