172 research outputs found
Evolution of Synchrotron X-rays in Supernova Remnants
A systematic study of the synchrotron X-ray emission from supernova remnants
(SNRs) has been conducted. We selected a total of 12 SNRs whose synchrotron
X-ray spectral parameters are available in the literature with reasonable
accuracy, and studied how their luminosities change as a function of radius. It
is found that the synchrotron X-ray luminosity tends to drop especially when
the SNRs become larger than ~5 pc, despite large scatter. This may be explained
by the change of spectral shape caused by the decrease of the synchrotron
roll-off energy. A simple evolutionary model of the X-ray luminosity is
proposed and is found to reproduce the observed data approximately, with
reasonable model parameters. According to the model, the total energy of
accelerated electrons is estimated to be 10^(47-48) ergs, which is well below
the supernova explosion energy. The maximum energies of accelerated electrons
and protons are also discussed.Comment: 6 pages, 2 figures, ApJ, in pres
Origin of the "Disk-Line" Feature in the X-Ray Energy Spectrum of a Seyfert Galaxy, NGC4151
We have studied the origin of the broad and skewed feature at 4.5-7.5 keV in
the energy spectra of NGC4151 using the ASCA and RXTE data. The feature
consists of a narrow peak at 6.4 keV and a broad wing extended between 4.5-7.5
keV. An analysis of the long-term variations revealed that the feature became
variable only on a time scale longer than 1.5x10^6 s. Through a comparison with
the continuum variabilities, we found that the emission region of the excess
flux at 4.5-7.5 keV has an extent of 10^17 cm. The broad and skewed feature at
4.5-7.5 keV may be explained by the so-called ``disk-line'' model. If so, the
size of the line-emitting region, 10^17 cm, should be equal to several or
ten-times the Schwarzschild radius of the central black hole. This results in a
black hole mass of 10^11 solar mass, which may be too large for NGC4151. We
propose an alternative explanation for the broad and skewed feature, i.e. a
``reflection'' model, which can also reproduce the overall energy spectra very
well. In this model, cold matter with a sufficiently large column density is
irradiated by X-rays to produce a reflected continuum, which constitutes the
broad wing of the feature, and narrow fluorescent lines. The equivalent width
of the iron fluorescent line (~2 keV) and the upper limit of its width (sigma <
92 eV) are also consistent with this model. From these results and
considerations, we conclude that the ``disk-line'' model has difficulty to
explain the spectral variations of NGC4151, and the reflection model is more
plausible.Comment: 19 pages, 16 figures, PASJ accepte
Suzaku observation of the unidentified VHE gamma-ray source HESS J1702-420
A deep X-ray observation of the unidentified very high energy (VHE) gamma-ray
source HESS J1702-420, for the first time, was carried out by Suzaku. No bright
sources were detected in the XIS field of view (FOV) except for two faint
point-like sources. The two sources, however, are considered not to be related
to HESS J1702-420, because their fluxes in the 2-10 keV band (~ 10^-14 erg s^-1
cm^-2) are ~ 3 orders of magnitude smaller than the VHE gamma-ray flux in the
1-10 TeV band (F_{TeV} = 3.1 x 10^-11 erg s^-1 cm^-2). We compared the energy
spectrum of diffuse emission, extracted from the entire XIS FOV with those from
nearby observations. If we consider the systematic error of background
subtraction, no significant diffuse emission was detected with an upper limit
of F_X <2.7 x 10^-12 erg s^-1 cm^-2 in the 2-10 keV band for an assumed
power-law spectrum of \Gamma=2.1 and a source size same as that in the VHE
band. The upper limit of the X-ray flux is twelve times as small as the VHE
gamma-ray flux. The large flux ratio (F_{TeV}/F_X) indicates that HESS
J1702-420 is another example of a "dark" particle accelerator. If we use a
simple one-zone leptonic model, in which VHE gamma-rays are produced through
inverse Compton scattering of the cosmic microwave background and interstellar
far-infrared emission, and the X-rays via the synchrotron mechanism, an upper
limit of the magnetic field (1.7 \mu G) is obtained from the flux ratio.
