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首页> 外文期刊>The Astrophysical journal >DETECTION OF SOFT GAMMA-RAY EMISSION FROM THE SEYFERT 2 GALAXY NGC 4507 BY THE OSSE TELESCOPE
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DETECTION OF SOFT GAMMA-RAY EMISSION FROM THE SEYFERT 2 GALAXY NGC 4507 BY THE OSSE TELESCOPE

机译：OSSE望远镜检测SEYFERT 2 GALXY NGC 4507的软伽玛射线发射

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摘要

We report the first soft gamma-ray observation by the OSSE experiment on board the Compton Gamma Ray Observatory (CGRO) of the optically selected Seyfert 2 galaxy NGC 4507. The source was observed on two separate weeks in 1993 and detected between 50 and 200 keV at a confidence level of 7 σ. There is no evidence of flux variability within or between the two observation periods which sample timescales from days to months. The source spectrum obtained from combining the two sets of data is best described by spectral forms including some exponential cutoff. For example, the best-fit thermal bremsstrahlung model provides a temperature of kT = 95_(-25)~(+31), while a thermal Comptonization model gives a plasma temperature of 44_(-9)~(+7), keV for an optical depth of 3; the 100 keV intensity is typically 5 x 10~(-6) photons cm~(-2) s~(-1) keV~(-1). The observed shape is steeper than the one observed by Ginga in the X-ray band in 1990. The joint Ginga-OSSE spectrum is best described by an absorbed power law exponentially cut off at high energies and having an iron Kα emission line; acceptable fits can be obtained without (f_r = 0) or with (f_r = 1) a Compton reflection component. In the first case we find a harder photon index (Γ = 1.3 ± 0.2) and a lower cutoff energy (E_c = 73_(-24)~(+48) keV) than in Seyfert 1 galaxies, while in the second case the fit parameters are within the range of values observed in type 1 objects (Γ = 1.7 ± 0.2 and E_c = 150_(-66)~(+290) keV); the statistics are, however, insufficient to discriminate between these two spectral shapes, although a slightly better fit is obtained with reflection included. Comparison between NGC 4507 and other Seyfert galaxies indicates that although at low energies a difference between type 1 and type 2 objects is possible (but still weak in this specific case), at high energies the observed shapes are indeed similar. We interpret this as evidence that the primary source emission (i.e., not reprocessed by material in the source) is the same for both types; this finding is in agreement with the unified scheme of Seyfert galaxies. Apart from its own interest, the OSSE detection of hard X-ray emission from a Seyfert 2 galaxy is also important because of the implications it has for the contribution of Seyfert galaxies of both types to the cosmic diffuse background, since type 2 objects are expected to outnumber type 1 sources.
机译：我们报告了OSSE实验在光学选择的Seyfert 2星系NGC 4507的康普顿伽玛射线天文台（CGRO）上进行的OSSE实验的首次软伽玛射线观测。在1993年的两周中分别观测到了该源，并在50至200 keV之间进行了探测。置信水平为7σ。没有证据表明两个观测周期内或之间的通量变化，这两个采样周期从几天到几个月不等。结合两组数据获得的源光谱最好用包括一些指数截止的光谱形式来描述。例如，最适合的热致辐射模型提供的温度为kT = 95 _（-25）〜（+31），而热Comptonization模型提供的等离子体温度为44 _（-9）〜（+7），keV为光学深度为3; 100 keV强度通常为5 x 10〜（-6）个光子cm〜（-2）s〜（-1）keV〜（-1）。观测到的形状比Ginga在1990年在X射线波段观察到的形状陡。联合Ginga-OSSE光谱最好用高能时呈指数截止的吸收功率定律来描述，并具有铁Kα发射线。可以不使用（f_r = 0）或使用（f_r = 1）康普顿反射分量而获得可接受的拟合。在第一种情况下，我们发现比Seyfert 1星系更硬的光子指数（Γ= 1.3±0.2）和更低的截止能量（E_c = 73 _（-24）〜（+48）keV），而在第二种情况下，拟合参数在类型1对象中观察到的值的范围内（Γ= 1.7±0.2和E_c = 150 _（-66）〜（+290）keV）;然而，尽管包括反射在内，获得的拟合度稍好一些，但统计数据仍不足以区分这两个光谱形状。 NGC 4507与其他塞弗特星系之间的比较表明，尽管在低能量下，类型1和类型2的物体之间可能存在差异（但在这种特定情况下仍然微弱），但在高能量下，观察到的形状确实相似。我们将其解释为以下证据：两种类型的主要排放源（即未经排放源中的材料重新处理）是相同的;这一发现与塞弗特星系的统一方案是一致的。除了自身的兴趣外，从塞弗2星系对硬X射线发射进行OSSE检测也很重要，因为它对两种类型的塞弗特星系对宇宙扩散背景的贡献都具有影响，因为预计会出现2型物体超过类型1来源。

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