Supernova

Supernova (SN) observations play a pivotal role in unraveling the mysteries of stellar evolution, the mechanisms underlying supernova explosions, the chemical enrichment of galaxies, and the fundamental physics governing our universe. The WFST (Wide-Field Survey Telescope) operates a high-cadence deep imaging survey, enabling us to anticipate the detection of tens of thousands of supernovae during the 6-year survey project. The abundance of meticulously observed supernova data collected by WFST holds the promise of establishing a more robust link between evolving stars and stars meeting their fate as supernovae. WFST will primarily focus its observational research efforts on the following aspects of supernovae.

(1) Early-phase Supernovae

The exploration of EExSNe Ia ("early-excess SNe Ia") has gained significant traction in the field of time-domain astronomy since 2012. Multiple instances of EExSNe Ia have been discovered. However, previous observations only hinted at the diverse origins of these phenomena. Shallow, low-cadence wide-field surveys have offered limited insights into the early-excess feature's origin. The deep and expansive imaging capabilities of WFST will significantly contribute to unraveling the progenitor enigma of SNe Ia. This will be achieved by systematically documenting their light curves within a remarkably short timeframe—within one day following the explosion.

The anticipation is that we will identify over one hundred "early-phase supernovae" within a few days after their explosions each year. Among these, WFST is expected to discover dozens of SNe Ia within approximately one day of their eruptions. This unprecedented dataset will enable us to establish the most precise constraints on their rise times, representing the final piece of the puzzle regarding the earliest emissions from SN Ia explosions. Furthermore, we look forward to obtaining compelling evidence of the single-degenerate (SD) progenitor system by identifying genuine instances of companion-ejecta interaction EExSNe Ia in the near future.

WFST is also poised to reveal early-phase CCSNe, providing valuable insights into the structural characteristics of dying massive stars and exploring potential relationships between their final activity and progenitor mass. Additionally, the survey's capabilities will extend to detecting or setting upper limits for precursors associated with nearby SNe at much greater depths than current time-domain surveys. This capability will enable the investigation of the last-minute stellar activities of massive stars.

(2) Fast Transients and Their Relationship with Core-Collapse Supernovae

Transients characterized by rapid fluctuations in UV and optical flux hold a special place within the scientific community. Their extraordinary photometric behaviors offer a unique opportunity to delve into the physical characteristics of their progenitors, which include core-collapse supernovae (CCSNe), Fast Blue Optical Transients (FBOTs), Fast Blue UV Transients (FBUTs), Fast Evolving Luminous Transients (FELTs), and kilonovae. Over the next few years, the wealth of photometric and spectral data collected by WFST will provide crucial insights into the explosion mechanisms of typical FBOTs and the mass-loss histories of massive stars.

Given the notably low occurrence rate of these phenomena within our local universe and the remarkable UV luminosity associated with FBUTs, the WFST DHS (Deep High-Cadence Survey) stands as the most promising survey project of the 2020s for systematically investigating these extreme transient events.

(3) Extreme Supernovae

The WFST WFS project, spanning six years, will conduct regular monitoring of the northern sky with a cadence of just a few days. This strategic approach ensures the high-completeness detection of Superluminous Supernovae (SLSNe) at redshifts (z) less than or equal to 1. SLSNe, characterized by their enduring and radiant light curves, will be prominently featured in this survey.

Leveraging the exceptional UV luminosity of SLSNe and their heightened incidence at greater redshifts (z ≤ 2), WFST, with its superior u-band sensitivity and well-designed telescope aperture, is poised to become the preeminent telescope for discovering SLSNe at z > 0.5 in the northern hemisphere.

Pair-Instability Supernovae (PISNe) and Pair-Instability Pulsational Pair-Instability Supernovae (PPISNe) are both classes of extraordinarily luminous supernovae. However, their identification in the low-redshift universe has remained challenging, largely due to the difficulties associated with pinpointing their massive progenitors. Nevertheless, the extensive field of view and deep imaging capabilities of WFST will significantly augment the pool of PISN/PPISN candidates throughout the planned six-year transient survey.

(4) Cosmology and Gravitational Lensing

A comprehensive and unbiased collection of Supernova Type Ia (SN Ia) data, observed with a relatively high cadence (i.e., ≲ 3 days), holds the potential to significantly diminish uncertainties in the measurement of dark energy density within the redshift range of 0.1 < z < 0.3. This, in turn, will enable a precise comparison with well-established measurements in the 0.1 < z < 0.1 bin. The WFST SN Ia dataset also offers the prospect of refining and extending SN Ia standardization models while enhancing our understanding of the connection between SN Ia distance measurements and the characteristics of their host galaxies.

Anticipating the discovery of thousands of WFST Supernova Type II (SN II) events at z > 0.1 in the near future, we will readily engage in direct comparisons with SN Ia measurements at 0.1 < z < 0.3, revealing pertinent implications.

To date, only five strongly-lensed supernovae (SNe) have been identified. A deep, wide-field imaging survey conducted with WFST is poised to substantially augment the sample size of strongly-lensed SNe. We can anticipate the discovery of more than 20 strongly-lensed SNe throughout the six-year WFST WFS program. Armed with dozens of WFST strongly-lensed SNe in the 2020s, we are poised to embark on exciting journeys at the forefront of cosmology and early-phase SN studies.

Transient surveys rely on three pivotal parameters: depth, area, and cadence. The exceptional design of WFST, with its large Field of View (FoV) and telescope aperture, positions it effectively for the efficient discovery of these intriguing celestial objects through high-cadence deep-imaging surveys.

We illustrate one-year WFST survey simulations conducted at two cadences: 3-day and 1-day intervals, corresponding to the wide-field and deep high-cadence surveys, respectively (refer to Figure 1). Figure 1 depicts the distribution of Supernova Type Ia (SNe Ia) on the discovery magnitude versus redshift plane, based on these two survey modes. It’s important to note that the time variable ’t' in the figure is defined as the moment of the second detection of an SN.

Our primary targets are SNe with ’t' less than 4 days, often referred to as early-phase SNe (indicated by solid symbols). These will undergo intensive observation by other observatories over the ensuing months, enabling the creation of detailed multi-band light curves and spectral evolution. Additionally, SNe Ia with ’t' less than 7 days (represented by open symbols) are also of considerable interest. This category predominantly consists of SNe for which we anticipate robust multi-color light curve coverage starting approximately 10 to 14 days before the peak. This data will greatly facilitate statistical analyses of SN light curve behaviors and SN cosmology across a broad spectrum of redshifts.

In our simulated one-year WFST observation, we anticipate the discovery of over 1000 SNe Ia at z ≲ 0.25 with ’t' less than 7 days, particularly identifying approximately 100 early-phase SNe Ia at z ≲ 0.15 through WFST DHS (Deep High-Cadence Survey). Remarkably, the number of early-phase SNe Ia discovered through this approach is approximately three times greater than that obtained from WFST WFS (Wide-Field Survey). This underscores the critical role of a deep high-cadence survey in the search for early-phase SNe and other rapid transients.