On the assessment of the disk truncation and detection of type-II bursts from the accreting millisecond X-ray Pulsar IGR J17062-6143

Abstract

We present a spectral analysis of the NuSTAR and NICER observations of the accreting millisecond X-ray pulsar IGR J17062-6143, performed in 2022. The source remained in the hard spectral state during the observations, with a luminosity of about 0.2-1.3$\%$ of the Eddington luminosity. The continuum emission of the NuSTAR spectrum is entirely dominated by a power-law component or by Comptonized emission of disk photons by a plasma with a high electron temperature ($\gtrsim100$ keV). The NuSTAR spectrum also reveals clear evidence of disk reflection, a broad Fe K line around 6-8 keV, and a Compton hump peaking at 20 keV, irrespective of the choice of the continuum models. Our spectral studies suggest a disk extending close to the neutron star surface ($\sim$7-17 $R_{\rm g}$) at low inclination angles ($\sim$20$^\circ$-40$^\circ$), as revealed by a couple of self-consistent relativistic reflection models, relxill and relxillCP. In addition, we detected type-II bursts for the first time in the NICER observation of this source. Light curve profiles of type-II bursts exhibit different patterns, mostly associated with the so-called mode-0 and mode-1 type-II bursts. The energy spectra of the persistent (pre-burst) and burst emission are well described by an absorbed Comptonization component, scattering diskbb- and blackbody-distributed photons, respectively, by a corona with a temperature of 1-3 keV. Although the origin of the type-II burst is not very clear, it has been substantially linked to magnetospheric gating of the accretion flow.