17245( 3) - 31-Aug-2022 22:06:11 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17245 Version: 3 Check-in Time: 31-Aug-2022 22:06:11 STScI Edit Number: 0 Title Unveiling the UV pulse emission mechanism in the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2022 outburst ------------------------------------------------------------------------------------ Type Cycle GO/DD 29 ------------------------------------------------------------------------------------ Investigators Contact? PI: Ms. Arianna Miraval Zanon INAF, Osservatorio Astronomico di Roma Y CoI: Dr. Filippo Ambrosino INAF, Osservatorio Astronomico di Roma N CoI: Dr. Alessandro Papitto INAF, Osservatorio Astronomico di Roma N CoI: Dr. Sergio Campana INAF, Osservatorio Astronomico di Brera, N Merate CoI: Dr. Paolo D'Avanzo INAF, Osservatorio Astronomico di Brera, N Merate CoI: Francesco Coti Zelati Institute of Space Sciences (CSIC-IEEC) N CoI: Giulia Illiano INAF, Osservatorio Astronomico di Roma N CoI: Maria Cristina Baglio New York University N ------------------------------------------------------------------------------------ Abstract Millisecond pulsars (MSPs) are the fastest spinning compact stars known to date. They can appear as radio pulsars powered by the rotation of the NS magnetic field or accreting X-ray pulsars powered by the accretion of mass transferred by a companion star. The accreting millisecond X-ray pulsar SAX J1808.4-3658 has revealed an abrupt increase in intensity beginning in 19 August 2022, indicating the onset of a new outburst. This activity has never been observed since the last outburst in 2019. During the outburst this source emits X-ray pulsations at the neutron star spin period, which have been already detected with NICER. Ultraviolet millisecond pulsations have also been detected from SAX J1808.4-3658 during its 2019 outburst with HST/STIS, representing the first discovery of this kind in accreting MSPs. For the first time in forty years of pulsar studies, we might have witnessed that electrons are accelerated to relativistic energies by the rotation of the neutron star magnetosphere even during the accretion phase. To probe the nature of this new phenomenon, we require disruptive activation time to observe SAX J1808.4-3658 with HST STIS/NUV-MAMA instrument combination to study UV pulses during the 2022 outburst. The detection of UV pulsations close to the peak of the outburst, when the mass accretion rate is maximum, would allow us to probe the UV pulse emission mechanism over a range of mass accretion rates spanning 2-3 orders of magnitude. ------------------------------------------------------------------------------------ Observations Description ------------------------ We ask to observe SAX J1808.4-3658 with HST during its 2022 outburst. This visit will consist in one HST orbits 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 17245( 3) - 31-Aug-2022 22:06:11 - [ 2] the 2022 outburst measured by the timing analysis of X-ray NICER data to study the pulses in NUV band. ------------------------------------------------------------------------------------ 17245( 3) - 31-Aug-2022 22:06:11 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SAXJ1808.4-3 STAR, ACCRETION DISK, RA=18H08M27.6200S, J2000 V = 18 658 BINARY PULSAR, LMXB, DEC=-36D58'43.30" NEUTRON STAR, PULSAR Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 17245( 3) - 31-Aug-2022 22:06:11 - [ 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 SAXJ1808.4- STIS/CCD ACQ 50CCD MIRROR 1 5 S (5 S) 3658 ------------------------------------------------------------------------------------------------------------------------------------ 2 SAXJ1808.4- STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=685 1 2240 S 3658 -MAMA G (2240 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 | ------------------------------------------------------------------------------------------------------------------------------------ SAXJ1808.4-3658 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A SAXJ1808.4-3658 2 STIS/NUV TIME-TA 52X0.2 G230L 2376 none none none none 1 N/A -MAMA G 17245( 3) - 31-Aug-2022 22:06:11 - [ 5] Summary Form for Proposal 17245 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 50CCD, 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=685 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names SAXJ1808.4-3658 ------------------------------------------------------------------------------------------------------------------------------------