0 ----- Attempting Submission 1 (Fri Aug 16 05:24:44 GMT 2019) ----- HST Phase II Proposal 15987 (phase2-GO_SAXJ1808.apt) successfully submitted. Receipt: # 15987-1 ----- Attempting Submission 2 (Sat Aug 17 09:40:00 GMT 2019) ----- 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. 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 the system is nowhere near bright enough to threaten detector damage, then there are no safety concerns for the STIS detectors. 0 false SUPPORTED STAR BINARY PULSAR LMXB NEUTRON STAR PULSAR X-RAY TRANSIENT 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. J2000 1566034427093 /tmp/OPVisit5176970510300586565.xml The scheduling data was generated by CASM 27.2. CASM was run in Phase II mode. 1567025414000 59.0 (Tic-Visit :proposal-id 15987 :visit-id "01" :proposal-version -1 :trans-version "20190514" :date "17-Aug-2019 11:33:46.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 226544 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15987 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15987 :visit-id "01" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2804 :al-overhead-at-end 0 :internal-dur-at-end 440 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3200 :pcs-gap 0 ) true 1 0 For our SNR estimate we started from the conservative assumption that the source may have faded at the time of the HST observation.