Search

We present simulated observations to assess the ability of the Large Synoptic Survey Telescope (LSST) and the wide-fast-deep (WFD) survey to detect and characterize kilonovae—the optical emission associated with binary neutron star (and possibly black hole–neutron star) mergers. We expand on previous studies in several critical ways by exploring a range of kilonova models and several choices of cadence, as well as by evaluating the information content of the resulting light curves. We find that, depending on the precise choice of cadence, the WFD survey will achieve an average kilonova detection efficiency of ≈1.6%–2.5% and detect only ≈3–6 kilonovae per year. The detected kilonovae will be within the detection volume of the Advanced LIGO/Virgo (ALV). By refitting the best resulting LSST light curves with the same model used to generate them, we find that the model parameters are generally weakly constrained, and are accurate to at best a factor of 2–3. Motivated by the finding that the WFD will yield a small number of kilonova detections, with poor light curves and marginal information content, and that the detections are in any case inside the ALV volume, we argue that target-of-opportunity follow-up of gravitational-wave triggers is a much more effective approach for kilonova studies. We outline the qualitative foundation for such a program with the goal of minimizing the impact on LSST operations. We argue that observations in the gz-bands with a total time investment per event of ≈1.5 hr per 10 deg2 of a search area is sufficient to rapidly detect and identify kilonovae with ≳90% efficiency. For an estimated event rate of ∼20 per year visible to LSST, this accounts for ∼1.5% of the total survey time. In this regime, LSST has the potential to be a powerful tool for kilonovae discovery, with detected events handed off to other narrow-field facilities for further monitoring. [ABSTRACT FROM AUTHOR]

We present an empirical study of contamination in wide-field optical follow-up searches of gravitational wave sources from Advanced LIGO/Virgo using dedicated observations with the Dark Energy Camera. Our search covered ∼56 deg2, with two visits per night, in the i and z bands, followed by an additional set of griz images three weeks later to serve as reference images for subtraction. We achieve 5σ point-source limiting magnitudes of i ≈ 23.5 and z ≈ 22.4 mag in the coadded single-epoch images. We conduct a search for transient objects that mimic the i − z color behavior of both red (i−z > 0.5 mag) and blue (i−z < 0 mag) kilonova emission, finding 11 and 10 contaminants, respectively. Independent of color, we identify 48 transients of interest. Additionally, we leverage the rapid cadence of our observations to search for sources with characteristic timescales of ≈1 day and ≈3 hr, finding no potential contaminants. We assess the efficiency of our search with injected point sources, finding that we are 90% (60%) efficient when searching for red (blue) kilonova-like sources to a limiting magnitude of i ≲ 22.5 mag. Using our efficiencies, we derive sky rates for kilonova contaminants of deg−2 and deg−2. The total contamination rate is deg−2. We compare our results to previous optical follow-up efforts and comment on the outlook for gravitational wave follow-up searches as additional detectors (e.g., KAGRA, LIGO India) come online in the next decade. [ABSTRACT FROM AUTHOR]

We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor superluminous supernova (SLSN) at redshift z = 0.5215 discovered in the last months of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out because of its extremely slow evolution, with an observed rise of rest-frame days, and exponential decline out to ∼250 days past peak at a measured rate of , consistent with fully trapped 56Co decay. This is the longest rise time measured in an SLSN to date, and the first SLSN to show a rise time consistent with pair-instability supernova (PISN) models. Compared to other slowly evolving SLSNe, it is spectroscopically similar to the prototype SN 2007bi at maximum light, although lower in luminosity () and with a flatter peak than previous events. PS1-14bj shows a number of peculiar properties, including a near-constant color temperature for days past peak, and strong emission lines from [O iii] λ5007 and [O iii] λ4363 with a velocity width of ∼3400 km s−1 in its late-time spectra. These both suggest there is a sustained source of heating over very long timescales, and are incompatible with a simple 56Ni-powered/PISN interpretation. A modified magnetar model including emission leakage at late times can reproduce the light curve, in which case the blue continuum and [O iii] features are interpreted as material heated and ionized by the inner pulsar wind nebula becoming visible at late times. Alternatively, the late-time heating could be due to interaction with a shell of H-poor circumstellar material. [ABSTRACT FROM AUTHOR]

We present observations of SN 2015bn (=PS15ae = CSS141223-113342+004332 = MLS150211-113342+004333), a Type I superluminous supernova (SLSN) at redshift z = 0.1136. As well as being one of the closest SLSNe I yet discovered, it is intrinsically brighter () and in a fainter galaxy () than other SLSNe at . We used this opportunity to collect the most extensive data set for any SLSN I to date, including densely sampled spectroscopy and photometry, from the UV to the NIR, spanning −50 to +250 days from optical maximum. SN 2015bn fades slowly, but exhibits surprising undulations in the light curve on a timescale of 30–50 days, especially in the UV. The spectrum shows extraordinarily slow evolution except for a rapid transformation between +7 and +20–30 days. No narrow emission lines from slow-moving material are observed at any phase. We derive physical properties including the bolometric luminosity, and find slow velocity evolution and non-monotonic temperature and radial evolution. A deep radio limit rules out a healthy off-axis gamma-ray burst, and places constraints on the pre-explosion mass loss. The data can be consistently explained by a M stripped progenitor exploding with erg kinetic energy, forming a magnetar with a spin-down timescale of ∼20 days (thus avoiding a gamma-ray burst) that reheats the ejecta and drives ionization fronts. The most likely alternative scenario—interaction with ∼20 M of dense, inhomogeneous circumstellar material—can be tested with continuing radio follow-up. [ABSTRACT FROM AUTHOR]

The first direct detection of gravitational waves (GWs) by the ground-based Advanced LIGO/Virgo interferometers is expected to occur within the next few years. These interferometers are designed to detect the mergers of compact object binaries composed of neutron stars and/or black holes to a fiducial distance of ∼200 Mpc and a localization region of ∼100 deg2. To maximize the science gains from such GW detections it is essential to identify electromagnetic counterparts. Among the wide range of proposed counterparts, the most promising is optical/IR emission powered by the radioactive decay of r-process elements synthesized in the neutron-rich merger ejecta—a “kilonova.” Here we present detailed simulated observations that encompass a range of strategies for kilonova searches during GW follow-up. We utilize these simulations to assess both the detectability of kilonovae and our ability to distinguish them from a wide range of contaminating transients in the large GW localization regions. We find that if pre-existing deep template images for the GW localization region are available, then nightly observations to a depth of i ≈ 24 mag and z ≈ 23 mag are required to achieve a 95% detection rate; observations that commence within ∼12 hr of trigger will also capture the kilonova peak and provide stronger constraints on the ejecta properties. We also find that kilonovae can be robustly separated from other known and hypothetical types of transients utilizing cuts on color (i − z ≳ 0.3 mag) and rise time (trise ≲ 4 days). In the absence of a pre-existing template the observations must reach ∼1 mag deeper to achieve the same kilonova detection rate, but robust rejection of contaminants can still be achieved. Motivated by the results of our simulations we discuss the expected performance of current and future wide-field telescopes in achieving these observational goals, and find that prior to LSST the Dark Energy Camera on the Blanco 4 m telescope and Hyper Suprime-Cam on the Subaru 8 m telescope offer the best potential for kilonova discovery. [ABSTRACT FROM AUTHOR]