Abstract for Proposal 090112
What has caused the dimming of the neutron star ULX NGC 7793 P13?
NGC 7793 P13 has been an ideal target to study super-Eddington accretion
in ultra-luminous X-ray pulsars thanks to its strong pulsations
and high flux. However, in 2019 its X-ray flux started to decline,
and the source entered an off-state. It quickly recovered from that
but has been in a very low state since then. During the decline, the
spin-up was continuing at almost the same rate as during the high state,
indicating that accretion was continuing unabated. During the current
low state the flux is too low to see pulsations and we do not know the
current spin-period evolution. We therefore ask for an XMM-Newton ToO
observation within a few weeks from when P13 emerges from the low state,
in order to obtain another spin measurement and understand the nature
of the off- and low state.
Details on Observing Strategy and Trigger Criteria
We will trigger the observation as soon as two consecutive weekly
Swift/XRT measurements show a flux above 0.01cts/s. After the trigger,
XMM-Newton should observe within one week.