15987( 3) - 17-Aug-2019 18:11:18 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 15987 Version: 3 Check-in Time: 17-Aug-2019 18:11:18 STScI Edit Number: 0 Title Searching for NUV pulses of SAX J1808.4-3658 during its 2019 outburst ------------------------------------------------------------------------------------ Type Cycle GO/DD 26 ------------------------------------------------------------------------------------ Investigators Contact? PI: Ms. Arianna Miraval Zanon INAF, Osservatorio Astronomico di Brera, Y Merate CoI: Dr. Sergio Campana INAF, Osservatorio Astronomico di Brera, Y Merate CoI: Dr. Paolo D'Avanzo INAF, Osservatorio Astronomico di Brera, Y Merate CoI: Dr. Mario Cadelano Universita di Bologna Y CoI: Dr. Alessandro Papitto INAF, Osservatorio Astronomico di Roma N CoI: Dr. Filippo Ambrosino INAF - Istituto di Astrofisica e N Planetologia Spaziali ------------------------------------------------------------------------------------ Abstract The accreting millisecond X-ray pulsar (AMXP) SAX J1808.4-3658 has revealed an abrupt increase in intensity beginning August 5, likely indicating the onset of a new transient outburst. The optical brightening reached V~16.6 mag in few days. X-ray pulsations at the neutron star (NS) spin period have been detected on 7 August with NICER, we thus want to explore the presence of pulsations in the Near-UV band with HST. We require to observe SAX J1808.4-3658 with HST STIS/NUV-MAMA instrument combination in order: (i) to investigate the presence or absence of the Near-UV pulses during the ongoing outburst; (ii) to compare the NUV and X-ray pulse profiles and their fractional amplitude. These observations can only be carried out using instruments with a high temporal resolution (<1ms), necessary to resolve pulsations at 2.5 ms in NUV band. This represents the first timing analysis of an AMXP in the NUV band during outburst. The discovery of UV pulsation in this class of objects would constitute a crucial step toward a better understanding of the evolution of binary millisecond pulsars. ------------------------------------------------------------------------------------ Observations Description ------------------------ We ask to observe SAX J1808.4-3658 with HST during its 2019 outburst. This visit will consist in one HST orbit (~2250 s) during which we will carry out time-resolved NUV TIME-TAG spectroscopy with the STIS/NUV-MAMA/G230L instrument/grating combination. We will use the 2376 \AA setting for this observation to achieve continuous wavelength coverage between 1570 \AA and 3180 \AA, with the 52X0.2 slit. The TIME-TAG mode of STIS will provide a maximum time resolution of 125 $\mu$s, fast enough to resolve pulsations at 2.5 ms (NS pulse period) in the Near-UV band. We will use precise and updated ephemeris of the 2019 outburst measured by the timing analysis of X-ray NICER data to investigate the presence of the pulses in NUV band. Swift/UVOT monitoring observations (acquired in the current outburst) show that 15987( 3) - 17-Aug-2019 18:11:18 - [ 2] the system is nowhere near bright enough to threaten detector damage, then there are no safety concerns for the STIS detectors. ------------------------------------------------------------------------------------ 15987( 3) - 17-Aug-2019 18:11:18 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SAX-J1808.4- STAR, BINARY PULSAR, RA=18H08M27.5400S, J2000 V = 16.8 3658 LMXB, NEUTRON STAR, DEC=-36D58'44.30" PULSAR, X-RAY TRANSIENT Reference Frame: ICRS Comments: The reference magnitude in V band is the minimum reached during the outburst. Observations of SAX J1808.4-3658 at the end of the outburst will likely detect a lower brightness. ------------------------------------------------------------------------------------------------------------------------------------ 15987( 3) - 17-Aug-2019 18:11:18 - [ 4] Visit: 01 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 SAX-J1808.4 STIS/CCD ACQ 50CCD MIRROR 1 5 S (5 S) -3658 ------------------------------------------------------------------------------------------------------------------------------------ 2 SAX-J1808.4 STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=755 1 2240 S -3658 -MAMA G (2240 S) Comments: For our SNR estimate we started from the conservative assumption that the source may have faded at the time of the HST observation. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ SAX-J1808.4-3658 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A SAX-J1808.4-3658 2 STIS/NUV TIME-TA 52X0.2 G230L 2376 none none none none 1 N/A -MAMA G 15987( 3) - 17-Aug-2019 18:11:18 - [ 5] Summary Form for Proposal 15987 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 50CCD, 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=755 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names SAX-J1808.4-3658 ------------------------------------------------------------------------------------------------------------------------------------