0 Assigned ID: 6047 ----- Attempting Submission 1 (Wed Dec 18 19:18:43 GMT 2024) ----- HST Phase I Proposal 6047 successfully submitted. Receipt: # 6047-1 This target is not used in this proposal. 0 ----- Attempting Submission 1 (Wed Jan 08 15:18:34 GMT 2025) ----- HST Phase II Proposal 17899 successfully submitted. Receipt: # 17899-1 ----- Attempting Submission 2 (Thu Jan 09 09:04:30 GMT 2025) ----- Generic target used as placeholder, fixed target to be updated upon ToO trigger Unmasking the central engine in a Cow-like transient with XMM, VLT, CXO and HST Luminous Fast Blue Optical Transients (LFBOTs, or Cow-like transients, after the prototypical AT2018cow) are characterised by luminous multi-wavelength emission, blue optical colours and rapid evolution that is inconsistent with Ni56-powered supernovae (SNe). Although the number of LFBOTs is growing thanks to high-cadence, deep and wide-field surveys, detailed studies of individual events are lacking. These are necessary to determine the central engine and progenitor. We propose a joint programme, utilising XMM- Newton, the Very Large Telescope (VLT), Chandra X-ray Observatory (CXO) and Hubble Space Telescope (HST) to firmly identify a single, low redshift LFBOT and to determine the nature of its central engine. We will monitor the long-timescale X-ray/UV light-curve, measure short-timescale X-ray variability and use environmental clues to distinguish between neutron star, stellar mass and intermediate mass black holes as the power source - placing the strongest constraints yet on the progenitors of these puzzling events. Two orbits (2 epochs, non-disruptive ToOs, 1 orbit per epoch) of HST/WFC3 imaging to pinpoint the transient location, determine the host galaxy morphology, and to follow the UV light-curve up to 3-4 months post-explosion. The first epoch will be at ~25 days. The second epoch, at ~3-4 months, will enable us to follow the lightcurve as it starts to plateau (assuming AT2018cow-like behaviour). Even if not, we will obtain an unobstructed view of the environment directly under the transient. Difference imaging between epochs 1 and 2 will reveal the precise location within the host, and along with the early-time XMM/OM UV data, we will obtain a UV light-curve spanning 3-4 months and several magnitudes in luminosity. In each 20-min (half-orbit) observation we will reach m(F275W)=26 and m(F555W)=26.5 (AB) for SNR= 5 point source detections. A Cow-like event at z = 0.1 would have m(UV)~22 (26) at ~30 days (3-4 months). The team has extensive experience using both the specific observatories requested in this programme, and generally in multi-wavelength transient follow-up. The PI (currently an independent Excellence Fellow) has previously handled a range of datasets from Chandra, HST and the VLT (Chrimes et al. 2019a, Chrimes et al. 2019b, Chrimes et al. 2022, Chrimes et al. 2024a,b). Our team overall has successfully obtained hundreds of hours of time on these observatories. Using these facilities and more, our outputs include important research contributions on the topics of Galactic compact objects, both in the X-ray (Jonker et al. 2004, Jonker et al. 2014) and radio (Miller-Jones et al. 2009, Stanway et al. 2018), tidal disruption events (Levan et al. 2011, Jonker eal. 2020), gamma-ray bursts (Groot et al. 1998, Fruchter et al. 2006, Levan et al. 2014), and transient host galaxies (Stanway et al. 2015, Angus et al. 2016, Levan et al. 2017), among others. We also have experience in modeling the progenitor systems of explosive transients (Chrimes et al. 2020), and galactic stellar populations (Eldridge et al. 2017). Some members of the team play a leading role in the Blackgem wide-field, high cadence southern sky survey, for which P. Groot is Principal Investigator and P. Jonker is Project Scientist. Our wide portfolio of experience means that we have the ability to lead this joint observing programme, and efficiently handle the data obtained, resulting in timely publication. Stellar Physics and Stellar Types Galaxies 6 false false false false false false false true SECONDARY XMM_NEWTON 096349 false SUPPORTED STAR GAMMA RAY BURSTER SUPERNOVA SUPERNOVA TYPE IB X-RAY TRANSIENT NO Coordinates and magnitude unknown in advance. J2000 J2000 STAR SUPERNOVA Newly discovered LFBOT WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F555W POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 1 false /tmp/OPVisit12765640529613356512.xml Timing link suitability includes constraints ([02 After 01 by 60.0 to 365.0 days]) The scheduling data was generated by CASM 32.1.3. CASM was run in Phase II mode. 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(Tic-Visit :proposal-id 17899 :visit-id "02" :proposal-version -1 :trans-version "20240604" :date "09-Jan-2025 09:49:22.0" :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 106960 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17899 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17899 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2622 :al-overhead-at-end 79 :internal-dur-at-end 153 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3005 :pcs-gap 0 ) true 1 0 To be excuted >~2 months after visit 01.