HEAPOW: The Spin of SpaceTime (2011 Nov 28)

[bystander]

HEAPOW: The Spin of SpaceTime (2011 Nov 28)

Black Holes are produced when matter is crushed to infinite density, producing a "rip" in the space-time continuum surrounded by an "event horizon" within which no occurrence or characteristic can be communicated to the extant Universe. There are only three properties of black holes that can be directly measured, at least in principle: how massive they are; whether they possess electrical charge or not; and how quickly they are rotating. Admittedly trying to understand the spin of an object that has no spatial dimension is a difficult undertaking, but spinning black holes can have measurable effects on the spacetime around them, at least according to Einstein. The best-studied black hole is also the first black hole to be discovered, an X-ray bright binary star system called Cygnus X-1. The black hole in Cygnus X-1 is swallowing material from its companion, a bright, massive, normal star called HDE 226868, and as this material spirals onto the black hole's event horizon (and effectively leaves our Universe), it heats up and generates X-rays. Astronomers have now used improved measures of the distance and mass of the black hole in Cygnus X-1, along with X-ray observations from the Rossi X-ray Timing Explorer, the Advanced Satellite for Cosmology and Astrophysics, and the Chandra X-ray Observatory to determine a precise value of the spin of Cyg X-1's black hole. The graph above shows the X-ray behavior of Cyg X-1 as measured by RXTE's All Sky Monitor, and the red stars indicate those rare times when the spin period of Cyg X-1's black hole can be determined. Astronomers have found the surprising result that the black hole in Cyg X-1 is spinning nearly as rapidly as the laws of physics allow.

CXC: Cygnus X-1: Chandra Adds to Black Hole Birth Announcement
NRAO: VLBA Helps with First 'Complete Description' of a Black Hole

The Extreme Spin of the Black Hole in Cygnus X-1 - Lijun Gou et al

• Astrophysical Journal 742(2) 85 (2011 Dec 01) DOI: 10.1088/0004-637X/742/2/85
  arXiv.org > astro-ph > arXiv:1106.3690 > 18 Jun 2011 (v1), 30 Sep 2011 (v2)

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Author: Dr. Michael F. Corcoran

[Ann]

An artist's eonception of Cygnus X-1.
Image: Chandra X-ray Observatory/NASA

Gemma Lavender of Astronomy Now online made a post about Cygnus X-1 on November 28, 2011. Here she describes how a team of astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) and San Diego State University have gone about to make a full description of the black hole as well as the normal massive companion of binary system Cygnus X-1.

Emma Lavender wrote:

“Because no other information can escape from a black hole, knowing its mass, spin and electrical charge gives a complete description of it,” says Mark Reid of the CfA.

The charge of the black hole is zero, so the only outstanding characteristics to be measured were its mass and spin.

With the assistance of the National Science Foundation’s Very Long Baseline Array – a continent-wide radio telescope system – Reid and his team set to work on making a direct trigonometric measurement of the distance to Cygnus X-1, which served as a basis for uncovering the long-awaited details of the black hole that nestles in the far away system.
...
With their VLBA observations, the team were left with a distance of 6,070 light years

“To measure the mass of the black hole and O-star [the companion star], we need to measure the true orbital velocities and the size of the orbit,” says Jerry Orosz of San Diego State University, who is lead author of the remaining two papers alongside Lijun Gou also from CfA.

“We can measure the radial velocities of the O-star from the optical spectra.

Since the binary orbit is inclined to our line of sight, the radial velocities will be less than the actual orbital velocities. If we know the angle of inclination, then we can make a simple correction to find the actual orbit velocity.”

However, since the gravity of the black hole distorts the companion star, tugging it into a teardrop shape, the binary components’ dance leads to changes that the astronomers find useful. “As it [the O-star] moves in its orbit we see its cross section [or projection] on the sky change, which leads to a change in apparent brightness,” says Orosz. The brightness changes that the scientist speaks of are called ellipsoidal variations and in binary systems like Cygnus X-1, it is the alteration in luminosity over the orbit that allows scientists to measure the inclination of the binary.

In order to obtain the correct range of inclinations that lead to the correct mass measurements the team were then able to put their accurate distance to good use. “Using the inverse square law and the Stefan-Boltzman law [the luminosity of a star is proportional to the square of its radius and the fourth power of its temperature], we can figure out the radius of the O-star,” says Orosz. “Once we know what the size of the O-star should be, we can figure out which range of inclination angles to choose from the light curve models. The correct range of inclinations then gives the correct mass measurements and their uncertainties.”

