Amongst the many rich returns of ULTRASAT survey data, we discuss three examples:
Star-Planet Connection
An important and little studied issue is the impact of stellar flares on life-bearing planet searches. Enhanced flare- and coronal-mass-ejection activity around both young solar analogs or low-mass M stars, puts severe limits on their habitability. An HST study investigated the UV temporal variability of a sample of planet-hosting M and K dwarfs, classified as inactive according to optical activity tracers. Over 50% of the sample were found to regularly produce UV flares with flare/quiescent flux ratios of >10. Thus, systematic characterization of prodigiously UV flaring stellar hosts is required to select the most promising, UV-quiet targets for expensive biomarker searches, e.g., by JWST or the ELT. In addition, photolysis (photo-dissociation) of CO2 and H2O by Lyα and other bright stellar chromospheric and transition region emission lines can result in a substantial O3 atmosphere from photochemical processes alone, suggesting that planets orbiting these flaring stars can also constitute false positives in the search for biomarkers.
ULTRASAT will monitor ~106 stars and will be sensitive also to smaller (and more abundant) flares. We will measure, for the first time, the NUV flare frequency and luminosity distribution for host stars as functions of both spectral subclass, and stellar rotation period, determined from ULTRASAT or TESS (Transiting Exoplanet Survey Satellite) photometry. This will then inform the selection of the best habitable planet candidates (e.g., from TESS) for expensive spectroscopic bio-marker searches, envisaged for example by JWST and ELT. An extended mission lifetime for ULTRASAT would determine the flaring rate and luminosity over the full stellar activity cycle for a subset of target stars.
Type Ia Supernovae
Type Ia SNe made the iron in our blood and provided the first evidence for an accelerating Universe. Despite these two amazing gifts, the origin of these SNe is controversial. Direct observational studies have shown that the exploding object is a white dwarf (WD). But there is still a debate whether the explosion is caused by accretion from a non-degenerate companion star (single degenerate, SD model), or by the merger or collision of a pair of WDs (double degenerate; DD).
A strong discriminator between these two models is the early UV flare predicted in the SD model when the SN explosion ejecta interacts with the non-degenerate companion. There is already good evidence for such a decaying UV flare (measured by Swift) from iPTF14atg, an event belonging to a rare subclass of type Ia SNe. Claims that additional normal (“classical”) SN Ia also shows a binary-interaction flare suggesting an SD origin (for example based on Kepler data) have been made and criticized.
At peak, the spectra of type Ia SNe are heavily blanketed at wavelengths <350 nm. Even despite this, we expect a mission yield of >30 photospheric detections of classical Ia SNe. In contrast, at early times (<1 d) SNe like iPTF14atg would be bright enough in the NUV so that ULTRASAT could detect them in a volume containing 70 SNe Ia. However, only a fraction of SNe Ia are expected to arise via the SD channel and only those whose line-of-sight is favorable will have a strong NUV emission. The number of iPTF14atg-like detections can thus constrain this fraction. A null detection would constrain the SD fraction to be <10% at >3σ (a significant improvement on ground based limits). The expected few detections will certainly serve as a Rosetta stone for SD model(s). Separately, radio and X-ray observations, triggered by early alerts from ULTRASAT, will provide crucial diagnostics for the DD model.
Tidal Disruption Events (TDEs)
Only a few percent of low-redshift galaxies have active nuclei, but in the nuclei of the otherwise silent majority of galaxies, stars will be tidally disrupted by their massive BHs (M<108Mʘ), at a rate of about 1 per 104 years per Milky-Way like galaxy. Emission comes from four phases: (1) breakout of shocks resulting from compression of the star at pericenter, (2) collisions between returning tidal streams of the bound half of the disrupted star, whose properties and timing vary greatly with black hole and star mass, spin and orbit, (3) accretion of the bound gas onto the black hole, and (4) collision of the hypervelocity unbound half of the disrupted star with gas in the galactic nucleus.
Current surveys discover a few UV-bright good tidal disruption event (TDE) candidates per year. Extrapolation from optical surveys shows ULTRASAT should find over 100 TDEs per year. TDEs and weak but highly variable AGN can easily be confused with each other. The one unambiguous way to confirm a TDE is to see the flash of phase (1): X-ray for sun-like stars and UV for giants.
Because of its continuous UV monitoring of a wide field, ULTRASAT is uniquely capable of convincingly confirming TDEs by detecting both the shock breakout of strongly compressed giant stars at pericenter [phase (1)] and the much later stream collisions and accretion [phase (2) and phase (3)]. This will probe nuclear black holes and the population and dynamics of their stellar cusps, and the ULTRASAT early alerts will enable intensive ground-based follow up. LSST will provide sparse optical lightcurves in the southern field.
The high cadence of ULTRASAT is ideal for finding and studying the rapidly-evolving TDEs expected to occur in lower-mass black holes (M<105Mʘ). Observational evidence for the existence of such BHs would fill a critical missing link for BH evolution models.