2017
01 JunCosmic explosions revealed during the search for light from LIGO’s newest detection
Research teams from the international GROWTH collaboration used telescopes around the world and covering vast portions of the spectrum to conduct a rapid and extensive search for an electromagnetic afterglow following a trigger by LIGO on Jan 4 this year. After several months of vigorous analysis, the LIGO/Virgo collaboration confirmed today the third detection of a gravitational wave event, dubbed GW170104. During those same months, GROWTH astronomers sieved through their data hoping to find the first ever electromagnetic counterpart of a gravitational wave event and in the process, they stumbled upon two, unrelated to LIGO, but nonetheless curious high-energy transients.
Gamma Ray Burst
On Wednesday, 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 Caltech 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 spread 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.
“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,” – says Mansi Kasliwal. “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 Palomar 200-inch, Discovery Channel Telescope, Lijiang Observatory and Akeno Observatory suggested that ATLAS17aeu was fading away very fast but implied an explosion time of Jan 5 and not Jan 4. Varun Bhalerao, an assistant professor at IIT-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.
“Analysing the data, I concluded that ATLAS17aeu must be related to some explosion on 5th January, not the 4th”, says Varun Bhalerao who is also the lead author in the first of two GROWTH papers describing the follow up campaign, published on Arxiv today.
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 hour 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.
Relativistic Supernova
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 came from. Finally, on Jan 7, the 48-inch telescope resumed its regular scanning of the skies. Alessandra Corsi at Texas Tech University, 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.” – says Alessandra Corsi, lead author of the second GROWTH study, published on Arxiv today, which describes the collaboration’s follow up campaign of GW170104.
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 US 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 back in 1998 astronomers 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. 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.” – says Brad Cenko, a co-author on the study.
Gamma-ray bursts eject material traveling at extremely high speeds (very close to the speed of light) in narrow jets. 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-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.
The search is on
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 confirmed today by the LIGO team, 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 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 effectively coordinated network of scientists and observatories that can swiftly collect multi-wavelength data on potential candidates and weed out false positives” – says Mansi Kasliwal. “We are getting better with each trigger and ready to finally catch a neutron star merger when it comes. And that 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.”
Contacts
Iva (Ivona) Kostadinova
GROWTH Communications & Media Contact
ivonata@caltech.edu
+1 626 395 2952
Mansi Kasliwal
Assistant Professor of Astronomy, Caltech
mansi@astro.caltech.edu
+1 626 395 1575
Varun Bhalerao
Assistant Professor of Astronomy, IIT-Bombay
varunb@iitb.ac.in
Alessandra Corsi
Assistant Professor of Astronomy, Texas Tech University
alessandra.corsi@ttu.edu