With their extensive research, the team found that the black hole weighed a hefty 15 solar masses with a turbo spin of more than 800 revolutions per second.

That was not all: in addition to measuring the distance to the binary system, the VLBA observations made during 2009 and 2010 also analysed Cygnus X-1’s movement through the Milky Way galaxy.

Since the slow movement of the binary system implies that the black hole was not a product of a supernova, astronomers believe that its creation could have possibly been the result of the dark collapse of a giant progenitor star that initially had a mass of more than 100 times that of the Sun before losing it in a vigorous stellar wind.

“If there is an asymmetric ejection of a significant amount of mass at high speed during a supernova explosion, then the stellar remnant will recoil,” says Reid. “Alternatively, if a star in a tight binary system explodes and loses enough mass to unbind the system, both stars can move apart at roughly the same escape speeds but since the binary is still intact, this didn’t happen for Cygnus X-1. Our results support these suggestions.”

Read Gemma Lavender's complete blog post here.

Ann

[bystander]

A Black Hole Unmasked
Smithsonian Astrophysical Observatory
Weekly Science Update | 2011 Nov 25

[i](Left) An optical image of the sky showing the location of the black hole, Cygnus X-1. (Right) An artist's conception of the black hole system, showing the black hole drawing material towards it from a massive, blue companion star. This material forms a disk and jets that emit radiation. (Credit: Optical: DSS; Illustration: NASA/CXC/M.Weiss)[/i]

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Black holes are among the most amazing and bizarre predictions of Einstein's theory of gravity. A black hole is thought to be point-like in dimension, but it is surrounded by an imaginary surface, or "edge," of finite size (its "event horizon") within which anything that ventures becomes lost forever to the rest of the universe.

Despite their reputation as implacable sinks for matter and energy, the regions around black holes are often sources of powerful emission. They can be ringed by a disk of matter, for example contributed by gas from an orbiting companion star; when infalling material interacts with that disk, radiation and matter can be ejected.

A black hole is so simple that it can be completely described by only three parameters: its mass, its spin, and its electric charge, but measuring these values is far from simple. The charge is usually considered to be negligibly small, leaving only two parameters. The mass can be found when the black hole has an orbiting companion, since the periodic orbital motions of the pair are precisely determined by their masses and the orbit's size. Infalling material provides a source of radiation to measure the period, but ascertaining the size of the orbit requires knowing the distance to the source.

All these difficulties have been overcome in a set of three papers appearing together this month. CfA astronomers Mark Reid, Jeff McClintock, Ramesh Narayan, Lijun Gou, James Steiner, and Jingen Xiang, together with their colleagues, used radio wavelength parallax techniques to measure the precise distance to the first discovered black hole, Cygnus X-1: it is 6060 light-years away, with an uncertainty of about 6%. The firm distance estimate enabled the scientists to infer the mass of the black hole: 14.8 solar masses with about the same uncertainty, 7%. Not least, the sole remaining parameter of a black hole -- its spin -- could now also be determined. The team calculated that the black hole is rotating at about 95% of the maximum rotation permitted in Einstein's theory, corresponding to its event horizon whirling around about 800 times per second. While these objects are still just as amazing and bizarre, these new papers are a dramatic step forward in our understanding of their basic properties.

The Trigonometric Parallax of Cygnus X-1 - Mark J. Reid et al

• Astrophysical Journal 742(2) 83 (2011 Dec 01) DOI: 10.1088/0004-637X/742/2/83
  arXiv.org > astro-ph > arXiv:1106.3688 > 18 Jun 2011 (v1), 30 Sep 2011 (v2)

The Mass of the Black Hole in Cygnus X-1 - Jerome A. Orosz et al

• Astrophysical Journal 742(2) 85 (2011 Dec 01) DOI: 10.1088/0004-637X/742/2/84
  arXiv.org > astro-ph > arXiv:1106.3689 > 18 Jun 2011 (v1), 30 Sep 2011 (v2)

The Extreme Spin of the Black Hole in Cygnus X-1 - Lijun Gou et al

• Astrophysical Journal 742(2) 85 (2011 Dec 01) DOI: 10.1088/0004-637X/742/2/85
  arXiv.org > astro-ph > arXiv:1106.3690 > 18 Jun 2011 (v1), 30 Sep 2011 (v2)

Cygnus X-1: A Black Hole Confirmed
Centauri Dreams | Paul Gilster | 2011 Nov 29

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