# Presentation: 325728

## X-ray Binaries

- Tim Kallman
- NASA/GSFC

## What is an X-ray binary?

- Binary consisting of non-degenerate star + accreting degenerate star
- Mass transfer fuels accretion, creating X-rays
- Degenerate star is either neutron star or black hole (WD systems = cataclysmic variables)
- White dwarf+neutron star systems are included

## XRB classification

- Classified according to type of non-degenerate star
- Early-type star (A or earlier) = ‘high mass X-ray binary’ (HMXB, MXB)
- Later-type =‘low mass X-ray binary’ (LMXB)
- Unique, but interesting, source Her X-1 straddles the boundary

## Science questions

- What is the nature of the degenerate object: BH orNS?
- BH:  what is the mass? How does accretion occur? How are jets formed?
- Ns: what is the equation of state?
- What can we learn about the companion star?
- What is the XRB role in stellar evolution? Can we learn about evolution from XRB statistics or abundances?
- “Physics of extremes”
- Strong gravity
- High magnetic field
- XRBs can be a laboratory for study of more distant sources

## outline

- Background
- List of subclasses
- Science examples
- What have we learned?

## High-mass X-ray binaries (HMXB)

- Among the first discovered extra-solar sources (eg. Vela X-1, Cyg X-1, Cyg X-3, Her X-1)
- Often contain pulsar
- Often eclipsing
- Pulse timing + stellar radial velocity +eclipses = mass, orbital separation, inclination determination
- Accretion can occur from wind from primary, or from Roche-lobe overflow
- Two different subtypes:
- Be binaries
- Supergiant binaries
- Statistics: ~50 known in galaxy
- Young population, lifetime ~105 yrs: mass transfer is unstable

## Pulse arrival times for Cen X-3

- Kelley et al., 1983

## High-mass X-ray binaries (HMXB)

- Among the first discovered extra-solar sources (eg. Vela X-1, Cyg X-1, Cyg X-3, Her X-1)
- Often contain pulsar
- Often eclipsing
- Pulse timing + stellar radial velocity +eclipses = mass, orbital separation, inclination determination
- Accretion can occur from wind from primary, or from Roche-lobe overflow
- Two different subtypes:
- Be binaries
- Supergiant binaries
- Statistics: ~50 known in galaxy
- Young population, lifetime ~105 yrs: mass transfer is unstable

## Evolution of a supergiant hmxb

## Evoution of Be binary

## (Corbet 1986)

- Be binaries show a strong correlation between X-ray pulse period and orbital period

## Theory of wind accretion: (Davidson and Ostriker 1974):

- vw2/2 = GM/r

## Too much

- Not enough

- Lx~Mwind c2G2M2/(a2 vw4)

- We can test whether the X-ray source is fueled by wind accretion

**Observed X-ray luminosity**

**From similar stars**

**separation**

**Efficiency Guess: ~0.1**

**Wind speed**

- (Conti 1976)

## Slide 14

## LMXB

- Many well-known sources (eg. Sco X-1, ‘microquasars’: GRO J1655, 4U1916)
-  <10 Hz pulsars are less common
- Eclipses less common
- Orbit determinations more difficult
- Accretion can occur from Roche-lobe overflow
- Many subtypes: Z sources, atoll sources, dippers, adc sources, pulsars, microquasars, black hole transients
- Statistics: ~200 known in galaxy
- bulge population, lifetime ~106 yrs

## Evolution of a low-mass binary

## Slide 17

## Slide 18

## Z-sources

- Among best-known LMXBs: Sco X-1, Cyg X-2,
- Trace out ‘Z’ shape in color-color diagram (Hasinger, 199x; Vander Klis 199x); QPO behavior follows

- Hasinger and VanderKlis 1989

## Atoll sources

- Trace out ‘island’ or ‘banana’ in color-color diagram
- Truly different from Z-sources?

- Hasinger and VanderKlis 1989

## ADC sources

- LMXBs which are viewed at high inclination
- Appearance is dominated by scattered emission from (hot) gas at high lattitudes above the plane of the orbit
- Potential ‘rosetta stone’ for studying accretion disk, heating
- Rare: <5  known

- White and Holt 1982

## dippers

- Show ‘dips’ in orbital lightcurve
- Likely Similar to adc sources, but slightly lower inclination
- More numerous than adc sources
- Provides complementary insight into disk above and below the orbital plane

- Smale et al. 1988

## ‘Pulsars’

- LMXBs showing pulsations:  cluster near ~6 seconds
- + msec burst oscillations observed from some burst sources
- Many spun up neutron stars are expected

## Microquasars and black hole transients

- Gierlinski and Done 2005

## Microquasars and black hole transients

- BHT show outbursts, factor ~108, on timescale > yrs.
- ~20 black hole candidates known
- Microquasars show jets in the radio and X-ray + superluminal expansion

- Mirabel and Rodriguez 1995

## X-ray burst sources

- Accreted material can burn He explosively on NS surface
- Requires slow rate of accretion --> atoll sources
- ‘relaxation oscillator’: burst fluence, interburst interval anticorrelated

