Speaker
Description
From the discovery of the low-mass X-ray binary (LMXB) MAXI J1820+070, it has been observed almost simultaneously by several telescopes, e.g. NICER, NuSTAR, INTEGRAL during different epochs of its outburst. Due to the very good quality of the data, several groups have analysed the timing as well as spectral properties of the data till date. Amazingly it is found that hard and hard-intermediate phase (rising phase) data can be explained by two different scenarios, one with the disk extending to the ISCO and other with the disk trucated quite far from ISCO.
We use our recently developed KYNSED model that couples the source of the soft photons, the disk, and the source of the hard photons, the corona, through their mutual interaction - the corona up-scatters the seed photons from the disc while the accretion disc is heated by the illumination from the corona. The model takes into account the photon number conservation and energy balance. We are trying to fit the hard state of MAXI J1820+070 with this model and this is the first attempt to fit any X-ray binary observation with this model. Assuming the corona is powered by the inner accretion flow and the disk's thermal emission is enhanced due to being heated by the corona, we investigate these two scenarios, disc reaching down to the ISCO and truncated disk. However, we find that the observation can not be matched with any of the scenarios. Even with truncated disk, illumination of the disk by the hot corona is leading to much larger temperature than observed. We believe that the overestimation of temperature by the model is generic and will be inconsistent for observations of other X-ray binaries in similar state. The above mentioned effect depends strongly on the fraction of the illumination which is reflected back as the rest will be absorbed and heat up. Currently we are investigating the reason for such inconsistency.
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