Understanding LMXBs in Elliptical Galaxies

Vicky Kalogera

Low-Mass X-Ray Binaries

CXC Image Archive Accretors: NS or BH

RLOF Donors: MS, RG, WD/degenerate

low-mass: < 1Mo

Binary Periods: minutes to ~10 days

Ages: old, ~ 0.1 - 10 Gyr

Persistent X-rays:~10 Myr - ~1 Gyr

LMXBs form in both galactic fields (isolated binaries) globulars (dynamical interactions)

courtesy Sky & TelescopeFeb 2003 issue

How doLow-Mass X-ray binaries form in galactic fields ?

primordial binary

Common Envelope:orbital contractionand mass loss

NS or BH formation

X-ray binary at Roche Lobeoverflow

LMXB Population Modeling

Population Synthesis Calculations: necessary Basic Concept of a Statistical Description: evolution of an ensemble of binary and single stars with focus on XRB formation and their evolution through the X-ray phase

(ideally in both galactic field and globulars).

Population Synthesis Elements Star formation conditions: > time and duration, metallicity, IMF, binary properties

Population Synthesis Elements Star formation conditions: > time and duration, metallicity, IMF, binary properties

Modeling of single and binary evolution > mass, radius, core mass, wind mass loss > orbital evolution: e.g., tidal synchronization and circularization, mass loss, mass transfer > mass transfer modeling: stable driven by nuclear evolution or angular momentum loss thermally unstable or dynamically unstable > compact object formation: masses and supernova kicks > X-ray phase: evolution of mass-transfer rate and X-ray luminosity

Population Synthesis Elements Star formation conditions: > time and duration, metallicity, IMF, binary properties

Modeling of single and binary evolution > mass, radius, core mass, wind mass loss > orbital evolution: e.g., tidal synchronization and circularization, mass loss, mass transfer > mass transfer modeling: stable driven by nuclear evolution or angular momentum loss thermally unstable or dynamically unstable > compact object formation: masses and supernova kicks > X-ray phase: evolution of mass-transfer rate and X-ray luminosity

Population Synthesis Elements Star formation conditions: > time and duration, metallicity, IMF, binary properties

Modeling of single and binary evolution > mass, radius, core mass, wind mass loss > orbital evolution: e.g., tidal synchronization and circularization, mass loss, mass transfer > mass transfer modeling: stable driven by nuclear evolution or angular momentum loss thermally unstable or dynamically unstable > compact object formation: masses and supernova kicks > X-ray phase: evolution of mass-transfer rate and X-ray luminosity

Population Synthesis Elements Star formation conditions: > time and duration, metallicity, IMF, binary properties

Modeling of single and binary evolution > mass, radius, core mass, wind mass loss > orbital evolution: e.g., tidal synchronization and circularization, mass loss, mass transfer > mass transfer modeling: stable driven by nuclear evolution or angular momentum loss thermally unstable or dynamically unstable > compact object formation: masses and supernova kicks > X-ray phase: evolution of mass-transfer rate and X-ray luminosity

Population Synthesis Elements Star formation conditions: > time and duration, metallicity, IMF, binary properties

Modeling of single and binary evolution > mass, radius, core mass, wind mass loss > orbital evolution: e.g., tidal synchronization and circularization, mass loss, mass transfer > mass transfer modeling: stable driven by nuclear evolution or angular momentum loss thermally unstable or dynamically unstable > compact object formation: masses and supernova kicks > X-ray phase: evolution of mass-transfer rate and X-ray luminosity

Population Synthesis Elements Star formation conditions: > time and duration, metallicity, IMF, binary properties

Modeling of single and binary evolution > mass, radius, core mass, wind mass loss > orbital evolution: e.g., tidal synchronization and circularization, mass loss, mass transfer > mass transfer modeling: stable driven by nuclear evolution or angular momentum loss thermally unstable or dynamically unstable > compact object formation: masses and supernova kicks > X-ray phase: evolution of mass-transfer rate and X-ray luminosity

Our population synthesis code:StarTrack

Belcynski et al. 2006

including (simple) cluster dynamics:Ivanova et al. 2005

XLFs in Elliptical Galaxies

(3-4)x1036 - (5-6)x1038 erg/s

XLF slope: 0.9 +- 0.1

Fabbiano et al., Kim et al. 2006

Field LMXB models for NGC 3379 and NGC4278

Star Formation: delta-function at t=0Population Age: 9-10 GyrMetallicity: Z=0.03 (1.5 x solar)Total Stellar Mass:3 x 1010 Mo