## The lmxb zoo..

| name | CO | i | # | remarks |
| --- | --- | --- | --- | --- |
| adc | (ns) | hi | <10 | Sinusoidal lightcurve |
| dipper | (ns) | Med-hi | 10-20 | dips |
| burster | ns | ? | many | X-ray bursts |
| quasar | bh | Med-hi | <10 | jets |
| BHT | bh | ? | many | outbursts |
| Z-source | (ns) | Low? | <10 | Color-color:Z |
| atoll | (ns) | Low? | many | Color-color:atoll |
| pulsar | ns | ? | <10 | 1s<P<10s |
| msec pulsar | ns | ? | ? | P~.01 s |

## XRB taxonomy

- Compact object

- Other factors

- Science questions

- early

- late

- bh

- ns

- High i

- Med-hi i

- HMXB

- LMXB

- BHT

- burster

- dipper

- ADC

- Be

- supergiant

- Bursts

- Outburst mechanism?
- BH mass?
- Jet/wind

- Disk/adc structure?
- Abundances?

- Ns equation of state

- Pulsar physics
- Wind structure

- z

- atoll

- Disk/corona structure

## Ionization and Thermal Balance

- For each ion:
-  Ionization         =      recombination
-   ~photon flux          ~electron density
- For the gas as a whole
-   Heating            =       cooling
-  ~photon flux             ~electron density
- => All results depend on the ratio photon flux/gas density or "ionization parameter"

## Consequences of Photoionization

- Temperature lower for same ionization than coronal, T~0.1 Eth/k
- Temperature is not a free parameter
- Temperature depends on global shape of spectrum
- At high ionization parameter, the gas is fully ionized, and the temperature is determined by Compton scattering and inverse T=<E>/4k
- Ionization balance is more 'democratic'
- Microphysical processes, such as dielectronic recombination, differ
- Observed spectrum differs

## Ionization fractions of elements in a photoionized gas

- Neutral <----------->fully stripped

- =X-ray flux/gas density=4F/n

## Comparison of ionization balance in photoionized and coronal gas

- => Photoionized gas is more ionized at a given temperature, and each ion exists over a broader range of temperatures

**photoionized**

**coronal**

## Observed Spectrum: Emission

- In coronal gas, need kTe~E to collisionally excite lines.
- In a photoionized gas there are fewer lines which satisfy this condition.
- Excitation is often by recombination cascade
- Also get recombination continua (RRCs) due to recombination by cold electrons directly to the ground state.  The width of these features is directly proportional to temperature
- Due to the democratic ionization balance, it is more likely that diverse ions such as N VII, O VIII, Si XIV can coexist and emit efficiently than it would be in a coronal gas
- Inner shell ionization and fluorescence is also important in gases where the ionization state is low enough to allow ions with filled  shells to exist.

## Density dependence of He-like lines

- Coronal                   photoionized

- (Porquet and Dubau 1998)

## Cyg X-3 Chandra HETG (Paerels et al. 2000)

## Vela X-1 (Watanabe et al. 2006)

## Cyg X-3 Chandra HETG (Paerels et al. 2000)

## Vela X-1: line offsets (Watanabe et al. 2006)

## X-ray spectra of HMXBs

- Clearly show evidence of photoionization:
- RRCs
- Inner shell lines
- He-like triplet
- RRC detection allows temperature determination: ~1-2 eV
- He-like triplet shows evidence for resonance scattering
- Velocity offsets of lines vs. orbital phase
- Inner shell lines of Si, S allow X-ray probe of low ionization material
- Interpretation is complicated, requires detailed modeling of gas dynamics and ionization

## XRB taxonomy

- Compact object

- Other factors

- Science questions

- early

- late

- bh

- ns

- High i

- Med-hi i

- HMXB

- LMXB

- BHT

- burster

- dipper

- ADC

- Be

- supergiant

- Bursts

- Outburst mechanism?
- BH mass?
- Jet/wind

- Disk/adc structure?
- Abundances?

- Ns equation of state

- Pulsar physics
- Wind structure

- z

- atoll

- Disk/corona structure

## Geometry of  heated accretion disk + coronal in LMXB

- Jimenez-Garate et al. 2002

## Effect of X-rays on atmosphere structure

- Jimenez-Garate et al. 2002

## Chandra Spectrum of Her X-1

- (Jimenez et al. 2005)

## Chandra Spectrum of Her X-1 in the iron line region

- (Jimenez et al. 2005)

- From H/He-like ratio
- + models ionization balance
- ==> can set limits on location of Fe emitting gas

## (Jimenez et al. 2005)

- Spectrum of Her X-1: Mg XI line

- R ratio < 1 => upper limit on density
- Photoexcitation => lower limit on EUV flux, distance

## (Schulz et al. 2002)

- Chandra spctrum of 4U1626-67 (6 sec pulsar)

- Ne X L

## detail of 4U1626-67 spectrum, 

- (Schulz et al. 2002)

- detail of 4U1626-67 spectrum,

- => Lines show double peaks, characteristic of accretion disk

## XRB taxonomy

- Compact object

- Other factors

- Science questions

- early

- late

- bh

- ns

- High i

- Med-hi i

- HMXB

- LMXB

- BHT

- burster

- dipper

- ADC

- Be

- supergiant

- Bursts

- Outburst mechanism?
- BH mass?
- Jet/wind

- Disk/adc structure?
- Abundances?

