Kavli Affiliate: Renxin Xu
| First 5 Authors: Zhaosheng Li, Zhaosheng Li, , ,
| 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 $Delta phiapprox0.21$, 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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