位于银河系和麦哲伦云中的X射线双星的长期X射线调制已被广泛研究。对于其他附近的星系，只有最明亮的X射线源可以通过专门的观测进行监测。本文首次系统研究了Swift监测的四个超亮X射线源（ESO 243-49 HLX-1、Holmberg IX X-1、M81 X-6和NGC 5408 X-1）的长期X射线变化。通过使用各种动力学技术分析它们的光变曲线，我们发现了几个有趣的低频准周期性。虽然周期性信号可能不代表任何稳定的轨道调制，这些检测揭示了这种长期的规律性模式可能与超轨道周期和吸积盘的结构有关。特别是，我们表明，ESO 243-49 HLX-1的爆发复发时间随时间而变化，并建议它可能不是轨道周期。相反，它可能是由于某些类型的岁差，而真正的双星周期预计要短得多。
英文摘要：
Long-term X-ray modulations on timescales from tens to hundreds of days have been widely studied for X-ray binaries located in the Milky Way and the Magellanic Clouds. For other nearby galaxies, only the most luminous X-ray sources can be monitored with dedicated observations. We here present the first systematic study of long-term X-ray variability of four ultraluminous X-ray sources (ESO 243-49 HLX-1, Holmberg IX X-1, M81 X-6, and NGC 5408 X-1) monitored with Swift. By using various dynamic techniques to analyse their light curves, we find several interesting low-frequency quasi-periodicities. Although the periodic signals may not represent any stable orbital modulations, these detections reveal that such long-term regular patterns may be related to superorbital periods and structure of the accretion discs. In particular, we show that the outburst recurrence time of ESO 243-49 HLX-1 varies over time and suggest that it may not be the orbital period. Instead, it may be due to some kinds of precession, and the true binary period is expected to be much shorter.
X-ray spectral variability in the ultraluminous X-ray source Holmberg IX X-1
我们使用XMM-牛顿和雨燕数据来研究超铝X射线源（ULX），Holmberg IX X-1的光谱变化。光源的光度变化3-4倍，引起相应的光谱变化，这些变化是显著的，但微妙的，并且不能很好地通过简单的硬度比来跟踪。相反，我们共同添加的雨燕数据的强度箱，并做全光谱拟合光盘加上热康普顿模型。所有的数据都很好地拟合了一个低温，光学厚Comptonising日冕，和可变性可以大致的特点是降低温度和增加光学深度的源变得更亮，如预期的那样，如果日冕是逐渐质量加载的材料吹离超爱丁顿内盘。在具有类似光谱的其他ULX中观察到这种可变性行为，但与在具有更软光谱的ULX中观察到的趋势相反。这支持了ULX中存在两种不同的物理机制的想法，其中光谱从由盘状日冕主导到由风主导。
We present the results of long-term monitoring of the X-ray emission from the ultraluminous X-ray source XMMUJ122939.9+075333 in the extragalactic globular cluster RZ2109. The combination of the high X-ray luminosity, short term X-ray variability, X-ray spectrum, and optical emission suggest that this system is likely an accreting black hole in a globular cluster. To study the long-term behavior of the X-ray emission from this source, we analyze both new and archival Chandra and XMM-Newton observations, covering 16 years from 2000 to 2016. For all of these observations, we fit extracted spectr…
Variability is a powerful tool to investigate properties of X-ray binaries (XRB), in particular for Ultraluminous X-ray sources (ULXs) that are mainly detected in the X-ray band. For most ULXs the nature of the accretor is unknown, although a few ULXs have been confirmed to be accreting at super-Eddington rates onto a neutron star (NS). Monitoring these sources is particularly useful both to detect transients and to derive periodicities, linked to orbital and super-orbital modulations. Here we present the results of our monitoring campaign of the galaxy NGC 925, performed with the Neil Gehrels…
A number of ultraluminous X-ray sources (ULXs) are physically associated with extragalactic globular clusters (GCs). We undertake a systematic X-ray analysis of eight of the brightest of these sources. We fit the spectra of the GC ULXs to single power law and single disk models. We find that the data never require that any of the sources change between a disk and a power law across successive observations. The GC ULXs best fit by a single disk show a bimodal distribution: they either have temperatures well below 0.5 keV, or variable temperatures ranging above 0.5 keV up to 2~keV. The GC ULXs w…
