One of the most intriguing results of the gamma-ray instruments currently in orbit has been the detection of powerful flares from the Crab Nebula in September 2010. In April 2011 a similar flare, lasting several days, made the nebula the brightest source in the gamma-ray sky. A critical reassessment of long term behavior of the Crab flux clearly showed that both Agile and Fermi had already detected similar events in October 2007 and February 2009, pointing to a recurrence time of once per year. A HST observing strategy must be set up to react promptly to any possible new brightening of the Crab in gamma rays. In September 2010 we requested a DD observation which was promptly accepted and carried out. However, the lack of a suitable reference image hampered our efforts to pinpoint the sites of possible variability inside the nebula. Thus, while now we ask for a triggered TOO observation of the Crab Nebula with ACS/WFC in case a gamma-ray flare is announced by the Agile and/or Fermi missions, we are also organizing a regular (monthly) monitoring of the source both in X-ray and optical through a joint Chandra-HST proposal. We ask to perform simple imaging of the Crab Pulsar and Nebula, reacting with the fastest possible response time to a trigger provided by the detection of a new gamma-ray flare from the Crab nebula by Fermi-LAT and/or AGILE. We will use the same setup we adopted in our ongoing HST monitoring campaign of the Crab. We will use the ACS/WFC with the WFC1 aperture, with the large-band F550M filter, well suited to sample the continuum emission from the system, with almost no contamination from line emission from the SNR filaments. A box dithering pattern will be implemented to fill the inter-chip gap as well as to reject cosmic ray hits. Setting an exposure of 500 s for each position of the dithering pattern results in 2000 s exposure time per visit, which is well suited to achieve our goals and allows for an optimal use of the orbital visibility window. The new image will be compared to the most recent reference one, acquired with an identical instrument setup within a monthly multiwavelength X-ray and optical monitoring campaign proposed as a joint Chandra-HST program. Thanks to the sharp angular resolution of HST, the proposed fast TOO observation will be crucial to pinpoint the region responsible for the puzzling gamma-ray flare. HOT STARS 12 false false 0.0 0.0 false false false SUPPORTED STAR NEUTRON STAR PULSAR J2000 J2000 ACS/WFC Imaging No No F550M POS-TARG DITHER 3.5 Dithering is aimed at filling the inter-chip gap as well as at rejecting CR hits LINE 0.5 false false 85.28 4 85.28 ACS-WFC-DITHER-BOX 1 false The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12912 :visit-id "01" :proposal-version -1 :trans-version "20120615" :date "10-Jul-2012 11:36:58.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12912 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12912 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2704 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3244 :pcs-gap 0 ) true 1 0