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

Angular Momentum Transfer in the Binary X-ray Pulsar GX 1+4

We describe three presentations relating to the X-ray pulsar GX 1+4 at a workshop on magnetic fields and accretion at the Astrophysical Theory Centre, Australian National University on 1998, November 12-13. Optical and X-ray spectroscopy indicate that GX 1+4 is seen through a cloud of gravitationaly bound matter. We discuss an unstable negative feedback mechanism (originally proposed by Kotani et al, 1999), based on X-ray heating of this matter which controls the accretion rate when the source is in a low X-ray luminosity state. A deep minimum lasting ~6 hours occurred during observations with the RXTE satellite over 1996, July 19-21. The shape of the X-ray pulses changed remarkably from before to after the minimum. These changes may be related to the transition from neutron star spin-down to spin-up which occurred at about the same time. Smoothed particle hydrodynamic simulations of the effect of adding matter with opposite angular momentum to an existing disc, show that it is possible for a number of concentric rings with alternating senses of rotation to co-exist in a disc. This could provide an explanation for the step-like changes in Pdot which are observed in GX 1+4. Changes at the inner boundary of the disc occur at the same timescale as that imposed at the outer boundary. Reversals of material torque on the neutron star occur at a minimum in L_X.Comment: 10 pages, 5 figures; accepted for publication by PAS

Cessation of X-ray Pulsation of GX 1+4

We report results from our weekly monitoring campaign on the X-ray pulsar GX 1+4 with the {\em Rossi X-ray Timing Explorer} satellite. The spin-down trend of GX 1+4 was continuing, with the pulsar being at its longest period ever measured (about 138.7 s). At the late stage of the campaign, the source entered an extended faint state, when its X-ray (2-60 keV) flux decreased significantly to an average level of $\sim 3 \times 10^{-10} ergs cm^{-2} s^{-1}$. It was highly variable in the faint state; the flux dropped to as low as $\sim 3 \times 10^{-11} ergs cm^{-2} s^{-1}$. In several observations during this period, the X-ray pulsation became undetectable. We can, therefore, conclude conservatively that the pulsed fraction, which is normally $\gtrsim$ 70% (peak-to-peak), must have decreased drastically in those cases. This is very similar to what was observed of GX 1+4 in 1996 when it became similarly faint in X-ray. In fact, the flux at which the cessation of X-ray pulsation first occurred is nearly the same as it was in 1996. We suggest that we have, once again, observed the propeller effect in GX 1+4, a phenomenon that is predicted by theoretical models of accreting X-ray pulsars.Comment: 13 pages, 9 figures (available at http://www.physics.purdue.edu/~cui/ftp/cuifigs.tar.gz). To appear in Ap

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