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Most Massive Stellar Black Hole Found

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Graphic featuring an artist's representation of M33 X-7, a binary system in the nearby galaxy M33. Image Courtesy Chandra X-ray Observatory

Figure 1. The main component of this graphic is an artist's representation of M33 X-7, a binary system in the nearby galaxy M33. In this system, a blue supergiant O7 star (large blue object) and a 16 solar mass stellar black hole, are revolving around their common center of mass in 3.45 days. This black hole is almost 16 times the Sun’s mass, a record for black holes created from the collapse of a giant star. Other black holes at the centers of galaxies are much more massive, but this object is the record-setter for a so-called "stellar mass" black hole.

In the illustration, an orange disk surrounds the black hole. This depicts material, fed by a wind from the blue companion star, which has been swept into orbit around the black hole. Rather than flowing unimpeded and uniformly into space, wind from the star is pulled towards the black hole by its powerful gravity. The wind that does make it past the black hole is disrupted, causing turbulence and ripples beyond the disk. The companion star itself is also distorted by the gravity from the black hole. The star is stretched slightly in the direction of the black hole, causing it to become less dense in this region and to appear darker.

The inset shows a composite of data from NASA’s Chandra X-ray Observatory and the Hubble Space Telescope. The bright objects in the inset image are young, massive stars around M33 X-7, and the bright, blue Chandra source is M33 X-7 itself. X-rays from Chandra reveals how long the black hole is eclipsed by the companion star, which indicates the size of the companion. Observations by the Gemini telescope on Mauna Kea, Hawaii track the orbital motion of the companion around the black hole, giving information about the mass of the two members of the binary. Other observed properties of the binary were also used to help constrain the mass estimates of both the black hole and its companion.

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Table showing selected parameters for M33 X-7.

Table 1: Selected parameters for M33 X-7. The uncertainties correspond to one standard deviation.

Chart showing Mean optical spectrum of M33 X-7.

Figure 2. Mean optical spectrum of M33 X-7. The spectrum shown here (connected dots), which was extracted with the SPECRES package in IRAF22, is the sum of the 22 individual spectra that have been velocity-shifted to the rest frame of the secondary star. The solid line is the model spectrum described in the paper. The data were obtained using GMOS on the Gemini North. Twenty-four 40-minute spectra were acquired in service mode between 2006 August 18 and November 16 in good seeing of always < 0.8 arcseconds. In the two-dimensional spectra, the overlap of the profiles of M33 X-7 and the nearby pair of stars was modest.

Chart showing phased X-ray light curve and radial velocity curve for M33 X-7.

Figure 3. Phased X-ray light curve and radial velocity curve for M33 X-7. (a) The Chandra ACIS light curve in the 0.5 − 5 keV energy band. (b) The radial velocity curve derived from the Gemini spectra (extracted using the GMOS IRAF package) with the best-fitting model shown as a solid line. The error bars 1σ (s.d.) statistical. The dashed line is the best-fitting sinusoid.

Observations that combine data from the ground-based Gemini North telescope and NASA’s orbiting Chandra X-ray observatory have led to the discovery of the most massive known stellar black hole.

Intriguingly, the black hole orbits an exceptionally large companion star, which by an extremely fortunate coincidence also eclipses the black hole from our perspective on Earth. As the only known eclipsing binary black hole/star pair the team was able to determine fundamental parameters of the pair with high precision. The ramifications of this pairing could have a profound effect on our understanding of how the largest stars evolve.

The black hole-star pair is located about 3 million light years from Earth in a neighboring galaxy named M33. The combination of X-ray data with Gemini North’s optical images and spectroscopy (see Figure 2) led the international team to conclude that the black hole is 15.7 times the mass of our Sun making it the most massive stellar black hole known.

Known as M33 X-7, the black hole orbits its over-sized companion in an orientation that creates an eclipse from our perspective on Earth every 3.5 days. The companion star weights in at about 70 times the mass of the Sun putting it near the top of the stellar weight limit.

To have such a large black hole partnered with such a portly “normal” star is an exceptional situation that will eventually result in a pair of orbiting black holes when the massive star eventually goes supernova. However, there is a challenge to understanding how a system like this could have formed. Since it is known that that more massive stars evolve more rapidly than less massive ones, the star that created the existing black hole in the pair must have already gone supernova, implying that it was even heavier than the 70 solar mass behemoth that remains in the system. This is puzzling since at that size the progenitor star of the black hole would have been large enough that it would have shared its atmosphere with its companion. Current theories of mass exchange between binary pairs lead to scenarios very different from what is seen in this system, meaning that how such an unusual binary formed will, for now, remain an enigma.

The result was announced in the October 18th issue of the British journal Nature. A press release from Chandra X-ray Observatory can be found here.

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