5,061 research outputs found
The Plasma Structure of the Cygnus Loop from the Northeastern Rim to the Southwestern Rim
The Cygnus Loop was observed from the northeast to the southwest with
XMM-Newton. We divided the observed region into two parts, the north path and
the south path, and studied the X-ray spectra along two paths. The spectra can
be well fitted either by a one-component non-equilibrium ionization (NEI) model
or by a two-component NEI model. The rim regions can be well fitted by a
one-component model with relatively low \kTe whose metal abundances are
sub-solar (0.1--0.2). The major part of the paths requires a two-component
model. Due to projection effects, we concluded that the low kTe (about 0.2 keV)
component surrounds the high kTe (about 0.6 keV) component, with the latter
having relatively high metal abundances (about 5 times solar). Since the Cygnus
Loop is thought to originate in a cavity explosion, the low-kTe component
originates from the cavity wall while the high-kTe component originates from
the ejecta. The flux of the cavity wall component shows a large variation along
our path. We found it to be very thin in the south-west region, suggesting a
blowout along our line of sight. The metal distribution inside the ejecta shows
non-uniformity, depending on the element. O, Ne and Mg are relatively more
abundant in the outer region while Si, S and Fe are concentrated in the inner
region, with all metals showing strong asymmetry. This observational evidence
implies an asymmetric explosion of the progenitor star. The abundance of the
ejecta also indicates the progenitor star to be about 15 M_sun.Comment: 24 pages, 9 figures, Astrophysical Journal in pres
Chandra Observations of the Northeastern Rim of the Cygnus Loop
We present results from spatially resolved spectral analyses of the
northeastern (NE) rim of the Cygnus Loop supernova remnant (SNR) based on two
Chandra observations. One pointing includes northern outermost
abundance-enhanced regions discovered by recent Suzaku observations, while the
other pointing is located on regions with "normal" abundances in the NE rim of
the Cygnus Loop. The superior spatial resolving power of Chandra allows us to
reveal that the abundance-enhanced region is concentrated in an about
200"-thickness region behind the shock front. We confirm absolute metal
abundances (i.e., relative to H) as well as abundance ratios between metals are
consistent with those of the solar values within a factor of about 2. Also, we
find that the emission measure in the region gradually decreases toward the
shock front. These features are in contrast with those of the ejecta fragments
around the Vela SNR, which leads us to believe that the abundance enhancements
are not likely due to metal-rich ejecta. We suggest that the origin of the
plasma in this region is the interstellar medium (ISM). In the "normal"
abundance regions, we confirm that abundances are depleted to the solar values
by a factor of about 5 that is not expected in the ISM around the Cygnus Loop.
Introduction of non-thermal emission in our model fitting can not naturally
resolve the abundance-depletion problem. The origin of the depletion still
remains as an open question.Comment: 18 pages, 6 figure
On the Nature of AX J2049.6+2939 and AX J2050.0+2914
AX J2049.6+2939 is a compact X-ray source in the vicinity of the southern
blow-up region of the Cygnus Loop supernova remnant (Miyata et al. 1998a). This
source was the brightest X-ray source inside the
Cygnus Loop observed during the ASCA survey project. The X-ray spectrum was
well fitted by a power-law function with a photon index of . Short-term timing analysis was performed and no coherent pulsation
was found. Follow-up observations with ASCA have revealed a large variation in
X-ray intensity by a factor of 50, whereas the spectral shape did not
change within the statistical uncertainties. In the second ASCA observation, we
found another X-ray source, AX J2050.0+2941, at the north east of AX
J2049.6+2939. During the three ASCA observations, the X-ray intensity of AX
J2050.0+2941 varied by a factor of 4. No coherent pulsations could be
found for AX J2050.0+2941.
We have performed optical photometric and spectroscopic observations in the
vicinity of AX J2049.6+2939 at the Kitt Peak National Observatory
(KPNO). As a result, all objects brighter than -band magnitude of 22 in
the error box can be identified with normal stars. Combined with the
X-ray results and the fact that there are no radio counterparts, AX
J2049.6+2939 is not likely to be either an ordinary rotation-powered pulsar
or an AGN. The nature of AX J2049.6+2939 is still unclear and further
observations over a wide energy band are strongly required.
