0 ----- Attempting Submission 1 (Fri May 26 20:02:23 GMT 2017) ----- HST Phase II Proposal 14934 (psr_1023.apt) successfully submitted. Receipt: # 14934-1 ----- Attempting Submission 2 (Fri May 26 20:02:50 GMT 2017) ----- HST Phase II Proposal 14934 (psr_1023.apt) successfully submitted. Receipt: # 14934-2 ----- Attempting Submission 3 (Tue May 30 06:26:29 GMT 2017) ----- PSR J1023+0038 is one of the first discovered transitional millisecond pulsars. These systems switch between a low-mass X-ray binary and a radio millisecond pulsar state and link these two source classes, in turn, confirming the pulsar recycling mechanism. Observation of PSR J1023+0038 and other tMSPs shown that this recycling occurs at luminosity levels usually considered as quiescent for other LMXBs. Hence, tMSPs also serve as unique avenues to test low level accretion onto neutron star surface, and interaction between accretion inflow and the pulsar magnetosphere. This low-level accretion regime has remained highly stable for the last 3.75 yr as evidenced by steady X-ray light curves characterised by `low' and `high' modes and rapid switches between the two. Recently, we have also seen similar moding behaviour at longer wavelengths including UV and optical. This is very intriguing as the X-ray moding originates in an inner accretion flow closer to the neutron star and constrained within the light cylinder. Whereas, UV variability is thought to emanate from colder, outer parts of the accretion disk. We request HST/NUV coverage to complement our XMM-Newton observation in order to search and characterise lags between and X-ray and UV emission. This simultaneity will allow us to probe two distinct regions of the system that seem to share a common variability. Ultimately we aim to understand what controls the accretion flow and state transitions in tMSPs. We aim to observe PSR J1023+0038 with HST in any one of the observation windows scheduled by XMM-Newton. This visit will consist of 4 consecutive HST orbits (spanning $\sim$ 22.8 ks) 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 all of these observations to achieve continuous wavelength coverage between 1570 \AA and 3180 \AA, with the 52 - 0.2 slit. The TIME-TAG mode of STIS will provide a maximum time resolution of 125 $\mu$s, fast enough to capture with great detail the mode changing (as shown in Fig. 2) as well as search for variability on the dynamical time-scale within the accretion disk and NS pulse period. There are no safety concerns for the STIS detectors. In the NUV, our existing HST data as well as Swift/UVOT monitoring observations (taken in the current high state) show that the system is nowhere near bright enough to threaten detector damage. We will use an imaging target acquisition with Mirror B as the best compromise between efficiency and instrument safety. With this set-up, a 3s ACQ/IMAGE target acquisition exposure with the CCD and the 50 CCD aperture yields S/N $\sim$ 30, and the count rate in the brightest pixel never exceeds $\sim8$ c/s, well below the bright object limit. Coordinated Observations Our observations require strict simultaneity between HST and XMM-Newton observatories. Our 24 ks XMM-Newton observations are scheduled for early June 2017. We have requested for these XMM-Newton observations with EPIC-MOS1/2 in small window mode and EPIC-pn in fast timing mode, using the thin filter. The existing EPIC-pn lightcurves shows that MSP is easily detectable even in the event of strong background flaring due to the high source count rate ($\sim$ 3 cnts s$^{-1}$ on an average). Stellar Physics 0 SUPPORTED STAR NEUTRON STAR X-RAY TRANSIENT This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 Swift/UVOT UVW2 = 18.1+/-0.5, UMW1=18.0+/-0.2 (errors are standard deviation due to variability) STIS/NUV-MAMA Spectroscopic No No G230L 2376 1495829373567 /Users/juan/astro/Proposals/HST/Cycle_25/OP-cache/psr_1023-v01.xml The scheduling data was generated by CASM 25.0. CASM was run in Phase II mode. 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