ULTRASAT (the Ultraviolet Transient Astronomy Satellite) will revolutionize our understanding of the transient ultraviolet (UV) universe by undertaking the first wide-field, UV time-domain survey of the skies. ULTRASAT has two primary goals:
- to understand how the lives of massive stars end;
- to determine the masses of accreting supermassive black holes and map surrounding structures.
Due to ULTRASAT’s inherent capabilities it will also: i) provide rapid transient alerts to the global astronomical community for follow-up observations; ii) promptly respond to Targets of Opportunity (ToO).
ULTRASAT will explore the rich UV transient sky uncovering tidal disruption events, the counterparts to gravitational-wave sources, cosmic relativistic explosions, variable stars, and other exciting phenomena.
Death of Massive Stars
The explosive death of massive stars as supernovae (SNe) is a complex unsolved astrophysical question, defined as a science frontier question by the 2010
decadal survey. Determining the physical properties of massive stars prior to explosion is a critical step towards solving this problem; the pre-explosion stellar state sets the
initial conditions to any computational investigation of the explosive process. Direct identification of SN progenitor stars in pre-explosion images is limited, as it can only be applied to explosions in nearby galaxies (typically ~20Mpc away) and requires that high-spatial-resolution and deep images (mostly by HST) were acquired prior to the explosion. To date, only around ten such relatively nearby massive stars have been confirmed as SN progenitors.
Early UV observations of SN explosions provide a powerful method to study the properties (e.g., radius, surface composition) of exploding massive stars. Following the supernova explosion a shock wave propagates outward from the core of the star through its optically thick envelope. When the shock wave reaches the outer regions where the optical depth is such that the photon diffusion time scale is shorter than the hydrodynamical time scale, the photons can escape the star; this is usually called the shock breakout flare, and would constitute the first electromagnetic signal for the explosion that an outside observer can detect. For supergiant stars the initial shock breakout signal is expected to be in the X-ray/UV and its duration is directly proportional to the radius of the progenitor star. Following this initial flare, the thermal energy deposited by the shock in the expanding envelope continues to diffuse out; we call this the shock cooling emission. The bolometric luminosity of the shock cooling emission is almost constant, while the temperature of the radiating gas declines. The shock cooling signal will be prominent in the UV. The measured flux will rise as the peak of the emitted spectrum cools and passes though the observed band and will then decline as further cooling drives the emission peak to redder wavelengths (see link). The rate of cooling (and thus the time it takes for the flux to peak in a given band) depends on the stellar radius and the composition of the envelope which determines the opacity. For supergiant star explosions with thick hydrogen envelopes, the opacity is known (Thomson scattering) and time independent, so the radius is straightforwardly inferred. For compact Wolf-Rayet (W-R) stars the opacity is time-dependent and a function of the surface composition (mass fraction of He, C and O). It has been shown that, given a well-sampled UV light curve, one can infer both the stellar radius and the surface composition. Given additional early observations (e.g, in the optical) one can measure the ratio of explosion energy to ejected mass (E/M) and the relative extinction towards an explosion. See Rabinak & Waxmann, 2011, Ganot et al. 2014 for more information.
The shock-cooling emission lasts a few hours for compact W-R stars and approximately a day for red supergiants. This early emission provides constraints on the progenitor radius and chemical composition, which can not be derived from later (>few days) ground observations, since by the time the shock cools enough for visible light to be below the peak emissivity, complicating radiation from other sources (e.g.,radioactivity, recombination) interferes with these measurements, and the total emission from shock cooling in compact stars become very faint and difficult to observe.
