黑洞天體活動的基本面研究

董愛軍

(華中科技大學(xué)物理學(xué)院 武漢 430074)

活動星系核(中心黑洞質(zhì)量MBH～106?1010M⊙)和黑洞X射線雙星(MBH～10M⊙)普遍被認(rèn)為具有相似的中心引擎:黑洞、吸積盤和噴流.類似的中心引擎、質(zhì)量相卻差如此之大(6–9個數(shù)量級)的兩類黑洞系統(tǒng)是否具有相似的物理仍不清楚.本文圍繞不同尺度黑洞天體的物理性質(zhì)和觀測特性展開,主要研究了不同尺度黑洞天體活動的基本面關(guān)系以及黑洞X射線雙星的能譜演化.

本文首先研究了黑洞X射線雙星不同射電-X射線關(guān)系及其可能的物理本質(zhì).近年來,射電-X射線的“例外”分支被發(fā)現(xiàn),它的擬合斜率比正常分支更陡,表明可能具有與正常分支不同的吸積模式.該工作首先利用同時或準(zhǔn)同時性的射電-X射線觀測,研究了4個黑洞X射線雙星光子指數(shù)隨流量的演化.發(fā)現(xiàn)在3–9 keV的流量F3?9keV在低于和高于臨界流量FX,crit處,射電-X射線相關(guān)性也發(fā)生了明顯的變化:當(dāng)F3?9keVFX,crit時,射電-X射線流量的擬合斜率b較陡(～1.1?1.4),處于例外分支;當(dāng)F3?9keVFX,crit時,處于正常分支或轉(zhuǎn)換分支.該工作通過分析硬X射線譜指數(shù)的演化,驗證了射電-X射線相關(guān)性的正常分支和例外分支的X射線輻射分別來自于輻射低效和輻射高效的吸積模式,表明Γ-F3?9keV正、反相關(guān)性的變化是吸積模式轉(zhuǎn)換的結(jié)果,其中Γ為X射線譜指數(shù).

其次,研究了高輻射效率黑洞天體的基本面關(guān)系lgLR=ξXlgLX+ξMlgMBH+c,其中LR和LX分別為射電光度和X射線光度,MBH是黑洞質(zhì)量,ξX和ξM是擬合系數(shù),c為擬合常數(shù).該工作首先構(gòu)建了1個輻射高效黑洞天體的樣本,包括高光度射電寧靜活動星系核和例外源(熱光度Lbol和愛丁頓光度LEdd滿足Lbol1%LEdd).樣本的Γ-F3?9keV都是正相關(guān)的,且它們的5 GHz射電光度L5GHz與愛丁頓的比值L5GHz/LEdd和2–10 keV X射線光度L2?10keV與愛丁頓光度的比值L2?10keV/LEdd具有相似的相關(guān)性:L5GHz/LEdd∝(L2?10keV/LEdd)c,c～1.50.既然這兩類源具有相似的性質(zhì),本文分析并給出了它們的基本面關(guān)系:擬合彌散度為σR=0.51 dex.與輻射低效黑洞基本面關(guān)系相比,該基本面關(guān)系適用于輻射高效的黑洞天體.

再次,研究了寧靜態(tài)黑洞天體物理性質(zhì).寧靜態(tài)時射電-X射線的相關(guān)性是否延續(xù)低/硬態(tài)時的演化一直存在著兩種不同的觀點.我們首先構(gòu)建了1個比較完備的樣本,包括黑洞X射線雙星和低光度活動星系核,其光度范圍為LX=(10?9?10?3)LEdd.然后,分析了黑洞活動的基本面關(guān)系,得到了與輻射低效黑洞基本面關(guān)系相似的結(jié)果(ξX～0.60,ξM～0.80).該結(jié)果表明寧靜態(tài)黑洞的基本面關(guān)系仍然在低/硬態(tài)時的延長線上.同時,本文還利用黑洞質(zhì)量小范圍變化、X射線光度相差數(shù)個量級的活動星系核模擬了單個活動星系核從低/硬態(tài)向?qū)庫o態(tài)的射線-X射線相關(guān)性的演化并得到了相似的結(jié)果,表明寧靜態(tài)和低/硬態(tài)時的黑洞天體具有相似物理機(jī)制.

