A pulsar reveals a strong magnetic field near our galaxy’s center

At the core of most galaxies is a supermassive black hole (SBH) thousands or millions of times heavier than the Sun. They’re generally seen in radio or x-ray emission powered by the black hole’s accretion of surrounding material. Some actively accreting galactic nuclei are the brightest steady lights in the distant cosmos. Although the nearest SBH, 27 000 light-years away at the center of the Milky Way, is surprisingly quiescent, its proximity makes it attractive for the study of accretion flow in galactic nuclei.

Five months ago that study received a serendipitous gift. Several observing teams discovered a radio pulsar less than a light-year from the Milky Way’s SBH. Radio pulsars had long been sought in the galaxy’s innermost precincts. But until now, none had been found within 70 ly of the center.

Radio pulsars, rapidly spinning neutron stars with sweeping radio beams, are superb celestial clocks. This new one is still too far from the SBH to probe its distortion of spacetime. But it is close enough to probe the magnetic field intensity B in the outskirts of the enveloping hot gas that feeds the SBH.

Now Ralph Eatough and colleagues at the Max Planck Institute for Radio Astronomy (MPIR) in Bonn, Germany, report a direct measurement of B in the region of the hot gas where its accretion flow begins. ¹ That’s an important, previously unknown parameter for simulating and understanding the accretion dynamics of our local SBH. The team determined B in the pulsar’s vicinity by measuring the Faraday rotation of the pulsar beam’s polarization direction with MPIR’s 100-meter radio telescope in Effelsberg near Bonn.

Discovery

The position of the 4 × 10⁶ solar-mass (M_⊙) black hole at the center of our galaxy is marked by the prominent radio source Sgr A* in the constellation Sagittarius. On 25 April NASA’s Swift orbiter detected transient x-ray flaring from a new source about 3” (arcseconds) from the direction of Sgr A*. The next day NASA’s new NuSTAR x-ray telescope found that the source was emitting x-ray pulses with a steady periodicity of 3.76 seconds. It seemed to have the hallmarks of a magnetar.

Magnetars are a rare subset of x-ray pulsars whose ultrahigh magnetic fields cause them to have episodic outbursts, probably due to cracking in the neutron star’s crust (see Physics Today, May 2005, page 19 ). Because three of the Milky Way’s 20 known magnetars are also radio pulsars, radio telescopes around the world were promptly trained on the new source. By 2 May, the MPIR team and a team using an interferometric detector array in Australia ² had discovered 3.76-second pulsation at radio frequencies from 4.5 GHz to 20 GHz.

”When we first looked on 28 April, the radio signal was unconvincingly weak,” recalls Eatough. “But four days later, the 3.76-second pulsation was quite distinct, and it got stronger over the following weeks. That’s the kind of erratic behavior you’d expect from a magnetar.”

At Sgr A*’s distance, the magnetar’s 3” angular offset corresponds to a projected distance between them of 0.4 ly. Their angular proximity might, of course, be a coincidence, with the magnetar far behind or in front of Sgr A*. But statistical arguments and the measured free-electron column density from the magnetar make such a coincidence very unlikely. “We’re convinced,” says Eatough, “that it’s within about a light-year of Sgr A*.”

”That’s roughly Sgr A*’s radius of influence,” comments Harvard theorist Avi Loeb. “It’s effectively the outer limit of the region where the black hole’s gravity dominates the dynamics of the hot gas. So measuring the magnetic field there helps characterize the boundary conditions of the accretion flow.”

Faraday rotation

In 1845 Michael Faraday discovered that a longitudinal magnetic field can rotate the polarization direction of light passing through transparent dielectric material. That discovery was, in fact, the first empirical connection between light and magnetism.

In interstellar space, Faraday rotation is most easily seen in linearly polarized radio beams traversing a region of magnetized plasma. The resulting rotation Δθ of the beam’s polarization direction is given by ℜλ², where λ is the radio wavelength and ℜ, the so-called rotation measure, is an integral over the observer’s line of sight to the radio source. In Gaussian units, it’s given by

ℜ = (e³/2πm²c⁴) ∫n_e(s) B_s(s) ds,

where n_e is the local density of free electrons, m is the electron mass, and B_s is the local magnetic field’s component along the line of sight.