- Ns equation of state

- Pulsar physics
- Wind structure

- z

- atoll

- Disk/corona structure

## (Cottam et al., 2001)

- Chandra HETG spectrum of ADC source 4U1822-37

- Emission lines are strongest at phase 0.25
- => Bulge on disk edge

## XRB taxonomy

- Compact object

- Other factors

- Science questions

- early

- late

- bh

- ns

- High i

- Med-hi i

- HMXB

- LMXB

- BHT

- burster

- dipper

- ADC

- Be

- supergiant

- Bursts

- Outburst mechanism?
- BH mass?
- Jet/wind

- Disk/adc structure?
- Abundances?

- Ns equation of state

- Pulsar physics
- Wind structure

- z

- atoll

- Disk/corona structure

## Dipper iron line spectrum 

- Boirin and Parmar 2002)

- Dipper iron line spectrum

- Essentially every dipper shows evidence for highly ionized iron obsorption
- Discovered with XMM CCD detectors
- Absorption is in addition to gas resposible for dips
- It is likely that we are seeing the accretion disk corona in absorption
- In one case, possible indication of outflow

## XRB taxonomy

- Compact object

- Other factors

- Science questions

- early

- late

- bh

- ns

- High i

- Med-hi i

- HMXB

- LMXB

- BHT

- burster

- dipper

- ADC

- Be

- supergiant

- Bursts

- Outburst mechanism?
- BH mass?
- Jet/wind

- Disk/adc structure?
- Abundances?

- Ns equation of state

- Pulsar physics
- Wind structure

- z

- atoll

- Disk/corona structure

## Iron line spectrum from GRS 1915+105

- (Lee et al. 2002)

- Many black hole transients show absorption by highly ionized iron during outblurst
- Lines are blueshifted, indicating outflows with v~500-1000 km/s
- Such outflows are similar to those observed in AGN
- It’s likely that most of the outflow is fully stripped
- Mass fluxes are difficult to estimate

## spectrum of GRO J1655-304 obtained during outburst

- It shows absorption lines from H- and He-like species of every element between Ne and Ni
- This is the first time many of these elements have been detected in X-ray astronomy
- Presents interesting questions about the evolutionary origins of the companion star
- Lines of Fe XXII allow density determination => test of dynamics

## spectrum of GRO J1655-304 obtained during outburst

- It shows absorption lines from H- and He-like species of every element between Ne and Ni
- This is the first time many of these elements have been detected in X-ray astronomy
- Presents interesting questions about the evolutionary origins of the companion star
- Lines of Fe XXII allow density determination => test of dynamics

## Iron K line from Cyg X-1

- Iron K lines are diagnostic of reprocessing in very optically thick material, eg. The accretion disk
- Relativistically broadened iron K lines are found in some AGN
- The galactic black hole candidate Cyg X-1 shows a line with is composite: it has both broad and narrow components, but only in low/hard states

- Miller et al., 2003

## summary

- X-ray binaries exhibit a wide range of behaviors, but much of the interesting physics/astrophysics is common to all
- Understanding of accretion disks, accretion flows, X-ray induced winds, compact obect evolution are all in a primitive state.
- Spectra provide new insights: importance of outflows, broad and narrow iron lines, importance of X-ray heating of accretion disks

## references

- Dippers: Smale et al. 1988 MNRAS 232 647
- Black hole transient lmxbs:  Remillard and McClintock, 2006 ARAA 44, 49
- Color-color diagrams for atoll/Z sources : Hasinger and VanderKlis 1989
- Microquasar GRS 1915+105: Mirabel and Rodriguez 1995 PNAS 92 11390
- ADC sources:  White and Holt 1982 Ap. J. 257 318
- Iron line from Cyg X-1:  Miller et al. 2003 Ap. J. 578, 348
- Cyg X-3 Chandra HETG: Paerels et al. 2000 Ap. J. 533, 135
- Accretion disk corona modeling: Jimenez-Garate et al. 2002 Ap. J. 558, 458
- 4U1822-37 spectrum :Cottam et al., 2001 Ap. J. 557, 101
- ‘Accretion power in Astrophysics’ Frank, King and Raine
- Catalog of X-ray Binaries, Liu Van Paradijs and Lewin 2007 A&A 469, 807
- GRO J1655 chandra spectrum:  Miller et al., 2006 Nature 441, 953
- Hydrodynamics of HMXB winds:  Blonding 1994 Ap. J.

## Slide 59