Kavli Affiliate: Renxin Xu
| First 5 Authors: Zhaosheng Li, Lucien Kuiper, Mingyu Ge, Maurizio Falanga, Juri Poutanen
| Summary:
We studied the broadband X-ray timing and spectral behaviors of the newly
confirmed accreting millisecond X-ray pulsar MAXI J1816$-$195 during its 2022
outburst. We used the data from Insight-HXMT ME/HE, NICER and NuSTAR which
cover the energy range between 0.8$-$210 keV. A coherent timing analysis of
solely Insight-HXMT HE data across the full outburst revealed a complex
behavior of the timing residuals, also prominently visible in independent
Insight-HXMT ME and NICER data, particularly at rising part of the outburst and
at the very end in NICER data. Therefore, we broke down the full outburst into
a (noisy) rising part, covering only about five days from MJD 59737.0 to
59741.9, and a decaying part lasting for 19 days across MJD 59741.9$-$59760.6.
Fitting for the decaying part a timing model including a frequency $nu$ and
frequency time derivative $dot{nu}$ component yielded a value of
$(+9.0pm2.1)times10^{-14}~{rm Hz~s^{-1}}$ for $dot{nu}$, which could be
interpreted as a spin-up under our model assumptions. We detected the X-ray
pulsations up to $sim$95 keV in a combination of Insight-HXMT HE observations.
The pulse profiles were quite stable over the whole outburst and could be well
described by a truncated Fourier series using two harmonics, the fundamental
and the first overtone. Both components kept alignment in the range 0.8$-$64
keV. The joint and time-averaged NICER and Insight-HXMT spectra in the energy
range 1$-$150 keV were well fitted by the absorbed Comptonization model compps
plus disk blackbody with two additional Gaussian components. Using the
bolometric flux and spin-up values both evaluated during the decay phase, we
determined a magnetic field strength of $(0.2-2)times10^8$ G for MAXI
J1816$-$195.
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