最后,以GX 339-4為例初步研究了黑洞X射線雙星的X射線能譜演化.本文利用RXTE 2007–2008年的觀測,研究了GX 339-4整個爆發(fā)過程譜指數(shù)Γ隨X射線流量F2?10keV的演化.結(jié)果表明,在光度上升和下降階段Γ-F2?10keV相關(guān)性遵循兩個不同的演化軌跡.該工作僅僅分析了GX 339-4一個完整的爆發(fā),后續(xù)工作將對更多的黑洞X射線雙星、更多的完整的爆發(fā)進(jìn)行研究,這對于進(jìn)一步限定黑洞吸積盤的物理機(jī)制具有重要意義.

It is generally believed that active galactic nuclei(AGNs)and black hole X-ray binaries(BHXRBs)have a similar central engine:black hole(BH),accretion disc,and jet.Whether the two kinds of sources with 6-10 orders magnitude differences in BH masses have a similar physical properties is still unknown.In this paper,we explore the physical properties in these BH sources,and especially investigate the possible universal correlations for these different-scale BH sources.

We firstly explore the possible physical reason that regulated the different radio-X-ray correlations as found in X-ray binaries(XRBs).We firstly explore the hard X-ray spectral evolution in four black-hole X-ray binaries with multiple quasi-simultaneous radio and X-ray observations,and find that the hard X-ray photon indices,Γ,are negatively and positively correlated with the X-ray fluxes respectively when the X-ray fluxF3?9keVis below and above a critical valueFX,crit,which is consistent with predictions of the advectiondominated accretion flow and the Shakura & Sunyaev disc-corona(SSD-corona)model,respectively.Most importantly,the radio-X-ray correlations are also changed simultaneously at this critical X-ray flux.As we know,the anti-and positive correlations of hard X-ray photon index and Eddington ratio are most possibly triggered by different accretion processes.Therefore,this work gives the direct evidence that the different tracks of radio-X-ray correlation may be triggered by different accretion processes.

Since the former fundamental plane(lgLR=ξXlgLX+ξMlgMBH+c,whereLRandLXare the radio and X-ray luminosity,respectively,MBH is the BH mass,the fitting slopesξX～0.60,ξM～0.80)is mainly applied to the radiatively inefficient BHs(e.g.,lowluminosity AGNs and low/hard state of XRBs),we next explore this issue for the radiatively efficient BHs.We firstly construct a sample consisting of the bright radio-quiet AGNs and outliers of XRBs withLX1%LEdd,whereLXis the X-ray luminosity,andLEddis the Eddington luminosity.We find these two populations follow a similar correlation between the hard X-ray photon indices and Eddington ratios.Basing on these similarities,we present a new fundamental planewith a scatter ofσR=0.51 dex,which is suitable for the radiatively efficient BHs.

In order to investigate whether theLR-LXcorrelation as defined in low/hard state of XRBs can extend to the quiescent state,or has a break when the X-ray luminosityLXis less than a critical valueLX,crit,we construct a sample consisting of XRBs and low luminosity active galactic nuclei(LLAGNs)with a wide distribution of the Eddington ratios.We find that there is a similar fundamental plane(ξX～0.6)forLXLX,critandLXLX,crit.We further simulate a single AGN with a narrow mass range evolving from the sub-Eddington to quiescent state,and find that the highly sub-Eddington sources still roughly stay on the extension of radio-X-ray correlation as defined by the sub-Eddington BHs.Therefore,ourresults support that the properties of quiescent BHs are similar to those of low/hard state.

Finally,we study the spectral evolution of XRBs in a whole outburst period,and try to constrain the possible accretion physics based on the spectral evolution.Because part of the work is still ongoing,we only take the whole 2007–2008 outburst of GX 339-4 as an example.We find that there are two different spectral evolutions in the rising and the decay periods of the 2007–2008 outburst.We will further study the spectral evolution of more complete outburst of XRBs to constrain the physical mechanism of BH accretion mode.

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