A gamma-ray powerhouse’s smoking gun? Gas trail points to microquasar jets

In 2016, a binary star system emitting unusually strong gamma rays was discovered in our galactic neighborhood. Now, astronomers have found the smoking gun, or rather, a smoking trail of cool molecular gas that suggests jets emanating from a microquasar—one of the few known outside the Milky Way—are responsible.

Unlike the Milky Way, which shrouds objects behind dust when looking toward the galactic core, the Large Magellanic Cloud (LMC), a dwarf satellite galaxy to our own, offers unhindered views of interesting and relatively nearby astrophysical phenomena.

LMC P3 is one such stellar puzzle. It is the most luminous gamma-ray binary star system, composed of a compact object—either a black hole or a neutron star—alongside a massive star that is more than 30 times heavier than the sun. The two stars take roughly 10 days to orbit each other.

It lies within the ashes left behind by the supernova that created its compact object. This optically attractive cloud of molecular gas, also known as a supernova remnant, is called DEM L241.

A smoking radio trail

Ever since its discovery, unusually strong gamma-ray emission from LMC P3 has puzzled astronomers. Gamma rays lie at the highest energy limit of the electromagnetic spectrum. But an important clue to LMC P3’s mystery was hidden at the other end of the spectrum: radio waves.

Starting in 2022, a team led by astronomers from Nagoya University and Gifu University in Japan recorded observations of DEM L241/LMC P3 using the ALMA observatory in Chile, one of the largest radio telescopes in the world.

Specifically, they used ALMA bands 6 and 7 to measure carbon monoxide (CO) emission from the surrounding cold molecular gas, at wavelengths around 1.3 mm and 0.87 mm. This helped researchers measure the gas’s structure and motion and estimate its temperature.

What they saw was revealing. “We discovered a gigantic molecular jet,” said Yasuo Fukui, professor emeritus at Nagoya University and lead author of this new discovery, which has been published in The Astrophysical Journal Letters.

The newly discovered jet signature is a strikingly straight and narrow trail of “heated” molecular gas about 26 light-years long and three light-years wide. Even though very cold compared to room temperature, this gas trail stands out because it is considerably hotter, at a temperature of 35 to 60 kelvin, compared to 15 kelvin for the surrounding gas.

Fukui said the relatively high temperature observed over a large fraction of the molecular cloud makes it “a very strong case” that the gas is interacting with a high-energy jet coming from the compact object—precisely the kind of jet that can also power LMC P3’s gamma rays.

Microquasar mechanism

The new study, which was co-led by Bhuvana Gadikere Rajendra, a researcher, and Kisetsu Tsuge, a designated assistant professor, both at the Institute for Advanced Study at Gifu University, together with Hidetoshi Sano, an associate professor at the Graduate School of Natural Science and Technology at Gifu University, strengthens support for one of two competing theories of gamma-ray binary emission for LMC P3: microquasar jets.

Previously, pulsar winds were thought to be a possible mechanism. This scenario requires LMC P3’s compact object to be a pulsar, or a rapidly spinning neutron star that sends out a wind of energetic particles that in turn collides with its companion star’s own stellar wind. This creates a shock wave that accelerates particles to produce gamma rays.

But the newly found jet signature indicates a quasar-like mechanism is the driving process. Quasars are some of the universe’s most luminous objects found in distant galaxies, powered by supermassive black holes at galactic cores accreting material and unleashing powerful jets of matter.

In LMC P3’s case, the compact object could be a stellar-mass black hole or neutron star that draws in material from its companion to form an accretion disk. This is a small-scale version of a quasar, hence termed a microquasar, that can power jets of high-speed particles that can similarly produce gamma rays.

Since black holes drive more powerful jets than neutron stars, the fact that LMC P3’s gamma-ray emission is particularly high “provides very strong support for the possibility that its central object is a black hole” instead of a neutron star, Fukui said.

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