An IceCube neutrino alert triggers the discovery of a supernova

On March 30, 2012, IceCube detected two high-energy neutrino events. They were less than two seconds apart and within 1.32 degrees of each other. The detection of a high-energy neutrino is always good news, so having two this close in space and time is incredibly exciting since it could indicate the existence of a transient source, such as a supernova explosion or a gamma-ray burst (GRB).

IceCube immediately sent an alert to several optical and X-ray telescopes—the Robotic Optical Transient Search Experiment (ROTSE), the Palomar Transient Factory (PTF) and the Swift satellite—and a core-collapse supernova was discovered in the PTF images. However, physicists have shown that this was a coincidental discovery and that this supernova is not likely to be the source of the neutrinos in IceCube. These results have been submitted today to the Astrophysical Journal and are the outcome of a joint study between the IceCube Collaboration and members of the PTF Collaboration, the Swift Collaboration and the Pan-STARRS1 Science Consortium.

New image, reference image and post-subtraction image of the PTF discovery of PTF12csy from 2012 April April 09, with the location of PTF12csy in the center. The image from the Sloan Digital Sky Survey (SDSS) is shown for reference, showing a faint host galaxy to the south of the SN. Image: PTF Collaboration.

The 21st century has seen the birth of a new age in astronomy. High-energy neutrinos have joined the electromagnetic spectrum as tools to explore the universe. New cosmic messengers and an increased number of facilities looking at the skies allow for the observation of sources of all kinds, including transient cosmic phenomena. The images from our universe are evolving from fairly static views to scenes where bursts, explosions, jets, flares, collapses, and all sort of transient phenomena are the norm.

Neutrinos, unlike light or other electromagnetic radiation, are almost completely undisturbed by matter. Even dense environments, such as the atmosphere of a star, are transparent to neutrinos. This means that we can use neutrinos to look at the interior of astronomical objects in a similar way as we use X-rays to look inside human bodies, but on much larger scales.

The transparency of matter to neutrinos may also grant our telescopes a glimpse of the ephemeral universe. Many transient sources are expected to emit neutrinos as well as different sorts of electromagnetic radiation. But neutrinos will escape first, reaching the Earth a few hours before the electromagnetic burst. This effect was recorded in supernova SN 1987A, which remains the only neutrino source outside our solar system ever identified. The majority of neutrinos from supernovas have energies of 10-40 megaelectronvolts (MeV), and such low-energy neutrinos are difficult for IceCube to detect unless the supernova is in our galaxy or close to it.

But scientists have theorized that neutrino emission also happens at energies a million times higher, in the teraelectronvolt (TeV) regime, where IceCube is more sensitive and can detect sources at much greater distances. Supernova explosions could emit neutrinos at these energies as well, for example, if they host choked jets that accelerate particles that in the end produce neutrinos. These phenomena would be very similar to long GRBs, which are also thought to be driven by powerful jets.

Neutrino sources at TeV energies have not yet been discovered. The IceCube Collaboration, in a joint effort with optical, X-ray and gamma-ray astronomers, has set up special programs to look for transient sources of neutrinos. Following an IceCube alert from promising neutrino detections, other telescopes will search in the direction of the observed neutrinos looking for a possible transient source.

The two neutrinos of March 30 were the most significant neutrino alert from IceCube so far. Several follow-up observations were made in the direction of the neutrino alert, searching for the electromagnetic counterpart of, for example, a long GRB or a choked jet supernova. PTF images found a core-collapse supernova in the right direction, only 0.2 degrees away from the average neutrino direction. The discovery per se was a thrill, and for a while researchers thought they might have observed a photon-neutrino coincident emission. However, a search of the Pan-STARRS1 archive revealed that the optical emission was at least 169 days old. If the neutrinos were not produced immediately after the core collapse, then maybe they were created during a steady emission in the following months.

The new supernova, named PTF12csy, is a member of the Type IIn class. These supernovas can emit neutrinos for 1 to 10 months. But, due to the distance of PTF12csy, it was very unlikely that two neutrinos from this source were seen within less than 2 seconds. To rule out this possibility, a one-year search for neutrinos from PTF12csy was performed. The results show no significant neutrino signal from this source.

The intensity of the astrophysical neutrino flux discovered by IceCube and the previous searches for neutrino sources support a scenario with plenty of but faint sources, pushing the discovery of the first neutrino source into the future. However, transient sources that emit all their neutrinos during a short time stand out above a steady background. Finding a transient source, bursting at the same time, with another telescope allows the attribution of a few neutrinos—even a single neutrino—to a given source. “This is why searching for transient sources is very exciting,” says Markus Voge, a graduate student at the University of Bonn and one of the corresponding authors of this paper. “Neutrino data alone only tells us half of the story. We need data from other telescopes to confirm and identify the source. But IceCube neutrinos can also help other telescopes to find a source,” adds Voge.

The present study is a nice example of the emerging synergies between neutrino- and photon-based telescopes. Finding PTF12csy was just a coincidence, but it also proved the potential for discovery of collaborative follow-up observation programs.

+ Info “Detection of a Type IIn Supernova in Optical Follow-up Observations of IceCube Neutrino Events,” IceCube Collaboration: M.G. Aartsen et al. Astrophysical Journal 811 (2015) 52,