Gravitational lensing, the bending of light which travels near massive objects, can result in distortion, magnification, and even multiple images of the original light source. The closer the light travels to the massive object the stronger the lensing effect. The magnification that results from this process allows scientists to observe objects which would normally not be visible due to their distance and brightness.
Researchers at the Oskar Klein Centre, and their international collaborators, have observed for the first time a type Ia supernova that is strongly gravitationally lensed producing multiple images. The light from the original supernova explosion (at a redshift z=0.4) is bent as it travels through a galaxy (at a redshift z=0.2) which is almost perfectly aligned between the supernova and the Earth. This alignment produced a magnification of the light from the supernova as well as four images of the source.
Type Ia supernovae are thought to always have the same brightness when they explode and thus have been used to measure distances in the Universe. Distance measurements to a large number of Type Ia supernovae led to the discovery that the expansion of the Universe is accelerating. The team will now use the difference in the time it takes the light from each of the four images of the single supernova to measure the current expansion rate of the Universe, the Hubble Constant.
What do you do when you are studying an exciting transient optical phenomenon and the Sun rises, rendering further observations impossible from your observatory? Well, there is always dark sky somewhere else!
A project dubbed Global Relay of Observatories Watching Transients Happen (GROWTH), a collaboration among twelve institutions spread around the globe including OKC, has been awarded a $4.5 million over five years from NSF to perform coordinated follow-up studies of optical transients, as shown by the figure below.
The project, led by the Caltech astrophysicist Mansi Kasliwal, is an important addition to the OKC involvement in iPTF and its successor, the Zwicky Transient Facility (ZTF). ZTF discoveries will be followed around the clock ensuring we can study phenomena with characteristic time-scales shorter than one day.
The GROWTH network will include ground- and space-based telescopes to observe cosmic transients at X-ray, ultraviolet, infrared, optical, and radio wavelengths. The rapid and efficient temporal coverage will vastly facilitate the identification of new types of transients. This is particularly timely, in view that the gravitational wave (GW) horizon has been expanded by Advanced LIGO.
One of the main goals of GROWTH is to identify electromagnetic counterparts of GW signals: ripples in the fabric of space and time that are predicted to accompany violent events in the universe such as the merger of a neutron star with a black hole. These cosmic collisions are expected to also produce short bursts of optical light that could be discovered already today by iPTF and by ZTF after 2017. GROWTH will help distinguishing these events from other variable phenomena. A secure identification of a GW signal would be a major scientific breakthrough.
The GROWTH project has also a strong educational component: students from the twelve involved universities will have a chance to visit the other host institutions. A great opportunity for students at OKC!
Supernova 2014J in the nearby galaxy M82 -less than 12 million light-years away- exploded on January 14, 2014 and was the closest ”standard candle” supernova since (at least) 42 years. An impressive coordinated observational effort orchestrated by the intermediate Palomar Transient Factory (iPTF) team and led by Ariel Goobar from the Oskar Klein Centre at Stockholm University (Goobar et al. 2014, The Astrophysical Journal Letters, 784, L12) provides important new clues into the nature of these explosions, as well as the environments where they take place. The proximity of SN2014J allowed the iPTF team to study this important class of stellar explosions, known as Type Ia supernovae, over a very wide wavelength range, starting just hours after the deduced explosion time.
Furthermore, Goobar and collaborators used pre-explosion images of the region of M82 where the supernova went off, both from the Hubble Space Telescope and from the Palomar Oschin Telescope, to search for a star in the location of the explosion, or possible earlier nova eruptions. The lack of pre-explosion detections suggests that the supernova may have originated in the merging of compact faint objects, e.g., two white dwarf stars, i.e., the kind of Earth size stars that our sun will evolve to once it runs out of nuclear fuel.
“Until very recently, the leading model for standard candle supernovae was thought to include a companion star from which material was stripped by the white dwarf until the accumulated mass could no longer be sustained by the outwards pressure, leading to a runaway thermonuclear explosion. The observations of SN2014J are challenging for this theoretical picture”, says Goobar.
Type Ia supernovae are among the best tools to measure cosmological distances. Thanks to their consistent peak brightness, these ”standard candles” are used to map the expansion history of the Universe. In 1998 distance measurements using supernovae lead to the a paradigm shift in cosmology and fundamental physics: the expansion of the Universe is speeding up, contrary to the expectations from the attractive nature of gravitational forces: a mysterious new cosmic component, ”dark energy”, has been invoked to explain this unexpected phenomenon. This discovery was awarded the 2011 Nobel Prize in physics.
“Since Type Ia supernovae are very rare, occurring only once every several hundred years in a galaxy like ours, there have been very few opportunities to study these explosions in great detail. SN2014J in the nearby galaxy M82 is a very welcome exception”, says Rahman Amanullah a researcher at OKC.
A better understanding of the physics behind Type Ia supernovae and the material surrounding the explosion and dimming some of the light is crucial to further refine the measurements of the expansion history of the Universe. Joel Johansson, a PhD student at OKC that played an essential role in the analysis fills in “many supernovae explode in clean environments, free of dust in the line of sight. This is not the case for SN2014J, which gives us a unique opportunity to study both the properties of the supernova explosion but also of the intervening dust”.
The lessons learned by the studies of SN2014J may be very useful for the analysis of the large Type Ia SN sample that scientists have collected over decades, especially the astrophysical corrections needed to make accurate distance estimates. Only then may we be able to tell what is causing the accelerated expansion of the cosmos.
The iPTF project is a scientific collaboration between Caltech; Los Alamos National Laboratory; the University of Wisconsin, Milwaukee; the Oskar Klein Centre in Sweden; the Weizmann Institute of Science in Israel; the TANGO Program of the University System of Taiwan; and the Kavli Institute for the Physics and Mathematics of the Universe in Japan.
The observations were carried out using multiple astronomical facilities. Besides the Palomar telescopes, data of SN2014J and M82 were obtained at the Nordic Optical Telescope, the Keck Telescope, the Faulkes Telescope North, the Mount Abu 1.2m Infrared telescope in India, the1.93m telescope of Haute-Provence Observatory, CNRS, France, the Spitzer Space Telescope and the Hubble Space Telescope.
Contact: Prof. Ariel Goobar (firstname.lastname@example.org)
Prof. Ariel Goobar, Dept of Physics, Stockholms universitet, tfn +46 8-55 37 86 59, e-mail email@example.com
Rahman Amanullah, researcher, Dept of Physics,, Stockholms universitet, tfn +46 8-55 37 88 48 e-mail: firstname.lastname@example.org
Joel Johansson, PhD student, Dept of Physics, Stockholms universitet, tfn +46 8-55 37 86 61, e-mail email@example.com
It is the Knut och Alice Wallenbergs foundation that grants a 5-year long project for finding and studying supernovae. The OKC are already since the beginning of this year members of the intermediate Palomar
Transient Factory (iPTF) – a supernova search aimed at finding supernovae soon after explosion. This is a pathfinder for the next generation of this project – the Zwicky Transient Facility (ZTF). The 30 million grant from KAW will now enable OKC astronomers and physicists to play a leading role in that project.
Jesper Sollerman, from the department of astronomy, is leading the application: – Previous supernova surveys have often discovered supernovae days or weeks after explosion. We want to find them on the very first night. In this way we hope to learn more about their progenitors, the stars that actually exploded.
The deaths of massive stars is the focus for the supernova astronomers at the department of astronomy, including both observers such as Sollerman and modelers as co-applicant Claes Fransson.