Binary Fraction: 50%Initial Mass Fn: power-law index -2.7 (Scalo/Kroupa)

also -2.35 (Salpeter)CE efficiency: 50%

also: 100%

Fragos, VK, Belczynski, et al. 2007

See poster by Fragos et al. (#155.01)

Field LMXB models for NGC 3379 and NGC4278

best-fit XLF slope: 0.9

NS accretors dominate over BHs

Transients in outburstmore numerous than Persistent sources

XLF shape depends ontransient Duty Cycle:Lout=min (LX/DC, 2LEDD)i.e., empty disk massaccumulated duringquiescenceDC ~ 15-20% favored

Field LMXB models for NGC 3379 and NGC 4278

NS accretors dominate over BHs

Transients in outburstmore numerous than Persistent sourcesLout=min (LX/DC, 2LEDD)DC ~ 15-20% favored

Lout dependent on Porb (claimed for MW BHs) clearly inconsistent with data

Field LMXB models for NGC 3379 and NGC 4278

Dominant LMXB Donor Types:

< ~5x1036 erg/stransient LMXBs with MS donors5x1036 - 2x1037 persistent LMXBs withRG donors> ~2x1037 transient LMXBs withRG donors

(not just transient RGas in Piro & Bildsten 2002)

Field LMXB models for NGC 3379 and NGC 4278

LMXBs contributingto the observed XLF:

LX > 5x1036 erg/s

Field LMXB models for NGC 3379 and NGC 4278

Short & old (10Gyr ago)star formation episodedoes NOT lead to similar LMXB formationpattern

LMXB formation rate: very high at ~500Myr but continues at lower levels for 10Gyr to present

Short-lived LMXBs (e.g., persistent ultra-compacts)follow the LMXB formation rate pattern and NOT the star formation of the galaxy

Field LMXB models for NGC 3379 and NGC 4278

Model Normalization depends on: assumed total galaxy mass (3x1010 Mo)assumed binary fraction (50%)

Total Galaxy Mass depends on: total stellar lightassumed mass-to-light ratio (uncertain by ~2)

NGC 3379: 1-3 x 1010 Mo (uncertain by ~3)NGC 4278: same (within 25%) total stellar light

Models favored based on XLF slope naturallygive normalization consistent with observations: NGC 3379: within ~3NGC 4278: within 15%

LMXBs in Globular Clusters

Bildsten & Deloye 2002:

NS with WD donors in ultra-compact binaries ( ~10 min orbital periods)

persistent, short-lived (1-10Myr), continually formed through dynamical interactions

XLF slope (~ 0.8) and normalization consistent with observations (within uncertainties)up to ~5x1038 erg/s

LMXBs Above the 'Break' ...

Ivanova & Kalogera 2006:BH transients in outburstRG or MS donors

XLF slope possible tracer of BH mass spectrum

... @ (4-5)x1038 erg/s (i.e., NS Eddington limit for He)

King 2002:BH transients in outburstwide orbits, RG donors

Sarazin et al. 2001: LMXBs with BH accretors

Bright XRBs in GCs ??

Kalogera et al. 2004: 1-2 BH LMXBs per clusterBUT low detection probability(transients)

LMXBs in Elliptical Galaxies

Slope and Normalization of XLF in ~5x1036 – 5x1038 erg/s can be explained by both:

Field NS-LMXBs with low-mass MS and RG donors (transient & persistent) GC ultra-compact NS-LMXBs (persistent)

Current Conclusions – Open Issues

Q: Points to contributions from both field and clusters, but how can different LMXB types give similar XLF slope &normalization? Bright-end XLF could be due to transient BH-LMXBs in outburst

Field and GC XLFs similar, but note: small-N sample

Q: Given BH evolution in GCs and transient nature, are there too many bright point sources in GCs ?Q: Could bright sources in GCs be due to superposition ?

Q: Could all bright sources be simply super-Eddington NS-LMXBs (by x10!) ? Where are the BH-LMXBs, similar to transients in the Milky Way?

LMXBs in Elliptical Galaxies

Models of Field NS-LMXBs are favored with: Transient DC ~15% Outburst Lx connected to long-term mass transfer rate and DC:

empty disk mass accumulated during quiescence Moderate CE efficiencies Shape changes at ~1x1037 erg/s could be connected to outburst Lx and DC

Current Conclusions – Open Issues

Even in the field LXMB formation rate is sustained over long timescales after an early phase of enhanced formation