Broadband X-ray timing and spectral characteristics of the accretion-powered millisecond X-ray pulsar MAXI J1816$-$195

Abstract

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 $(+9.0\pm2.1)\times10^{-14}~{\rm Hz~s^{-1}}$ 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.