Broadband X-Ray Timing and Spectral Characteristics of the Accretion-powered Millisecond X-Ray Pulsar MAXIJ1816-195

Abstract

<div>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 data from the Insight-HXMT Medium Energy (ME) and High Energy (HE) telescopes, NICER, and NuSTAR that cover the energy range between 0.8 and 210 keV. A coherent timing analysis of solely the Insight-HXMT HE data across the full outburst revealed a complex behavior of the timing residuals, also prominently visible in the independent Insight-HXMT ME and NICER data, particularly at the rising part of the outburst and at the very end in the 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 <em>ν</em> and frequency time derivative component yielded a value of (+9.0 ± 2.1) <strong>×</strong> 10⁻¹⁴ Hz s⁻¹ for , which could be interpreted as a spinup under our model assumptions. We detected X-ray pulsations up to ∼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 <span>compps</span> plus disk blackbody with two additional Gaussian components. Using the bolometric flux and spinup values both evaluated during the decay phase, we determined a magnetic field strength of (0.2–2) <strong>×</strong> 10⁸ G for MAXI J1816−195.<br></div>