Kavli Affiliate: Renxin Xu
| First 5 Authors: Tao Fu, Zhaosheng Li, Yuanyue Pan, Long Ji, Yupeng Chen
| Summary:
We report analysis of $textit{Insight}$-HXMT observations of the newly
discovered accreting millisecond pulsar SRGA J144459.2$-$604207. During the
outburst, detected in 2024 February by $textit{eROSITA}$, the broadband
persistent spectrum was well fitted by an absorbed Comptonization model. We
detected 60 type I X-ray bursts in the $textit{Insight}$-HXMT medium energy
(ME) data, and 37 were also detected with the low-energy (LE) telescope. By
superimposing the $textit{Insight}$-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
$bar{X} = 0.342 pm 0.033$, and constrain the burst fuel composition as
$X_0lesssim0.4$. We found that 14 out of 60 X-ray bursts exhibited
photospheric expansion, and thus we estimated the distance to the source as
$10.03pm 0.71$ kpc. Combined with $textit{IXPE}$ observations, the burst
recurrence time were increasing from 1.55 to 8 hr as the local mass accretion
rate decreasing, which can be described as $Delta T_{rm rec}sim
dot{m}^{-0.91pm0.02}$.
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