Physics of activity in the "wisps" of the Crab nebula remains poor understood, with shocks, in situ particle acceleration and magnetic field all proposed to be the driving factors. A recent flare in the Crab Nebula discovered with AGILE offers an excellent opportunity to shed the light on this issue. The flare has been successfully observed with Chandra, HST, and VLBI (EVN+MERLIN), with the flaring region (located at ~5" west from the Crab pulsar) remaining in a high state for over two months and showing a brightness temperature of >10^7 . Such a peculiar behavior is difficult to explain with the usual shock scenario. Instead it may imply a magnetic field build up or reconnection, which may turn out a common mechanism driving the activity in the "wisps" inside the Crab nebula. New observations with VLBI (23-24 November) and Chandra (27 November) are aiming at further investigating this possibility. We propose here to image the flaring region concurrently with the HST in order to be able to address radio-optical-X-ray spectral properties and also to study the evolution of the flaring region in all three bands by comparing the new images with the ones made during the previous round of concurrent observations (made about one month ago).

We propose to use the HST/ASC instrument to image optical continuum emission from the Crab nebula and to attempt detecting the region of enhanced emission resulting from the ongoing flare. In order to cover a sufficiently large area and avoid contamination from line emission, we propose to use the WFC-FIX aperture, observing with the broad-band F550M filter. This choice will, in addition, facilitate the comparison with the image from the previous HST ToO observation of the flare (Project 12381), made on October 2, 2010. We propose to use the default 4-point dither box, allocating 550 seconds per single dither pointing. This choice will result in the total on-target time of 2200 seconds, and it will minimize the amount of unused visibility during the observing run. The flaring region has been observed recently with VLBI (November 23-24) and Chandra (November 27). Scheduling the HST observation is as closely in time to these observations as possible is desired for improving the accuracy physical modeling and reducing the effect of (likely) emission variability in the flaring region. ISM AND CIRCUMSTELLAR MATTER 0 false 0.0 0.0 false false false false false false SUPPORTED ISM EJECTA SNR WIND This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 05 34 31.9500 +22 00 52.10 J2000 ACS/WFC Imaging No No F550M POS-TARG DITHER 0.265 LINE 0.187 false false 69.05 4 20.67 ACS-WFC-DITHER-BOX 1 false The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12434 :visit-id "01" :proposal-version -1 :trans-version "20100902" :date "29-Nov-2010 16:33:22.0" :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 12434 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12434 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2894 :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 3249 :pcs-gap 0 ) false 1 0