From x-ray measurements of the hot gas surrounding Sgr A*, one can estimate that n_e at the magnetar’s distance r from the SBH is high (on the order of 100 free electrons/cm³) and that it falls off roughly like 1/r. Concluding, therefore, that ℜ between the magnetar and us would be dominated by the yet unmeasured magnetic field in its neighborhood, the MPIR team set out to measure the Faraday rotation of the new source in three different RF bands—from 2.5 GHz to 8.35 GHz.

Happily, to that end, the linear polarization of the new magnetar’s 0.1-second-wide radio pulses, as shown in figure 1, turns out to be close to 100%. One measures the magnetar’s ℜ by sweeping through an RF band of its radio emission and recording the resulting oscillation, with changing λ, of the Stokes parameters that characterize the radio signal’s polarization state. For example, the Stokes parameter Q plotted in figure 2 is the difference E_x² − E_y² between the squares of the electric-field components in two orthogonal directions transverse to the beam direction.

The magnetic field

From the best sinusoidal fits to the oscillations of the Stokes parameters Q and U in the three MPIR RF bands and in one band recorded at the Very Large Array in New Mexico, the team found that ℜ = (−6.696 ± 0.005) × 10⁴ rad/m². (The minus sign means counterclockwise rotation.)

If one then invokes x-ray measurements of n_e in the hot gas around Sgr A* and posits that the free-electron density does indeed fall off like 1/r, then the measured ℜ translates into a lower limit of about 8 milligauss on the magnetic field in the new magnetar’s vicinity. It’s a lower limit primarily because local magnetic field reversals along the line of sight would imply an even stronger mean field intensity for the measured ℜ. But 8 mG is already surprisingly large, which speaks against a disordered local field with many reversals.

In what sense is 8 mG “surprisingly large” at the magnetar’s distance from Sgr A*? A plausible upper limit might be the equipartition value, about 2.6 mG, which would locally equalize the magnetic, gravitational, and thermal energies of the 2 × 10⁷ K gas. In fact, many Sgr A* accretion models assume negligible B at the periphery of the accretion flow.

“Accretion modelers are nervous about super-equipartition B fields,” says MPIR team member Heino Falcke. “But they can occur when the magnetic field is anchored in something other than the accreting gas itself. And this one might explain why Sgr A* is currently on such a starvation diet.” Under some circumstances, super-equilibrium fields can inhibit accretion. The black hole’s present accretion rate is only 10⁻⁸M_⊙ per year. At that puny rate it would have taken Sgr A* ten thousand times the age of the universe to reach its present mass.

Models that begin with negligible B at the periphery generally posit that dynamo mechanisms within the flow are sufficient to generate the strong B fields (of order 100 G) known from Sgr A*’s synchrotron radiation to prevail near the black hole’s event horizon—its boundary of no return at a distance r of about half a light-minute.

“But now we know for the first time that there’s already a dynamically significant magnetic field out where the accretion flow begins,” says Falcke. “Naive 1/r scaling of the field we measured yields the requisite field strength near Sgr A*’s event horizon. We haven’t excluded the dynamo mechanisms, but they’re no longer urgently needed.”

More to come

Only a tiny fraction of all radio pulsars are magnetars. So there are probably lots more radio pulsars waiting to be discovered and exploited in the immediate vicinity of Sgr A*. Magnetars, with their episodic flaring, may be the showiest, but they’re not the most precise timers. “To test general relativity near Sgr A*’s event horizon,” says Eatough, “we’d prefer an ordinary radio pulsar.”

Meanwhile, the new magnetar, orbiting Sgr A* with a period on the order of 1000 years, might before long reveal interesting small-scale variations in the gas enveloping the black hole at the Milky Way’s heart.