* Astronomy

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V1357 Cygni

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RE: Cygnus X-1

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Title: Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states
Authors: S. Sabatini, M. Tavani, P. Coppi, G. Pooley, M. Del Santo, R. Campana, A. Chen, Y. Evangelista, G. Piano, A. Bulgarelli, P. W. Cattaneo, S. Colafrancesco, E. Del Monte, A. Giuliani, M. Giusti, F. Longo, A. Morselli, A. Pellizzoni, M. Pilia, E. Striani, M. Trifoglio, S. Vercellone

We present the results of multi-year gamma-ray observations by the AGILE satellite of the black hole binary system Cygnus X-1. In a previous investigation we focused on gamma-ray observations of Cygnus X-1 in the hard state during the period mid-2007/2009. Here we present the results of the gamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 carried out for which includes a remarkably prolonged 'soft state' phase (June 2010 -- May 2011). Previous 1--10 MeV observations of Cyg X-1 in this state hinted at a possible existence of a non-thermal particle component with substantial modifications of the Comptonised emission from the inner accretion disk. Our AGILE data, averaged over the mid-2010/mid-2011 soft state of Cygnus X-1, provide a significant upper limit for gamma-ray emission above 100 MeV of F_soft < 20 x 10^{-8} ph/cm²/s, excluding the existence of prominent non-thermal emission above 100 MeV during the soft state of Cygnus X-1. We discuss theoretical implications of our findings in the context of high-energy emission models of black hole accretion. We also discuss possible gamma-ray flares detected by AGILE. In addition to a previously reported episode observed by AGILE in October 2009 during the hard state, we report a weak but important candidate for enhanced emission which occurred at the end of June 2010 (2010-06-30 10:00 - 2010-07-02 10:00 UT) exactly in coincidence with a hard-to-soft state transition and before an anomalous radio flare. An appendix summarises all previous high-energy observations and possible detections of Cygnus X-1 above 1 MeV.

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Title: Comprehensive Spectral Analysis of Cyg X-1 using RXTE Data
Authors: Rizwan Shahid, R. Misra, S. N. A. Jaaffrey

We analyse a large number (> 500) pointed RXTE observations of Cyg X-1 and model the spectrum of each one. A subset of the observations for which there is simultaneous reliable measure of the hardness ratio by the All Sky Monitor, shows that the sample covers nearly all the spectral shapes of Cyg X-1. The relative strength, width of the Iron line and the reflection parameter are in general correlated with the high energy photon spectral index \Gamma. This is broadly consistent with a geometry where for the hard state (low \Gamma ~ 1.7) there is a hot inner Comptonising region surrounded by a truncated cold disk. The inner edge of the disk moves inwards as the source becomes softer till finally in the soft state (high \Gamma > 2.2) the disk fills the inner region and active regions above the disk produce the Comptonised component. However, the reflection parameter shows non-monotonic behaviour near the transition region (\Gamma ~ 2), suggestive of a more complex geometry or physical state of the reflector. Additionally, the inner disk temperature, during the hard state, is on the average higher than in the soft one, albeit with large scatter. These inconsistencies could be due to limitations in the data and the empirical model used to fit them. The flux of each spectral component is well correlated with \Gamma which shows that unlike some other black hole systems, Cyg X-1 does not show any hysteresis behaviour. In the soft state, the flux of the Comptonised component is always similar to the disk one, which confirms that the ultra-soft state (seen in other brighter black hole systems) is not exhibited by Cyg X-1. The rapid variation of the Compton Amplification factor with \Gamma, naturally explains the absence of spectra with \Gamma < 1.6, despite a large number having \Gamma ~ 1.65.

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Title: Millisecond X-Ray Pulses from Cygnus X-1
Authors: J. F. Dolan

X-ray pulses with millisecond-long FWHM have been detected in RXTE (Rossi X-Ray Timing Explorer) observations of Cyg X-1. Their identity as short-timescale variations in the X-ray luminosity of the source, and not stochastic variability in the X-ray flux, is established by their simultaneous occurrence and similar pulse structure in two independent energy bandpasses. The light-time distance corresponding to the timescale of their FWHM indicates that they originate in the inner region of the accretion disk around the system's black hole component. The fluence in the pulses can equal or exceed the fluence of the system's average continuous flux over the duration of the pulses' FWHM in several different bandpasses between 1 and 73 keV. Millisecond pulses are detected during both high and low luminosity states of Cyg X-1, and during transitions between luminosity states.

