Matthew Kerr

University of Washington

On behalf of the Fermi LAT Collaboration

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 1

47 Tucanae: A New Laboratory for GeV Millisecond Pulsars

Globular Clusters: Overview

• Tightly-bound, roughly spherical collections of 10^4 to 10^6 Population II stars 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 metallicity 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 Omega Centauri (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 Moon

• Distance:– 4kpc (+/- 10%) kinematically estimated (McLaughlin et al, APJS Sept 2006)

– 4kpc from white dwarf cooling

– 4.5-5 kpc from main sequence fitting

7/14/2009 TeV Particle Astrophysics 2009 - SLAC 4

47 Tuc Corein

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 accretion 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:

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

Freire et al., APJL July 2001

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 Pulsar 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 star 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 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

PRELIMINARY

Spectrum

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

PRELIMINARY

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

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

• Detection of additional clusters in DC

• Detection of pulsations from brightest individual MSPs

• Tighter constraints on spectral parameters, gamma-ray luminosities