Abstract for Proposal 094378

Unveling the geometry of 4U 1728-34 by probing the X-ray/IR variability at 20Hz

In the last few years our team has carried out several fast X-ray+IR observations of neutron star low mass X-ray binaries (NS LMXBs), discovering a wide range of new exciting phenomena. One of the most important is arguably the discovery of correlated X-ray/IR variability down to sub-second timescales from the archetypal NS LMXB 4U 1728-34. Our results indicate that to fully constrain the disk/jet geometry we need a time resolution of a few tens of ms: this would enable us to probe regions just a few gravitational radii from the NS surface. The Very Large Telescope has recently commissioned a new fast IR photometer which can reach such timescales: ERIS. Thus, we propose to perform one Target of Opportunity observation of 4U 1728-34 with XMM-Newton (7ks) and ERIS@VLT (1hr).

Details on Observing Strategy and Trigger Criteria
Swift-BAT count rate<0.01 cts-1 and MAXI 2-4 keV band <0.2 photons cm-2 s-1