Science Overview
ZTF's high-cadence data stream enables new investigations in a wide variety of fields. The ZTF survey averages more than 300 epoch each year over the entire Northern sky, giving nearly four times the number of exposures of SDSS Stripe 82 over 100 times the sky area. Public access to the ZTF data provides a wide variety of community science, much unanticipated. Within the partnership, we have six working groups.
A new collection of signals emitted by stars that are falling into black holes. The ZTF survey has been able to capture a large number of these rare events. Each symbol denotes a different star that meets this unfortunate fate. The shape of these light curves encodes important information about the properties of the black hole that disrupted the star (van Velzen et al., 2020).
The centers of galaxies can host a variety of transient events. This figure shows two important properties of the full collection of nuclear transients found during the first year of ZTF operations. We see that supernova (SN) typically have a much faster rise time compared to flares from black holes that are actively accreting gas (labelled AGN). The rare class of tidal disruption events (TDE) show blue colors and relatively slow rise times. Combining this information with other properties measured by ZTF allows for efficient selection of these valuable transients (van Velzen et al., 2020).
The ZTF lightcurves of three newly discovered double-degenerate ultracompact binary systems. ZTF J1539+5027 is a double white dwarf binary which will be one of the strongest sources of gravitational radiation detectable by the Laser Space Interferometer Antenna (Burdge et al. 2019). ZTF J2130+4420 is the first known example of a semi-degenerate hot subdwarf accreting onto a white dwarf (Kupfer et al. 2020), a potential class of Type Ia supernova progenitors. Shown on the right, ZTF J1901+5309 is an eclipsing double white dwarf binary with narrow eclipses discovered due to ZTF's dense sampling (Coughlin et al. 2020).
Part of ZTF's scientific goal is to discover gravitational wave sources detectable by the Laser Space Interferometer Antenna (LISA). So far we have discovered two of these systems, and measured their orbital decay due to gravitational radiation. The left hand panel shows this decay for ZTF J1539+5027 (Burdge et al. 2019a), the middle panel for PTF J0533+0209 (Burdge et al. 2019b), and the right-hand panel illustrates the gravitational wave strain of these sources with respect to other known LISA sources (Kupfer et al. 2018). The solid black curve in the right panel represents LISA's sensitivity threshold after 4 years of observations.
An artist impression of ZTF J1539+5027, a 6.91 minute orbital period double white dwarf binary which will be one of the strongest gravitational wave sources detectable by the Laser Space Interferometer Antenna (LISA) (Burdge et al. 2019).
Type Ia supernova Hubble diagram (only in i-band for now) using the Type Ia supernovae in the RCF sample with adequate i-band data. This is the current largest I-band dataset from a single telescope with search and follow-up in an untargeted survey, important for minimising selection and environment biases for SNe Ia, and extends the Hubble diagram to a median redshift that is twice that of the most recent sample in the literature.
ZTF coverage maps of two BNS triggers (GW190901 and GW190425) and two NSBH triggers (GW190426 and GW190814) during the third observing run of LIGO/Virgo. Despite both BNS triggers being localized to worse than a pi of the sky, ZTF was able to map the accessible localization area in a few hours. ZTF followed up 13 out of 15 triggers during O3 (all except one that was too far South and one that was too close to the moon).
Response time, defined as the time lag between merger time and earliest possible observation time at a given site, for all GW alerts in O3. We assume observation begins when at least 30% of the enclosed probability of a GW localization contour is above airmass 1.5 and the sun is below 12 degrees at a given site. Note that Palomar can respond within four hours for most of the triggers while Chile has systematic time lags.
Pre-discovery detection of interstellar comet 2I/2019 Q4 (Borisov), the second known interstellar object, identified in ZTF data. (Ye et al. 2020)
Collage of flash spectra going from H-rich (SN II) to He-less (SN Ic). This brings us to a total of 271 infant SNe so far in ZTF discovered with an age less than 2.5 days.
The phase-space of rise time and peak luminosity (cf. Kasliwal et al. 2011). The colored points represent individual objects observed by ZTF.
High-velocity ("broad-lined") stripped-envelope Type Ic supernovae (Ic-BL SNe) are a rare subclass of core-collapse supernovae of particular interest because of their association with long-duration gamma-ray bursts (GRBs). In the local universe (z<0.05), Ic-BL SNe associated with GRBs have had an early (<2d) peak in the optical light curve: the best-studied example is SN2006aj associated with GRB060218, shown as a grey line. With ZTF, we have the first sample of early high-cadence light curves of Ic-BL SNe: the first test of how many Ic-BL SNe have light curves similar to SN2006aj.
