来源 中科院紫金山天文台

广义相对论预言引力波在真空中的速度为光速。2015年9月之前引力波从未被直接探测到，也就不可能直接测量其速度。如果引力波速度小于光速，现有的极高能宇宙线观测要求该相对偏差小于10-15。但在很多的引力理论模型中引力波速度可以超过光速，目前对此相关限制很弱。所以引力波速度的直接测量具有非常重要的科学意义。

引力波速度的直接测量目前主要有两种方案：一种是基于各引力波探测器接收到引力波信号的时间差来计算，目前这种测量的精度非常低；另一种测量的原理是：如果引力波速度不同于光速，那么宇宙学距离上的引力波爆发信号与几乎同时发出的电磁辐射信号到达观测者时将产生一定的时差，通过这个时差就可以给出引力波速度偏离光速的幅度的上限。

2015年9月14日，在 LIGO 引力波探测器成功探测到 GW150914 后大约0.4秒，Fermi 卫星上搭载的伽马射线暴监视器（GBM）探测到了一个持续约1秒的微弱伽马射线暂现源。尽管还存在一定的争议，但 Fermi/GBM 的一个专门的数据分析小组认为这个信号的显著性达到3倍标准差，它在时间与空间上与 GW150914 也基本吻合，极有可能成协。中国科学院紫金山天文台李翔、张富文、袁强等人第一时间对该成协的物理意义进行了深入探讨，通过计算得到：如果引力波速度超过光速，那么其相对偏差应小于10-17（arXiv:1602.04460）。如果 GW150914 与 Fermi-GBM 的伽玛射线信号的成协是真实的，那么这是人类首次对引力波的速度做出高精度直接测量。该研究成果发表在《天体物理杂志快报》(ApJL)上。该文还被列入 ApJL 双黑洞并合电磁对应体研究合集。

尽管上述结论依赖 Fermi/GBM 伽马射线信号的真实性，但在2016年8月《天体物理杂志》（ApJ）的一篇论文中，紫金山天文台李翔、胡一鸣、范一中等人研究指出短伽马暴与引力波事件的成协性很有可能在未来几年内确立，并且引力波速度与光速的相对偏差可望被直接限制到10-18 的精度（如图）。该文中他们还首次给出了“同时进行引力波速度高精度直接测量及爱因斯坦弱等效原理检验”的研究方案。

2020年前后对“引力波速度与光速相对偏差”测量预期达到的精度。图取自 Li et al. (2016 ApJ)

这两项研究工作得到科技部“973”计划、国家自然科学基金、中科院B类先导专项的资助。

原文链接：

http://www.cas.cn/syky/201608/t20160811_4570997.shtml

论文基本信息

标题  GRB/GW association: long–short GRB candidates, time lag, measuring gravitational wave velocity, and testing Einstein's equivalence principle

作者  Xiang Li, Yi-Ming Hu, Yi-Zhong Fan, and Da-Ming Wei

刊期  The Astrophysical Journal (2016)

日期  09 August 2016

doi   10.3847/0004-637X/827/1/75

论文摘要

Short-duration gamma-ray bursts (SGRBs) are widely believed to be powered by the mergers of compact binaries, such as binary neutron stars or possibly neutron star–black hole binaries. Though the prospect of detecting SGRBs with gravitational wave (GW) signals by the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)/VIRGO network is promising, no known SGRB has been found within the expected advanced LIGO/VIRGO sensitivity range for binary neutron star systems. We find, however, that the two long–short GRBs (GRB 060505 and GRB 060614) may be within the horizon of advanced GW detectors. In the upcoming era of GW astronomy, the merger origin of some long–short GRBs, as favored by the macronova signature displayed in GRB 060614, can be unambiguously tested. The model-dependent time lags between the merger and the onset of the prompt emission of the GRB are estimated. The comparison of such time lags between model predictions and the real data expected in the era of the GW astronomy would be helpful in revealing the physical processes taking place at the central engine (including the launch of the relativistic outflow, the emergence of the outflow from the dense material ejected during the merger, and the radiation of gamma rays). We also show that the speed of GWs, with or without a simultaneous test of Einstein's equivalence principle, can be directly measured to an accuracy of 3*10^8 cm/s or even better in the advanced LIGO/VIRGO era.

链接

http://iopscience.iop.org/article/10.3847/0004-637X/827/1/75/meta

标题 Implications of the tentative association between GW150914 and a FERMI-GBM transient

作者 Xiang Li, Fu-Wen Zhang, Qiang Yuan, Zhi-Ping Jin, Yi-Zhong Fan, Si-Ming Liu, and Da-Ming Wei

刊期 The Astrophysical Journal Letters (2016)

日期 08 August 2016

doi 10.3847/2041-8205/827/1/L16

论文摘要

The merger-driven gamma-ray bursts (GRBs) and their associated gravitational-wave (GW) radiation, if both are successfully detected, have some far-reaching implications, including, for instance: (i) the statistical comparison of the physical properties of the short/long-short GRBs with and without GW detection can test the general origin model; (ii) revealing the physical processes taking place at the central engine; (iii) measuring the velocity of the gravitational wave directly/accurately. In this work, we discuss these implications in the case of a possible association of GW150914/Gamma-ray Burst Monitor (GBM) transient 150914. We compared GBM transient 150914 with other SGRBs and found that such an event may be a distinct outlier in some statistical diagrams, possibly due to its specific binary black hole merger origin. However, the presence of a "new" group of SGRBs with "unusual" physical parameters is also possible. If the outflow of GBM transient 150914 was launched by the accretion onto the nascent black hole, the magnetic activity rather than the neutrino process is likely responsible for the energy extraction, and the accretion disk mass is estimated to be ~10^−5 M ⊙. The GW150914/GBM transient 150914 association, if confirmed, would provide the first opportunity to directly measure the GW velocity, and its departure from the speed of the light should be within a factor of ~10^−17.

链接

http://iopscience.iop.org/article/10.3847/2041-8205/827/1/L16

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