Astronomers are perplexed by a clockwork pattern of ultra-bright flashes spotted at a distance of 15,000 light years away from Earth. The regular 162-day period of this “gamma ray heartbeat,” as it is called in a new study, may reveal a mysterious link to a rare “microquasar” star system called SS 433, which is used to study fundamental cosmic processes.
The radiant heartbeat “challenges obvious interpretations and is unexpected from previously published theoretical models” and is yet another reason why “SS 433 continues to amaze observers at all frequencies and theoreticians alike,” reports a study published on Monday in Nature Astronomy.
SS 433 became the first known microquasar ever discovered back in 1977, and only a few dozen more have been identified in the decades since. A microquasar consists of a massive star that orbits a compact dead star, such as a black hole or neutron star. The gravitational pull of the dead star tugs stellar material off of the live star, causing an enormous disk of hot gas to form around the compact object.
The disk becomes so intensely heated that it shoots out energetic twin jets made of plasma and particles in opposite directions at relativistic velocities that approach the speed of light. In this way, microquasars are like cute mini-versions of quasars, a type of energetic galactic core that also produces relativistic jets, though on far more massive scales.
Quasars, which are among the most luminous phenomena in the universe, can shed light on unresolved questions about galaxy formation and the high-energy cosmos. But the closest one to Earth is nearly 600 million light years away. Microquasars inside the Milky Way, therefore, can serve as a handy model to study these exotic processes up close.
“Microquasar systems are the local siblings for the quasar systems in far away galaxies,” said Jian Li, who co-led the new study and serves as Humboldt Fellow at Deutsches Elektronen-Synchrotron (DESY) in Germany, in a call. “When we study microquasars, we also want to understand the physics processes in quasars.”
Li and his colleagues examined observations of SS 433 taken over a full decade by NASA's Fermi Gamma-ray Space Telescope. Gamma rays are the most energetic form of light in the universe, and SS 433 has all the ingredients to produce them. Indeed, the shock from one of the microquasar’s jets appears to be lighting up the sky in gamma rays in a fairly predictable way.
However, there is another odd source of gamma rays, called Fermi J1913+0515, about 100 light years from the microquasar. The gamma rays coincide with the location of a large cloud of hydrogen gas, which is clearly being charged with some kind of energetic emission, causing the flashes of light.
To figure out whether SS 433 and Fermi J1913+0515 could be connected, Li’s team focused on the precessional period of the microquasar. The disk of SS 433 wobbles like a spinning top over a period of about 162 days, causing the jets to get twisted into spiral shapes. By scouring the Fermi data, Li’s team were surprised to find that the period of the gamma ray heartbeat matched SS 433’s precessional period.
“We did a timing analysis and we found that there is a peak at 162 days which is consistent with the precession period of the jet,” Li said. “To see this timing signal is not predicted by our previous knowledge because it’s so far away.”
“The question is how this periodicity is produced,” he continued. “The first thing that comes to our mind is that maybe this is illuminated by the jet.”
However, the gamma ray heartbeat should be too distant for a connection with SS 433, plus it is not located in the direct path of its jets. Models predict that the helical structure of the jets would collapse long before they could stretch across 100 light years, suggesting that a jet is not the reason this region of sky lights up with such intense energy.
Another potential explanation is that the heartbeat is illuminated by more diffuse and unstructured outflows of gas and particles generated by the disk’s precession. These outflows are not as concentrated and luminous as the jets, but they could potentially ripple out to Fermi J1913+0515 and light it up in this unique way.
The team is in the midst of collecting follow-up observations with the IRAM 30m millimeter radio telescope in Spain that might constrain the origins of the strange gamma ray heartbeat.
“We discovered the source, and discovered its periodicity, but we do not know what it means or how it is produced, so we need more observations to continue the study,” Li said.
“There’s maybe too many coincidences in our theory to work, so we have to prove them one by one,” he concluded. “We only have the first step.”