June 1, 2017
Sky Map of LIGO's Black-Hole Mergers This three-dimensional projection of the Milky Way galaxy onto a transparent
globe shows the probable locations of the three confirmed LIGO black-hole merger events—GW150914
(blue), GW151226 (orange), and the most recent detection GW170104 (magenta)—and a
fourth possible detection, at lower significance (LVT151012, green). The outer contour
for each represents the 90 percent confidence region; the innermost contour signifies
the 10 percent confidence region.
LIGO/Caltech/MIT/Leo Singer (Milky Way image: Axel Mellinger)
Research teams from the international Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration, including Texas Tech University’s own Alessandra Corsi, used telescopes around the world covering vast portions of the spectrum to conduct a rapid and extensive search for an electromagnetic afterglow following a trigger by the Laser Interferometer Gravitational-wave Observatory (LIGO) on Jan. 4.
After several months of vigorous analysis, LIGO and the European-based Virgo Collaboration confirmed today the third detection of a gravitational wave event, dubbed GW170104. During those same months, GROWTH astronomers sifted through their data hoping to find the first electromagnetic counterpart of a gravitational-wave event and, in the process, they stumbled upon two unrelated, but nonetheless curious, high-energy transients.
On Jan. 4, temperatures in the northern hemisphere and after-holiday spirits were running low. Everything changed in an instant when Mansi Kasliwal, GROWTH principal investigator and an assistant professor of astronomy at the California Institute of Technology, received an email from the LIGO/Virgo collaboration for a possible detection of a gravitational-wave event. She forwarded the announcement to colleagues from GROWTH around the world, triggering a noticeable wave of excitement. Mobilizing the global network of GROWTH observatories, multiple telescopes spanning frequencies from gamma-rays to radio peered into the region of the sky identified by LIGO as the possible source location of the gravitational-wave trigger.
This image shows the most common type of gamma-ray burst, thought to occur when a
massive star collapses, forms a black hole, and blasts particle jets outward at nearly
the speed of light.
NASA's Goddard Space Flight Center
“Here in California, we were quite unlucky because our transient discovery engine in Palomar Observatory was clouded out and we couldn’t use it for three nights after the LIGO trigger,” Kasliwal said. “However, colleagues using the ATLAS telescopes in Hawaii discovered an intriguing transient, called ATLAS17aeu, in the sky location specified by LIGO. We quickly triggered many of our GROWTH telescopes to solve the mystery by studying it in detail.”
Photometry from the Palomar 200-inch Discovery Channel Telescope in Arizona, Lijiang Observatory in China and Akeno Observatory in Japan suggested that ATLAS17aeu was fading away very fast but implied an explosion time of Jan. 5, not Jan 4. Varun Bhalerao, an assistant professor at the Indian Institute of Technology Bombay and a GROWTH co-investigator had already looked into the X-ray data from the Indian satellite AstroSat for anything that could be associated with the GW170104 but found nothing.
Digging into AstroSat data from Jan. 5, Varun and his students found the explosion they were looking for. It appeared that the optical ATLAS17aeu was associated with a gamma-ray burst – short-lived bursts of the most energetic form of light – dubbed GRB 170105A, which occurred in the same part of the sky 21 hours after the LIGO trigger and therefore completely unrelated to the gravitational-wave event.
Additional optical, X-ray and radio data from GROWTH facilities confirmed the connection between the two events. Using these data to analyze the behavior of the gamma-ray burst, astronomers concluded they had witnessed the birth of a black hole after the collapse of an extremely massive star in a galaxy several billion light years away.
The few days between the LIGO trigger on Jan. 4 and the moment when clouds finally dispersed at Palomar felt like an eternity for most GROWTH team members, who eagerly waited to point the observatory telescopes towards the patch of sky where the tiny ripples in spacetime detected by LIGO originated. Finally, on Jan. 7, the 48-inch telescope resumed its regular scanning of the skies.
Position of the relativistic supernova iPTF17cw (Corsi et al. 2017, ApJ, submitted to) superimposed on the final LIGO localization of GW170104 (https://gcn.gsfc.nasa.gov/gcn3/21056.gcn3 and Abbott et al. 2017 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.221101).
Color represents probability density. A black contour is drawn around the LIGO 90% credible region. iPTF17cw falls just outside this region, so we rule out any association between iPTF17cw and GW170104.
Corsi, an assistant professor in the Texas Tech Department of Physics & Astronomy and the newest partner in the GROWTH collaboration, noticed an interesting transient event, which appeared slightly away from the LIGO-identified region.
“This event fell on the outside border of the error circle, but I wanted to follow it nonetheless as it showed a peculiar spectrum with high velocities,” said Corsi, lead author of the second GROWTH study, published on Arxiv today.
Multiple optical spectra – the amount of light per given frequency – of the transient, dubbed iPTF17cw, were gathered using telescopes at Palomar, the Liverpool Telescope on the Canary Islands, the Discovery Channel Telescope in the U.S. and the Keck telescopes in Hawaii, all of which are part of the GROWTH network of observatories. These spectra were used to classify iPTF17cw as a broad-line Type Ic supernova. These supernovae are interesting because astronomers in 1998 discovered a connection between them and gamma-ray bursts – the most powerful electromagnetic explosions observed in the universe lasting from milliseconds to hours.
Corsi and her team swiftly gathered radio data from the Very Large Array and X-ray data from the Swift satellite and Chandra X-ray Observatory. They also searched for gamma-ray detections that coincide with iPTF17cw in data from NASA’s Fermi Gamma-Ray Space Telescope and the POLAR Satellite run jointly by Europe and China. Extensive analysis performed by the team pointed to iPTF17cw being associated with a weak gamma-ray burst GRB 161228B.
“We need further data to confirm with certainty that the supernova we discovered and the gamma-ray burst are associated,” said Brad Cenko, a co-author on the study. “However, what is truly exciting about iPTF17cw is that it is one of the few highly energetic events detected independent of a gamma-ray trigger.”
Gamma-ray bursts eject material traveling in narrow jets very close to the speed of light. They are typically discovered via bright but short-lived gamma-ray emission, which can be readily detected by space-based satellites such as Fermi and POLAR. However, these gamma-ray satellites can only discover events with their narrow jets pointed towards Earth, and these represent only a small fraction of the total sample.
Furthermore, recent discoveries suggest some stellar explosions may generate bright optical and radio emission but no discernible gamma rays, even when viewed on an axis. By discovering more of these relativistic explosions via their optical and radio emission, astronomers hope to understand what causes material to be accelerated to such high speeds in the first place.
LIGO and its European cousin Virgo have now firmly opened a new window into the universe, especially powerful for observing the dynamics of strong gravity events. Black hole mergers, such as GW170104, may not result in detectable electromagnetic radiation – the tool we have used for centuries to study the universe.
A truly exciting chapter will open up when astronomers catch light from a gravitational wave event detected by LIGO/Virgo. Models suggest that electromagnetic radiation is expected from a neutron star merger or the clash between a neutron star and a black hole. Learning how to routinely make such detections will shine light on fundamental questions about the physics and evolution of our universe.
“The recent search by our GROWTH team underscores the importance of an effectively coordinated network of scientists and observatories that can swiftly collect multi-wavelength data on potential candidates and weed out false positives,” Kasliwal said. “We are getting better with each trigger and ready to finally catch a neutron star merger when it comes – and it will. The universe has never yielded its secrets easily and astronomers are used to grand challenges. LIGO is a perfect example. We continue our search.”
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