Every couple of seconds a star explodes somewhere in the universe.These tremendously energetic events called supernovae shape the galaxies they live in and seed the cosmos with the raw materials of life. Telescopes located everywhere around the world - from remote,high altitude professional observatories to the backyards of amateur enthusiasts - find and study as many supernovae as possible. Sometimes it's possible to find two or more supernovae in random locations of the same galaxy over the course of years or decades. This past year,however, astronomers were surprised to learn that sometimes supernovae can even be found right on top of one another!
Excitement started in July 2013 when dairy farmer and amateur astronomer Stuart Parker in New Zealand discovered a new bright source in the galaxy NGC 6984. A seasoned supernova hunter with 50 discoveries under his belt who operates as part of the Backyard Observatory Supernova Search group, Parker quickly realized that his discovery was located at virtually the same position as another supernova named SN 2012im that had been found the year before.Parker immediately shared his peculiar result with postdoctoral fellow Dan Milisavljevic at the Harvard-Smithsonian Center for Astrophysics(CfA) who initiated a cascade of follow-up observations spanning the entire electromagnetic spectrum. First to respond within an hour of Parker's original dispatch was the new Southern African LargeTelescope (SALT) in South Africa. These data confirmed the reality of the supernova and it was given the name SN 2013ek by the International Astronomical Union.
Following the confirmation by SALT, the Swift satellite in space that has a suite of instruments sensitive to X-ray, optical and ultraviolet wavelengths was triggered into action by Raffaella Margutti (CfA).Eric Hsiao and Nidia Morrell of the Carnegie Supernova Project thencoordinated near-infrared and optical observations at Las Campanas Observatory, and supporting observations at radio wavelengths were arranged by Atish Kamble (CfA) with the Giant Metrewave Radio Telescope in India.
While these observations were taking place, emails zipped back and forth between inboxes around the globe as astronomers attempted to understand the nature of this "double supernova." Were the two explosions related? Or was this a chance alignment of two completely independent supernovae? Contributing to the analysis were Harvard University Professors Edo Berger, Robert Kirshner, and Alicia Soderberg, Harvard graduate students Maria Drout and Nathan Sanders, Professor Robert Fesen of Dartmouth College, and Caltech research scientist Andrew Drake who is co-PI of the Catalina Real-Time Sky Survey that enabled the discovery of the first supernova SN 2012im.Archival data on SN 2012im was easily retrieved from the "Bright Supernova" website that computer-programmer-by-day and supernova-enthusiast-by-night David Bishop manages in his free time.The website hosted many of the images of SN 2012im taken and investigated by Joseph Brimacombe and Stan Howerton that helped to pinpoint the locations of the two supernovae.
Milisavljevic believed that the rare discovery of two supernovae located so close to one another warranted a closer look with the razor sharp resolution of the Hubble Space Telescope (HST). He and collaborators applied for use of Director's Discretionary time, which meant justifying an urgent need to interrupt regular operations with HST. Their request was successful and the resulting images obtained August 2013 showed two sources in the region of Parker's discovery.One source is easily identifiable as SN 2013ek, but the other is something unknown and possibly the previous supernova SN 2012im.Though the offset between the two sources is very small, at the distance of the host galaxy NGC 6984 (approximately 200 million light years) it implies a large physical distance that makes an association between the two events nearly impossible.
Milisavljevic, who is presenting these results at the 223rd annual meeting of the American Astronomical Society taking place in Washington, D.C. between January 5-9, 2014, stresses that the nature of the second source is not certain and could easily be an unrelated nearby cluster of stars. A return visit with the Hubble Space Telescope could firmly resolve whether the second source is actuallySN 2012im. "By waiting a year to take another set of images with the Hubble Space Telescope," says Milisavljevic, "we can do a nose count to see what is left behind." If both sources fade, then this would be proof positive that the two events are unrelated and that Parker had uncovered a curious case of two closely neighboring supernovae occurring within a year of each other: Strange but not impossible. Alternatively, if both sources don’t fade, then the jury would still be out and it would remain possible that the two events SN 2012im and SN 2013ek could be related in some unknown and potentially very exciting way.