A variable ultraluminous X-ray source in a globular cluster in NGC 4649
本文报道了在椭圆星系NGC 4649中发现的一个与球状星团有关的新的超亮X射线源（ULX）。最初探测到的X射线源的光度低于5 x 10^38 erg/s，但在7年和11年后的后续观测中，它已大幅增亮至2 ~ 3 x 10^39 erg/s。在六个独立的观测过程中，它显示出显着的光谱变化，在连续斜率和吸收柱。在至少一次观察中，还存在X射线通量的短期变异性。这个天体的性质似乎与以超爱丁顿速率（即在超铝吸积状态）吸积的恒星质量黑洞一致，尽管不能排除高度超爱丁顿中子星。在至少一次观测中，吸收柱的增加与短期变率的可能增强相一致，这表明有一股由clusifully辐射驱动的风穿过我们的视线到达该物体
Most ultraluminous X-ray sources (ULXs) are thought to be powered by neutron stars and black holes accreting beyond the Eddington limit. If the compact object is a black hole or a neutron star with a magnetic field $\lesssim10^{12}$ G, the accretion disc is expected to thicken and launch powerful winds driven by radiation pressure. Evidence of such winds has been found in ULXs through the high-resolution spectrometers onboard XMM-Newton, but several unknowns remain, such as the geometry and launching mechanism of these winds. In order to better understand ULX winds and their link to the accret…
Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1
NGC 55 ULX 1是一个明亮的超铝X射线源，距离地球1.78 Mpc。我们分析了2013年4月至8月期间拍摄的20个Swift观测样本，以及2001年9月和2004年6月拍摄的两个Chandra观测样本。我们只发现了有限数量的光变曲线的下降，以前报道发生在这个源的边缘提示，虽然由于数据的低计数统计的不确定性很大。Chandra和Swift光谱显示出清晰的光谱变异性，这与在其他ULX中观察到的相似。我们可以解释这种光谱的变化方面的变化，在两个标准化和固有的柱密度的两个组件的模型组成的黑体（软组件）和多色吸积盘（硬组件）。我们讨论的可能性，强大的外流喷出的光盘是部分负责这种光谱的变化。
Using simultaneous optical (VLT/FORS2) and X-ray (XMM-Newton) data of NGC 5408, we present the first ever attempt to search for a reverberation signal in an ultraluminous X-ray source (NGC 5408 X-1). The idea is similar to AGN broad line reverberation mapping where a lag measurement between the X-ray and the optical flux combined with a Keplerian velocity estimate should enable us to weigh the central compact object. We find that although NGC 5408 X-1's X-rays are variable on a timescale of a few hundred seconds (RMS of 9.0$\pm$0.5%), the optical emission does not show any statistically signif…
We report X-ray spectral variability in an ultraluminous X-ray source NGC 1042 ULX1, using archival XMM-Newton and recent NuSTAR observations. In long-term evolution, the source has shown a trend of variation in spectral hardness. The variability in different XMM-Newton observations is prominent above $\sim 1$ keV. Cool thermal disk component with a characteristic temperature of $\sim 0.2$ keV manifests that the spectral state of NGC 1042 ULX1 in all epochs is similar to that of the ultraluminous state sources. An apparent anti-correlation between luminosity and powerlaw index demonstrates tha…
Transient ultraluminous X-ray sources (ULXs) provide an important link bridging transient low-mass X-ray binaries and ULXs. Here we report the first discovery of both a canonical sub-Eddington outburst and an ultraluminous super-Eddington outburst from an unusual transient ULX, NGC 7793 P9 with a variability factor higher than $10^3$. Its X-ray spectrum switches between the typical high/soft state and the steep power-law state during the canonical outburst. The inner radius of the accretion disk and the disk temperature--luminosity correlation suggest that P9 harbors a stellar-mass black hole …
The study of ultraluminous X-ray sources (ULXs) has changed dramatically over the last decade. In this review we first describe the most important observations of ULXs in various wavebands, and across multiple scales in space and time. We discuss recent progress and current unanswered questions. We consider the range of current theories of ULX properties in the light of this observational progress. Applying these models to neutron-star ULXs offers particularly stringent tests, as this is the unique case where the mass of the accretor is effectively fixed.