Because the magnetic field is weaker than the typical value in the Galactic
plane (3-10 \mu G), the simple one-zone model may not work for HESS J1702-420
and a significant fraction of the VHE gamma-rays may originate from protons.Comment: 7 pages, accepted for publication in PASJ (Suzaku and MAXI special
issue
A Flare of AE Aquarii Observed with XMM-Newton
We present the results of analyzing the XMM-Newton data obtained in 2001
November 7 - 8. A flare is observed simultaneously in X-ray and UV together
with a quiescence. We find that during the flare event X-ray flux varies with
UV with no significant time lag, indicating a close correlation of flux
variation for X-ray and UV flares. An upper limit of the lag is estimated to be
\~1 min. From a timing analysis for X-ray data, we find that both pulsed and
unpulsed flux increase clearly as the flare advances in the entire energy band
0.15 - 10 keV. The net increase of pulsed flux to the quiescence is, however,
small and corresponds to about 3 - 4% of the increase in unpulsed flux,
confirming that a flux variation of flare in AE Aqr is dominated by unpulsed
X-rays. A spectral analysis reveals that the energy spectrum is similar to that
of the quiescence at the beginning of the flare, but the spectrum becomes
harder as the flare advances. Based on these results, we discuss the current
issues that need to be clarified, e.g., the possible flaring site and the mass
accretion problem of the white dwarf. We also discuss the flare properties
obtained in this study.Comment: 15 pages, 3 tables, 9 figures, accepted for publication in Ap
Study of the Long Term Stability of two Anomalous X-ray Pulsars 4U 0142+61 and 1E 1048.1-5937 with ASCA
We present new observations of two anomalous X-ray pulsars (AXP) 4U 0142+61
and 1E 1048.1-5937 made in 1998 with the ASCA. The energy spectra of these two
AXPs are found to consist of two components, a power-law and a blackbody
emission from the neutron star surface. These observations, when compared to
earlier ASCA observations in 1994 show remarkable stability in the intensity,
spectral shape and pulse profile. However, we find that the spin-down rate in
1E 1048.1-5937 is not constant. In this source, we have clearly identified
three epochs with spin-down rates different from each other and the average
value. This has very strong implications for the magnetar hypothesis of AXPs.
We also note that the spin-down rate and its variations in 1E 1048.1-5937 are
much larger than what can normally be produced by an accretion disk with very
low mass accretion rate corresponding to its low X-ray luminosity.Comment: 12 pages, 7 postscript figures included, To appear in the
Astrophysical Journa
High resolution X-ray spectrum of the accreting binary X-ray pulsar GX 1+4
We present here high resolution X-ray spectrum of the accreting binary X-ray
pulsar GX 1+4 obtained with the High Energy Transmission Grating (HETG)
instrument of the Chandra X-ray Observatory. This was supplemented by a
simultaneous observation with the Proportional Counter Array (PCA) of the Rossi
X-ray Timing Explorer (RXTE). During this observation, the source was in a
somewhat low intensity state and the pulse profile with both Chandra and RXTE
shows a narrow dip, characteristic of GX 1+4 in medium and low intensity
states. The continuum X-ray spectrum obtained with the HETG and PCA can be
fitted well with a high energy cutoff power-law model with line of sight
absorption. Interestingly, we find that this low state is accompanied by a
relatively small absorption column density. A 6.4 keV narrow emission line with
an equivalent width of 70 eV is clearly detected in the HETG spectrum. The
fluorescence iron line, or at least part of it is produced in the neutral or
lowly ionized iron in the circumstellar material that also causes most of the
line of sight absorption. In the HETG spectrum, we have found evidence for a
weak (equivalent width ~30 eV) emission line at 6.95 keV. This line is
identified as Ly_alpha emission line from hydrogen-like iron and the spectrum
does not show emission lines from helium-like iron. We discuss various emission
regions for the hydrogen-like iron emission line, like gas diffused into the
Alfven sphere or an accretion curtain flowing from the inner accretion disk to
the magnetic poles.Comment: 15 pages, 4 postscript figures, accepted for publication in The
Astrophysical Journa
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