ANS observations of Cygnus X-1

Cygnus X-1 was observed from 3 November until 9 November, 1974, using the hard X-ray (1–28 keV) and soft X-ray (1–8 keV) experiment on board the Astronomical Netherlands Satellite. On three occasions, on 4 and 5 November, the X-ray spectrum was observed to be harder, while the flux intensity in the 1–7 keV ranges decreased by 50% from its quiescent value. These events occurred near and following the time of superior conjunction of the likely optical counterpart HDE 226 868. These events appear to be the same as previously reported absorption dips and reveal this phenomenon to be more complex than had been believed. A systematic study of the X-ray spectral variations in these energy bands, on a time scale of 64 seconds over the period of the spectroscopic binary, is presented.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.On leave from Institute of Space and Aeronautical Science, University of Tokyo.     

X-ray spectroscopy of Cygnus X-3

We have analysed the X-ray spectra of the highly variable X-ray source Cygnus X-3 over a wide energy range from 5 keV to 150 keV using data selected from the RXTE archives. Separate analysis of the low and hard states show the presence of a hard powerlaw tail in both the states. Here we present the result of the wide band spectral study of the source.     

Interstellar absorption and variable soft X-ray component in Cygnus X-1

The hydrogen column density along the line of sight to Cyg X-1 is 7×10²¹ cm^–2 as determined from the extinction of its optical counterpart HD 226 868. This value may be used to interpret soft X-ray measurements, including those previously reported, where it is not possible to determine the column density independently from the intrinsic spectral function. The correction for interstellar absorption is larger than previously thought. Application to an old observation suggests that an intense soft X-ray component was present in Cyg X-1, even though the data at 1 keV suggests that it was probably in a low state. This is consistent with the picture of Cyg X-1 suggested by Price and Thorne, in which transitions in Cyg X-1 are attributed to changes in the high energy cut-off of an intense soft component.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

X-ray emission behaviour of Her X-1

The paper presents an evidence for a steady-state flux of high-energy photons from Her X-1. The results are obtained from the analysis of the existing spectral measurements of the X-ray source. The continuum emission has three times more energy than that emitted during the pulse mode. The possible circumstances in which the source can simultaneously emit in two modes are presented within the framework of standard model generally accepted for this binary source.On leave from TATA Institute of Fundamental Research, Bombay, India.     

The aperiodic broad-band X-ray variability of Cygnus X-3

We study the soft X-ray variability of Cygnus X-3. By combining data from the All-Sky Monitor and Proportional Counter Array instruments on the RXTE satellite with EXOSAT /Medium Energy (ME) detector observations, we are able to analyse the power density spectrum (PDS) of the source from 10⁻⁹ to 0.1 Hz, thus covering time-scales from seconds to years. As the data on the longer time-scales are unevenly sampled, we combine traditional power spectral techniques with simulations to analyse the variability in this range. The PDS at higher frequencies  (≳10⁻³ Hz)  are for the first time compared for all states of this source. We find that it is for all states well described by a power law, with index  ∼−2  in the soft states and a tendency for a less steep power law in the hard state. At longer time-scales, we study the effect of the state transitions on the PDS, and find that the variability below  ∼10⁻⁷ Hz  is dominated by the transitions. Furthermore, we find no correlation between the length of a high/soft-state episode and the time since the previous high/soft state. On intermediate time-scales, we find evidence for a break in the PDS at time-scales of the order of the orbital period. This may be interpreted as evidence for the existence of a tidal resonance in the accretion disc around the compact object, and constraining the mass ratio to   M ₂/ M ₁≲ 0.3  .     

A transient relativistic radio jet from Cygnus X-1

We report the first observation of a transient relativistic jet from the canonical black hole candidate, Cygnus X-1, obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN). The jet was observed in only one of six epochs of MERLIN imaging of the source during a phase of repeated X-ray spectral transitions in 2004 Jan–Feb, and this epoch corresponded to the softest 1.5–12 keV X-ray spectrum. With only a single epoch revealing the jet, we cannot formally constrain its velocity. Nevertheless, several lines of reasoning suggest that the jet was probably launched 0.5–4.0 d before this brightening, corresponding to projected velocities of  0.2 c ≲ v _app≲ 1.6 c   , and an intrinsic velocity of  ≳0.3 c   . We also report the occurrence of a major radio flare from Cyg X-1, reaching a flux density of ∼120 mJy at 15 GHz, and yet not associated with any resolvable radio emission, despite a concerted effort with MERLIN. We discuss the resolved jet in terms of the recently proposed 'unified model' for the disc–jet coupling in black hole X-ray binaries, and tentatively identify the 'jet line' for Cyg X-1. The source is consistent with the model in the sense that a steady jet appears to persist initially when the X-ray spectrum starts softening, and that once the spectral softening is complete the core radio emission is suppressed and transient ejecta/shock observed. However, there are some anomalies, and Cyg X-1 clearly does not behave like a normal black hole transient in progressing to the canonical soft/thermal state once the ejection event has happened.     

On possible long-period pulsations in the hard X-ray flux of Cygnus X-1

Fourier analysis performed on the data of three balloon observations (20–200 keV) of Cygnus X-1 has shown a significant peak of the power spectral density at about 17 s. Possible periodic pulsations, sustained over periods of at least one hour, could be responsible for the peak we detected. Results of computer simulations in terms of a superposition of randomly occurring pulses give some support to this interpretation.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held in Varna, Bulgaria, May 1975.     

