17602( 5) - 04-Dec-2024 18:58:56 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17602 Version: 5 Check-in Time: 04-Dec-2024 18:58:56 STScI Edit Number: 1 Title Understanding the Rapid Evolution in the Light Curve of the White Dwarf Pulsar AR Scorpii ------------------------------------------------------------------------------------ Type Cycle GO 31 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Peter M. Garnavich University of Notre Dame Y CoI: Dr. Stephen Potter South African Astronomical Observatory N CoI: Prof. David Buckley South African Astronomical Observatory N CoI: Mr. Colin Littlefield University of Notre Dame N ------------------------------------------------------------------------------------ Abstract The optical light curve of AR Scorpii has recently been found to be evolving at a rapid pace. AR Sco is the nearest and brightest white dwarf pulsar -- a binary system where synchrotron emission is powered by the spin-down of its magnetic degenerate primary. The white dwarf rotates every 1.95 minutes and generates two synchrotron pulses per spin, one from each of its magnetic poles. New observations in 2023 found that these two pulses were nearly equal in amplitude, while in 2015 the secondary pulse was half the strength of the primary. This rapid evolution has been explained as precession of the white dwarf spin axis caused by an obliquity to the binary orbital plane. However, the mechanism that accelerates the relativistic electrons is uncertain, so other physical processes could be driving the evolution. We propose COS spectroscopy of this dynamic system to test for differences in the evolution between the far-UV and optical wavelengths. By directly comparing with archival COS spectra taken in 2016, we will also search for changes in the strong emission lines that would imply evolution in the magnetic interaction between the white dwarf and secondary star. ------------------------------------------------------------------------------------ Observations Description ------------------------ The optical light curve of AR Sco has evolved over the past decade so that the beat pulses from each pole are now nearly equal. We will make observations with COS in time-tag mode to directly compare the UV light curve and spectra of AR Sco with the COS data from 2015. The position of AR Sco is from the GSC2 in the ICRS frame. The proper motion and parallax is from GAIA EDR3. The light curve of AR Sco shows pulses separated by about 1 minute as well as an orbital modulation. The minimum flux observed in the 2015 COS data was between the pulses and around orbital phase 0. This minimum flux was 5e-16 erg/cm2/s/A. The spectrum is approximately flat, so we use this flux as that expected for acquistion using ACQ/image. From the COS acquistion ETC (COS.ta.1901985) we will reach a S/N ratio of 25 in 6s of exposure using MIRRORA/PSA. So we will take a 30s acquistion image to be certain of a high-quality image. 17602( 5) - 04-Dec-2024 18:58:56 - [ 2] To protect COS from a flare on the M-dwarf, we have switched to acquistion using MirrorB and extended the acqusition exposure to 60s. The original plan was to repeat the 2015 COS setup to directly compare the changes in the light curve and spectra over the decade. But the possibility of a flare from the M5 secondary required that we switch to the 1280 cenwave position. This places the Lyman alpha line between the detector segments and avoids exceeding the count-rate limit at Lyman alpha if a super-flare did occur. From the 2015 visit, the average flux of AR Sco over several orbits was 4.9e-15 erg/cm2/s/A. From the COS ETC (COS.sp.1902107), the time to fill the buffer is 10102 s. Reducing that number by 2/3 gives 6700 s. The maximum flux during the 2015 visit was a pulse that peaked at 2e-14 erg/cm2/s/A and lasted for 25s. The COS ETC estimated the buffer fill time for this flux as 4400s. Two-thirds of this is 2900s. To be safe, we are setting the Buffer Time to 1200 to 1400s depending on the exposure time. The four FP-POS have been spread over the 5 HST orbits. ------------------------------------------------------------------------------------ 17602( 5) - 04-Dec-2024 18:58:56 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 V-AR-SCO EXT-STAR, INTERMEDIATE RA=16H21M47.2735S +/- 0.1", J2000 V = 14.75 +/- 1.0 POLAR DEC=-22D53'10.10" +/- 0.1" Reference Frame: ICRS Extended: NO Comments: The position is from the GSC2 in ICRS reference frame. The proper motion and parallax are from GAIA EDR3. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 7.011987009334056E-4 -0.05148900002041046 0.0085437 ------------------------------------------------------------------------------------------------------------------------------------ 17602( 5) - 04-Dec-2024 18:58:56 - [ 4] Visit: 01 (Visit) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 V-AR-SCO COS/NUV ACQ/IMA PSA MIRRORB 1 60 S (60 GE S) Comments: The minimum flux observed during the 2015 visit was 5e-16 erg/cm2/s/A. Using that in the ETC provides and esimate of 6s to reach a S/N=25. ------------------------------------------------------------------------------------------------------------------------------------ 2 V-AR-SCO COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=1200, FP-POS=1 1 2309 S G see below (2209 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 V-AR-SCO COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=1400, FP-POS=2 1 2683 S G see below (2584 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 V-AR-SCO COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=1400, FP-POS=2 1 1100 S G see below (1055 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 V-AR-SCO COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=1400, FP-POS=3 1 1411 S G see below (1366 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 V-AR-SCO COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=1200, FP-POS=3 1 1530 S G see below (1472 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 V-AR-SCO COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=1400, FP-POS=4 1 1038 S G see below (988 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 V-AR-SCO COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=1400, FP-POS=4 1 2683 S G see below (2584 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ 17602( 5) - 04-Dec-2024 18:58:56 - [ 5] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ V-AR-SCO 1 COS/NUV ACQ/IMA PSA MIRRORB none none none none 1 N/A GE V-AR-SCO 2 COS/FUV TIME-TA PSA G140L 1280 none none none none 1 2209.0 S G V-AR-SCO 3 COS/FUV TIME-TA PSA G140L 1280 none none none none 2 2584.0 S G V-AR-SCO 4 COS/FUV TIME-TA PSA G140L 1280 none none none none 3 1055.0 S G V-AR-SCO 5 COS/FUV TIME-TA PSA G140L 1280 none none none none 3 1366.0 S G V-AR-SCO 6 COS/FUV TIME-TA PSA G140L 1280 none none none none 4 1472.0 S G V-AR-SCO 7 COS/FUV TIME-TA PSA G140L 1280 none none none none 4 988.0 S G V-AR-SCO 8 COS/FUV TIME-TA PSA G140L 1280 none none none none 5 2584.0 S G 17602( 5) - 04-Dec-2024 18:58:56 - [ 6] Summary Form for Proposal 17602 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1200, BUFFER-TIME=1400, FP-POS=1, FP-POS=2, FP-POS=3, FP-POS=4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, MIRRORB ------------------------------------------------------------------------------------------------------------------------------------ Target Names V-AR-SCO ------------------------------------------------------------------------------------------------------------------------------------