In our recent work, we demonstrated that a novel X-ray scaling method, originally introduced for Galactic black holes, could be reliably extended to estimate the mass of supermassive black holes accreting at moderate to high level. Here, we apply this X-ray scaling method to ultraluminous X-ray sources (ULXs) to constrain their \mbh. Using 49 ULXs with multiple \xmm\ observations, we infer that ULXs host both stellar mass BHs and intermediate mass BHs. The majority of the sources of our sample seem to be consistent with the hypothesis of highly accreting massive stellar BHs with $M_{\rm BH}\si…
The origin of Ultraluminous X-ray sources (ULXs) in external galaxies whose X-ray luminosities exceed those of the brightest black holes in our Galaxy by hundreds and thousands of times is mysterious. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. Here we review the ULX properties, their X-ray spectra indicate a presence of hot winds in their accretion disks supposing the supercritical accretion. However, the strongest evidences come from optical spectroscopy. The spectra of the ULX count…
Ultraluminous X-ray bursts (hereafter ULXBs) are ultraluminous X-ray flares with a fast rise ($\sim$ one minute) and a slow decay ($\sim$ an hour), which are commonly observed in extragalactic globular clusters. Most ULXBs are observational one-off bursts, whereas five flares from the same source in NGC 5128 were discovered by Irwin et al. (2016). In this Letter, we propose a neutron star (NS)-white dwarf (WD) binary model with super-Eddington accretion rates to explain the repeating behavior of the ULXB source in NGC 5128. With an eccentric orbit, the mass transfer occurs at the periastron wh…
Ultraluminous X-ray sources (ULXs) represent a class of binary systems that are more luminous than any black hole in our Galaxy. The nature of these objects remained unclear for a long time. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. In the last few years our understating of these objects was significantly improved, which made the model of super-Eddington accretion much preferable. Both the X-ray and optical spectra provide evidence for the strong outflows coming from supercritical ac…
The mechanism that can be responsible for the bimodal luminosity distribution of super-Eddington X-ray pulsars in binary systems is pointed out. The transition from the high to low state of these objects is explained by accretion flow spherization due to the radiation pressure at certain (high) accretion rates. The transition between the states can be associated with a gradual change in the accretion rate. The complex behavior of the recently discovered ultraluminous X-ray pulsars M 82 X-2, NGC 5907 ULX-1, and NGC 7793 P13 is explained by the proposed mechanism. The proposed model also natural…
We report a possible detection of a 55-day X-ray modulation for the ultraluminous accreting pulsar M82 X-2 from archival Chandra observations. Because M82 X-2 is known to have a 2.5-day orbital period, if the 55-day period is real, it will be the superorbital period of the system. We also investigated variabilities of other three nearby ultraluminous X-ray sources in the central region of M82 with the Chandra data and did not find any evidence of periodicities. Furthermore, we re-examined the previously reported 62-day periodicity near the central region of M82 by performing a systematic timin…
We analyzed the X-ray power density spectra of five ultraluminous X-ray sources (ULXs) NGC5408 X-1, NGC6946 X-1, M82 X-1, NGC1313 X-1 and IC342 X-1 that are the only ULXs which display both flat-topped noise (FTN) and quasi-periodic oscillations (QPO). We studied the QPO frequencies, fractional root-mean-square (rms) variability, X-ray luminosity and spectral hardness. We found that the level of FTN is anti-correlated with the QPO frequency. As the frequency of the QPO and brightness of the sources increase, their fractional variability decreases. We propose a simple interpretation using the s…
We review observations of ultraluminous X-ray sources (ULXs). X-ray spectroscopic and timing studies of ULXs suggest a new accretion state distinct from those seen in Galactic stellar-mass black hole binaries. The detection of coherent pulsations indicates the presence of neutron-star accretors in three ULXs and therefore apparently super-Eddington luminosities. Optical and X-ray line profiles of ULXs and the properties of associated radio and optical nebulae suggest that ULXs produce powerful outflows, also indicative of super-Eddington accretion. We discuss models of super-Eddington accretio…
We investigate a sample of seven edge-on spiral galaxies using Chandra observations. Edge-on spiral galaxies allow us to clearly separate source associated with their star-forming regions versus the outer edges of the system; offering a clear advantage over other systems. We uncover a number of X-ray point sources across these galaxies, and after eliminating contaminating foreground and background sources, we identify 12 candidate ultraluminous X-ray sources. All of these sources are projected onto the central regions, implying that the majority of ULXs in this sample of spiral galaxies are di…
Ultraluminous X-ray sources (ULXs) are bright extragalactic sources with X-ray luminosities above 10^39 erg/s powered by accretion onto compact objects. According to the first studies performed with XMM-Newton ULXs seemed to be excellent candidates to host intermediate-mass black holes (10^2-4 solar masses). However, in the last years the interpretation of super-Eddington accretion onto stellar-mass black holes or neutron stars for most ULXs has gained a strong consensus. One critical missing piece to confirm the super-Eddington scenario was the direct detection of the massive, radiatively-dri…
