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We now know how gamma rays spawn, but where do they come from?
Blazars are often the answer. These rare galaxies are fueled by supermassive black holes that blast jet streams of matter towards Earth at almost the speed of light. You don’t mess with supermassive black holes that are millions to billions of solar masses. They accelerate particles so fast that they emit light equivalent to the most energetic gamma rays –– and so do some unknown things that act like blazars.
Because the most known sources of gamma rays are actual blazars, researchers using China’s Large Sky Area Multi-Object Fabre Spectroscopic Telescope (LAMOST) and data from the Fermi satellite have now identified some strange BCUs, or Blazar Candidates of Uncertain Type, as a potential low-energy sources of gamma rays. They recently published two studies with their findings in The Astronomical Journal. Francesco Massaro of the University of Turin is one of those researchers.
“BCUs are unidentified gamma-ray sources with unknown or unassociated counterparts at low energies,” Massaro told SYFY WIRE. “They have most of the properties blazars do, such as flat radio spectra, variability and polarized emission, but they lack the optical classification that can certify they are blazars.”
Traveling at almost the speed of light, the extremely energetic particles in the jets being emitted by the BCUs were energetic enough (think bright enough) for the Fermi satellite to see. Its Large Area Telescope was previously able to find many high-energy gamma ray sources but had trouble detecting low-energy sources. Optical spectroscopy was needed to seek them out. This method looks into light that is absorbed and emitted by matter. Emission and absorption lines depend on the wavelength, which was gamma radiation, comparable to the most intense X-rays.
There were BCUs later classified as blazars by analyzing the spectroscopic data LAMOST had archived. Some of these were BL Lacertae or BL Lac objects, elliptical galaxies which can be difficult to identify because their emission lines are weak to nonexistent. They do vomit out enormous radio jets with enough kinetic energy — energy they get from motion — to make up most of their power output across all wavelengths. However, The emission lines of BCUs, which show up if they emit certain wavelengths of radiation, are faint or even undetectable.
“Over time, we have found that most BCUs are BL Lacs, the most elusive class of blazars and the most elusive class of active galactic nuclei,” Massaro said. “BL Lacs have no features in their optical spectra, and this is their main signature — a blue continuum with only and very rarely weak lines overlaid.”
BCU and especially Bl Lac redshifts, or wavelengths of light that reach or “shift” towards the red end of the spectrum when stretched, are also nearly impossible to make out when their emission lines are so weak to begin with. LAMOST still came through on many of these objects. Not only that, but it was also able to find hundreds more blazars while searching for emission and absorption lines. This doesn’t mean that all BCUs are necessarily blazars. However, such a strong association between blazars and gamma ray emissions means something is going right.
“We recently started investigating their large scale environment of blazars, and in the near future, we will keep investigating their optical properties,” Massaro said.
As Massaro and his colleagues continue to investigate blazars hiding in the dark depths of space, keep an eye out for the things you can’t see.