Free Astronomy Magazine May-June 2019

55 MAY-JUNE 2019 SPACE CHRONICLES team that used telescopes ranging from gamma rays to radio waves, in- cluding the National Science Founda- tion’s Karl G. Jansky Very Large Array (VLA), to study the object. “In addi- tion, AT2018cow’s distance of 200 million light-years, is nearby, by as- tronomical standards, making it an excellent target for study,” Margutti said. Astronomers presented their findings about the object at the Amer- ican Astronomical So- ciety’s meeting in Seattle, Washington. After watching the ob- ject and measuring its changing characteris- tics with a worldwide collection of ground- based and orbiting tel- escopes, scientists still are not sure exactly what it is, but they have two leading explanations. It may be, they suspect, either a very unusual supernova, or the shredding of a star that passed too close to a massive black hole, called a Tidal Dis- ruption Event (TDE). Researchers are quick to point out, however, that the object’s character- istics don’t match previously-seen ex- amples of either one. “If it is a super- nova, then it is unlike any supernova we have ever seen,” Ho said. The ob- ject’s range of colors, or spectrum, she said, “doesn’t look like a super- nova at all.” In addition, it was brighter in millimeter waves — those seen by ALMA — than any other su- pernova. It also differs from previ- ously-seen Tidal Disruption Events. “It’s off-center in its host galaxy,” Deanne Coppejans, of Northwestern University, said, meaning it can’t be a star shredded by the supermassive black hole at the galaxy’s center. “If it’s a TDE, then we need an inter- mediate mass black hole to do the shredding, and those are expected to form in stellar clusters,” Kate Alexan- der, an Einstein Fellow at Northwest- ern, added. The problem with that, she pointed out, is that AT2018cow appears to be inside a high-density interstellar medium, which “is diffi- cult to reconcile with the density of gas in stellar clusters.” Most of the researchers agree that AT2018cow’s behavior requires a cen- tral source of ongoing energy unlike those of other supernova explosions. The best candidate, they said, is a black hole that is drawing material from its surroundings. The inflowing material forms a rotating disk around the black hole and that disk radiates prolific amounts of energy. This is the type of “central engine” that powers quasars and radio galaxies through- out the Universe as well as smaller ex- amples such as microquasars. When a star much more massive than the Sun ceases thermonuclear fusion and collapses of its own grav- ity, producing a “normal” supernova explosion, no such central engine is produced. However, in the extreme cases called hypernovas, which pro- duce gamma ray bursts, such a cen- tral engine produces the superfast jets of material that generate the gamma rays. That engine, however is very short-lived, lasting only a matter of seconds. If such a central engine powered AT2018cow, it lasted for weeks, making this event distinct from the collapse-induced explosions of su- pernovas and the more-energetic such explosions that produce gamma ray bursts. In the case of a TDE, the “engine” would come to life as the black hole drew in material from the star shredded by its gravitational pull. Alternatively, the “en- gine” resulting from a supernova explosion might be a rapidly-ro- tating neutron star with an extremely powerful magnetic field — a magnetar. “We know from the- ory that black holes and neutron stars form when a star dies, but we’ve never seen them right after they are born. Never,” Margutti said. “This is very exciting, since it would be the first time that astronomers have witnessed the birth of a central engine,” Ho said. However, because of AT2018cow’s strange behavior, the verdict still is unclear, the scientists said. The central energy source could be a powerful shock wave hitting a dense shell of material at the ob- ject’s core. Either the strange super- nova or the TDE explanation still is viable, Ho’s team said. The astronomers look forward to more work on AT2018cow and to more objects like it. “During the first few weeks, this ob- ject was very bright at millimeter wavelengths, so that means that, with ALMA now available, we may be able to find and study others,” Ho said. “The peak strength of the radio emission starts at ALMA wavelengths, and only moved to VLA wavelengths after a few weeks,” she added. ! A LMA and VLA im- ages of the mysteri- ous new type of cosmic blast, AT2018cow at left. Visible-light image of outburst in its host galaxy at right. Images not to same scale. Im- ages of the blast itself do not indicate its size, but are the result of its brightness and the char- acteristics of the tele- scopes. [Sophia Dagnel- lo, NRAO/AUI/NSF; R. Margutti, W.M. Keck Observatory; Ho, et al.]