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