Pan-STARRS1 far vision at the service of neutrino sources

Pan-STARSS1, a 1.8-meter-diameter optical telescope on the island of Maui is the world’s leading near-Earth object discovery telescope. However, its large digital camera, with almost 1.4 billion pixels, can also detect galactic and extragalactic transient phenomena and has a great potential for the discovery of supernovas, some of which could be sources of high-energy neutrinos.

In a recent publication submitted to Astronomy and Astrophysics, the IceCube Collaboration and Pan-STARRS1 scientists have searched for counterpart transient optical emission associated with IceCube high-energy neutrino alerts. When following five alerts sent during 2016-17, researchers found one supernova worth studying, SN PS16cgx. However, a more detailed analysis showed that it is most likely a Type Ia supernova, i.e., the result of a white dwarf explosion, which is not expected to produce neutrinos.

Pan-STARRS1 riz-band false-colour 1′ × 1′subsection of the field of PS16cgx. North is up, east is left.

Neutrino emission is expected in large amounts from supernovas, but in many cases these neutrinos have typical energies in the MeV range and are not associated with high-energy cosmic rays.

Very high energy neutrinos, which point to cosmic-ray sources, can be produced in some types of supernovas and usually only during a very short time. Astronomers have detected more than ten thousand extragalactic supernovas, and a few more in the Milky Way––if we take into account early observations by the naked eye or with the first telescopes––but to date none of them has proven to be a source of astrophysical TeV-and-above neutrinos.

In a recent multimessenger partnership, IceCube researchers have joined efforts with Pan-STARRS1 astronomers to follow up high-energy neutrino alerts, looking for counterpart electromagnetic emission. For small flares of neutrinos, such as the case of individual IceCube alerts, the associated electromagnetic emission can be the only way to single out a potential neutrino source. This was the case, for example, in the identification of the first likely source of high-energy neutrinos and cosmic rays following a 290-TeV neutrino detected in IceCube in September 2017.

Moreover, only a detailed understanding of mulimessenger and multiwavelength emission can reveal the processes that power the most extreme environments in the universe.

In fact, IceCube’s high-energy realtime alerts program was launched in 2016 to boost these types of follow-ups, trying to catch transient phenomena that would otherwise be only serendipitously observed by several telescopes at the same time.

Pan-STARRS1 followed five of the IceCube alerts sent during the first two years of operation of the realtime program. The first alert was sent on April 27, 2016, and turned out to be the only one with a prospective counterpart emission from Pan-STARRS1 observations.

Transient PS16cgx showed a rising light curve over two days, which is a typical signature of a young supernova, possibly undergoing a potential explosion epoch where very high energy neutrinos could be produced.

Initial spectral observations were not able to clarify whether this was a Type Ia supernova, which is not expected to emit high-energy neutrinos, or a Type Ic supernova, a stripped core-collapse supernova that could be a cosmic-ray generator and, thus, an emitter of high-energy neutrinos.

After further inspection, looking for more detailed features of the electromagnetic emission spectrum, researchers concluded that the observations are in reasonable agreement with emission expected from a Type Ia supernova and that, at the same time, there is no specific argument to support a classification as a Type Ic supernova. Therefore, scientists think that the IceCube neutrino and PS16cgx are not related.

Looking at the rate of high-energy alerts with good pointing resolution in IceCube––currently, fewer than ten per year––researchers estimate that one could expect a true association of a supernova and a high-energy neutrino once every two years, assuming that all IceCube alerts can be followed up. These results also support expanding the redshift range, i.e., the distance of the transient sources, of these joint searches, which would increase the number of transient phenomena observed and, thus, the discovery potential of neutrino and cosmic-ray sources.

info “A search for transient optical counterparts to high-energy IceCube neutrinos with Pan-STARRS1,” Pan-STARRS and the IceCube Collaboration: E. Kankare et al. Accepted to Astronomy and Astrophysics