银河系中心超大质量黑洞Sgr A*的光变研究进展

Progress in Research on the Variability of Sgr A*

越来越多的观测证据表明,位于银河系中心的致密射电源Sgr A*是一个质量约为400万个太阳质量的超大质量黑洞。本文介绍在射电、毫米/亚毫米、近红外、X射线波段对SgrA*的光变观测和理论研究进展。自1974年被射电干涉测量发现伊始,SgrA*就被检测到时标从天到年不等的射电光变,随着新的观测设备的出现和观测技术的发展,天文学家先后在所有的对银河系中心可观测波段上检测到了SgrA*的光变,包括在毫米/亚毫米波段的时标为小时的快速光变,近红外波段上时标不到1h的显著光变,以及空间X射线卫星记录到的幅度变化几十倍以上的巨大耀变。这些光变观测数据可以用于估算产生SgrA*耀变辐射的区域范围(假设光变是由整个源产生的,辐射区域的尺度l与光变时标τ之间存在线性关系l≤cτ),并对SgrA*辐射机制研究提供了间接的限制。近年来开展的一些多波段联测显示,近红外和X射线光变是同步发生的,并且都比射电和毫米波光变先出现,这类时间延迟支持SgrA*光变的绝热膨胀理论模型。但目前成功的多波段耀变观测非常有限,如何获得从射电厘米波到X射线的SgrA*光变间的内在关联将是未来几年中SgrA*光变的一个主要研究方向。

There is compelling evidence that Sgr A＊, the extremely compact radio source at the dynamical center of the Galaxy, is associated with a four million solar masses massive black hole. This paper deals with the temporal variability observed in the emission from Sgr A＊. The total flux density variation has been puzzling ever since the discovery of Sgr A＊ in 1974. At radio bands, Sgr A＊ appears to have significant fluctuations in flux density at shorter centimeter wavelengths and the fractional variation appears to be higher toward the short wavelengths. This trend continues toward higher frequency bands including millimeter, sub-mm, near-infrared and X-ray wavelengths. The observed outbursts exhibit a typical time scale of hours at mm/sub-mm, less than an hour at nearinfrared and X-rays. Both Chandra and XMM-Newton X-ray observatories have detected some very strong and highly variable flares from Sgr A＊. These observations provide unique constraints on the emission geometry of Sgr A＊, and can be use to study the origin of the variability of Sgr A＊. Some multi-frequency observations of Sgr A＊ have detected flares simultaneously at both near-infrared and X-ray bands, and a time delay of the radio flare emission with respect to those at near-infrared and X-rays. These measurements of time delay seem to favor the adiabatically expanding plasmon model. However, more reliable simultaneous multi-frequency campaign is still needed to well establish the relationship between flaring emission at different theoretical