Scorpius X-1

If you could see X-rays, maybe you’d see this.

Near the Galactic Center, the Fermi bubbles would glow bright… but the supernova remnant Vela, the neutron star Scorpius X-1 and a lot of activity in the constellation of Cygnus would stand out.

Scorpius X-1 was the first X-ray source in space to be found after the Sun. It was discovered by accident when a rocket launched to detect X-rays from the Moon went off course!

But why is it making so many X-rays?

Scorpius X-1 is a double star about 9,000 light-years away from us. It’s a blue-hot star orbiting a neutron star that’s three times as heavy. As gas gets stripped off from the lighter star and sucked into the neutron star, it first forms a spinning disk. As it spirals down into the neutron star, it releases a tremendous amount of energy.

This gas is near the ‘Eddington limit’, where the pressure of radiation pushing outward and the gravitational force pulling inward are in balance!

Scorpius X-1 puts out about 23000000000000000000000000000000 watts of power in X-rays! Yes, that’s 2.3 × 10³¹ watts. This is 60,000 times the X-ray power of our Sun.

Scorpius X-1 is considered a low-mass X-ray binary: the neutron star is roughly 1.4 solar masses, while the lighter star is only 0.42 solar masses. These stars were probably not born together: the binary may have been formed by a close encounter inside a globular cluster.

The lighter star orbits about once every 19 days.

Puzzle. Why is such a light star blue-hot, rather than a red dwarf?

I want to read more about Scorpius X-1 and similar X-ray binaries! Besides the Wikipedia article:

• Wikipedia, Scorpius X-1.

I’m finding technical papers like this:

• Danny Steeghs and Jorge Casares, The mass donor of Scorpius X-1 revealed, The Astrophysical Journal 568 (2002), 273.

which gets into details like “The insertion of the calcite slab in the light path results in the projection of two target beams on the detector.” But I’d like to read a synthesis of what we know, like an advanced textbook.

3 Responses to Scorpius X-1

  1. cronmorgan says:

    Interesting that a neutron star could be that low in mass.

    • John Baez says:

      Yes! Apparently neutron star “masses range between 1.18 and 1.97 times that of the Sun, but most are 1.35 times that of the Sun.” A bunch, at least, are left over from more massive stars that shot off their outer layers.

  2. Wyrd Smythe says:

    Up until just now, if you’d asked me what the first x-ray source we detected was, I’d have answered Cygnus X-1. But Scorpius X-1 beats it by almost a decade (1962 versus 1971). Cygnus X-1 is merely the first black hole we discovered.

    As an aside, it’s long caught my fancy that, for those in the Northern Hemisphere, our winter nights face away from the galactic center while our summer nights face inwards. On a cold winter night in Minnesota, it adds to the chill to think one is looking out of the galaxy into intergalactic space.

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