ULTRASAT can detect the shock breakout flare from the largest stars, and those exploding within an extended circum-stellar medium, as well as the shock cooling signal from numerous supergiant and W-R massive stars, as predicted by theory and demonstrated by available observations. Combined space-UV and ground-based observations triggered by a UV transient explorer would yield an unprecedented wealth of data about massive star explosions, going beyond the stellar radius and surface composition (and thus the stellar class of the progenitor: red or blue supergiant, or W-R star). Such information includes direct measurements of the dust extinction curve toward the progenitor location, removing uncertainties in measured quantities due to extinction, and the amount of radioactive 56Ni mixed into the ejecta (a probe of the explosion mechanism and geometry). As the early UV data measure the ratio of explosion energy to ejected mass, E/M, derivation of the ejecta mass M from modeling of late-time data (light curves and nebular spectra) would provide information about the explosion energy. Measurements of early-UV photometry and early spectroscopic velocity measurements that diverge from predictions of simple models would indicate non-standard stellar density profiles while early UV observations of explosions occurring in thick circum-stellar medium would probe the final stages of massive star evolution just prior to explosion.
Given the mission parameters, the known UV signals (Figure) and local SN rates, ULTRASAT is expected to detect more than 100 events within a day of explosion per year (Ganot et al. 2014). Analysis of such a sample of massive star explosions would shed new light on the final stages of massive-star evolution and the explosive deaths of these stellar giants.
Supermassive Black Hole Masses and Environments
Supermassive black holes (SMBHs) are understood to play a pivotal role in galaxy evolution, but the physical processes responsible for the correlations between BH mass and host galaxy properties (including the interplay between the mass of the galaxy, the stellar velocity dispersion and feedback from black hole activity) are not yet understood. To confront theoretical models with data requires accurate SMBH masses over a range of cosmic distances and environments. However, direct dynamical measurements of SMBH masses are only possible for galaxies within tens of Mpc. Extending measurements to cosmological distances is only possible by observing accreting SMBHs or Active Galactic Nulclei (AGN).
From five decades of observations we have developed a basic picture of the structure of the inner regions of AGN. The black hole accretes gas through a disk, which is illuminated by an X-ray-emitting corona. The disk radiates variable continuum emission (hot UV on the inside, cooler optical further out). The UV disk emission ionizes fast-moving clouds of gas that revolve around the SMBH and shine in broad emission lines (broad line region; BLR). However, considerable debate remains about the geometry, the size and the dynamics of these structures. The method of Reverberation Mapping (RM) provides both a probe of the structure of the SMBH environment as well as a method to derive its mass. RM measures the time delay between fluctuations in the UV continuum arising in the central region of the disk and the subsequent response of the lines from the BLR. RM is the only method that can probe the structure of the spatially unresolved BLR on scales of light-days to light-months. Modeling of “high-fidelity” RM data can probe the BLR geometry and dynamics as well as provide direct constraints on the mass of the central SMBH. “Hi-Fi” observations providing both BLR geometric constraints and accurate masses require (1) a high-cadence, long-duration, high signal-to-noise (S/N) light curve of the AGN’s UV continuum flux, and (2) simultaneous, high signal-to-noise (S/N) spectroscopic monitoring of the broad emission lines to detect variations in fluxes and line profile shapes. The gold standard for this technique is NGC 5548 – the subject of an unprecedented 6-month UV and optical monitoring program in 2014 including 180 orbits of HST time and large time investments on numerous ground-based telescopes. Given the expense of extensive UV observations, optical continuum measurements have been used as a proxy for the ionizing continuum, and these have provided mass constraints for ~50 AGN. However, a recent exhaustive Swift campaign, spread over 750 days, indicates a surprisingly long delay between the near-UV (290 nm) and optical continuum variations in NGC 5548. The delay amounts to ~25% of the reverberation delay between the optical continuum and the BLR emission lines, introducing a significant uncertainty into the BH mass measurements. Additionally, micro-lensing measurements have indicated that AGN accretion disk sizes inferred from the optical band are five times larger than predicted by standard disk theory. If the size of the disk optical continuum-emitting region is indeed a significant fraction of the BLR radius, this would require a fundamental re-interpretation of all recent optical reverberation-mapping observations and the derived black hole masses, which are based on the assumption that the accretion disk size is negligible compared with the BLR radius. Measuring UV-optical time delays for AGN across a broad range in luminosity is therefore essential for establishing the correct normalization of optical-only reverberation-derived black hole masses. Optical methods will continue to be the primary means for extending the RM method to redshifts z>1.
