Pulsars have been nicknamed “cosmic lighthouses” after the narrow beams of radio light they sweep through space. For astronomers, one newly-discovered pulsar is casting light on an unexplored and mysterious region of the cosmic “ocean”: the immediate neighborhood of the supermassive black hole at the Milky Way galaxy’s heart.

Pulsars are rapidly rotating neutron stars that emit regular pulses of radio waves. The pulsar in question (formally known as PSR J1745-2900) was discovered last year by NASA’s NuSTAR satellite and lies less than a light year from the supermassive black hole (SMBH) at the center of our galaxy [1]. The black hole, known as Sagittarius A*(or Sag A* for short), is 4 million times the sun’s mass, and plays an important role in most theories of the evolution of our galaxy. These theories deal with how complex structure like the Milky Way evolved from a uniform gas of fundamental particles just after the Big Bang. In fact, almost all large galaxies are observed to have SMBH at their centers. As our closest SMBH neighbor, Sag A* could provide an important clues as to how these objects influenced the formation and evolution of galaxies over the lifetime of the universe.

 

Figure 1 ~  Artist’s impression of the radio magnetar PSR J1745-2900 near the supermassive black hole Sagittarius A* [Image Credit: MPIfR/Ralph Eatough http://scitechdaily.com/astronomers-discover-a-magnetar-at-the-galactic-center/]

The discovery of the pulsar PSR J1745-2900

The discovery of PSR J1745-2900 excited astronomers, who had been searching for pulsars around Sagittarius A* since at least the early 90s [3]. This is because pulsars, with their extremely regular pulses, are excellent probes of their surrounding environment, which in this case includes both the magnetic and gravitational fields of the SMBH. However, finding pulsars in this region is difficult  due to the relatively high density of free electrons in the gas around the galactic center. Radio waves scatter off of these electrons, smearing out the sharp pulses from a pulsar in a phenomenon known as interstellar dispersion. Because searches for pulsars rely on detecting periodic bursts, if the pulses are smeared out over the entire pulse period, a pulsar becomes essentially undetectable without intensive computer analysis. Since high frequency radiation is more resilient to such smearing from dispersion, the pulsar near Sag A* was first discovered in high frequency X-rays by the NuSTAR satellite, and only later observed at lower frequency radio wavelengths by radio telescopes on the ground.  These later radio measurements by astronomers at the Effelsberg radio telescope in Germany and the Australia Telescope Compact Array have shown that of the more than 4000 pulsars currently known, this pulsar has the highest level of interstellar dispersion confirming its proximity to the SMBH at the center of our galaxy [1,4].

Using the pulsar to understand black holes

Once the pulsar was discovered, astronomers turned to using its signals to probe the environment near the black hole.  Pulsars produce linearly polarized radiation, where the direction of the oscillating electric field is stable and changes slowly across a pulse. In contrast, light from the sun is unpolarized; the electric field direction changes quickly and randomly, since the light is produced by many atoms whose emissions are completely uncorrelated. Although the origin of polarized light is not understood despite years of study, astronomers can use the polarization to measure the local magnetic field of the pulsar using a fundamental phenomenon of electricity and magnetism known as Faraday rotation.

Faraday rotation describes how, when a linearly polarized electromagnetic wave like a pulsar beam passes through a magnetic field, the direction of the beam’s electric field rotates at a speed proportional to the magnetic field strength. When discovered by Faraday in 1845, this effect was the first evidence linking light to electricity and magnetism; now, it is the most commonly used technique in mapping cosmic magnetic fields [5].

