Magnified or multiply imaged?-Search strategies for gravitationally lensed supernovae in wide-field surveys

Abstract

Strongly lensed supernovae (SNe) can be detected as multiply imaged or highly magnified transients. In order to compare the performances of these two observational strategies, we calculate expected discovery rates as a function of survey depth in five grizy filters and for different classes of SNe (types Ia, IIP, IIL, Ibc, and IIn). We find that detections via magnification is the only effective strategy for relatively shallow pre-LSST (Large Synoptic Sky Survey) surveys. For survey depths about the LSST capacity, both strategies yield comparable numbers of lensed SNe. SN samples from the two methods are to a large extent independent and combining them increases detection rates by about 50 per cent. While the number of lensed SNe detectable via magnification saturates at the limiting magnitudes of LSST, detection rates of multiply imaged SN still go up drastically at increasing survey depth. Comparing potential discovery spaces, we find that lensed SNe found via image multiplicity exhibit longer time delays and larger image separations making them more suitable for cosmological constraints than their counterparts found via magnification. We provide useful fitting functions approximating the computed discovery rates for different SN classes and detection methods. We find that the Zwicky Transient Factory will find about two type Ia and four core-collapse lensed SNe per year at a limiting magnitude of 20.6 in the r band. Applying a hybrid method which combines searching for highly magnified or multiply imaged transients, we find that LSST will detect 89 type Ia and 254 core-collapse lensed SNe per year. In all cases, lensed core-collapsed SNe will be dominated by type IIn SNe contributing to 80 per cent of the total counts, although this prediction relies quite strongly on the adopted spectral templates for this class of SNe. Revisiting the case of the lensed SN iPTF16geu, we find that it is consistent within the 2σ contours of predicted redshifts and magnifications for the intermediate Palomar Transient Factory survey.