17604( 4) - 14-Nov-2024 15:35:01 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17604 Version: 4 Check-in Time: 14-Nov-2024 15:35:01 STScI Edit Number: 1 Title An unbound accretion flow in the quiescent binary Cen X-4 ------------------------------------------------------------------------------------ Type Cycle GO 31 ------------------------------------------------------------------------------------ Investigators Contact? PI: Ms. Stefanie Fijma Universiteit van Amsterdam Y CoI: Dr. Nathalie Degenaar Universiteit van Amsterdam Y CoI: Dr. Noel Castro Segura University of Warwick Y CoI: Prof. Christian Knigge University of Southampton N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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. ------------------------------------------------------------------------------------ 17604( 4) - 14-Nov-2024 15:35:01 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 V-V822-CEN STAR, ACCRETION DISK, RA=14H58M21.9348S +/- 0.00011 ", J2000 V = 18.78 +/- 0.13 uvot uvw2=18.3 LMXB, NEUTRON STAR DEC=-31D40'7.52" +/- 0.00009" Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 6.611071532056265E-5 -0.05568200001562218 0.0005 ------------------------------------------------------------------------------------------------------------------------------------ 17604( 4) - 14-Nov-2024 15:35:01 - [ 3] Visit: 01 (Orbit 1) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 V-V822-CEN STIS/CCD ACQ F28X50LP MIRROR 1 20 S (20 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 V-V822-CEN STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1050 1 3110 S -MAMA G see below (2047 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 V-V822-CEN STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1350 1 3110 S -MAMA G see below (2626 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ V-V822-CEN 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A V-V822-CEN 2 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 1 2047.0 S -MAMA G V-V822-CEN 3 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 2 2626.0 S -MAMA G 17604( 4) - 14-Nov-2024 15:35:01 - [ 4] Visit: 02 (Orbit 2) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 V-V822-CEN STIS/CCD ACQ F28X50LP MIRROR 1 20 S (20 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 V-V822-CEN STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1050 1 3110 S -MAMA G see below (2047 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 V-V822-CEN STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1350 1 3110 S -MAMA G see below (2626 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ V-V822-CEN 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A V-V822-CEN 2 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 1 2047.0 S -MAMA G V-V822-CEN 3 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 2 2626.0 S -MAMA G 17604( 4) - 14-Nov-2024 15:35:01 - [ 5] Summary Form for Proposal 17604 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1050, BUFFER-TIME=1350 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names V-V822-CEN ------------------------------------------------------------------------------------------------------------------------------------