Discovering a pulsar in the Milky Way’s center has been one of the biggest and most exciting challenges recently in galactic astronomy. Pulsars can be used as precise clocks to investigate the interesting conditions found near the supermassive black hole at the galactic center, Sgr A*. Just recently, a new magnetar was discovered in the center of the Milky Way. Magnetars are very similar to pulsars, and therefore can also be used as clocks to learn about their surroundings. But as exciting as this discovery is, the road that led to the discovery is also very interesting.
Magnetars, how do they work?
Before diving into the discovery, it helps to have an understanding of what makes magnetars different from pulsars1. Magnetars and pulsars both originate from neutron stars. Neutron stars are stars a little more massive than the Sun supported by neutron degeneracy pressure with a radius of about ten kilometers (approximately the size of a small city). They are the end products of type II supernovae, left behind after the explosion has taken place.
If the neutron star is spinning fast enough, it can develop a strong magnetic field through a process called dynamo action. Ionized gas inside the star circulates by convection (as the hot gases rise and cool gases fall towards the center of the star), and the movement of this gas can create a magnetic field. The spin of the star itself can help increase the magnetic field by facilitating the dynamo action insde the star. This process converts some of the star’s rotational energy into energy stored in the magnetic field. In a pulsar, the spin is not strong enough to cause this conversion to happen significantly, while in magnetars, this process gives them their very strong magnetic fields: 1015 – 1017 Gauss, over a 1000 times stronger than that of a pulsar. This property is the key differentiator between magnetars and pulsars, and also provides their name.
When the magnetic field of the star shifts can bend the crust and heat up the interior of the star, leading to starquakes on the star’s crust. Gamma rays can be released during this process. Occasionally, the magnetar’s magnetic field undergoes a huge rearrangement, leading to large flares (similar to, but much more powerful than, the solar flares on the Sun). These flares can cause fireballs that emit a large amount of X-ray radiation. It is with one of these large X-ray flares that the magnetar in the galactic center was first discovered.
It’s a black hole … it’s a gas cloud … no, it’s a magnetar!
On April 24, a large X-ray flare was detected from the region surrounding the supermassive black hole in the galactic center by the Swift satellite. Immediately, predictions for what caused the flare jumped to the G2 gas cloud. G2 is a gas cloud discovered last year approaching towards an encounter with Sgr A*. The only problem is that this encounter is predicted for sometime late this year or early next year, and not this quickly.
Follow-up observations were quickly conducted using other telescopes. Using the superior timing resolution of the NuSTAR satellite on April 26, astronomers were able to measure a fluctuating signal coming in X-ray coming from the location, with a 3.76 second period. This period (and a later measurement of a spin-down rate) led to the discovery of the magnetar, one spinning every 3.76 seconds. Meanwhile, the Chandra X-ray Observatory was able to better resolve the location of the magnetar on April 29, identifying its location to be just 0.12 parsecs from Sgr A*.
So why is this small distance from the supermassive black hole very exciting? The answer stems from a pulsar or a magnetar’s precise measurements of period, allowing them to behave as clocks (I discussed this briefly for pulsars in this previous post). If we could go out to Sgr A* and place a precise clock in an elliptical orbit around the supermassive black hole, we would observe the clock’s ticking rate to fluctuate due to effects caused by general relativity as its distance from the black hole changes. Unfortunately, we can’t do that2, but if we find a magnetar or a pulsar in an elliptical orbit around the supermassive black hole whose period we can precisely measure, it would be as if we conveniently happened to find a precise clock just as we wanted. As the pulsar or magnetar orbits around the supermassive black hole, we would observe different spin rates due to relativistic effects. In the process, we can conduct tests on general relativity. Hopefully, the new magnetar in the galactic center will prove useful in allowing these relativistic effects to be seen in future observations.
Sources and Further Exploration
- Degenaar, N., M. T. Reynolds, J. M. Miller, et al. “Large Flare from Sgr A* Detected by Swift”, 25 April 2013, The Astronomer’s Telegram.
- Gotthelf, Eric V., Kaya Mori, Jules P. Halpern, et al. “Spin-down Measurement of PSR J1745-2900: a New Magnetar”, 4 May 2013, The Astronomer’s Telegram.
- Kouveliotou, Chryssa, Robert C. Duncan, and Christopher Thompson. “Magnetars”, 2003, Scientific American 0203:34-41.
- Mori, Kaya, Eric V. Gotthelf, Nicolas M. Barriere, et al. “NuSTAR discovery of a 3.76 second pulsar in the Sgr A* region”, 27 April 2013, The Astronomer’s Telegram.
- Rea, N., P. Esposito, G. L. Israel, et al. “Chandra localization of the soft gamma repeater in the Galactic Center region”, 30 April 2013, The Astronomer’s Telegram.
- Reich, Eugenie Samuel. “Magnetar found at giant black hole”, 14 May 2013, Nature News & Comment.