Rapid and Extreme Explosions
WFM’s combination of sensitivity, angular and spectral resolution, and instantaneous wide-field coverage will enable unique studies of black hole transients, tidal disruption events, gamma-ray bursts (GRBs), and extreme stellar flaring events. Its positional uncertainty, smaller than the fields of view of modern integral-field instruments, will allow immediate optical spectroscopic follow-up with a single pointing. WFM’s wide and shallow survey will yield a large rate of transient events both near and bright enough for multiwavelength follow-up.
WFM will have unique capabilities relative to the existing GRB missions. In particular, WFM will detect the softest GRBs, referred to as X-ray flashes (XRFs), reexamining a population first explored with BeppoSAX and HETE but with better grasp and angular resolution. Some XRFs are the subluminous, less-extreme tail of ordinary GRB distribution. These may, in fact, be the dominant population of GRBs, since XRFs tend to be found at lower redshift than classical GRBs, so a small fraction of them is sampled in typical surveys for GRBs. In contrast, a small fraction of XRFs may instead be the highest-redshift GRBs. Additionally, with WFM’s 300eV spectral resolution, it will be possible to search for absorption edges and emission lines in prompt GRB emission, which was marginally detected with BeppoSAX, and can yield key information about the progenitor’s abundances.
Additionally, the progenitor donor stars of type Ia SNe may be probed with WFM. The blast wave of a type Ia SN interacting with its donor star should produce a shock with Lx of about 6 x 1e44 erg/sec for duration that scales roughly linearly with donor radius. For 1 M_sol donors, this emission should last 100 seconds, allowing detections out to about 200 Mpc, while they could be seen to about 130 Mpc for 0.2 M_sol donors. Thus, even for the lowest-mass main sequence donors, the event rate should be ~ 1 event per week, given known SN rates. Thus, an absence of any signatures of such events would point toward a double-degenerate channel dominating, since for them, the shocks with the white dwarf donors should be too short to detect. The donor size distribution will be probed immediately with the X-ray flare durations, while the flares will also enable triggers for early-time spectroscopy.
Supernova shock breakouts
WFM will be a unique discovery machine for the earliest stages of supernova (SN) shock breakouts. A few breakouts per year should be detectable from type Ibc SNe within 20 Mpc. This will allow much more rapid spectroscopic follow- up than other means of discovering SNe (especially compared with the 3-day cadence of LSST), allowing crucial studies of the early stages of the explosions that can be used to probe details of the explosion mechanisms and the binarity of SN progenitors.