fundamental re-interpretation of all recent optical reverberation-mapping observations and the derived black hole masses, which are based on the assumption that the accretion disk size is negligible compared with the BLR radius. Measuring UV-optical time delays for AGN across a broad range in luminosity is therefore essential for establishing the correct normalization of optical-only reverberation-derived black hole masses. Optical methods will continue to be the primary means for extending the RM method to redshifts z>1. ULTRASAT will carry out high-fidelity RM on a sample of > 45 low redshift AGN by measuring time delays between the primary UV disk continuum and BLR emission with intensive and optimized campaigns. The Hi-Fi sample spanning three orders of magnitude in BH mass will provide the fundamental anchor for secondary methods which are critical for tracing the growth of SMBH over cosmic time.
ULTRASAT combined with our ground-based program will also measure time lags between near-UV, g, R and i bands for >1,000 bright AGN. This will test models of lags resulting from reprocessing of the X-ray coronal continuum in different temperature zones of a cold accretion disk, will constrain basic theories of disk accretion in AGN, and will provide critical new data for interpreting RM results and SMBH masses from data that relies on optical photometry as a proxy for the inner disk emission.
Tidal Disruption Events (TDEs)
Stars are expected to be tidally disrupted by massive black holes (M<108 solar mass) in galactic nuclei. Accretion converts potential energy to radiation primarily in the UV. Such Tidal Disruption Events (TDEs) have become a part of astronomers’ routine lexicon.
While only ~1% of low-redshift galaxies harbor AGN, TDEs probe the most common dormant nuclear black holes. Furthermore, these cosmic gravity laboratories offer opportunities to study accretion physics “in real time” over a wide range of accretion rate and in some cases observe the formation and the decay of a relativistic jet. As the orbital angular momentum of the debris and the BH spin are almost never aligned, unique General Relativistic effects occur with potentially observable consequences.
ULTRASAT will transform the studies of these long lived and UV luminous events, that stand out in the UV against the red bulges of galaxies. Furthermore, the light curves are distinct from SNe, and high cadence ULTRASAT monitoring allows for rejecting AGN by their characteristic variability. TDEs will be discovered early during their rise and ULTRASAT alerts will enable intensive ground-based follow up. ULTRASAT is expected to find over 100 TDEs per year and thereby provide benchmark lightcurves for comparison with physical modelsincluding theories of TDE rates.
Gravitational Wave Electromagnetic counterparts
With the Advanced LIGO and Virgo projects ramping up from 2015 to 2019, and given the expected great returns, enormous resources are being invested to prepare for detecting electromagnetic (EM) counterparts of neutron star coalescences. Electromagnetic counterparts will not only identify the host galaxy but shed light on the origin of “r-process” elements (e.g. Au, Pt and U).
By 2019, coarse GW localizations (55-180 squared degrees) is well-matched to ULTRASAT’s FoV. The UV is a unique diagnostic of the fastest-moving ejecta since this can be powered by decay of free neutrons. Even if only 2.5% of the neutrons remain in the ejecta with opacity lower than 30 cm2 gm-1, ULTRASAT will detect UV emission decaying on few hour timescales (within the LIGO detection horizon of 200 Mpc). Within <1 hour of a ToO trigger, ULTRASAT can slew to any point in a third of the sky and sensitively image the entire GW localization. Thus, ULTRASAT is a powerful probe (and the only probe) of this key physics of neutron star mergers.
