0 Assigned ID: 6937 ----- Attempting Submission 1 (Wed Nov 19 11:57:40 GMT 2025) ----- HST Phase I Proposal 6937 successfully submitted. Receipt: # 6937-1 0 ----- Attempting Submission 1 (Mon Dec 01 17:25:05 GMT 2025) ----- HST Phase II Proposal 18217 successfully submitted. Receipt: # 18217-1 ----- Attempting Submission 2 (Mon Dec 01 21:18:17 GMT 2025) ----- X-ray Flashes as Dirty Fireballs or Off-axis GRBs The collapsar model provides a very good model for the energetics, prompt gamma-ray emission, and the subsequent afterglow emission of long-duration GRBs. However, it does not provide a ready explanation for the apparent variations in the opening angle of the jet and in the explosive energy release. It has been speculated that there exist classes of relativistic explosions which have a lower Lorentz factor and wider opening angle than classical GRBs that peak in the X-ray band, known as dirty fireballs, with afterglows similar to classical GRBs. A decade long search with ground- based optical surveys has found a dozen afterglows without a gamma-ray trigger, but their physical interpretation have been limited by a lack of coverage by high-energy missions, as well as a dearth of late-time sensitive afterglow observations. The recently launched Einstein Probe (EP) and Space Variable Objects Monitor (SVOM) are changing this landscape, offering a clear path to identify dirty fireballs. Here, we propose a comprehensive program with the VLA, XMM-Newton, and HST to help distinguish dirty fireballs from their primary false positives, GRBs viewed slightly off-axis. The discovery of a single dirty fireball would have a dramatic impact on our view of the GRB phenomenon. This is a target-of-opportunity proposal. We will trigger on one object with HST. For that object, we will obtain three epochs of ToO observations with WFC3. The first will be one orbit at 1-2 months after discovery (i.e., non-disruptive) in the F606W filter. The second will be at 2-4 months, also in F606W, this time two orbits. And the third will be after the transient light has faded, 2 additional orbits in F606W for the purpose of subtracting out the host-galaxy contribution. So, the total HST observation comes out to 5 orbits of WFC3/F606W observations. N/A Stellar Physics and Stellar Types 6 false false false false false false true SECONDARY NRAO VLA/26A-488 false SUPPORTED Soft X-ray flash (Ep<25 keV, Eiso>1E51 erg) WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F606W POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false /private/var/folders/kf/b04lmqv50dd7ydhwb_qbmtf80000gq/T/OPVisit5949998481344362200.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18217 :visit-id "01" :proposal-version -1 :trans-version "20250929" :date "01-Dec-2025 16:17:57.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154992 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18217 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18217 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 /private/var/folders/kf/b04lmqv50dd7ydhwb_qbmtf80000gq/T/OPVisit7585847242657230094.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18217 :visit-id "02" :proposal-version -1 :trans-version "20250929" :date "01-Dec-2025 16:17:36.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2310000 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18217 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18217 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18217 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 /private/var/folders/kf/b04lmqv50dd7ydhwb_qbmtf80000gq/T/OPVisit12525745579774637015.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18217 :visit-id "03" :proposal-version -1 :trans-version "20250929" :date "01-Dec-2025 16:17:47.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2310000 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18217 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18217 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18217 :visit-id "03" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0