Using the Faraday method, astronomers found that the magnetic field at the pulsar’s position is about a hundred times stronger than the average magnetic field in our galaxy. The field measures around 8 milligauss, which is puny compared to the 6,000 times stronger 50 gauss field of a refrigerator magnet, but this is nonetheless significant enough to affect Sag A* [1]. Magnetic fields in the accretion disks of gas around supermassive black holes are thought to be extremely important in regulating how quickly matter in the accretion disk falls through the event horizon, the black hole’s “point of no return,” and thus how quickly the black hole grows. Sag A* is, cosmically speaking, extremely quiet; it accretes material at a small rate and doesn’t burn as fantastically bright as the supermassive black holes at the centers of distant galaxies.  Measurements of the magnetic field around Sag A* will help astronomers better understand the accretion process and the co-evolution of supermassive black holes and galaxies [1].

Einstein’s general relativity and other impacts

The discovery of PSR J1745-2900 also excited astronomers  working to test Einstein’s theory of general relativity. Because pulsars are such accurate clocks, they are ideal for making precise measurements of the spacetime-distorting gravitational field around the SMBH. Accurate measurements of these effects would impose stringent new tests on Einstein’s general relativity and test the feasibility of competing models of gravity [2].

Unfortunately, the newly-discovered pulsar is too far away and its rotation is not stable enough to directly  probe the warped spacetime around the black hole. This is because the pulsar belongs to a rare subclass of pulsars called magnetars for their extremely high magnetic fields. While the magnetic field of the surrounding gas is only 8 milligauss, the magnetar’s own magnetic field reaches extraordinary strengths of 1014 gauss, 100,000 billion times the strength of the earth’s magnetic field at the surface.  These high magnetic fields can trigger “starquakes” in the neutron star’s crust, which disrupt the highly regular pulsar “clock” [5].

The good news is that since magnetars are extremely rare in the galactic pulsar population, the discovery of this particular pulsar indicates that there are likely many more ordinary radio pulsars near Sag A*. In fact, the new pulsar is only the 4th known radio pulsar that is also a magnetar out of 4000 total.  More stable radio pulsars in the region would allow astronomers to sample more areas of the accretion disk and begin to make accurate measurements of the curvature of spacetime [1].   To find these pulsars and overcome the high dispersion of pulses near the galactic center, astronomers will use further searches in high frequency X-rays as well as computer-intensive attempts to “de-disperse” observations by testing different estimates of the density of free electrons between earth and the pulsar at each observed point. Estimates of the pulsar population around Sag A* range from the hundreds to the thousands [6]. With luck, astronomers will soon have many more pulsar lighthouses with which to map out the strange territory around the supermassive black hole.

Andrew Chael is a graduate student in the Physics department at Harvard University.

REFERENCES

[1] R.P. Eatough et al. (2013, September), A strong magnetic field around the supermassive black hole at the centre of  the Galaxy. Nature 501 391-394. http://www.nature.com/nature/journal/v501/n7467/full/nature12499.html

[2] R.M. Shannon et al. (2013, October), Gravitational-Wave Limits from Pulsar Timing Constrain Supermassive Black Hole Evolution. Science 341 334-337. http://www.sciencemag.org/content/342/6156/334.full

[3] J.M. Cordes and T.J. Lazio (1997, February), Finding Radio Pulsars in and beyond the Galactic Center. The Astrophysical Journal 475 557. http://iopscience.iop.org/0004-637X/475/2/557/

[4] R.M. Shannon and S. Johnston (2013, June), Radio Properties of the magnetar near Sagittarius A* from observations with the Australia Telescope Compact Array. Monthly Notices of the Royal Astronomical Society. 435 L29-L32. http://mnrasl.oxfordjournals.org/content/435/1/L29

[5] B.M. Schwarzschild (2013, October), A pulsar reveals a strong magnetic field near our galaxy’s center. Physics Today. http://scitation.aip.org/content/aip/magazine/physicstoday/article/66/10/10.1063/PT.3.2132

[6] J. Chennamangalam and D.R. Lorimer (2014, February), The Galactic Centre Pulsar Population. Monthly Notices of the Royal Astronomical Society Letters. http://mnrasl.oxfordjournals.org/content/early/2014/02/27/mnrasl.slu025

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