From W.M. Keck Observatory: “Unexplained Brightness from Colossal Explosion”

One view of the interior of one of the telescopes.

W.M. Keck Observatory, operated by Caltech and the University of California, Maunakea Hawaii USA, altitude 4,207 m (13,802 ft). Credit: Caltech.

Mauna Kea Observatory, Hawaii USA, altitude 4,213 m (13,822 ft).

From W.M. Keck Observatory

November 12, 2020

Mari-Ela Chock, Communications Officer
W. M. Keck Observatory
(808) 554-0567
mchock@keck.hawaii.edu

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This image shows the glow from a kilonova caused by the merger of two neutron stars. the kilonova, whose peak brightness reaches up to 10,000 times that of a classical nova, appears as a bright spot (indicated by the arrow) to the upper left of the host galaxy. the merger of the neutron stars is believed to have produced a magnetar, which has an extremely powerful magnetic field. the energy from that magnetar brightened the material ejected from the explosion. Credit: NASA, ESA, W. Fong (Northwestern University), and T. Laskar (University of Bath, UK)

Astronomers have discovered the brightest infrared light from a short gamma-ray burst ever seen, with a bizarre glow that is more luminous than previously thought was possible.

Its half-second flash of light, detected in May of this year, came from a violent explosion of gamma rays billons of light-years away that unleashed more energy in a blink of an eye than the Sun will produce over its entire 10-billion-year lifetime.

The study has been accepted in The Astrophysical Journal.

“It’s amazing to me that after 10 years of studying the same type of phenomenon, we can discover unprecedented behavior like this,” said Wen-fai Fong, assistant professor of physics and astronomy at Northwestern University and lead author of the study. “It just reveals the diversity of explosions that the universe is capable of producing, which is very exciting.”

NASA’s Hubble Space Telescope quickly captured the glow within just three days after the burst and determined its near-infrared emission was 10 times brighter than predicted, defying conventional models.

NASA/ESA Hubble Telescope.

“These observations do not fit traditional explanations for short gamma-ray bursts,” said Fong. “Given what we know about the radio and X-rays from this blast, it just doesn’t match up. The near-infrared emission that we’re finding with Hubble is way too bright.”

To zero in on this new phenomenon’s exact brightness, the team used W. M. Keck Observatory on Maunakea in Hawaii to pinpoint the precise distance of its host galaxy.

“Distances are important in calculating the burst’s true brightness as opposed to its apparent brightness as seen from Earth,” said Fong. “Just as the brightness of a light bulb when it reaches your eye depends on both its luminosity and its distance from you, a burst could be really bright because either it is intrinsically luminous and distant, or not as luminous but much closer to us. With Keck, we were able to determine the true brightness of the burst and thus the energy scale. We found it was to be much more energetic than we originally thought.”


This animation shows the sequence for forming a magnetar-powered kilonova, whose peak brightness reaches up to 10,000 times that of a classical nova. In this sequence, two orbiting neutron stars spiral closer and closer together before colliding and merging. This triggers an explosion that unleashes more energy in a half-second than the Sun will produce over its entire 10-billion-year lifetime. The merger forms an even more massive neutron star called a magnetar, which has an extraordinarily powerful magnetic field. The magnetar deposits energy into the ejected material, causing it to glow unexpectedly bright at infrared wavelengths. Credit: NASA, ESA, and D. Player (STScI).

Using Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS) [below] and DEep Imaging and Multi-Object Spectrograph (DEIMOS) [below], the team determined the burst came from a galaxy located at a redshift of z = 0.55 – quite a bit farther than the initial calculated distance.

Lasting less than two seconds, short gamma-ray bursts are among the most energetic, explosive events known; they live fast and die hard. Scientists think they’re caused by the merger of two neutron stars, extremely dense objects about the mass of the Sun compressed into the volume of a small city. A neutron star is so dense that on Earth, one teaspoonful would weigh a billion tons!

Neutron star mergers are very rare and extremely important because scientists think they are one of the main sources of heavy elements in the universe, such as gold and uranium.

Along with a short gamma-ray burst, scientists expect to see a “kilonova” whose peak brightness typically reaches 1,000 times that of a classical nova. Kilonovae are an optical and infrared glow from the radioactive decay of heavy elements and are unique to the merger of two neutron stars, or the merger of a neutron star and a black hole.

What Fong and her team saw was too bright to be explained even by a traditional kilonova. They provide one possible explanation for the unusually bright blast. While most short gamma-ray bursts probably result in a black hole, the neutron star merger in this case may have instead formed a magnetar, a supermassive neutron star with a very powerful magnetic field. The magnetar deposited a large amount of energy into the ejected material of the kilonova, causing it to glow even brighter.

