0 Assigned ID: 7393 ----- Attempting Submission 1 (Thu Apr 16 14:15:09 GMT 2026) ----- HST Phase I Proposal 7393 successfully submitted. Receipt: # 7393-1 Could not connect to APT Status Server Alternate ToO scenarios included: 12 candidate visits pre-loaded for rapid trigger response, only one will be activated. Confirmed acceptable by PC Alison Vick. 1868212102 ----- Attempting Submission 1 (Tue Aug 11 14:13:51 GMT 2026) ----- Alternate ToO scenarios included: 12 candidate visits pre-loaded for rapid trigger response, only one will be activated. Confirmed acceptable by PC Alison Vick. UV pulsations in accreting millisecond pulsars: a new window on the pulsar-disk interaction Accreting millisecond pulsars (AMSPs) are the fastest-spinning neutron stars known, typically powered by mass transfer from a companion. However, the 2019 HST discovery of ultraviolet millisecond pulsations in SAX J1808.4-3658 challenged this paradigm, suggesting that magnetospheric particle acceleration can occur even during active accretion. The physical origin of these UV pulses remains one of the most intriguing mysteries in pulsar physics. We propose a disruptive Target of Opportunity program to observe one AMSP in outburst using the HST/STIS NUV-MAMA in TIME-TAG mode for 1 HST orbit. By timing UV pulsations across different accretion regimes, we aim to constrain the pulsar-disk interaction and the evolution of the emission mechanism during the outburst cycle. HST/STIS in TIME-TAG mode is the only facility capable of providing the temporal resolution and UV sensitivity required to resolve these signals, which are currently inaccessible from the ground or other space telescopes. To maximize the scientific return, each HST trigger will be supported by (quasi-)simultaneous X-ray monitoring with EP/FXT through pre-established collaboration. This synergy will provide the X-ray spectral context and the precise ephemeris required to fold the UV photons and search for coherent pulsations. The unpredictable, short-lived nature of AMSP outbursts, which typically peak and decay within a few weeks (Di Salvo & Sanna 2022; Patruno & Watts 2021; Campana & Di Salvo 2018), poses a strict time-criticality requiring a disruptive ToO activation. While scientific success does not exclusively depend on catching the outburst peak (given that UV pulsations remain detectable during the decay phase (Ambrosino et al. 2021; Ballocco et al. 2026)) prompt execution is essential to capture the source in an accretion state before it inevitably returns to quiescence. We request the ToO observation to be performed using the STIS/NUV-MAMA detector in TIME-TAG mode (time resolution of 125 microseconds). This observation can only be carried out using instruments with a high temporal resolution (< 1 ms), capable of resolving millisecond pulsations in the UV band. There are no other instruments on ground-based telescopes suited to perform this type of UV-band study. STIS has successfully characterized fast variability in multiple fast pulsars (Sollerman et al. 2000; Romani et al. 2005; Shibanov et al. 2005; Mignani et al. 2019; Jaodand et al. 2021; Ambrosino et al. 2021; Miraval Zanon et al. 2022; Baglio et al. 2023; Ballocco et al. 2026). We request the G230L grating (1600-3100 A) with the 52x0.2 arcsec slit to ensure precise background subtraction for the pulsed component. We propose to observe 1 known or newly discovered AMSP in outburst for 1 HST orbit. Following guidance from the HST Help Desk, the APT uses a single generic "AMSP" placeholder. This entry covers all known AMSPs with confirmed optical counterparts and established ephemerides, as well as any new discoveries. The specific target will be selected at the time of the trigger based on the criteria described below. To assess the feasibility and ensure a conservative approach, we performed simulations using the STIS Exposure Time Calculator (ETC v. 34.1.1, STIS.sp.2327962), adopting the parameters of one of the most distant and faint AMSP, XTE J0929-314 (Giles et al. 2005). We assumed an average magnitude during the outburst of V = 18.8 mag, an extinction of E(B-V) = 0.2 mag, and a blackbody model with T = 15000 K to represent the disk contribution (see Supplementary Table 1 from Ambrosino et al. 2021). Our simulations show that with one orbit (~2700 s of net integration), we obtain a Signal-to-Noise Ratio (SNR) > 9 per resolution element at 2376 A. The total source counts integrated over the full NUV-MAMA bandpass exceed 6x10^4 photons. This photon statistics allow for a targeted single-trial search, exploiting the precise X-ray ephemeris from EP/FXT (obtained via priv. comm.), enabling the detection of millisecond pulsations with an amplitude as low as ~2% at a confidence level > 3sigma (Vaughan et al. 1994). This sensitivity threshold is well below the ~4% amplitude (~2.7% r.m.s.) estimated for SAX J1808.4-3658 (Ambrosino et al. 2021), ensuring that we can significantly detect UV pulsations even in the faintest AMSP with 1 HST orbit. To ensure the instrument safety according to the limits for irregularly variable sources (Table 7.8 from the STIS Instrument