Abstract for Proposal 086208
Searching for X-ray and Near-UV pulses in accreting millisecond X-ray pulsars
Millisecond pulsars are the fastest spinning compact stars known to
date. They are believed to be the descendants of accreting neutron
stars in low-mass X-ray binary systems (LMXBs). We have recently
discovered optical and UV pulsations from the accreting millisecond
X-ray pulsar (AMXP) SAX J1808.4-3658 during its outburst in August
2019. This discovery strongly motivates us to investigate the presence
of optical/UV pulsations in other AMXPs. We ask for ToO simultaneous
high-time resolution XMM/EPIC-pn and HST/STIS observations of a known or
newly identified AMXP, to be triggered in case of detection of a bright
X-ray outburst. The goals of the proposed multi-wavelength campaign are
to search for UV pulses, and study the mechanism that produces the pulsed
X-ray and UV emissions.
Details on Observing Strategy and Trigger Criteria
The small group of accreting millisecond X-ray pulsars form a rather
heterogeneous class of objects. Then, the outburst duration and recurrence
varies greatly depending on the system. In three years we will be able to
observe at least one outburst event simultaneously with XMM-Newton and
HST. We ask that the reaction time for the ToO execution is <7 days. We
request that one of the sources indicated below be triggered in case of
detection of a bright X-ray outburst for 60 ks with XMM-Newton and for
2 orbits with HST/STIS.