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Cosmic particle accelerator in the X-ray binary system revealed by Suzaku
X-ray binaries constitute the brightest class of X-ray sources in the sky, and were the main focus from the beginning of X-ray astronomy. An X-ray binary contains either a neutron star or a black hole accreting material from a compact star. The widely accepted idea of the emission mechanism was that X-ray binaries can effectively transform the gravitational energy of the compact object (a neutron star or a black hole) into thermal X-ray emission. However, recent observations detected TeV gamma-rays, which is one billion times higher than X-ray energies, from an X-ray binary system LS 5039. This result revealed that LS 5039 is a source of Very High Energy (VHE) γ-rays and hence, is able to accelerate particles to TeV energies.
More surprisingly, LS 5039 shows a clock-like stable modulation of the TeV γ-ray flux with its orbital period of 3.9-days. The discovery of the TeV γ-ray modulation provided us swirling debates for the particle acceleration and radiation mechanisms along the binary orbit.
In order to unveil the nature of this source, we performed a long-time, 6-days continuous observation on LS 5039, which covered a whole orbital period of 3.9 days. Figure 1 shows the flux light curves obtained with Suzaku XIS (X-ray Imaging Spectrometer). We detected strong X-ray flux modulation from the binary system LS 5039 for the first time, with its close resemblance to the TeV γ-ray light curve (Takahashi et al., ApJ, 697, pp. 592-600). The X-ray emission can be interpreted as a direct evidence of synchrotron radiation by the electrons accelerated to very high energies. The Suzaku data enables us to compare the X- ray variability with the past snapshot observations. Combining data of the archival ASCA observations provided by DARTS, and public XMM-Newton and Chandra observations, we investigated the long-time stability of the X-ray modulation. We found that the X-ray modulation curves are surprisingly stable over the past decade. It is remarkable that not only the overall modulation but also the fine structures in the light curves agrees well between the data from Suzaku and those from other X-ray observatories (Kishishita et al., ApJ, 697, L1-L5). The clock-like synchrotron modulation suggests the existence of stable particle acceleration and emission mechanisms which depends only on the binary orbit in this system.
The discovery of the particle acceleration in binary system not only changes the general view of X-ray binaries but also provides us important clues for the origin and acceleration mechanism of cosmic rays, which is one of the important unsolved problem in the field of astrophysics.
This study was performed in the joint research with ISAS/JAXA, SLAC National Accelerator Laboratory and Max-Planck-Institut fur Kernphysik.
Figure 1: (a) Orbital light curves of LS 5039 in the energy range of 1--10~keV. (Top) Suzaku XIS data with a time bin of 2~ks. Overlaid in the range of φ=0.0 to 2.0 is the same light curve but shifted by one orbital period (open circles). (Bottom) Comparison with the past observations. Each color corresponds to XMM-Newton (blue, cyan, and green), ASCA (red), and Chandra (magenta). (b) Closeup in 1.2 ≤φ<1.8.
Tetsuichi Kishishita (ISAS/JAXA)
October, 2009