A pulsar discovered last April is helping astronomers measure the magnetic field surrounding our galaxy’s central black hole.
The supermassive black hole hiding in Milky Way’s core just got a new neighbor.
After decades of searching, astronomers discovered a young pulsar in the galactic center a few months ago. Now Ralph Eatough (Max Planck Institute for Radio Astronomy, Germany) and his colleagues are putting the pulsar to work spying on the behemoth next door. In this week’s Nature, they announce that the strong magnetic field they find surrounding the black hole might put the beast on a permanent diet.
Like L. A. smog, thick clouds of gas and dust hang over our galaxy’s downtown, hiding millions of stars from view. Only very long and very short wavelengths penetrate the veil. A flash of X-rays did just that on April 24th (which by cosmic coincidence also happens to be my birthday). Follow-up observations showed X-ray and radio-wave pulses arriving every 3.76 seconds, confirming that astronomers had discovered a bona fide pulsar in the galactic center.
Such pulsars shouldn’t be uncommon — theorists think there ought to be more than a thousand within spitting distance of the black hole — but somewhat mysteriously, this pulsar is the first find after a three-decade hunt.
The emission from this pulsar, named PSR J1745–2900, is almost all linearly polarized, meaning the wavelengths tend to oscillate along a single direction. That’s an advantage astronomers can use, because magnetic fields between the pulsar and Earth twist the waves as they travel, rotating the direction that they oscillate. The effect is known as Faraday rotation.
So, measure the twist, measure the magnetic field? Well, almost. How much the waves rotate also depends on the amount of hot, ionized gas between the pulsar and Earth. And that’s a little harder to measure since, surprisingly enough, we don’t know exactly where the pulsar is.
Though the pulsar was found just 3 arcseconds away from the black hole on the plane of the sky, the two could still be quite far apart in space. “[Distance] is always a problem for pulsars,” explains Eatough. “We cannot get accurate [direct] distance measurements without a parallax.” And parallax is impossible to measure for a pulsar so far away.
So Eatough’s team used indirect measurements instead. For example, ionized gas between the pulsar and Earth causes pulses at lower frequencies to lag slightly behind pulses at higher frequencies. This so-called dispersion measure is higher than that measured for any other known pulsar and suggests the pulsar lies within 33 light-years of the black hole.
Eatough and his colleagues also compared the pulsar’s location to two nearby pockets of ionized gas. In front of the black hole, hot gas glows bright in the X-rays; in back, a cooler gas stream emits radio waves. One or both of these gas pockets could affect the pulsar’s light as it travels through. The gas stream (technically known as the Northern Arm of Sagittarius A West) is especially well studied, and its polarized emission is far more rotated than the pulsar’s. So the pulsar probably lies in front of the cooler gas stream.
That means it’s the hot gas that twists the pulsar’s polarized light, gas that will ultimately become the black hole’s meal. Eatough’s team estimates the magnetic field threading the ionized gas is roughly 8 milligauss. That’s about 1,000 times stronger than the fields crisscrossing the rest of the galaxy.
A strong magnetic field would have important consequences for how the black hole feeds. Although there’s enough gas in the black hole’s neighborhood to feed it the equivalent of 30 Earths every year, only 0.003 Earth’s worth of gas makes it past the event horizon. A strong magnetic field might help the black hole diet by holding the gas back and preventing it from crossing the point of no return.
Fred Baganoff (MIT), who measured the hot gas reservoir surrounding the black hole a decade ago, cautions that a strong magnetic field isn’t necessary to explain why the black hole fasts. Despite gravity’s strong pull, it’s harder than you might think to swallow matter away into eternity. Much of the gas might never reach the black hole, surging outwards instead in a wind or a jet.
“I think we need theoretical studies now,” explains Heino Falcke (Radboud University, the Netherlands). “We have the density of the material [around the black hole], its temperature, and the strength of the magnetic field. That’s the only accretion disk of all accretion disks where you can say that. Now based on the observations, we can say, look guys, . . . this is what you should start with, now tell us what comes out of your simulations.”