SN 2001 em：没有那么快
SN 2001em: Not so Fast
摘要：
SN 2001 em是一颗奇特的超新星，最初被归类为Ib/c型。在超新星爆发两年后，它在射电中被发现，显示出一个上升的射电通量，光谱斜率很薄，它也显示出一个很大的X射线光度（~10^{41} erg/s）。因此，它被怀疑藏有一个减速（当时，温和）的相对论性喷流指向远离我们。SN 2001 em在发现3年后的光谱中显示出一条宽的H-α谱线，因此被重新归类为IIn型。在这里，我们通过分析VLBI观测的四个时期来限制它的自行和膨胀速度，延伸到SN之后的5.4年。超新星爆炸后5.4年仍未得到解决。对于自行，我们得到（23，000 +/-30，000）km/s，而我们对膨胀速度的2-sigma上限是6000 km/s。这些限制比Bietenholz和Bartel得出的限制更严格，并证实了他们的结论，即SN 2001 em在爆炸后几年的后期排放不是由相对论性喷流驱动的。VLA在8.46 GHz观测到的射电通量密度表明，SN之后约2.7年开始衰减为t^{-1.23 +/- 0.40}。总的来说，观测结果表明SN喷出物与非常致密的星周介质相互作用，尽管隐含的不透明度约束仍然是一个挑战。
英文摘要：
SN 2001em is a peculiar supernova, originally classified as Type Ib/c. About two years after the SN it was detected in the radio, showing a rising radio flux with an optically thin spectral slope, and it also displayed a large X-ray luminosity (~10^{41} erg/s). Thus it was suspected to harbor a decelerating (by then, mildly) relativistic jet pointing away from us. About 3 years after its discovery the optical spectrum of SN 2001em showed a broad H-alpha line, and it was therefore reclassified as Type IIn. Here we constrain its proper motion and expansion velocity by analyzing four epochs of VLBI observations, extending out to 5.4 years after the SN. The supernova is still unresolved 5.4 years after the explosion. For the proper motion we obtain (23,000 +/- 30,000) km/s while our 2-sigma upper limit on the expansion velocity is 6000 km/s. These limits are somewhat tighter than those derived by Bietenholz & Bartel, and confirm their conclusion that late time emission from SN 2001em, a few years after the explosion, is not driven by a relativistic jet. VLA observations of the radio flux density, at 8.46 GHz, show a decay as t^{-1.23 +/- 0.40} starting ~2.7 years after the SN. Collectively, the observations suggest interaction of the SN ejecta with a very dense circumstellar medium, though the implied opacity constraints still present a challenge.
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