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This illustration shows the sequence for forming a magnetar-powered kilonova, whose peak brightness reaches up to 10,000 times that of a classical nova. 1) Two orbiting neutron stars spiral closer and closer together. 2) They collide and merge, triggering an explosion that unleashes more energy in a half-second than the Sun will produce over its entire 10-billion-year lifetime. 3) The merger forms an even more massive neutron star called a magnetar, which has an extraordinarily powerful magnetic field. 4) The magnetar deposits energy into the ejected material, causing it to glow unexpectedly bright at infrared wavelengths. Credit: NASA, ESA, and D. Player (STScI).

“What we detected even outshines the one confirmed kilonova discovered in 2017,” said co-author Jillian Rastinejad, a graduate student with Fong’s team at Northwestern University. “As a first-year graduate student working with real-time data for the first time when this burst happened, it’s remarkable to see our discovery motivate a new and exciting magnetar-boosted model.”

With such an event, the team expects the ejecta from the burst to produce light at radio wavelengths in the next few years. Follow-up radio observations may ultimately prove the origin of the burst was indeed a magnetar. The birth of a magnetar from a neutron star merger has never definitively been seen before, as they are expected to be rare outcomes.

The short gamma-ray burst was first detected with NASA’s Neil Gehrels Swift Observatory.

NASA Neil Gehrels Swift Observatory.

Once the alert went out, the team quickly enlisted other telescopes to conduct multi-wavelength observations. They analyzed the afterglow in X-ray with Swift Observatory, optical and near-infrared with Las Cumbres Observatory Global Telescope, Hubble, and Keck Observatory, and in radio wavelengths with the Very Large Array. This particular gamma-ray burst was one of the rare instances in which scientists were able to detect light across the entire electromagnetic spectrum.


LCOGT Las Cumbres Observatory Global Telescope Network, Haleakala Hawaii, USA, Elevation 10,023 ft (3,055 m).

NRAO Karl G Jansky Very Large Array, located in central New Mexico on the Plains of San Agustin, between the towns of Magdalena and Datil, ~50 miles (80 km) west of Socorro. The VLA comprises twenty-eight 25-meter radio telescopes.

NASA’s upcoming James Webb Space Telescope is particularly well-suited for this type of observation.

NASA James Webb Space Telescope annotated.

“We can’t wait to combine the power of Keck and JWST along with other facilities as a team to go after even more enigmatic events like these,” said Keck Observatory Chief Scientist John O’Meara. “This study shows that we have much left to learn.”

ABOUT LRIS [below]

The Low Resolution Imaging Spectrometer (LRIS) is a very versatile and ultra-sensitive visible-wavelength imager and spectrograph built at the California Institute of Technology by a team led by Prof. Bev Oke and Prof. Judy Cohen and commissioned in 1993. Since then it has seen two major upgrades to further enhance its capabilities: the 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 in 2011 for research determining that the universe was speeding up in its expansion.

ABOUT DEIMOS [below]

The DEep Imaging and Multi-Object Spectrograph (DEIMOS) boasts the largest field of view (16.7arcmin by 5 arcmin) of any of the Keck Observatory instruments, and the largest number of pixels (64 Mpix). It is used primarily in its multi-object mode, obtaining simultaneous spectra of up to 130 galaxies or stars. Astronomers study fields of distant galaxies with DEIMOS, efficiently probing the most distant corners of the universe with high sensitivity.

See the full article here .
See the full NASA/ESA Hubble article here


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Mission
To advance the frontiers of astronomy and share our discoveries with the world.

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometer and world-leading laser guide star adaptive optics systems. Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

Today Keck Observatory is supported by both public funding sources and private philanthropy. As a 501(c)3, the organization is managed by the
California Association for Research in Astronomy (CARA), whose Board of Directors includes representatives from the California Institute of Technology and the University of California, with liaisons to the board from NASA and the Keck Foundation.


Keck UCal

Instrumentation

Keck 1

HIRES – The largest and most mechanically complex of the Keck’s main instruments, the High Resolution Echelle Spectrometer breaks up incoming starlight into its component colors to measure the precise intensity of each of thousands of color channels. Its spectral capabilities have resulted in many breakthrough discoveries, such as the detection of planets outside our solar system and direct evidence for a model of the Big Bang theory.

Keck High-Resolution Echelle Spectrometer (HIRES), at the Keck I telescope.

LRIS – The Low Resolution Imaging Spectrograph is a faint-light instrument capable of taking spectra and images of the most distant known objects in the universe. The instrument is equipped with a red arm and a blue arm to explore stellar populations of distant galaxies, active galactic nuclei, galactic clusters, and quasars.

UCO Keck LRIS on Keck 1.

