Alternate Names

Nova Persei 1992, XN Persei 92

Type

Stellar mass

Location

In the constellation Perseus

Finder Chart

Distance

8,100 light-years (2,500 parsecs)

Mass

3 to 11 times the mass of the Sun

Size

Diameter roughly 10-40 miles (20-65 km), equal to the size of a large city

Discovery Methods

• Detection of an accretion disk
• Measuring the motions of a companion star

GRO J0422+32

It's tough to pin down the vital statistics of a black hole, even when it produces a spectacular lightshow.

A case in point is GRO J0422+32, a binary system about 8,000 light-years away. The more massive member of the system is probably a black hole a few times as massive as the Sun. Despite two decades of observations, though, astronomers have yet to settle on an exact mass.

The story began with a bang, when the orbiting Compton Gamma Ray Observatory discovered an X-ray nova in the constellation Perseus. Such a nova occurs when an accretion disk suddenly collapses onto a central neutron star or black hole. As the disk's gas spirals in, it's quickly heated to hundreds of millions of degrees, so it produces an outburst of X-rays and other energy.

Over the following few months, the GRO J0422+32 system faded to about one five-thousandth of its peak brightness, although it continues to produce occasional bright "flickers."

Astronomers quickly set about the difficult task of measuring the masses of the two stars. Such a calculation requires measuring the length of the orbit of the two stars, the angle at which the orbit is tilted relative to Earth, and the characteristics of the visible star, which is supplying gas to the accretion disk.

The first part was fairly straightforward: the stars orbit each other once every 5.1 hours. The second part proved more challenging, with different groups calculating widely varying viewing angles to the system. And even the third part was difficult, although today astronomers are in fairly close agreement that the companion star is less than half as massive as the Sun, and its surface is thousands of degrees cooler, giving it an orange color.

The different measurements have yielded a wide range of estimates for the mass of the black hole. A 1995 study, in fact, found a mass of just 2.2 times the mass of the Sun, which would make the object a neutron star, not a black hole. Over the next decade, though, other groups estimated 3.6 to 5 times the Sun's mass, which would make the object the smallest black hole yet discovered.

But a study released in 2008 suggested that the earlier estimates were fuzzied by the flickering of the accretion disk, making it difficult to measure the angle of the orbit. This study estimated that the black hole is at least 10.4 times as massive as the Sun.

Yet even this study noted that the case is not closed. Better observations over a longer time are needed to allow astronomers to finally log the vital statistics of GRO J0422+32.

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This document was last modified: February 12, 2012.