Abstract for Proposal 090439
The Hunt for New ULX Pulsars: Transient and Highly Variable ULXs
Following a series of remarkable recent discoveries, we now know that
some of the most luminous members of the ultraluminous X-ray source (ULX)
population are actually powered by highly super-Eddington neutron star
accretors. However, key questions remain about these remarkable systems,
and few ULX pulsars are known. Those that are appear to commonly show
higher amplitude variability on long timescales than other ULXs. Here,
we propose using this property to search for new ULX pulsars, using
Swift to monitor four nearby galaxies rich in ULXs and triggering up
to two XMM-Newton observations on luminous transients and/or variable
ULXs when they are sufficiently bright that we can perform accelerated
pulsation searches.
Details on Observing Strategy and Trigger Criteria
We will trigger the ToOs on detecting a new transient ULX or a known high
variability ULX with a Swift XRT count rate in excess of 0.02 count/s. We
ask for a reaction time of a week to optimise the chances of the trigger
source still being in a high flux state. The Swift monitoring that we
trigger from will be scheduled such that one observation is taken in
the week before an XMM-Newton visibility window opens, and another is
scheduled half-way through. Hence we require four Swift observations
per target ULX-rich galaxy to cover each of the two visibility windows.