Low-Mass X-Ray Binary Models for Ellipticals NGC3379 and NGC4278

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1 Low-Mass X-Ray Binary Models for Ellipticals NGC3379 and NGC4278 Tassos Fragos with V. Kalogera, K. Belczynski, G. Fabbiano et al. Department of Physics and Astronomy Northwestern University MODEST 7b Philadelphia 2007 T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

2 Low-Mass X-Ray Binaries (LMXBs) Illustration Credit: Tony Piro LMXBs form in both: galactic field (isolated binaries) globular clusters (dynamical interactions) Accretors: NS or BH RLOF Donors: MS, RG, WD/degenerate low-mass: < 1M Binary Periods: minutes to 10 days Persistent or Transient Persistent phase: 10 Myr 1 Gyr Transient phase: DC = T outburst T outburst +T quiescent T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

3 Observationally... Detect X-ray source populations Spectra consistent with LMXBs Study X-ray Luminosity Functions (XLF) Shape (components, evolution) Normalization (what drives the LMXB content of a galaxy?) Study spatial distributions Sources in the Field Sources in GCs Study of source variability Transients? T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

4 Kim, E. et al GC vs. Field Kim & Fabbiano 2004 E and S0 XLFs have similar shapes (> erg/s) Break at erg/s seen in composite XLF Slope consistent with Galactic and M31 LMXB XLFs Overall cumulative slope -1 XLFs of GC and field LMXBs are consistent? T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

5 XLFs in ellipticals NGC3379 and NGC4278 Fabbiano et al., Kim et al (3 4) 6 (5 6) 8 erg/s XLF slope: 0.9 ± 0.1 T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

6 How do Low-Mass X-ray binaries form in galactic fields? Start from a primordial binary The system undergoes a Common Envelope (CE) phase, which results to orbital contraction and mass loss. An LMXB is born when the secondary star overflows its Roche lobe and mass gets accreted onto the already formated compact object (BH or NS). T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

7 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 (Belczynski et al. 2006) T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

8 Models for NGC3379 and NGC4278 Star Formation: delta-function at t=0 Population Age: Metallicity: Total Stellar Mass: Binary Fraction: Initial Mass Function: CE efficiency: 9 10 Gyr Z=0.03 (1.5 x solar) 3 0 M 50% power-law index -2.7 (Scalo/Kroupa) or (Salpeter) 50% or 100% T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

9 Field LMXB models: XLF N(>L X )! CE = 0.5 Obs. NGC 3379 Obs. NGC 4278 DC = 30% DC = 15% DC = 1% DC = 0% best fit slope N(>L X )! CE = 1.0 Obs. NGC 3379 Obs. NGC 4278 DC = 30% DC = 15% DC = 1% DC = 0% L X (ergs/sec) L X (ergs/sec) NS binaries are dominating the population Transients in outburst are more numerous than persistent sources. A large duty cycle (DC) 10 20% is favored. T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

10 Number of systems Field LMXB models: Period Distribution! CE = 0.5 DC = 15% L X > 5x6 all systems MS donors giant donors H deg. donors He WD donors Number of systems! CE = 1.0 DC = 15% L X > 5x6 all systems MS donors giant donors H deg. donors He WD donors Orbital period (hours) Orbital period (hours) Only systems with L x > 5 6 erg/s, which contribute to the observed XLF are taken into account. α CE = 1 allows for Transient LMXBs with RG donors in very wide orbits and makes the XLF flatter. T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

11 Models for LMXBs in Globular Clusters Bildsten & Deloye 2002 NS with WD donors in ultra-compact binaries ( 10 min periods) persistent, short-lived (1-10Myr), continually formed through dynamical interactions XLF slope ( 0.8) and normalization consistent with observations (within uncertainties) up to 5 8 erg/s Future work: StarTrack + FewBody (Ivanova, N. et al. 2004) Single and binary star evolution with StarTrack. Simplistic treatment of cluster dynamics: Two zone structure (halo and core), cluster properties do not change with time. Close interactions are calculated with FewBody (Fregeau, J. 2004), a numerical toolkit for small-n gravitational dynamics. T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

12 Open questions... How can different LMXB types give similar XLF slope & normalization? Is it a problem of small N statistics in the observations? How should we treat the outburst luminosity of transient systems? What are the sources with L x > 5 8 erg/s? Are there too many bright point sources in GCs? Are they BH or NS? Could bright sources in GCs be due to superposition? Could all bright sources be simply super-eddington NS-LMXBs? T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

13 Current conclusions Slope and normalization of XLF in erg/s can be explained, within the known uncertainties, by both: Field NS-LMXBs with low-mass MS and RG donors GC ultra-compact NS-LMXBs The field LXMB formation rate is sustained over long timescales. Models of Field NS-LMXBs are favored with: Transient DC 15 % Moderate CE efficiencies 50 % T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

14 LMXB donor types in galactic field models N(>L X )! CE = 0.5 DC = 15% total XLF MS donors giant donors H degenerate donors He WD donors N(>L X )! CE = 1.0 DC = 15% total XLF MS donors giant donors H deg. donors He WD donors L X (ergs/sec) L X (ergs/sec) T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

15 Field LMXB models: formation rate! CE = 0.5 Formation rate (systems/myrs) all XRBs persistent XRBs 0 2x 4x 6x 8x 10 4 Time (Myrs) T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16

16 Field LMXB models: Period Distribution II Number of systems! CE = 0.5 DC = 15% L X > 5 all systems MS donors giant donors H deg. donors He WD donors Number of systems! CE = 1.0 DC = 15% L X > 5 all systems MS donors giant donors H deg. donors He WD donors Orbital period (hours) Orbital period (hours) T. Fragos (Northwestern University) LMXB models for Ellipticals Philadelphia / 16