четверг, 4 июля 2019 г.

Fast Radio Burst Pinpointed to Distant Galaxy

Owens Valley Radio Observatory, together with w. m. keck observatory, is providing new clues in an ongoing cosmic mystery. Credit: Caltech/Ovro/G. Hallinan

Maunakea, Hawaii – Fast radio bursts (FRBs) are among the most enigmatic and powerful events in the cosmos. Around 80 of these events—intensely bright millisecond-long bursts of radio waves coming from beyond our galaxy—have been witnessed so far but their causes remain unknown.

In a rare feat, researchers at Caltech’s Owens Valley Radio Observatory (OVRO) have now caught a new burst, called FRB 190523, and, together with the W. M. Keck Observatory in Hawaii, have pinpointed its origins to a galaxy 7.9 billion light-years away. Identifying the galaxies from which these radio bursts erupt is a critical step toward solving the mystery of what triggers them.

Finding the host galaxies of FRBs is not easy. Before this new discovery, only one other burst, called FRB 121102, had been localized to a host galaxy. FRB 121102 was reported in 2014 and then later, in 2017, was pinpointed to a galaxy lying 3 billion light-years away. Recently, a second localized FRB was announced on June 27, 2019. Called FRB 180924, this burst was discovered by a team using the Australian Square Kilometer Array Pathfinder and traced to a galaxy about 4 billion light-years away.

FRB 121102 was easiest to find because it continues to burst every few weeks. Most FRBs, however—including the Australian and OVRO finds—just go off once, making the job of finding their host galaxies harder.

“Finding the locations of the one-off FRBs is challenging because it requires a radio telescope that can both discover these extremely short events and locate them with the resolving power of a mile-wide radio dish,” says Vikram Ravi, a new assistant professor of astronomy at Caltech who works with the radio telescopes at OVRO, which is situated east of the Sierra Nevada mountains in California.

The team then conducted follow-up observations using Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS) to determine the properties of FRB 190523’s host galaxy.

The LRIS data revealed that the host galaxy for FRB 190523 is similar to our Milky Way. This is a surprise because the previously located FRB 121102 originates from a dwarf galaxy that is forming stars more than a hundred times faster than the Milky Way.

“This finding tells us that every galaxy, even a run-of-the-mill galaxy like our Milky Way, can generate an FRB,” says Ravi.

The discovery also suggests that a leading theory for what causes FRBs—the eruption of plasma from young, highly magnetic neutron stars, or magnetars—may need to be rethought.

“The theory that FRBs come from magnetars was developed in part because the earlier FRB 121102 came from an active star-forming environment, where young magnetars can be formed in the supernovae of massive stars,” says Ravi. “But the host galaxy of FRB 190523 is more mellow in comparison. “

The Deep Synoptic Array ten-antenna prototype (DSA-10) searches for fast radio bursts within a sky-area the size of 150 full moons (left). Within this area, the DSA-10 can locate these bursts with immense resolving power, isolating them to regions containing just one galaxy (middle). This feat was achieved for the fast radio burst called FRB 190523, detected by DSA-10 on May 23, 2019. The right panel shows the time profile of the burst above its radio spectrum. Credit: Caltech/OVRO/V. Ravi 

Ultimately, to solve the mystery of FRBs, astronomers hope to uncover more examples of their host galaxies.  

“With the full Deep Synoptic Array, we are going to find and localize FRBs every few days,” says Gregg Hallinan, the director of OVRO and a professor of astronomy at Caltech. “This is an exciting time for FRB discoveries.”

“We’re very excited about the new capabilities to find these enigmatic bursts and we look forward to continuing to provide the critical follow-up data that tell us how distant these objects are and what environments they live in,” says John O’Meara, chief scientist at Keck Observatory.

The researchers also say that FRBs can be used to study the amount and distribution of matter in our universe, which will tell us more about the environments in which galaxies form and evolve. As radio waves from FRBs head toward Earth, intervening matter causes some of the wavelengths to travel faster than others; the wavelengths become dispersed in the same way that a prism spreads apart light into a rainbow. The amount of dispersion tells astronomers exactly how much matter there is between the FRB sources and Earth. 

“Most matter in the universe is diffuse, hot, and outside of galaxies,” says Ravi. “This state of matter, although not ‘dark,’ is difficult to observe directly. However, its effects are clearly imprinted on every FRB, including the one we detected at such a great distance.” 

The Nature study, titled, “A fast radio burst localized to a massive galaxy,” was funded by NSF and Caltech. Other Caltech authors include: Morgan Catha, electronics engineer at OVRO; Larry D’Addario, system engineer; George Djorgovski, professor of astronomy; Richard Hobbs, software developer at OVRO; Jonathon Kocz, digital research engineer; Shri Kulkarni, the George Ellery Hale Professor of Astronomy and Planetary Science; Jun Shi, postdoctoral scholar; Harish Vedantham, a former postdoctoral scholar now at ASTRON, the Netherlands Institute for Radio Astronomy; Sandy Weinreb, visiting associate in astronomy; and David Woody, assistant director of OVRO.

About LRIS

The Low Resolution Imaging Spectrometer (LRIS) is a very versatile visible-wavelength imaging and spectroscopy instrument commissioned in 1993 and operating at the Cassegrain focus of Keck I. Since it has been commissioned it has seen two major upgrades to further enhance its capabilities: addition of a second, blue arm optimized for shorter wavelengths of light; and the installation of detectors that are much more sensitive at the longest (red)wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe.  LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in2011 for research determining that the universe was speeding up in its expansion.

About W.M. Keck Observatory

The W. M. Keck Observatory telescopes are the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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