Gamma Ray Burst (GRB) Geometry
More than half of all Gamma-Ray Bursts (GRB) are associated with an optical/UV counterpart signal that lasts between minutes to few days. The afterglow is supposedly generated by the interaction of relativistic expanding shells with the surrounding medium. The resulting light curve is a complex, time-dependent combination of several components (reverse shock, jet break, density bumps, late energy injection) and unraveling them provides valuable information regarding the physics and energetics of the explosion.
Detection of the afterglow in the near UV is limited to the closest GRBs since above z~1.4 host galaxy Lyman limit absorption will suppress the signal. Of the ~1000 GRBs occurring every year, about 10 will happen within ULTRASAT's field of view, approximately four will be in the redshift range such that their afterglow can be detected in the NUV and two will be brighter than NUV 21.5 mag AB. These GRBs will therefore be observed regardless of a high-energy trigger, and their early afterglow emission will be followed continuously at minute-timescale temporal resolution.
Gamma-Ray Bursts are assumed to be collimated explosions, powered by ultra-relativistic jets that are a few degrees wide. Although indirect evidence supports this model, a direct observational demonstration of the collimated nature of the outflow would be very valuable. A testable prediction of the narrow jet model is that the radiation beaming angle should become wider with time as the jet decelerates. Thus, low-energy afterglow emission recorded hours-days after the burst should be seen by observers out of the initial opening angle of the prompt gamma-ray emission cone. The hypothesized event of afterglow emission seen without a high-energy (gamma- or X-ray) emission has been termed "orphan GRB".
The orphan events are 100 time more abundant but are orders of magnitude fainter than the GRB prompt emission and have not been detected so far, with a single possible exception . In flux limited surveys we expect a ~1:1 ratio of orphan to regular GRBs. ULTRASAT will thus be able to detect a few events per year. Assuming future high energy missions maintain the current sky coverage provided by Swift and Fermi/GBM (~50%), we expect a handful of bona-fide orphan afterglows per year, i.e., events which are detected as UV transients (with precise temporal information and spatial localization) and yet have no high-energy detection, even though they have occurred within sky areas covered by sensitive space missions. Later optical/radio observations would be useful to confirm the identity of such transients, e.g., by identification of an associated GRB-SN, or a long-lived radio afterglow. Detection of even a single orphan afterglow will provide a valuable direct confirmation of the GRB jet model. The ratio of orphan GRB afterglows (prompt UV without high-energy emission) to normal events (prompt gamma-ray emission and UV afterglow) will measure the GRB jet opening angle. A measurement of the average jet opening angle will allow translation of observed to isotropic energy, settling the true energy budget of GRBs.
Planet Transits
The majority of exoplanet were detected by their transit light curve in visible light. Discovery of transiting planets relies on the detection of a deficit in the photon flux during the eclipse, a signal whose magnitude scales with the square ratio
of the planetary to stellar radii, the eclipse duration, and the observation time. For a significant detection the flux deficit must exceeds a threshold noise level.
ULTRASAT offers opportunities to detect planets. Planetary transit detections are challenging and require precision measurements, so a conservative approach is warranted in
evaluating the potential for such detections.
Several possible subclasses of sources are of particular interest. First, we consider planets orbiting UV-bright stars, particularly those orbiting O, B, and A type stars, for which no extensive survey has
been performed, and whose radiative envelopes are likely to be photometrically quiet.
However, we note that the exact level of UV activity of early type stars is poorly explored. Since these types of stars are considerably shorter lived than Sun-like stars, such detections will provide a snapshot of solar systems
in their early stages of formation (e.g. before or during planet migration), and probe planet formation around massive stars.
Secondly, white dwarfs (WD) are rare and small, making the detection of orbiting planets challenging. However they are bright in the UV, and because of their small size, even relatively small planets can obscure a detectable fraction of the stellar light. While no such systems have been reported to date, ULTRASAT will include more than 20,000 WD and may enable the first such discovery (or set a limit on the planetary abundance around WDs).