0 Assigned ID: 1602 ----- Attempting Submission 1 (Mon Oct 09 15:48:50 GMT 2017) ----- HST Phase I Proposal 1602 (burst6440_1.apt) successfully submitted. Receipt: # 1602-1 0 ----- Attempting Submission 1 (Tue Oct 17 13:18:48 GMT 2017) ----- Low mass X-ray binaries (LMXBs) and radio Millisecond Pulsars (MSPs) are thought to be the initial and ending points of the evolution of a binary system where a neutron star (NS) accretes matter from a companion. Many crucial phases of this scenario still need to be understood and a new, unexpected tile has been added to the mosaic very recently: a few systems (named "transitional MSPs") are observed to swing between the accretion-powered and the rotation-powered states. The case of the X-ray burster MAXI J1749-200 in the globular cluster NGC 6440 can act the part of the (still unexplored) stage immediately preceding the swinging phase. Swift observations located the source with an uncertainty of 3.5" and suggest a positional coincidence with two previously known x-ray sources, among which SAX J1748.9-2021, an already known X-ray accreting Millisecond pulsar. Since the X-ray bursts are expected to be accompanied by a significant enhancement of the optical luminosity, here we propose to invest only 2 HST orbits to acquire multi-filter images of the region. The comparison between these data (secured during the outburst) and those already available in the archive (acquired during quiescence) will immediately pinpoint the source, allowing to identify the companion to the NS and to characterize the nature of the on-going mass-transfer. The optical emission is expected to fade away in a time-scale of 1-2 months after the X-ray burst. Hence, the proposed DDT observations represent a unique but temporary opportunity to unveil the nature of this possibly new X-ray transient Observations are organized in 1 visit of 2 orbits. In each orbit 2 long exposures in the optical filters F606W and F814W and 2 long exposures in the narrow filter F656N are planned. Dithering is performed by applying small offsets (via POS TARG keyword) to each exposure using a path already tested in the past. The target is located at the aperture UVIS1; with this configuration we are also able to sample the entire cluster core. We also added proper post-flashes when necessary. Stellar Physics Stellar Populations 0 SUPPORTED STAR ACCRETION DISK INTERACTING BINARY NEUTRON STAR PULSAR X-RAY TRANSIENT J2000 J2000 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F656N 1508235566402 /Users/cristina/LAVORO/PROPOSAL/DDTHST25/PHASE2/OP-cache/phase2_burst6440_1-v01.xml The scheduling data was generated by CASM 25.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 15403 :visit-id "01" :proposal-version -1 :trans-version "20170613" :date "17-Oct-2017 12:19:25.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2309952 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15403 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15403 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2884 :al-overhead-at-end 121 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3217 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15403 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2995 :al-overhead-at-end 121 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3217 :pcs-gap 0 ) true 2 0