Rodney, S. A., Balestra, I., Bradac, M., Brammer, G., Broadhurst, T., Caminha, G. B., Chirivi, G., Diego, J. M., Filippenko, A. V., Foley, R. J., Graur, O., Grillo, C., Hemmati, S., Hjorth, J., Hoag, A., Jauzac, M., Jha, S. W., Kawamata, R., Kelly, P. L., McCully, C., Mobasher, B., Molino, A., Oguri, M., Richard, J., Riess, A. G., et al
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.