As to AX J2050.0+2941, the long-term X-ray variability and the radio
counterpart suggests that it is an AGN.Comment: 23 pages, 4 figures, Accepted for publication by Astrophysical
Journa
Subpixel Spatial Resolution of the X-Ray Charge-Coupled Device Based on the Charge Cloud Shape
When an X-ray photon lands into a pixel (event pixel), the primary charge is
mainly collected into the event pixel. If the X-ray landing position is
sufficiently close to the pixel boundary, the primary charge spills over to the
adjacent pixel forming split events. We can easily understand that there are
three parameters coupled together; the X-ray landing position inside the pixel,
the X-ray event pattern and the primary charge cloud shape. We can determine
any one of them from the other two parameters. Since we know the charge cloud
shape using the multi-pitch mesh experiment, we can calculate the X-ray landing
position with subpixel resolution using the event pattern. We applied our
method to Ti-K X-rays for the charge-coupled device with m square
pixel. Once the primary charge splits into the adjacent pixel, we can determine
the X-ray landing position with subpixel resolution. Using three- or four-pixel
split events, we can determine the X-ray landing position with an accuracy of
less than m. For a two-pixel split event, we obtained a similar position
accuracy in the split direction with no improvement in the direction
perpendicular to it. We will discuss the type of CCD which can achieve the
subpixel resolution for the entire area of the CCD.Comment: 16pages, 5 figures. To appear in Jpn. J. Appl. Phys. March, 200
Spectroscopic Study of the Vela-Shrapnel
Several shrapnels have been detected in the vicinity of Vela SNR by the ROSAT
all-sky survey. We present here the spectral properties of shrapnel `A'
observed with the ASCA satellite. A prominent Si-K emission line with
relatively weak emission lines from other elements have been detected,
revealing that the relative abundance of Si is a few ten-times higher than
those of other elements. Combining with the ROSAT PSPC results, we obtained the
electron temperature, , to be keV. The total mass
of shrapnel `A' is estimated to be . If it is an ejecta of a
supernova explosion, the interstellar matter (ISM) would be swept up in the
leading edge while the ejecta material would be peeled off in the trailing
edge, which should be confirmed by future observations.Comment: 19pages, 5 figures, 1 table; Accepted for Publications of the
Astronomical Society of Japa
Ionization States and Plasma Structures of Mixed-morphology SNRs Observed with ASCA
We present the results of a systematic study using ASCA of the ionization
state for six ``mixed-morphology'' supernova emnants (MMSNRs): IC 443, W49B,
W28, W44, 3C391, and Kes 27. MMSNRs show centrally filled thermal X-ray
emission, which contrasts to shell-like radio morphology, a set of
haracteristics at odds with the standard model of SNR evolution (e.g., the
Sedov model). We have therefore studied the evolution of the MMSNRs from the
ionization conditions inferred from the X-ray spectra, independent of X-ray
morphology. We find highly ionized plasmas approaching ionization equilibrium
in all the mmsnrs. The degree of ionization is systematically higher than the
plasma usually seen in shell-like SNRs. Radial temperature gradients are also
observed in five remnants, with cooler plasma toward the limb. In IC 443 and
W49B, we find a plasma structure consistent with shell-like SNRs, suggesting
that at least some MMSNRs have experienced similar evolution to shell-like
SNRs. In addition to the results above, we have discovered an ``overionized''
ionization state in W49B, in addition to that previously found in IC 443.
Thermal conduction can cause the hot interior plasma to become overionized by
reducing the temperature and density gradients, leading to an interior density
increase and temperature decrease. Therefore, we suggest that the
``center-filled'' X-ray morphology develops as the result of thermal
conduction, and should arise in all SNRs. This is consistent with the results
that MMSNRs are near collisional ionization equilibrium since the conduction
timescale is roughly similar to the ionization timescale. Hence, we conclude
that MMSNRs are those that have evolved over yr. We call this phase
as the ``conduction phase.''Comment: 34 pages, 20 figures, 9 tables, accepted for publication in The
Astrophysical Journa
Chandra X-ray Observation of a Mature Cloud-Shock Interaction in the Bright Eastern Knot Region of Puppis A
We present Chandra X-ray images and spectra of the most prominent cloud-shock
interaction region in the Puppis A supernova remnant. The Bright Eastern Knot
(BEK) has two main morphological components: (1) a bright compact knot that
lies directly behind the apex of an indentation in the eastern X-ray boundary
and (2) lying 1' westward behind the shock, a curved vertical structure (bar)
that is separated from a smaller bright cloud (cap) by faint diffuse emission.
Based on hardness images and spectra, we identify the bar and cap as a single
shocked interstellar cloud. Its morphology strongly resembles the ``voided
sphere'' structures seen at late times in Klein et al.'s experimental
simulations of cloud-shock interactions, when the crushing of the cloud by
shear instabilities is well underway. We infer an interaction time of roughly 3
cloud-crushing timescales, which translates to 2000-4000 years, based on the
X-ray temperature, physical size, and estimated expansion of the shocked cloud.
This is the first X-ray identified example of a cloud-shock interaction in this
advanced phase. Closer to the shock front, the X-ray emission of the compact
knot in the eastern part of the BEK region implies a recent interaction with
relatively denser gas, some of which lies in front of the remnant. The complex
spatial relationship of the X-ray emission of the compact knot to optical [O
III] emission suggests that there are multiple cloud interactions occurring
along the line of sight.Comment: 22 pages LaTeX with multiple figures, to appear in Ap
Nonthermal X-Ray Emission from G266.2-1.2 (RX J0852.0-4622)
The newly discovered supernova remnant G266.2-1.2 (RX J0852.0-4622), along
the line of sight to the Vela SNR, was observed with ASCA for 120 ks. We find
that the X-ray spectrum is featureless, and well described by a power law,
extending to three the class of shell-type SNRs dominated by nonthermal X-ray
emission. Although the presence of the Vela SNR compromises our ability to
accurately determine the column density, the GIS data appear to indicate
absorption considerably in excess of that for Vela itself, indicating that
G266.2-1.2 may be several times more distant. An unresolved central source may
be an associated neutron star, though difficulties with this interpretation
persist.Comment: 4 pages, 5 figures, uses aipproc.sty & epsfig.sty. To appear in
"Young Supernova Remnants" (11th Annual Astrophysics Conference in Maryland),
S. S. Holt & U. Hwang (eds), AIP, New York (2001
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
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