The low accretion rate Polar EF Eri has been in a low state for more than 9 years. Our recent GALEX photometry revealed a source of UV light that is producing more flux than the white dwarf and which is highly modulated on the 81 min orbital period of the system. We request UV spectra with the SBC on the ACS to resolve whether limb darkening or cyclotron emission can explain the observed phenomena and provide insight on the long term heating effects under low accretion scenarios.

Our scientific goals require UV observations that cover all orbital phases of EF Eri. To do this, we request 4 contiguous HST orbits using the ACS with the SBC and Prism PR110L. This will give us UV coverage from about 1170-1800\AA. We have prior experience with the SBC observations with PR110L from our Cycle 13 observations of three pulsating white dwarfs. Given the 81 min orbital period of EF Eri and the 96 min HST orbit cycle, the first orbit on EF Eri (with about 4 min for setup) will cover phases 0-0.53 (arbitrary zero), the second HST orbit phases 0.12-0.70, the 3rd phases 0.31-0.89 and the 4th 0.50-0.08. Thus, 4 orbits will ensure total phase coverage, with some phases having multiple overlaps so they can be added to increase the S/N. We will be able to create light curves throughout the wavelength range as well as analyze spectra at both the high and low fluxes of the UV modulation in order to resolve the source of FUV light. Our analysis of the pulsating white dwarf in the system SDSS0131-09 (Szkody et al. 2007, ApJ, 658, 1188) showed that we can determine temperatures to an accuracy of 1000K using the SBC low resolution prism. SDSSJ0131 had a comparable V mag of 18 and a slightly lower 1500A flux (1.1X10^{-15} ergs/cm^2/s/A) compared to EF Eri where GALEX measured a 1500A flux of 1.3X10^{-15}ergs/cm^2$/s/A. Our access to the SMARTS consortium guarantees several observations per week to monitor the state of EF Eri. We can supplement this with AAVSO observers, APO observers and other worldwide monitors to ensure that EF Eri would be in a low state throughout the HST observations. Due to the inactivity during the last 9 yrs, it is very unlikely that EF Eri would suddenly enter a high state with no warning. HOT STARS 12 false 0.0 false false false false false false SUPPORTED STAR POLAR This object was generated by the targetselector and retrieved from the SIMBAD database. only observe in low state near 18 J2000 03 14 13.0300 -22 35 41.40 J2000 ACS/SBC Spectroscopic No No PR110L Ground observations will be used to ensure EF Eri at low state of 18th mag. AFTER BY requirement needed to verify the brightness of the target using ground observations. Also, uplink opportunities are needed in the ~2 orbit interval to clear the NSSC-I Event Flag 2 and allow high-voltage turn-on, if the target is determined to be safe. (Tic-Visit :proposal-id 11162 :visit-id "01" :proposal-version -1 :trans-version "20070325" :date "13-Apr-2007 16:31:55.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 759056 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11162 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11162 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2792 :al-overhead-at-end 0 :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 11162 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2790 :al-overhead-at-end 0 :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 11162 :visit-id "01" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2790 :al-overhead-at-end 0 :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 11162 :visit-id "01" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2790 :al-overhead-at-end 0 :internal-dur-at-end 322 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3147 :pcs-gap 0 ) false 4 0 Set NSSC-I Event Flag 2 to allow high-voltage turn-on. Insert SQL for qasi_states ACS start/end = WFHROPER, DOOR start/end = OPEN, HCOR start/end = HOLD, LAMP start/end = HOLD, and WCOR start/end = HOLD. This Visit sets the NSSC-I Event Flag 2 that disables the high-voltage turn-on. The high-voltage will be enabled, to allow SBC observations in the corresponding Visit, using real-time commanding if the target is determined to be safe. false -1 -1 Set NSSC-I Event Flag 2 to prevent high-voltage turn-on. Insert SQL to ensure that the SBC is off when the flag is set, and for qasi_states ACS start/end = WFHROPER, DOOR start/end=OPEN, HCOR start/end=HOLD,LAMP start/end=HOLD, and WCOR start/end=HOLD.