Object 15: J0510+1800
Podcast release date: 24 February 2020
Right ascension: 05:10:02.4
Corresponding Earth location: Farmland outside Daddapur, India
J0510+1800 is another object identified by its coordinates. The J refers to the coordinate system set up in the year 2000, the first four digits refer to the right ascension, and the second four coordinates refer to the declination. This object, which is located at a distance of about 4.5 billion light years away , is a blazar, a type of galaxy with an active galactic nucleus (AGN). What's surprising about this galaxy is that it might be something that I have worked with before in my research, but I'm not absolutely certain. I'll explain if a few minutes, but first, let me describe what an AGN and a blazar are.
An AGN consists of a supermassive black hole millions of times more massive than the Sun, a disk of gas and dust falling into that black hole, and a jet of ionized gas that emerges from above the poles of the black hole through complex magentohydrodynamics effects. The center of the disk can get very hot and will produce a broad range of electromagnetic radiation. When the disk is viewer from the side, we don't see the central region from Earth because it's hidden by all the gas and dust in the disc, but we still see the jets on either side of the disk. Because the jets produce very large amounts of radiowaves and because these types of AGN are usually identified by the radio emission from these jets, these objects are called radio galaxies. When we can see the top of the disk (although we may not be looking directly down the axis of the disk), we can also see into the really hot part of the center of the disk near the black hole, and so we will see a very bright, pointlike source in some or all parts of the electromagnetic spectrum. We very frequenty see the jets as well, although they will appear to be moving closer to along our line of sight. These objects are called quasars because, when astronomers first saw them, they thought they looked sort-of like stars and called them "quasi stars" or quasars for short. Blazars are quasars where we are looking directly down the jet coming out the top of the AGN, and so they look exceptionally bright. I would be inclined to think that the word blazar combines the words "blazing" and "quasar", but I found a reference that seemed to indicate that the "bl" in blazar comes from BL Lacertae, the first object identified as a blazar .
So, J0510+1800, like other blazars, emits a huge amount of radiation in all parts of the electromagnetic spectrum. However, the gamma radiation from J0510+1800 attracts a lot of attention. On Earth, gamma rays are rare. They are produced mostly by nuclear bombs, although a few gamma rays can potentially be produced by lightning and by some radioactive decay processes [3,4]. Comic books about the Hulk and other Marvel comic books characters who got their powers from gamma radiation are actually much more common than gamma rays themselves. In space, gamma rays are also rare. Any object that produces gamma radiation will receive a lot of attention, and that includes J0510+1800. The gamma rays from this blazar as well as other blazars come from electrons oscillating within magnetic fields near the base of the jets in the AGN. The electrons are moving towards us at close to the speed of light, which causes the radiation to be Doppler shifted and boosts the amount of gamma radiation that we see from these blazars . The gamma radiation from blazars in general and J0510+1800 specifically is also very variable; gamma radiation flares are observed very frequently from these objects. Astronomers have been monitoring many blazars including J0510+1800 with the Fermi Gamma-ray Space Telescope so that they can see these flares when they happen . Since this radiation comes from the base of the jets, it can potentially be used to understand the environments near the supermassive black holes in AGN, although it still seems like people are working on trying to understand exactly what is happening in these locations.
Despite the variability of J0510+1800, particularly in gamma rays, people working with Atacama Large Millimeter/submillimeter Array (ALMA) have decided to use it as a calibration source. This sounds insane, but it actually makes logical sense, as I will explain.
ALMA is a telescope in Chile consisting of 66 antennas that observe millimeter and submillimeter radiation, which is on the opposite side of the electromagnetic spectrum from gamma rays. ALMA works as an interferometer, which means that astronomers do not record the signals from the 66 individual antennas but instead record the signals from various pairs of antennas within the array. The waves of radiation will reach the different antennas at different times. Either the waves could be in phase, which will lead to the creation of a bigger wave in a process called constructive interference, or they could be out of phase, which would lead to the waves cancelling out in a process called destructive interference. While gamma radiation is rare, millimeter and submillimeter radiation is very common. On Earth, the type of thermal radiation that people normally as1sociate with infrared light is also produced at millimeter and submillimeter wavelengths by just about everything. In space, the primary sources of this radiation are interstellar dust and ionized interstellar gas. Molecules both in space and on Earth also produce a lot of millimeter and submillimeter radiation. On Earth, the most common molecule that produces this type of radiation, at least as far as I know, is water. In space, the most common molecule that produces this type of radiation is carbon monoxide.
In any case, astronomers using ALMA, which includes me because my job is to help other astronomers using ALMA, need a way to calibrate their data so that they know exactly how much energy they are measuring from their astronomical objects. This is called flux calibration. Surprisingly, the best objects to do this are Solar System objects, including Mars, all of the gas giants, a few of the largest moons of the gas giants, and a couple of the largest asteroids. In visible light, these types of objects mainly reflect sunlight, but at millimeter and submillimeter wavelengths, they emit thermal radiation that is easy to model. The Solar System objects are also really bright at submillimeter and millimeter wavelengths, which makes it easy to detect and to accurately measure the radiation from them.
However, Solar System objects have two drawbacks as calibration sources for ALMA. First, they move around a lot, and sometimes there aren't any convenient Solar System objects in the sky that can be used for flux calibrating an observation. Second, ALMA is not designed to measure emission from objects as large as the larger planets, moons, and asteroids in our Solar System. Strangely enough, it is possible to place two ALMA antennas at a certain distance where the combination of the size of the planets and the effects of interferometry lead to the planet looking invisible.
To solve these problems, ALMA organized a system for the flux calibration of ALMA that did not require observing a Solar System object for every set of science observations [7,8]. ALMA support scientists identified about 40 AGN that look very bright and very point-like at millimeter and submillimeter wavelengths. This set of objects, which are evenly spaced around the sky, are called grid sources by ALMA, and J0510+1800 is one of these grid sources. Once every two weeks, ALMA observes these grid sources along with a Solar System object using a subset of antennas which are spaced close enough together that the Solar System object does not disappear. This allows ALMA scientists to calibrate the brightness of the grid sources. Then, when observations for a science program are performed, a grid source can be observed for flux calibration the data for the other objects. By observing the grid sources frequently, ALMA scientists are able to track the variability in J0510+1800 and the other grid sources, which makes it possible to use these sources for flux calibration.
As I mentioned before, I work as an ALMA support scientist, and I have processed a lot of ALMA data. This means that I have looked at lots of flux calibration data as well as quite a few images of flux calibration sources. It's quite possible that I have either used J0510+1800 to calibrate ALMA data, I have made images of J0510+1800, or I have inspected images of J0510+1800 created by the ALMA data processing pipeline to ensure that it worked properly. However, these calibration sources have names that are not easy to memorize, so it's hard for me to remember working with any specific grid source.
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 University of Tokyo, Thunderbolts of lightning, gamma rays exciting: Researchers connect lightning with gamma-ray phenomena in clouds, 2019, ScienceDaily
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