Galactic Black Hole Binaries

1. GBH Galactic Black Hole Binaries Miniature versions of AGNs AGN Emily Alicea-Muñoz Astro 597A 18 October 2004

2. Outline • Overview • SXTs, LMXBs, HMXBs • The case for BHBs and BHCs and not for NSBs • Accretion Mechanisms • BHXRB states and their oh-so-confusing classification schemes (ugh, so many acronyms) • Luminosity classification scheme • McClintock-Remillard classification scheme • Cyg X-1: NGC 4051’s Mini-me? • Microquasars – AGN/QSO’s Mini-me’s; are they related to GRBs too?

3. Overview

4. Overview: SXTs • Soft X-ray Transients (SXT) • short period binaries (5hrs – 6 days) • late-type (K-M) type secondaries • rare, dramatic x-ray outbursts (L > 1038erg s-1) • LMXB: low-mass x-ray binaries • most of them strong black hole candidates (BHC) • optical spectra are hot, blue continua (U-B ~ -1) with superposed broad H and He emission lines arising from the inner disk region • HMXB: high-mass x-ray binaries • early-type (O-B) secondaries • most have NS instead of BH (with a few exceptions, e.g. Cyg X-1)

5. SXT Observations • Outbursts • once every ~10-20 yrs • fast rise followed by exponential decay • mini-outbursts sometimes in their way to quiescence • ultra-soft x-ray spectra during outburst, with blackbody color temperature of kT ~ 0.5-1 keV superposed on hard power-law extending to higher energies • Quiescence • mass transfer continues at a very low rate • inner accretion disk turns into hot, low density corona • radiatively inefficient advective accretion • SXTs with BH fainter than those with NS because of event horizon (energy is lost)

6. SXT Observations • X-ray Spectroscopy and Variability • presence of type-I x-ray bursts indicates primary is NS • rapid quasi-periodic oscillations (QPOs) allow study of inner accretion disk; often indicates presence of BH • two different x-ray states • hard power-law spectrum extending to high energies • explained by Comptonization • predicts energy-dependent time delay • soft blackbody spectrum • arising from inner region of accretion disk • Multicolor Disk Model (MCD) – temperature varies with radius • situation not so simple as “two clearly defined” states; more details later...

7. SXT Observations • Black Hole Spin • affects innermost stable circular orbit (ISCO) • maximal spin makes ISCO smaller • Radial Velocity Curves • measured during quiescence • can also determine secondary’s spectral type and orbital period • mass function can be calculated from K-velocity amplitude • inclination is most uncertain parameter

8. BHB & BHC vs. NSB

9. Accretion Mechanisms • Best Model: Thin Accretion Disk • secondary fills its Roche equipotential lobe • narrow stream of gas escapes through first Lagrangian point (L1) • gas with high angular momentum cannot directly accrete onto BH, thus forming an accretion disk • gas in disk moves in Keplerian orbits with angular velocity (GM/R3)1/2 • viscosity transports angular momentum outward • gas gets hotter closer to the BH • disk terminates at ISCO: • RISCO = 6Rg = 6GM/c2, for a Schwarzschild BH • RISCO = RG = GM/c2, for a Kerr BH

10. Accretion Mechanisms • Another Model: MCD • multi-temperature (multicolor) disk • used to describe thermal component in x-ray spectra • total disk luminosity in steady state • limitation: neglect of torque-free boundary condition at ISCO • MCD temperature profile T(R) R-3/4 (maximum at ISCO) • proper boundary condition sets maximum at R > RISCO • relativistic MHD corrections – add a magnetic field and you extract energy from very near the horizon

11. X-Ray States: Luminosity • Very High State (VHS) • strong ultra-soft (US) component and unbroken power-law (PL) component; strong QPOs at ~ 10Hz • High/Soft State (HS) • US dominates; very weak PL component; high luminosity; MCD • Intermediate State • US and steeper PL at high energies • Low/Hard State (LH) • no US component; hard power-law PDS (power density spectrum); G ~ 1.7 (2-20keV); low luminosity; radio emission • Quiescent State • truncated disk; ADAF down to the ISCO

12. X-Ray States: Luminosity “Unified MCD and ADAF model”

13. X-Ray States • Limitations of luminosity classifications • unified MCD and ADAF model don’t account well for the VHS’s unbroken power-law • ordering states by accretion rate or luminosity is “naïve” • model does not account for dynamical behavior of corona (flares, QPOs, radio emission) • no quantitative model relating disk truncation to accretion rate • McClintock & Remillard (2004) propose new scheme, based on a model consisting of a MCD and a power law component

14. X-Ray States: McC & R • Quiescent State • extraordinarily faint (Lx = 1030.5-1033.5 erg s-1) • distinctly non-thermal, hard spectrum (G = 1.5 – 2.1) • long period systems brighter than short period systems • ADAF/MCD model accounts for observed properties • hard PL spectra • faintness of BHBs relative to NSBs • optical/UV time delay of X-ray novae • broadband spectrum • truncated accretion disk

