Blazars
Blazars are a specific type of active galactic nucleus (AGN) that are notable for their intense brightness and the jets of matter they emit, which are directed toward Earth. These jets, powered by supermassive black holes at the center of their galaxies, can travel at nearly the speed of light and generate more light than an entire galaxy of stars. Blazars are characterized by a flat radio spectrum, high optical polarization, and significant variations in optical brightness over short periods. There are two main subcategories of blazars: flat-spectrum radio quasars (FSRQs), which have broad emission spectra, and BL Lacertæ objects (BL Lacs), known for lacking strong spectral features.
NASA and other space research organizations actively study blazars using advanced instruments and telescopes to understand the composition and formation of their jets, which are believed to emit gamma rays, the most powerful form of electromagnetic radiation. The investigation of blazars provides insights into the nature of black holes and their role in galaxy formation, as well as the workings of the universe at large. Data from missions like the James Webb Space Telescope and previous projects like the Wide-Field Infrared Survey Explorer (WISE) have significantly contributed to the detection and understanding of blazars, revealing valuable information about these fascinating cosmic phenomena.
Blazars
FIELDS OF STUDY: Astronomy; Extragalactic Astronomy
ABSTRACT: Blazars are a type of active galactic nucleus (AGN). Like all AGNs, blazars are thought to be powered by supermassive black holes. Blazars are among the most energetic objects in the universe and emit large jets or bursts of matter at nearly the speed of light. Scientists study blazars to understand how they generate such fast and powerful jets. They may also reveal information about the early universe.
Active Galactic Nuclei
The universe is full of galaxies of many sizes and compositions. It is thought that most if not all massive galaxies have an immense black hole, known as a supermassive black hole, at their center. In some cases, the center of the galaxy is extremely luminous, brighter than all light sources in the rest of the galaxy combined and emitting a great deal of energy along the entire electromagnetic spectrum. Such galaxies are known as active galaxies, and their centers are known as active galactic nuclei (AGNs). Scientists believe that AGNs are the result of the sheer amount of matter accumulated by the supermassive black holes at the center of active galaxies. Research indicates that AGNs contain millions or even billions of times more material than Earth’s sun.
The enormous gravitational force of a black hole constantly draws in matter. This creates bands of gas and space debris called accretion disks. Eventually, the black hole pulls all this matter in like a drain emptying a tub. Sometimes before gas disappears into the black hole, it is released in blasts or jets of atomic matter and debris. Fueled by energy from the supermassive black hole, some of these jets can shoot out at nearly the speed of light. Galactic jets can create more light than an entire galaxy of stars, making AGNs among the brightest and most luminous objects in space.
Several types of AGNs exist, including quasars, radio galaxies, Seyfert galaxies, and blazars.All of these AGNs blast out jets of gas, debris, and other matter. The distinguishing feature of blazars is that their jets are directed toward Earth. Scientists have theorized that this head-on view of the light explains why blazars appear much brighter than quasars and Seyfert galaxies to observers on Earth.
To qualify as a blazar, an AGN must meet certain criteria. It must have a flat radio spectrum with high radio brightness. It must be highly optically polarized. Finally, it must display major optical variation over a period of a few days.
Composition of Blazars
The National Aeronautics and Space Administration (NASA) has conducted several missions to study AGNs. NASA is particularly interested in the electromagnetic radiation that is released by blazars because they point toward Earth. Scientists are not sure how these jets are formed and what they are made of. One theory is that the magnetic field of the black hole interacts with the band of debris around it in a way that causes energy to be released in a certain direction.
Because of their great power and the amount of light they generate, blazar jets are believed to emit gamma rays along with other forms of electromagnetic radiation. Gamma rays are the brightest and highest-powered form of electromagnetic radiation known to exist.
Scientists use an array of powerful instruments and telescopes to study blazars. They are working to determine the percentages of electrons, protons, and positrons contained in the electromagnetic rays they emit. This will help scientists better understand the power behind blazars and other AGNs, including how blazars generate jets with such force and speed. Data collected by the James Webb Space Telescope, which was launched December 25, 2021, is expected to further scientists' knowledge about what is occurring near AGNs.
Detecting Blazars
Space-research organizations around the world have designed several missions to detect and study blazars. Between its launch in December 2009 and its temporary retirement in February 2011, NASA’s Wide-Field Infrared Survey Explorer (WISE) identified more than two hundred blazars. Researchers believe that thousands more will be found.
WISE identified blazars by their infrared signatures in areas where large concentrations of gamma rays had been detected. Infrared light is outside the visible spectrum of light and requires special equipment to be seen. Blazars can be classified into two main subgroups based on their emission spectra. Flat-spectrum radio quasars (FSRQs) are blazars that produce emission spectra with strong, broad features. BL Lacertæ objects, or BL Lacs, are those with emission spectra that lack strong features. These differences are detected by variations in luminosity and redshift distributions. In 2016 astronomers discovered a connection between two wavelengths of light emitted by monster black holes. Examining WISE data, they found the mid-infrared colors of blazars correlate to gamma-ray output.
NASA’s Fermi Gamma-Ray Space Telescope discovered 709 suspected AGNs within a year of its activation in 2008. About three hundred are considered FSRQs, another three hundred are BL Lacs, and the rest are other types of AGNs.
A Glimpse into Galactic History
All the AGNs studied by scientists are millions of miles away from Earth, so even the light from the extremely fast blazar jets takes a very long time to be seen. This means that AGNs provide information about the nature of black holes and the role they played in the formation of galaxies. It is possible that blazars will reveal how black holes affect the current state of the universe. In particular, the jets from blazars are clues to the differences between the supermassive black holes at the center of AGNs and the smaller black holes found in other parts of the universe.
PRINCIPAL TERMS
- active galactic nucleus (AGN): a relatively compact region at the center of a galaxy that releases immense amounts of energy and electromagnetic radiation, believed to be the result of mass accumulation by a supermassive black hole.
- black hole: a region of space with a gravitational pull so strong that even light cannot escape it.
- gamma ray: a form of electromagnetic radiation with high energy and a very short wavelength.
- jet: a plume of electrons and positrons that is emitted from a black hole when its magnetic field interacts with other space objects.
Bibliography
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