Samir Mandal

Indian Institute of Space Science & Technology

IISER, 18th November 2011

Accretion and Radiative Processes Around Black Accretion and Radiative Processes Around Black Hole CandidatesHole Candidates

Plan of the TalkPlan of the Talk

Black Hole Accretion

Accretion Disk Models

Radiative Processes

Consequences of Shocks

Model Spectra

Fitting of Observed Data

Conclusions

IntroductionBlack holes, unlike other astrophysical objects, cannot be observed directly. Therefore the only way to discover its presence is through its gravitational interaction with the surrounding matter. Black holes accrete matter from the ambient medium or from the companion star due to its strong gravity. The in-falling matter has angular momentum and forms a disk-like structure, widely known as the accretion disk.

Boundary Conditions for BHs:

At r ∞: Flow radial velocity, v ~ 0 sub-sonic

At the horizon: Flow radial velocity, v =>Super sonic

Flow angular momentum, < kep => sub- Keplerian

Accretion on to BH is necessarily transonic

Black Hole Accretion

• Bondi Accretion: Spherically symmetric flow, high radial velocity, low luminosity

• Standard Accretion Disk: Geometrically thin and optically thick flow with a Keplerian angular momentum distribution

• Sub-Keplerian Disk: Optically thin flow and flow angular momentum is less than Keplerian angular momentum

Cartoon Diagram of a General Model

Advective Accretion Process

FGR ~ -1/(r - rg)2

FCEN ~ 2/ r3; =Constant

FGR >> FCEN at r ~ rg and r ~

At intermediate distance FGR ~ FCEN

v and

Slowed down inner part of the disk effective boundary layer of the BH.

If slowing down is discontinuous

=) SHOCK!!!

The hot puffed up post-shock region commonly known as CENBOL (CENtrifugal pressure supported Boundary Layer)

CENBOL Compton Cloud

Consequences of Shock

The hot puffed up CENBOL intercepts & inverse-Comptonizes a significant portion of the soft photons from the outer part of the disk, to produce the hard power-law tail

Excess thermal gradient force at CENBOL drives apart of the incoming matter as outflow/jet along axis

Origin of QPO can be explained by shock

It can produce non-thermal electrons through shock acceleration, which is essential in explaining the non-thermal power-law spectrum in high energy

Generic spectrum of a black hole binary(Chakrabarti, et al. 2002)

PDS and QPO(Chakrabarti, et al. 2000)

Time lag between hard and soft photons(Smith, et al. 2007)

Units and Governing Equations

v =Radial velocity =Angular momentum

=Density =Vertical averaged density

P =Pressure Q+ =Heat gain

=Adiabatic index Q- =Heat loss

Radiative Processes

Accelerated or decelerated electrons around ions emit bremsstrahlung radiation.

Relativistic electrons moving in a magnetic field emit synchrotron radiation.

Locally soft photons and that supplied by Keplerian disk is inverse-Comptonized by hot electrons in the CENBOL.

Coulomb coupling tries to maintain the temperature difference between electron and proton.

Synchrotron self-Comptonization due to non-thermal electrons produced by the shock acceleration.

Solution Topology

Typical temperature profile

Model Spectrum

Chakrabarti & Mandal, 2007, Astrophysics and Space Science

Shock Explains Spectral Properties of Cyg X-1

Observation

o Soft State

• Hard State

Model Fit

Soft State

Hard State

Chakrabarti & Mandal, 2006, APJL

Variation of the emergent spectrum with the variation of the halo rate.

Variation of the emergent spectrum with shock location.

Spectral Transition

Mandal & Chakrabarti, 2005, A&A

Jet + CENBOL

• The jet is produced from a location close to central region is expected from our CENBOL paradigm for the origin of jet.

• Jets are not separable from the accretion disk. So, the spectrum from the core will always have some contribution from the jet.

• We have assumed that 10% of matter from the accretion disk is launched from the location of the CENBOL as jet with the same temperature as that of CENBOL.

Spectral Fit of M87 Nucleus

Observations

Biretta et al. 1991; Spark et al. 1996;

Stiavelli et al. 1997; Ho et al, 1999;

Perlman et al. 2005; Maoz et. al 2005

Model Fit1 -- Pre-shock synchrotron2 -- pre-shok bremsstrahlung3 -- post-shock synchrotron4 -- SSC (thermal)5 -- SSC (non-thermal)6 -- synchrotron from jet

Mandal & Chakrabarti, 2008, ApJL

A fit of hardness ratio - photon intensity diagram of GRO J1655-40 during its outburst (solid line) of 2005

Mandal & Chakrabarti, 2010

ConclusionsAccretion disk spectrum around stellar mass black holes as well as supper-massive black holes can be explained from a single model.

Sub-Keplerian flow is very important for interpreting disk spectrum.

We have shown that the temperature of the electrons could be much cooler than that of protons.

Shocks heat up the accretion flow and produces a dense puffed region (CENBOL). This intercept the low energy photons from the Keplerian disk and reprocess them to generate high energy photons.

Shocks are very important to accelerate particles and produces non-thermal electrons which could explain the observed high energy non-thermal tail in black hole candidates.

Thank You

(a) Variation of the energy spectral index for thermal electrons (th) as well as due to convergent flow with Keplerian disk rate. (b) Variation of energy spectral index for non-thermal electrons (nth) with the Keplerian disk rate.