15. X-Ray States: McC & R • Thermal-Dominant (TD) • new name for the HS state • soft x-rays represent thermal emission from inner disk, dominant below 10keV • steep PL (G = 2.1 – 4.8) • PDS shows weak variability and power scaling roughly as n-1, indicative of turbulence • “The set of conditions for which the disk-flux fraction is above 75% (2-20keV), the PDS shows no QPOs, and weak power continuum”

16. X-Ray States: McC & R • Hard X-Ray State • take out “low” from name (some sources show high luminosity) • PL with G ~ 1.7 • broad enhancement at 20-100keV (reflection of PL from surface of inner disk) • steep cut-off near 100keV • compact quasi-steady radio jets present (disappear upon return to TD state) • physical conditions that give rise to this state are still debated

17. X-Ray States: McC & R • Hard X-Ray State • blackbody radiation truncated at large radius ~100Rg • what’s going on inside this radius? • truncated disk, inner region filled by ADAF? • relativistic flow entrained in a jet? • disk intact but depleted of energy in some sort of Compton corona? • answer could be found by • optical/x-ray variability studies • spectral analysis focused on broad Fe emission features • origin of x-ray PL also debated – many possible mechanisms • “Association of hard state with radio jet is an important step forward. […] HS is well characterized by three conditions: spectrum dominated (>80% at 2-20keV) by power law, spectral index in the range 1.5 < G < 2.1, and a strong integrated power continuum”

18. X-Ray States: McC & R • Steep Power-Law (SPL) • new name for VHS • often exceedingly bright (Lx>0.2LEdd), but not always • very steep unbroken (x-ray to gamma-ray) PL (G≥ 2.4) • QPOs in 0.1–30 Hz range • no evidence for high-energy cutoff • transitions between TD and H states usually pass through SPL state • essentially radio-quiet; though sometimes shows impulsive jets

19. X-Ray States: McC & R • Steep Power-Law (SPL) • physical origin still an outstanding problem • spectrum extends to ~1MeV, maybe higher • possible model: • inverse Compton scattering for a radiation mechanism • scattering occurs in a non-thermal corona • where do the Comptonizing electrons come from? • magnetic instabilities in accretion disk? • strongly magnetized disk, as in AGNs? • PL gets stronger and steeper as disk luminosity and radius decrease, while keeping high temperature • Intermediate States • “State transitions and hybrid emission properties are to be expected; x-ray spectra and PDS should be interpreted as intermediate states when necessary, while specifying which states can be combined to yield he observed x-ray properties”

20. Cyg X-1 NGC 4051 Cyg X-1 • Unusual spectrum • soft state dominated by PL instead of TD spectrum • transition from hard to soft x-ray spectrum considered as one from the hard state to the SPL state • weird SPL: no QPOs and low luminosity • reminiscent of NGC 4051 (flashback to Week 5) • hints that the same physical mechanism generating variability regardless of BH size

21. Microquasars • BHXRBs that eject plasma at relativistic speeds (jets) • Fossil sources of GRBs? • Analogy with AGN/QSO • length and time scales are proportional to BH mass

22. Microquasars

23. Microquasars • A connection between x-ray flux and jets has been observed • jets appear when disk x-ray flux drops • jets are produced during replenishment of inner accretion disk • time delay between jet flares at different wavelengths consistent with adiabatically expanding cloud model • delay of few min between drop in x-ray flux and onset of jets could indicate absence of material border, thus making the case for a BH event horizon • however, absence of evidence is not evidence of absence, so this observation could have an alternative explanation

24. Microquasars

25. Microblazars • Microquasars with jet axis with <10° angle with line of sight should be analogous to blazars • Should appear as intense sources of high-energy photons with very fast variations in flux • Very hard to observe • It has been proposed that microblazars may be more frequently linked to HMXBs • gamma-rays produced by inverse Compton of the jet particles with the UV photons radiated by massive secondary

26. Microquasars • X-ray/radio correlations • hard x-ray state w/ radio jets also proposed for AGNs • Time variation correlations • duration of x-ray flares from stellar-mass BHs and AGNs (e.g. Sgr A*) seem proportional to BH mass • minimum frequencies of QPOs expected to be proportional to BH mass, for a given BH spin • Iron Ka correlations • AGN: broad Fe Ka line, skewed to low energies • consistent with emission from surface of accretion disk • also observed for BHXRBs (post-Chandra era)

27. Microquasars and GRBs? • Mirabel (2004) adopts the theory that long GRBs might be caused by formation of BH in collapsars or highly magnetized neutron stars • The case for the microquasar connection: • spin-orbit interactions provide enough power for collapsar • GRBs seem associated with SN Ic, the ones that show no H or He lines • progenitor lost outer layers way before event • could be due to progenitor being in a binary system which underwent a common-envelope phase

28. Microquasars/AGNs/GRBs

29. GBH Questions? (hopefully I’ll be able to answer them) References: P.A. Charles: astro-ph/9806217 J.E. McClintock & R.A. Remillard: astro-ph/0306213 I.F. Mirabel: astro-ph/0405433