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

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?

Overview

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 > erg 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)

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 ~ 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)

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...

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

BHB & BHC vs. NSB

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/R 3 ) 1/2 –viscosity transports angular momentum outward –gas gets hotter closer to the BH –disk terminates at ISCO: R ISCO = 6R g = 6GM/c 2, for a Schwarzschild BH R ISCO = R G = GM/c 2, for a Kerr BH

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 > R ISCO –relativistic MHD corrections – add a magnetic field and you extract energy from very near the horizon

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);  ~ 1.7 (2-20keV); low luminosity; radio emission Quiescent State –truncated disk; ADAF down to the ISCO

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

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

X-Ray States: McC & R Quiescent State –extraordinarily faint (L x = erg s -1 ) –distinctly non-thermal, hard spectrum (  = 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

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 (  = 2.1 – 4.8) –PDS shows weak variability and power scaling roughly as -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”

X-Ray States: McC & R Hard X-Ray State –take out “low” from name (some sources show high luminosity) –PL with  ~ 1.7 –broad enhancement at keV (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

X-Ray States: McC & R Hard X-Ray State –blackbody radiation truncated at large radius ~100R g –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 <  < 2.1, and a strong integrated power continuum”

X-Ray States: McC & R Steep Power-Law (SPL) –new name for VHS –often exceedingly bright (L x >0.2L Edd ), but not always –very steep unbroken (x-ray to gamma-ray) PL (  ≥ 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

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”

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 NGC 4051 Cyg X-1

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

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

Microquasars

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

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 K  correlations –AGN: broad Fe K  line, skewed to low energies –consistent with emission from surface of accretion disk –also observed for BHXRBs (post-Chandra era)

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

Microquasars/AGNs/GRBs

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