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Home , 47 Tucanae

47 Tucanae: A New Laboratory for GeV Millisecond

Matthew Kerr University of Washington On behalf of the Fermi LAT Collaboration

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 1 Globular Clusters: Overview

• Tightly-bound, roughly spherical collections of 10^4 to 10^6 Population II orbiting in Galactic halo; 158 known • Sharp main sequence turnoff in HR diagram indicates all stars have roughly the same age  distance calibration – Uniform age, distance, and make globular clusters unique stellar laboratories • Dynamics: a collisional stellar gas dominated by gravitational interactions – Relaxation time (time for stellar encounters to erase initial conditions) of ~10^8 years – Mass segregation, high sphericity, very high central densities (up to 10^6 stars/ pc^3) • Binaries are a natural consequence of high density: – Formation via tidal capture – Exchange through three-body interactions – Store energy: “hard binaries get harder” • Thus, globular clusters with dense centers are expected (and observed) to host many binaries and binary “products”.

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 2 7/14/2009 TeV Particle Astrophysics 2009 - SLAC 3 47 Tucanae: in Optical; History

• 47 Tucanae (Flamsteed; 18th C designation); NGC 104 (New General Catalog, 19th C designation) • 10^6 stars; second brightest cluster to (Bayer; 17th C designation) • ~11 Gyr old (from isochrone fitting, Gratton et al. A&A 408, 529–543 (2003)) • Main sequence turnoff around 0.85-0.9 solar masses • Half-mass radius: 1-2 arcminutes • Full extension: about 0.5 degrees, the size of the • Distance: – 4kpc (+/- 10%) kinematically estimated (McLaughlin et al, APJS Sept 2006) – 4kpc from cooling – 4.5-5 kpc from main sequence fitting

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 4 47 Tuc Core in X-rays

(Heinke et al. APJ June 2005)

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 5 47 Tuc in X-rays

• X-rays reveal the dramatically different nature of GCs from the field – binaries, binaries, binaries! • ~300 Chandra sources (including ~70 background): • LMXBs – Neutron stars sink to center, pick up a MS companion; CE phase  hard binary – neutron stars accreting from low mass companions; quiescent and active – generally most luminous, a hard spectrum • MSPs – neutron stars sufficiently spun up from to re-establish magnetospheric gaps – many systems extremely short period (< 1 day) – typically a soft thermal spectrum from particle heating of polar cap, but non-thermal emission from magnetosphere or interactions with a companion possible • CVs – accreting white dwarfs; potentially Type 1A SN progenitor – also among the more luminous objects, hard spectrum • chromospherically-active binaries

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 6 47 Tuc in Radio

• 23 millisecond pulsars have been detected in 47 Tuc • Drive discoveries in other wavebands and their own:

Freire et al., APJL July 2001 7/14/2009 TeV Particle Astrophysics 2009 - SLAC 7 Millisecond Pulsars in Clusters

• Radio detection is generally sensitivity limited • 140 detected so far in 26 clusters (recall 23 in 47 Tuc; see Paulo Freire’s webpage) • Globular clusters provide – a well-determined distance, – nearly-identical dispersion measures, – a large sample (particularly compared to field) – Millisecond Laboratory! • For example, X-ray studies have shown the X-ray/spindown luminosity relationship for MSPs in 47 Tuc is flatter than for those in the field, pointing to possible differences in the surface magnetic field or neutron properties – Caveat: acceleration in cluster potential makes determining intrinsic period derivative difficult

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 8 GeV Millisecond Pulsars

• New class! Fermi detects 8 millisecond pulsars in the field • Low luminosities: 10^32 – 10^33 erg / s • Spectra similar to young GeV pulsars – Photon indices 1.3-2.1 – Cutoff energies 1-3 GeV • Sensitivity-limited: we detect MSPs within ~1 kpc – But, also limited by radio sensitivity (need ephemeris for detection of pulsations) • Predictions from models depend strongly on geometry (line of sight, orientation of magnetic axis and spin axis) and distance (flux) – Many (GeV) MSPs have parallax measurements of distance – But geometry still uncertain • Globular clusters provide a natural ensemble of GeV MSPs, reducing uncertainties by perhaps an order of magnitude over field pulsars -> a robust connection of models to observations 7/14/2009 TeV Particle Astrophysics 2009 - SLAC 9 GeV Detection of 47 Tucanae

• GeV Emission (>17 PRELIMINARY sigma) consistent with core of 47 Tucanae – point source; LAT cannot resolve core at GeV energies • Emission is truly DC – No variability detected on daily, weekly, fortnightly, or monthly scales – No significant pulsations found in search for 21 MSPs with timing solutions

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 10 Spectrum

PRELIMINARY

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 11 Interpretation

• Spectral shape consistent with field MSPs • Emission from pulsar wind shocks possible but disfavored due to low spindown luminosity and spectral break observed at GeV scale – H.E.S.S. upper limit of 7e33 erg/s (>800 GeV) • Observed isotropic gamma-ray luminosity (4.8 x 10^34 erg /s) and spindown power (1.8 x 10^34 erg / s) give the efficiency

• Observed value agrees with population-averaged predictions of 6% (3D GR polar cap) to 10% (polar cap) • If we take the efficiency of the closest field pulsars, 8%, this gives

• Combined with X-ray estimates, suggests a population of 50 MSPs in 47 Tuc and a radio beaming fraction ~0.5 that of GeV emission

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 12 Eye for the Future

• Detection of additional clusters in DC • Detection of pulsations from brightest individual MSPs • Tighter constraints on spectral parameters, gamma-ray luminosities

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 13