Note - This phase-2 proposal describes a revised HST program to repeat two failed cycle-16 visits of 8 orbits in total with the WFPC2 to the globular cluster M28. The revised program consists of 4 orbits with the ACS/WFC. The original abstract is included below. ----------------------------------------------------------------------------------------- We propose a deep X-ray survey of the globular cluster M28 which will yield a wealth of important and unique science, ranging from the first direct measurement of the magnetic field of a millisecond pulsar and constraints on the neutron star equation of state to likely X-ray/optical (HST) detection of a re-exchanged binary MSP. The proposed joint HST WFPC2 observation will tie the X-ray, optical, and radio data to a common astrometric frame allowing an unambiguous identification of numerous cataclysmic variables and active binaries in M28 as well as making possible the first direct optical detection of a millisecond pulsar.

The goal of this program is to image the core of the globular cluster M28 with the ACS/WFC in order to look for optical counterparts of the milli-second pulsars (MSPs) and other sources that were detected in our joint Chandra cycle-9 observation. In particular, our aim is to identify the optical counterparts to MSP-H and MSP-A; our observational setup is thus optimized to get good photometry in the region around these objects. From archival HST observations and the WFPC2 images obtained during the first attempt at executing this program, we identified faint candidate counterparts to MSP-H. To avoid that the diffraction spikes and saturation columns of a nearby bright star contaminate the photometry of these faint stars we impose constraints on the ORIENT angle. These constraints also leave the region around another high-priority object, MSP-A, free of diffraction/saturation spikes. The current pointing of the ACS/WFC is preliminary and will be refined once we know the approximate date when our observations will be scheduled. The reason is that for a given ORIENT angle, some Chandra sources will fall inside the chip gap. By refining the coordinates of the pointing we can avoid that high-priority objects fall outside the field of view. This program comprises 1 visit of 4 orbits that contains exposures in the F435W, F625W and F658N filters. All exposures are part of dither patterns that make use of integer and sub-pixel offsets to improve the intrinsic spatial resolution of the ACS/WFC and mitigate the effect of bad pixels. The exposures in F435W (4x464s) and F625W (4x60s) are made in the standard 2x2 ACS-WFC-DITHER-BOX pattern. The exposures through the F658N filter (3x717s+6x724s) are made in a 3x3 dither pattern that uses 5-pixel integer offsets combined with 1/3 sub-pixel offsets. HOT STARS 12 false 0.0 0.0 false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER J2000 18 24 32.9000 -24 52 11.00 J2000 STELLAR CLUSTER GLOBULAR CLUSTER The V-mag refers to the brightest UCAC2 star projected within 2 arcmin from the cluster center. The reddening towards the cluster is E(B-V)=0.42. NGC6626 J2000 18 24 32.9000 -24 52 11.00 J2000 WFPC2 Imaging No F675W WFPC2 Imaging No F439W POS-TARG DITHER 0.3535 LINE false false 2 45.0 WFPC2-LINE 1 false POS-TARG DITHER 0.559017 LINE 0.559017 false false 143.1301 4 26.56505 WFPC2-BOX 2 false POS-TARG DITHER 0.265 LINE 0.187 false false 69.05 4 20.67 ACS-WFC-DITHER-BOX 3 false 1212939932000 300.0 (Tic-Visit :proposal-id 11340 :visit-id "01" :proposal-version -1 :trans-version "20070709" :date "03-Dec-2007 10:57:23.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 1030768 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11340 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11340 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2780 :al-overhead-at-end 100 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3147 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11340 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2780 :al-overhead-at-end 100 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3147 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11340 :visit-id "01" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3147 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11340 :visit-id "01" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2760 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3147 :pcs-gap 0 ) false 4 0 1213095292000 300.0 (Tic-Visit :proposal-id 11340 :visit-id "02" :proposal-version -1 :trans-version "20070709" :date "03-Dec-2007 10:57:54.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 985952 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11340 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11340 :visit-id "02" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2780 :al-overhead-at-end 100 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3147 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11340 :visit-id "02" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2780 :al-overhead-at-end 100 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3147 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11340 :visit-id "02" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3147 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11340 :visit-id "02" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2640 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3147 :pcs-gap 0 ) false 4 0 The scheduling data was generated by CASM 18.1. CASM was run in Phase II mode. The ORIENT constraint is set by the requirement that the saturation and diffraction spikes of two bright stars should avoid the locations of candidate counterparts near Chandra sources. To compute the allowed ranges, we calculated the position angles of the lines connecting the bright stars and their nearby candidate counterparts (261.8deg and 265.6 deg, measured North to East). Then we looked up the angle between the U3-axis and the ACS/WFC detector y-axis (2 deg) to compute the ORIENT angles that should be avoided: 261.8+2deg=263.8deg and 265.6+2deg=267.6deg. We adopt a margin of +-15deg around these angles, which results in a range that should be avoided of 248.8-282.6deg. Since saturation and diffraction occur at 90deg-intervals, the final range that should be avoided is: 68.8-102.6deg, 158.8-192.6deg, 248.8-282.6deg and 338.8-12.6deg. Therefore, the allowed ORIENTangles are in the range: 12.6-68.8deg, 102.6-158.8deg, 192.6-248.8deg, 282.6-338.8deg. Note added on March 5, 2010: Given the plan windows for this visit (April 19 - May 24, 2010), we have selected an ORIENT angle of 283 deg, which will optimize the schedulability of the visit, and put all our prime targets in the ACS/WFC field of view. 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