0 Assigned ID: 2152 ----- Attempting Submission 1 (Tue Mar 19 14:15:26 GMT 2013) ----- 0 ----- Attempting Submission 1 (Fri Mar 22 23:03:39 GMT 2013) ----- 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. With the aim of locating the site(s) of the flares, we have organized a multiwavelength program, including a regular monitoring of the source both in X-ray and optical through a joint Chandra-HST project, coupled to pre-planned ToOs with both telescopes in order to react promptly in case of a new brightening in gamma rays. We could take advantage of our program, activating the HST ToO at the onset of a new flare on March 4, 2013. Comparing this ToO observation with a recent monitoring observation allowed us to spot a peculiar, active region, possibly associated to the flaring gamma-ray activity. We ask here for a new HST/ACS observation to be performed on March 25-30, i.e. about 20 days after the flare and 20 days before the next planned observation of our monitoring program (scheduled on April 18, just before the end of the visibility window of the target). Such a sequence of observations will allow us to search for the expected dynamical aftermath of the flare, yielding a crucial piece of information about the location of the flaring site. We ask to observe the Crab pulsar and nebula system to search for surface brightness changes in the inner nebula possibly associated to the dynamical aftermath of a large gamma-ray flare. We will use the same setup we adopted in our ongoing HST monitoring campaign of the Crab, as well as in our recent HST pre-planned ToO. 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 one collected in our ToO (performed on March 6, when the flare was ongoing) as well as to the most recent pre-flare image (collected on February 24), as well as to the next observation of our monitoring program, planned for April 18. Such a multi-epoch dataset, thanks to the superb angular resolution of HST, will allow to perform a definitive search for the expected variability related to the aftermath of a gamma-ray flare. HOT STARS 12 false false 0.0 0.0 false false false SUPPORTED STAR NEUTRON STAR PULSAR J2000 ACS/WFC Imaging No No F550M POS-TARG DITHER 3.5 Dithering is aimed at filling the inter-chip gap as well as to rejecting CR hits LINE 0.5 false false 85.28 4 85.28 ACS-WFC-DITHER-BOX 1 false 1363993394664 /tmp/OPVisit5272912048251918924.xml The scheduling data was generated by CASM 21.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 13196 :visit-id "01" :proposal-version -1 :trans-version "20121031" :date "23-Mar-2013 00:03:14.0" :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 13196 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13196 :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