Our Spitzer/IRAC discovery of a dusty debris disk around the young neutron star 4U 0142+61 is the first detection of a fallback disk around a neutron star. Fallback, the notion that some material from a supernova explosion could remain bound to the compact remnant, is a general prediction of core?collapse supernova models, yet until now there were few if any meaningful observational constraints on the size (or existence) of such disks. Here we propose to test the ubiquity of such disks with deep IRAC observations of two young neutron stars, each a representative of a different class of object (a rotation?powered pulsar and a radio?quiet neutron star in a supernova remnant). We will examine whether fallback disks are common to all neutron stars or depend on the factors that separate these classes. Our proposed observations will pave the way for future surveys and in? depth investigations of individual objects.

With HST, we choose to use ACS/WFC with the i' filter in the optical and NICMOS/Nic2 with the F160W filter (approximately H-band) in the near-IR. These will allow very deep exposures that have wavelength coverage to complement the Spitzer observations. We used a visual extinction A_V=1.6 for exposure-time calculations based on the column density inferred from the X-ray. Using the ACS imaging ETC, we find that we will need 150 min of HST time (3 orbits, with a visibility time of 53 min per orbit) to reach the desired level of i'=27.5 mag with S/N=5. With ACS, we will use only a single WFC chip for a 100x200 arcsec field-of-view; this will allow us to avoid the V=9.7 star HD 70745. Given that the WFC is only marginally sampled, we will use four dithered exposures of 9 min in each orbit for spatial sampling and psf reconstruction. Using the NICMOS imaging ETC, we find that we need 285 min on-source (6 orbits) to reach H=25.3 mag with S/N=5. (Both of these include instrumental overheads). We will use MULTIACCUM readout with Step=16, Nsamp=25, a 8-point spiral dither and 304-s exposures to make sure that we achieve full spatial sampling and can correctly measure the background. The full HST request is then 9 orbits. 12 false 0.0 false false false false false SUPPORTED STAR PULSAR NEUTRON STAR J2000 00 00 0.0000 +00 00 0.00 J2000 POS-TARG DITHER DITHER 1.537 NIC-XSTRIP-DITH false false 2 0 POS-TARG NIC-SPIRAL-DITH 0 1 false POS-TARG DITHER 0.265 LINE 0.187 false false 69.050003 4 20.67 ACS-WFC-DITHER-BOX 2 false (Tic-Visit :proposal-id 11001 :visit-id "01" :proposal-version -1 :trans-version "20060322" :date "01-Aug-2006 18:40:24.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "23" :si-bright-earth-limb-angle 20.0 :data-volume 172256 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11001 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11001 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2653 :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 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11001 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2654 :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 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11001 :visit-id "01" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2651 :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 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11001 :visit-id "01" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2656 :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 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11001 :visit-id "01" :orbit-number 5 :pcs-time 355 :si-vis-utilization 2651 :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 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11001 :visit-id "01" :orbit-number 6 :pcs-time 355 :si-vis-utilization 2631 :al-overhead-at-end 0 :internal-dur-at-end 121 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3169 :pcs-gap 0 ) true 6 0 (Tic-Visit :proposal-id 11001 :visit-id "02" :proposal-version -1 :trans-version "20060322" :date "31-Jul-2006 15:44:47.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 3410688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11001 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11001 :visit-id "02" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2814 :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 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11001 :visit-id "02" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2812 :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 3169 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11001 :visit-id "02" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2812 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3169 :pcs-gap 0 ) true 3 0