椭圆星系NGC 4278中心的紫外线耀斑
The ultraviolet flare at the center of the elliptical galaxy NGC 4278
摘要：
大部分正常的星系显示出微弱的核活动。低光度活动星系核（LLAGN）的特征之一是在一些低电离核发射线区（LINER）星系中心偶然发现的紫外光变率。迫切需要获得关于星系核紫外线耀斑和变异性的更好统计数据，包括目标的数量和监测。对哈勃空间望远镜的科学数据档案进行了查询，以找到所有椭圆星系，这些星系具有用宽视场行星照相机2（WFPC 2）在不同时期获得的紫外图像，以及可能具有用空间望远镜成像光谱仪（STIS）在H α发射线区域获得的核光谱。这些数据只在椭圆形射电星系NGC 4278中发现。利用1994年6月至1995年1月期间获得的WFPC 2/F218 W图像，用孔径光度法测量了NGC 4278核源的紫外通量。通过测量STIS/G750 M光谱中观测到的发射线的宽分量，并假设气体均匀分布在球体中，估算了中心超大质量黑洞（SBH）的质量。NGC 4278的核心拥有一个几乎无法分辨但变化很大的紫外源。它的紫外线亮度在6个月内增加了1.6倍。NGC 4278中紫外耀斑的振幅和标度时间与早期在其他LLAGN中发现的最亮紫外核瞬变非常相似。SBH的质量在7 × 10^7到2 × 10^9太阳质量之间。这与以前的发现基于不同的假设的气体分布和预测的基础上的星系速度色散。
英文摘要：
A large fraction of otherwise normal galaxies shows a weak nuclear activity. One of the signatures of the low-luminosity active galactic nuclei (LLAGNs) is the ultraviolet variability which was serendipitously discovered in the center of some low-ionization nuclear emission-line region (LINER) galaxies. There is a pressing need to acquire better statistics about UV flaring and variability in galaxy nuclei, both in terms of the number and monitoring of targets. The Science Data Archive of the Hubble Space Telescope was queried to find all the elliptical galaxies with UV images obtained in different epochs with the Wide Field Planetary Camera 2 (WFPC2) and possibly with nuclear spectra obtained with the Space Telescope Imaging Spectrograph (STIS) in the region of the Halpha emission line. These data were found only for the elliptical radiogalaxy NGC 4278. The UV flux of the nuclear source of NGC 4278 was measured by means of aperture photometry on the WFPC2/F218W images obtained between June 1994 and January 1995. The mass of the central supermassive black hole (SBH) was estimated by measuring the broad components of the emission lines observed in the STIS/G750M spectrum and assuming that the gas is uniformly distributed in a sphere. The nucleus of NGC 4278 hosts a barely resolved but strongly variable UV source. Its UV luminosity increased by a factor of 1.6 in a period of 6 months. The amplitude and scale time of the UV flare in NGC 4278 are remarkably similar to those of the brightest UV nuclear transients which were earlier found in other LLAGNs. The mass of the SBH was found to be in the range between 7x10^7 and 2x10^9 M_sun. This is in agreement with previous findings based on different assumptions about the gas distribution and with the predictions based on the galaxy velocity dispersion.
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