0 Assigned ID: 4918 ----- Attempting Submission 1 (Wed Nov 15 20:10:19 GMT 2023) ----- To repeat the failed orbit as approved by the TTRB, I've copied the failed visit and prepared 1 orbit of observations. Please let me know if I can be of further help. -2144566043 ----- Attempting Submission 1 (Wed Feb 21 21:17:47 GMT 2024) ----- ----- Attempting Submission 2 (Wed Nov 13 23:49:51 GMT 2024) ----- ----- Attempting Submission 3 (Fri Jun 06 11:34:22 GMT 2025) ----- Our target has a well known position An unbound accretion flow in the quiescent binary Cen X-4 Accretion is a fundamental aspect of many different astronomical objects, including black holes and neutron stars. These compact objects predominantly accrete at very low rates. Therefore, understanding this regime is of vital scientific importance. However, it is currently not established how accretion flows organise themselves at these low rates. Accretion flows are proposed to be radiatively inefficient (RIAFs) in quiescence, but there is no self-consistent model for these flows, and there is little to no observational evidence for RIAFs or their properties. Some proposed models predict bound or unbound RIAFs, but neither of these solutions have ever been confirmed. We have recently discovered a tentative P-Cygni outflow feature in the low-ionisation O I triplet in archival FUV data of the quiescent neutron star X-ray binary Cen X-4, which we propose could be a hint of an unbound RIAF. If this outflow feature can be confirmed, this will both be one of the first observational studies proving the existence of RIAFs, as well as the first ever study to detect an unbound RIAF, which would have a vast impact of our understanding of accretion at low rates. Therefore, we propose HST-STIS FUV spectroscopic observations of Cen X-4, in order to confirm and characterise the outflow identified for the O I triplet. Moreover, the proposed HST-STIS observations will allow us to place better constraints on the origin of the UV emission, the morphology of the accretion flow, and the evolutionary stage and history of the binary. This programme consists of 4 HST orbits, in order to obtain a continuum signal-to-noise around the O I 1305A line of S/N~3. To obtain the required SNR, we spread this over 2 visits that consist of 2 orbits each (STIS.sp.1897134). Each first orbit will consist of a STIS/CCD ACQ, and complemented with the spectroscopic exposures taken with the STIS/FUV G140L The source position is accurately known (to within 0.11 mas; Gaia EDR3), so target acquisition can be performed using ACQ (a S/N of 55 is obtained for a 20-s exposure for V~18.8; ECT ID STIS.ta.1903019). Since the UV emission can vary with a factor of ~2 on a timescale of days, we ensure that the TA exposure time is sufficient for the low end (V~18.8) and high end (V~17.7) of the expected brightness. PI Ms. Stefanie Fijma and Co-I Dr. Noel Castro Segura have ample expertise in HST FUV spectral data reduction and analysis for STIS, and published several papers using HST data over the past years. They are the data analysis experts in this project. Co-I Dr. Nathalie Degenaar is an expert in X-ray binary research, including disc physics and outflows, and will thus have a key role in interpreting the data in view of the quiescent behaviour of X-ray binaries and the morphology of the accretion flow. Co-I Prof. Christian Knigge is an expert in accretion physics, as well as on analysis with HST-STIS specifically, which he worked on when he was at STScI. He will have a key role in interpreting the data in view of the origin of the UV emission and the accretion regimes. Stellar Physics and Stellar Types 6 false true false false false false SUPPORTED STAR ACCRETION DISK LMXB NEUTRON STAR NO This object was generated by the targetselector and retrieved from the SIMBAD database. CENX-4 V822CEN J2000 J2000 uvot uvw2=18.3 STIS/FUV-MAMA Spectroscopic No No G140L 1425 The scheduling data was generated by CASM 32.0. 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CASM was run in Phase II mode. 1740213002000 280.0 (Tic-Visit :proposal-id 17604 :visit-id "02" :proposal-version -1 :trans-version "20240604" :date "18-Jun-2024 18:34:52.0" :trans-errors '(:WARNINGS 3 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 313504 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17604 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17604 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2654 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3034 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17604 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2654 :al-overhead-at-end 0 :internal-dur-at-end 582 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3034 :pcs-gap 0 ) true 2 0