2025ApJ...990...15L

SRGA J144459.2-604207 is a newly confirmed accreting millisecond X-ray pulsar and type I X-ray burster. We present the broadband X-ray timing and spectral behaviors of SRGA J144459.2-604207 during its 2024 outburst. The data were collected from NICER, Einstein Probe, Imaging X-ray Polarimetry Explorer (IXPE), Insight Hard X-ray Modulation Telescope (Insight-HXMT), NuSTAR, and INTEGRAL observations. X-ray pulsations have been detected for the 1.5-90 keV energy range throughout the “ON” phase of the outburst from MJD ∼ 60355-60385. We refined the orbital and spin ephemerides assuming a circular orbit, and found that the pulsar was in a spin-up state during MJD ∼ 60361-60377 showing a significant spin-up rate $\dot{\nu }$ of (3.15 ± 0.36) × 10^–13 Hz s^–1. Around MJD ∼ 60377 a swing was detected in the spin evolution accompanied by significantly enhanced pulsed emission. We studied the pulse-profile morphology during the X-ray bursts as observed by Insight-HXMT, IXPE, and NuSTAR. During the bursts, pulsations were detected across the 2-60 keV with shapes broadly consistent with those observed for the persistent emission. We found, however, that the “burst” pulse profiles exhibit significant phase offsets relative to the pre- and postburst profiles. These offsets systematically decrease with increasing energy, Δϕ ≈ 0.15, 0.11, and 0.02 for IXPE, Insight-HXMT ME, and HE in 2-8, 5-30, and 20-60 keV, respectively, and Δϕ ≈ 0.21, 0.10, and 0.07 for NuSTAR in 3-10, 20-35, and 35-60 keV, respectively, compared to the pre- and postburst profiles. We performed a joint spectral analysis of quasi-simultaneous NICER, NuSTAR, and Insight-HXMT data for two epochs. The resulting spectra from both observations were consistent and well described by an absorbed thermal Comptonization model, nthcomp, plus relativistic reflection, relxillCp.