Kavli Affiliate: Renxin Xu
| First 5 Authors: Tao Fu, Zhaosheng Li, Yuanyue Pan, Long Ji, Yupeng Chen
| Summary:
We report an analysis of Insight-HXMT observations of the newly discovered
accreting millisecond pulsar SRGA J144459.2$-$604207. During the outburst,
detected in 2024 February by SRG/ART-XC, the broadband persistent spectrum was
well fitted by an absorbed Comptonization model. We detected 60 type I X-ray
bursts in the Insight-HXMT medium energy (ME) data, and 37 were also detected
with the low-energy (LE) telescope. By superimposing the Insight-HXMT/LE/ME/HE
light curves of 37 bursts with similar profiles and intensities, we measured a
deficit of X-rays in the $40-70$ keV energy band. By analyzing the
time-resolved X-ray burst spectra, we determine the mean ratio of persistent to
burst flux of $alpha=71pm7$. We estimate the average hydrogen mass fraction
in the fuel at ignition, as $bar{X} =0.342pm0.033$, and constrain the burst
fuel composition as $X_0leq0.4$. We found that 14 out of 60 X-ray bursts
exhibited photospheric expansion, and thus we estimated the distance to the
source as $10.0pm0.71$ kpc. Combined with IXPE observations, the burst
recurrence time increased from 1.55 to 8 hr as the local mass accretion rate
decreased, which can be described as $Delta T_{rm rec}sim
dot{m}^{-0.91pm0.02}$.
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