0 0 ----- Attempting Submission 1 (Fri Jan 19 21:45:42 GMT 2024) ----- HST Phase II Proposal 17587 successfully submitted. Receipt: # 17587-1 ----- Attempting Submission 2 (Tue Jan 23 09:40:35 GMT 2024) ----- Transitional millisecond pulsars exhibit a peculiar X-ray sub-luminous disk state that features characteristics of both rotation-powered radio pulsars and accretion-powered X-ray pulsars. In this state, switching between two distinct intensity levels, dubbed high and low modes, is observed in both X-rays and optical/UV bands. For the prototype PSR J1023+0038, it was suggested that fast variations of the inner accretion flow might be at the origin of the observed mode-switching phenomenon. To better understand the physical mechanisms driving these transitions, we are organizing the first high-time resolution multi-wavelength campaign on the robust candidate transitional millisecond pulsar 3FGL J1544.6-1125, exploiting recently approved high-time resolution observations with the quintuple-beam imager HiPERCAM, at the 10.4m GTC, and ToO observing programs with XMM-Newton and NuSTAR. High-time resolution UV observations have never been performed on this source, but we expect, given its brightness, to allow a similar study as performed for PSR J1023+0038. We will investigate the timescales of mode-switching in 3FGL J1544.6-1125, assessing their compatibility with the proposed model for PSR J1023+0038. We will model broadband spectral energy distributions, separately extracted during the high and low modes. Such analyses are pivotal in refining and constraining the considered model, enhancing our efforts to understand this intriguing phenomenon. We found a candidate pulsed signal with the fast optical photometer TNG/SiFAP2. The proposed observing campaign also aims at confirming the hint of detection or put stringent upper limits on the fractional amplitude. We ask to observe 3FGL J1544.6-1125 in the time interval: 2024-02-08, 04:30-06:30 UTC (60348.18750000 - 60348.27083333 MJD) This visit will consist in one HST orbit 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 A setting for this observation to achieve continuous wavelength coverage between 1570 A and 3180 A, with the 52X0.2 slit. The TIME-TAG mode of STIS will provide a maximum time resolution of 125 micro-s, fast enough to resolve high/low mode transitions and to search for pulsations at millisecond periods in the Near-UV band. We will observe simultaneously with 10.4m Gran Telescopio Canarias and X-ray telescopes. Several members within our group have effectively employed the Hubble Space Telescope in projects focused on the study of millisecond pulsars. They possess an advanced understanding of timing analysis across different bands of the electromagnetic spectrum (Ambrosino F., Miraval Zanon A., et al. 2021, Nature Astronomy, 5, 552; Miraval Zanon A., et al. 2022, A&A, 660, A63; Baglio M. C., Coti Zelati F., et al., 2023, A&A, 677, A30). Consequently, our team has the necessary skills to handle HST data reduction, investigate the mode-switching simultaneously with observations in other bands, search for NUV pulsations, and interpret the obtained results (Ambrosino, F., Papitto A., et al., 2017, Nature Astronomy, 1, 854; Papitto A., et al., 2019, ApJ, 882, 104; Miraval Zanon A., et al. 2022, A&A, 660, A63; Illiano G., et al., 2023, A&A, 669, A26; Baglio M. C., Coti Zelati F., et al., 2023, A&A, 677, A30). Our collaboration has consolidated experience in timing analysis of pulsars in X-ray/UV/optical/radio bands, having also implemented different codes to search for pulsations in different bands, even in systems with high orbital variations as transitional pulsars and binary millisecond pulsars (Ambrosino, F., Papitto A., et al., 2017, Nature Astronomy, 1, 854; Miraval Zanon A., et al., 2020, A&A, 635, A30; Coti Zelati F., et al., 2019, A&A, 622, A211; Illiano G., et al., 2023, ApJL, 942, L40; Baglio M. C., Coti Zelati F., et al., 2023, A&A, 677, A30). 6 false false false false false false false false SUPPORTED STAR ACCRETION DISK BINARY PULSAR NEUTRON STAR PULSAR J2000 1706002614700 The scheduling data was generated by CASM 32.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 17587 :visit-id "01" :proposal-version -1 :trans-version "20230626" :date "23-Jan-2024 10:36:54.0" :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 156960 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17587 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17587 :visit-id "01" :orbit-number 1 :pcs-time 443 :si-vis-utilization 2693 :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 3136 :pcs-gap 0 ) false 1 0