Mid-cycle Cycle 25 proposal. Fixed typo on resubmit #1. -287404636 Assigned ID: 1664 ----- Attempting Submission 1 (Tue Mar 20 22:29:43 GMT 2018) ----- HST Phase I Proposal 1664 (GW17817_midcycle_scijust_4visit.apt) successfully submitted. Receipt: # 1664-1 ----- Attempting Submission 2 (Tue Mar 20 22:49:02 GMT 2018) ----- Version 1 for checking of dithering and scheduling strategy 282032225 ----- Attempting Submission 1 (Thu May 17 15:46:24 GMT 2018) ----- Unveiling the nature of GW170817 through late-time observations: A normal short GRB viewed off-axis or a unique event? The first multi-messenger gravitational wave event, GW170817, continues to intrigue long after the fading of the fabled kilonova. The source is now detectable as a gamma-ray burst (GRB) afterglow, emitting synchrotron radiation across a wide range of electromagnetic frequencies. The continued, long rise of the afterglow thus far has shown the canonical beamed jet model, often used to interpret cosmological GRBs, is not correct for this event. Currently, competing models are able to reproduce the observed afterglow - these include the 'cocoon' and 'structured-jet' models. Distinguishing between these scenarios is vital to understanding the intrinsically subluminous GRB that accompanied GW170817, by either reconciling it with cosmological short GRBs (due to it being viewed off-axis), or confirming an entirely new means of gamma-ray production for this event. The coming months represent a crucial period of evolution for the afterglow light curve, where we have predicted it will peak and begin to decay. Observing the time of peak and the steepness of the decay are a means to directly distinguish between the competing models. Our proposed cadenced deep mid-cycle observations will provide the necessary coverage over this opportune epoch and will answer one of the outstanding mysteries of this unique event: the nature of the emission mechanism responsible for the GRB and afterglow. These results will shape future theoretical and observational effort in the new field of multi-messenger GW astrophysics. This proposal will perform 2 deep observations of GW170817 at very late times to constrain its optical flux and investigate the associated GRB afterglow emission mechanism. Deep multi-orbit WFC3/UVIS imaging will be performed in the F606W filter utilising dithers. The strategy differs slightly from phase I in that we will perform 2 epochs of observations for 5 orbits each. We favour less epochs and better depth compared to phase I, owing to the increased fading of the source since proposal submission. Each observation will be split into 2+3 orbit visits. The 2-orbit visits have a window of 1 week and the subsequent associated 3-orbit visit can be performed anytime within several days afterward, to aid scheduability. Note CTE aperture will be used owing to the desire to cover the whole galaxy, and the fact that the object is in a region of reasonable background brightness. The WFC3-UVIS-MOS-DITH-LINE dither pattern has been used with adjusted point spacing to create a 6-point dither pattern since psf sampling, rather than chip gap coverage is desired. Observation 1 (to be scheduled around start of July) Visit 1A: 2 orbits - F606W - 4 point box dither spread over visit, 2 exposures per orbit Visit 1B: 3 orbits - F606W - 6 point dither (3 line x 2 point) spread over visit, 2 exposures per orbit Observation 2 (to be scheduled around middle of August) Visit 2A: 2 orbits - F606W - 4 point box dither spread over visit, 2 exposures per orbit Visit 2B: 3 orbits - F606W - 6 point dither (3 line x 2 point) spread over visit, 2 exposures per orbit Stellar Physics 6 false false false false false false SUPPORTED GALAXY UNDESIGNATED Actual flux at time of observations not known, is likely to be fading and will be somewhere 26.5-27 in V. J2000 J2000 The scheduling data was generated by CASM 25.2. CASM was run in Phase I mode. WFC3/UVIS Imaging No No F606W The scheduling data was generated by CASM 25.2. CASM was run in Phase I mode. WFC3/UVIS Imaging No No F606W POS-TARG DITHER 0.173 WFC3-UVIS-DITHER-BOX 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false POS-TARG DITHER DITHER 2 0.135 WFC3-UVIS-MOS-DITH-LINE false true 0.119 3 85.754 POS-TARG WFC3-UVIS-MOS-DITH-LINE 33.606 3 true /tmp/OPVisit5627112419248842832.xml Timing link suitability includes constraints ([1B After 1A by at most 4.0 days]) The scheduling data was generated by CASM 25.2. CASM was run in Phase II mode. 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CASM was run in Phase II mode. (Tic-Visit :proposal-id 15482 :visit-id "2A" :proposal-version -1 :trans-version "20171009" :date "17-May-2018 16:45:50.0" :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 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15482 :visit-id "2A" :obset-id "2A" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15482 :visit-id "2A" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2884 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3217 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15482 :visit-id "2A" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2994 :al-overhead-at-end 116 :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 /tmp/OPVisit2340121000794690317.xml Timing link suitability includes constraints ([2B After 2A by at most 4.0 days]) The scheduling data was generated by CASM 25.2. CASM was run in Phase II mode. 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