Kavli Affiliate: Renxin Xu
| First 5 Authors: , , , ,
| 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 $dotnu$ 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, $Delta
phiapprox0.15$, 0.11 and 0.02 for IXPE, Insight-HXMT ME and HE in 2–8, 5–30
and 20–60 keV, respectively, and $Delta phi approx 0.21$, 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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