Modelling multiwavelength emission of ULXs

Transcription

1 Modelling multiwavelength emission of ULXs Elena Ambrosi University of Padova Luca Zampieri, Michela Mapelli INAF-Astronomical Observatory of Padova ULXs and Their Environments-Workshop June june 2016

2 MOTIVATION Compute the multiwavelength emission of ULX binary systems during their evolution allowing for super-eddington accretion Develop a TOOL to: compare theoretical CMDs with position of observed optical counterparts compare computed spectra to the observed ones Constrain accretion mechanism and disc structure Constrain donor mass Determine effects of the geometry of the disc on emission properties

3 MODEL : ULXs as Binary Systems in Roche Lobe overflow powered by Super-Eddington accretion onto stellar-mass or massive stellar BHs (e.g. Zampieri & Roberts, 2009; Mapelli et al., 2009; Belzinsky et al, 2010; Feng & Soria, 2011) Starting Point: model of Patruno & Zampieri (2008, 2010) Computation of optical emission with a standard Eddington limited disc Credits: Image created by Tom Russell (ICRAR) using the software created by Rob Hynes (Louisiana State University) This model: Computation of optical emission allowing for super- Eddington accretion and considering a bimodal disc structure

4 Starting Point: Model of Patruno & Zampieri (PZ) Calculation of the evolutionary tracks of binary systems provide donor mass, orbital period, T eff, ṁ,donor radius as a function of time Initial parameters: M BH, M donor, orbital separation, A standard irradiated accretion disc (Shakura & Sunyaev, 1973) and irradiated donor are assumed irradiating flux comes from ALL the inner region of the disc Each anulus of the disc emits as a black body with a temperature T tot = T visc + T self-irr (Copperwheat et al. 2005) Star surface temperature: T* = T eff + T irr CM diagrams are calculated using the UBVRI photometric system. Colors and bolometric correction come from an interpolation of tabulated values (taken from Cox 1999)

5 Radial Disc structure Inner radius: 3r g Outer radius: 2r circ Accretion Efficiency: L= L Edd Ṁ ṁ=6 M Edd η= r g 2 r i = 1 6 Maximum mass transfer rate supported by entire standard disc If ṁ 6 standard disc If ṁ>6 bimodal disc structure AS IN PZ r 0 Boundary radius CM diagrams in UBVRI and HST photometric systems

6 Radial disc structure Boundary radius (outer radius of the advection dominated disc; Watarai et al., 2000) r 0 = ṁ 2 r g Inner slim disc does not irradiate the outer part H r =c 1/ 2 Watarai & Fukue (1999) 3 c 3 c 2 s (r) v 2 ff (r) =1.2 V ff marginally subsonic

7 Disc self-irradiation Self-irradiation is due to the UV-Xray flux produced in the region between r 0 and r x, where r x is the radius of the last annulus emitting in the UV-Xray band ( ν max =2.8 k ) h T (r ) o If r 0 = r x there is no self-irradiation r x L irr = 2 π R F (R)dR Depends on r 0 Treatment of transition region will be implemented

8 Results Expected behaviour of luminosity as a function of Mdot L 10 L Edd

9 Results: photometry Star photometry: calculated with the code of Girardi et al., 2002 L, T Eff, M, metallicity Magnitudes in different photometric systems - More accurate than the tabulated BC in PZ (taken from Cox et al.) BC Cox BC Gir 30 % - Accuracy of the fit in PZ is 15% - Total different with PZ 40% Must be taken into account when comparing the two models

10 Results: evolutionary tracks in the color magnitude diagram Donor ascending along the giant branch is highly super-eddington After MS: Luminosity is larger than that in PZ Colours are initially bluer and then become comparatively redder MS Donor during MS is mildly super-eddington The two models tracks overlap when the donor is on the MS Sub-Eddington accretion

11 Evolutionary tracks on cmds: contributions Irradiation luminosity increases with ṁ Disc radius increases

12 Post Main Sequence evolution

13 Single star evolution versus donor in binary system PARSEC evolutionary tracks (Bressan et al.,2012): Donor stars in ULXs have different properties with respect to single stars: - they loose a significant part of their initial mass - their stellar surface gravity is higher These facts need to be taken into account when estimating donor masses from the optical properties of the counterpart Our tracks

14 Conclusions - When bimodal disc structure sets in at super-eddington rates, disc selfirradiation is different - Irradiation luminosity depends on mass transfer rate - Optical emission of ULXs depends on interplay between the irradiation luminosity and the disc size: systems bluer when donor leaves the MS, then they become comparatively redder when the disc is more extended - The inner slim disc region does not illuminate the outer ones, but selfirradiation from the region of the standard disc close to the boundary radius is enhanced by the higher flux produced by highly super-eddington rates - Evolution of donor star is markely different from that of a single star - Position on CM diagram useful to constrain the donor mass. It will be used for constraining self-consistent ULX binary configurations computed through N-body simulations (as in Mapelli & Zampieri, 2014) - Model will be improved with: - treatment of transition region - inclusion of the wind