KITP: γ-rays from globulars

Fermi Gamma-ray Telescope has results.
We hear about them.

why, yes, that really was 47 Tuc


"Discovery of γ-ray sources from the globular cluster 47 Tuc" or something similar, in preparation, at a glossy journal near you soon

in the mean time, browse the LAT bright source catalog (for high galactic latitude - none has dared descend into the plane yet...)

this will really screw up starry eyed particle (astro)physicists who were desperately hoping to find cold dark matter annihilation radiation from substructure in the halo

we temporarily ignore gamma-ray bursts...
we will return to your normal schedule soon, in which we really do desperately care about GRBs

So the LAT - Large Area Telescope - has sensitivity between 100MeV and 300 GeV
PSF is about 3 degrees at 100 MeV and 0.1 degrees at GeV... not much to be done about that...
gratuitous taunts of [other] telescopes deleted... ;-)

2.5 steradian field of view, operates in sky-scanning mode covering whole sky every three hours.
Need about a day of integration to see even the brighter sources; but you build up signal and get variability on longer time scales

There is always the Crab - provides nice calibration source;
sensitivity after ~ 5 years integration you get to maybe a milliCrab (or better, if they're seeing 10 milliCrabs in 1/4 of a year with high confidence) and expect to see 1000-10,000 sources.

So, why go to these energies? Since sensitivity and resolution kinda suck...
well, there are no thermal sources up there - so probe directly relativistic acceleration and annihilation radiation
and straddle possible energy range in which you might see photon pairs from cold dark matter; assuming cold dark matter annihilates and there is significant branching ratio to photon pairs, and the cross-section for annihilation is large enough for the rate to be finite from anywhere close enough...

so we see (first 3 months):

bright sources:
galactic plane - photons from high energy cosmic rays hitting gas, and SNRs

faint sources:
globular cluster(s)
dark matter clumps (maybe)
other - like starbursts, star forming galaxies, elliptical galaxies, clusters of galaxies, unknown

Robust identification of isotropic diffuse gamma rays from Galactic dark matter"

so the hope was to see dark matter annihilation in halo subclumps and the galactic center
basic premise is the somewhat naive hope that CDM follows divergent density profiles, as seen in dark matter cosmology simulations, and, better still, that the growth of supermassive black holes provides adiabatic contraction of dark matter halos to higher densities with steeper denisty profiles. (like so)

Problem is, that dwarf spheroidals don't show dark matter cusps: they have a lot of dark matter, and high mass-to-light ratios, but generally they have rigid rotation profiles in the inner regions, consistent with constant dark matter density profiles at relatively low densities.
Yes, it is a puzzle, most likely to do with baryonic matter evolution and violent relaxation, or possibly some more exotic dark matter physics, but that is how it is.

If you get spectra, or lots of photons and good timing, you may discriminate - CDM annihilation should have sharp cutoff in the spectra at the CDM mass*c^2, and pulsars shold pulsate - but with low photon statistics and unknown periods finding a good period through blind search will be very hard at short periods
of course we don't know (the) mass of cold dark matter particles, or the scattering cross-section, or the density, or the branching ratio

so do we feel lucky?
well, don't know till you look...

oh, and faint blazars are also a problem - their (power law) spectrum at high energy is poorly constrained; their population is unknown; and depending on the poorly known extragalactic photon background you may get cutoffs of blazar spectra at high energy due to photon-photon scattering which mimic the CDM annihilation spectra

they look for angular correlations to see if there is structure in faint γ ray sources consistent with a halo population of annihilating CDM in cores of subhalos

hm, you care about the centers and maybe were streams intersect
we expect few thousand +- subhalos, if you believe low mass predictions of extended Press-Schechter spread over the sky
so maybe one unresolved source per few tens of square degree
so see excess energy at l ~ 100 +-
statistically robust, IF the signal is there and there is nothing else that mimics that sky distribution

which brings us to pulsars in the halo, specifically in globulars

so they see 47 Tuc in high energy gamma rays
presumably from the pulsars (47 Tuc famously has lots of pulsars...)

10 milliCrabs --- 5*10^34 erg/s
details are classified and will not be reported here - read the paper

don't know γ-ray luminosity function of psrs so we don't know if we're seeing lucky brightest psr or integrated emission of all of them

but this is not an issue, probably, since they can cut out the plane and bulge and there are few enough globulars left that the angular correlation scale will be at much lower l and the brighter clusters will be seen as point sources and can be subtracted



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