Observation of soft X-ray emission from a region near Per X-1

Soft X-ray emission from the X-ray source Per X-1 was observed in the 0.4–2 keV energy interval from a rocket borne X-ray detector. Spectral analysis of the data indicates that in the 0.4–2 keV band the X-ray emission from Per X-1 can be fitted either with a power law of slope-(4.8±1.2) or a thermal bremsstrahlung spectrum with akT value of (0.26 _?0.08 ^+0.12 ) keV. Such a steep spectrum is inconsistent with the spectrum measured above 2 keV. The measured flux in 0.4–2 keV band corresponds to X-ray luminosity of 3×10⁴⁵ ergs s^?1 for Per X-1.     

High resolution images of Cygnus X-1

The classic black hole candidate Cygnus X-1 exhibits many similarities to Galactic jet sources (micro-quasars) in soft gamma and hard X-ray rays. This has fuelled suspicions that radio jets also may be present in Cygnus X-1. The lack of radio flaring in Cygnus X-1 has been associated with the presence of continuous jets rather than multiple plasmon ejection however no evidence for this has been observed with the VLA, or with MERLIN. Recent VLBA observations at 15 GHz have detected extended emission and further multi frequency observations are required to confirm the presence of jets on a milliarcsecond level.     

X-ray timing behaviour of Cygnus X-2 at low intensities

It is known that the overall (mean) intensity of the low-mass X-ray binary Cyg X-2 varies on time-scales from a day to months, independently of the variations on time-scales of hours to a day by which the source moves between the horizontal, normal and flaring branches.
We present RXTE PCA observations of Cyg X-2, taken when its overall intensity was near its lowest values, in 1996 October and 1997 September. For the first time we perform a study of the fast timing behaviour at such low intensities. During the 1996 October observations, the source was in the left part of the horizontal branch, and during the 1997 September observations was most likely in the lower parts of the normal branch and flaring branch.
We find that the properties of the very low-frequency noise during the 1997 September observations are consistent with a monotonic decrease in its strength and power-law index as a function of overall intensity. In contrast, the strength of the ∼6 Hz normal branch quasi-periodic oscillations does not vary monotonically with overall intensity. They are strongest at medium overall intensity and weaker both when the overall intensity is low and when the overall intensity is high.     

Absorption of gamma rays by X-ray photons around Cygnus X-3

We calculated the absorption of gamma rays of Cygnus X — 3 by the ambient X — ray photons through photon-photon pair production and inverse Compton scattering. Differences in the gamma ray spectrum between the X — ray high state and low state indicate the presence of this absorption.     

Cyg X-1频谱的细微结构

利用线性状态空间模型（Ｌｉｎｅａｒ Ｓｔａｔｅ Ｓｐａｃｅ Ｍｏｄｅｌ－ＬＳＳＭ）,将 Ｃｙｇ Ｘ－１的功率谱分解为ＬＳＳＭ产生的模型功率谱和剩余功率谱两部分．集中分析剩余功率谱,结果显示, Ｃｙｇ Ｘ－１在低／硬态、转换态和高／软态三种状态的剩余功率谱有着不同的结构特征．其中高／软态和低／硬态的剩余功率谱存在一个宽峰结构,中心频率分别为～ ８ Ｈｚ和～ ５ Ｈｚ,低／硬态的剩余功率诺在～ ０．０７ Ｈｚ达到极大;转换态的剩余功率谱有双峰结构,其中心频率分别在～０．５—４Ｈｚ和～５—１１Ｈｚ,与我们以前分析的 ＱＰＯ的中心频率很接近,说明 ＱＰＯ与线性状态空间所要求的自回归模型可能属于不同的物理过程．与总功率谱比较,剩余功率谱对谱的变化部分（例如ＱＰＯ、截断频率等）更为敏感,本文对剩余功率谱中谱峰变化的原因做了初步的探讨．     

Effects of the stellar wind on X-ray spectra of Cygnus X-3

We study X-ray spectra of Cyg X-3 from BeppoSAX , taking into account absorption and emission in the strong stellar wind of its companion. We find the intrinsic X-ray spectra are well modelled by disc blackbody emission, its upscattering by hot electrons with a hybrid distribution, and by Compton reflection. These spectra are strongly modified by absorption and reprocessing in the stellar wind, which we model using the photoionization code cloudy . The form of the observed spectra implies the wind is composed of two phases. A hot tenuous plasma containing most of the wind mass is required to account for the observed features of very strongly ionized Fe. Small dense cool clumps filling ≲0.01 of the volume are required to absorb the soft X-ray excess, which is emitted by the hot phase but not present in the data. The total mass-loss rate is found to be  (0.6–1.6) × 10⁻⁵ M_⊙ yr⁻¹  . We also discuss the feasibility of the continuum model dominated by Compton reflection, which we find to best describe our data. The intrinsic luminosities of our models suggest that the compact object is a black hole.     

Possible transient emission from Cygnus X-3 at TeV energies during October 1985 radio flare

The Gulmarg gamma-ray telescope (threshold energy 6 TeV) was tracking Cygnus X-3 on four days in October 1985, around the time the source produced one of its most outstanding radio flares. Examined here is the behaviour of the event rates recorded during this period with a time-resolution of 216 s. Two episodes of higher event rates, each several minutes in duration and having a Poissonian significance of 4, were recorded on 10 and 12 October, suggesting possible activity at TeV energies during the radio flare period of Cygnus X-3.