Globular clusters, those dense clusters of ancient stars that
are the oldest stellar systems in galaxies and our Milky
Way, are rich targets for study with the Chandra
X-ray Observatory. Their stellar densities are so high (a million times
more stars per unit volume than in the solar neighborhood) that
encounters between individual stars and between binaries (pre-existing)
and single stars are frequent enough that binary populations
including exotic objects (neutron stars and white dwarfs) are
created ~100X more efficiently than in
the Galaxy as a whole. Chandra has revealed the
rich population of X-ray binary systems containing neutron stars
in quiescent low mass X-ray binaries (qLMXBs) and white
dwarfs in cataclysmic variables (CVs). Deep Chandra images of
globulars have also discovered the much larger population
of qLMXB descendents, the millisecond pulsars (MSPs), with neutron
stars spinning some 500 times per second and detected by the
X-ray emission from their hot polar caps. Some of these MSPs
have acquired new binary partners to replace those that spun them
up originally: Chandra (and correlative HST) observations have
revealed the partner swapping in these crowded stellar swarms.
And finally the active binaries -- normal cluster stars in close
binary pairs which in some cases are surprisingly luminous X-ray
sources showing highly active and flaring coronae never anticipated
for such ancient stars.
The remarkable high resolution X-ray (Chandra) and optical (HST)
observations of globulars reveal reveal entirely new facets of compact
objects, stellar binary systems and the very evolution of the clusters
Deep Chandra image of the well-studied globular 47 Tuc
in which some 300 compact binaries are detected (the brightest ~150 are
visible in this image) of varying X-ray color (temperature). During the
4 days Chandra pointed to make this image, many of these objects showed
remarkable variability. (Chandra image from Heinke et al 2005, ApJ, 625, 796
and astro-ph/0503132). Click on image to enlarge.
Jonathan Grindlay, Slavko Bogdanov, Peter Edmonds, Maureen van den Berg
Moderately deep Chandra image of a "core collapsed" globular
cluster, NGC 6397 which has both created and destroyed compact binaries.
X-ray sources are color coded by temperature (blue are hottest);
classes of optical counterparts (from HST) are marked. (Chandra image from
Grindlay 2006, Adv. Sp. Res., 38, 2923 and astro-ph/0605133).Click on image to enlarge.
Illustration (to scale) of the millisecond pulsar 47Tuc-W
(blue dot to right) that is blasting its "normal" star binary companion
star (orbiting it every 3.2 hours) with high energy particles. The
gas that otherwise might fall on the neutron star is instead heated by the shock produced by the pulsar. wind and produces high energy
X-ray emission. (image from Bogdanov, Grindlay and van den Berg 2005,
ApJ, 630, 1029 and astro-ph/0506031). Click on image to enlarge.