by Ke Fang

Consider for a moment some of Nature’s most violent objects, called active galactic nuclei (AGN). Despite the rather bland name, these objects are actually voracious supermassive black holes at the centers of certain galaxies, ravenously sucking in gas and the odd unlucky star passing by. Their gravity compresses all that matter into a superheated plasma and shoots some of it out in jets along its spin axis before the material passes beyond the event horizon, to be lost forever. (AGN were discussed in two previous KIPAC research highlights by Dan Wilkins in 2017 and Krzysztof Nalewajko in 2015.) The most powerful AGN, with the most luminous jets coming from the supermassive black holes at their centers, are called quasars.

Now let’s dial it down many notches, to a stellar-mass black hole (or perhaps a neutron star) which is hungrily accreting the material of a more normal companion star. This miniature version of an AGN gets the miniaturized version of the name: microquasar. Like its bigger cousin, a microquasar also develops an accretion disk of hot material circling it and has jets of particles that shoot out along its poles. Some of the particles in these jets are extremely energetic (more on that in a bit) and when they eventually arrive here on Earth after tens of thousands of years, they unveil the history of acceleration, cooling, and interaction that they’ve experienced on their long journeys.

Now to step back for a moment: cosmic rays are high-energy charged particles from outer space, unique messengers from distant sources (and were discussed in this earlier 2019 KIPAC research highlight by Noemie Globus). First detected more than a century ago, the origin of these mysterious particles is still not fully understood because their trajectories easily bend in magnetic fields, making it difficult to track them back to their source. Luckily, they produce secondary particles by interacting with material surrounding their sources, or between the source and the Earth, and some of their children are neutral particles such as gamma-rays and high-energy neutrinos. These particles are not affected by the magnetic fields, and thus travel almost linearly from where they are created. When they are captured by observatories here on the Earth, they help us uncover the secrets of their sources. 

Going back to the sources: astrophysical jets have been proposed as promising acceleration sites, given that most of the highest-energy photons in the sky come from blazars, which are a type of AGN with jets pointing toward us. Blazars are extragalactic (outside the Milky Way) and usually located at a great distance from us. Therefore, they appear as point sources in the eyes of the current generation of gamma-ray telescopes, which typically have sub-degree- to degree-level angular resolutions. As a result, resolving astrophysical jets in the gamma-ray band to find out where the acceleration of cosmic rays happens has been an impossible task.

Luckily, with similar disks and jets—albeit thousands of times smaller—galactic microquasars are resolved much more easily. Indeed, the first and one of the most exotic objects in the Milky Way, SS 433 (the first discovered microquasar), has been well-measured from radio to X-ray. As shown in Figure 1, it has a roughly piscine shape when viewed in radio waves, with a spherical belly likely coming from a supernova explosion approximately 40,000 years ago, and a head and tail generated by the interaction of a set of jets with the environment.

The central object of SS 433 seems to be a roughly 30-solar mass black hole, although a neutron star center cannot yet be excluded. The jets are illuminated in the X-ray map, as shown by the black contours in the bottom panel of Figure 1. Specifically, several hot spots have been identified in the hard X-ray map, as indicated by the white cross markers. These hotspots were first seen in 1990s by the RXTE satellite. Spectra of these hotspots, especially e1 and w1, suggest that the emission is non-thermal (in other words, not like the emissions from hot glowing gas) and could be synchrotron emissions—radiation given off by extreme-energy particles that are spiraling around magnetic fields. Based on these hard X-ray hotspots, researchers have predicted that several hundred-TeV electrons should exist in the region, and they should emit TeV photons.

After almost thirty years, these forecasted VHE (very high energy) gamma rays were eventually found by the High-Altitude Water Cherenkov (HAWC) Gamma-ray Observatory. HAWC is a facility designed to observe some of the highest-energy gamma rays and cosmic rays, with energies between 0.1 and 100 TeV. For comparison, the Large Area Telescope, the main instrument of the Fermi Gamma-ray Space Telescope, detects photons with energies from about 20 million to about 300 billion electronvolts (20 MeV to 0.3 TeV).

HAWC is located on the Sierra Negra volcano near Puebla, Mexico at an altitude of 13,500 feet (4100 meters). In contrast to the traditional imaging atmospheric Cherenkov technique, it uses water tanks to measure air showers generated by high-energy particles. Therefore it has a wide field-of-view and is specifically good at observing extended sources and sources with bright backgrounds.

Thanks to its wide field of view, HAWC can identify the SS 433 lobes in data obtained over 1017 days of observation. The major difficulty in finding the lobes is due to the nearby TeV monster source MGRO J1908+06—an extended bright pulsar wind nebula. The statistical significance of the map of the signal minus the background is shown in the color contours in the bottom panel of Figure 1. The best-fit locations of the TeV gamma rays align with the X-ray map of the jets, and are close to the hard X-ray hotspots. The first resolved VHE emission in jets proves that VHE cosmic rays can be accelerated in the jets, not just near the compact object.

My colleagues and I studied the origin of these TeV photons. As shown in Figure 4, the observed gamma rays and hard X-rays can be explained by the same population of primary or secondary electrons. In contrast, gamma rays from pion decay of cosmic-ray protons are insufficient due to the low gas density in the lobes. Currently, I am leading a joint analysis using HAWC and Fermi data, aiming to unveil the maximum energy with which particles can be accelerated by the jets. 

The fact that sub-relativistic jets can accelerate particles to such high energies is surprising, considering that the jets of SS 433 are not very powerful, and sub-relativistic outflows are only observed close to the central compact object. Based on the detection of high-energy photons in the microquasar, we conclude that astrophysical jets can be very efficient cosmic accelerators. This leads us to believe that the much larger and more powerful jets in AGN could be capable of accelerating the highest-energy cosmic rays in the Universe—bringing us a step closer to resolving this decades-long mystery in astrophysics.