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

TL;DR

This study analyzed the broadband X-ray timing and spectral properties of the accreting millisecond X-ray pulsar MAXI J1816-195 during its 2022 outburst, revealing complex timing behavior, stable pulse profiles, and estimating its magnetic field strength.

Contribution

It provides the first detailed broadband timing and spectral analysis of MAXI J1816-195, including a spin-up measurement and magnetic field estimation, across multiple X-ray observatories.

Findings

Detected X-ray pulsations up to 95 keV.

Measured a spin-up rate of (9.0 ± 2.1) × 10^{-14} Hz s^{-1}.

Estimated magnetic field strength of (0.2-2) × 10^8 G.

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.