VISIBLE BAND (0.3-1.0 Micron)

MOSFIRE – The Multi-Object Spectrograph for Infrared Exploration gathers thousands of spectra from objects spanning a variety of distances, environments and physical conditions. What makes this huge, vacuum-cryogenic instrument unique is its ability to select up to 46 individual objects in the field of view and then record the infrared spectrum of all 46 objects simultaneously. When a new field is selected, a robotic mechanism inside the vacuum chamber reconfigures the distribution of tiny slits in the focal plane in under six minutes. Eight years in the making with First Light in 2012, MOSFIRE’s early performance results range from the discovery of ultra-cool, nearby substellar mass objects, to the detection of oxygen in young galaxies only 2 billion years after the Big Bang.

Keck/MOSFIRE on Keck 1, Mauna Kea, Hawaii, USA.

OSIRIS – The OH-Suppressing Infrared Imaging Spectrograph is a near-infrared spectrograph for use with the Keck I adaptive optics system. OSIRIS takes spectra in a small field of view to provide a series of images at different wavelengths. The instrument allows astronomers to ignore wavelengths where the Earth’s atmosphere shines brightly due to emission from OH (hydroxl) molecules, thus allowing the detection of objects 10 times fainter than previously available.

Keck OSIRIS on Keck 1

Keck 2

DEIMOS – The Deep Extragalactic Imaging Multi-Object Spectrograph is the most advanced optical spectrograph in the world, capable of gathering spectra from 130 galaxies or more in a single exposure. In ‘Mega Mask’ mode, DEIMOS can take spectra of more than 1,200 objects at once, using a special narrow-band filter.

Keck/DEIMOS on Keck 2.

NIRSPEC – The Near Infrared Spectrometer studies very high redshift radio galaxies, the motions and types of stars located near the Galactic Center, the nature of brown dwarfs, the nuclear regions of dusty starburst galaxies, active galactic nuclei, interstellar chemistry, stellar physics, and solar-system science.

NIRSPEC on Keck 2.

ESI – The Echellette Spectrograph and Imager captures high-resolution spectra of very faint galaxies and quasars ranging from the blue to the infrared in a single exposure. It is a multimode instrument that allows users to switch among three modes during a night. It has produced some of the best non-AO images at the Observatory.

KECK Echellette Spectrograph and Imager (ESI) on Keck II.

KCWI – The Keck Cosmic Web Imager is designed to provide visible band, integral field spectroscopy with moderate to high spectral resolution, various fields of view and image resolution formats and excellent sky-subtraction. The astronomical seeing and large aperture of the telescope enables studies of the connection between galaxies and the gas in their dark matter halos, stellar relics, star clusters and lensed galaxies.

Keck Cosmic Web Imager on Keck 2 schematic.

Keck Cosmic Web Imager on Keck 2.

NEAR-INFRARED (1-5 Micron)

ADAPTIVE OPTICS – Adaptive optics senses and compensates for the atmospheric distortions of incoming starlight up to 1,000 times per second. This results in an improvement in image quality on fairly bright astronomical targets by a factor 10 to 20.

UCO Keck Laser Guide Star Adaptive Optics,Keck Observatory.

LASER GUIDE STAR ADAPTIVE OPTICS – The Keck Laser Guide Star expands the range of available targets for study with both the Keck I and Keck II adaptive optics systems. They use sodium lasers to excite sodium atoms that naturally exist in the atmosphere 90 km (55 miles) above the Earth’s surface. The laser creates an “artificial star” that allows the Keck adaptive optics system to observe 70-80 percent of the targets in the sky, compared to the 1 percent accessible without the laser.

NIRC-2/AO – The second generation Near Infrared Camera works with the Keck Adaptive Optics system to produce the highest-resolution ground-based images and spectroscopy in the 1-5 micron range. Typical programs include mapping surface features on solar system bodies, searching for planets around other stars, and analyzing the morphology of remote galaxies.

Keck NIRC2 Camera on Keck 2.

NIRES

Keck Near-Infrared Echellette Spectrometer on Keck 2.

Future Instrumentation

KCRM – The Keck Cosmic Reionization Mapper will complete the Keck Cosmic Web Imager (KCWI), the world’s most capable spectroscopic imager. The design for KCWI includes two separate channels to detect light in the blue and the red portions of the visible wavelength spectrum. KCWI-Blue was commissioned and started routine science observations in September 2017. The red channel of KCWI is KCRM; a powerful addition that will open a window for new discoveries at high redshifts.

KCRM – Keck Cosmic Reionization Mapper KCRM on Keck 2.

KPF – The Keck Planet Finder (KPF) will be the most advanced spectrometer of its kind in the world. The instrument is a fiber-fed high-resolution, two-channel cross-dispersed echelle spectrometer for the visible wavelengths and is designed for the Keck II telescope. KPF allows precise measurements of the mass-density relationship in Earth-like exoplanets, which will help astronomers identify planets around other stars that are capable of supporting life.

KPF Keck Planet Finder on Keck 2