Anatomy of a Blazar

What is the brightest object in the Arts & Sciences professor Somehow, researchers think, night sky? Paradoxically, the most of astronomy and director those behemoth black holes of BU’s Institute for Astro- must be fi ring up the blazars. luminous things in the cosmos are physical Research (IAR), But even though nearly invisible to the naked eye. They are and Jorstad, an IAR senior every galaxy has a supermas- “blazars,” mysterious objects that research scientist, are trying sive black hole, only a small to resolve. fraction of galaxies—about glow not just with visible light—the When the fi rst blazar was one in ten—are “active,” kind our eyes can see—but with discovered in 1962, astrono- radiating a huge amount mers were stumped: they of energy. And fewer than every kind of radiation, from had never seen anything like one in a thousand active gal- radio waves to gamma rays. it. But time and technology, axies is a blazar. What makes like NASA’s Hubble Space them diff erent? Telescope, have yielded It all starts with the black T THE BU BLAZAR LAB, ASTRONOMERS some clues. hole’s diet. Black holes gob- Alan Marscher and Svetlana Jorstad and First, astronomers tracked ble up anything that gets too A their students are trying to understand how blazars to ancient galaxies close. When a black hole is blazars work and where they get their tremendous located hundreds of mil- energy. They think that blazars are powered by lions, or even billions, of supermassive black holes containing the mass of light years from Earth. Each ▼ A jet of particles and radia- hundreds of millions of suns. But how do black of these galaxies, like our tion shoots out from near holes—where gravity is so strong that nothing, own Milky Way, is centered the supermassive black hole at the heart of an active galaxy. not even light, can escape—power the brightest on a supermassive black When the jet is pointed right objects in the cosmos? hole that’s engulfed millions at Earth, telescopes see it as a That is the puzzle that Marscher, a College of of suns’ worth of matter. blazar. (Still image is from a computer animation.) Anatomy of a Blazar Astronomers are learning how the mysterious objects work, and where they get their energy / BY KATE BECKER 24 BOSTONIA Summer 2016 20-25_Bostonia_SU16_r1.indd 24 6/6/16 4:18 PM satellite, which can be rap- idly pointed toward the fl are ◀ Astronomers Alan Marscher and Svetlana Jorstad and their source to capture ultraviolet students are trying to under- and X-ray readings, and taps stand how blazars work into publicly available data and where they get their tremendous energy. from the Fermi Gamma-ray Space Telescope. By scru- tinizing diff erences in the also ram into particles of shape and timing of the fl are light, called photons, and at diff erent wavelengths, give them the extra boost of she, Marscher, and their energy to make gamma rays. colleagues can deduce the But what, exactly, kicks physics behind the fl are. the electrons up to such Marscher and Jorstad also high speeds? Astrophysi- enlist a network of radio tele- cists are still debating, but scopes, called the Very Long many think that the elec- Baseline Array (VLBA), to trons are whorled through a take pictures of the fl are as it corkscrew-shaped magnetic moves and changes. Because fi eld that shoots them out at the telescopes that make blinding velocity. up the VLBA are located on If that hypothesis is right, opposite sides of the Earth, the twisty magnetic fi eld the VLBA can pick out, or should leave a characteristic “resolve,” fi ne details about imprint, called polarization, 1,000 times better than the on light coming out of the Hubble Space Telescope. In jet. To isolate that signature, fact, even though the jets “well fed,” says Marscher, Marscher, Jorstad, and a of the fl are are enormous— matter on its way down team of international col- many light years long, in the gullet will congeal in a laborators had to wait for a some cases—they are so far pancake-shaped disk cen- blazar to discharge a fl are— from Earth that the VLBA is tered on the black hole. temporary, concentrated the only instrument in the Friction in the disk heats it emission—that would give world that can actually see up and makes it glow and them a chance to trace out bright spots moving through fl icker with ultraviolet and the shape of the magnetic the jets. visible light. That explains fi eld. In 2005, they found Now the researchers in one part of the mystery— just what they were seek- BU’s Blazar Lab are trying why some galaxies are ing: while peering nearly to understand the source of “active” when others aren’t— straight down the barrel of the blazars’ most energetic but something more seems the jet of a powerful, fl aring gamma ray fl ares. Astro- to happen to transform an blazar called BL Lacertae, physicists expected that the ordinary active galaxy into they caught the polarization gamma rays would all be a blazar capable of fi ring off within the fl are rotating by coming from very close to high-energy gamma rays one-and-a-half turns, map- the black hole at the center and X-rays. ping out exactly the spiral of the blazar. But to every- Astronomers think that shape astronomers had pre- one’s surprise, the BU team “something” is a jet: a fi re dicted. They presented their found that a major fraction hose of charged particles, results in Nature in 2008. of the gamma rays is coming magnetic fi elds, and radia- Now, thanks to a part- from a point that is light tion that shoots out from the nership between BU and years away. How does such top and bottom of the rotat- the Lowell Observatory, an extreme burst of energy ing disk. When fast-moving Marscher and Jorstad have happen so far from the electrons near the black hole near-continuous coverage blazar’s central engine? meet the strong magnetic of more than three dozen The BU team is testing fi eld inside the jet, they give blazars on the Perkins tele- out a variety of ideas using off a broad spectrum of ra- scope, a 1.8-meter optical computer models, and they diation, from low-frequency telescope near Flagstaff , hope to put them to an actual radio waves all the way up to Ariz. When a fl are erupts, test soon with the Discovery high-energy X-rays. Mean- Jorstad quickly notifi es the Channel Telescope at Low- while, those electrons can managers of NASA’s ell Observatory. MELISSA OSTROW (TOP); COURTESY OF NASA/GODDARD SPACE FLIGHT CENTER CONCEPTUAL IMAGE LAB IMAGE CENTER CONCEPTUAL FLIGHT SPACE NASA/GODDARD OF COURTESY (TOP); OSTROW MELISSA Swift Summer 2016 BOSTONIA 25 20-25_Bostonia_SU16.indd 25 6/1/16 1:03 PM.

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