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VLBA Distance Measurement Is Key to Producing First "Complete Description" of a Black Hole

For the first time, astronomers have produced a complete description of a black hole, a concentration of mass so dense that not even light can escape its powerful gravitational pull. Their precise measurements have allowed them to reconstruct the history of the object from its birth some six million years ago.
Using several telescopes, both ground-based and in orbit, the scientists unravelled longstanding mysteries about the object called Cygnus X-1, a famous binary-star system discovered to be strongly emitting X-rays nearly a half-century ago. The system consists of a black hole and a companion star from which the black hole is drawing material. The scientists' efforts yielded the most accurate measurements ever of the black hole's mass and spin rate.
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A Black Hole - Cygnus X-1

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NASA's Chandra Adds to Black Hole Birth Announcement

On the left, an optical image from the Digitized Sky Survey shows Cygnus X-1, outlined in a red box. Cygnus X-1 is located near large active regions of star formation in the Milky Way, as seen in this image that spans some 700 light years across. An artist's illustration on the right depicts what astronomers think is happening within the Cygnus X-1 system. Cygnus X-1 is a so-called stellar-mass black hole, a class of black holes that comes from the collapse of a massive star. The black hole pulls material from a massive, blue companion star toward it. This material forms a disk (shown in red and orange) that rotates around the black hole before falling into it or being redirected away from the black hole in the form of powerful jets.
A trio of papers with data from radio, optical and X-ray telescopes, including NASA's Chandra X-ray Observatory, has revealed new details about the birth of this famous black hole that took place millions of years ago.
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Cygnus X-1 was identified as a likely black hole in 1972, but its distance from Earth has been maddeningly difficult to pin down. This in turn has made it hard to determine basic properties like its mass and spin.
Mark Reid of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, and colleagues used the Very Long Baseline Array of radio telescopes spanning the US to measure the object's parallax - tiny shifts in its apparent position due to Earth's motion around the sun. Based on the size of the shifts, Cygnus X-1 is 6000 light years away, give or take a few hundred light years, the team reports
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Title: The Trigonometric Parallax of Cygnus X-1
Authors: Mark J. Reid, Jeffrey E. McClintock, Ramesh Narayan, Lijun Gou, Ronald A. Remillard, Jerome A. Orosz

We report a direct and accurate measurement of the distance to the X-ray binary Cygnus X-1, which contains the first black hole to be discovered. The distance of 1.86(-0.11,+0.12) kpc was obtained from a trigonometric parallax measurement using the Very Long Baseline Array. The position measurements are also sensitive to the 5.6 d binary orbit and we determine the orbit to be clockwise on the sky. We also measured the proper motion of Cygnus X-1 which, when coupled to the distance and Doppler shift, gives the three-dimensional space motion of the system. When corrected for differential Galactic rotation, the non-circular (peculiar) motion of the binary is only about 21 km/s, indicating that the binary did not experience a large "kick" at formation.

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Title: The Extreme Spin of the Black Hole in Cygnus X-1
Authors: Lijun Gou, Jeffrey E. McClintock, Mark J. Reid, Jerome A. Orosz, James F. Steiner, Ramesh Narayan, Jingen Xiang, Ronald A. Remillard, Keith A. Arnaud, Shane W. Davis

The compact primary in the X-ray binary Cygnus X-1 was the first black hole to be established via dynamical observations. We have recently determined accurate values for its mass and distance, and for the orbital inclination angle of the binary. Building on these results, which are based on our favoured (asynchronous) dynamical model, we have measured the radius of the inner edge of the black hole's accretion disk by fitting its thermal continuum spectrum to a fully relativistic model of a thin accretion disk. Assuming that the spin axis of the black hole is aligned with the orbital angular momentum vector, we have determined that Cygnus X-1 contains a near-extreme Kerr black hole with a spin parameter a/M>0.97 (3 \sigma). For a less probable (synchronous) dynamical model, we find a/M>0.91 (3 \sigma). Our results take into account all significant sources of observational and model-parameter uncertainties, which are dominated by the uncertainties in black hole mass, orbital inclination angle and distance. The uncertainties introduced by the thin-disk model we employ are particularly small in this case, given the disk's low luminosity (L/L_{Edd} ~ 0.02).

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