Two peculiar fast transients in a strongly lensed host galaxy
Sharon, K.; Diego, J. M.; Molino, A.; Zitrin, A.; Filippenko, A. V.; Foley, R. J.; Hoag, A.; Brammer, G.; Treu, T.; Grillo, C.; Balestra, I.; Rosati, P.; Richard, J.; Weiner, B. J.; Mobasher, B.; Broadhurst, T.; Graur, O.; McCully, C.; Caminha, G. B.; Schmidt, K. B.; Hjorth, J.; Strolger, L. -G.; Riess, A. G.; Kelly, P. L.; Rodney, S. A.; Jha, S. W.; Suyu, S. H.; Bradac, M.; Chirivı, G.; Hemmati, S.; Jauzac, M.; Kawamata, R.; Oguri, M.; Selsing, J.; Williams, L. L. R.
United States, Germany, Spain, Italy, Netherlands, Denmark, United Kingdom, South Africa, Japan, Brazil, France, Taiwan, Israel
Abstract
A massive galaxy cluster can serve as a magnifying glass for distant stellar populations, as strong gravitational lensing magnifies background galaxies and exposes details that are otherwise undetectable. In time-domain astronomy, imaging programmes with a short cadence are able to detect rapidly evolving transients, previously unseen by surveys designed for slowly evolving supernovae. Here, we describe two unusual transient events discovered in a Hubble Space Telescope programme that combined these techniques with high-cadence imaging on a field with a strong-lensing galaxy cluster. These transients were faster and fainter than any supernovae, but substantially more luminous than a classical nova. We find that they can be explained as separate eruptions of a luminous blue variable star or a recurrent nova, or as an unrelated pair of stellar microlensing events. To distinguish between these hypotheses will require clarification of the cluster lens models, along with more high-cadence imaging of the field that could detect related transient episodes. This discovery suggests that the intersection of strong lensing with high-cadence transient surveys may be a fruitful path for future astrophysical transient studies.