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arXiv 1403.7204astro-ph.HEastro-ph.SR
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球状星团M15中射电星和其它致密射电源的精密天体测量

Precision astrometry of pulsars and other compact radio sources in the globular cluster M15

Franz Kirsten, Wouter Vlemmings, Paulo Freire, Michael Kramer, Helge Rottmann, Robert M. Campbell

摘要:

球状星团(GC)M15(NGC 7078)是至少八颗双星和两颗低质量X射线双星(LMXB)的宿主,其中一颗在射电区域也可以看到。在这里,我们提出了一个多历元的全球甚长基线干涉测量(VLBI)活动的结果,旨在i)测量的自行已知的紧凑的无线电源,ii)发现和分类迄今未检测到的紧凑的无线电源的GC,和iii)检测的签名推定的中等质量黑洞(IMBH)提出居住在M15的核心。我们测量了恒星M15 A和M15 C以及LMXB AC 211的天球赤经运动($μ_α$)和赤经运动($μ_δ$),分别为$(μ_α,\,μ_δ)_{\text{M15A}}=(-0.54\pm0.14,\,-4.33\pm0.25)\,$ mas $\,$ yr $^{-1}$,$(μ_α,\,μ_δ)_{\text{M15C}}=(-0.75\pm0.09,\,-3.52\pm0.13)\,$ mas $\,$ yr $^{-1}$和$(μ_α,\,μ_δ)_{\text{AC211}}=(-0.46\pm0.08,\,-4.31\pm0.20)\,$ mas $\,$ yr $^{-1}$。根据这些测量结果,我们估计M15的整体自行为$(μ_α,\,μ_δ)=(-0.58\pm0.18,\,-4.05\pm0.34)\,$ mas $\,$ yr $^{-1}$。我们检测到两个以前已知的,但未分类的紧凑的来源在我们的视野。我们的观测表明,其中一个是河外起源,而另一个是前景源,很可能是LMXB。双中子星系统M15C在观测过程中变暗,消失了一年,现在又可以观测到了,这可能是大地岁差引起的。LMXB AC 211在一次观测中显示了一个双瓣结构,表明在这次活动中发生了一次爆发。随着最后两个共7个观测结果的列入,我们确认了一个假定的IMBH的质量上限为M $_{\bullet}<500$ M $_{\odot}$。

英文摘要:

The globular cluster (GC) M15 (NGC 7078) is host to at least eight pulsars and two low mass X-ray binaries (LMXBs) one of which is also visible in the radio regime. Here we present the results of a multi-epoch global very long baseline interferometry (VLBI) campaign aiming at i) measuring the proper motion of the known compact radio sources, ii) finding and classifying thus far undetected compact radio sources in the GC, and iii) detecting a signature of the putative intermediate mass black hole (IMBH) proposed to reside at the core of M15. We measure the sky motion in right ascension ($μ_α$) and declination ($μ_δ$) of the pulsars M15A and M15C and of the LMXB AC211 to be $(μ_α,\,μ_δ)_{\text{M15A}}=(-0.54\pm0.14,\,-4.33\pm0.25)\,$mas$\,$yr$^{-1}$, $(μ_α,\,μ_δ)_{\text{M15C}}=(-0.75\pm0.09,\,-3.52\pm0.13)\,$mas$\,$yr$^{-1}$, and $(μ_α,\,μ_δ)_{\text{AC211}}=(-0.46\pm0.08,\,-4.31\pm0.20)\,$mas$\,$yr$^{-1}$, respectively. Based on these measurements we estimate the global proper motion of M15 to be $(μ_α,\,μ_δ)=(-0.58\pm0.18,\,-4.05\pm0.34)\,$mas$\,$yr$^{-1}$. We detect two previously known but unclassified compact sources within our field of view. Our observations indicate that one them is of extragalactic origin while the other one is a foreground source, quite likely an LMXB. The double neutron star system M15C became fainter during the observations, disappeared for one year and is now observable again---an effect possibly caused by geodetic precession. The LMXB AC211 shows a double lobed structure in one of the observations indicative of an outburst during this campaign. With the inclusion of the last two of a total of seven observations we confirm the upper mass limit for a putative IMBH to be M$_{\bullet}<500$ M$_{\odot}$.

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