Kavli Affiliate: Renxin Xu
| Summary:
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, IXPE, 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 $sim 60355-60385$. We refined the orbital and spin ephemerides assuming a circular orbit, and found that the pulsar was in a spin-up state during MJD $sim$ 60361–60377 showing a significant spin-up rate $dotν$ of $(3.15pm 0.36)times10^-13~rm Hz~s^-1$. Around MJD $sim 60377$ 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 post-burst profiles. These offsets systematically decrease with increasing energy, $Δφapprox0.15$, 0.11 and 0.02 for IXPE, Insight-HXMT ME and HE in 2–8, 5–30 and 20–60 keV, respectively, and $Δφapprox 0.21$, 0.10 and 0.07 for NuSTAR in 3–10, 20–35 and 35–60 keV, respectively, compared to the pre- and post-burst 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.
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