FERMI LAT OBSERVATIONS OF LS I +61°303: FIRST DETECTION OF AN ORBITAL MODULATION IN GeV GAMMA RAYS
FERMI LAT OBSERVATIONS OF LS I +61°303: FIRST DETECTION OF AN ORBITAL MODULATION IN GeV GAMMA RAYS
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DOI：
10.1088/0004-637x/701/2/l123
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发表时间：
2009-07
期刊：
影响因子：
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通讯作者：
A. Abdo;et al.;for the Belle collaboration
中科院分区：
文献类型：
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作者：
A. Abdo;et al.;for the Belle collaboration
这封信介绍了2008年8月至2009年3月期间使用费米伽马射线太空望远镜的大面积望远镜数据对LS I +61 ° 303进行观测的第一批结果。我们的研究结果表明，与双星周期一致的变化，发射在26.6 ± 0.5天被调制。这是第一次探测到高能伽马射线（20 MeV-100 GeV，HE）的轨道周期性。光变曲线的特点是在近星点之后有一个宽峰，在远日点之前有一个较小的峰。该光谱最好用指数截止的幂律表示，得到100 MeV以上的总通量为0.82 ± 0.03（stat）± 0.07（syst）10−6 ph cm−2 s−1，截止为6.3 ± 1.1（stat）± 0.4（syst）GeV，光子指数Γ = 2.21 ± 0.04（stat）± 0.06（syst）。光谱随轨道相位的变化不明显。最大发射的相位，接近近星点，暗示逆康普顿散射作为主要的辐射机制。然而，MAGIC和VERITAS先前的甚高能量伽马射线（>100 GeV，VHE）观测显示峰值发射接近于远射。这一点和费米实验中所看到的能量截止点表明，HE和VHE伽马射线之间的联系是不平凡的。
This Letter presents the first results from the observations of LS I +61°303 using Large Area Telescope data from the Fermi Gamma-Ray Space Telescope between 2008 August and 2009 March. Our results indicate variability that is consistent with the binary period, with the emission being modulated at 26.6 ± 0.5 days. This constitutes the first detection of orbital periodicity in high-energy gamma rays (20 MeV–100 GeV, HE). The light curve is characterized by a broad peak after periastron, as well as a smaller peak just before apastron. The spectrum is best represented by a power law with an exponential cutoff, yielding an overall flux above 100 MeV of 0.82 ± 0.03(stat) ± 0.07(syst) 10−6 ph cm−2 s−1, with a cutoff at 6.3 ± 1.1(stat) ± 0.4(syst) GeV and photon index Γ = 2.21 ± 0.04(stat) ± 0.06(syst). There is no significant spectral change with orbital phase. The phase of maximum emission, close to periastron, hints at inverse Compton scattering as the main radiation mechanism. However, previous very high-energy gamma ray (>100 GeV, VHE) observations by MAGIC and VERITAS show peak emission close to apastron. This and the energy cutoff seen with Fermi suggest that the link between HE and VHE gamma rays is nontrivial.