OBSERVATIONAL PROPERTIES OF PUFFY ACCRETION DISC ABSTRACT We have performed a general rela/vis/c radia/ve magnetohydrodynamics (GRRMHD) simula/ons of a stable accre/on disc around a stellar-mass black hole (BH) with sub-Eddington mass accre/on rates. The numerical solu/ons revealed an elevated ver/cal structure - above a dense geometrically thin core of dimensionless thickness ℎ!/𝑟∼0.1, resembling a classic thin accre/on disc, a puﬀed-up, op/cally and geometrically thick layer of lower density and ℎ"/𝑟∼ 1.0 is formed. We refer to this solu/on as the Puﬀy disc. We discuss the observa/onal proper/es of a puﬀy discs, in par/cular the geometrical obscura/on at higher observing inclina/ons of the inner area by the puﬀed-up region, and collima/on of radia/on along the accre/on disc spin axis. These eﬀects may explain the apparent super-Eddington luminosity of some ultraluminous X-ray binaries. We also present synthe/c spectra of puﬀy discs, and show that they are qualita/vely similar to Comptonized thin disc spectra. We demonstrate that the exis/ng XSPEC spectral models provide good ﬁts to synthe/c spectra of puﬀy discs but cannot correctly recover the input luminosity, nor the black hole spin. We suggest that puﬀy discs may correspond to X-ray binary systems in the intermediate spectral state with luminosi/es above 0.3 of the Eddington luminosity. Debora Lančová1, Maciek Wielgus2,5, Odele Straub3, Włodek Kluźniak4, Ramesh Narayan5, David Abarca4, Gabriel Török1 and Marek Abramowicz1,4 KORAL CODE The simula*ons were performed using GRRMHD code KORAL (Sądowski et al. 2013a, 2014a), a ﬁnite-diﬀerence Godunov type code solving coevolu*on of gas, radia*on and magne*c ﬁeld in an arbitrary metric. Coupling of gas and radia*on is solved semi-implicitly using M1 closure scheme. We obtain in total 30 mil. corehours on Polish supercomputer Prometheus, part of the PLgrid infrastructure. This computa*onal resources enables us to run a number of simula*ons with diﬀerent ini*al condi*ons to obtain three simula*ons with diﬀerent mass accre*on rates. 1Ins*tute of Physics, Silesian University in Opava, Czech Republic 2Max-Planck-Ins*tut für Radioastronomie, Germany 3ORIGINS Excellence Cluster, Germany 4Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Poland 5Black Hole Ini*a*ve at Harvard University, USA We are studying a mildly sub-Eddington accre*on onto a non-rota*ng stellar mass BH with a mildly sub- Eddintgon accre*on rate. The classical models of accre*on discs repor*ng a thermal and viscous instability in this regime, however, our solu*on is stable thanks to the strong ver*cal net ﬂux of magne*c ﬁeld. The magne*c pressure inside the disc is at the same order as the radia*on pressure, both strongly dominates over the gas pressure. This also cause the accre*on disk to be “puﬀy” - with geometrically thin dense core (1) and geometrically and op*cally thick turbulent puﬀy region (2). Although the core resembles a standard thin disc, the op*cally thick puﬀy region and the posi*on of the photosphere has a strong inﬂuence on the observa*onal proper*es and thus the disc observa*onal appearance is very diﬀerent from a standard models. On the ﬁgure, the thick white line denotes the posi*on of the photosphere, while the dashed line is the density scale-height. The color contours are showing the gas temperature. 2 1 OBSERVATIONAL APPEARANCE Radia*on from the disk is beamed to the direc*on of the axis and obscured for viewing angles close to the equatorial plane, in contrast with the standard thin disc model. The inner parts of the disc and the central BH is completely hidden from observers with high inclina*on. At the same *me for low inclina*on observer the disc radiates all the way to the BH horizon, leaving no trace of the innermost stable circular orbit. Three columns on the le` are showing the picture of a puﬀy disk for three diﬀerent mass-accre*on rate compared to the 0.6 ̇𝑀#$$ standard thin disc image in the right column, with i