Gas Trail 26 Light-Years Long Points to Black Hole Jet Behind Brightest Gamma-Ray Binary
ALMA images of the Large Magellanic Cloud system LMC P3 reveal a narrow stream of heated gas that favors a 'microquasar' explanation over a pulsar wind.
Astronomers have found what looks like the smoking gun for the most luminous gamma-ray binary ever discovered: a straight, narrow trail of heated gas stretching about 26 light-years, which they say was carved by a high-speed jet. The result, published in The Astrophysical Journal Letters, favors a black hole or neutron star feeding on its companion over a competing explanation based on a spinning pulsar.
The system, known as LMC P3, lies in the Large Magellanic Cloud, a satellite galaxy of the Milky Way about 163,000 light-years away. Found in 2016, it was the first gamma-ray binary identified in another galaxy. It pairs a compact object, either a black hole or a neutron star, with a massive companion star of more than 30 solar masses. The two orbit each other about every 10 days, inside a supernova remnant called DEM L241.
Beginning in 2022, a team led by astronomers from Nagoya University and Gifu University used the ALMA radio observatory in Chile to measure carbon monoxide emission from the cold molecular gas around the system. Observations in two radio bands, near 1.3 millimeters and 0.87 millimeters, revealed the gas's structure, motion and temperature.
They found a straight trail of heated molecular gas about 26 light-years long and three light-years wide. The gas in the trail measures 35 to 60 kelvin, compared with about 15 kelvin in the surrounding cloud. Lead author Yasuo Fukui, professor emeritus at Nagoya University, called it a large molecular jet. The heating over such a large part of the cloud, he said, makes a "very strong case" that the gas is interacting with a high-energy jet from the compact object. Bhuvana Gadikere Rajendra and Kisetsu Tsuge of Gifu University's Institute for Advanced Study co-led the work with Hidetoshi Sano of Gifu University.
Scientists had offered two ways to explain the gamma rays. In the pulsar-wind model, the compact object is a rapidly spinning neutron star whose wind collides with the companion's stellar wind and accelerates particles to gamma-ray energies. In the microquasar model, a black hole or neutron star pulls gas from its companion into an accretion disk and launches fast jets that can also produce gamma rays. A jet signature in the gas favors the second picture.
Fukui argued that because black holes drive more powerful jets than neutron stars, the system's unusually high gamma-ray output "provides very strong support" for a black hole as the central object. Phys.org's summary notes that the study does not settle whether the compact object is a black hole or a neutron star.
The paper, titled "ALMA Observations of DEM L241/LMC P3 in the Large Magellanic Cloud: Evidence for the Formation of Cool Molecular Jets Driven by a Microquasar," carries the DOI 10.3847/2041-8213/aea0cb. It follows other recent work suggesting microquasar jets can reshape their surroundings, including a possible microquasar origin for the Cygnus Bubble reported in August.