Average CENBOL temperature

List of Publications

1. S. K. Chakrabarti, A. Nandi, S. Manickam, S. Mandal & A. R. Rao, 2002, ApJ. Letter, 579, 21L

2. S. K. Chakrabarti, S. Pal, K. Acharya, S. Mandal, S. Chakrabarti, R. Khan & B. Bose, 2002, Ind. J. Phys., 76B(6), 693

3. S. K. Chakrabarti, K. Acharya, B. Bose, S. Mandal, A. Chatterjee, N. M. Nandi, S. Pal & R. Khan, 2003, Ind. J. Phys., 77B(2), 173

4. S. Mandal & S. K. Chakrabarti, 2004, Ind. J. Phys., 78(B), 145

5. S. Mandal & S. K. Chakrabarti, 2005, Astronomy and Astrophysics, 434, 839

6. S. Mandal & S. K. Chakrabarti, 2005, Astrophysics and Space Science, 297, 269

7. S. Mandal & S. K. Chakrabarti, 2005, Int. J. Mod. Phys. D, 14(6), 933

8. S. K. Chakrabarti, M. Saha, R. Khan, S. Mandal, K. Acharya & R. Saha, 2005, Ind. J. of Radio and Space Phys., 34, 314

9. S. K. Chakrabarti, B. G. Anandarao, S. Pal, S. Mondal, A. Nandi, A. Bhattacharyya, S. Mandal, R. Sagar, J. Pandey, A. Pati & S. K. Saha, 2005, MNRAS, 362, 957

10. S. K. Chakrabarti & S. Mandal, 2006, ApJ Letter, 642(1), 49

11. S. Pal, S. K. Chakrabarti, A. Kraus & S. Mandal, 2006, Bull. Astr. Soc. India, 34, 1

12. S. Mandal & S. K. Chakrabarti, 2007, Astrophysics and Space Science, 309, 305

13. S. K. Chakrabarti & S. Mandal, 2007, Astrophysics and Space Science, 309, 163

14. S. Mandal & S. K. Chakrabarti, 2008, ApJ Letter, 689, L17

15. D. Debnath, Sandip K. Chakrabarti, A. Nandi & S. Mandal, 2008, Bull. Astr. Soc. India, 36(4), 151

16. S. Mandal & S. K. Chakrabarti, 2009, ApJ Letter (Submitted)

17. S. Mandal & D. Eichler, 2009, ApJ Letter (Submitted)

18. R. Sarkar, S. Mandal, D. Debnath, T. C. Kotoch, A. Nandi, A. R. Rao & S. K. Chakrabarti, 2009, Exp. Astron. (Submitted)

Future Plan

• The radiation spectrum from an accretion disk have calculated in a parametric way without considering the hydrodynamical transonic solutions. We wish to extend the transonic study including all heating and cooling processes to calculate the spectrum from an accretion disk.

• So far we have studied only the spectral properties. In future we have plan to study both timing and spectral properties simultaneously using time dependent hydrodynamic simulation code.

• In case of accretion or wind flow, magnetic field is the most difficult parameter to determine. I have plan to involve myself in magneto-hydrodynamics simulation study to couple our spectrum calculation code with the magneto-hydrodynamics of the flow and try to calculate the more realistic spectrum from the accretion disk.

• We will extend our work in Kerr geometry using transonic flow solutions in presence of cooling and viscosity (Mandal & Mondal 2009 in preparation), so as to identify the effect of black hole spin parameter on the spectrum.

• Moreover, I will continue to work on GRBs. We will calculate the spectrum of the prompt emission as well as the afterglow of GRB in the realm of radiative shocks and scattering due to baryonic matter which is necessary to understand the anisotropy in the burst as well as the long hard tail of short GRBs. Already we have started using FERMI data for GRB light curves. Since FERMI GBM and LAT have very good energy resolution and provide the GRB spectrum upto few GEV, we will have a good opportunity to verify our work with the observations.

Current Works

Mandal & Eichler, 2009, ApJL (Submitted)

An Explanation of GRB 060218 light curves as seen by an off-axis observer due to the scattering of radiation by an accelerating baryonic cloud

Convergent flow

Application to M87

• The elliptical galaxy M87 contains a super-massive black hole of mass M = (3.2 ± 0.9) × 109 M⊙.

• The inclination angle of the accretion disk with the line of sight is i = (42 ±5)◦.

• It is a low luminosity AGN located in the Virgo cluster at a distance of D = (16 ± 1.2) Mpc having a prominent one-sided jet.

• The central luminosity of the accretion disk is ∼ 1042 ergs/s

• The jet is produced from a central region not more than 50 rg.

Why Sub-Keplerian Disk for M87

The low-ionization nuclear emission-line regions (LINER) of M87 are produced due to the shock excitation in a dissipative accretion (Dopita et al. 1997).

From kinematical arguments, based on Doppler shifts of several lines emitted from the disk and assuming a Keplerian motion of the emitting gas, they conclude that the mass of the nucleus M = (2.4 ± 0.7) x109 M⊙.

No indication of big blue bump in the observed spectrum.

The variability of the core of M87 in optical/x-ray wavelength is reported to be of the order of few months.