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Two Peculiar Fast Transients in a Strongly Lensed Host Galaxy

Abstract

Two unusual transient events were observed by the Hubble Space Telescope in 2014, appearing in a galaxy at z=1.0054$$\pm$$0.0002 that is gravitationally lensed by the galaxy cluster MACS J0416.1-2403. These transients—collectively nicknamed “Spock”— were faster and fainter than any supernova, but significantly more luminous than a classical nova: they reached peak luminosities of $$\sim 10^{41}$$ erg s$${}^{-1}$$ in $$\lesssim$$5 rest-frame days, then faded below detectability in roughly the same timespan. Lens models of the foreground cluster suggest that it is entirely plausible that the two events are spatially coincident at the source plane, but very unlikely that they were also temporally coincident. We find that Spock can plausibly be classified as a recurrent nova, a luminous blue variable, or a stellar caustic crossing event—but all three explanations require either an extreme astrophysical source or highly unusual gravitational lensing.

Introduction

\label{sec:Introduction}

The Spock transient events—separately designated HFF14Spo-1 and HFF14Spo-2—appeared in Hubble Space Telescope (HST) imaging collected in January and August of 2014, respectively (Figure \ref{fig:SpockDetectionImages}). These images were centered on the galaxy cluster MACS J0416.1-2403 (hereafter, MACS0416) and were collected as part of the Hubble Frontier Fields (HFF) survey (HST-PID:13496, PI:Lotz), a multi-cycle program for deep imaging of 6 massive galaxy clusters and associated “blank sky” fields observed in parallel.

Combining the HST imaging and lens models of the MACS0416 gravitational lens leads to three key observables for the HFF14Spo events: (1) they are both more luminous than a classical nova, but less luminous than almost all supernovae (SNe), reaching a peak luminosity of roughly $$10^{41}$$ erg s$${}^{−1}$$ ($$M_{V}=−14$$); (2) both transients exhibited fast light curves, with rise and decline timescales of $$\sim 2--5$$ days in the rest frame; and (3) it is likely that both events arose from the same physical location but were not coincident in time—they were probably separated by 3-5 months in the rest frame. These peculiar transients thus present an intriguing puzzle: they are broadly consistent with the expected behavior of stellar explosions (they each exhibit a single isolated rise and decline in brightness), but they can not be trivially classified into any of the common categories of explosive or eruptive astrophysical transients.

The HFF survey was not designed with discovery of peculiar extragalactic transients as a core objective, but it has unintentionally opened an unusual window of discovery for such events. Very faint sources at relatively high redshift in these fields are made detectable by the substantial gravitational lensing magnification of the foreground galaxy clusters. Very rapidly evolving sources are also more likely to be found, due to the necessity of a rapid cadence for repeat imaging in the HFF program. These unusual characteristics for an HST survey contributed to the early detection and characterization of SN Refsdal, the first SN that is strongly lensed into multiple resolved images (Kelly et al., 2015). The HFF imaging program has also enabled a precise measurement of the lensing magnification for SN Tomas, a high-redshift Type Ia SN (Rodney et al., 2015). Both of those SNe constitute fairly common astrophysical phenomena, and the possibility of discovering such transients was anticipated at the start of the HFF program. HFF14Spo, however, appears to be sui generis. No single astrophysical model is clearly sufficient to explain all of the available observational data, and the best available models require either an extreme stellar source or a very unusual gravitational lensing configuration.

\label{fig:SpockDetectionImages}The detection of HFF14Spo-1 and HFF14Spo-2 in HST imaging from the Hubble Frontier Fields. The central panel shows the full field of the MACSJ0416 cluster, in a combined image using optical and infrared bands from HST. Two boxes within the main panel demarcate the regions where the HFF14Spo host galaxy images appear. These regions are shown as two inset panels on the left, highlighting the three images of the host galaxy (labeled 11.1, 11.2, and 11.3), whcih are caused by the gravitational lensing of the cluster. Two columns on the right side show the discovery of the two transient events in optical and infrared light, respectively. In these final two columns the top row is a template image, the center row shows the epoch when each transient appeared, and the bottom row is the difference image.