Most ultraluminous X-ray sources (ULXs) are argued to be powered by supercritical accretion onto compact objects. One of the key questions regarding these objects is whether or not the hard X-rays are geometrically beamed toward the symmetric axis. We propose to test the scenario using disk irradiation, to see how much the outer accretion disk sees the central hard X-rays. We collect a sample of 11 bright ULXs with an identification of a unique optical counterpart, and model their optical fluxes considering two irradiating sources: soft X-rays from the photosphere of the optically thick wind d…
NGC 4472 is home to five ultraluminous X-ray sources hosted by globular clusters. These sources have been suggested as good black hole candidates in extragalactic globular clusters$-$ a highly sought after population that may provide observational information regarding the progenitors of merging black hole binaries. In this body of work, we present X-ray and optical follow up to one of these sources, CXOUJ1229410+075744 (GCU1). We find no evidence of [OIII] optical emission in GCU1 which indicates a lack of significant evidence for super-Eddington outflows, unlike what is seen in a handful of …
The nature of ultraluminous X-ray sources (ULXs) is not yet fully understood but it can be done in the most efficient way by analyzing its optical emission and environment. We present preliminary results of photometric and spectral studies of some newly identified ULX optical counterparts. Based on the X-ray catalog of ULX candidates by Liu 2011 we have unequivocally identified in the optical range more than 100 sources, for 49 of them we carried out a long slit spectroscopy. According to these results, the majority of the most brightest candidates (m$_V < 22^m$) turned out to be background ob…
Swift observations of the ultraluminous X-ray source Holmberg IX X-1
Holmberg IX X-1是一个著名的超亮X射线源，其X射线亮度约为1 e40 erg/s。斯威夫特的X射线望远镜定期监测该源。自2009年4月以来，该源一直处于扩展的低光度状态。我们利用在不同光度状态下的共加光谱来研究源的光谱行为。简单的幂律和多色盘黑体模型可以排除。然而，最好的整体拟合是由具有冷黑体和暖盘黑体的双热模型提供的。这表明Holmberg IX X-1可能是一个10个太阳质量的黑洞，在比爱丁顿极限高7倍的地方吸积，或者是一个100个太阳质量的最大旋转黑洞，在爱丁顿极限处吸积，我们正在观察吸积盘的内部区域和致密物体的流出物。
The discovery of the ultraluminous X-ray pulsar M82 X-2 has stimulated lively discussion on the nature of the accreting neutron star. In most of the previous studies the magnetic field of the neutron star was derived from the observed spin-up/down rates based on the standard thin, magnetized accretion disk model. However, under super-Eddington accretion the inner part of the accretion disk becomes geometrically thick. In this work we consider both radiation feedback from the neutron star and the sub-Keplerian rotation in a thick disk, and calculate the magnetic moment - mass accretion rate rel…
Ultraluminous X-ray sources are extragalactic, off-nucleus, point sources in galaxies with an X-ray luminosity above 3x10^39 erg/s, thought to be powered by accretion onto a compact object. Possible explanations include accretion onto neutron stars with strong magnetic fields, stellar-mass black holes (< 20 Msolar) at or in excess of the classical Eddington limit or intermediate-mass black holes (10^3-5 Msolar). The lack of sufficient energy resolution in previous analyses has hitherto prevented an unambiguous identification of any emission or absorption lines in the X-ray band, thereby preclu…
We report the detection of coherent pulsations from the ultraluminous X-ray source NGC 7793 P13. The ~0.42s nearly sinusoidal pulsations were initially discovered in broadband X-ray observations using XMM-Newton and NuSTAR taken in 2016. We subsequently also found pulsations in archival XMM-Newton data taken in 2013 and 2014. The significant (>>5 sigma) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star with an observed peak luminosity of ~1e40 erg/s (assuming isotropy), well above the Eddington limit for a 1.4 M_sun accretor. This makes P13 the seco…
We have identified three ultraluminous X-ray sources (ULXs) hosted by globular clusters (GCs) within NGC 1316's stellar system. These discoveries bring the total number of known ULXs in GCs up to 20. We find that the X-ray spectra of the three new sources do not deviate from the established pattern of spectral behaviour of the other known GC ULXs. The consistency of the X-ray spectral behaviour for these sources points to multiple paths of formation and evolution mechanisms for these rare and unique sources. Using the now larger sample of GC ULXs, we compare the optical properties of the entir…
Most ultraluminous X-ray sources (ULXs) are believed to be stellar mass black holes or neutron stars accreting beyond the Eddington limit. Determining the nature of the compact object and the accretion mode from broadband spectroscopy is currently a challenge, but the observed timing properties provide insight into the compact object and details of the geometry and accretion processes. Here we report a timing analysis for an 800 ks XMM-Newton campaign on the supersoft ultraluminous X-ray source, NGC 247 ULX-1. Deep and frequent dips occur in the X-ray light curve, with the amplitude increasing…
Pulsations were recently detected from the ultraluminous X-ray source X-2 in M82. The newly discovered pulsar has been described as a common neutron star with a 1 TG magnetic field that accretes above the Eddington rate and as a magnetar-like pulsar with a 100 TG magnetic field that is above the quantum limit. We show here that this X-ray source is not exotic in any sense. The standard equations of accretion along field lines predict that, for the measured spin period $P_S$ and spinup rate $\dot{P_S}$, the isotropic X-ray luminosity $L_X$ must be near the Eddington limit (i.e., $L_{X}\approx 3…
