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arXiv 1702.08602astro-ph.HE
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用费米大面积望远镜观测M31和M33:仙女座星系的中心过剩?

Observations of M31 and M33 with the Fermi Large Area Telescope: a galactic center excess in Andromeda?

LAT Collaboration

摘要:

费米LAT为比较研究银河系(MW)和其他外部恒星形成星系中的宇宙射线(CR)和高能物体开辟了道路。使用2年的观测与费米LAT,本地组星系M31被检测到作为一个边缘扩展的伽马射线源,而只有一个上限(UL)已被推导出的其他附近的星系M33。我们重新审视了伽马射线发射的方向M31和M33使用超过7年的LAT Pass 8数据的能量范围为0.1-100 GeV,提出了详细的形态和光谱分析。M33仍然未被探测到,我们计算了它的UL。这个修订后的UL仍然与观测到的伽马射线光度和恒星形成速率示踪剂之间的相关性一致,并意味着M33的平均CR密度至多是MW的一半。检测到M31的显著性接近10 sigma,并扩展为4 sigma。它的频谱符合幂律。发射的空间分布与半径为0.4度且与M31中心无偏移的均匀圆盘一致,但不能排除不均匀的强度分布。来自M31的辐射流似乎局限于星系内部区域,并没有填满星系盘,也没有远离星系盘。伽马射线信号与富含气体或恒星形成活动的区域无关,这表明辐射不是星际起源的,除非伽马射线辐射的高能粒子不是起源于最近的恒星形成。另一种非排他性的解释是,这种发射是由被破坏的球状星团分散在M31的凸起和盘中的毫秒级恒星群造成的,或者是由暗物质粒子的衰变或湮灭造成的,类似于在费米-LAT观测中发现的所谓银河中心过剩。

英文摘要:

The Fermi LAT has opened the way for comparative studies of cosmic rays (CRs) and high-energy objects in the Milky Way (MW) and in other, external, star-forming galaxies. Using 2 yr of observations with the Fermi LAT, local Group galaxy M31 was detected as a marginally extended gamma-ray source, while only an upper limit (UL) has been derived for the other nearby galaxy M33. We revisited the gamma-ray emission in the direction of M31 and M33 using more than 7 yr of LAT Pass 8 data in the energy range 0.1-100 GeV, presenting detailed morphological and spectral analyses. M33 remains undetected and we computed an UL for it. This revised UL remains consistent with the observed correlation between gamma-ray luminosity and star-formation rate tracers and implies an average CR density in M33 that is at most half of that of the MW. M31 is detected with a significance of nearly 10 sigma and to be extended with 4 sigma. Its spectrum is consistent with a power law. The spatial distribution of the emission is consistent with a uniform disk with a radius of 0.4 deg and no offset from the center of M31, but nonuniform intensity distributions cannot be excluded. The flux from M31 appears confined to the inner regions of the galaxy and does not fill the disk or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star-formation activity suggesting that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. Alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of M31 by disrupted globular clusters or from the decay or annihilation of dark matter particles, similar to what has been proposed to account for the so-called Galactic Center excess found in Fermi-LAT observations of the MW.

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