Handbook), we simulated an extremely bright AMSP at its outburst peak (V = 15 mag, E(B-V) = 0.1), with a T = 25000 K blackbody model to account for the maximum expected UV flux (STIS ETC v.32.2; STIS.sp.2320384). The global and peak local count rates are ~2953 counts/s and a ~1.1 counts/s/pixel, respectively, well below the required limits. The proposing team combines leading expertise in high-resolution timing, multi-wavelength spectroscopy, and rapid response to transients, ensuring the successful execution of this program. The team has established itself as a leader in using HST/STIS for the study of millisecond pulsars. Our group has proven experience in the STIS TIME-TAG data reduction and NUV pulsation searches, as evidenced by a consistent track record of high-impact publications (e.g., Ambrosino et al. 2021, Nat. Ast.; Miraval Zanon et al. 2022; Baglio et al. 2024, Illiano et al. 2025, Ballocco et al. 2026). Beyond standard analysis, we have developed proprietary custom algorithms specifically designed to detect coherent signals in binary systems. This capability has recently led to the discovery of UV burst oscillations in the low-mass X-ray binary EXO 0748-676 (Miraval Zanon et al. 2025) and the successful identification of the transitional MSP candidate 3FGL J1544.6-1125 using HST archival data (Illiano et al. 2025). Multi-wavelength timing: the group (Illiano, Papitto, Marino, Coti Zelati, Ballocco) possesses extensive experience in the timing analysis of accreting millisecond pulsars in the X-ray band. This is complemented by a strong expertise in fast optical timing (Ambrosino, Illiano, Papitto, Miraval Zanon, La Placa, Turner, Malacaria, Ballocco), supported by guaranteed access to the fast optical photometer SiFAP2 at the Telescopio Nazionale Galileo (TNG). The PI (G. Illiano) leads an active ToO program for accreting millisecond pulsars at TNG for the current and upcoming semesters. Einstein Probe: a key strength of this collaboration is the privileged access to the Einstein Probe (EP) mission. Team members (N. Rea, F. Coti Zelati, and A. Marino) are part of the EP collaboration, with access to EP data, proprietary reduction pipelines, and guaranteed observing time. This is critical for the rapid identification of accreting millisecond pulsars in outbursts and the derivation of the high-precision X-ray ephemeris required for the HST trigger. Las Cumbres Observatory (LCO): optical monitoring and rapid identification of counterparts are guaranteed by M. C. Baglio, who has access to the LCO network and its dedicated pipelines for automated analysis and alert dissemination. Multi-band observations: the team is perfectly positioned to provide a global view of the outburst evolution. A. Papitto leads active ToO programs for accreting millisecond pulsars with IXPE, XMM-Newton, and NuSTAR, while F. Coti Zelati has active programs with the Gran Telescopio Canarias (GTC). Spectroscopic support is guaranteed by S. Campana, PI of the SOXS instrument, in the case of a transient event. Furthermore, the team (D. de Martino, S. Campana, M.C. Baglio, A. Miraval Zanon, F. Coti Zelati, M. M. Messa) has a proven track record in multi-band spectral modeling from the IR to the UV. Stellar Physics and Stellar Types 6 false false false false false false false SUPPORTED STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. IGR-J00291+5934 J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. XTE-J0929-314 J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. XTE-J1814-338 J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. XTE-J1751-305 J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. XTE-J1807-294 J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 STAR BINARY PULSAR LMXB NEUTRON STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 STAR BINARY PULSAR LMXB NEUTRON STAR The target will be a known or newly discovered accreting millisecond pulsar (AMSP) in outburst, satisfying all of the following conditions at the time of trigger: Classification and ephemeris: The source is confirmed to be an AMSP in outburst via X-ray monitoring (EP/WXT, MAXI) or optical monitoring (LCO). For newly discovered transients, EP/FXT observations establish the AMSP nature and provide the timing ephemeris. For known sources, existing timing solutions are updated via EP/FXT as needed. Flux and extinction: The source has a detected optical counterpart and NH < 4x10^21 cm^-2 (E(B-V) < 0.6 mag). The 0.5-10 keV X-ray flux is FX >= 10^-10 erg cm^-2 s^-1 (LX ~ 8x10^35 erg s^-1 at 8 kpc), ensuring a representative outburst state. Visibility: The source is visible to HST (verified in APT) and satisfies all scheduling constraints (e.g., no solar constraints) at the time of the trigger. The specific target is selected from the list of known AMSPs included in this proposal, or corresponds to a newly discovered AMSP meeting the above criteria, whichever occurs first and is confirmed by our EP/FXT and LCO collaboration. see comments STIS/NUV-MAMA Spectroscopic No No G230L 2376 The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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